feat(grpc): PSK-authenticate the site gRPC control plane; drop the vestigial management receptionist registration

Phase 0 of the ClusterClient→gRPC migration
(docs/plans/2026-07-22-clusterclient-to-grpc-plan.md). Standalone hardening: it
closes a gap that exists today and is a precondition for moving command/control
onto gRPC in later phases.

T0.1 — delete the ManagementActor ClusterClientReceptionist registration.
It was built for an out-of-cluster CLI that was never written: the shipped CLI
speaks HTTP Basic to /management, which asks the actor in-process through
ManagementActorHolder. Nothing in the repo ever sent to /user/management. The
actor still runs there; only the cross-boundary advertisement is gone. Six
documents claimed the CLI used ClusterClient — including the CLI's own README
"Architecture Notes" — and are corrected here rather than left to rot.

T0.2 — record, do not port, the dead integration-routing path.
IntegrationCallRequest is unwired at BOTH ends: RouteIntegrationCallAsync has
zero callers anywhere, and RegisterLocalHandler(Integration, …) appears only in
a test, so production always answers "Integration handler not available". It is
excluded from the gRPC contract (28 of 29 commands migrate) rather than
enshrined on an additive-only wire format, and deleting it during a
transport migration would mix a behavioural change into a change whose whole
value is that behaviour is identical. See
docs/known-issues/2026-07-22-integration-call-routing-is-dead-code.md.

T0.3 — preshared-key authentication on SiteStreamService.
The service shipped with no auth at all: plaintext h2c, no interceptor, so
anything that could reach a site node's :8083 could open a live data stream or
read audit rows back via PullAuditEvents/PullSiteCalls. ControlPlaneAuthInterceptor
now gates /sitestream.SiteStreamService/ — modeled on LocalDbSyncAuthInterceptor
(constant-time compare, fail-closed, PermissionDenied) but gating a SET of
service prefixes so phases 1A/1B add services rather than interceptors. LocalDb
sync keeps its own separate key: it authenticates the pair partner, not central,
and collapsing the two would make a site's central-facing key also admit writes
into its database.

Keys are per site (SB-GRPC-PSK-<siteId>), never fleet-wide, so a compromised
site yields only its own. Central attaches them through ControlPlaneCredentials,
which binds CallCredentials to the channel — covering unary and streaming
uniformly, and letting the key resolve asynchronously, which a client
interceptor could not do without blocking. All three central→site channel
creation sites go through it (SiteStreamGrpcClient and both audit pull invokers);
the pull invokers' channel caches are re-keyed by (site, endpoint) because
credentials are per-site and bound to the channel.

Two decisions beyond the plan:

  * StartupValidator now requires GrpcPsk on Site nodes. The plan specified only
    the runtime gate, but fail-closed with no boot check produces a node that
    joins, answers heartbeats and reports healthy while refusing every stream,
    audit pull and telemetry ingest — silent and total. Same reasoning as the
    existing inbound API-key pepper rule.

  * Added Communication:SitePsks as a central-side key map. The plan assumed
    central would read the store, seeded via a dev KEK; the docker rig
    deliberately boots with no master key, so store-only resolution would leave
    it unable to dial its own sites. The store stays primary — it is the only
    source that can serve a site added at runtime — with the map covering
    key-less hosts and one-off pins. Neither source falling back to
    "unauthenticated" is the invariant.

T0.4 — dev keys on both rigs and tests.
34 tests. The seven that matter most exercise a real in-process gRPC stack over
TestServer: the unit tests on either side of the wire would both stay green if
the halves disagreed, and gRPC refuses call credentials on a plaintext channel
by default — the UnsafeUseInsecureChannelCallCredentials opt-in is only provable
by making a real call. They confirm correct key passes on unary AND streaming,
wrong key and no-credentials both get PermissionDenied, and an unresolvable key
fails the call with nothing reaching the service.

OPERATIONAL: a site node upgraded to this build without a key will not boot.
That includes the gitignored deploy/wonder-app-vd03/ overlay.

CLAUDE.md

@@ -138,7 +138,7 @@ Other peers in the `scadaproj` family (see `scadaproj/CLAUDE.md` for details): `
 - **All clusters share ONE ActorSystem name**, `"scadabridge"` — hardcoded at `AkkaHostedService.cs:191`. Central and each site are separate clusters *only* by seed-node partitioning. This is required, not incidental: Akka.Remote address matching means a ClusterClient could not reach a differently-named system.
 - **`ActiveNodeEvaluator.SelfIsOldestUp` is THE single definition of "active node"** (`Communication/ClusterState/ActiveNodeEvaluator.cs`) — the **oldest Up member** in a role scope, and explicitly **never `cluster.State.Leader`**: leadership (lowest address) is an Akka-internal concept that diverges from singleton placement permanently once the original first node restarts and rejoins, and both sides claim it during a partition. The equivalence *oldest-Up == where `ClusterSingletonManager` places singletons* **is** the design. `ClusterActivityEvaluator.SelfIsOldest`, the S&F delivery gate, `/health/active` and the heartbeat `IsActive` stamp all delegate here.
 - Site nodes carry **two Akka roles**: the base `Site` plus a site-specific `site-{SiteId}` (`BuildRoles`, `AkkaHostedService.cs:386`). Singletons scope to the **site-specific** role.
-- **Neither inter-cluster transport is authenticated or encrypted.** Akka remoting sets no `enable-ssl`, no secure cookie, no `trusted-selection-paths`; the gRPC listener is **h2c** and `LocalDbSyncAuthInterceptor` gates *only* `/localdb_sync.v1.LocalDbSync/`, so the whole `SiteStreamService` surface — including the `PullAuditEvents`/`PullSiteCalls` RPCs that return audit rows — is callable by anyone who can reach :8083. The boundary assumes a trusted network. (The HTTP fetch token and the LocalDb sync interceptor are the only authenticated pieces.)
+- **The gRPC boundary is authenticated (PSK) as of 2026-07-22; Akka remoting still is not, and nothing is encrypted.** Akka remoting sets no `enable-ssl`, no secure cookie, no `trusted-selection-paths` — so the ClusterClient command/control path remains open to anyone who can reach the remoting port, and the boundary still assumes a trusted network. The gRPC listener stays **h2c**, but `SiteStreamService` is no longer open: `ControlPlaneAuthInterceptor` (`Host/ControlPlaneAuthInterceptor.cs`) gates `/sitestream.SiteStreamService/` — including the `PullAuditEvents`/`PullSiteCalls` RPCs that return audit rows — against a **per-site preshared key**, fail-closed, constant-time compared, alongside the separate `LocalDbSyncAuthInterceptor` on `/localdb_sync.v1.LocalDbSync/` with its own separate key. **Site side:** `ScadaBridge:Communication:GrpcPsk`, in production `${secret:SB-GRPC-PSK-<siteId>}`, and **`StartupValidator` refuses to boot a site node without it** (an unset key would leave the node healthy-looking but serving nothing). **Central side:** `SitePskProvider` resolves `SB-GRPC-PSK-{siteId}` from the secrets store at channel-build time (sites are added at runtime, so no boot-time expansion is possible), with `ScadaBridge:Communication:SitePsks:{siteId}` as an override for hosts running without a master key — the docker rig uses the latter. One key per site, never fleet-wide. A bearer token over h2c is readable and replayable on-path; TLS is the follow-on hardening and needs no change to this design. Introduced by Phase 0 of the ClusterClient→gRPC migration (`docs/plans/2026-07-22-clusterclient-to-grpc-plan.md`).
 - **Akka frame size is the default 128 KB with `log-frame-size-exceeding` off**, and no custom serializer is configured — so payload carried over ClusterClient is JSON-escaped a second time by the default Newtonsoft serializer, roughly doubling it. Over the limit the transport drops **that one message** without tearing down the association (heartbeats keep flowing, the site still reports healthy) and the central Ask simply times out. See `docs/known-issues/2026-06-26-deploy-config-exceeds-akka-frame-size.md`; `DeployArtifactsCommand` still carries payload and remains exposed.
 - gRPC streaming channel — **note the direction is inverted from the data flow**: data moves site→central, but each **site node hosts the gRPC server** (`SiteStreamGrpcServer`, Kestrel h2c, port 8083, mapped **only in the Site branch** of `Program.cs`) and **central is the client**, dialling in. There is **no gRPC server on central at all**, which is why the two `Ingest*` unary RPCs — documented as a "central-side ingest surface" — are dead in practice (acknowledged in `AkkaHostedService.cs:510-519`); sites reach central over ClusterClient instead. Central creates per-site `SiteStreamGrpcClient` via `SiteStreamGrpcClientFactory`, keyed **`(siteId, endpoint)`** — the key was widened from site-only to fix an arch-review High where one session's NodeA→NodeB flip disposed a channel another session was still using. Site entity has GrpcNodeAAddress/GrpcNodeBAddress fields. Proto: `sitestream.proto`, **6 RPCs** (2 server-streaming: `SubscribeInstance`, `SubscribeSite` (site-wide, alarm-only); 4 unary: `IngestAuditEvents`, `IngestCachedTelemetry`, `PullAuditEvents`, `PullSiteCalls`), `SiteStreamEvent` (oneof: AttributeValueUpdate, AlarmStateUpdate). Field numbers are never reused; evolution is additive only (`AlarmStateUpdate` grew 7→23 fields for the native-alarm mirror). Generated C# is **vendored** under `Communication/SiteStreamGrpc/` with the `<Protobuf>` include commented out — regeneration is a manual toggle-build-copy-untoggle. DebugStreamEvent message removed (no longer flows through ClusterClient).
 - Native alarms: a read-only mirror of native alarms from OPC UA Alarms & Conditions servers and the MxAccess Gateway, unified onto an A&C-style condition model (`AlarmConditionState`: orthogonal Active/Acked/Confirmed/Shelved/Suppressed + 0–1000 severity) plus an `AlarmKind` discriminator (Computed/NativeOpcUa/NativeMxAccess). New DCL capability seam `IAlarmSubscribableConnection` (implemented by the OPC UA and MxGateway adapters); the `DataConnectionActor` opens ONE alarm feed per connection and routes transitions to instances by source-object reference. A `NativeAlarmActor` (peer to the computed `AlarmActor` under `InstanceActor`) mirrors one source binding: snapshot atomic-swap on (re)subscribe, retention (drops once inactive+acked), per-source cap, and site SQLite persistence (`native_alarm_state`, survives failover, cleared on redeploy/undeploy — mirrors static overrides). State streams to central over the additively-enriched gRPC `AlarmStateUpdate` (the existing computed `AlarmStateChanged` was enriched additively) and seeds via the DebugView snapshot. Authoring: `TemplateNativeAlarmSource` / `InstanceNativeAlarmSourceOverride` entities flatten to `ResolvedNativeAlarmSource` (inherit/compose/override); management commands + ManagementActor handlers + CLI (`template/instance native-alarm-source`) + Central UI (template editor tab + instance override panel) + enriched DebugView alarm table. Read-only — no ack-back; no central tables.

docker-env2/docker-compose.yml

@@ -16,6 +16,10 @@ services:
       # pepper per the "different per environment" guidance; real deployments inject a
       # true secret out-of-band, never from source control. Both Central nodes share it.
       ScadaBridge__InboundApi__ApiKeyPepper: "dev-only-insecure-pepper-env2-cluster-0001"
+      # DEV-ONLY gRPC control-plane preshared key for site-x — NOT a real secret.
+      # Must match ScadaBridge:Communication:GrpcPsk in site-x-node-*/appsettings.Site.json.
+      # Production seeds SB-GRPC-PSK-<siteId> into the secret store instead.
+      ScadaBridge__Communication__SitePsks__site-x: "dev-grpc-psk-docker-env2-site-x"
     ports:
       - "9101:5000"   # Web UI + Inbound API
       - "9111:8081"   # Akka remoting
@@ -43,6 +47,10 @@ services:
       # pepper per the "different per environment" guidance; real deployments inject a
       # true secret out-of-band, never from source control. Both Central nodes share it.
       ScadaBridge__InboundApi__ApiKeyPepper: "dev-only-insecure-pepper-env2-cluster-0001"
+      # DEV-ONLY gRPC control-plane preshared key for site-x — NOT a real secret.
+      # Must match ScadaBridge:Communication:GrpcPsk in site-x-node-*/appsettings.Site.json.
+      # Production seeds SB-GRPC-PSK-<siteId> into the secret store instead.
+      ScadaBridge__Communication__SitePsks__site-x: "dev-grpc-psk-docker-env2-site-x"
     ports:
       - "9102:5000"   # Web UI + Inbound API
       - "9112:8081"   # Akka remoting

docker-env2/site-x-node-a/appsettings.Site.json

@@ -40,6 +40,13 @@
       "SqliteDbPath": "/app/data/store-and-forward.db"
     },
     "Communication": {
+      // DEV-ONLY control-plane preshared key — NOT a real secret. Must be
+      // IDENTICAL on both nodes of the pair and match the central-side entry in
+      // ScadaBridge__Communication__SitePsks__<siteId> (docker-compose.yml).
+      // Production supplies this as ${secret:SB-GRPC-PSK-<siteId>}. Without it the
+      // node fails StartupValidator: the gate is fail-closed, so an unset key would
+      // refuse every SiteStream call while the node still looked healthy.
+      "GrpcPsk": "dev-grpc-psk-docker-env2-site-x",
       "CentralContactPoints": [
         "akka.tcp://scadabridge@scadabridge-env2-central-a:8081",
         "akka.tcp://scadabridge@scadabridge-env2-central-b:8081"

docker-env2/site-x-node-b/appsettings.Site.json

@@ -40,6 +40,13 @@
       "SqliteDbPath": "/app/data/store-and-forward.db"
     },
     "Communication": {
+      // DEV-ONLY control-plane preshared key — NOT a real secret. Must be
+      // IDENTICAL on both nodes of the pair and match the central-side entry in
+      // ScadaBridge__Communication__SitePsks__<siteId> (docker-compose.yml).
+      // Production supplies this as ${secret:SB-GRPC-PSK-<siteId>}. Without it the
+      // node fails StartupValidator: the gate is fail-closed, so an unset key would
+      // refuse every SiteStream call while the node still looked healthy.
+      "GrpcPsk": "dev-grpc-psk-docker-env2-site-x",
       "CentralContactPoints": [
         "akka.tcp://scadabridge@scadabridge-env2-central-a:8081",
         "akka.tcp://scadabridge@scadabridge-env2-central-b:8081"

docker/README.md

@@ -120,6 +120,31 @@ docker/
     └── logs/
 ```
 
+## gRPC control-plane keys (dev)
+
+The site gRPC service (`SiteStreamService` on 8083 — live subscriptions, audit pull,
+cached-telemetry ingest) is gated by a preshared key, and the gate is **fail-closed**: a site node
+with no key refuses every call, and `StartupValidator` refuses to boot it at all. So the rig
+carries dev keys, one per site:
+
+| Where | Setting | Value |
+|---|---|---|
+| `site-{a,b,c}-node-*/appsettings.Site.json` | `ScadaBridge:Communication:GrpcPsk` | `dev-grpc-psk-docker-site-{a,b,c}` |
+| `docker-compose.yml`, both central nodes | `ScadaBridge__Communication__SitePsks__site-{a,b,c}` | same value |
+
+Both nodes of a pair carry the same key; each site's key is different from the others'. The
+central half lives in compose env rather than the mounted `appsettings.Central.json`, which by
+convention holds no plaintext credentials. Production uses `${secret:SB-GRPC-PSK-<siteId>}` on
+the site and the matching secret in central's store — see
+[`docs/deployment/topology-guide.md`](../docs/deployment/topology-guide.md).
+
+**These are not real secrets and are committed deliberately**, exactly like the LocalDb sync key
+(`dev-site-a-localdb-sync-key`) beside them. The two are separate keys on purpose: the LocalDb one
+authenticates the *pair partner* for database replication, not central.
+
+If you add a site to the rig, add its key in both places or its streams will fail with
+`PermissionDenied`.
+
 ## Commands
 
 ### Initial Setup

docker/docker-compose.yml

@@ -27,6 +27,15 @@ services:
       ScadaBridge__Database__MachineDataDb: "Server=scadabridge-mssql,1433;Database=ScadaBridgeMachineData;User Id=scadabridge_app;Password=ScadaBridge_Dev1#;TrustServerCertificate=true"
       ScadaBridge__Security__Ldap__ServiceAccountPassword: "serviceaccount123"
       ScadaBridge__Security__JwtSigningKey: "scadabridge-dev-jwt-signing-key-must-be-at-least-32-characters-long"
+      # DEV-ONLY gRPC control-plane preshared keys, one per site — NOT real secrets.
+      # Central verifies/presents these; each site node carries the same value as
+      # ScadaBridge:Communication:GrpcPsk in its mounted appsettings.Site.json. Kept as
+      # env overrides (not in the mounted central appsettings) so that file stays free of
+      # plaintext credentials. Production instead seeds SB-GRPC-PSK-<siteId> into the
+      # secret store, which is also the only source that can serve a site added at runtime.
+      ScadaBridge__Communication__SitePsks__site-a: "dev-grpc-psk-docker-site-a"
+      ScadaBridge__Communication__SitePsks__site-b: "dev-grpc-psk-docker-site-b"
+      ScadaBridge__Communication__SitePsks__site-c: "dev-grpc-psk-docker-site-c"
     ports:
       - "9001:5000"   # Web UI + Inbound API
       - "9011:8081"   # Akka remoting (host access for CLI/debugging)
@@ -65,6 +74,15 @@ services:
       ScadaBridge__Database__MachineDataDb: "Server=scadabridge-mssql,1433;Database=ScadaBridgeMachineData;User Id=scadabridge_app;Password=ScadaBridge_Dev1#;TrustServerCertificate=true"
       ScadaBridge__Security__Ldap__ServiceAccountPassword: "serviceaccount123"
       ScadaBridge__Security__JwtSigningKey: "scadabridge-dev-jwt-signing-key-must-be-at-least-32-characters-long"
+      # DEV-ONLY gRPC control-plane preshared keys, one per site — NOT real secrets.
+      # Central verifies/presents these; each site node carries the same value as
+      # ScadaBridge:Communication:GrpcPsk in its mounted appsettings.Site.json. Kept as
+      # env overrides (not in the mounted central appsettings) so that file stays free of
+      # plaintext credentials. Production instead seeds SB-GRPC-PSK-<siteId> into the
+      # secret store, which is also the only source that can serve a site added at runtime.
+      ScadaBridge__Communication__SitePsks__site-a: "dev-grpc-psk-docker-site-a"
+      ScadaBridge__Communication__SitePsks__site-b: "dev-grpc-psk-docker-site-b"
+      ScadaBridge__Communication__SitePsks__site-c: "dev-grpc-psk-docker-site-c"
     ports:
       - "9002:5000"   # Web UI + Inbound API
       - "9012:8081"   # Akka remoting

docker/site-a-node-a/appsettings.Site.json

@@ -41,6 +41,13 @@
       "SqliteDbPath": "/app/data/store-and-forward.db"
     },
     "Communication": {
+      // DEV-ONLY control-plane preshared key — NOT a real secret. Must be
+      // IDENTICAL on both nodes of the pair and match the central-side entry in
+      // ScadaBridge__Communication__SitePsks__<siteId> (docker-compose.yml).
+      // Production supplies this as ${secret:SB-GRPC-PSK-<siteId>}. Without it the
+      // node fails StartupValidator: the gate is fail-closed, so an unset key would
+      // refuse every SiteStream call while the node still looked healthy.
+      "GrpcPsk": "dev-grpc-psk-docker-site-a",
       "CentralContactPoints": [
         "akka.tcp://scadabridge@scadabridge-central-a:8081",
         "akka.tcp://scadabridge@scadabridge-central-b:8081"

docker/site-a-node-b/appsettings.Site.json

@@ -41,6 +41,13 @@
       "SqliteDbPath": "/app/data/store-and-forward.db"
     },
     "Communication": {
+      // DEV-ONLY control-plane preshared key — NOT a real secret. Must be
+      // IDENTICAL on both nodes of the pair and match the central-side entry in
+      // ScadaBridge__Communication__SitePsks__<siteId> (docker-compose.yml).
+      // Production supplies this as ${secret:SB-GRPC-PSK-<siteId>}. Without it the
+      // node fails StartupValidator: the gate is fail-closed, so an unset key would
+      // refuse every SiteStream call while the node still looked healthy.
+      "GrpcPsk": "dev-grpc-psk-docker-site-a",
       "CentralContactPoints": [
         "akka.tcp://scadabridge@scadabridge-central-a:8081",
         "akka.tcp://scadabridge@scadabridge-central-b:8081"

docker/site-b-node-a/appsettings.Site.json

@@ -41,6 +41,13 @@
       "SqliteDbPath": "/app/data/store-and-forward.db"
     },
     "Communication": {
+      // DEV-ONLY control-plane preshared key — NOT a real secret. Must be
+      // IDENTICAL on both nodes of the pair and match the central-side entry in
+      // ScadaBridge__Communication__SitePsks__<siteId> (docker-compose.yml).
+      // Production supplies this as ${secret:SB-GRPC-PSK-<siteId>}. Without it the
+      // node fails StartupValidator: the gate is fail-closed, so an unset key would
+      // refuse every SiteStream call while the node still looked healthy.
+      "GrpcPsk": "dev-grpc-psk-docker-site-b",
       "CentralContactPoints": [
         "akka.tcp://scadabridge@scadabridge-central-a:8081",
         "akka.tcp://scadabridge@scadabridge-central-b:8081"

docker/site-b-node-b/appsettings.Site.json

@@ -41,6 +41,13 @@
       "SqliteDbPath": "/app/data/store-and-forward.db"
     },
     "Communication": {
+      // DEV-ONLY control-plane preshared key — NOT a real secret. Must be
+      // IDENTICAL on both nodes of the pair and match the central-side entry in
+      // ScadaBridge__Communication__SitePsks__<siteId> (docker-compose.yml).
+      // Production supplies this as ${secret:SB-GRPC-PSK-<siteId>}. Without it the
+      // node fails StartupValidator: the gate is fail-closed, so an unset key would
+      // refuse every SiteStream call while the node still looked healthy.
+      "GrpcPsk": "dev-grpc-psk-docker-site-b",
       "CentralContactPoints": [
         "akka.tcp://scadabridge@scadabridge-central-a:8081",
         "akka.tcp://scadabridge@scadabridge-central-b:8081"

docker/site-c-node-a/appsettings.Site.json

@@ -41,6 +41,13 @@
       "SqliteDbPath": "/app/data/store-and-forward.db"
     },
     "Communication": {
+      // DEV-ONLY control-plane preshared key — NOT a real secret. Must be
+      // IDENTICAL on both nodes of the pair and match the central-side entry in
+      // ScadaBridge__Communication__SitePsks__<siteId> (docker-compose.yml).
+      // Production supplies this as ${secret:SB-GRPC-PSK-<siteId>}. Without it the
+      // node fails StartupValidator: the gate is fail-closed, so an unset key would
+      // refuse every SiteStream call while the node still looked healthy.
+      "GrpcPsk": "dev-grpc-psk-docker-site-c",
       "CentralContactPoints": [
         "akka.tcp://scadabridge@scadabridge-central-a:8081",
         "akka.tcp://scadabridge@scadabridge-central-b:8081"

docker/site-c-node-b/appsettings.Site.json

@@ -41,6 +41,13 @@
       "SqliteDbPath": "/app/data/store-and-forward.db"
     },
     "Communication": {
+      // DEV-ONLY control-plane preshared key — NOT a real secret. Must be
+      // IDENTICAL on both nodes of the pair and match the central-side entry in
+      // ScadaBridge__Communication__SitePsks__<siteId> (docker-compose.yml).
+      // Production supplies this as ${secret:SB-GRPC-PSK-<siteId>}. Without it the
+      // node fails StartupValidator: the gate is fail-closed, so an unset key would
+      // refuse every SiteStream call while the node still looked healthy.
+      "GrpcPsk": "dev-grpc-psk-docker-site-c",
       "CentralContactPoints": [
         "akka.tcp://scadabridge@scadabridge-central-a:8081",
         "akka.tcp://scadabridge@scadabridge-central-b:8081"

docs/components/Communication.md

@@ -30,7 +30,13 @@ The transports are independent. A gRPC stream interruption does not affect in-fl
 
 **The gRPC dial direction is inverted from its data direction.** Values flow site → central, but each **site node hosts the gRPC server** and **central is the client**. `MapGrpcService<SiteStreamGrpcServer>()` appears exactly once in the tree, inside the Site branch of `Program.cs` (`Host/Program.cs:542`); there is **no gRPC server on a central node at all**. That is why the two `Ingest*` unary RPCs — nominally a central-side ingest surface — are dead in the shipped topology (acknowledged in `AkkaHostedService.cs:505-508`: "when the gRPC server is not registered (current central topology)"); sites push audit telemetry to central over `ClusterClient` instead, and central pulls with `PullAuditEvents` / `PullSiteCalls` by dialling the site.
 
-**None of the three transports carries a mutual-auth or transport-encryption story.** `BuildHocon` emits no Akka `enable-ssl`, no secure cookie and no `trusted-selection-paths`, so remoting and `ClusterClient` are plaintext and unauthenticated. The site gRPC listener is **h2c** — `ListenAnyIP(grpcPort, o => o.Protocols = HttpProtocols.Http2)` with no `UseHttps` (`Host/Program.cs:501-506`) — and `SiteStreamGrpcClient` opens a bare `GrpcChannel.ForAddress(endpoint, …)` with no credentials. `LocalDbSyncAuthInterceptor` shares that listener but gates **only** methods under `/localdb_sync.v1.LocalDbSync/`, explicitly letting every `SiteStreamService` call through untouched, so the whole surface — including the `PullAuditEvents` / `PullSiteCalls` RPCs that return audit rows — is callable by anything that can reach the site's gRPC port. The one authenticated piece is the HTTP config fetch, and its per-deployment token is the *entire* security boundary (the endpoint is `AllowAnonymous`). The design assumes a trusted network between central and sites.
+**No transport carries transport encryption; two of the three now carry authentication.**
+
+- **Akka remoting / `ClusterClient` — unauthenticated.** `BuildHocon` emits no `enable-ssl`, no secure cookie and no `trusted-selection-paths`, so the command/control path is plaintext and open to anything that can reach the remoting port.
+- **gRPC — authenticated by preshared key since 2026-07-22.** The listener is still **h2c** — `ListenAnyIP(grpcPort, o => o.Protocols = HttpProtocols.Http2)` with no `UseHttps` — but `ControlPlaneAuthInterceptor` gates every method under `/sitestream.SiteStreamService/`, including the `PullAuditEvents` / `PullSiteCalls` RPCs that return audit rows. It is fail-closed (no key ⇒ everything refused, and `StartupValidator` will not boot a site node in that state), compares with `CryptographicOperations.FixedTimeEquals`, and rejects with `PermissionDenied`. Central attaches the key via `ControlPlaneCredentials`, which binds `CallCredentials` to each channel so unary and streaming calls are covered uniformly — `SiteStreamGrpcClient`, `GrpcPullAuditEventsInvoker` and `GrpcPullSiteCallsInvoker` all build their channels through it. Keys are **per site** (`SB-GRPC-PSK-<siteId>`), so a compromised site yields only its own. `LocalDbSyncAuthInterceptor` shares the listener and keeps its own separate key on `/localdb_sync.v1.LocalDbSync/` — the two authenticate different peers (central vs. the pair partner) and are never shared.
+- **HTTP config fetch — token-authenticated.** Its per-deployment token is the *entire* security boundary (the endpoint is `AllowAnonymous`).
+
+A bearer PSK over plaintext h2c is readable and replayable by anyone on the path, so the design still assumes a trusted network between central and sites — but the bar is now "read the traffic" rather than "reach the port". TLS on these listeners is the follow-on hardening and would not change the key design. Operational detail: [`docs/deployment/topology-guide.md`](../deployment/topology-guide.md).
 
 ### Notify-and-fetch: the deployment-config HTTP path
 
@@ -262,7 +268,7 @@ Three layers of dead-client detection protect the gRPC stream path:
 - [Notification Outbox (#21)](./NotificationOutbox.md) — `CentralCommunicationActor` routes `NotificationSubmit` / `NotificationStatusQuery` messages from sites to the `NotificationOutboxActor` proxy. `CommunicationService` Asks the proxy directly for central-UI outbox management calls.
 - [Site Call Audit (#22)](./SiteCallAudit.md) — `CommunicationService` Asks the `SiteCallAuditActor` proxy directly for query and relay operations. `SiteCallAuditActor` issues `RetryParkedOperation` / `DiscardParkedOperation` relay commands to sites via `SiteEnvelope`; `SiteCommunicationActor` dispatches them to `_parkedMessageHandler`.
 - [Store-and-Forward Engine (#6)](./StoreAndForward.md) — the site S&F Engine drives `NotificationSubmit` forwarding and cached-call telemetry emission through `SiteCommunicationActor`. Parked-message queries and retry/discard relay commands flow back the other way.
-- [Management Service (#18)](./ManagementService.md) — `ManagementActor` is registered with `ClusterClientReceptionist` at `/user/management` on central; the CLI connects via its own separate `ClusterClient`. This is a distinct `ClusterClient` usage from the inter-cluster hub-and-spoke connections managed by this component. Management Service also hosts `DeploymentConfigEndpoints` — the `GET /api/internal/deployments/{id}/config` route that terminates the third (HTTP) transport, mapped in the central-role block alongside `/api/audit/*` and `/management`.
+- [Management Service (#18)](./ManagementService.md) — `ManagementActor` runs at `/user/management` on central and is reached **in-process** through `ManagementActorHolder`; the CLI connects over HTTP, not `ClusterClient`. (It was `ClusterClientReceptionist`-registered until 2026-07-22, for a CLI that was never built that way.) So this component's `ClusterClient` usage is exclusively the inter-cluster hub-and-spoke connections. Management Service also hosts `DeploymentConfigEndpoints` — the `GET /api/internal/deployments/{id}/config` route that terminates the third (HTTP) transport, mapped in the central-role block alongside `/api/audit/*` and `/management`.
 - Design spec: [Component-Communication.md](../requirements/Component-Communication.md).
 
 ## Troubleshooting

docs/components/ManagementService.md

@@ -56,17 +56,18 @@ Mutating handlers that call repositories directly invoke `AuditAsync` (backed by
 
 ### Actor lifecycle and registration
 
-`AkkaHostedService` (in the Host) creates the `ManagementActor` under the path `/user/management` and registers it with `ClusterClientReceptionist`:
+`AkkaHostedService` (in the Host) creates the `ManagementActor` under the path `/user/management` and publishes it to `ManagementActorHolder`, which is the only way anything reaches it:
 
 ```csharp
 var mgmtActor = _actorSystem!.ActorOf(
     Props.Create(() => new ManagementActor(_serviceProvider, mgmtLogger)),
     "management");
-ClusterClientReceptionist.Get(_actorSystem).RegisterService(mgmtActor);
 var mgmtHolder = _serviceProvider.GetRequiredService<ManagementActorHolder>();
 mgmtHolder.ActorRef = mgmtActor;
 ```
 
+A `ClusterClientReceptionist.Get(_actorSystem).RegisterService(mgmtActor)` call sat between those two statements until 2026-07-22. It was deleted because nothing ever sent to it: the CLI it was built for uses HTTP, not ClusterClient.
+
 `ClusterClientReceptionist` advertises the actor to `ClusterClient` senders without requiring them to join the Akka cluster. The `ManagementActorHolder.ActorRef` property is then the bridge from the HTTP endpoint (which runs in ASP.NET Core middleware) into the Akka actor world.
 
 The actor declares an explicit supervisor strategy — one-for-one with Resume and no retry limit — to match the coordinator-actor convention and remain correct if child actors are added later.
@@ -154,9 +155,11 @@ Content-Type: application/json
 
 A successful response is HTTP 200 with the JSON result. An authorization failure is HTTP 403 with `{ "error": "...", "code": "UNAUTHORIZED" }`.
 
-### Sending a command via ClusterClient
+### Sending a command in-process
 
-The `ManagementActor` is also reachable from any `ClusterClient` that has a contact point into the central cluster. The actor is registered under `/system/receptionist` with the path `/user/management`. Callers construct and `Tell` a `ManagementEnvelope` and expect one of `ManagementSuccess`, `ManagementError`, or `ManagementUnauthorized` in reply.
+`ManagementEnvelope` is also the in-process contract: a caller holding `ManagementActorHolder.ActorRef` asks the actor directly and expects one of `ManagementSuccess`, `ManagementError`, or `ManagementUnauthorized` in reply. `ManagementEndpoints` is that caller.
+
+There is **no** out-of-process actor path. The actor was advertised via `ClusterClientReceptionist` until 2026-07-22, so a `ClusterClient` with a contact point into the central cluster could `Tell` it a `ManagementEnvelope`; no caller ever did, and the registration is gone. The HTTP endpoints above are the only remote management surface.
 
 ## Command Groups
 

docs/deployment/topology-guide.md

@@ -178,9 +178,45 @@ database from its peer rather than letting it rejoin.
 
 ### Central-Site Communication
 
-- Sites connect to central via Akka.NET remoting.
-- The `Communication:CentralSeedNode` setting in the site config points to one of the central nodes.
-- If that central node is down, the site's communication actor will retry until it connects to the active central node.
+Three transports cross the boundary, not one:
+
+- **Akka ClusterClient** — command/control. Sites list every central node in
+  `ScadaBridge:Communication:CentralContactPoints`; contact rotation reaches whichever node
+  answers, so no "active central" needs to be identified. (There is no `Communication:CentralSeedNode`
+  setting — earlier revisions of this guide named one that never existed in the code.)
+- **gRPC** — real-time data and audit pull. Note the direction is inverted from the data flow:
+  each **site node hosts the gRPC server** on `GrpcPort` (default 8083, h2c) and central dials in.
+- **Plain HTTP** — the deploy config itself, fetched by the site with a per-deployment token.
+
+#### gRPC control-plane preshared key (required)
+
+Every site node must set `ScadaBridge:Communication:GrpcPsk`, and central must hold the same
+value for that site. **`StartupValidator` refuses to boot a site node without it**, deliberately:
+the gate is fail-closed, so an unset key would leave the node joined, healthy-looking and
+answering heartbeats while refusing every gRPC call — no live subscriptions, no audit pull, no
+cached-telemetry ingest.
+
+| Side | Where the key lives |
+|---|---|
+| Site node (both nodes of the pair, identical) | `ScadaBridge:Communication:GrpcPsk`, in production `${secret:SB-GRPC-PSK-<siteId>}` |
+| Central | secret `SB-GRPC-PSK-<siteId>` in its store — **or** `ScadaBridge:Communication:SitePsks:<siteId>` |
+
+The store is the source that matters in production, because sites are added at runtime and their
+keys cannot be enumerated in configuration at boot; `SitePsks` covers a host running without a
+master key (the docker rig) and one-off pins.
+
+One key **per site**, never one for the fleet: a compromised site must not yield another site's
+key. And never share it with `LocalDb:Replication:ApiKey` — that authenticates the *pair partner*
+for database replication, a different trust relationship on the same listener.
+
+**Rotation:** set the new value on both sides, then restart the pair (pairs restart together
+anyway — see above). **Upgrading to a build that has this gate requires seeding the key first**,
+including in the on-host `deploy/` overlays.
+
+The key is a bearer token over plaintext h2c, so it is readable and replayable by anyone on the
+path. That is the accepted posture today — the same trusted-network assumption the boundary
+already made, now with authentication rather than none. TLS on these listeners is follow-on
+hardening and needs no change to the key design.
 
 ## Scaling Guidelines
 

docs/known-issues/2026-07-22-integration-call-routing-is-dead-code.md

@@ -0,0 +1,59 @@
+# Integration call routing (`IntegrationCallRequest`) is dead on both ends
+
+**Date:** 2026-07-22 · **Status:** OPEN (decision needed: wire or delete) · **Severity:** Low (no
+runtime impact — the path cannot be reached) · **Area:** Central–Site Communication
+
+## What
+
+"Pattern 4: Integration Routing" — `CommunicationService.RouteIntegrationCallAsync` →
+`SiteEnvelope(IntegrationCallRequest)` → `SiteCommunicationActor` → an integration handler — is
+plumbed end to end but connected at neither end.
+
+- **No producer.** `RouteIntegrationCallAsync` (`CommunicationService.cs`, "Pattern 4") has **zero
+  callers** in `src/` or `tests/`. It is the only one of `CommunicationService`'s command methods
+  with none.
+- **No handler.** `SiteCommunicationActor` forwards to `_integrationHandler` when one is
+  registered, but `RegisterLocalHandler(LocalHandlerType.Integration, …)` appears **only** in
+  `SiteCommunicationActorTests.cs`. `AkkaHostedService` registers the other three handler types
+  (`Artifacts`, `EventLog`, `ParkedMessages`) and never this one.
+
+So if anything ever did call it, the site would answer
+`IntegrationCallResponse(Success: false, Error: "Integration handler not available")`
+(`SiteCommunicationActor.cs`, Pattern 4) — and the two tests that exercise the path both register
+the handler themselves first, which is why the suite has never noticed.
+
+Do not confuse this with the **Inbound API**'s routed-site-script path, which is live, tested, and
+uses different messages entirely. This is a separate, unused routing pattern that predates it.
+
+## Why it is recorded rather than fixed
+
+Found during the recon for the ClusterClient→gRPC transport migration
+([`docs/plans/2026-07-22-clusterclient-to-grpc-plan.md`](../plans/2026-07-22-clusterclient-to-grpc-plan.md),
+T0.2), which had to enumerate every command crossing the site↔central boundary. Of the **29**
+command types, this is the one that is excluded: **28 migrate to the gRPC contract.**
+
+Porting it would mean designing a proto contract, a `oneof` slot and round-trip mapper tests for a
+verb no caller can invoke and no site can service — and enshrining it on a wire format whose
+evolution rules are additive-only, so an unused RPC slot is permanent. Deleting it during a
+transport migration would mix an unrelated behavioural change into a change whose whole value is
+that behaviour is identical. Hence: excluded from the contract, behaviour untouched, decision
+deferred to its own change.
+
+## Decision needed
+
+Either:
+
+1. **Delete** — remove `RouteIntegrationCallAsync`, the `IntegrationCallRequest`/`Response`
+   messages, the `SiteCommunicationActor` receive block, `LocalHandlerType.Integration`, and the
+   three tests that cover them. This is the default if no consumer is planned.
+2. **Wire** — register a real integration handler on site nodes and give the method a caller. This
+   only makes sense if there is a requirement it serves; none is recorded in
+   `docs/requirements/`.
+
+Whichever is chosen, do it **before Phase 4** of the migration, since Phase 4 deletes the Akka
+transport underneath this path. If it is still dead at that point, option 1 is forced.
+
+## Filing
+
+To be filed as a Gitea issue on `dohertj2/scadabridge` by the repo owner — this note is the
+in-repo record of the finding and of the migration exclusion it justifies.

docs/plans/2026-07-22-clusterclient-to-grpc-plan.md

@@ -302,10 +302,10 @@ Critical path ≈ 1B: **~4–6 weeks total**, matching the design estimate.
 ## Task checklist (tick as you go; IDs reference the sections above)
 
 **Phase 0 — PSK + dead code** (branch `feat/grpc-phase0-psk`)
-- [ ] T0.1 Delete `/user/management` receptionist registration (`AkkaHostedService.cs:459`) + Component-Host.md update
-- [ ] T0.2 File dead-`IntegrationCallRequest` issue; record exclusion (28 of 29 migrate)
-- [ ] T0.3 `ControlPlaneAuthInterceptor` + `CommunicationOptions.GrpcPsk` + `SitePskProvider`; gate SiteStream; PSK attached on central's streaming + pull clients
-- [ ] T0.4 Rig dev keys (all 3 sites + central store seeds) + interceptor/provider/wiring tests
+- [x] T0.1 Delete `/user/management` receptionist registration (`AkkaHostedService.cs:459`) + Component-Host.md update
+- [x] T0.2 File dead-`IntegrationCallRequest` issue; record exclusion (28 of 29 migrate)
+- [x] T0.3 `ControlPlaneAuthInterceptor` + `CommunicationOptions.GrpcPsk` + `SitePskProvider`; gate SiteStream; PSK attached on central's streaming + pull clients
+- [x] T0.4 Rig dev keys (all 3 sites + central store seeds) + interceptor/provider/wiring tests
 - [ ] Phase 0 DoD: suite green; rig unauthenticated ⇒ `PermissionDenied`, authenticated paths work; PR merged
 
 **Phase 1A — central control plane** (worktree, `feat/grpc-central-control`)

docs/plans/2026-07-22-clusterclient-to-grpc-plan.md.tasks.json

@@ -0,0 +1,241 @@
+{
+  "plan": "docs/plans/2026-07-22-clusterclient-to-grpc-plan.md",
+  "design": "~/Desktop/scadaproj/scadabridge_clusterclient_to_grpc.md",
+  "worktrees": {
+    "feat/grpc-phase0-psk": "/Users/dohertj2/Desktop/ScadaBridge-phase0"
+  },
+  "tasks": [
+    {
+      "id": "T0.1",
+      "phase": "0",
+      "subject": "Delete the vestigial /user/management receptionist registration",
+      "status": "completed",
+      "activeForm": "Deleting the /user/management receptionist registration",
+      "files": [
+        "src/ZB.MOM.WW.ScadaBridge.Host/Actors/AkkaHostedService.cs",
+        "docs/requirements/Component-Host.md",
+        "docs/requirements/Component-ManagementService.md",
+        "docs/requirements/Component-Communication.md",
+        "docs/components/ManagementService.md",
+        "docs/components/Communication.md",
+        "src/ZB.MOM.WW.ScadaBridge.CLI/README.md"
+      ],
+      "notes": "Actor stays; only the ClusterClientReceptionist.RegisterService call goes. Verified: CLI is Akka-free, so 6 docs claiming the CLI reaches ManagementActor over ClusterClient are stale and must be corrected in the same change."
+    },
+    {
+      "id": "T0.2",
+      "phase": "0",
+      "subject": "Record the dead IntegrationCallRequest exclusion (28 of 29 commands migrate)",
+      "status": "completed",
+      "activeForm": "Recording the dead IntegrationCallRequest exclusion",
+      "files": [
+        "docs/known-issues/"
+      ],
+      "notes": "Plan says 'file a Gitea issue' \u2014 outward-facing, handed to the user. In-repo half is a known-issues note + the exclusion record."
+    },
+    {
+      "id": "T0.3",
+      "phase": "0",
+      "subject": "ControlPlaneAuthInterceptor + CommunicationOptions.GrpcPsk + SitePskProvider; gate SiteStream; attach PSK on central's clients",
+      "status": "completed",
+      "activeForm": "Building the control-plane PSK auth",
+      "files": [
+        "src/ZB.MOM.WW.ScadaBridge.Host/ControlPlaneAuthInterceptor.cs",
+        "src/ZB.MOM.WW.ScadaBridge.Communication/CommunicationOptions.cs",
+        "src/ZB.MOM.WW.ScadaBridge.Communication/Grpc/ISitePskProvider.cs",
+        "src/ZB.MOM.WW.ScadaBridge.Host/SitePskProvider.cs",
+        "src/ZB.MOM.WW.ScadaBridge.Communication/Grpc/SiteStreamGrpcClient.cs",
+        "src/ZB.MOM.WW.ScadaBridge.Communication/Grpc/SiteStreamGrpcClientFactory.cs",
+        "src/ZB.MOM.WW.ScadaBridge.AuditLog/Central/GrpcPullAuditEventsClient.cs",
+        "src/ZB.MOM.WW.ScadaBridge.AuditLog/Central/GrpcPullSiteCallsClient.cs",
+        "src/ZB.MOM.WW.ScadaBridge.Host/Program.cs"
+      ],
+      "notes": "Service prefix verified from sitestream.proto: /sitestream.SiteStreamService/. Grpc 2.76 -> CallCredentials.FromInterceptor + UnsafeUseInsecureChannelCallCredentials is the async-safe attach path on h2c."
+    },
+    {
+      "id": "T0.4",
+      "phase": "0",
+      "subject": "Rig dev keys (3 sites + central secret seeds) + interceptor/provider/wiring tests",
+      "status": "completed",
+      "activeForm": "Seeding rig dev keys and writing the auth tests",
+      "files": [
+        "docker/site-a-node-a/appsettings.Site.json",
+        "docker/site-a-node-b/appsettings.Site.json",
+        "docker/site-b-node-a/appsettings.Site.json",
+        "docker/site-b-node-b/appsettings.Site.json",
+        "docker/site-c-node-a/appsettings.Site.json",
+        "docker/site-c-node-b/appsettings.Site.json",
+        "tests/ZB.MOM.WW.ScadaBridge.Host.Tests/ControlPlaneAuthInterceptorTests.cs"
+      ],
+      "notes": "Fail-closed from day one \u2014 every environment (incl. docker-env2 and the gitignored deploy/wonder-app-vd03 overlay) needs its key before upgrade. Ops item for the user."
+    },
+    {
+      "id": "P0.DoD",
+      "phase": "0",
+      "subject": "Phase 0 DoD: suite green; rig unauthenticated => PermissionDenied, authenticated paths work; PR merged",
+      "status": "in_progress",
+      "activeForm": "Verifying the Phase 0 DoD",
+      "blockedBy": [
+        "T0.1",
+        "T0.2",
+        "T0.3",
+        "T0.4"
+      ],
+      "notes": "Rig half needs user approval (docker/deploy.sh recreates 8 containers)."
+    },
+    {
+      "id": "T1A.1",
+      "phase": "1A",
+      "subject": "central_control.proto (7 RPCs, checked-in codegen) + CentralControlDtoMapper + round-trip golden tests",
+      "status": "pending",
+      "activeForm": "Authoring central_control.proto and its mappers",
+      "blockedBy": [
+        "P0.DoD"
+      ]
+    },
+    {
+      "id": "T1A.2",
+      "phase": "1A",
+      "subject": "Central hosting: AddGrpc + per-site-PSK interceptor, CentralGrpcPort h2c listener, CentralControlGrpcService, readiness gate",
+      "status": "pending",
+      "activeForm": "Hosting CentralControlService on central",
+      "blockedBy": [
+        "T1A.1"
+      ]
+    },
+    {
+      "id": "T1A.3",
+      "phase": "1A",
+      "subject": "ICentralTransport (Akka extract + Grpc impl), CentralChannelProvider, CentralTransport flag, CentralGrpcEndpoints option",
+      "status": "pending",
+      "activeForm": "Building the site->central transport seam",
+      "blockedBy": [
+        "T1A.1"
+      ]
+    },
+    {
+      "id": "T1A.4",
+      "phase": "1A",
+      "subject": "Tests: actor-with-fake-transport x7, TestServer transport tests, S&F/audit/health suites pass unmodified",
+      "status": "pending",
+      "activeForm": "Testing the central control plane",
+      "blockedBy": [
+        "T1A.2",
+        "T1A.3"
+      ]
+    },
+    {
+      "id": "P1A.DoD",
+      "phase": "1A",
+      "subject": "1A DoD: rig site-a on Grpc proves all 5 site->central paths while site-b/c stay Akka; PR merged before 1B",
+      "status": "pending",
+      "activeForm": "Verifying the 1A DoD",
+      "blockedBy": [
+        "T1A.4"
+      ]
+    },
+    {
+      "id": "T1B.1",
+      "phase": "1B",
+      "subject": "site_command.proto (6 oneof RPCs / 28 commands) + SiteCommandDtoMapper + round-trip golden tests",
+      "status": "pending",
+      "activeForm": "Authoring site_command.proto and its mappers",
+      "blockedBy": [
+        "P0.DoD"
+      ]
+    },
+    {
+      "id": "T1B.2",
+      "phase": "1B",
+      "subject": "SiteCommandDispatcher refactor (actor + SiteCommandGrpcService share it; parked stays node-local)",
+      "status": "pending",
+      "activeForm": "Extracting the site command dispatcher",
+      "blockedBy": [
+        "T1B.1"
+      ]
+    },
+    {
+      "id": "T1B.3",
+      "phase": "1B",
+      "subject": "ISiteCommandTransport in CentralCommunicationActor (Akka extract + Grpc impl), SitePairChannelProvider, SiteTransport flag",
+      "status": "pending",
+      "activeForm": "Building the central->site transport seam",
+      "blockedBy": [
+        "T1B.1"
+      ]
+    },
+    {
+      "id": "T1B.4",
+      "phase": "1B",
+      "subject": "Tests: dispatcher routing x28, actor envelope/reply plumbing, TestServer service tests, existing suites green",
+      "status": "pending",
+      "activeForm": "Testing the site command plane",
+      "blockedBy": [
+        "T1B.2",
+        "T1B.3"
+      ]
+    },
+    {
+      "id": "P1B.DoD",
+      "phase": "1B",
+      "subject": "1B DoD: rig central on Grpc for site-a proves full command matrix incl. standby parked retry; rebased on 1A; PR merged",
+      "status": "pending",
+      "activeForm": "Verifying the 1B DoD",
+      "blockedBy": [
+        "T1B.4",
+        "P1A.DoD"
+      ]
+    },
+    {
+      "id": "P2",
+      "phase": "2",
+      "subject": "All sites CentralTransport=Grpc; central-kill S&F soak, failback observed, health sequences clean",
+      "status": "pending",
+      "activeForm": "Running the site->central cutover soak",
+      "blockedBy": [
+        "P1A.DoD"
+      ]
+    },
+    {
+      "id": "P3",
+      "phase": "3",
+      "subject": "Central SiteTransport=Grpc all sites; full UI command matrix; site-kill mid-command clean; zero ClusterClient activity",
+      "status": "pending",
+      "activeForm": "Running the central->site cutover soak",
+      "blockedBy": [
+        "P1B.DoD"
+      ]
+    },
+    {
+      "id": "P4.1",
+      "phase": "4",
+      "subject": "Flip both flag defaults to Grpc + soak; delete Akka transports, ClusterClient creation, DefaultSiteClientFactory, receptionist registrations, CentralContactPoints, then the flags",
+      "status": "pending",
+      "activeForm": "Deleting the ClusterClient transport",
+      "blockedBy": [
+        "P2",
+        "P3"
+      ]
+    },
+    {
+      "id": "P4.2",
+      "phase": "4",
+      "subject": "Grep-gates pass + docs updated (grpc_streams.md, Component-Host.md, Component-StoreAndForward.md, known-issues cross-ref)",
+      "status": "pending",
+      "activeForm": "Running the deletion grep-gates and doc sweep",
+      "blockedBy": [
+        "P4.1"
+      ]
+    },
+    {
+      "id": "P5",
+      "phase": "5",
+      "subject": "Live gate, 8 checks, recorded in docs/plans/2026-07-22-clusterclient-to-grpc-live-gate.md",
+      "status": "pending",
+      "activeForm": "Running the live gate",
+      "blockedBy": [
+        "P4.2"
+      ]
+    }
+  ]
+}

docs/requirements/Component-Communication.md

@@ -109,6 +109,31 @@ The streaming protocol is defined in `sitestream.proto` (`src/ZB.MOM.WW.ScadaBri
 - The `oneof event` pattern is extensible — future event types (health metrics, connection state changes) are added as new fields without breaking existing consumers.
 - Proto field numbers are never reused; new RPCs and message fields are appended additively. Old clients ignore unknown `oneof` variants.
 
+##### Authentication (preshared key, 2026-07-22)
+
+Every RPC on this service — streaming and unary alike — requires a preshared key, presented as
+`authorization: Bearer <key>` metadata and verified by `ControlPlaneAuthInterceptor` on the site
+node. The gate is **fail-closed**: with no key configured, every call is refused with
+`PermissionDenied`, and `StartupValidator` refuses to boot a site node in that state (an unset key
+would otherwise leave the node joined and reporting healthy while serving nothing).
+
+Keys are scoped **one per site** — secret `SB-GRPC-PSK-<siteId>`, so a compromised site never
+yields another site's key. The site node reads its own key from
+`ScadaBridge:Communication:GrpcPsk` (in production a `${secret:}` reference expanded before the
+host is built); central resolves each site's key at channel-build time via `SitePskProvider`,
+because sites are created at runtime and cannot be enumerated in configuration at boot.
+`ControlPlaneCredentials` binds the key to the channel as `CallCredentials`, so no individual call
+site can omit it.
+
+This is distinct from `LocalDb:Replication:ApiKey`, which gates
+`/localdb_sync.v1.LocalDbSync/` on the same listener via its own interceptor: that key
+authenticates the *pair partner* for database replication, a different trust relationship, and the
+two are never shared.
+
+The transport remains h2c, so the key is readable and replayable by anyone on the path — the
+boundary still assumes a trusted network, with the bar raised from "can reach the port" to "can
+read the traffic". TLS is follow-on hardening and does not change this design.
+
 #### Enriched AlarmStateUpdate (Native Alarm Mirror)
 
 `AlarmStateUpdate` carries the read-only native alarm mirror (Computed, native OPC UA, and native MxAccess Gateway alarms) to central over the **existing gRPC real-time stream** — no new transport, no command/control round-trip. The message was extended **additively**: existing fields 1–7 are unchanged, and fields 8–23 carry the enriched native-alarm state. Old clients that only read fields 1–7 continue to work; new fields are populated only where the source provides them.
@@ -306,7 +331,9 @@ Akka.NET guarantees message ordering between a specific sender/receiver actor pa
 
 ## ManagementActor and ClusterClient
 
-The ManagementActor is registered at the well-known path `/user/management` on central nodes and advertised via **ClusterClientReceptionist**. External tools (primarily the CLI) connect using Akka.NET ClusterClient, which contacts the receptionist to discover the ManagementActor. This is a separate ClusterClient usage from the inter-cluster ClusterClient connections used for central-site messaging — the CLI does not participate in cluster membership or affect the hub-and-spoke topology.
+The ManagementActor runs at the well-known path `/user/management` on central nodes. It is **not** advertised via ClusterClientReceptionist, and no ClusterClient reaches it.
+
+That registration existed until 2026-07-22 for an out-of-cluster CLI that was never built. The shipped CLI speaks **HTTP Basic to the central `/management` endpoints**; those endpoints ask the ManagementActor **in-process** through `ManagementActorHolder` (`ManagementEndpoints.cs`). Nothing in the repo ever sent to `/user/management` across the boundary, so the registration was deleted — leaving exactly one receptionist registration per cluster role (`CentralCommunicationActor` on central, `SiteCommunicationActor` on sites), both of which serve inter-cluster central↔site messaging and are themselves scheduled for removal by the gRPC transport migration (`docs/plans/2026-07-22-clusterclient-to-grpc-plan.md`).
 
 ## Connection Failure Behavior
 

docs/requirements/Component-Host.md

@@ -125,7 +125,11 @@ The Host bootstraps the Akka.NET actor system from a **hand-assembled, injection
 
 ### REQ-HOST-6a: ClusterClientReceptionist (Central Only)
 
-On central nodes, the Host must configure the Akka.NET **ClusterClientReceptionist** and register the ManagementActor with it. This allows external processes (e.g., the CLI) to discover and communicate with the ManagementActor via ClusterClient without joining the cluster as full members. The receptionist is started as part of the Akka.NET bootstrap (REQ-HOST-6) on central nodes only.
+On central nodes, the Host must configure the Akka.NET **ClusterClientReceptionist** and register the **CentralCommunicationActor** with it, so that site clusters' ClusterClients can reach the central command/control endpoint without joining the central cluster.
+
+**The ManagementActor is NOT registered with the receptionist** (removed 2026-07-22). That registration was written for an out-of-cluster CLI that was never built: the shipped CLI speaks HTTP Basic to the central `/management` endpoints, which ask the ManagementActor **in-process** via `ManagementActorHolder` (`ManagementEndpoints.cs`). No sender to `/user/management` existed anywhere in the repo, so the registration only widened the cluster-client surface for nothing. The actor itself still runs at `/user/management`; only its cross-boundary advertisement is gone.
+
+> **Migration note.** This receptionist registration — and the `CentralCommunicationActor` one that remains — are scheduled for deletion once the site↔central transport moves to gRPC. See `docs/plans/2026-07-22-clusterclient-to-grpc-plan.md`.
 
 ### REQ-HOST-7: ASP.NET Web Endpoints
 

docs/requirements/Component-ManagementService.md

@@ -24,7 +24,7 @@ Central cluster only. The ManagementActor runs as a plain actor on **every** cen
 
 ### ManagementActor
 
-The central actor that receives and processes all management commands. Registered at a well-known actor path (`/user/management`) and with ClusterClientReceptionist.
+The central actor that receives and processes all management commands. Created at the well-known actor path (`/user/management`) and handed to `ManagementActorHolder`, which is how every in-process caller (notably `ManagementEndpoints`) reaches it. It is **not** advertised via ClusterClientReceptionist — see the note under "ManagementActor and ClusterClient" in Component-Communication.
 
 ### ManagementEndpoints
 

src/ZB.MOM.WW.ScadaBridge.AuditLog/Central/GrpcPullAuditEventsClient.cs

@@ -145,7 +145,7 @@ public sealed class GrpcPullAuditEventsClient : IPullAuditEventsClient
     {
         try
         {
-            var reply = await _invoker.InvokeAsync(endpoint, request, ct).ConfigureAwait(false);
+            var reply = await _invoker.InvokeAsync(siteId, endpoint, request, ct).ConfigureAwait(false);
             return (reply, false);
         }
         catch (RpcException ex) when (IsTolerable(ex.StatusCode))
@@ -226,11 +226,17 @@ public sealed class GrpcPullAuditEventsClient : IPullAuditEventsClient
         /// May throw <see cref="RpcException"/> / <see cref="HttpRequestException"/>
         /// on transport faults — the caller classifies and swallows tolerable ones.
         /// </summary>
+        /// <param name="siteId">
+        /// The site being pulled from. Selects which preshared key the call presents —
+        /// <c>PullAuditEvents</c> is gated by the site's <c>ControlPlaneAuthInterceptor</c>, and
+        /// keys are per-site, so the endpoint alone is not enough to authenticate.
+        /// </param>
         /// <param name="endpoint">The site gRPC authority (e.g. <c>http://site-a:8083</c>).</param>
         /// <param name="request">The wire-format pull request.</param>
         /// <param name="ct">Cancellation token.</param>
         /// <returns>The wire-format pull response.</returns>
-        Task<ProtoPullResponse> InvokeAsync(string endpoint, ProtoPullRequest request, CancellationToken ct);
+        Task<ProtoPullResponse> InvokeAsync(
+            string siteId, string endpoint, ProtoPullRequest request, CancellationToken ct);
     }
 }
 
@@ -251,8 +257,9 @@ public sealed class GrpcPullAuditEventsClient : IPullAuditEventsClient
 public sealed class GrpcPullAuditEventsInvoker
     : GrpcPullAuditEventsClient.IPullAuditEventsInvoker, IDisposable
 {
-    private readonly ConcurrentDictionary<string, GrpcChannel> _channels = new(StringComparer.Ordinal);
+    private readonly ConcurrentDictionary<(string Site, string Endpoint), GrpcChannel> _channels = new();
     private readonly CommunicationOptions _options;
+    private readonly ISitePskProvider? _pskProvider;
 
     /// <summary>
     /// Creates the invoker using default <see cref="CommunicationOptions"/>.
@@ -268,15 +275,27 @@ public sealed class GrpcPullAuditEventsInvoker
     /// </summary>
     /// <param name="options">Communication options supplying gRPC keepalive timings.</param>
     public GrpcPullAuditEventsInvoker(CommunicationOptions options)
+        : this(options, pskProvider: null)
+    {
+    }
+
+    /// <summary>
+    /// Creates the invoker with per-site call credentials, the production shape: the site's
+    /// <c>ControlPlaneAuthInterceptor</c> refuses an unauthenticated <c>PullAuditEvents</c>.
+    /// </summary>
+    /// <param name="options">Communication options supplying gRPC keepalive timings.</param>
+    /// <param name="pskProvider">Resolves each site's preshared key; null dials unauthenticated.</param>
+    public GrpcPullAuditEventsInvoker(CommunicationOptions options, ISitePskProvider? pskProvider)
     {
         _options = options ?? throw new ArgumentNullException(nameof(options));
+        _pskProvider = pskProvider;
     }
 
     /// <inheritdoc />
     public async Task<ProtoPullResponse> InvokeAsync(
-        string endpoint, ProtoPullRequest request, CancellationToken ct)
+        string siteId, string endpoint, ProtoPullRequest request, CancellationToken ct)
     {
-        var channel = GetOrCreateChannel(endpoint);
+        var channel = GetOrCreateChannel(siteId, endpoint);
         var client = new SiteStreamService.SiteStreamServiceClient(channel);
         using var call = client.PullAuditEventsAsync(request, cancellationToken: ct);
         return await call.ResponseAsync.ConfigureAwait(false);
@@ -288,12 +307,13 @@ public sealed class GrpcPullAuditEventsInvoker
     // pool) and the loser would leak. Create-then-GetOrAdd-then-dispose-if-lost
     // mirrors SiteStreamGrpcClientFactory: only the channel actually installed
     // survives; a channel that lost the race is disposed immediately.
-    private GrpcChannel GetOrCreateChannel(string endpoint)
+    private GrpcChannel GetOrCreateChannel(string siteId, string endpoint)
     {
-        if (!_channels.TryGetValue(endpoint, out var channel))
+        var key = (siteId, endpoint);
+        if (!_channels.TryGetValue(key, out var channel))
         {
-            var created = CreateChannel(endpoint);
-            channel = _channels.GetOrAdd(endpoint, created);
+            var created = CreateChannel(siteId, endpoint);
+            channel = _channels.GetOrAdd(key, created);
             if (!ReferenceEquals(channel, created))
             {
                 created.Dispose();
@@ -302,7 +322,10 @@ public sealed class GrpcPullAuditEventsInvoker
         return channel;
     }
 
-    private GrpcChannel CreateChannel(string endpoint) =>
+    // Keyed by (site, endpoint) rather than endpoint alone: the call credentials are bound to
+    // the channel, and they are per-site, so two sites sharing an endpoint string would
+    // otherwise share one channel carrying the first site's key.
+    private GrpcChannel CreateChannel(string siteId, string endpoint) =>
         GrpcChannel.ForAddress(endpoint, new GrpcChannelOptions
         {
             HttpHandler = new SocketsHttpHandler
@@ -311,7 +334,7 @@ public sealed class GrpcPullAuditEventsInvoker
                 KeepAlivePingTimeout = _options.GrpcKeepAlivePingTimeout,
                 KeepAlivePingPolicy = HttpKeepAlivePingPolicy.Always,
             },
-        });
+        }.WithSiteCredentials(_pskProvider, siteId));
 
     /// <summary>Disposes all cached channels.</summary>
     public void Dispose()

src/ZB.MOM.WW.ScadaBridge.AuditLog/Central/GrpcPullSiteCallsClient.cs

@@ -174,7 +174,7 @@ public sealed class GrpcPullSiteCallsClient : IPullSiteCallsClient
     {
         try
         {
-            var reply = await _invoker.InvokeAsync(endpoint, request, ct).ConfigureAwait(false);
+            var reply = await _invoker.InvokeAsync(siteId, endpoint, request, ct).ConfigureAwait(false);
             return (reply, false);
         }
         catch (RpcException ex) when (IsTolerable(ex.StatusCode))
@@ -254,11 +254,17 @@ public sealed class GrpcPullSiteCallsClient : IPullSiteCallsClient
         /// May throw <see cref="RpcException"/> / <see cref="HttpRequestException"/>
         /// on transport faults — the caller classifies and swallows tolerable ones.
         /// </summary>
+        /// <param name="siteId">
+        /// The site being pulled from. Selects which preshared key the call presents —
+        /// <c>PullSiteCalls</c> is gated by the site's <c>ControlPlaneAuthInterceptor</c>, and
+        /// keys are per-site, so the endpoint alone is not enough to authenticate.
+        /// </param>
         /// <param name="endpoint">The site gRPC authority (e.g. <c>http://site-a:8083</c>).</param>
         /// <param name="request">The wire-format pull request.</param>
         /// <param name="ct">Cancellation token.</param>
         /// <returns>The wire-format pull response.</returns>
-        Task<ProtoPullResponse> InvokeAsync(string endpoint, ProtoPullRequest request, CancellationToken ct);
+        Task<ProtoPullResponse> InvokeAsync(
+            string siteId, string endpoint, ProtoPullRequest request, CancellationToken ct);
     }
 }
 
@@ -277,8 +283,9 @@ public sealed class GrpcPullSiteCallsClient : IPullSiteCallsClient
 public sealed class GrpcPullSiteCallsInvoker
     : GrpcPullSiteCallsClient.IPullSiteCallsInvoker, IDisposable
 {
-    private readonly ConcurrentDictionary<string, GrpcChannel> _channels = new(StringComparer.Ordinal);
+    private readonly ConcurrentDictionary<(string Site, string Endpoint), GrpcChannel> _channels = new();
     private readonly CommunicationOptions _options;
+    private readonly ISitePskProvider? _pskProvider;
 
     /// <summary>Creates the invoker using default <see cref="CommunicationOptions"/>.</summary>
     public GrpcPullSiteCallsInvoker()
@@ -292,15 +299,27 @@ public sealed class GrpcPullSiteCallsInvoker
     /// </summary>
     /// <param name="options">Communication options supplying gRPC keepalive timings.</param>
     public GrpcPullSiteCallsInvoker(CommunicationOptions options)
+        : this(options, pskProvider: null)
+    {
+    }
+
+    /// <summary>
+    /// Creates the invoker with per-site call credentials, the production shape: the site's
+    /// <c>ControlPlaneAuthInterceptor</c> refuses an unauthenticated <c>PullSiteCalls</c>.
+    /// </summary>
+    /// <param name="options">Communication options supplying gRPC keepalive timings.</param>
+    /// <param name="pskProvider">Resolves each site's preshared key; null dials unauthenticated.</param>
+    public GrpcPullSiteCallsInvoker(CommunicationOptions options, ISitePskProvider? pskProvider)
     {
         _options = options ?? throw new ArgumentNullException(nameof(options));
+        _pskProvider = pskProvider;
     }
 
     /// <inheritdoc />
     public async Task<ProtoPullResponse> InvokeAsync(
-        string endpoint, ProtoPullRequest request, CancellationToken ct)
+        string siteId, string endpoint, ProtoPullRequest request, CancellationToken ct)
     {
-        var channel = GetOrCreateChannel(endpoint);
+        var channel = GetOrCreateChannel(siteId, endpoint);
         var client = new SiteStreamService.SiteStreamServiceClient(channel);
         using var call = client.PullSiteCallsAsync(request, cancellationToken: ct);
         return await call.ResponseAsync.ConfigureAwait(false);
@@ -310,12 +329,13 @@ public sealed class GrpcPullSiteCallsInvoker
     // concurrent first dials of the same endpoint can both build a GrpcChannel;
     // only the channel actually installed survives, the loser is disposed.
     // Mirrors SiteStreamGrpcClientFactory / GrpcPullAuditEventsInvoker.
-    private GrpcChannel GetOrCreateChannel(string endpoint)
+    private GrpcChannel GetOrCreateChannel(string siteId, string endpoint)
     {
-        if (!_channels.TryGetValue(endpoint, out var channel))
+        var key = (siteId, endpoint);
+        if (!_channels.TryGetValue(key, out var channel))
         {
-            var created = CreateChannel(endpoint);
-            channel = _channels.GetOrAdd(endpoint, created);
+            var created = CreateChannel(siteId, endpoint);
+            channel = _channels.GetOrAdd(key, created);
             if (!ReferenceEquals(channel, created))
             {
                 created.Dispose();
@@ -324,7 +344,10 @@ public sealed class GrpcPullSiteCallsInvoker
         return channel;
     }
 
-    private GrpcChannel CreateChannel(string endpoint) =>
+    // Keyed by (site, endpoint) rather than endpoint alone: the call credentials are bound to
+    // the channel, and they are per-site, so two sites sharing an endpoint string would
+    // otherwise share one channel carrying the first site's key.
+    private GrpcChannel CreateChannel(string siteId, string endpoint) =>
         GrpcChannel.ForAddress(endpoint, new GrpcChannelOptions
         {
             HttpHandler = new SocketsHttpHandler
@@ -333,7 +356,7 @@ public sealed class GrpcPullSiteCallsInvoker
                 KeepAlivePingTimeout = _options.GrpcKeepAlivePingTimeout,
                 KeepAlivePingPolicy = HttpKeepAlivePingPolicy.Always,
             },
-        });
+        }.WithSiteCredentials(_pskProvider, siteId));
 
     /// <summary>Disposes all cached channels.</summary>
     public void Dispose()

src/ZB.MOM.WW.ScadaBridge.AuditLog/ServiceCollectionExtensions.cs

@@ -511,9 +511,14 @@ public static class ServiceCollectionExtensions
             var options = sp
                 .GetService<Microsoft.Extensions.Options.IOptions<
                     ZB.MOM.WW.ScadaBridge.Communication.CommunicationOptions>>();
+            // The PSK provider is central-only and optional in DI, so GetService (not
+            // GetRequiredService): a host without one dials unauthenticated and the site
+            // refuses it, which is the fail-closed outcome we want rather than a
+            // resolution crash at composition time.
+            var psk = sp.GetService<ZB.MOM.WW.ScadaBridge.Communication.Grpc.ISitePskProvider>();
             return options is null
-                ? new GrpcPullAuditEventsInvoker()
-                : new GrpcPullAuditEventsInvoker(options.Value);
+                ? new GrpcPullAuditEventsInvoker(new ZB.MOM.WW.ScadaBridge.Communication.CommunicationOptions(), psk)
+                : new GrpcPullAuditEventsInvoker(options.Value, psk);
         });
         services.TryAddSingleton<GrpcPullAuditEventsClient.IPullAuditEventsInvoker>(
             sp => sp.GetRequiredService<GrpcPullAuditEventsInvoker>());
@@ -536,9 +541,14 @@ public static class ServiceCollectionExtensions
             var options = sp
                 .GetService<Microsoft.Extensions.Options.IOptions<
                     ZB.MOM.WW.ScadaBridge.Communication.CommunicationOptions>>();
+            // The PSK provider is central-only and optional in DI, so GetService (not
+            // GetRequiredService): a host without one dials unauthenticated and the site
+            // refuses it, which is the fail-closed outcome we want rather than a
+            // resolution crash at composition time.
+            var psk = sp.GetService<ZB.MOM.WW.ScadaBridge.Communication.Grpc.ISitePskProvider>();
             return options is null
-                ? new GrpcPullSiteCallsInvoker()
-                : new GrpcPullSiteCallsInvoker(options.Value);
+                ? new GrpcPullSiteCallsInvoker(new ZB.MOM.WW.ScadaBridge.Communication.CommunicationOptions(), psk)
+                : new GrpcPullSiteCallsInvoker(options.Value, psk);
         });
         services.TryAddSingleton<GrpcPullSiteCallsClient.IPullSiteCallsInvoker>(
             sp => sp.GetRequiredService<GrpcPullSiteCallsInvoker>());

src/ZB.MOM.WW.ScadaBridge.CLI/README.md

@@ -1987,9 +1987,11 @@ scadabridge --url <url> cached-call discard --site-id <string> --tracked-operati
 
 ## Architecture Notes
 
-The CLI connects to the Central cluster using Akka.NET's `ClusterClient`. It does not join the cluster — it contacts the `ClusterClientReceptionist` on one of the configured Central nodes and sends commands to the `ManagementActor` at path `/user/management`.
+The CLI connects to the Central cluster over **HTTP** (`ManagementHttpClient`), posting to the `/management` endpoints at the configured `managementUrl` — normally the Traefik load balancer, which routes to the active central node. It carries HTTP **Basic** credentials from `--username`/`--password`. Central's endpoint handler asks the `ManagementActor` in-process via `ManagementActorHolder`.
 
-The connection is established per-command invocation and torn down cleanly via `CoordinatedShutdown` when the command completes.
+There is no Akka dependency in the CLI at all: it does not join the cluster, does not use `ClusterClient`, and does not contact a `ClusterClientReceptionist`. (Earlier revisions of this document described a ClusterClient transport that was never built.)
+
+An `HttpClient` is created per command invocation and disposed when the command completes.
 
 Role enforcement is applied by the ManagementActor on the server side. The CLI authenticates against LDAP using `--username` / `--password`, resolves LDAP group memberships, then maps groups to ScadaBridge roles (Admin, Design, Deployment) via role mappings configured in the security settings. Operations require the appropriate role — for example, creating templates requires `Design`, deploying requires `Deployment`. In the test environment, use the `multi-role` user (password: `password`) which has all three roles.
 

src/ZB.MOM.WW.ScadaBridge.Communication/CommunicationOptions.cs

@@ -41,6 +41,54 @@ public class CommunicationOptions
     /// </summary>
     public List<string> CentralContactPoints { get; set; } = new();
 
+    /// <summary>
+    /// Preshared key authenticating this node's gRPC control plane — the site↔central
+    /// boundary. On a site node this is the key its inbound gate
+    /// (<c>ControlPlaneAuthInterceptor</c>) expects on every <c>SiteStreamService</c> call, and
+    /// which central must present; central resolves the matching value per site from its own
+    /// secret store under the name <c>SB-GRPC-PSK-{siteId}</c>.
+    /// </summary>
+    /// <remarks>
+    /// <para>
+    /// In production this is supplied as <c>${secret:SB-GRPC-PSK-&lt;siteId&gt;}</c> and expanded
+    /// out of the secrets store before the host is built, so the plaintext never sits in
+    /// appsettings. Development rigs set a literal, mirroring the LocalDb replication key.
+    /// </para>
+    /// <para>
+    /// <b>Empty means closed, not open.</b> With no key set the interceptor rejects every gated
+    /// call. This is not optional configuration: a node that ships without a key serves no
+    /// streams and no audit pulls.
+    /// </para>
+    /// <para>
+    /// Distinct from <c>LocalDb:Replication:ApiKey</c>, which authenticates the pair partner for
+    /// database replication over the same listener. The two are never shared.
+    /// </para>
+    /// </remarks>
+    public string GrpcPsk { get; set; } = "";
+
+    /// <summary>
+    /// Central-side per-site gRPC preshared keys, keyed by site identifier — the mirror image
+    /// of <see cref="GrpcPsk"/>, which is the single key a site node expects on its own inbound
+    /// gate. An entry here takes precedence over the secret store for that site.
+    /// </summary>
+    /// <remarks>
+    /// <para>
+    /// <b>Why both a config map and a secret store.</b> The store is the primary source and the
+    /// only one that works for the real case: sites are added at runtime from the Central UI, so
+    /// their keys cannot be enumerated in configuration at boot, and <c>SitePskProvider</c>
+    /// resolves <c>SB-GRPC-PSK-{siteId}</c> on demand. This map covers the cases the store
+    /// cannot or should not: a development rig that runs with no master key and injects every
+    /// credential as an environment override, and an operator pinning one site's key without
+    /// touching the store. Values may themselves be <c>${secret:…}</c> references, since a map
+    /// declared in configuration IS enumerable at boot.
+    /// </para>
+    /// <para>
+    /// Absence is not a fallback to "unauthenticated" in either source — a site with no key in
+    /// the map and none in the store cannot be dialed at all.
+    /// </para>
+    /// </remarks>
+    public Dictionary<string, string> SitePsks { get; set; } = new();
+
     /// <summary>gRPC keepalive ping interval for streaming connections.</summary>
     public TimeSpan GrpcKeepAlivePingDelay { get; set; } = TimeSpan.FromSeconds(15);
 

src/ZB.MOM.WW.ScadaBridge.Communication/Grpc/ISitePskProvider.cs

@@ -0,0 +1,132 @@
+using Grpc.Core;
+using Grpc.Net.Client;
+
+namespace ZB.MOM.WW.ScadaBridge.Communication.Grpc;
+
+/// <summary>
+/// Resolves the preshared key that authenticates gRPC control-plane traffic for one site
+/// relationship. Central holds one key per site; each site holds only its own.
+/// </summary>
+/// <remarks>
+/// <para>
+/// <b>Why per-site rather than one fleet-wide key.</b> A compromised site yields only its own
+/// key, never another site's. A single shared key would be simpler to seed and strictly worse
+/// on blast radius, which is why it was rejected in the design.
+/// </para>
+/// <para>
+/// <b>Fail-closed.</b> Implementations throw when the key cannot be resolved. They must never
+/// fall back to "no key means no authentication" — that is the failure mode this whole
+/// mechanism exists to remove, and it would silently disable auth on exactly the default
+/// configuration. A dial that cannot be authenticated does not happen.
+/// </para>
+/// <para>
+/// The interface lives in Communication (not Host) because both sides need it: central's
+/// site-dialing clients live here and in AuditLog, while the implementation over
+/// <c>ISecretResolver</c> lives in Host, which owns the secrets container.
+/// </para>
+/// </remarks>
+public interface ISitePskProvider
+{
+    /// <summary>
+    /// Resolves the preshared key for <paramref name="siteId"/>, caching the result.
+    /// </summary>
+    /// <param name="siteId">Site identifier, as used in the <c>Site.SiteIdentifier</c> column.</param>
+    /// <param name="ct">Cancellation token.</param>
+    /// <returns>The preshared key. Never null or empty.</returns>
+    /// <exception cref="InvalidOperationException">
+    /// The key is not configured, not resolvable, or empty — the fail-closed path.
+    /// </exception>
+    ValueTask<string> GetAsync(string siteId, CancellationToken ct);
+
+    /// <summary>
+    /// Drops any cached key for <paramref name="siteId"/>, so the next
+    /// <see cref="GetAsync"/> re-reads the store. Called when a site is removed, and after a
+    /// key rotation.
+    /// </summary>
+    /// <param name="siteId">Site identifier whose cached key should be discarded.</param>
+    void Invalidate(string siteId);
+}
+
+/// <summary>
+/// Builds the call credentials that carry a site's preshared key (and the site's own identity)
+/// on every gRPC call central makes to that site — and, from Phase 1A, on the calls a site makes
+/// to central.
+/// </summary>
+/// <remarks>
+/// <para>
+/// <b>Why <see cref="CallCredentials.FromInterceptor(AsyncAuthInterceptor)"/> rather than a
+/// client <c>Interceptor</c>.</b> The key is resolved asynchronously from the secrets store, and
+/// this is the one extension point in gRPC that is async by design. A client interceptor would
+/// have to block on the resolve inside a synchronous <c>AsyncServerStreamingCall</c> path.
+/// Credentials also apply uniformly to unary and streaming calls, so no call site can forget one.
+/// </para>
+/// <para>
+/// <b>Why <c>UnsafeUseInsecureChannelCallCredentials</c>.</b> gRPC refuses to attach call
+/// credentials to a plaintext channel by default, precisely because a bearer token on h2c is
+/// readable and replayable by anyone on the path. That is a real and accepted limitation here:
+/// these listeners are h2c today and the boundary assumes a trusted network. The PSK raises the
+/// bar from "anyone who can reach the port" to "anyone who can read the traffic"; TLS on these
+/// listeners is the follow-on hardening and requires no change to this code.
+/// </para>
+/// </remarks>
+public static class ControlPlaneCredentials
+{
+    /// <summary>
+    /// Metadata header naming the site a call belongs to. Central needs it to pick which
+    /// per-site key to verify against; a site's own inbound gate ignores it (a site has exactly
+    /// one key). Required by central's interceptor from Phase 1A.
+    /// </summary>
+    public const string SiteHeader = "x-scadabridge-site";
+
+    /// <summary>The bearer metadata header. Lowercase — gRPC lowercases header keys on the wire.</summary>
+    public const string AuthorizationHeader = "authorization";
+
+    /// <summary>
+    /// Creates call credentials that attach <c>authorization: Bearer &lt;psk&gt;</c> and
+    /// <c>x-scadabridge-site: &lt;siteId&gt;</c> to every call.
+    /// </summary>
+    /// <param name="provider">Resolves the site's preshared key.</param>
+    /// <param name="siteId">The site this channel talks to (or, site-side, this site's own id).</param>
+    /// <returns>Call credentials for a channel bound to that site.</returns>
+    public static CallCredentials ForSite(ISitePskProvider provider, string siteId)
+    {
+        ArgumentNullException.ThrowIfNull(provider);
+        ArgumentException.ThrowIfNullOrWhiteSpace(siteId);
+
+        return CallCredentials.FromInterceptor(async (context, metadata) =>
+        {
+            // A throw here fails the call, which is the point: an unauthenticated dial must
+            // not happen. Callers classify the resulting fault the same way they classify any
+            // other — the pull clients degrade to an empty batch and log, the streaming
+            // subscribers retry.
+            var psk = await provider.GetAsync(siteId, context.CancellationToken).ConfigureAwait(false);
+            metadata.Add(AuthorizationHeader, $"Bearer {psk}");
+            metadata.Add(SiteHeader, siteId);
+        });
+    }
+
+    /// <summary>
+    /// Applies per-site call credentials to channel options, if a provider is available.
+    /// A null provider leaves the options untouched — the shape used by test-only and
+    /// default constructors that never dial a gated endpoint.
+    /// </summary>
+    /// <param name="options">Channel options being built.</param>
+    /// <param name="provider">Key provider, or null to leave the channel unauthenticated.</param>
+    /// <param name="siteId">The site this channel talks to.</param>
+    /// <returns>The same options instance, for chaining.</returns>
+    public static GrpcChannelOptions WithSiteCredentials(
+        this GrpcChannelOptions options, ISitePskProvider? provider, string? siteId)
+    {
+        ArgumentNullException.ThrowIfNull(options);
+
+        if (provider is null || string.IsNullOrWhiteSpace(siteId))
+        {
+            return options;
+        }
+
+        options.Credentials = ChannelCredentials.Create(
+            ChannelCredentials.Insecure, ForSite(provider, siteId));
+        options.UnsafeUseInsecureChannelCallCredentials = true;
+        return options;
+    }
+}

src/ZB.MOM.WW.ScadaBridge.Communication/Grpc/SiteStreamGrpcClient.cs

@@ -60,6 +60,27 @@ public class SiteStreamGrpcClient : IAsyncDisposable, IDisposable
     /// <param name="logger">Logger for diagnostics and errors.</param>
     /// <param name="options">Communication options including keepalive settings.</param>
     public SiteStreamGrpcClient(string endpoint, ILogger logger, CommunicationOptions options)
+        : this(endpoint, logger, options, pskProvider: null, siteIdentifier: null)
+    {
+    }
+
+    /// <summary>
+    /// Creates a client that authenticates every call with the site's preshared key.
+    /// This is the production shape: <c>SiteStreamService</c> is gated by
+    /// <c>ControlPlaneAuthInterceptor</c> on the site node, so a client without credentials
+    /// gets <see cref="StatusCode.PermissionDenied"/> on every call.
+    /// </summary>
+    /// <param name="endpoint">The gRPC endpoint address for the site.</param>
+    /// <param name="logger">Logger for diagnostics and errors.</param>
+    /// <param name="options">Communication options including keepalive settings.</param>
+    /// <param name="pskProvider">Resolves the site's preshared key; null leaves the channel unauthenticated.</param>
+    /// <param name="siteIdentifier">Site this channel talks to; null leaves the channel unauthenticated.</param>
+    public SiteStreamGrpcClient(
+        string endpoint,
+        ILogger logger,
+        CommunicationOptions options,
+        ISitePskProvider? pskProvider,
+        string? siteIdentifier)
     {
         Endpoint = endpoint;
         KeepAlivePingDelay = options.GrpcKeepAlivePingDelay;
@@ -72,7 +93,7 @@ public class SiteStreamGrpcClient : IAsyncDisposable, IDisposable
                 KeepAlivePingTimeout = options.GrpcKeepAlivePingTimeout,
                 KeepAlivePingPolicy = HttpKeepAlivePingPolicy.Always
             }
-        });
+        }.WithSiteCredentials(pskProvider, siteIdentifier));
         _client = new SiteStreamService.SiteStreamServiceClient(_channel);
         _logger = logger;
     }

src/ZB.MOM.WW.ScadaBridge.Communication/Grpc/SiteStreamGrpcClientFactory.cs

@@ -26,9 +26,11 @@ public class SiteStreamGrpcClientFactory : IAsyncDisposable, IDisposable
     private readonly ConcurrentDictionary<(string Site, string Endpoint), SiteStreamGrpcClient> _clients = new();
     private readonly ILoggerFactory _loggerFactory;
     private readonly CommunicationOptions _options;
+    private readonly ISitePskProvider? _pskProvider;
 
     /// <summary>
-    /// Test/default constructor — uses default <see cref="CommunicationOptions"/>.
+    /// Test/default constructor — uses default <see cref="CommunicationOptions"/> and creates
+    /// unauthenticated channels.
     /// </summary>
     /// <param name="loggerFactory">Logger factory passed to created clients.</param>
     public SiteStreamGrpcClientFactory(ILoggerFactory loggerFactory)
@@ -37,16 +39,36 @@ public class SiteStreamGrpcClientFactory : IAsyncDisposable, IDisposable
     }
 
     /// <summary>
-    /// DI constructor — flows <see cref="CommunicationOptions"/> into every created
-    /// <see cref="SiteStreamGrpcClient"/> so the configured gRPC keepalive settings
-    /// are applied rather than hard-coded defaults.
+    /// Constructor without a key provider — creates unauthenticated channels, which a gated
+    /// site will refuse. Retained for tests and for hosts that never dial a site.
     /// </summary>
     /// <param name="loggerFactory">Logger factory passed to created clients.</param>
     /// <param name="options">Communication options applied to each created client.</param>
     public SiteStreamGrpcClientFactory(ILoggerFactory loggerFactory, IOptions<CommunicationOptions> options)
+        : this(loggerFactory, options, pskProvider: null)
+    {
+    }
+
+    /// <summary>
+    /// DI constructor — flows <see cref="CommunicationOptions"/> into every created
+    /// <see cref="SiteStreamGrpcClient"/> so the configured gRPC keepalive settings are applied
+    /// rather than hard-coded defaults, and attaches the per-site preshared key that the site's
+    /// <c>ControlPlaneAuthInterceptor</c> requires.
+    /// </summary>
+    /// <param name="loggerFactory">Logger factory passed to created clients.</param>
+    /// <param name="options">Communication options applied to each created client.</param>
+    /// <param name="pskProvider">
+    /// Resolves each site's preshared key. Optional in DI so a host that registers no provider
+    /// (a site node, which never dials another site) still resolves this factory.
+    /// </param>
+    public SiteStreamGrpcClientFactory(
+        ILoggerFactory loggerFactory,
+        IOptions<CommunicationOptions> options,
+        ISitePskProvider? pskProvider)
     {
         _loggerFactory = loggerFactory;
         _options = options.Value;
+        _pskProvider = pskProvider;
     }
 
     /// <summary>
@@ -59,7 +81,7 @@ public class SiteStreamGrpcClientFactory : IAsyncDisposable, IDisposable
     /// <param name="grpcEndpoint">gRPC endpoint (second half of the cache key) the client is bound to.</param>
     /// <returns>The cached or newly-created client bound to <paramref name="grpcEndpoint"/>.</returns>
     public virtual SiteStreamGrpcClient GetOrCreate(string siteIdentifier, string grpcEndpoint) =>
-        _clients.GetOrAdd((siteIdentifier, grpcEndpoint), _ => CreateClient(grpcEndpoint));
+        _clients.GetOrAdd((siteIdentifier, grpcEndpoint), key => CreateClient(key.Site, key.Endpoint));
 
     /// <summary>
     /// Returns the cached client for <c>(site, endpoint)</c>, or <c>null</c> — never creates.
@@ -77,12 +99,13 @@ public class SiteStreamGrpcClientFactory : IAsyncDisposable, IDisposable
     /// can substitute a tracking client while still exercising the factory's real
     /// caching and disposal machinery.
     /// </summary>
+    /// <param name="siteIdentifier">Site the new client talks to; selects which preshared key it presents.</param>
     /// <param name="grpcEndpoint">gRPC endpoint the new client will connect to.</param>
     /// <returns>A new <see cref="SiteStreamGrpcClient"/> connected to <paramref name="grpcEndpoint"/>.</returns>
-    protected virtual SiteStreamGrpcClient CreateClient(string grpcEndpoint)
+    protected virtual SiteStreamGrpcClient CreateClient(string siteIdentifier, string grpcEndpoint)
     {
         var logger = _loggerFactory.CreateLogger<SiteStreamGrpcClient>();
-        return new SiteStreamGrpcClient(grpcEndpoint, logger, _options);
+        return new SiteStreamGrpcClient(grpcEndpoint, logger, _options, _pskProvider, siteIdentifier);
     }
 
     /// <summary>
@@ -99,6 +122,10 @@ public class SiteStreamGrpcClientFactory : IAsyncDisposable, IDisposable
             if (_clients.TryRemove(key, out var client))
                 await client.DisposeAsync();
         }
+
+        // Drop the cached preshared key too, so a site removed and re-added under the same
+        // identifier (with a rotated key) is not dialed with the stale one.
+        _pskProvider?.Invalidate(siteIdentifier);
     }
 
     /// <summary>

src/ZB.MOM.WW.ScadaBridge.Communication/ServiceCollectionExtensions.cs

@@ -1,5 +1,6 @@
 using Microsoft.Extensions.DependencyInjection;
 using Microsoft.Extensions.DependencyInjection.Extensions;
+using Microsoft.Extensions.Logging;
 using Microsoft.Extensions.Options;
 using ZB.MOM.WW.ScadaBridge.Communication.Grpc;
 
@@ -23,7 +24,15 @@ public static class ServiceCollectionExtensions
             ServiceDescriptor.Singleton<IValidateOptions<CommunicationOptions>, CommunicationOptionsValidator>());
 
         services.AddSingleton<CommunicationService>();
-        services.AddSingleton<SiteStreamGrpcClientFactory>();
+
+        // Explicit factory rather than AddSingleton<T>(): the ISitePskProvider dependency is
+        // optional (central registers one, a site node does not), and constructor selection
+        // over a nullable interface parameter is exactly the case the container cannot decide
+        // for itself — GetService returns null cleanly where constructor injection would throw.
+        services.AddSingleton(sp => new SiteStreamGrpcClientFactory(
+            sp.GetRequiredService<ILoggerFactory>(),
+            sp.GetRequiredService<IOptions<CommunicationOptions>>(),
+            sp.GetService<ISitePskProvider>()));
         services.AddSingleton<DebugStreamService>();
 
         // Aggregated live alarm cache (plan #10, Task 4): transient, in-memory, shared

src/ZB.MOM.WW.ScadaBridge.Host/Actors/AkkaHostedService.cs

@@ -450,16 +450,22 @@ akka {{
         siteAlarmLiveCache?.SetActorSystem(_actorSystem!);
 
 
-        // Management Service — accessible via ClusterClient
+        // Management Service — reached IN-PROCESS only, via ManagementActorHolder.
+        //
+        // This actor used to be registered with the ClusterClientReceptionist as well, for
+        // an out-of-cluster CLI that was never built (REQ-HOST-6a). The shipped CLI speaks
+        // HTTP Basic to /management, which asks this actor through the holder below
+        // (ManagementEndpoints), so the registration had no sender anywhere in the repo —
+        // it only advertised a management surface across the cluster-client boundary for
+        // free. Removed 2026-07-22 (ClusterClient→gRPC migration, T0.1).
         var mgmtLogger = _serviceProvider.GetRequiredService<ILoggerFactory>()
             .CreateLogger<ZB.MOM.WW.ScadaBridge.ManagementService.ManagementActor>();
         var mgmtActor = _actorSystem!.ActorOf(
             Props.Create(() => new ZB.MOM.WW.ScadaBridge.ManagementService.ManagementActor(_serviceProvider, mgmtLogger)),
             "management");
-        ClusterClientReceptionist.Get(_actorSystem).RegisterService(mgmtActor);
         var mgmtHolder = _serviceProvider.GetRequiredService<ZB.MOM.WW.ScadaBridge.ManagementService.ManagementActorHolder>();
         mgmtHolder.ActorRef = mgmtActor;
-        _logger.LogInformation("ManagementActor registered with ClusterClientReceptionist");
+        _logger.LogInformation("ManagementActor started at /user/management (in-process access via ManagementActorHolder)");
 
         // Notification Outbox — cluster singleton so exactly one node owns ingest,
         // the dispatch sweep and the purge loop. Central actors run on the base

src/ZB.MOM.WW.ScadaBridge.Host/ControlPlaneAuthInterceptor.cs

@@ -0,0 +1,211 @@
+using System.Security.Cryptography;
+using System.Text;
+using Grpc.Core;
+using Grpc.Core.Interceptors;
+using Microsoft.Extensions.Logging;
+using Microsoft.Extensions.Options;
+using ZB.MOM.WW.ScadaBridge.Communication;
+using ZB.MOM.WW.ScadaBridge.Communication.Grpc;
+
+namespace ZB.MOM.WW.ScadaBridge.Host;
+
+/// <summary>
+/// Gates the site↔central gRPC control plane with a preshared key.
+/// </summary>
+/// <remarks>
+/// <para>
+/// <b>The gap this closes.</b> <c>SiteStreamService</c> shipped with no authentication at all —
+/// plaintext h2c, no interceptor. Anything that could reach a site node's gRPC port could open a
+/// live data stream or call <c>PullAuditEvents</c>/<c>PullSiteCalls</c> and read audit rows back.
+/// The only gated surface on that listener was LocalDb sync, and only for its own service. That
+/// gap exists independently of the ClusterClient→gRPC migration; it becomes indefensible once
+/// every command crosses this listener.
+/// </para>
+/// <para>
+/// <b>Modeled on <see cref="LocalDbSyncAuthInterceptor"/>,</b> deliberately: same four server
+/// handlers funnelling into one <c>Authorize</c>, same <c>authorization: Bearer</c> extraction,
+/// same <see cref="CryptographicOperations.FixedTimeEquals"/> comparison, same fail-closed
+/// posture, same <see cref="StatusCode.PermissionDenied"/> rejection. Two differences:
+/// </para>
+/// <list type="number">
+/// <item>It gates a <b>set</b> of service prefixes rather than one, so later phases can add the
+/// new command/control services without a second interceptor.</item>
+/// <item>Its expected key comes from <see cref="CommunicationOptions.GrpcPsk"/> — the site's own
+/// key, supplied in production as <c>${secret:SB-GRPC-PSK-&lt;siteId&gt;}</c> and expanded before
+/// the host is built.</item>
+/// </list>
+/// <para>
+/// <b>The two keys are separate on purpose.</b> LocalDb sync keeps its own
+/// <c>LocalDb:Replication:ApiKey</c>, which authenticates a different peer (the pair partner, not
+/// central) over a different trust relationship. Sharing one key would mean a site's central-facing
+/// key also admits writes into its database.
+/// </para>
+/// <para>
+/// <b>Fail-closed, and not optional.</b> With no <c>GrpcPsk</c> configured, every gated call is
+/// rejected — including the ones that work today. That is a deliberate break: LocalDb replication
+/// is an opt-in feature whose "off" state is "no peer", whereas streaming and audit pull are
+/// core paths, so "no key" must not silently mean "no authentication". Every environment must
+/// carry a key before upgrading to this build.
+/// </para>
+/// </remarks>
+public sealed class ControlPlaneAuthInterceptor : Interceptor
+{
+    /// <summary>
+    /// Service prefixes gated by default. Read from the generated <c>sitestream.proto</c>
+    /// package/service names — <c>package sitestream; service SiteStreamService</c>.
+    /// Later phases append their own services here.
+    /// </summary>
+    public static readonly IReadOnlyList<string> DefaultGatedPrefixes =
+        new[] { "/sitestream.SiteStreamService/" };
+
+    private readonly IReadOnlyList<string> _gatedPrefixes;
+    private readonly IOptions<CommunicationOptions> _options;
+    private readonly ILogger<ControlPlaneAuthInterceptor> _logger;
+
+    /// <summary>Creates the interceptor gating <see cref="DefaultGatedPrefixes"/>.</summary>
+    /// <param name="options">Communication options; <c>GrpcPsk</c> is the expected bearer token.</param>
+    /// <param name="logger">Logger for denial diagnostics.</param>
+    public ControlPlaneAuthInterceptor(
+        IOptions<CommunicationOptions> options,
+        ILogger<ControlPlaneAuthInterceptor> logger)
+        : this(options, logger, DefaultGatedPrefixes)
+    {
+    }
+
+    /// <summary>Creates the interceptor gating an explicit set of service prefixes.</summary>
+    /// <param name="options">Communication options; <c>GrpcPsk</c> is the expected bearer token.</param>
+    /// <param name="logger">Logger for denial diagnostics.</param>
+    /// <param name="gatedPrefixes">Method-path prefixes to gate, e.g. <c>/sitestream.SiteStreamService/</c>.</param>
+    public ControlPlaneAuthInterceptor(
+        IOptions<CommunicationOptions> options,
+        ILogger<ControlPlaneAuthInterceptor> logger,
+        IReadOnlyList<string> gatedPrefixes)
+    {
+        ArgumentNullException.ThrowIfNull(options);
+        ArgumentNullException.ThrowIfNull(logger);
+        ArgumentNullException.ThrowIfNull(gatedPrefixes);
+
+        _options = options;
+        _logger = logger;
+        _gatedPrefixes = gatedPrefixes;
+    }
+
+    /// <inheritdoc />
+    public override Task<TResponse> UnaryServerHandler<TRequest, TResponse>(
+        TRequest request,
+        ServerCallContext context,
+        UnaryServerMethod<TRequest, TResponse> continuation)
+    {
+        Authorize(context);
+        return continuation(request, context);
+    }
+
+    /// <inheritdoc />
+    public override Task DuplexStreamingServerHandler<TRequest, TResponse>(
+        IAsyncStreamReader<TRequest> requestStream,
+        IServerStreamWriter<TResponse> responseStream,
+        ServerCallContext context,
+        DuplexStreamingServerMethod<TRequest, TResponse> continuation)
+    {
+        Authorize(context);
+        return continuation(requestStream, responseStream, context);
+    }
+
+    /// <inheritdoc />
+    public override Task<TResponse> ClientStreamingServerHandler<TRequest, TResponse>(
+        IAsyncStreamReader<TRequest> requestStream,
+        ServerCallContext context,
+        ClientStreamingServerMethod<TRequest, TResponse> continuation)
+    {
+        Authorize(context);
+        return continuation(requestStream, context);
+    }
+
+    /// <inheritdoc />
+    public override Task ServerStreamingServerHandler<TRequest, TResponse>(
+        TRequest request,
+        IServerStreamWriter<TResponse> responseStream,
+        ServerCallContext context,
+        ServerStreamingServerMethod<TRequest, TResponse> continuation)
+    {
+        Authorize(context);
+        return continuation(request, responseStream, context);
+    }
+
+    /// <summary>
+    /// Throws <see cref="RpcException"/> with <see cref="StatusCode.PermissionDenied"/> if this
+    /// is a gated call that does not carry the configured bearer token. Calls to services
+    /// outside <c>gatedPrefixes</c> — notably LocalDb sync, which has its own interceptor —
+    /// return immediately.
+    /// </summary>
+    private void Authorize(ServerCallContext context)
+    {
+        if (!IsGated(context.Method))
+        {
+            return;
+        }
+
+        var expected = _options.Value.GrpcPsk;
+        if (string.IsNullOrEmpty(expected))
+        {
+            _logger.LogWarning(
+                "Rejected a control-plane call to {Method}: no ScadaBridge:Communication:GrpcPsk is "
+                + "configured, so the control plane is closed. Set the same key here (in production, "
+                + "as ${{secret:SB-GRPC-PSK-<siteId>}}) and in central's secret store.",
+                context.Method);
+            throw new RpcException(new Status(
+                StatusCode.PermissionDenied,
+                "Control plane is not accepting calls: no preshared key is configured on this node."));
+        }
+
+        var presented = ExtractBearerToken(context.RequestHeaders);
+        if (presented is null || !FixedTimeEquals(presented, expected))
+        {
+            _logger.LogWarning(
+                "Rejected a control-plane call to {Method}: {Reason}.",
+                context.Method,
+                presented is null ? "no bearer token presented" : "bearer token did not match");
+            throw new RpcException(new Status(
+                StatusCode.PermissionDenied,
+                "Control plane authentication failed."));
+        }
+    }
+
+    private bool IsGated(string method)
+    {
+        foreach (var prefix in _gatedPrefixes)
+        {
+            if (method.StartsWith(prefix, StringComparison.Ordinal))
+            {
+                return true;
+            }
+        }
+        return false;
+    }
+
+    private static string? ExtractBearerToken(Metadata headers)
+    {
+        // gRPC lowercases header keys on the wire; compare case-insensitively anyway so a
+        // hand-built Metadata in a test behaves the same as a real request.
+        foreach (var entry in headers)
+        {
+            if (!string.Equals(entry.Key, ControlPlaneCredentials.AuthorizationHeader,
+                    StringComparison.OrdinalIgnoreCase))
+            {
+                continue;
+            }
+
+            var value = entry.Value;
+            if (value is not null && value.StartsWith("Bearer ", StringComparison.OrdinalIgnoreCase))
+            {
+                return value["Bearer ".Length..];
+            }
+        }
+
+        return null;
+    }
+
+    private static bool FixedTimeEquals(string presented, string expected)
+        => CryptographicOperations.FixedTimeEquals(
+            Encoding.UTF8.GetBytes(presented), Encoding.UTF8.GetBytes(expected));
+}

src/ZB.MOM.WW.ScadaBridge.Host/Program.cs

@@ -100,6 +100,14 @@ try
         // Shared components
         builder.Services.AddClusterInfrastructure();
         builder.Services.AddCommunication();
+
+        // Per-site gRPC preshared keys. Central-only: it is the side that dials sites, and
+        // the only side whose key set is dynamic (sites come from the configuration
+        // database, so there is no fixed list of ${secret:} references to expand at boot —
+        // hence a runtime resolver rather than the pre-host SecretReferenceExpander a site
+        // node uses for its single key). Registered before the clients that consume it.
+        builder.Services.AddSingleton<
+            ZB.MOM.WW.ScadaBridge.Communication.Grpc.ISitePskProvider, SitePskProvider>();
         builder.Services.AddHealthMonitoring();
         builder.Services.AddCentralHealthAggregation();
         builder.Services.AddExternalSystemGateway();
@@ -519,12 +527,21 @@ try
             });
         });
 
-        // gRPC server registration
-        // The interceptor gates ONLY /localdb_sync.v1.LocalDbSync/ — SiteStream calls on
-        // this same pipeline pass through untouched. It is fail-closed: with no
-        // LocalDb:Replication:ApiKey configured, no sync stream is accepted at all.
+        // gRPC server registration. Two interceptors, two disjoint service prefixes, two
+        // separate keys — neither one's absence weakens the other:
+        //
+        //   LocalDbSyncAuthInterceptor  gates /localdb_sync.v1.LocalDbSync/  (LocalDb:Replication:ApiKey)
+        //   ControlPlaneAuthInterceptor gates /sitestream.SiteStreamService/ (ScadaBridge:Communication:GrpcPsk)
+        //
+        // Both are fail-closed: an unset key closes that surface rather than opening it. For
+        // LocalDb that means replication simply does not start; for the control plane it means
+        // this node serves no streams and no audit pulls until a key is configured, which is
+        // why every environment must carry one before running this build.
         builder.Services.AddGrpc(options =>
-            options.Interceptors.Add<LocalDbSyncAuthInterceptor>());
+        {
+            options.Interceptors.Add<LocalDbSyncAuthInterceptor>();
+            options.Interceptors.Add<ControlPlaneAuthInterceptor>();
+        });
         builder.Services.AddSingleton<ZB.MOM.WW.ScadaBridge.Communication.Grpc.SiteStreamGrpcServer>();
 
         // Existing site service registrations (this is also where LocalDb and its

src/ZB.MOM.WW.ScadaBridge.Host/SitePskProvider.cs

@@ -0,0 +1,149 @@
+using System.Collections.Concurrent;
+using Microsoft.Extensions.Logging;
+using Microsoft.Extensions.Options;
+using ZB.MOM.WW.ScadaBridge.Communication;
+using ZB.MOM.WW.ScadaBridge.Communication.Grpc;
+using ZB.MOM.WW.Secrets.Abstractions;
+
+namespace ZB.MOM.WW.ScadaBridge.Host;
+
+/// <summary>
+/// Central's <see cref="ISitePskProvider"/>: resolves each site's gRPC preshared key at
+/// channel-build time — from <c>ScadaBridge:Communication:SitePsks</c> if the site is listed
+/// there, otherwise from the secrets store under the name <c>SB-GRPC-PSK-{siteId}</c>.
+/// </summary>
+/// <remarks>
+/// <para>
+/// <b>Why resolved at runtime rather than expanded into config at boot.</b> Site nodes get their
+/// single key through the pre-host <c>SecretReferenceExpander</c> — a <c>${secret:…}</c> in
+/// appsettings, resolved once before the host is built. Central cannot do that: its set of sites
+/// comes from the configuration database and changes while the process runs, so there is no
+/// fixed list of references to expand at boot. This mirrors the pattern the MxGateway data
+/// connection already uses for its per-connection API keys.
+/// </para>
+/// <para>
+/// <b>Caching.</b> Successful resolves are cached indefinitely; a site's key changes only on
+/// rotation, and rotation restarts the pair. Failures are deliberately NOT cached, so a key
+/// seeded after the first (failed) dial is picked up on the next attempt instead of requiring a
+/// restart. <see cref="Invalidate"/> drops an entry when a site is removed or a key is rotated
+/// in place.
+/// </para>
+/// <para>
+/// <b>Fail-closed.</b> A key absent from BOTH sources throws, and the dial that needed it fails.
+/// It never degrades to an unauthenticated call. The message names the exact config key and the
+/// exact secret name, so the fix is one setting or one <c>secret</c> CLI seed.
+/// </para>
+/// </remarks>
+public sealed class SitePskProvider : ISitePskProvider
+{
+    /// <summary>Prefix of the per-site secret name; the site identifier is appended verbatim.</summary>
+    public const string SecretNamePrefix = "SB-GRPC-PSK-";
+
+    private readonly ConcurrentDictionary<string, string> _cache = new(StringComparer.Ordinal);
+    private readonly ISecretResolver _resolver;
+    private readonly IOptionsMonitor<CommunicationOptions> _options;
+    private readonly ILogger<SitePskProvider> _logger;
+
+    /// <summary>Creates the provider over the host's secret resolver.</summary>
+    /// <param name="resolver">Runtime secret resolver from the host container.</param>
+    /// <param name="options">
+    /// Communication options supplying the optional <c>SitePsks</c> map. Read through an
+    /// <see cref="IOptionsMonitor{TOptions}"/> so a reloaded configuration is honoured on the
+    /// next uncached resolve rather than pinned at construction.
+    /// </param>
+    /// <param name="logger">Logger for resolution diagnostics.</param>
+    public SitePskProvider(
+        ISecretResolver resolver,
+        IOptionsMonitor<CommunicationOptions> options,
+        ILogger<SitePskProvider> logger)
+    {
+        ArgumentNullException.ThrowIfNull(resolver);
+        ArgumentNullException.ThrowIfNull(options);
+        ArgumentNullException.ThrowIfNull(logger);
+
+        _resolver = resolver;
+        _options = options;
+        _logger = logger;
+    }
+
+    /// <summary>Builds the secret name for a site.</summary>
+    /// <param name="siteId">Site identifier.</param>
+    /// <returns>The secret name, e.g. <c>SB-GRPC-PSK-site-a</c>.</returns>
+    public static string SecretNameFor(string siteId) => SecretNamePrefix + siteId;
+
+    /// <inheritdoc />
+    public async ValueTask<string> GetAsync(string siteId, CancellationToken ct)
+    {
+        ArgumentException.ThrowIfNullOrWhiteSpace(siteId);
+
+        if (_cache.TryGetValue(siteId, out var cached))
+        {
+            return cached;
+        }
+
+        // Configured map first: it is the explicit, operator-stated answer for this site, and
+        // it is the only source available to a host running without a secrets master key.
+        var configured = _options.CurrentValue.SitePsks;
+        if (configured.TryGetValue(siteId, out var fromConfig) && !string.IsNullOrEmpty(fromConfig))
+        {
+            _cache[siteId] = fromConfig;
+            return fromConfig;
+        }
+
+        // Otherwise the store, which is the only source that can serve a site added at runtime.
+        var secretName = SecretNameFor(siteId);
+        var value = await ResolveFromStoreAsync(secretName, siteId, ct).ConfigureAwait(false);
+
+        if (string.IsNullOrEmpty(value))
+        {
+            _logger.LogError(
+                "gRPC control-plane key for site {SiteId} could not be resolved: it is absent from "
+                + "ScadaBridge:Communication:SitePsks and secret '{SecretName}' is missing, empty or "
+                + "tombstoned. Calls to this site are refused until one of the two is set (the site "
+                + "node must carry the same value in ScadaBridge:Communication:GrpcPsk).",
+                siteId, secretName);
+            throw new InvalidOperationException(
+                $"gRPC preshared key for site '{siteId}' could not be resolved from "
+                + $"ScadaBridge:Communication:SitePsks or secret '{secretName}'.");
+        }
+
+        // Two concurrent first dials may both resolve; the read is idempotent and the values
+        // identical, so the loser simply overwrites with the same string.
+        _cache[siteId] = value;
+        return value;
+    }
+
+    /// <summary>
+    /// Reads the key from the secrets store, treating a store fault as "not found" rather than
+    /// letting it propagate. A host with no master key configured (the development rig) throws
+    /// from the resolver; that must produce the same clear "key not configured" error as a
+    /// missing secret, not an opaque cryptographic one.
+    /// </summary>
+    private async Task<string?> ResolveFromStoreAsync(string secretName, string siteId, CancellationToken ct)
+    {
+        try
+        {
+            return await _resolver.GetAsync(new SecretName(secretName), ct).ConfigureAwait(false);
+        }
+        catch (OperationCanceledException)
+        {
+            throw;
+        }
+        catch (Exception ex)
+        {
+            _logger.LogDebug(ex,
+                "Secret store lookup for '{SecretName}' (site {SiteId}) faulted; treating as not found.",
+                secretName, siteId);
+            return null;
+        }
+    }
+
+    /// <inheritdoc />
+    public void Invalidate(string siteId)
+    {
+        if (!string.IsNullOrWhiteSpace(siteId))
+        {
+            _cache.TryRemove(siteId, out _);
+        }
+    }
+}

src/ZB.MOM.WW.ScadaBridge.Host/StartupValidator.cs

@@ -126,6 +126,23 @@ public static class StartupValidator
                 p.Require("ScadaBridge:Node:MetricsPort", _ => metricsPort != port, "must differ from RemotingPort");
                 p.Require("ScadaBridge:Node:MetricsPort", _ => metricsPort != grpcPort, "must differ from GrpcPort");
 
+                // The gRPC control-plane preshared key. Same argument as the inbound
+                // API-key pepper above: without it the node boots and looks healthy, but
+                // ControlPlaneAuthInterceptor is fail-closed, so every SiteStream call —
+                // live subscriptions, audit pulls, cached-telemetry ingest — is refused
+                // with PermissionDenied. A silent, total loss of the site's central-facing
+                // surface is far worse than a loud boot failure, so require it here.
+                // Central holds the matching value per site in its secret store as
+                // SB-GRPC-PSK-{SiteId}; production supplies this one as
+                // ${secret:SB-GRPC-PSK-<siteId>}, expanded before the host is built.
+                p.Require("ScadaBridge:Communication:GrpcPsk",
+                    value => !string.IsNullOrWhiteSpace(value),
+                    "is required for Site nodes: it is the preshared key the gRPC control "
+                    + "plane authenticates with, and the interceptor is fail-closed, so an "
+                    + "unset key refuses every SiteStream call. Set the same value here (in "
+                    + "production as ${secret:SB-GRPC-PSK-<siteId>}) and under the secret "
+                    + "name SB-GRPC-PSK-<siteId> in central's secret store");
+
                 // ScadaBridge:Database:SiteDbPath was required here until LocalDb
                 // Phase 2. The site's tables now live in the consolidated LocalDb
                 // database (LocalDb:Path, which SiteServiceRegistration requires),

src/ZB.MOM.WW.ScadaBridge.Host/appsettings.Site.json

@@ -42,6 +42,8 @@
       "SqliteDbPath": "./data/store-and-forward.db"
     },
     "Communication": {
+      "_grpcPsk": "REQUIRED on Site nodes (StartupValidator fails the boot without it). The preshared key the gRPC control plane authenticates with: ControlPlaneAuthInterceptor is fail-closed, so an unset key refuses every SiteStream call — live subscriptions, audit pulls, cached-telemetry ingest — while the node still reports healthy. Supply it as ${secret:SB-GRPC-PSK-<siteId>} so the plaintext never sits in this file, and seed the SAME value on central: either as the secret SB-GRPC-PSK-<siteId> in its store, or as ScadaBridge:Communication:SitePsks:<siteId>. BOTH nodes of the pair carry the same key. Distinct from LocalDb:Replication:ApiKey, which authenticates the pair partner, not central — never share the two.",
+      "GrpcPsk": "${secret:SB-GRPC-PSK-site-1}",
       "_centralContactPoints": "Host-016: each entry MUST be a central node's remoting endpoint, NOT this site's own remoting port. The single dev-loopback default below points only at central-a (localhost:8081). In a multi-central deployment add the second central node here (e.g. 'akka.tcp://scadabridge@central-b-host:8081') so ClusterClient can fail over when central-a is down. The previous template listed localhost:8082 as the second contact — that is THIS site's own RemotingPort and is a permanent failure in the initial-contact rotation.",
       "CentralContactPoints": [
         "akka.tcp://scadabridge@localhost:8081"

tests/ZB.MOM.WW.ScadaBridge.AuditLog.Tests/Central/GrpcPullAuditEventsClientTests.cs

@@ -58,7 +58,7 @@ public class GrpcPullAuditEventsClientTests
         public static FakeInvoker Throwing(Exception ex) => new(null, ex);
 
         public Task<ProtoPullResponse> InvokeAsync(
-            string endpoint, ProtoPullRequest request, CancellationToken ct)
+            string siteId, string endpoint, ProtoPullRequest request, CancellationToken ct)
         {
             CallCount++;
             Endpoint = endpoint;
@@ -84,7 +84,7 @@ public class GrpcPullAuditEventsClientTests
             _byEndpoint = byEndpoint;
 
         public Task<ProtoPullResponse> InvokeAsync(
-            string endpoint, ProtoPullRequest request, CancellationToken ct)
+            string siteId, string endpoint, ProtoPullRequest request, CancellationToken ct)
         {
             Dialed.Add(endpoint);
             return Task.FromResult(_byEndpoint[endpoint]());

tests/ZB.MOM.WW.ScadaBridge.AuditLog.Tests/Central/GrpcPullSiteCallsClientTests.cs

@@ -50,7 +50,7 @@ public class GrpcPullSiteCallsClientTests
         public static FakeInvoker Throwing(Exception ex) => new(null, ex);
 
         public Task<ProtoPullResponse> InvokeAsync(
-            string endpoint, ProtoPullRequest request, CancellationToken ct)
+            string siteId, string endpoint, ProtoPullRequest request, CancellationToken ct)
         {
             CallCount++;
             Endpoint = endpoint;
@@ -77,7 +77,7 @@ public class GrpcPullSiteCallsClientTests
             _byEndpoint = byEndpoint;
 
         public Task<ProtoPullResponse> InvokeAsync(
-            string endpoint, ProtoPullRequest request, CancellationToken ct)
+            string siteId, string endpoint, ProtoPullRequest request, CancellationToken ct)
         {
             Dialed.Add(endpoint);
             // The Func may throw (transport fault) — the client's try/catch handles it.

tests/ZB.MOM.WW.ScadaBridge.Communication.Tests/Grpc/SiteStreamGrpcClientFactoryDisposeTests.cs

@@ -41,7 +41,7 @@ public class SiteStreamGrpcClientFactoryDisposeTests
 
         public IReadOnlyCollection<TrackingClient> Created => _created.ToList();
 
-        protected override SiteStreamGrpcClient CreateClient(string grpcEndpoint)
+        protected override SiteStreamGrpcClient CreateClient(string siteIdentifier, string grpcEndpoint)
         {
             var client = new TrackingClient();
             _created.Add(client);

tests/ZB.MOM.WW.ScadaBridge.Communication.Tests/Grpc/SiteStreamGrpcClientFactoryTests.cs

@@ -155,7 +155,7 @@ public class SiteStreamGrpcClientFactoryTests
     {
         public TrackingEndpointFactory() : base(NullLoggerFactory.Instance) { }
         public int CreatedCount { get; private set; }
-        protected override SiteStreamGrpcClient CreateClient(string grpcEndpoint)
+        protected override SiteStreamGrpcClient CreateClient(string siteIdentifier, string grpcEndpoint)
         {
             CreatedCount++;
             return new TrackingEndpointClient(grpcEndpoint);

tests/ZB.MOM.WW.ScadaBridge.Host.Tests/ControlPlaneAuthEndToEndTests.cs

@@ -0,0 +1,225 @@
+using Grpc.Core;
+using Grpc.Net.Client;
+using Microsoft.AspNetCore.Builder;
+using Microsoft.AspNetCore.Hosting;
+using Microsoft.AspNetCore.TestHost;
+using Microsoft.Extensions.DependencyInjection;
+using Microsoft.Extensions.Hosting;
+using Microsoft.Extensions.Logging.Abstractions;
+using Microsoft.Extensions.Options;
+using ZB.MOM.WW.ScadaBridge.Communication;
+using ZB.MOM.WW.ScadaBridge.Communication.Grpc;
+
+namespace ZB.MOM.WW.ScadaBridge.Host.Tests;
+
+/// <summary>
+/// End-to-end proof that the two halves of the control-plane PSK actually interoperate:
+/// <see cref="ControlPlaneCredentials"/> on the client and
+/// <see cref="ControlPlaneAuthInterceptor"/> on the server, over a real gRPC stack.
+/// </summary>
+/// <remarks>
+/// <para>
+/// The unit tests either side of this file each test one half against a hand-built input, and
+/// would both stay green if the halves disagreed — if the credentials never attached to a
+/// streaming call, if the metadata key case differed, or if attaching call credentials to a
+/// plaintext channel were rejected outright (gRPC refuses that by default; the code opts in with
+/// <c>UnsafeUseInsecureChannelCallCredentials</c>, and nothing but a real call proves the opt-in
+/// works). Getting that wrong takes down every site's streaming and audit-pull path at once,
+/// which is a bad thing to discover on the rig.
+/// </para>
+/// <para>
+/// Runs entirely in-process over <see cref="TestServer"/>: no ports, no containers. The service
+/// is a stub rather than the real <c>SiteStreamGrpcServer</c> — this is a test of the auth
+/// pipeline, and the real server would drag in an actor system for no added coverage. The method
+/// paths and message types are the real generated ones.
+/// </para>
+/// </remarks>
+public class ControlPlaneAuthEndToEndTests : IAsyncLifetime
+{
+    private IHost _host = null!;
+    private TestServer _server = null!;
+
+    /// <summary>Boots the in-process gRPC host with the real interceptor.</summary>
+    public async Task InitializeAsync()
+    {
+        _host = await new HostBuilder()
+            .ConfigureWebHost(web => web
+                .UseTestServer()
+                .ConfigureServices(services =>
+                {
+                    services.AddGrpc(o => o.Interceptors.Add<ControlPlaneAuthInterceptor>());
+                    services.AddSingleton(Options.Create(
+                        new CommunicationOptions { GrpcPsk = SiteKey }));
+                    services.AddSingleton<ControlPlaneAuthInterceptor>();
+                    services.AddSingleton<EchoSiteStreamService>();
+                })
+                .Configure(app =>
+                {
+                    app.UseRouting();
+                    app.UseEndpoints(e => e.MapGrpcService<EchoSiteStreamService>());
+                }))
+            .StartAsync();
+
+        _server = _host.GetTestServer();
+    }
+
+    /// <inheritdoc />
+    public async Task DisposeAsync()
+    {
+        await _host.StopAsync();
+        _host.Dispose();
+    }
+
+    private const string SiteKey = "the-site-a-preshared-key";
+
+    /// <summary>
+    /// Builds a channel through the test server, credentialed exactly as production does.
+    /// </summary>
+    private GrpcChannel Channel(string? key, string siteId = "site-a")
+    {
+        var options = new GrpcChannelOptions { HttpHandler = _server.CreateHandler() };
+        if (key is not null)
+        {
+            options.WithSiteCredentials(new FixedPskProvider(key), siteId);
+        }
+        return GrpcChannel.ForAddress(_server.BaseAddress, options);
+    }
+
+    private sealed class FixedPskProvider(string key) : ISitePskProvider
+    {
+        public ValueTask<string> GetAsync(string siteId, CancellationToken ct) => new(key);
+        public void Invalidate(string siteId) { }
+    }
+
+    /// <summary>Stub service: echoes back what the auth pipeline let through.</summary>
+    private sealed class EchoSiteStreamService : SiteStreamService.SiteStreamServiceBase
+    {
+        /// <summary>The site header the last accepted call carried.</summary>
+        public string? LastSiteHeader { get; private set; }
+
+        public override Task<PullAuditEventsResponse> PullAuditEvents(
+            PullAuditEventsRequest request, ServerCallContext context)
+        {
+            LastSiteHeader = context.RequestHeaders
+                .FirstOrDefault(h => h.Key == ControlPlaneCredentials.SiteHeader)?.Value;
+            return Task.FromResult(new PullAuditEventsResponse { MoreAvailable = false });
+        }
+
+        public override async Task SubscribeInstance(
+            InstanceStreamRequest request,
+            IServerStreamWriter<SiteStreamEvent> responseStream,
+            ServerCallContext context)
+        {
+            await responseStream.WriteAsync(new SiteStreamEvent { CorrelationId = request.CorrelationId });
+        }
+    }
+
+    [Fact]
+    public async Task CorrectKey_IsAccepted_OnAUnaryCall()
+    {
+        using var channel = Channel(SiteKey);
+        var client = new SiteStreamService.SiteStreamServiceClient(channel);
+
+        var reply = await client.PullAuditEventsAsync(new PullAuditEventsRequest { BatchSize = 1 });
+
+        Assert.False(reply.MoreAvailable);
+    }
+
+    [Fact]
+    public async Task WrongKey_IsRejected_WithPermissionDenied()
+    {
+        using var channel = Channel("some-other-sites-key");
+        var client = new SiteStreamService.SiteStreamServiceClient(channel);
+
+        var ex = await Assert.ThrowsAsync<RpcException>(
+            async () => await client.PullAuditEventsAsync(new PullAuditEventsRequest()));
+
+        Assert.Equal(StatusCode.PermissionDenied, ex.StatusCode);
+    }
+
+    [Fact]
+    public async Task NoCredentialsAtAll_IsRejected()
+    {
+        // The pre-T0.3 client shape. This is the case that proves the gap is actually closed.
+        using var channel = Channel(key: null);
+        var client = new SiteStreamService.SiteStreamServiceClient(channel);
+
+        var ex = await Assert.ThrowsAsync<RpcException>(
+            async () => await client.PullAuditEventsAsync(new PullAuditEventsRequest()));
+
+        Assert.Equal(StatusCode.PermissionDenied, ex.StatusCode);
+    }
+
+    [Fact]
+    public async Task CredentialsApplyToStreamingCalls_NotJustUnaryOnes()
+    {
+        // CallCredentials cover every call on the channel; a client interceptor that only
+        // handled the unary path would pass the test above and still break every subscription.
+        using var channel = Channel(SiteKey);
+        var client = new SiteStreamService.SiteStreamServiceClient(channel);
+
+        using var call = client.SubscribeInstance(
+            new InstanceStreamRequest { CorrelationId = "c1", InstanceUniqueName = "i1" });
+
+        Assert.True(await call.ResponseStream.MoveNext(CancellationToken.None));
+        Assert.Equal("c1", call.ResponseStream.Current.CorrelationId);
+    }
+
+    [Fact]
+    public async Task WrongKey_IsRejected_OnStreamingCallsToo()
+    {
+        using var channel = Channel("wrong");
+        var client = new SiteStreamService.SiteStreamServiceClient(channel);
+
+        using var call = client.SubscribeInstance(new InstanceStreamRequest { CorrelationId = "c1" });
+
+        var ex = await Assert.ThrowsAsync<RpcException>(
+            async () => await call.ResponseStream.MoveNext(CancellationToken.None));
+
+        Assert.Equal(StatusCode.PermissionDenied, ex.StatusCode);
+    }
+
+    [Fact]
+    public async Task AnUnresolvableKey_FailsTheCall_RatherThanDialingWithoutOne()
+    {
+        // SitePskProvider throws when a site has no key anywhere. What matters here is that the
+        // throw stops the call: the alternative — swallowing it and sending the request
+        // unauthenticated — is the exact failure this design exists to prevent. The status code
+        // is gRPC's choice, so assert the RpcException and record what it actually is rather
+        // than pinning a guess: callers already treat every non-OK status as a failed call, and
+        // the diagnosable signal is SitePskProvider's own LogError, not this code.
+        var options = new GrpcChannelOptions { HttpHandler = _server.CreateHandler() }
+            .WithSiteCredentials(new ThrowingPskProvider(), "site-a");
+        using var channel = GrpcChannel.ForAddress(_server.BaseAddress, options);
+        var client = new SiteStreamService.SiteStreamServiceClient(channel);
+
+        var ex = await Assert.ThrowsAsync<RpcException>(
+            async () => await client.PullAuditEventsAsync(new PullAuditEventsRequest()));
+
+        Assert.NotEqual(StatusCode.OK, ex.StatusCode);
+
+        // And nothing reached the service.
+        Assert.Null(_host.Services.GetRequiredService<EchoSiteStreamService>().LastSiteHeader);
+    }
+
+    private sealed class ThrowingPskProvider : ISitePskProvider
+    {
+        public ValueTask<string> GetAsync(string siteId, CancellationToken ct)
+            => throw new InvalidOperationException($"no key for '{siteId}'");
+        public void Invalidate(string siteId) { }
+    }
+
+    [Fact]
+    public async Task TheSiteHeaderTravels_SoCentralCanPickAPerSiteKeyInPhase1A()
+    {
+        // Central's own interceptor (T1A.2) verifies against the key for the site named in this
+        // header. Shipping it now means Phase 1A adds a lookup, not a wire change.
+        using var channel = Channel(SiteKey, siteId: "site-a");
+        var client = new SiteStreamService.SiteStreamServiceClient(channel);
+
+        await client.PullAuditEventsAsync(new PullAuditEventsRequest());
+
+        var service = _host.Services.GetRequiredService<EchoSiteStreamService>();
+        Assert.Equal("site-a", service.LastSiteHeader);
+    }
+}

tests/ZB.MOM.WW.ScadaBridge.Host.Tests/ControlPlaneAuthInterceptorTests.cs

@@ -0,0 +1,218 @@
+using Grpc.Core;
+using Microsoft.Extensions.Logging.Abstractions;
+using Microsoft.Extensions.Options;
+using ZB.MOM.WW.ScadaBridge.Communication;
+using ZB.MOM.WW.ScadaBridge.Communication.Grpc;
+
+namespace ZB.MOM.WW.ScadaBridge.Host.Tests;
+
+/// <summary>
+/// The site↔central gRPC control plane's inbound gate (ClusterClient→gRPC migration, T0.3).
+/// </summary>
+/// <remarks>
+/// <para>
+/// <c>SiteStreamService</c> shipped unauthenticated: plaintext h2c with no interceptor, so
+/// anything that could reach a site node's gRPC port could open a live data stream or pull audit
+/// rows back with <c>PullAuditEvents</c>/<c>PullSiteCalls</c>. These tests pin the gate that
+/// closes it, and — just as importantly — pin that it does NOT gate LocalDb sync, which has its
+/// own interceptor and its own key.
+/// </para>
+/// <para>
+/// Sibling of <see cref="LocalDbSyncAuthInterceptorTests"/>; the two interceptors share a shape
+/// deliberately, so the cases mirror each other.
+/// </para>
+/// </remarks>
+public class ControlPlaneAuthInterceptorTests
+{
+    // Real method paths: package `sitestream`, service `SiteStreamService` (sitestream.proto).
+    private const string SubscribeMethod = "/sitestream.SiteStreamService/SubscribeInstance";
+    private const string PullAuditMethod = "/sitestream.SiteStreamService/PullAuditEvents";
+    private const string LocalDbSyncMethod = "/localdb_sync.v1.LocalDbSync/Sync";
+
+    private static ControlPlaneAuthInterceptor CreateInterceptor(string? psk)
+        => new(
+            Options.Create(new CommunicationOptions { GrpcPsk = psk ?? "" }),
+            NullLogger<ControlPlaneAuthInterceptor>.Instance);
+
+    private static ServerCallContext CreateContext(string method, string? authorizationHeader)
+    {
+        var headers = new Metadata();
+        if (authorizationHeader is not null)
+            headers.Add("authorization", authorizationHeader);
+
+        return new FakeServerCallContext(method, headers);
+    }
+
+    /// <summary>
+    /// Minimal <see cref="ServerCallContext"/> carrying just a method name and request headers —
+    /// the only two things the interceptor reads. Hand-rolled for the same reason the LocalDb
+    /// sibling hand-rolls one: <c>Grpc.Core.Testing.TestServerCallContext</c> lives in the
+    /// retired native package and does not exist on the grpc-dotnet stack.
+    /// </summary>
+    private sealed class FakeServerCallContext(string method, Metadata requestHeaders)
+        : ServerCallContext
+    {
+        protected override string MethodCore => method;
+        protected override string HostCore => "localhost";
+        protected override string PeerCore => "ipv4:127.0.0.1:12345";
+        protected override DateTime DeadlineCore => DateTime.UtcNow.AddMinutes(1);
+        protected override Metadata RequestHeadersCore => requestHeaders;
+        protected override CancellationToken CancellationTokenCore => CancellationToken.None;
+        protected override Metadata ResponseTrailersCore { get; } = [];
+        protected override Status StatusCore { get; set; }
+        protected override WriteOptions? WriteOptionsCore { get; set; }
+        protected override AuthContext AuthContextCore { get; } =
+            new(null, new Dictionary<string, List<AuthProperty>>());
+
+        protected override ContextPropagationToken CreatePropagationTokenCore(
+            ContextPropagationOptions? options)
+            => throw new NotSupportedException();
+
+        protected override Task WriteResponseHeadersAsyncCore(Metadata responseHeaders)
+            => Task.CompletedTask;
+    }
+
+    /// <summary>Invokes the interceptor's unary path with a trivial continuation.</summary>
+    private static Task<string> Invoke(
+        ControlPlaneAuthInterceptor interceptor, ServerCallContext context)
+        => interceptor.UnaryServerHandler<string, string>(
+            "request", context, (_, _) => Task.FromResult("ok"));
+
+    [Fact]
+    public async Task LocalDbSyncMethod_PassesThrough_BecauseItHasItsOwnGateAndItsOwnKey()
+    {
+        // Both interceptors sit on the same site AddGrpc pipeline and see every call. If this
+        // one also gated sync, a site would need its central-facing key to equal its pair-replication
+        // key — collapsing two distinct trust relationships into one secret.
+        var interceptor = CreateInterceptor("the-site-key");
+        var context = CreateContext(LocalDbSyncMethod, authorizationHeader: null);
+
+        Assert.Equal("ok", await Invoke(interceptor, context));
+    }
+
+    [Fact]
+    public async Task GatedMethod_WithNoKeyConfigured_IsDenied_EvenWithABearerToken()
+    {
+        // Fail-closed, and this is the case that differs in consequence from LocalDb's: an
+        // unset key here does not disable an optional feature, it closes the site's entire
+        // central-facing surface. Loud refusal beats silent unauthenticated service.
+        var interceptor = CreateInterceptor(psk: null);
+        var context = CreateContext(SubscribeMethod, "Bearer anything-at-all");
+
+        var ex = await Assert.ThrowsAsync<RpcException>(() => Invoke(interceptor, context));
+        Assert.Equal(StatusCode.PermissionDenied, ex.StatusCode);
+    }
+
+    [Fact]
+    public async Task GatedMethod_WithNoBearerToken_IsDenied()
+    {
+        var interceptor = CreateInterceptor("the-site-key");
+        var context = CreateContext(SubscribeMethod, authorizationHeader: null);
+
+        var ex = await Assert.ThrowsAsync<RpcException>(() => Invoke(interceptor, context));
+        Assert.Equal(StatusCode.PermissionDenied, ex.StatusCode);
+    }
+
+    [Fact]
+    public async Task GatedMethod_WithWrongBearerToken_IsDenied()
+    {
+        var interceptor = CreateInterceptor("the-site-key");
+        var context = CreateContext(SubscribeMethod, "Bearer some-other-sites-key");
+
+        var ex = await Assert.ThrowsAsync<RpcException>(() => Invoke(interceptor, context));
+        Assert.Equal(StatusCode.PermissionDenied, ex.StatusCode);
+    }
+
+    [Fact]
+    public async Task GatedMethod_WithCorrectBearerToken_PassesThrough()
+    {
+        var interceptor = CreateInterceptor("the-site-key");
+        var context = CreateContext(SubscribeMethod, "Bearer the-site-key");
+
+        Assert.Equal("ok", await Invoke(interceptor, context));
+    }
+
+    [Fact]
+    public async Task GatedMethod_WithCorrectKey_ButNoBearerScheme_IsDenied()
+    {
+        // A raw key with no "Bearer " prefix is not what ControlPlaneCredentials sends;
+        // accepting it would widen the accepted credential shape for nothing.
+        var interceptor = CreateInterceptor("the-site-key");
+        var context = CreateContext(SubscribeMethod, "the-site-key");
+
+        var ex = await Assert.ThrowsAsync<RpcException>(() => Invoke(interceptor, context));
+        Assert.Equal(StatusCode.PermissionDenied, ex.StatusCode);
+    }
+
+    [Fact]
+    public async Task GatedMethod_TokenComparison_IsNotAPrefixMatch()
+    {
+        // A StartsWith comparison would accept a truncated key and make the secret recoverable
+        // one character at a time. FixedTimeEquals also rejects on length.
+        var interceptor = CreateInterceptor("the-site-key");
+        var context = CreateContext(SubscribeMethod, "Bearer the-site-ke");
+
+        var ex = await Assert.ThrowsAsync<RpcException>(() => Invoke(interceptor, context));
+        Assert.Equal(StatusCode.PermissionDenied, ex.StatusCode);
+    }
+
+    [Fact]
+    public async Task ServerStreaming_IsGated_BecauseThatIsHowSubscriptionsActuallyRun()
+    {
+        // SubscribeInstance/SubscribeSite are server-streaming. Gating only the unary path
+        // would leave the live data feed wide open while every unary test still passed.
+        var interceptor = CreateInterceptor("the-site-key");
+        var context = CreateContext(SubscribeMethod, "Bearer the-wrong-key");
+
+        var ex = await Assert.ThrowsAsync<RpcException>(() =>
+            interceptor.ServerStreamingServerHandler<string, string>(
+                "request",
+                responseStream: null!,
+                context,
+                (_, _, _) => Task.CompletedTask));
+
+        Assert.Equal(StatusCode.PermissionDenied, ex.StatusCode);
+    }
+
+    [Fact]
+    public async Task PullRpcs_AreGated_BecauseTheyReturnAuditRows()
+    {
+        // The strongest reason this gate exists: PullAuditEvents/PullSiteCalls hand back audit
+        // content to anyone who asks. Unary, so it would be easy to miss in a streaming-focused
+        // reading of the service.
+        var interceptor = CreateInterceptor("the-site-key");
+        var context = CreateContext(PullAuditMethod, authorizationHeader: null);
+
+        var ex = await Assert.ThrowsAsync<RpcException>(() => Invoke(interceptor, context));
+        Assert.Equal(StatusCode.PermissionDenied, ex.StatusCode);
+    }
+
+    [Fact]
+    public async Task GatedPrefixes_AreConstructorProvided_SoLaterPhasesAddServicesNotInterceptors()
+    {
+        // Phases 1A/1B add CentralControlService and SiteCommandService to this same gate.
+        var interceptor = new ControlPlaneAuthInterceptor(
+            Options.Create(new CommunicationOptions { GrpcPsk = "k" }),
+            NullLogger<ControlPlaneAuthInterceptor>.Instance,
+            new[] { "/scadabridge.sitecommand.v1.SiteCommandService/" });
+
+        var gated = CreateContext("/scadabridge.sitecommand.v1.SiteCommandService/ExecuteQuery", null);
+        var ex = await Assert.ThrowsAsync<RpcException>(() => Invoke(interceptor, gated));
+        Assert.Equal(StatusCode.PermissionDenied, ex.StatusCode);
+
+        // ...and the default set is no longer implied once an explicit set is supplied.
+        var notGated = CreateContext(SubscribeMethod, authorizationHeader: null);
+        Assert.Equal("ok", await Invoke(interceptor, notGated));
+    }
+
+    [Fact]
+    public void DefaultGatedPrefixes_MatchTheRealSiteStreamServicePath()
+    {
+        // A typo here disables the whole gate silently: every call would simply pass through.
+        // Pin it against a path taken from the generated service, not from the proto text.
+        var method = SiteStreamService.Descriptor.FullName;
+        Assert.Contains(
+            ControlPlaneAuthInterceptor.DefaultGatedPrefixes,
+            p => p == $"/{method}/");
+    }
+}

tests/ZB.MOM.WW.ScadaBridge.Host.Tests/SitePskProviderTests.cs

@@ -0,0 +1,179 @@
+using Microsoft.Extensions.Logging.Abstractions;
+using Microsoft.Extensions.Options;
+using NSubstitute;
+using ZB.MOM.WW.ScadaBridge.Communication;
+using ZB.MOM.WW.Secrets.Abstractions;
+
+namespace ZB.MOM.WW.ScadaBridge.Host.Tests;
+
+/// <summary>
+/// Central's per-site gRPC preshared key resolution (ClusterClient→gRPC migration, T0.3).
+/// </summary>
+/// <remarks>
+/// The property that matters most here is the negative one: a site whose key cannot be found
+/// must produce a throw, never an unauthenticated channel. Everything else — the two sources,
+/// the caching, the invalidation — exists to make that behaviour usable in practice.
+/// </remarks>
+public class SitePskProviderTests
+{
+    private static SitePskProvider Create(
+        ISecretResolver resolver, CommunicationOptions? options = null)
+        => new(
+            resolver,
+            new StaticOptionsMonitor(options ?? new CommunicationOptions()),
+            NullLogger<SitePskProvider>.Instance);
+
+    private sealed class StaticOptionsMonitor(CommunicationOptions value)
+        : IOptionsMonitor<CommunicationOptions>
+    {
+        public CommunicationOptions CurrentValue => value;
+        public CommunicationOptions Get(string? name) => value;
+        public IDisposable? OnChange(Action<CommunicationOptions, string?> listener) => null;
+    }
+
+    [Fact]
+    public async Task ResolvesFromTheSecretStore_UnderTheSiteQualifiedName()
+    {
+        var resolver = Substitute.For<ISecretResolver>();
+        resolver.GetAsync(new SecretName("SB-GRPC-PSK-site-a"), Arg.Any<CancellationToken>())
+            .Returns("key-for-a");
+
+        var psk = await Create(resolver).GetAsync("site-a", CancellationToken.None);
+
+        Assert.Equal("key-for-a", psk);
+    }
+
+    [Fact]
+    public async Task ConfiguredMapWins_SoAHostWithNoMasterKeyCanStillDial()
+    {
+        // The development rig runs with no secrets master key at all — every credential
+        // arrives as an environment override. Without this source, the rig could not use the
+        // gated control plane and the fail-closed design would be untestable there.
+        var resolver = Substitute.For<ISecretResolver>();
+        resolver.GetAsync(Arg.Any<SecretName>(), Arg.Any<CancellationToken>())
+            .Returns("from-the-store");
+
+        var options = new CommunicationOptions();
+        options.SitePsks["site-a"] = "from-config";
+
+        var psk = await Create(resolver, options).GetAsync("site-a", CancellationToken.None);
+
+        Assert.Equal("from-config", psk);
+        await resolver.DidNotReceive().GetAsync(Arg.Any<SecretName>(), Arg.Any<CancellationToken>());
+    }
+
+    [Fact]
+    public async Task StoreIsStillConsulted_ForASiteMissingFromTheMap()
+    {
+        // Sites are added at runtime from the Central UI, so the map can never be complete.
+        var resolver = Substitute.For<ISecretResolver>();
+        resolver.GetAsync(new SecretName("SB-GRPC-PSK-site-b"), Arg.Any<CancellationToken>())
+            .Returns("key-for-b");
+
+        var options = new CommunicationOptions();
+        options.SitePsks["site-a"] = "from-config";
+
+        var psk = await Create(resolver, options).GetAsync("site-b", CancellationToken.None);
+
+        Assert.Equal("key-for-b", psk);
+    }
+
+    [Fact]
+    public async Task MissingSecret_Throws_AndNeverYieldsAnEmptyKey()
+    {
+        // Fail-closed. The alternative — returning "" — would build a channel that presents
+        // "Bearer " and gets PermissionDenied anyway, but with a far less diagnosable error.
+        var resolver = Substitute.For<ISecretResolver>();
+        resolver.GetAsync(Arg.Any<SecretName>(), Arg.Any<CancellationToken>())
+            .Returns((string?)null);
+
+        var ex = await Assert.ThrowsAsync<InvalidOperationException>(
+            async () => await Create(resolver).GetAsync("site-a", CancellationToken.None));
+
+        // The message must name both sources — either one fixes it.
+        Assert.Contains("SB-GRPC-PSK-site-a", ex.Message);
+        Assert.Contains("SitePsks", ex.Message);
+    }
+
+    [Fact]
+    public async Task EmptySecret_IsTreatedAsMissing()
+    {
+        var resolver = Substitute.For<ISecretResolver>();
+        resolver.GetAsync(Arg.Any<SecretName>(), Arg.Any<CancellationToken>()).Returns("");
+
+        await Assert.ThrowsAsync<InvalidOperationException>(
+            async () => await Create(resolver).GetAsync("site-a", CancellationToken.None));
+    }
+
+    [Fact]
+    public async Task AFaultingStore_SurfacesAsKeyNotConfigured_NotAsACryptoError()
+    {
+        // A host with no master key throws from the resolver. The operator's problem is the
+        // same either way — "this site has no key" — so the diagnosis must say that.
+        var resolver = Substitute.For<ISecretResolver>();
+        resolver.GetAsync(Arg.Any<SecretName>(), Arg.Any<CancellationToken>())
+            .Returns<string?>(_ => throw new InvalidOperationException("no master key configured"));
+
+        var ex = await Assert.ThrowsAsync<InvalidOperationException>(
+            async () => await Create(resolver).GetAsync("site-a", CancellationToken.None));
+
+        Assert.Contains("SB-GRPC-PSK-site-a", ex.Message);
+    }
+
+    [Fact]
+    public async Task SuccessfulResolvesAreCached_SoEveryCallDoesNotHitTheStore()
+    {
+        var resolver = Substitute.For<ISecretResolver>();
+        resolver.GetAsync(Arg.Any<SecretName>(), Arg.Any<CancellationToken>()).Returns("k");
+        var provider = Create(resolver);
+
+        await provider.GetAsync("site-a", CancellationToken.None);
+        await provider.GetAsync("site-a", CancellationToken.None);
+        await provider.GetAsync("site-a", CancellationToken.None);
+
+        await resolver.Received(1).GetAsync(Arg.Any<SecretName>(), Arg.Any<CancellationToken>());
+    }
+
+    [Fact]
+    public async Task FailuresAreNotCached_SoASeededKeyIsPickedUpWithoutARestart()
+    {
+        var resolver = Substitute.For<ISecretResolver>();
+        resolver.GetAsync(Arg.Any<SecretName>(), Arg.Any<CancellationToken>())
+            .Returns((string?)null, "seeded-later");
+        var provider = Create(resolver);
+
+        await Assert.ThrowsAsync<InvalidOperationException>(
+            async () => await provider.GetAsync("site-a", CancellationToken.None));
+
+        Assert.Equal("seeded-later", await provider.GetAsync("site-a", CancellationToken.None));
+    }
+
+    [Fact]
+    public async Task Invalidate_DropsTheCachedKey_SoARotatedKeyIsRead()
+    {
+        var resolver = Substitute.For<ISecretResolver>();
+        resolver.GetAsync(Arg.Any<SecretName>(), Arg.Any<CancellationToken>())
+            .Returns("old", "rotated");
+        var provider = Create(resolver);
+
+        Assert.Equal("old", await provider.GetAsync("site-a", CancellationToken.None));
+        provider.Invalidate("site-a");
+
+        Assert.Equal("rotated", await provider.GetAsync("site-a", CancellationToken.None));
+    }
+
+    [Fact]
+    public async Task KeysAreScopedPerSite_SoOneSiteNeverPresentsAnothersKey()
+    {
+        // The whole reason the design rejected a single fleet-wide key: blast radius.
+        var resolver = Substitute.For<ISecretResolver>();
+        resolver.GetAsync(new SecretName("SB-GRPC-PSK-site-a"), Arg.Any<CancellationToken>())
+            .Returns("key-a");
+        resolver.GetAsync(new SecretName("SB-GRPC-PSK-site-b"), Arg.Any<CancellationToken>())
+            .Returns("key-b");
+        var provider = Create(resolver);
+
+        Assert.Equal("key-a", await provider.GetAsync("site-a", CancellationToken.None));
+        Assert.Equal("key-b", await provider.GetAsync("site-b", CancellationToken.None));
+    }
+}

tests/ZB.MOM.WW.ScadaBridge.Host.Tests/StartupValidatorTests.cs

@@ -38,6 +38,9 @@ public class StartupValidatorTests
         ["ScadaBridge:Database:SiteDbPath"] = "./data/scadabridge.db",
         ["ScadaBridge:Cluster:SeedNodes:0"] = "akka.tcp://scadabridge@site-a-node1:8082",
         ["ScadaBridge:Cluster:SeedNodes:1"] = "akka.tcp://scadabridge@site-a-node2:8082",
+        // T0.3: the gRPC control plane is fail-closed, so a Site node without a preshared
+        // key serves nothing while still looking healthy. Required at boot for that reason.
+        ["ScadaBridge:Communication:GrpcPsk"] = "test-site-control-plane-key",
     };
 
     [Fact]
@@ -56,6 +59,44 @@ public class StartupValidatorTests
         Assert.Null(ex);
     }
 
+    [Fact]
+    public void SiteWithoutGrpcPsk_FailsValidation()
+    {
+        // The failure this prevents is silent: ControlPlaneAuthInterceptor refuses every
+        // SiteStream call without a key, so the node boots, joins its pair, reports healthy —
+        // and serves no live streams, no audit pulls and no cached-telemetry ingest. Central
+        // sees a site that is up and answering heartbeats but never sends anything.
+        var values = ValidSiteConfig();
+        values.Remove("ScadaBridge:Communication:GrpcPsk");
+        var config = BuildConfig(values);
+
+        var ex = Assert.Throws<InvalidOperationException>(() => StartupValidator.Validate(config));
+        Assert.Contains("GrpcPsk", ex.Message);
+    }
+
+    [Fact]
+    public void SiteWithBlankGrpcPsk_FailsValidation()
+    {
+        // Whitespace is not a key. An empty-string value would otherwise satisfy a
+        // key-present check while leaving the interceptor in its fail-closed state.
+        var values = ValidSiteConfig();
+        values["ScadaBridge:Communication:GrpcPsk"] = "   ";
+        var config = BuildConfig(values);
+
+        var ex = Assert.Throws<InvalidOperationException>(() => StartupValidator.Validate(config));
+        Assert.Contains("GrpcPsk", ex.Message);
+    }
+
+    [Fact]
+    public void CentralWithoutGrpcPsk_PassesValidation()
+    {
+        // Central holds one key PER SITE (SitePsks / the secret store), not a single key of
+        // its own, so this setting is meaningless there and must not be required.
+        var config = BuildConfig(ValidCentralConfig());
+
+        Assert.Null(Record.Exception(() => StartupValidator.Validate(config)));
+    }
+
     [Fact]
     public void MissingRole_FailsValidation()
     {

tests/ZB.MOM.WW.ScadaBridge.ManagementService.Tests/ManagementActorTests.cs

@@ -3643,7 +3643,8 @@ public class ManagementActorTests : TestKit, IDisposable
     private sealed class TrackingGrpcFactory : SiteStreamGrpcClientFactory
     {
         public TrackingGrpcFactory() : base(NullLoggerFactory.Instance) { }
-        protected override SiteStreamGrpcClient CreateClient(string grpcEndpoint) => new TrackingClient(grpcEndpoint);
+        protected override SiteStreamGrpcClient CreateClient(string siteIdentifier, string grpcEndpoint)
+            => new TrackingClient(grpcEndpoint);
     }
 
     /// <summary>