feat(inbound-api): standby central node relays POST /api/* to the active peer instead of 503

A caller addressing a standby central node directly (or hitting Traefik inside its
/health/active probe window after a failover) used to get 503 STANDBY_NODE. The
standby now relays the request verbatim to the active peer over HTTP
(IStandbyRequestForwarder / HttpStandbyRequestForwarder) and replays the response;
method scripts and Route.To() still execute only on the active node.

- ScadaBridge:InboundApi:StandbyForwardPeerEndpoints (static list of the OTHER
  central node's HTTP base URL; empty = old 503 behaviour) + StandbyForwardTimeout
  (60s), both startup-validated.
- Peers tried in order: peer 503 stamped X-ScadaBridge-Standby, connection failure
  or timeout → next peer; all fail → 503 ACTIVE_NODE_UNREACHABLE. A 503 without
  the marker is the active node's own answer and is replayed.
- One-hop loop guard via X-ScadaBridge-Forwarded; 413 body cap now runs BEFORE
  the active-node check so a standby never buffers/relays an oversized body.
- AuditWriteMiddleware skips its own ApiInbound row for a relayed request so a
  call is audited once, on the node that executed it.
- Rig + env2 central nodes configured; spec (Component-InboundAPI.md → Standby
  forwarding), Component-Host.md options row, CLAUDE.md updated.
- 20 new tests (filter decision + relay mechanics + validator).

Claude-Session: https://claude.ai/code/session_019ZxXT5sxZs5XrtdwYzJDPV

CLAUDE.md

@@ -117,6 +117,7 @@ spec for each is `docs/requirements/Component-<Name>.md`, and `README.md` carrie
 - `Notify.Send` is async and **enqueue-only** — it buffers the notification into the local SQLite S&F store and returns a `NotificationId` (GUID, idempotency key) status handle immediately; it never runs the forwarder's central Ask inline on the script thread (`deferToSweep: true` buffers due-immediately + kicks a background sweep), so its worst-case latency is the local insert whether central is up or down. It enqueues with `maxRetries: 0` (the "no limit" escape hatch), so notifications retry until central acks and are **never parked for retry exhaustion** — only a corrupt payload parks them (arch-review 02, Tasks 13/14). `Notify.Status(notificationId)` returns a status record (status, retry count, last error, key timestamps); answered site-locally as `Forwarding` while still in the site S&F buffer, otherwise round-trips to central.
 - SMS delivery adapter (T9/T10, 2026-06-19): `SmsNotificationDeliveryAdapter` — Twilio REST, no SDK, HTTP Basic auth (`AccountSid:AuthToken`), one `POST .../Messages.json` per recipient. `SmsConfiguration` entity (`AccountSid`, `AuthToken` encrypted via Data Protection, `FromNumber`/`MessagingServiceSid`, `ApiBaseUrl`, timeout/retry) stored centrally; managed Admin-only via CLI `notification sms list|update` + Central UI `/notifications/sms`. Central-only (never deployed to sites). Per-recipient rollup: all-accepted → Delivered; any-transient → retry/park; mix → delivered-to-good + note; all-permanent → Park. `SmsConfiguration` (Auth Token in SecretsBlock) travels in Transport bundles via `--sms-configs`.
 - Inbound API: `POST /api/{methodName}`, `X-API-Key` header, flat JSON, extended type system (Object, List).
+- **Inbound API on a STANDBY central node RELAYS to the active peer (2026-08-28)** instead of answering 503 `STANDBY_NODE`. Method scripts/`Route.To()` still run only on the active node (`IActiveNodeGate`); the standby's `InboundApiEndpointFilter` hands the request to `IStandbyRequestForwarder` (`HttpStandbyRequestForwarder`, plain HTTP, verbatim method/path/query/body/headers, peer response replayed unchanged) when `ScadaBridge:InboundApi:StandbyForwardPeerEndpoints` lists the *other* node's HTTP base URL — a static appsettings list, because the Akka member address carries the remoting port, not the HTTP one. Empty list = old 503 behaviour. Peers are tried in order; a peer 503 stamped `X-ScadaBridge-Standby: 1`, a connection failure or a `StandbyForwardTimeout` (60 s, must exceed the longest method timeout) expiry each move to the next, all failing → 503 `ACTIVE_NODE_UNREACHABLE`. **One-hop loop guard:** relayed requests carry `X-ScadaBridge-Forwarded` and are never relayed again. The 413 body cap runs *before* the relay, and the standby's `AuditWriteMiddleware` skips its own row for a relayed call (`InboundApiEndpointFilter.RelayedToActiveItemKey`) so it is audited once, on the node that executed it. Both rigs (`docker/`, `docker-env2/`) configure it on both central nodes; QA/prod appsettings are user-held and need the key added by hand. Spec: `Component-InboundAPI.md` → *Standby forwarding*.
 
 ### Templates & Deployment
 - Pre-deployment validation includes semantic checks (call targets, argument types, trigger operand types).

docker-env2/central-node-a/appsettings.Central.json

@@ -68,7 +68,9 @@
       "OfflineTimeout": "00:01:00"
     },
     "InboundApi": {
-      "DefaultMethodTimeout": "00:00:30"
+      "DefaultMethodTimeout": "00:00:30",
+      "_standbyForwarding": "2026-08-28 — a standby central node relays POST /api/{methodName} to the active peer instead of answering 503 STANDBY_NODE. List the OTHER node only.",
+      "StandbyForwardPeerEndpoints": [ "http://scadabridge-env2-central-b:5000" ]
     },
     "Notification": {
       "SmtpServer": "scadabridge-smtp",

docker-env2/central-node-b/appsettings.Central.json

@@ -68,7 +68,9 @@
       "OfflineTimeout": "00:01:00"
     },
     "InboundApi": {
-      "DefaultMethodTimeout": "00:00:30"
+      "DefaultMethodTimeout": "00:00:30",
+      "_standbyForwarding": "2026-08-28 — a standby central node relays POST /api/{methodName} to the active peer instead of answering 503 STANDBY_NODE. List the OTHER node only.",
+      "StandbyForwardPeerEndpoints": [ "http://scadabridge-env2-central-a:5000" ]
     },
     "Notification": {
       "SmtpServer": "scadabridge-smtp",

docker/central-node-a/appsettings.Central.json

@@ -76,7 +76,9 @@
       "OfflineTimeout": "00:01:00"
     },
     "InboundApi": {
-      "DefaultMethodTimeout": "00:00:30"
+      "DefaultMethodTimeout": "00:00:30",
+      "_standbyForwarding": "2026-08-28 — a standby central node relays POST /api/{methodName} to the active peer instead of answering 503 STANDBY_NODE. List the OTHER node only.",
+      "StandbyForwardPeerEndpoints": [ "http://scadabridge-central-b:5000" ]
     },
     "Notification": {
       "SmtpServer": "scadabridge-smtp",

docker/central-node-b/appsettings.Central.json

@@ -76,7 +76,9 @@
       "OfflineTimeout": "00:01:00"
     },
     "InboundApi": {
-      "DefaultMethodTimeout": "00:00:30"
+      "DefaultMethodTimeout": "00:00:30",
+      "_standbyForwarding": "2026-08-28 — a standby central node relays POST /api/{methodName} to the active peer instead of answering 503 STANDBY_NODE. List the OTHER node only.",
+      "StandbyForwardPeerEndpoints": [ "http://scadabridge-central-a:5000" ]
     },
     "Notification": {
       "SmtpServer": "scadabridge-smtp",

docs/requirements/Component-Host.md

@@ -59,7 +59,7 @@ The Host must bind configuration sections from `appsettings.json` to strongly-ty
 | `ScadaBridge:SiteEventLog` | `SiteEventLogOptions` | Site Event Logging | RetentionDays, MaxStorageMb, PurgeScheduleCron |
 | `ScadaBridge:Communication` | `CommunicationOptions` | Communication | DeploymentTimeout, LifecycleTimeout, QueryTimeout, TransportHeartbeatInterval, TransportFailureThreshold |
 | `ScadaBridge:Security` | `SecurityOptions` | Security & Auth | LdapServer, LdapPort, LdapUseTls, JwtSigningKey, JwtExpiryMinutes, IdleTimeoutMinutes |
-| `ScadaBridge:InboundApi` | `InboundApiOptions` | Inbound API | DefaultMethodTimeout |
+| `ScadaBridge:InboundApi` | `InboundApiOptions` | Inbound API | DefaultMethodTimeout, MaxRequestBodyBytes, ApiMethodCacheTtl, StandbyForwardPeerEndpoints (HTTP base URLs of the other central node — a standby relays `POST /api/*` there instead of answering 503), StandbyForwardTimeout |
 | `ScadaBridge:Notification` | `NotificationOptions` | Notification Service | (SMTP config is stored in the central config DB, not in appsettings) |
 | `ScadaBridge:NotificationOutbox` | `NotificationOutboxOptions` | Notification Outbox | Dispatcher poll interval, stuck-age threshold, retention window (delivery retry settings reuse the central SMTP configuration) |
 | `ScadaBridge:SiteCallAudit` | `SiteCallAuditOptions` | Site Call Audit | Reconciliation pull interval, stuck-age threshold, retention window |

docs/requirements/Component-InboundAPI.md

@@ -6,7 +6,7 @@ The Inbound API exposes a web API on the central cluster that external systems c
 
 ## Location
 
-Central cluster only (active node). Not available at site clusters.
+Central cluster only. Method scripts execute on the **active** central node; a **standby** central node accepts the same endpoint and relays the request to the active peer (see [Standby forwarding](#standby-forwarding)). Not available at site clusters.
 
 ## Responsibilities
 
@@ -16,7 +16,7 @@ Central cluster only (active node). Not available at site clusters.
 - Enforce per-method API key authorization (only approved keys can call a given method).
 - Execute the C# script implementation for the called method.
 - Return structured responses to the caller.
-- Failover: API becomes available on the new active node after central failover.
+- Failover: API becomes available on the new active node after central failover; during the handover a standby relays to whichever peer is active, so a caller addressing either central node directly is served without waiting for the load balancer's health probe to catch up.
 
 ## API Key Management
 
@@ -122,7 +122,8 @@ The `sbk_<keyId>_<secret>` design already supports zero-downtime rotation — a
 | `INVALID_JSON` | 400 | Request body was not valid JSON. |
 | `VALIDATION_FAILED` | 400 | Parameter validation failed (missing required field, wrong type, undeclared field). |
 | `BODY_TOO_LARGE` | 413 | Request body exceeded `MaxRequestBodyBytes`. |
-| `STANDBY_NODE` | 503 | Request hit a standby central node; the inbound API serves only the active node. |
+| `STANDBY_NODE` | 503 | Request hit a standby central node that could not relay it: no `StandbyForwardPeerEndpoints` configured, or the request was already relayed once (loop guard — neither node is active, e.g. mid-failover). Carries the `X-ScadaBridge-Standby: 1` response header. |
+| `ACTIVE_NODE_UNREACHABLE` | 503 | A standby tried every configured peer and none served the relay (unreachable, timed out after `StandbyForwardTimeout`, or also standby). |
 | `TIMEOUT` | 500 | The method script exceeded its execution timeout. |
 | `SITE_UNREACHABLE` | 500 | The target site did not respond to the routed request within the routing timeout (no contact / down / partitioned) — typed from the routed Ask expiring (`AskTimeoutException`), never inferred from an error-message substring. A `Success=false` response means the site answered and is classified `SCRIPT_ERROR`. |
 | `SCRIPT_COMPILE_FAILED` | 500 | The method's script failed to compile (returned consistently on every node until a compiling version is saved). |
@@ -282,6 +283,19 @@ For everything else, scripts interact with the system through the curated `Route
   3. The key's provisioned scopes include the requested method name (case-sensitive).
 - Failed authentication returns an appropriate HTTP error (401 Unauthorized for invalid/missing credentials; 403 Forbidden for a valid key not approved for the method). For enumeration-safety, both "method not found" and "key not in scope" return 403 with an identical body.
 
+## Standby forwarding
+
+Both central nodes expose `POST /api/{methodName}`, but only the active node (the oldest Up member — `ActiveNodeEvaluator.SelfIsOldestUp`, the same verdict behind `/health/active`) executes method scripts and `Route.To()` calls; running them on a standby could race the active node or read stale singleton state. Before 2026-08-28 a standby simply answered 503 `STANDBY_NODE`, and "both nodes work" relied entirely on the load balancer probing `/health/active` — a caller addressing a node directly, or hitting the balancer inside its probe interval after a failover, got the 503.
+
+A standby now **relays** the request to the active peer instead (`InboundApiEndpointFilter` → `IStandbyRequestForwarder` / `HttpStandbyRequestForwarder`):
+
+- **Configuration.** `ScadaBridge:InboundApi:StandbyForwardPeerEndpoints` — the HTTP base URL(s) of the *other* central node(s), e.g. `["http://scadabridge-central-b:5000"]` on central-a. Empty (default) keeps the old 503 behaviour. `StandbyForwardTimeout` (default 60 s) is the per-peer attempt deadline and must exceed the longest method timeout in use. Validated at startup (absolute http/https URLs, positive timeout). Note the standby discovers the active peer from **appsettings**, not from cluster state — the Akka member address carries the remoting port, not the HTTP port — so this is a static, restart-to-change list, like `CentralGrpcEndpoints` on sites.
+- **Pure relay.** Method, path, query string, body and headers travel verbatim (hop-by-hop headers and `Host` excepted), so the **active node** authenticates the caller's own `Authorization`/`X-API-Key` credential, applies its own body cap and method timeout, and writes the audit row. The peer's status, `Content-Type` and body are replayed unchanged. The standby's `AuditWriteMiddleware` skips its own `ApiInbound` row for a relayed request (`HttpContext.Items[RelayedToActiveItemKey]`) so a call is audited once, where it ran.
+- **Order of checks.** The body-size cap (413 `BODY_TOO_LARGE`) runs *before* the active-node check, so a standby never buffers or relays a body the active node would refuse. The body is buffered once on the standby (bounded by `MaxRequestBodyBytes`) so it can be re-sent to a second peer.
+- **Peer selection.** The standby knows only that it is *not* active. It tries the peers in order and accepts the first response that is not a standby refusal; a peer answering 503 with `X-ScadaBridge-Standby: 1`, a connection failure, or a `StandbyForwardTimeout` expiry each move to the next peer. A 503 *without* the marker is the active node's own answer and is replayed, not retried. All peers failing → 503 `ACTIVE_NODE_UNREACHABLE`.
+- **One-hop loop guard.** Every relayed request carries `X-ScadaBridge-Forwarded: <relaying host>`. A standby never relays a request that already carries it — it answers 503 `STANDBY_NODE` with the marker header — so a pair in which neither node is active (a transient during failover) bounces at most one hop and never ping-pongs. The active node ignores the header and serves the request normally.
+- **Still not a retry layer.** The relay makes one HTTP attempt per peer and does not retry a peer that answered; at-least-once callers keep their own retry loop and the idempotent-method authoring note above still applies.
+
 ## Error Handling
 
 - Invalid API key → 401 Unauthorized.
@@ -298,7 +312,7 @@ For everything else, scripts interact with the system through the curated `Route
 - **Configuration Database (via IAuditService)**: All API key and method definition changes are audit logged.
 - **Script Analysis (#25)**: `ForbiddenApiChecker` delegates to `ScriptTrustValidator.FindViolations` for the authoritative forbidden-API verdict during script compilation and validation.
 - **Audit Log (#23)**: Every inbound API request emits an `ApiInbound.Completed` row via `ICentralAuditWriter` from request middleware (non-blocking for the HTTP response). Request and response bodies are captured in full up to `AuditLog:InboundMaxBytes` (default 1 MiB) per the Audit Log Payload Capture Policy; redaction (headers + per-target body redactors) still applies before persistence.
-- **Cluster Infrastructure**: API is hosted on the active central node and fails over with it.
+- **Cluster Infrastructure**: method execution is hosted on the active central node and fails over with it; `IActiveNodeGate` (Host) tells the endpoint filter whether to serve locally or relay.
 
 ## Interactions
 

src/ZB.MOM.WW.ScadaBridge.InboundAPI/HttpStandbyRequestForwarder.cs

@@ -0,0 +1,238 @@
+using System.Net;
+using System.Net.Http.Headers;
+using Microsoft.AspNetCore.Http;
+using Microsoft.Extensions.Logging;
+using Microsoft.Extensions.Options;
+
+namespace ZB.MOM.WW.ScadaBridge.InboundAPI;
+
+/// <summary>
+/// Production <see cref="IStandbyRequestForwarder"/>: relays a standby-node inbound
+/// API request to the active central node over plain HTTP using the named
+/// <see cref="HttpClientName"/> client.
+///
+/// <para><b>Peer selection.</b> The standby does not know which peer is active — it
+/// only knows it is not. It therefore tries
+/// <see cref="InboundApiOptions.StandbyForwardPeerEndpoints"/> in order and accepts the
+/// first response that is not itself a standby refusal. A peer that is also a standby
+/// (mid-failover, or a misconfigured list that includes this node) answers 503 with the
+/// <see cref="StandbyResponseHeader"/> marker, which the forwarder treats like a
+/// connection failure: try the next peer. Connection errors and per-attempt timeouts
+/// (<see cref="InboundApiOptions.StandbyForwardTimeout"/>) also move to the next peer.
+/// When every peer fails the caller gets 503 <c>ACTIVE_NODE_UNREACHABLE</c>.</para>
+///
+/// <para><b>Loop guard.</b> Every relayed request carries
+/// <see cref="ForwardedRequestHeader"/>. <see cref="InboundApiEndpointFilter"/> never
+/// forwards a request that already carries it, so a pair in which neither node is
+/// active (a transient during failover) bounces at most one hop, never ping-pongs.</para>
+///
+/// <para><b>Fidelity.</b> Method, path, query string, body and headers are relayed
+/// verbatim (hop-by-hop headers and <c>Host</c> excepted), so the active node
+/// authenticates the caller's own <c>Authorization</c>/<c>X-API-Key</c> credential and
+/// applies its own body cap and method timeout. The active node's status, content type
+/// and body are replayed to the caller unchanged. The body is buffered once on the
+/// standby (bounded by <see cref="InboundApiOptions.MaxRequestBodyBytes"/>, which the
+/// filter enforces before calling this class) so it can be re-sent to a second peer.</para>
+/// </summary>
+public sealed class HttpStandbyRequestForwarder : IStandbyRequestForwarder
+{
+    /// <summary>Name of the <see cref="IHttpClientFactory"/> client used for relays.</summary>
+    public const string HttpClientName = "ScadaBridge.InboundApi.StandbyForward";
+
+    /// <summary>
+    /// Request header stamped on every relayed request (value: the forwarding node's
+    /// host name when known, else <c>1</c>). The filter refuses to forward a request
+    /// that already carries it.
+    /// </summary>
+    public const string ForwardedRequestHeader = "X-ScadaBridge-Forwarded";
+
+    /// <summary>
+    /// Response header the filter adds to its own 503 <c>STANDBY_NODE</c> reply so a
+    /// forwarding peer can distinguish "that node is also standby — try the next one"
+    /// from a genuine 503 the active node's method script produced.
+    /// </summary>
+    public const string StandbyResponseHeader = "X-ScadaBridge-Standby";
+
+    /// <summary>Machine-readable <c>code</c> when no configured peer served the relay.</summary>
+    public const string ActiveNodeUnreachableCode = "ACTIVE_NODE_UNREACHABLE";
+
+    private static readonly HashSet<string> HopByHopRequestHeaders = new(StringComparer.OrdinalIgnoreCase)
+    {
+        "Host", "Content-Length", "Transfer-Encoding", "Connection", "Keep-Alive",
+        "Proxy-Connection", "Upgrade", "TE", "Trailer", "Expect",
+    };
+
+    private readonly IHttpClientFactory _httpClientFactory;
+    private readonly IOptionsMonitor<InboundApiOptions> _options;
+    private readonly ILogger<HttpStandbyRequestForwarder> _logger;
+
+    /// <summary>Initializes a new <see cref="HttpStandbyRequestForwarder"/>.</summary>
+    /// <param name="httpClientFactory">Factory for the named relay client.</param>
+    /// <param name="options">Live-reloadable inbound API options (peer list + timeout are read per relay).</param>
+    /// <param name="logger">Logger for relay diagnostics.</param>
+    public HttpStandbyRequestForwarder(
+        IHttpClientFactory httpClientFactory,
+        IOptionsMonitor<InboundApiOptions> options,
+        ILogger<HttpStandbyRequestForwarder> logger)
+    {
+        _httpClientFactory = httpClientFactory;
+        _options = options;
+        _logger = logger;
+    }
+
+    /// <inheritdoc />
+    public bool IsConfigured => _options.CurrentValue.StandbyForwardPeerEndpoints.Count > 0;
+
+    /// <inheritdoc />
+    public async Task<IResult> ForwardAsync(HttpContext context, CancellationToken cancellationToken)
+    {
+        var opts = _options.CurrentValue;
+        var peers = opts.StandbyForwardPeerEndpoints;
+        if (peers.Count == 0)
+            return Unreachable();
+
+        // Buffer the body once so it can be replayed to a second peer. The filter has
+        // already enforced MaxRequestBodyBytes (Content-Length up front, and the
+        // per-request Kestrel cap for chunked bodies), so this is bounded.
+        byte[] body;
+        using (var buffer = new MemoryStream())
+        {
+            await context.Request.Body.CopyToAsync(buffer, cancellationToken).ConfigureAwait(false);
+            body = buffer.ToArray();
+        }
+
+        var client = _httpClientFactory.CreateClient(HttpClientName);
+        var pathAndQuery = context.Request.Path.Value + context.Request.QueryString.Value;
+        var forwardedBy = string.IsNullOrEmpty(context.Request.Host.Host) ? "1" : context.Request.Host.Host;
+
+        foreach (var peer in peers)
+        {
+            cancellationToken.ThrowIfCancellationRequested();
+
+            using var attemptCts = CancellationTokenSource.CreateLinkedTokenSource(cancellationToken);
+            attemptCts.CancelAfter(opts.StandbyForwardTimeout);
+
+            try
+            {
+                using var request = BuildRequest(context, peer, pathAndQuery, body, forwardedBy);
+                var response = await client
+                    .SendAsync(request, HttpCompletionOption.ResponseHeadersRead, attemptCts.Token)
+                    .ConfigureAwait(false);
+
+                if (response.StatusCode == HttpStatusCode.ServiceUnavailable
+                    && response.Headers.Contains(StandbyResponseHeader))
+                {
+                    response.Dispose();
+                    _logger.LogInformation(
+                        "Inbound API relay: peer {Peer} is also a standby — trying the next peer", peer);
+                    continue;
+                }
+
+                var responseBody = await response.Content
+                    .ReadAsByteArrayAsync(attemptCts.Token)
+                    .ConfigureAwait(false);
+                var contentType = response.Content.Headers.ContentType?.ToString();
+                var status = (int)response.StatusCode;
+                response.Dispose();
+
+                _logger.LogDebug(
+                    "Inbound API relay: served {Path} via active peer {Peer} → HTTP {Status}",
+                    context.Request.Path, peer, status);
+                return new RelayedResult(status, contentType, responseBody);
+            }
+            catch (OperationCanceledException) when (cancellationToken.IsCancellationRequested)
+            {
+                throw; // caller went away — nothing to answer
+            }
+            catch (OperationCanceledException)
+            {
+                _logger.LogWarning(
+                    "Inbound API relay: peer {Peer} did not answer within {Timeout} — trying the next peer",
+                    peer, opts.StandbyForwardTimeout);
+            }
+            catch (HttpRequestException ex)
+            {
+                _logger.LogWarning(ex,
+                    "Inbound API relay: peer {Peer} unreachable — trying the next peer", peer);
+            }
+        }
+
+        _logger.LogWarning(
+            "Inbound API relay: no configured peer served {Path} ({PeerCount} tried)",
+            context.Request.Path, peers.Count);
+        return Unreachable();
+    }
+
+    private static HttpRequestMessage BuildRequest(
+        HttpContext context, string peer, string pathAndQuery, byte[] body, string forwardedBy)
+    {
+        var request = new HttpRequestMessage(
+            new HttpMethod(context.Request.Method),
+            new Uri(new Uri(peer.TrimEnd('/') + "/"), pathAndQuery.TrimStart('/')));
+
+        var content = new ByteArrayContent(body);
+        foreach (var header in context.Request.Headers)
+        {
+            if (HopByHopRequestHeaders.Contains(header.Key)
+                || string.Equals(header.Key, ForwardedRequestHeader, StringComparison.OrdinalIgnoreCase))
+                continue;
+
+            var values = header.Value.ToArray();
+            if (!request.Headers.TryAddWithoutValidation(header.Key, values))
+                content.Headers.TryAddWithoutValidation(header.Key, values);
+        }
+
+        if (content.Headers.ContentType == null
+            && MediaTypeHeaderValue.TryParse(context.Request.ContentType, out var mediaType))
+            content.Headers.ContentType = mediaType;
+
+        request.Content = content;
+        request.Headers.TryAddWithoutValidation(ForwardedRequestHeader, forwardedBy);
+        return request;
+    }
+
+    private static IResult Unreachable() => Results.Json(
+        new
+        {
+            error = "Inbound API request could not be relayed to the active central node",
+            code = ActiveNodeUnreachableCode,
+        },
+        statusCode: StatusCodes.Status503ServiceUnavailable);
+
+    /// <summary>
+    /// Replays a peer's status, content type and body to the original caller.
+    /// Exposed so tests can assert on what the relay produced without a server.
+    /// </summary>
+    public sealed class RelayedResult : IResult, IStatusCodeHttpResult
+    {
+        /// <summary>Initializes a new <see cref="RelayedResult"/>.</summary>
+        /// <param name="statusCode">The peer's HTTP status.</param>
+        /// <param name="contentType">The peer's <c>Content-Type</c>, if any.</param>
+        /// <param name="body">The peer's response body.</param>
+        public RelayedResult(int statusCode, string? contentType, byte[] body)
+        {
+            StatusCode = statusCode;
+            ContentType = contentType;
+            Body = body;
+        }
+
+        /// <inheritdoc />
+        public int? StatusCode { get; }
+
+        /// <summary>The peer's <c>Content-Type</c>, replayed verbatim.</summary>
+        public string? ContentType { get; }
+
+        /// <summary>The peer's response body, replayed verbatim.</summary>
+        public byte[] Body { get; }
+
+        /// <inheritdoc />
+        public async Task ExecuteAsync(HttpContext httpContext)
+        {
+            httpContext.Response.StatusCode = StatusCode ?? StatusCodes.Status200OK;
+            if (ContentType != null)
+                httpContext.Response.ContentType = ContentType;
+            httpContext.Response.ContentLength = Body.Length;
+            await httpContext.Response.Body.WriteAsync(Body, httpContext.RequestAborted).ConfigureAwait(false);
+        }
+    }
+}

src/ZB.MOM.WW.ScadaBridge.InboundAPI/IStandbyRequestForwarder.cs

@@ -0,0 +1,33 @@
+using Microsoft.AspNetCore.Http;
+
+namespace ZB.MOM.WW.ScadaBridge.InboundAPI;
+
+/// <summary>
+/// Standby-node forwarding seam for <c>POST /api/{methodName}</c>. When a request
+/// lands on a standby central node, <see cref="InboundApiEndpointFilter"/> hands it
+/// here instead of returning HTTP 503, and the implementation relays it verbatim to
+/// the active central node over HTTP so callers that address a central node directly
+/// (or that hit the load balancer inside its health-probe window after a failover)
+/// still get a served response. Method scripts and <c>Route.To()</c> calls still run
+/// ONLY on the active node — the standby is a pure relay.
+/// </summary>
+public interface IStandbyRequestForwarder
+{
+    /// <summary>
+    /// <c>true</c> when at least one peer endpoint is configured
+    /// (<see cref="InboundApiOptions.StandbyForwardPeerEndpoints"/>). When <c>false</c>
+    /// the filter falls back to the pre-forwarding behaviour: HTTP 503 <c>STANDBY_NODE</c>.
+    /// </summary>
+    bool IsConfigured { get; }
+
+    /// <summary>
+    /// Relays the current request to the first peer that serves it and returns a
+    /// result that replays the peer's status, content type and body to the caller.
+    /// Never throws for peer failures: when no peer served the request the result is
+    /// HTTP 503 <c>ACTIVE_NODE_UNREACHABLE</c>.
+    /// </summary>
+    /// <param name="context">The inbound HTTP context on the standby node.</param>
+    /// <param name="cancellationToken">Cancelled when the caller aborts the request.</param>
+    /// <returns>A result replaying the peer's response, or a 503 when no peer served it.</returns>
+    Task<IResult> ForwardAsync(HttpContext context, CancellationToken cancellationToken);
+}

src/ZB.MOM.WW.ScadaBridge.InboundAPI/InboundApiEndpointFilter.cs

@@ -12,17 +12,32 @@ namespace ZB.MOM.WW.ScadaBridge.InboundAPI;
 ///
 /// <list type="bullet">
 /// <item><description>
-/// Active-node gating. The inbound API is central-active-node-only;
-/// a standby node returns HTTP 503 so it never executes method scripts.
+/// Request body size cap. Oversized bodies are rejected with HTTP
+/// 413 before being buffered into a <c>JsonDocument</c> — and before any relay,
+/// so a standby never buffers or forwards a body the active node would refuse.
 /// </description></item>
 /// <item><description>
-/// Request body size cap. Oversized bodies are rejected with HTTP
-/// 413 before being buffered into a <c>JsonDocument</c>.
+/// Active-node gating. Method scripts execute only on the active central node.
+/// A standby RELAYS the request to the active peer through
+/// <see cref="IStandbyRequestForwarder"/> when peers are configured
+/// (<see cref="InboundApiOptions.StandbyForwardPeerEndpoints"/>), and otherwise
+/// returns HTTP 503 <c>STANDBY_NODE</c>. A request that already carries
+/// <see cref="HttpStandbyRequestForwarder.ForwardedRequestHeader"/> is never
+/// relayed again (one-hop loop guard) — it gets the 503, stamped with
+/// <see cref="HttpStandbyRequestForwarder.StandbyResponseHeader"/> so the relaying
+/// peer knows to try its next endpoint.
 /// </description></item>
 /// </list>
 /// </summary>
 public sealed class InboundApiEndpointFilter : IEndpointFilter
 {
+    /// <summary>
+    /// <see cref="HttpContext.Items"/> key set when this node relayed the request to the
+    /// active peer. <c>AuditWriteMiddleware</c> reads it to skip its own audit row (the
+    /// active node audits the call it executed).
+    /// </summary>
+    public const string RelayedToActiveItemKey = "ScadaBridge.InboundApi.RelayedToActive";
+
     private readonly ILogger<InboundApiEndpointFilter> _logger;
     private readonly IOptionsMonitor<InboundApiOptions> _options;
 
@@ -37,32 +52,20 @@ public sealed class InboundApiEndpointFilter : IEndpointFilter
         _options = options;
     }
 
-    /// <summary>Applies active-node gating and request body size checks before delegating to the next filter or handler.</summary>
+    /// <summary>Applies the request body size check and active-node gating/relay before delegating to the next filter or handler.</summary>
     /// <param name="context">The endpoint filter invocation context containing the HTTP context and arguments.</param>
     /// <param name="next">The next filter or endpoint handler in the pipeline.</param>
-    /// <returns>A value task that resolves to the filter pipeline result, or an error result if gating fails.</returns>
+    /// <returns>A value task that resolves to the filter pipeline result, a relayed peer response, or an error result if gating fails.</returns>
     public async ValueTask<object?> InvokeAsync(
         EndpointFilterInvocationContext context,
         EndpointFilterDelegate next)
     {
         var httpContext = context.HttpContext;
 
-        // Refuse to serve the inbound API on a standby central node.
-        // The gate is optional — when no IActiveNodeGate is registered (non-clustered
-        // host / tests) the API is served, preserving prior behaviour.
-        var gate = httpContext.RequestServices.GetService<IActiveNodeGate>();
-        if (gate is { IsActiveNode: false })
-        {
-            _logger.LogWarning(
-                "Inbound API request rejected — this node is a standby (not the active central node)");
-            return Results.Json(
-                new { error = "Inbound API is only available on the active central node", code = "STANDBY_NODE" },
-                statusCode: StatusCodes.Status503ServiceUnavailable);
-        }
-
-        // Cap the request body size. Reject an over-limit body up
+        // Cap the request body size FIRST. Reject an over-limit body up
         // front via Content-Length; also lower the per-request max body size so a
-        // chunked/unknown-length stream is cut off by Kestrel as it is read.
+        // chunked/unknown-length stream is cut off by Kestrel as it is read. Runs
+        // before the active-node check so a standby never relays an oversized body.
         var maxBytes = _options.CurrentValue.MaxRequestBodyBytes;
         if (httpContext.Request.ContentLength is { } declaredLength && declaredLength > maxBytes)
         {
@@ -80,6 +83,33 @@ public sealed class InboundApiEndpointFilter : IEndpointFilter
             sizeFeature.MaxRequestBodySize = maxBytes;
         }
 
+        // Active-node gating. The gate is optional — when no IActiveNodeGate is
+        // registered (non-clustered host / tests) the API is served locally.
+        var gate = httpContext.RequestServices.GetService<IActiveNodeGate>();
+        if (gate is { IsActiveNode: false })
+        {
+            var alreadyForwarded = httpContext.Request.Headers
+                .ContainsKey(HttpStandbyRequestForwarder.ForwardedRequestHeader);
+            var forwarder = httpContext.RequestServices.GetService<IStandbyRequestForwarder>();
+
+            if (!alreadyForwarded && forwarder is { IsConfigured: true })
+            {
+                httpContext.Items[RelayedToActiveItemKey] = true;
+                _logger.LogDebug("Inbound API request on a standby node — relaying to the active peer");
+                return await forwarder.ForwardAsync(httpContext, httpContext.RequestAborted)
+                    .ConfigureAwait(false);
+            }
+
+            _logger.LogWarning(
+                alreadyForwarded
+                    ? "Inbound API request rejected — relayed here by a peer, but this node is also a standby (loop guard)"
+                    : "Inbound API request rejected — this node is a standby (not the active central node) and no relay peers are configured");
+            httpContext.Response.Headers[HttpStandbyRequestForwarder.StandbyResponseHeader] = "1";
+            return Results.Json(
+                new { error = "Inbound API is only available on the active central node", code = "STANDBY_NODE" },
+                statusCode: StatusCodes.Status503ServiceUnavailable);
+        }
+
         return await next(context);
     }
 }

src/ZB.MOM.WW.ScadaBridge.InboundAPI/InboundApiOptions.cs

@@ -48,4 +48,25 @@ public class InboundApiOptions
     /// serve minutes-old method definitions.
     /// </summary>
     public TimeSpan ApiMethodCacheTtl { get; set; } = TimeSpan.FromSeconds(5);
+
+    /// <summary>
+    /// Standby-node forwarding (2026-08-28): the HTTP base URLs of the OTHER central
+    /// node(s) — e.g. <c>["http://scadabridge-central-b:5000"]</c> on central-a — that a
+    /// standby relays <c>POST /api/{methodName}</c> to instead of answering 503
+    /// <c>STANDBY_NODE</c>. Tried in order; the first peer that is not itself a standby
+    /// serves the call. Empty (the default) keeps the pre-forwarding behaviour: a standby
+    /// refuses with 503. Method scripts still execute only on the active node — the
+    /// standby is a pure relay (see <see cref="HttpStandbyRequestForwarder"/>). Each entry
+    /// must be an absolute <c>http</c>/<c>https</c> URL; never list this node's own address
+    /// (it would cost one wasted round-trip per call — the loop guard stops it bouncing).
+    /// </summary>
+    public List<string> StandbyForwardPeerEndpoints { get; set; } = new();
+
+    /// <summary>
+    /// Per-peer deadline for one relay attempt (connect + the active node's full method
+    /// execution). Must comfortably exceed <see cref="DefaultMethodTimeout"/> and the
+    /// longest per-method timeout in use, otherwise the standby gives up on a peer that
+    /// is still legitimately working and moves on to the next one. Default 60 s.
+    /// </summary>
+    public TimeSpan StandbyForwardTimeout { get; set; } = TimeSpan.FromSeconds(60);
 }

src/ZB.MOM.WW.ScadaBridge.InboundAPI/InboundApiOptionsValidator.cs

@@ -31,5 +31,18 @@ public sealed class InboundApiOptionsValidator : OptionsValidatorBase<InboundApi
         builder.RequireThat(options.ApiMethodCacheTtl > TimeSpan.Zero,
             $"ScadaBridge:InboundApi:ApiMethodCacheTtl must be a positive duration " +
             $"(was {options.ApiMethodCacheTtl}); it bounds how long a resolved ApiMethod row is cached.");
+
+        builder.RequireThat(options.StandbyForwardTimeout > TimeSpan.Zero,
+            $"ScadaBridge:InboundApi:StandbyForwardTimeout must be a positive duration " +
+            $"(was {options.StandbyForwardTimeout}); it bounds each standby→active relay attempt.");
+
+        foreach (var peer in options.StandbyForwardPeerEndpoints)
+        {
+            var ok = Uri.TryCreate(peer, UriKind.Absolute, out var uri)
+                && (uri.Scheme == Uri.UriSchemeHttp || uri.Scheme == Uri.UriSchemeHttps);
+            builder.RequireThat(ok,
+                $"ScadaBridge:InboundApi:StandbyForwardPeerEndpoints entry '{peer}' must be an absolute " +
+                $"http/https URL (e.g. http://central-b:5000); it is the peer a standby relays inbound API calls to.");
+        }
     }
 }

src/ZB.MOM.WW.ScadaBridge.InboundAPI/Middleware/AuditWriteMiddleware.cs

@@ -219,6 +219,12 @@ public sealed class AuditWriteMiddleware
         {
             sw.Stop();
 
+            // Standby forwarding (2026-08-28): a request this node RELAYED to the active
+            // peer is audited there, by the node that actually authenticated and executed
+            // it. Emitting a second row here would double-count every relayed call, so the
+            // standby skips its own row when the filter flagged the relay.
+            var relayed = ctx.Items.ContainsKey(InboundApiEndpointFilter.RelayedToActiveItemKey);
+
             // Restore the original stream and resolve the captured audit copy.
             // The forwarding wrapper has already written every byte to the
             // original sink; this just pulls back the bounded UTF-8 string.
@@ -232,7 +238,8 @@ public sealed class AuditWriteMiddleware
             var responseBody = skipBody ? null : capturedResponseBody;
             var responseTruncated = skipBody ? false : capturedResponseTruncated;
 
-            EmitInboundAudit(
+            if (!relayed)
+                EmitInboundAudit(
                 ctx,
                 opts,
                 sw.ElapsedMilliseconds,

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

@@ -35,6 +35,15 @@ public static class ServiceCollectionExtensions
         // body size cap and active-node gating for POST /api/{methodName}.
         services.AddSingleton<InboundApiEndpointFilter>();
 
+        // Standby-node forwarding (2026-08-28): a standby relays POST /api/{methodName}
+        // to the active peer over HTTP instead of answering 503 STANDBY_NODE. The
+        // named client has no HttpClient.Timeout of its own — the forwarder's per-attempt
+        // CTS (InboundApiOptions.StandbyForwardTimeout) is the single deadline, so a
+        // long-running method on the active node is not cut off by a second, hidden cap.
+        services.AddHttpClient(HttpStandbyRequestForwarder.HttpClientName)
+            .ConfigureHttpClient(c => c.Timeout = Timeout.InfiniteTimeSpan);
+        services.AddSingleton<IStandbyRequestForwarder, HttpStandbyRequestForwarder>();
+
         // N1: the ApiMethod consumer of the script-artifact invalidation bus.
         // The bus parameter is optional — hosts without a registered
         // IScriptArtifactChangeBus (site roles, plain test hosts) start it as a no-op

tests/ZB.MOM.WW.ScadaBridge.InboundAPI.Tests/InboundApiOptionsValidatorTests.cs

@@ -83,4 +83,38 @@ public class InboundApiOptionsValidatorTests
         Assert.True(result.Failed);
         Assert.Contains("ApiMethodCacheTtl", result.FailureMessage);
     }
+
+    // --- Standby forwarding (2026-08-28) ---
+
+    [Fact]
+    public void StandbyForwardPeerEndpoints_AbsoluteHttpUrls_AreValid()
+    {
+        var result = Validate(new InboundApiOptions
+        {
+            StandbyForwardPeerEndpoints = { "http://central-b:5000", "https://central-b.example/" },
+        });
+        Assert.True(result.Succeeded, result.FailureMessage);
+    }
+
+    [Theory]
+    [InlineData("central-b:5000")]
+    [InlineData("/api")]
+    [InlineData("ftp://central-b")]
+    [InlineData("")]
+    public void StandbyForwardPeerEndpoints_NonHttpOrRelative_IsRejected(string peer)
+    {
+        var result = Validate(new InboundApiOptions { StandbyForwardPeerEndpoints = { peer } });
+
+        Assert.True(result.Failed);
+        Assert.Contains("StandbyForwardPeerEndpoints", result.FailureMessage);
+    }
+
+    [Fact]
+    public void ZeroStandbyForwardTimeout_IsRejected()
+    {
+        var result = Validate(new InboundApiOptions { StandbyForwardTimeout = TimeSpan.Zero });
+
+        Assert.True(result.Failed);
+        Assert.Contains("StandbyForwardTimeout", result.FailureMessage);
+    }
 }

tests/ZB.MOM.WW.ScadaBridge.InboundAPI.Tests/StandbyRequestForwardingTests.cs

@@ -0,0 +1,400 @@
+using System.Net;
+using System.Text;
+using Microsoft.AspNetCore.Http;
+using Microsoft.Extensions.DependencyInjection;
+using Microsoft.Extensions.Logging.Abstractions;
+using Microsoft.Extensions.Options;
+
+namespace ZB.MOM.WW.ScadaBridge.InboundAPI.Tests;
+
+/// <summary>
+/// Standby-node forwarding (2026-08-28): a standby central node must relay
+/// <c>POST /api/{methodName}</c> to the active peer instead of refusing with 503,
+/// while never executing the method itself and never relaying more than one hop.
+/// Covers both the <see cref="InboundApiEndpointFilter"/> decision and the
+/// <see cref="HttpStandbyRequestForwarder"/> relay mechanics.
+/// </summary>
+public class StandbyRequestForwardingTests
+{
+    // ------------------------------------------------------------------
+    // Filter: when does a standby relay, and when does it refuse?
+    // ------------------------------------------------------------------
+
+    [Fact]
+    public async Task Standby_WithForwarderConfigured_RelaysAndDoesNotRunHandler()
+    {
+        var forwarder = new StubForwarder(configured: true, result: Results.Text("relayed"));
+        var (ctx, invocation) = BuildInvocation(activeNode: false, forwarder);
+        var next = NextSentinel(out var handlerRan);
+
+        var result = await CreateFilter().InvokeAsync(invocation, next);
+
+        Assert.False(handlerRan());
+        Assert.Equal(1, forwarder.Calls);
+        Assert.Same(forwarder.Result, result);
+        Assert.True(ctx.Items.ContainsKey(InboundApiEndpointFilter.RelayedToActiveItemKey));
+    }
+
+    [Fact]
+    public async Task Standby_WithForwarderUnconfigured_Returns503StandbyNode_MarkedForPeers()
+    {
+        var forwarder = new StubForwarder(configured: false, result: Results.Ok());
+        var (ctx, invocation) = BuildInvocation(activeNode: false, forwarder);
+        var next = NextSentinel(out var handlerRan);
+
+        var result = await CreateFilter().InvokeAsync(invocation, next);
+
+        Assert.False(handlerRan());
+        Assert.Equal(0, forwarder.Calls);
+        AssertStatus(result, StatusCodes.Status503ServiceUnavailable);
+        Assert.Equal("1", ctx.Response.Headers[HttpStandbyRequestForwarder.StandbyResponseHeader]);
+        Assert.False(ctx.Items.ContainsKey(InboundApiEndpointFilter.RelayedToActiveItemKey));
+    }
+
+    [Fact]
+    public async Task Standby_AlreadyForwardedRequest_IsNotRelayedAgain_LoopGuard()
+    {
+        var forwarder = new StubForwarder(configured: true, result: Results.Ok());
+        var (ctx, invocation) = BuildInvocation(activeNode: false, forwarder);
+        ctx.Request.Headers[HttpStandbyRequestForwarder.ForwardedRequestHeader] = "central-b";
+        var next = NextSentinel(out var handlerRan);
+
+        var result = await CreateFilter().InvokeAsync(invocation, next);
+
+        Assert.False(handlerRan());
+        Assert.Equal(0, forwarder.Calls);
+        AssertStatus(result, StatusCodes.Status503ServiceUnavailable);
+        Assert.Equal("1", ctx.Response.Headers[HttpStandbyRequestForwarder.StandbyResponseHeader]);
+    }
+
+    [Fact]
+    public async Task Standby_OversizedBody_Returns413_BeforeRelaying()
+    {
+        var options = new InboundApiOptions();
+        var forwarder = new StubForwarder(configured: true, result: Results.Ok());
+        var (_, invocation) = BuildInvocation(activeNode: false, forwarder,
+            contentLength: options.MaxRequestBodyBytes + 1);
+        var next = NextSentinel(out var handlerRan);
+
+        var result = await CreateFilter(options).InvokeAsync(invocation, next);
+
+        Assert.False(handlerRan());
+        Assert.Equal(0, forwarder.Calls);
+        AssertStatus(result, StatusCodes.Status413PayloadTooLarge);
+    }
+
+    [Fact]
+    public async Task ActiveNode_ForwardedRequest_IsServedLocally()
+    {
+        // The active node receives the relay from its standby peer and serves it —
+        // the marker header only matters when the receiving node is NOT active.
+        var forwarder = new StubForwarder(configured: true, result: Results.Ok());
+        var (ctx, invocation) = BuildInvocation(activeNode: true, forwarder);
+        ctx.Request.Headers[HttpStandbyRequestForwarder.ForwardedRequestHeader] = "central-b";
+        var next = NextSentinel(out var handlerRan);
+
+        await CreateFilter().InvokeAsync(invocation, next);
+
+        Assert.True(handlerRan());
+        Assert.Equal(0, forwarder.Calls);
+    }
+
+    // ------------------------------------------------------------------
+    // Forwarder: relay mechanics against a fake HTTP peer
+    // ------------------------------------------------------------------
+
+    [Fact]
+    public async Task Forwarder_RelaysMethodPathQueryHeadersAndBody_AndReplaysPeerResponse()
+    {
+        HttpRequestMessage? seen = null;
+        string? seenBody = null;
+        var handler = new FakeHandler(async req =>
+        {
+            seen = req;
+            seenBody = await req.Content!.ReadAsStringAsync();
+            return new HttpResponseMessage(HttpStatusCode.Created)
+            {
+                Content = new StringContent("{\"ok\":true}", Encoding.UTF8, "application/json"),
+            };
+        });
+        var forwarder = CreateForwarder(handler, "http://central-b:5000/");
+        var ctx = BuildRequestContext("{\"x\":1}");
+        ctx.Request.Headers["X-API-Key"] = "sbk_key_secret";
+        ctx.Request.Headers["Host"] = "central-a:5000";
+
+        var result = await forwarder.ForwardAsync(ctx, CancellationToken.None);
+
+        var relayed = Assert.IsType<HttpStandbyRequestForwarder.RelayedResult>(result);
+        Assert.Equal(201, relayed.StatusCode);
+        Assert.StartsWith("application/json", relayed.ContentType);
+        Assert.Equal("{\"ok\":true}", Encoding.UTF8.GetString(relayed.Body));
+
+        Assert.NotNull(seen);
+        Assert.Equal(HttpMethod.Post, seen!.Method);
+        Assert.Equal("http://central-b:5000/api/GetRecipe?v=2", seen.RequestUri!.ToString());
+        Assert.Equal("{\"x\":1}", seenBody);
+        Assert.Equal("sbk_key_secret", Assert.Single(seen.Headers.GetValues("X-API-Key")));
+        Assert.Equal("central-a", Assert.Single(seen.Headers.GetValues(HttpStandbyRequestForwarder.ForwardedRequestHeader)));
+        Assert.Null(seen.Headers.Host); // hop-by-hop / origin headers are not relayed
+        Assert.Equal("application/json", seen.Content!.Headers.ContentType!.MediaType);
+    }
+
+    [Fact]
+    public async Task Forwarder_ReplaysPeerErrorStatusUnchanged()
+    {
+        var handler = new FakeHandler(_ => Task.FromResult(new HttpResponseMessage(HttpStatusCode.Forbidden)
+        {
+            Content = new StringContent("{\"code\":\"NOT_APPROVED\"}", Encoding.UTF8, "application/json"),
+        }));
+        var forwarder = CreateForwarder(handler, "http://central-b:5000");
+
+        var result = await forwarder.ForwardAsync(BuildRequestContext("{}"), CancellationToken.None);
+
+        var relayed = Assert.IsType<HttpStandbyRequestForwarder.RelayedResult>(result);
+        Assert.Equal(403, relayed.StatusCode);
+        Assert.Contains("NOT_APPROVED", Encoding.UTF8.GetString(relayed.Body));
+    }
+
+    [Fact]
+    public async Task Forwarder_PeerUnreachable_TriesNextPeer_WithSameBody()
+    {
+        var bodies = new List<string>();
+        var handler = new FakeHandler(async req =>
+        {
+            bodies.Add(await req.Content!.ReadAsStringAsync());
+            if (req.RequestUri!.Host == "dead")
+                throw new HttpRequestException("connection refused");
+            return new HttpResponseMessage(HttpStatusCode.OK) { Content = new StringContent("served") };
+        });
+        var forwarder = CreateForwarder(handler, "http://dead:5000", "http://alive:5000");
+
+        var result = await forwarder.ForwardAsync(BuildRequestContext("{\"n\":7}"), CancellationToken.None);
+
+        var relayed = Assert.IsType<HttpStandbyRequestForwarder.RelayedResult>(result);
+        Assert.Equal(200, relayed.StatusCode);
+        Assert.Equal(new[] { "{\"n\":7}", "{\"n\":7}" }, bodies);
+    }
+
+    [Fact]
+    public async Task Forwarder_PeerIsAlsoStandby_TriesNextPeer()
+    {
+        var handler = new FakeHandler(req =>
+        {
+            if (req.RequestUri!.Host == "standby")
+            {
+                var refusal = new HttpResponseMessage(HttpStatusCode.ServiceUnavailable);
+                refusal.Headers.TryAddWithoutValidation(HttpStandbyRequestForwarder.StandbyResponseHeader, "1");
+                return Task.FromResult(refusal);
+            }
+            return Task.FromResult(new HttpResponseMessage(HttpStatusCode.OK) { Content = new StringContent("served") });
+        });
+        var forwarder = CreateForwarder(handler, "http://standby:5000", "http://active:5000");
+
+        var result = await forwarder.ForwardAsync(BuildRequestContext("{}"), CancellationToken.None);
+
+        var relayed = Assert.IsType<HttpStandbyRequestForwarder.RelayedResult>(result);
+        Assert.Equal(200, relayed.StatusCode);
+        Assert.Equal("served", Encoding.UTF8.GetString(relayed.Body));
+    }
+
+    [Fact]
+    public async Task Forwarder_Genuine503FromActivePeer_IsReplayed_NotRetried()
+    {
+        // A 503 WITHOUT the standby marker is the active node's own answer (e.g. a
+        // method script returning 503) and must be replayed, not treated as a miss.
+        var calls = 0;
+        var handler = new FakeHandler(_ =>
+        {
+            calls++;
+            return Task.FromResult(new HttpResponseMessage(HttpStatusCode.ServiceUnavailable)
+            {
+                Content = new StringContent("busy"),
+            });
+        });
+        var forwarder = CreateForwarder(handler, "http://a:5000", "http://b:5000");
+
+        var result = await forwarder.ForwardAsync(BuildRequestContext("{}"), CancellationToken.None);
+
+        var relayed = Assert.IsType<HttpStandbyRequestForwarder.RelayedResult>(result);
+        Assert.Equal(503, relayed.StatusCode);
+        Assert.Equal("busy", Encoding.UTF8.GetString(relayed.Body));
+        Assert.Equal(1, calls);
+    }
+
+    [Fact]
+    public async Task Forwarder_AllPeersFail_Returns503ActiveNodeUnreachable()
+    {
+        var handler = new FakeHandler(_ => throw new HttpRequestException("down"));
+        var forwarder = CreateForwarder(handler, "http://a:5000", "http://b:5000");
+
+        var result = await forwarder.ForwardAsync(BuildRequestContext("{}"), CancellationToken.None);
+
+        Assert.IsNotType<HttpStandbyRequestForwarder.RelayedResult>(result);
+        AssertStatus(result, StatusCodes.Status503ServiceUnavailable);
+    }
+
+    [Fact]
+    public async Task Forwarder_PeerTimesOut_TriesNextPeer()
+    {
+        var handler = new FakeHandler(async (req, ct) =>
+        {
+            if (req.RequestUri!.Host == "slow")
+            {
+                await Task.Delay(Timeout.InfiniteTimeSpan, ct); // honours the per-attempt deadline
+            }
+            return new HttpResponseMessage(HttpStatusCode.OK) { Content = new StringContent("served") };
+        });
+        var forwarder = CreateForwarder(handler, TimeSpan.FromMilliseconds(100), "http://slow:5000", "http://fast:5000");
+
+        var result = await forwarder.ForwardAsync(BuildRequestContext("{}"), CancellationToken.None);
+
+        var relayed = Assert.IsType<HttpStandbyRequestForwarder.RelayedResult>(result);
+        Assert.Equal(200, relayed.StatusCode);
+    }
+
+    [Fact]
+    public async Task Forwarder_CallerAborts_Propagates_DoesNotTryNextPeer()
+    {
+        var calls = 0;
+        using var callerCts = new CancellationTokenSource();
+        var handler = new FakeHandler(async (_, ct) =>
+        {
+            calls++;
+            callerCts.Cancel();
+            await Task.Delay(Timeout.InfiniteTimeSpan, ct);
+            throw new InvalidOperationException("unreachable");
+        });
+        var forwarder = CreateForwarder(handler, "http://a:5000", "http://b:5000");
+
+        await Assert.ThrowsAnyAsync<OperationCanceledException>(
+            () => forwarder.ForwardAsync(BuildRequestContext("{}"), callerCts.Token));
+        Assert.Equal(1, calls);
+    }
+
+    [Fact]
+    public void Forwarder_IsConfigured_TracksPeerList()
+    {
+        Assert.False(CreateForwarder(new FakeHandler(_ => throw new InvalidOperationException())).IsConfigured);
+        Assert.True(CreateForwarder(new FakeHandler(_ => throw new InvalidOperationException()), "http://b:5000").IsConfigured);
+    }
+
+    [Fact]
+    public async Task RelayedResult_WritesStatusContentTypeAndBody()
+    {
+        var ctx = new DefaultHttpContext();
+        ctx.Response.Body = new MemoryStream();
+        var relayed = new HttpStandbyRequestForwarder.RelayedResult(418, "text/plain", Encoding.UTF8.GetBytes("tea"));
+
+        await relayed.ExecuteAsync(ctx);
+
+        Assert.Equal(418, ctx.Response.StatusCode);
+        Assert.Equal("text/plain", ctx.Response.ContentType);
+        Assert.Equal(3, ctx.Response.ContentLength);
+        ctx.Response.Body.Position = 0;
+        Assert.Equal("tea", new StreamReader(ctx.Response.Body).ReadToEnd());
+    }
+
+    // ------------------------------------------------------------------
+    // helpers
+    // ------------------------------------------------------------------
+
+    private static InboundApiEndpointFilter CreateFilter(InboundApiOptions? options = null) =>
+        new(NullLogger<InboundApiEndpointFilter>.Instance,
+            new StaticOptionsMonitor<InboundApiOptions>(options ?? new InboundApiOptions()));
+
+    private static (DefaultHttpContext ctx, DefaultEndpointFilterInvocationContext invocation)
+        BuildInvocation(bool activeNode, IStandbyRequestForwarder forwarder, long contentLength = 2)
+    {
+        var services = new ServiceCollection();
+        services.AddSingleton<IActiveNodeGate>(new StubActiveNodeGate(activeNode));
+        services.AddSingleton(forwarder);
+        var ctx = new DefaultHttpContext { RequestServices = services.BuildServiceProvider() };
+        ctx.Request.Method = "POST";
+        ctx.Request.ContentLength = contentLength;
+        return (ctx, new DefaultEndpointFilterInvocationContext(ctx));
+    }
+
+    private static EndpointFilterDelegate NextSentinel(out Func<bool> wasCalled)
+    {
+        var called = false;
+        wasCalled = () => called;
+        return _ =>
+        {
+            called = true;
+            return ValueTask.FromResult<object?>(Results.Ok("handler-ran"));
+        };
+    }
+
+    private static void AssertStatus(object? result, int expected)
+    {
+        var status = Assert.IsAssignableFrom<IStatusCodeHttpResult>(result);
+        Assert.Equal(expected, status.StatusCode);
+    }
+
+    private static DefaultHttpContext BuildRequestContext(string body)
+    {
+        var ctx = new DefaultHttpContext();
+        ctx.Request.Method = "POST";
+        ctx.Request.Path = "/api/GetRecipe";
+        ctx.Request.QueryString = new QueryString("?v=2");
+        ctx.Request.ContentType = "application/json";
+        ctx.Request.Body = new MemoryStream(Encoding.UTF8.GetBytes(body));
+        return ctx;
+    }
+
+    private static HttpStandbyRequestForwarder CreateForwarder(FakeHandler handler, params string[] peers) =>
+        CreateForwarder(handler, TimeSpan.FromSeconds(5), peers);
+
+    private static HttpStandbyRequestForwarder CreateForwarder(FakeHandler handler, TimeSpan timeout, params string[] peers) =>
+        new(new FakeHttpClientFactory(handler),
+            new StaticOptionsMonitor<InboundApiOptions>(new InboundApiOptions
+            {
+                StandbyForwardPeerEndpoints = peers.ToList(),
+                StandbyForwardTimeout = timeout,
+            }),
+            NullLogger<HttpStandbyRequestForwarder>.Instance);
+
+    private sealed class StubForwarder : IStandbyRequestForwarder
+    {
+        public StubForwarder(bool configured, IResult result) { IsConfigured = configured; Result = result; }
+        public bool IsConfigured { get; }
+        public IResult Result { get; }
+        public int Calls { get; private set; }
+        public Task<IResult> ForwardAsync(HttpContext context, CancellationToken cancellationToken)
+        {
+            Calls++;
+            return Task.FromResult(Result);
+        }
+    }
+
+    private sealed class StubActiveNodeGate : IActiveNodeGate
+    {
+        public StubActiveNodeGate(bool isActive) => IsActiveNode = isActive;
+        public bool IsActiveNode { get; }
+    }
+
+    private sealed class FakeHandler : HttpMessageHandler
+    {
+        private readonly Func<HttpRequestMessage, CancellationToken, Task<HttpResponseMessage>> _send;
+        public FakeHandler(Func<HttpRequestMessage, Task<HttpResponseMessage>> send) => _send = (r, _) => send(r);
+        public FakeHandler(Func<HttpRequestMessage, CancellationToken, Task<HttpResponseMessage>> send) => _send = send;
+        protected override Task<HttpResponseMessage> SendAsync(HttpRequestMessage request, CancellationToken ct) =>
+            _send(request, ct);
+    }
+
+    private sealed class FakeHttpClientFactory : IHttpClientFactory
+    {
+        private readonly HttpMessageHandler _handler;
+        public FakeHttpClientFactory(HttpMessageHandler handler) => _handler = handler;
+        public HttpClient CreateClient(string name) =>
+            new(_handler, disposeHandler: false) { Timeout = Timeout.InfiniteTimeSpan };
+    }
+
+    private sealed class StaticOptionsMonitor<T> : IOptionsMonitor<T>
+    {
+        public StaticOptionsMonitor(T value) => CurrentValue = value;
+        public T CurrentValue { get; }
+        public T Get(string? name) => CurrentValue;
+        public IDisposable? OnChange(Action<T, string?> listener) => null;
+    }
+}