# MTConnect Driver

Getting-started guide for the MTConnect Agent driver (P1 Agent MVP). This is
the short path — for the full design rationale read
[`docs/plans/2026-07-15-mtconnect-driver-design.md`](../plans/2026-07-15-mtconnect-driver-design.md)
and the build-vs-plan record in
[`docs/plans/2026-07-24-mtconnect-driver.md`](../plans/2026-07-24-mtconnect-driver.md); for the
fixture recipe read
[`tests/Drivers/ZB.MOM.WW.OtOpcUa.Driver.MTConnect.IntegrationTests/Docker/README.md`](../../tests/Drivers/ZB.MOM.WW.OtOpcUa.Driver.MTConnect.IntegrationTests/Docker/README.md)
(not duplicated here).

> **Live gate:** see the LIVE-GATE RESULT note in the plan's Task 21
> (`docs/plans/2026-07-24-mtconnect-driver.md`) for the docker-dev `/run` verification outcome
> (browse picker / typed editor / deploy / read / subscribe against the real Agent fixture).

## What it talks to

A **MTConnect Agent** — the vendor-neutral telemetry endpoint that
front-ends a machine tool (or fronts an adapter that itself talks to the
machine over SHDR). The driver speaks plain HTTP + XML against the Agent's
three standard REST paths:

- `/probe` — the static device model (Device → Component → DataItem tree)
- `/current` — a snapshot of every DataItem's latest observation
- `/sample` — a `multipart/x-mixed-replace` long-poll stream of observation
  deltas, keyed by a monotonic sequence number

Those three are the whole data plane: **agent-first** — Discover + Read +
Subscribe. There is a fourth path, and it is not part of the data plane: the
Agent's optional **`AllowPut`** surface, off by default in `agent.cfg`, which
this driver's `IWritable` uses (`b29623cf`, 2026-08-05). It sets what the Agent
*reports* for a DataItem; nothing reaches the machine. Read
[Write-back](#write-back--observation-injection-off-by-default) before enabling
it.

## Built vs. planned — read this before trusting the design doc's §2

The design doc ([`2026-07-15-mtconnect-driver-design.md`](../plans/2026-07-15-mtconnect-driver-design.md))
picked the **TrakHound `MTConnect.NET-Common` / `-HTTP`** NuGet packages (MIT,
`netstandard2.0`) as the primary path, with a hand-rolled fallback. **The
hand-rolled fallback is what shipped.** Task 6/7 of the implementation plan
found by reflection + live invocation that the pinned TrakHound packages
ship **no XML formatter** (`Document Formatter Not found for "xml"` — the
formatter lives in a separate `MTConnect.NET-XML` package the design didn't
pin) and that `MTConnectHttpClientStream` exposes no injectable `HttpClient`
handler, so it cannot be unit-tested behind the driver's `IMTConnectAgentClient`
seam. Both package references were dropped. There is **no TrakHound
dependency anywhere in the shipped driver** — probe/current/sample parsing is
hand-rolled `System.Xml.Linq` plus a multipart boundary reader, entirely
behind `IMTConnectAgentClient`. See the CORRECTION block under Task 0 of the
implementation plan for the full record.

**Namespace-agnostic parsing.** Every parser matches XML elements on
`LocalName` only, ignoring the document's XML namespace entirely, so
MTConnect 1.3 through 2.x documents all parse without a schema-version
branch. This is deliberate, not sloppy: a real Agent injects vendor-extension
`Component`s in a foreign namespace whose child `DataItems` elements inherit
the *default* namespace, and namespace-strict matching would silently drop
the whole vendor component (and every DataItem beneath it) rather than fail
loudly. Attributes are read **unqualified** for the same reason — so an
`xsi:type` attribute is never mistaken for a DataItem's own `type`.

## Project split

| Project | Target | Role |
|---------|--------|------|
| `src/Drivers/ZB.MOM.WW.OtOpcUa.Driver.MTConnect/` | net10.0 | In-process driver — `MTConnectDriver`, the hand-rolled `IMTConnectAgentClient` (probe/current/sample), the observation index, the factory |
| `src/Drivers/ZB.MOM.WW.OtOpcUa.Driver.MTConnect.Contracts/` | net10.0 | `MTConnectDriverOptions`, `MTConnectTagDefinition`, and the pure `MTConnectDataTypeInference` table shared by the driver, browse-commit, and the AdminUI typed editor |

No `.Browser` project — browse comes free from the Wave-0 universal
discovery browser (see [Browse](#browse--free-via-the-universal-browser)
below).

## Capability surface

`MTConnectDriver : IDriver, IReadable, IWritable, ISubscribable, ITagDiscovery, IHostConnectivityProbe, IRediscoverable`
(`src/Drivers/ZB.MOM.WW.OtOpcUa.Driver.MTConnect/MTConnectDriver.cs`).

`IWritable` landed 2026-08-05 (`b29623cf`), against the Agent's `AllowPut`
surface rather than MTConnect *Interfaces*. It is **observation injection, not
machine command**, and is off at two independent gates by default — read
[Write-back](#write-back--observation-injection-off-by-default) before
authoring a writable MTConnect tag. *Interfaces* itself — the protocol's own
request/response handshake — is a different mechanism and remains deferred; see
[Deferred](#deferred-not-in-this-build).

| Capability | Path | Notes |
|------------|------|-------|
| `ITagDiscovery` | `DiscoverAsync` — streams `/probe`'s Device→Component→DataItem tree into the address-space builder | `SupportsOnlineDiscovery = true`, `RediscoverPolicy = Once` |
| `IReadable` | `ReadAsync` → one `/current` per call | **Not the production data path** — see below |
| `IWritable` | `WriteAsync` → `POST {agent}/{device}?{dataItemId}={value}` on the Agent's `AllowPut` surface | **Off by default** — needs `Write.Enabled` **and** the Agent's `AllowPut`; CONDITION items are refused |
| `ISubscribable` | `SubscribeAsync`/`OnDataChange` — the shared `/sample` long-poll pump | **The production data path** |
| `IHostConnectivityProbe` | periodic `/probe` under `Probe.*` | No consumer wires it today — see [Known limitations](#known-limitations) |
| `IRediscoverable` | watches the Agent's `Header.instanceId` | No consumer wires it today — see [Known limitations](#known-limitations) |
| `IAlarmSource` | CONDITION observations off the same `/sample` pump → Part 9 transitions keyed by the alarm tag's RawPath | Opt-in per raw tag (`alarm` object) — see [CONDITION alarms](#condition-alarms-626) |

**`IReadable.ReadAsync` is never called by the running server.** The
production data plane is entirely `ISubscribable` — `DriverInstanceActor`
subscribes once and lives off `OnDataChange`. `ReadAsync` exists for the
Client CLI (`... read -n ...`) and for the unit/integration suites; it is
correct and tested, just not on the hot path.

## Minimum deployment

```jsonc
"Drivers": {
  "mtconnect-1": {
    "Type": "MTConnect",
    "Config": {
      "AgentUri": "http://10.100.0.35:5000",
      "DeviceName": null,
      "RequestTimeoutMs": 5000,
      "SampleIntervalMs": 1000,
      "SampleCount": 1000,
      "HeartbeatMs": 10000,
      "Probe":     { "Enabled": true, "IntervalMs": 5000, "TimeoutMs": 2000 },
      "Reconnect": { "MinBackoffMs": 0, "MaxBackoffMs": 30000, "BackoffMultiplier": 2.0 },
      "Write":     { "Enabled": false },
      "Tags": []
    }
  }
}
```

`RawTags[]` is not authored by hand — `DriverDeviceConfigMerger` injects it
at deploy time from the tags authored on the `/raw` tree.

### Config keys

Read off `MTConnectDriverOptions` / `MTConnectDriverConfigDto`
(`src/Drivers/ZB.MOM.WW.OtOpcUa.Driver.MTConnect.Contracts/MTConnectDriverOptions.cs`,
`src/Drivers/ZB.MOM.WW.OtOpcUa.Driver.MTConnect/MTConnectDriver.cs`):

| Key | Default | Notes |
|---|---|---|
| `AgentUri` | — (required) | Agent base URI; the driver appends `/probe`, `/current`, `/sample` |
| `DeviceName` | `null` | **Double duty.** (1) Scopes requests to `{AgentUri}/{DeviceName}/...`; `null` = whole Agent. (2) The write path's fallback device — the device a write is addressed to when the probe model does not say which device declared the DataItem (see [Write-back](#write-back--observation-injection-off-by-default)) |
| `RequestTimeoutMs` | `5000` | Per-call deadline for `/probe` and `/current` |
| `SampleIntervalMs` | `1000` | The Agent's `/sample?interval=` query param |
| `SampleCount` | `1000` | The Agent's `/sample?count=` query param |
| `HeartbeatMs` | `10000` | The Agent's `/sample?heartbeat=` query param — also the watchdog's liveness window |
| `Probe.Enabled` / `Probe.IntervalMs` / `Probe.TimeoutMs` | `true` / `5000` / `2000` | Background connectivity-probe knobs (mirrors `ModbusProbeOptions`) |
| `Reconnect.MinBackoffMs` / `MaxBackoffMs` / `BackoffMultiplier` | `0` / `30000` / `2.0` | Geometric backoff after a failed request or a dropped `/sample` stream |
| `Write.Enabled` | `false` | The driver-side half of the write-back opt-in. An absent block means disabled — see [Write-back](#write-back--observation-injection-off-by-default) |
| `Tags[]` | `[]` | Pre-v3 / CLI authoring surface — one `MTConnectTagDefinition` per DataItem `id` |
| `RawTags[]` | `[]` (deploy-injected) | The v3 data-plane binding — see below |

**All timing knobs are validated strictly positive** at `InitializeAsync`
(`RequirePositive`, mirroring the arch-review 01/S-6 lesson: a `0` timeout
does not mean "wait forever," it faults the driver). This applies to
`RequestTimeoutMs`, `HeartbeatMs`, `SampleIntervalMs`, `SampleCount`, and
(when the probe is enabled) `Probe.Interval` / `Probe.Timeout`.

**No save-time gate on a blank `AgentUri` in the AdminUI.** The driver form
renders an inline validation notice (`_form.Validate()`), but
`DriverConfigModal.SaveAsync` has no per-form validation seam to block the
Save button on it — no sibling driver form has one either. A blank
`AgentUri` saves cleanly and fails at deploy with the driver's own error
message, not at authoring time.

## Data plane

### FullName == DataItem `id`

`FullName` (both `MTConnectTagDefinition.FullName` and every `RawTags[]`
blob's identifier) is the MTConnect `DataItem@id` attribute — the value the
universal browser commits and the key the driver resolves reads/subscribes
against. `MTConnectTagConfigModel.FromJson` (the AdminUI typed editor) tries
three spellings in order — `fullName` → `dataItemId` → `address` — and takes
the first non-blank one, normalising onto `fullName` on save. `address` is
accepted because that is the field name `RawBrowseCommitMapper` writes for a
driver with no typed-editor branch of its own — MTConnect has had such a
branch since Gitea #536 and now commits under `fullName`, but blobs committed
before it, and hand-authored/CSV/API blobs, still carry `address` and must
stay readable.

### Coercion-type precedence

The driver keys its data plane by **RawPath** (the v3 raw-tag identity), not
by DataItem id directly, resolving RawPath → dataItemId internally. Each raw
tag's coercion type (the `DriverDataType` its Agent observation is parsed
into) is resolved in this order, first non-null wins:

1. The `RawTags[]` blob's `driverDataType` or `dataType` field (both
   spellings accepted — the typed editor writes `dataType`, the driver's own
   `tags[]` shape uses `driverDataType`). **`driverDataType` wins when it is
   non-blank**, and a present-but-invalid one rejects the tag rather than
   falling through to `dataType`. The AdminUI's typed editor mirrors that
   precedence on load and **normalises onto `dataType`, dropping the alias, on
   save** (Gitea #550) — so a tag edited through the editor keeps the type the
   driver read for it, and an unreadable spelling is refused at authoring
   instead of dying at the driver.
2. A matching `Tags[]` entry naming the same DataItem id.
3. The Agent's own `/probe` declaration, via `MTConnectDataTypeInference.Infer`
   (category/type/units/representation → `DriverDataType`).
4. `DriverDataType.String` — the coercion that cannot fail.

> **A browse-committed tag resolves at step 1, carrying the step-3 answer.** `RawBrowseCommitMapper`'s
> MTConnect branch emits a whole `MTConnectTagConfigModel`, whose `ToJson()` always writes `dataType`
> — so a committed blob always states a non-blank type and step 1 always wins. Since Gitea **#560**
> that stated type is the **browsed leaf's own inferred `DriverDataType`**, the very value step 3
> would compute, forwarded by `MapLeaf` into `BuildTagConfig` exactly as the AbCip / TwinCAT / FOCAS
> branches forward their browsed native type (Gitea #536). It therefore agrees by construction with
> the raw tag row's OPC UA `DataType`, which `MapLeaf` → `MapDataType` takes from the same inference.
>
> Before #560 the branch stamped the model's `String` default instead, so a browsed `SAMPLE` landed
> as a `Double` node whose driver coerced the observation to text — recorded as `Driver.MTConnect-003`
> in [`code-reviews/Driver.MTConnect/findings.md`](../../code-reviews/Driver.MTConnect/findings.md).
> A leaf the browser reports **no** type for still commits `String`, and a leaf legitimately inferred
> as `String` (an `EVENT` item) commits `String` too — in both cases the type the driver would have
> reached anyway.
>
> **Step 3 remains the rescue for a blob that states no type at all** — hand-authored, CSV-imported,
> or API-posted `RawTags[]` entries carrying only an id, which is the shape it was written for.

### Quality mapping

`UNAVAILABLE` — MTConnect's one explicit "I have no value" sentinel — maps to
**`BadNoCommunication`** (`0x80310000`). This is deliberately distinct from
the fleet-standard `BadCommunicationError` (`0x80050000`, used elsewhere for
the driver's *own* transport failure to the Agent): `UNAVAILABLE` means the
Agent is reachable and answered, it just has no device-backed value for that
item. A `CONDITION` observation's value is taken from its **element name**
(`<Normal/>` → the string `"Normal"`, `<Fault/>` → `"Fault"`); a `<Unavailable/>`
condition element normalises onto the same `UNAVAILABLE` sentinel as every
other category so one comparison covers all three.

Other status codes an observation can surface:

| Code | Meaning |
|---|---|
| `BadTypeMismatch` (`0x80740000`) | The Agent's text isn't a value of the tag's coerced type at all |
| `BadOutOfRange` (`0x803C0000`) | The Agent reported a number the coerced type can't represent |
| `BadDecodingError` (`0x80070000`) | A `TIME_SERIES` vector whose element count disagrees with the observation's own `sampleCount` (or the tag's fixed `ArrayDim`) — see [TIME_SERIES](#time_series-arrays-627) |
| `BadNotSupported` (`0x803D0000`) | A shape this build doesn't materialize — a structured `DATA_SET`/`TABLE` observation (decided: stays unsupported, #627), or an array tag declared as anything other than Float64 |
| `BadWaitingForInitialData` | Tag is authored but the Agent hasn't reported it yet |
| `BadNodeIdUnknown` | DataItem id is neither authored nor ever observed |

### TIME_SERIES arrays (#627)

A `SAMPLE` DataItem with `representation="TIME_SERIES"` is served as a
**variable-length `double[]`** (`ValueRank = 1`, no fixed `ArrayDimensions` —
a live Agent never declares `sampleCount` on the DataItem, see Known
limitations 4; a probe that does declare one yields a fixed `ArrayDim`).
Parsing: the observation text is split on any whitespace, each token parsed
invariant-culture. The result is **all-or-nothing** — one non-numeric /
non-finite token is `BadTypeMismatch`, a literal `UNAVAILABLE` token or an
empty body is `BadNoCommunication`, and a count that disagrees with the
observation's own `sampleCount` attribute (or a fixed `ArrayDim`) is
`BadDecodingError`. A partial vector is never served as Good. Only a
**Float64** array tag is served; any other declared element type stays
`BadNotSupported`. An authored tag needs `"isArray": true` and
`"dataType": "Float64"` in its TagConfig (the AdminUI editor shows an advisory
for any other array type; a browse-commit does not stamp `isArray`, so set it
in the Tag modal's array section).

**Sample rate is not exposed on the served node.** The observation's
`sampleRate` and `sampleCount` attributes are parsed (`MTConnectObservation`)
but the OPC UA node has no description/property slot through `DriverAttributeInfo`,
and a sibling property node would need a new address-space mechanism. Decision:
document it — the rate is a property of the Agent's DataItem (`sampleRate` in
`/probe`), read it there. **`DATA_SET` / `TABLE` stay `BadNotSupported`** (decision, #627): they are
key/value maps with no scalar or array OPC UA shape that preserves the keys.

### Subscribe — the `/sample` pump

One shared `/sample` long-poll stream per driver instance (the Agent streams
the whole device model regardless of which subset is subscribed, so
per-tag streams would be wasted round-trips), run under a heartbeat
watchdog — `HttpClient.Timeout` cannot bound a long-lived stream, so a
missing chunk **and** missing keep-alive heartbeat within
`HeartbeatMs × N` is what detects a frozen peer.

Ring-buffer overflow (the Agent's circular observation buffer wrapped past
what the driver's cursor expects) is detected **two ways**, both triggering a
`/current` re-baseline before the stream resumes:

1. `IMTConnectAgentClient.IsSequenceGap` — the next chunk's `firstSequence`
   is newer than the driver's expected cursor.
2. An `MTConnectError` document reporting `OUT_OF_RANGE` — real Agents return
   this both under HTTP 200 (a normal MTConnect error document) and as a bare
   HTTP 400, and the client handles both.

**An Agent `instanceId` change means the Agent restarted**, and is checked
**before** the sequence-gap check (a restart also usually trips the gap, so
the two need disambiguating, not just OR-ing together). On a changed
`instanceId` the driver clears its cached probe model and raises
`OnRediscoveryNeeded`, which surfaces a **re-browse prompt** on the AdminUI
`/hosts` page — see [Known limitations](#known-limitations) for what that does
and does not do.

## Write-back — observation injection, off by default

> ⚠️ **A write sets what the Agent *reports*, not what the machine does.** The value
> lands in the Agent's observation buffer exactly as if an adapter had reported it —
> visible to **every** consumer of that Agent, and **overwritten by the next
> adapter/SHDR update for that DataItem**. Nothing reaches the CNC. Never document,
> script, or operate this as a machine command. It is genuinely useful for
> operator-set EVENT values, for external systems feeding DataItems the adapter does
> not own, and for test rigs — and nothing else.

The mechanism is the Agent's **`AllowPut`** surface (`POST
{agent}/{device}?{dataItemId}={value}`), a standard cppagent feature that is **off by
default**. It is not MTConnect *Interfaces* — see [Deferred](#deferred-not-in-this-build)
for why Interfaces was rejected as the write-back shape. Design record:
[`2026-08-05-mtconnect-writeback-design.md`](../plans/2026-08-05-mtconnect-writeback-design.md)
§2.2; implementation `b29623cf`.

### Two independent gates, both off by default

1. **The Agent** — `AllowPut = true` in `agent.cfg`. Optionally scope it with
   `AllowPutFrom = <host list>` so only named hosts may write; that list is the Agent's
   own access control and this driver neither reads nor influences it.
2. **The driver** — `"Write": { "Enabled": true }` in the driver config (default
   `false`; an absent block is disabled). Authored in the AdminUI on the `/raw` driver
   config modal's **Write-back** section (`MTConnectDriverForm`), which writes the block
   explicitly in both states — so turning the switch back off clears a previously
   authored `true` rather than leaving it standing.

Either gate alone leaves the shipped read-only behaviour unchanged: with `Write.Enabled`
off every slot is refused `BadNotWritable` without an Agent being touched, and with
`AllowPut` off the Agent refuses and the driver reports `BadCommunicationError`.

### The OPC UA write bit is authored, not derived — read this before filing a bug

**Enabling write-back does not make any existing node writable over OPC UA.** The
`CurrentWrite` access bit is composed from the **authored `Tag.AccessLevel`** alone
(`AddressSpaceComposer` → `AddressSpaceApplier`, which additionally requires a
non-array tag → `OtOpcUaNodeManager.ComposeAccessLevel`); driver capability is never
consulted at compose time. An operator must set the raw tag to **ReadWrite** — a
browse-committed tag is created `Read` (`RawBrowseCommitMapper.MapLeaf`) whatever the
browse said.

Browse *does* stamp writable leaves `SecurityClassification.Operate` when
`Write.Enabled` is on (and `ViewOnly` otherwise, CONDITION always `ViewOnly`), on the
same "a node cannot advertise writability the driver would refuse" principle MQTT
pinned. But that stamp is **honest labelling, not the access bit**: nothing persists
`SecurityClass` — `Tag` has no such column — so it reaches the browse session's
attribute side-panel and stops there.

### What a write does, slot by slot

`WriteAsync` never throws and one bad slot never fails the batch (matching `ReadAsync`);
even cancellation returns a full-length result set. The refusal order is fixed, and each
step answers a different operator question:

| Condition | Status | Meaning |
|---|---|---|
| `Write.Enabled` off, **or** the DataItem is CONDITION-category | `BadNotWritable` (`0x803B0000`) | Nothing was attempted; no Agent was involved |
| No live Agent client (pre-`InitializeAsync`, faulted initialize, post-shutdown) | `BadNotConnected` (`0x808A0000`) | This driver is not running |
| The reference resolves to no DataItem, or to one whose owning device cannot be named | `BadNodeIdUnknown` (`0x80340000`) | There is no Agent URL the write could be addressed to |
| The value has no invariant string form — `null`, or non-scalar | `BadTypeMismatch` (`0x80740000`) | Arrays are refused rather than rendered (`Convert.ToString` would emit the literal `System.Double[]`, which the Agent stores happily) |
| The Agent refused, or could not be reached | `BadCommunicationError` (`0x80050000`) | Includes an `MTConnectError` document returned under HTTP 200 |

**CONDITION DataItems are excluded deliberately** — they carry alarm semantics and a
different `level|text` wire format, and are out of scope for v1.

**The Agent's refusal text is logged at Warning and never stored.** It stays out of
`DriverHealth.LastError`, which is type-only by repo-wide discipline, and a refused write
is not evidence the read path degraded — so a refusal leaves driver health untouched.

**Device resolution needs no extra config.** `TagBinding.WriteTargetsByDataItemId`,
built at bind time from a walk of the cached probe model, remembers which device declared
each DataItem, so a multi-device Agent is addressed correctly; the fallback is the
configured `DeviceName`, and an unresolvable device is `BadNodeIdUnknown` rather than a
guess.

**Values are rendered with `CultureInfo.InvariantCulture`.** A Boolean therefore writes
the invariant string `"True"`/`"False"`, **not** `"1"`/`"0"` — if the consuming adapter or
Agent vocabulary expects something else, author the tag as a `String` and write the exact
token.

## Browse — free via the universal browser

MTConnect ships **no bespoke browser project**. Setting
`ITagDiscovery.SupportsOnlineDiscovery => true` is the entire integration:
the Wave-0 `DiscoveryDriverBrowser` sees a driver whose `TryCreate` succeeds
and whose instance reports online discovery, renders the AdminUI **Browse**
button, constructs the driver, runs `InitializeAsync` (the `/probe` connect)
+ `DiscoverAsync` into a `CapturingAddressSpaceBuilder`, then tears the
throwaway instance down. Each captured leaf's NodeId is the DataItem `id`,
committed directly as `TagConfig.FullName` on pick. See
[`docs/plans/2026-07-15-universal-discovery-browser-design.md`](../plans/2026-07-15-universal-discovery-browser-design.md).

## CONDITION alarms (#626)

A `CONDITION` DataItem can be served two ways, and the choice is **per raw tag**:

- **As a String value** (the default — every existing CONDITION tag, and every browse-committed
  one). The value is the current state word — `Normal` / `Warning` / `Fault` / `UNAVAILABLE`
  (`UNAVAILABLE` → `BadNoCommunication`). Unchanged by #626.
- **As a native OPC UA Part 9 alarm**, by adding the cross-driver `alarm` object to the raw tag's
  `TagConfig` — exactly how a Galaxy alarm tag is made alarm-bearing (the `/raw` tag modal's native
  alarm section, or JSON):

  ```json
  { "address": "dev1_system_cond", "alarm": { "alarmType": "AlarmCondition", "severity": 800 } }
  ```

  The server then materialises **one Part 9 condition at the tag's RawPath** (ConditionId = RawPath),
  fanned to the raw device folder and every referencing equipment folder, and the driver feeds it
  through `IAlarmSource`. An alarm-bearing tag is a condition, not a value variable (the host excludes
  alarm tags from the value subscription), so to keep the String as well, author a second raw tag on
  the same DataItem without the `alarm` object — one DataItem may back several raw tags.

It is deliberately **not automatic** for `category=CONDITION`: that would silently turn every existing
CONDITION String tag into a condition node on upgrade.

**State mapping.**

| Agent level | Part 9 | Severity | Quality |
|---|---|---|---|
| `Normal` | inactive (return to normal) | — | Good |
| `Warning` | active, unacknowledged | 500 | Good |
| `Fault` | active, unacknowledged | 800 | Good |
| `Unavailable` | **unchanged** (an active condition stays active and acknowledged) | — | `BadNoCommunication` |

**`Unavailable` is comms loss one hop further out, not a return to normal.** A cppagent marks every
DataItem of an adapter UNAVAILABLE when the adapter socket drops, so it is treated exactly like the
host's own transport loss (#477/#481): the condition keeps its Active/Acked state and only its `Quality`
goes `BadNoCommunication`, as a **quality-only annotation** (no `/alerts` row, written through the host's
Quality-only node path — it can never rewrite message/severity or acknowledge anything). When data
returns: the **same** statement ⇒ quality-only Good (an ack survives an adapter blip); `Normal` ⇒ Clear;
a **different** active statement ⇒ Retrigger. An inactive condition going Unavailable is likewise a
quality-only Bad. Severity is the exact Part 9 value (`AlarmEventArgs.OpcUaSeverity`, #626), not one of
the four buckets. A clear keeps the severity of the alarm it clears.

**Message** = the condition's text content; without text, `"{type} {level}"` (e.g. `SYSTEM Fault`).
Whichever of `nativeCode` / `nativeSeverity` / `qualifier` the Agent supplied is appended —
`Spindle overtemperature [nativeCode=E17, nativeSeverity=2, qualifier=HIGH]` — because the Part 9
event model has no slot for them. `AlarmType` is the condition's `type`; `SourceNodeId` the DataItem id.

**Several simultaneous conditions.** MTConnect can hold several active conditions on one DataItem
(distinct `nativeCode`s). The OPC UA condition shows **the most severe active one**; equal levels tie-break deterministically
(ordinal `nativeCode`, then `type`), never by Agent document order, so a re-baseline that lists them
differently cannot re-raise.
The driver tracks them with MTConnect's own rules (`MTConnectConditionTracker`): `Warning`/`Fault`
activates its `nativeCode`; `Normal` with a `nativeCode` clears only that code, `Normal` without one
clears all; `Unavailable` clears all; a `/current` re-baseline replaces the DataItem's set. So a
`/sample` chunk clearing code A while code B is active is **not** a return to normal. An escalation
(Warning → Fault) or a different winning code is a **Retrigger** — active and unacknowledged again with
the new severity/message. By design that includes **clearing the worst of several codes**: the next one
down is re-raised un-acked, because it is a statement the operator has not seen. A **text-only change**
on the same level + `nativeCode` (an adapter embedding a live value in the text) updates nothing — no
retrigger, no un-ack — and the message shown stays the statement's first text. (The String value surface still reconciles per
document, so on a delta it shows the state word of that chunk; the alarm is the authoritative view.)

**Acknowledge.** MTConnect has no acknowledgement upstream. Native conditions support Acknowledge and
AddComment only; the ack is applied server-side and the driver's `AcknowledgeAsync` is a no-op success
(nothing is sent to the Agent). Off-Primary acks are refused by `NativeAlarmAckPrimaryGate` as for every
native source.

**Restart / reconnect / comms loss.** The driver diffs each condition against what the server was last
**told** — dedup on level + `nativeCode` + `nativeSeverity` + `qualifier` (not text, never the value) —
and records a state only when an alarm handler was attached to receive it. Consequences:

- A replayed `/current` (reconnect, re-initialize, Agent restart, ring-buffer re-baseline) re-raises
  nothing that is unchanged — re-raising would un-acknowledge an operator's ack.
- A transition that happened while the host was detached (it detaches while it reconnects) is delivered
  on the re-subscribe that follows; a fault cleared during an outage is cleared on recovery.
- Comms loss emits nothing from the driver, matching every native source: the host annotates every
  condition the driver owns `BadNoCommunication` on the connectivity edge (#477) and keeps its Active
  state; the reconnect edge re-annotates Good, after which the driver re-asserts any still-`Unavailable`
  condition's Bad quality on its re-subscribe.
- A re-subscribe with a **changed** alarm set clears the whole told record: only an alarm-bearing raw-tag
  delta changes the set, the server applies that as a full rebuild, and a rebuild recreates every
  condition node inactive — so every active condition is re-asserted onto its new node. The **same** set
  (a reconnect) keeps the record. (A rebuild that leaves this driver's alarm set unchanged triggers no
  re-subscribe; that is a host-wide gap shared with every native source.)
- An alarm-only driver (no value tags) still runs the `/sample` pump.

**EVENT controlled vocabularies as OPC UA enumerations — won't fix.** EVENT values (`Execution`,
`ControllerMode`, …) stay Strings carrying the Agent's own word; an enumeration would need a per-type
vocabulary table that vendors extend freely.

## Known limitations

These are real, not placeholders — read them before relying on the driver
for anything beyond values-and-conditions.

1. **A restarted Agent prompts an operator; it does not re-shape the address
   space.** `DriverInstanceActor` now consumes `OnRediscoveryNeeded`
   (2026-07-27), so a changed `instanceId` raises a "re-browse" chip against
   this driver on `/hosts` carrying the reason. It is **advisory**: v3 authors
   raw tags through the `/raw` browse-commit flow, so until an operator
   re-browses the Agent and commits, any DataItem that appeared or vanished is
   not reflected in the served tree. A runtime graft was deliberately rejected
   — it would materialise nodes nobody approved and no deployment artifact
   records.

   **`IHostConnectivityProbe` is consumed** (Gitea #521, closed) —
   `DriverInstanceActor` subscribes `OnHostStatusChanged`, pulls
   `GetHostStatuses()` fresh, and the result rides `DriverHealthChanged` to
   the AdminUI `/hosts` page (no SQL; the `DriverHostStatus` table was
   dropped). See `docs/Telemetry.md`.
2. **One MTConnect driver instance = one Agent client = at most one `/raw`
   Device (#628, decision (b)).** The driver never reads
   `RawTagEntry.DeviceName`; every raw tag binds through the single Agent client
   scoped by `MTConnectDriverOptions.DeviceName`. The v3 raw tree always has a
   Device level (a new driver auto-creates `Device1`), and that single-Device
   layout is the normal, valid shape — its `DeviceConfig` is merged up into the
   driver config by `DriverDeviceConfigMerger` (this is how the endpoint lands).
   With **two or more** Devices under one MTConnect driver there is no merge-up
   and no routing, so which Device supplies the endpoint is ambiguous. That is
   now a **deploy-time `DraftValidator` error** (`MTConnectMultipleDevices`),
   and the AdminUI refuses to *create* the second Device
   (`RawTreeService.CreateDeviceAsync`, same message): "MTConnect binds all tags
   through one agent client per driver instance; author one MTConnect driver per
   agent device." Option (a), a client per Device, was rejected as a design
   change with no demand. A multi-device Agent is served by several driver
   instances, each with its own `DeviceName` scope. The driver's init-time
   Warning for differing `RawTagEntry.DeviceName` values stays as defence in
   depth for configs that predate the rule.

   ⚠️ **Upgrade note:** an MTConnect driver that *already* has two or more Device
   rows fails its next deploy with `MTConnectMultipleDevices` (the rule counts every
   Device row, enabled or not, because nothing at runtime reads `Device.Enabled`).
   Before upgrading, move each extra Device's tags under its own MTConnect driver
   and delete the extra Device rows.

   **The write path is the one place a device name is resolved per DataItem**,
   and it does not use `RawTagEntry.DeviceName` either: it reads the owning
   device out of the cached probe model and falls back to the top-level
   `DeviceName` (see [Write-back](#write-back--observation-injection-off-by-default)).
   So `MTConnectDriverOptions.DeviceName` does double duty — request scope for
   reads/browse, fallback write target for writes.

3. **Authoring only *warns* when a declared coercion type cannot hold the
   inferred one (#613).** The AdminUI tag editor (`MTConnectTagConfigModel.Warnings()`)
   compares the declared type with `MTConnectDataTypeInference` over the
   `/probe` metadata a browse-commit stamped (`mtCategory`/`mtType`), and shows an
   advisory line — it never blocks a save (the deploy gate does not refuse it) and
   is a no-op for hand-authored blobs with no stamped metadata. `representation`
   is not on the model; TIME_SERIES arrays get their own `isArray` advisory (declare Float64). Beyond that,
   nothing validates a raw tag's declared OPC UA `DataType` against the
   blob's coercion type. The `/probe`-sourced inference
   (`MTConnectDataTypeInference`) narrows how often an author picks a wrong
   type, but it doesn't close the gap — an authored `DataType` mismatched
   against the actual Agent observation still surfaces as `BadTypeMismatch`
   /`BadOutOfRange` at runtime rather than at authoring time.
4. **`sampleCount` is not a legal `DataItem` attribute.** It belongs on a
   `TIME_SERIES` *observation*, not the device-model declaration. A real
   Agent that meets `sampleCount` on a `DataItem` declaration logs `The
   following keys were present and not expected: sampleCount` and **drops
   the entire data item** from the served model. A live TIME_SERIES tag
   therefore always resolves to `ArrayDim = null` (variable-length array) in
   practice — the fixture's canned `probe.xml`/`Devices.xml`, which does
   declare a `sampleCount`, is unrealistic on this point and exists only to
   pin the parsing rule itself.
5. **A CONDITION alarm shows one condition per DataItem** — the most severe active `nativeCode`, not one
   Part 9 condition per code (see [CONDITION alarms](#condition-alarms-626)).
6. **Structured `DATA_SET`/`TABLE` observations surface as
   `BadNotSupported`**, not an approximation — see the Quality mapping table.
   `TIME_SERIES` is served as `double[]` (#627, see
   [TIME_SERIES arrays](#time_series-arrays-627)); the sample rate is not
   surfaced on the node.

## Deferred (not in this build)

- **MTConnect *Interfaces*.** Still deferred, and for the reason design
  [§3.6](../plans/2026-07-15-mtconnect-driver-design.md#36-iwritable--not-implemented-v1)
  gave: it is a rare, optional request/response **state-machine handshake**
  requiring adapter-side cooperation, not a "set value" operation, and would
  mislead if modelled as `IWritable`.
  **Write-back itself is no longer deferred** — `b29623cf` shipped `IWritable`
  over the Agent's `AllowPut` observation-injection surface instead, a different
  mechanism entirely. See [Write-back](#write-back--observation-injection-off-by-default)
  and
  [`2026-08-05-mtconnect-writeback-design.md`](../plans/2026-08-05-mtconnect-writeback-design.md)
  §2.1.
- **P1.5 fast-follow** (design
  [§9](../plans/2026-07-15-mtconnect-driver-design.md#9-phasing--effort)): CONDITION → native Part 9
  alarms **shipped** (#626, see [CONDITION alarms](#condition-alarms-626)); EVENT
  controlled-vocabulary values → OPC UA enumerations is **won't-fix**.
- **P2 (on demand): SHDR adapter ingest.** A `SourceMode: "Agent" | "Shdr"`
  switch that opens the raw pipe-delimited SHDR TCP socket directly. Loses
  auto-discovery (no device model without an Agent in front, so
  `SupportsOnlineDiscovery` would have to report `false` and tags would be
  authored by hand, like Modbus). Niche; not built.

## Testing

- **Unit tests** — `tests/Drivers/ZB.MOM.WW.OtOpcUa.Driver.MTConnect.Tests/`
  (canned-XML fixtures, no network) — 491/491 at time of writing. Covers
  discovery tree shape, `MTConnectDataTypeInference`, observation indexing,
  `UNAVAILABLE` → `BadNoCommunication`, CONDITION state-word mapping,
  multipart chunk framing, and ring-buffer re-baseline paging (both the
  sequence-gap and the `OUT_OF_RANGE` legs). The write path adds
  `MTConnectAgentClientWriteTests` (`9f17d7ef` — verb, URL, agent-error-in-200
  detection) and `MTConnectWriteTests` (`b29623cf` — every refusal leg, the
  batch shape, the browse-stamping flip).
- **Integration tests** —
  `tests/Drivers/ZB.MOM.WW.OtOpcUa.Driver.MTConnect.IntegrationTests/` — env-gated
  on `MTCONNECT_AGENT_ENDPOINT` (default `http://10.100.0.35:5000`), skips
  cleanly (12 Skipped) when the fixture is unreachable, 12/12 against a real
  Agent. Read the fixture's own
  [`Docker/README.md`](../../tests/Drivers/ZB.MOM.WW.OtOpcUa.Driver.MTConnect.IntegrationTests/Docker/README.md)
  for the full recipe — summary only, here:
  - Image is **`mtconnect/agent:2.7.0.12`** — not `mtconnect/cppagent`, which
    does not exist on Docker Hub (the design's original assumption).
  - The stack is **two services**: the Agent plus a stdlib-only SHDR adapter
    (`Docker/adapter.py`, on `python:3.13-alpine`). An Agent with no adapter
    reports every observation `UNAVAILABLE` forever, which proves nothing.
  - Deployed at `/opt/otopcua-mtconnect/` on the shared docker host
    (`10.100.0.35`, `project=lmxopcua` label). Endpoint
    `http://10.100.0.35:5000`.
  - `agent.cfg` must be **pure ASCII** — one non-ASCII byte anywhere,
    including a comment, makes the config parser reject the whole file with
    a bare `Failed / Stopped at line: N`.
  - Bring-up: `lmxopcua-fix sync mtconnect` then `lmxopcua-fix up mtconnect`
    (PowerShell helper, Windows-only) — or, directly on the docker host,
    `rsync` the repo's `Docker/` dir to `/opt/otopcua-mtconnect/` and run
    `docker compose up -d --wait`.
- **CONDITION alarms (#626)** — `MTConnectConditionAlarmMappingTests` (tracker semantics + state
  mapping/sequencing), `MTConnectAlarmSourceTests` (driver wiring: Normal→Fault→Normal, escalation,
  several `nativeCode`s, Unavailable, replay after re-subscribe/re-initialize/Agent restart with no
  duplicate, the missed-while-detached delivery, comms loss), `MTConnectConditionAlarmWireTests` in
  `OpcUaServer.IntegrationTests` (a scripted Agent's Fault/Normal reaching a client's event subscription on
  the raw device folder). **Live check:** the fixture's adapter only ever writes `normal` for its two
  CONDITION items (`fixture_x_travel` / SHDR key `Xtravel`, `fixture_logic` / `Plogic`, see
  `Docker/Devices.xml`). To see a transition, make `send_conditions()` in `Docker/adapter.py` alternate
  `Xtravel|fault|1001|2|HIGH|X overtravel` with `Xtravel|normal||||` every few ticks, sync + restart the
  adapter, author a raw tag `{"address":"fixture_x_travel","alarm":{"alarmType":"AlarmCondition"}}` on an
  MTConnect driver, deploy, and subscribe events on its raw device folder (Client CLI `alarms`): expect a
  Fault event (severity 800, message `X overtravel [nativeCode=1001, nativeSeverity=2, qualifier=HIGH]`),
  then a return-to-normal event, repeating.

## Further reading

- [`docs/plans/2026-07-15-mtconnect-driver-design.md`](../plans/2026-07-15-mtconnect-driver-design.md) — full design: capability wiring, browse reconciliation, typed-editor spec, resilience/timeout rules, phasing
- [`docs/plans/2026-07-24-mtconnect-driver.md`](../plans/2026-07-24-mtconnect-driver.md) — the executable implementation plan and the task-by-task build record, including where it diverged from the design (TrakHound → hand-rolled)
- [`docs/plans/2026-07-24-driver-expansion-tracking.md`](../plans/2026-07-24-driver-expansion-tracking.md) — Wave-2 program tracking
- [Docker fixture README](../../tests/Drivers/ZB.MOM.WW.OtOpcUa.Driver.MTConnect.IntegrationTests/Docker/README.md) — the authoritative fixture recipe (seeded device model, endpoint, Mac AirPlay port-5000 gotcha)