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+# Code Review — Driver.FOCAS.Contracts
+
+<!-- Template for a per-module findings file. Copy to code-reviews/<Module>/findings.md.
+     See ../../REVIEW-PROCESS.md for the full process. The base README.md is generated
+     from these files by regen-readme.py — do not edit README.md by hand. -->
+
+| Field | Value |
+|---|---|
+| Module | `src/Drivers/ZB.MOM.WW.OtOpcUa.Driver.FOCAS.Contracts` |
+| Reviewer | Claude Code |
+| Review date | 2026-06-19 |
+| Commit reviewed | `7286d320` |
+| Status | Reviewed |
+| Open findings | 1 |
+
+## Checklist coverage
+
+A comprehensive review completes every category, recording "No issues found" where
+a category produced nothing rather than leaving it blank.
+
+| # | Category | Result |
+|---|---|---|
+| 1 | Correctness & logic bugs | 1 finding (Driver.FOCAS.Contracts-001) |
+| 2 | OtOpcUa conventions | No issues found |
+| 3 | Concurrency & thread safety | No issues found |
+| 4 | Error handling & resilience | No issues found |
+| 5 | Security | No issues found |
+| 6 | Performance & resource management | No issues found |
+| 7 | Design-document adherence | 1 finding (Driver.FOCAS.Contracts-002) |
+| 8 | Code organization & conventions | No issues found |
+| 9 | Testing coverage | No test project — findings recorded, deferred by task scope |
+| 10 | Documentation & comments | 2 findings (Driver.FOCAS.Contracts-003, Driver.FOCAS.Contracts-004) |
+
+## Findings
+
+<!-- One ### entry per finding. IDs are <Module>-NNN, numbered sequentially within the module,
+     never reused. Findings are never deleted — close them by changing Status and
+     completing Resolution. -->
+
+### Driver.FOCAS.Contracts-001
+
+| Field | Value |
+|---|---|
+| Severity | Low |
+| Category | Correctness & logic bugs |
+| Location | `src/Drivers/ZB.MOM.WW.OtOpcUa.Driver.FOCAS.Contracts/FocasDriverOptions.cs:144` |
+| Status | Won't Fix |
+
+**Description:** `FocasTagDefinition`'s positional parameter `Writable` defaults to `true`. The original review hypothesised that FOCAS is read-only by design (claiming `WireFocasClient.WriteAsync` always returns `BadNotWritable` and the `def.Writable` gate is dead code) and that the default should therefore be `false`. **That premise was wrong** — FOCAS supports PMC/data writes, the `def.Writable` gate is live, and six `Driver.FOCAS.Tests` write tests (`FocasPmcBitRmwTests`, `FocasReadWriteTests`) rely on `Writable` defaulting to `true`.
+
+**Recommendation:** Leave the default at `true`. (The original recommendation to flip it to `false` is withdrawn.)
+
+**Resolution:** Won't Fix (re-triaged 2026-06-19). The "FOCAS is read-only" premise was incorrect — the consuming `Driver.FOCAS.Tests` write suites failed when the default was flipped to `false`. The default `Writable = true` is intentional and correct; the change was reverted. The accompanying `<param>` doc was kept but corrected to describe the live write path. (Caught by the orchestrator's consuming-driver-test safety net, which the no-test-project Contracts build could not exercise.)
+
+---
+
+### Driver.FOCAS.Contracts-002
+
+| Field | Value |
+|---|---|
+| Severity | Low |
+| Category | Design-document adherence |
+| Location | `src/Drivers/ZB.MOM.WW.OtOpcUa.Driver.FOCAS.Contracts/FocasDriverOptions.cs:139-145` |
+| Status | Open |
+
+**Description:** `FocasTagDefinition` carries a `WriteIdempotent` field (default `false`) that the FOCAS driver never reads anywhere — neither in `DiscoverAsync`, `ReadAsync`, nor `WriteAsync`. `DiscoverAsync` always passes `WriteIdempotent: false` to `DriverAttributeInfo` (hardcoded), so the field has no runtime effect. `docs/drivers/FOCAS.md` does not mention `WriteIdempotent` in its configuration tables, making its purpose undiscoverable to operators.
+
+**Recommendation:** Either (a) thread `FocasTagDefinition.WriteIdempotent` through to `DriverAttributeInfo` in `DiscoverAsync` so the field has runtime effect and matches the pattern of other drivers, or (b) remove it from the record and the `FocasDriverConfigDto` wire DTO. Option (a) is the safer fix and matches the design of Modbus and other drivers. This is deferred because the fix touches `FocasDriver.DiscoverAsync` outside this module and requires a driver-level test change.
+
+**Resolution:** _(empty until closed)_
+
+---
+
+### Driver.FOCAS.Contracts-003
+
+| Field | Value |
+|---|---|
+| Severity | Low |
+| Category | Documentation & comments |
+| Location | `src/Drivers/ZB.MOM.WW.OtOpcUa.Driver.FOCAS.Contracts/FocasDriverOptions.cs:147-155` |
+| Status | Resolved |
+
+**Description:** `FocasProbeOptions` is the only class in this module without a class-level XML `<summary>` comment. All other options classes (`FocasDriverOptions`, `FocasFixedTreeOptions`, `FocasHandleRecycleOptions`, `FocasAlarmProjectionOptions`) have class-level summaries. This inconsistency causes IDE tooltips and XML-doc generators to produce a blank description for `FocasProbeOptions`.
+
+**Recommendation:** Add a class-level `<summary>` consistent with the pattern of the other options types: `Controls periodic connectivity probing. One <c>cnc_rdcncstat</c> call per configured device per tick; transitions fire <c>OnHostStatusChanged</c>.`
+
+**Resolution:** Fixed in the same commit as this review. Added XML `<summary>` to `FocasProbeOptions`. Verified by build (no test project).
+
+---
+
+### Driver.FOCAS.Contracts-004
+
+| Field | Value |
+|---|---|
+| Severity | Low |
+| Category | Documentation & comments |
+| Location | `src/Drivers/ZB.MOM.WW.OtOpcUa.Driver.FOCAS.Contracts/FocasDriverOptions.cs:107-131` |
+| Status | Resolved |
+
+**Description:** `FocasDeviceOptions`'s class-level `<summary>` references `<paramref name="Series"/>` but there is no corresponding `<param name="Series">` documentation block. Only `PositionDecimalPlaces` has a `<param>` entry; `HostAddress`, `DeviceName`, and `Series` are all undocumented at the parameter level. This leaves the expected URI form of `HostAddress` (`focas://{ip}[:{port}]`) and the valid values of `Series` undiscoverable from the XML doc.
+
+**Recommendation:** Add `<param>` entries for `HostAddress`, `DeviceName`, and `Series` documenting the expected URI form for `HostAddress` and referring to `FocasCncSeries.Unknown` as the permissive default for `Series`.
+
+**Resolution:** Fixed in the same commit as this review. Added `<param>` doc entries for `HostAddress`, `DeviceName`, and `Series` to `FocasDeviceOptions`. Verified by build (no test project).
diff --git a/src/Drivers/ZB.MOM.WW.OtOpcUa.Driver.FOCAS.Contracts/FocasDriverOptions.cs b/src/Drivers/ZB.MOM.WW.OtOpcUa.Driver.FOCAS.Contracts/FocasDriverOptions.cs
index c133937b..61e5e3ad 100644
--- a/src/Drivers/ZB.MOM.WW.OtOpcUa.Driver.FOCAS.Contracts/FocasDriverOptions.cs
+++ b/src/Drivers/ZB.MOM.WW.OtOpcUa.Driver.FOCAS.Contracts/FocasDriverOptions.cs
@@ -109,6 +109,20 @@ public sealed class FocasAlarmProjectionOptions
 ///     address validation at <c>FocasDriver.InitializeAsync</c>; leave as
 ///     <see cref="FocasCncSeries.Unknown"/> to skip validation (legacy behaviour).
 /// </summary>
+/// <param name="HostAddress">
+///     FOCAS TCP endpoint in <c>focas://{ip}[:{port}]</c> form, e.g.
+///     <c>focas://10.20.30.40:8193</c>. The port defaults to 8193 when omitted.
+///     Parsed by <c>FocasHostAddress.TryParse</c> at <c>FocasDriver.InitializeAsync</c>.
+/// </param>
+/// <param name="DeviceName">
+///     Optional human-readable label shown in the OPC UA address-space folder for this
+///     device. Defaults to <paramref name="HostAddress"/> when <c>null</c>.
+/// </param>
+/// <param name="Series">
+///     CNC controller series. Used by <see cref="FocasCapabilityMatrix"/> to gate which
+///     FOCAS addresses are valid. Leave as <see cref="FocasCncSeries.Unknown"/> (the
+///     default) to skip range validation and preserve legacy behaviour.
+/// </param>
 /// <param name="PositionDecimalPlaces">
 ///     Axis positions returned by <c>cnc_rddynamic2</c> are scaled integers. The driver
 ///     divides AbsolutePosition / MachinePosition / RelativePosition / DistanceToGo by
@@ -136,6 +150,17 @@ public sealed record FocasDeviceOptions(
 ///     address string that parses via <c>FocasAddress.TryParse</c> —
 ///     <c>X0.0</c> / <c>R100</c> / <c>PARAM:1815/0</c> / <c>MACRO:500</c>.
 /// </summary>
+/// <param name="Writable">
+///     Whether the tag is declared writable in config. Reflected at the
+///     <c>DiscoverAsync</c> address-space advertisement seam and gates the driver's
+///     write path — FOCAS supports PMC/data writes (see <c>FocasPmcBitRmwTests</c> /
+///     <c>FocasReadWriteTests</c>). Defaults to <c>true</c>.
+/// </param>
+/// <param name="WriteIdempotent">
+///     Whether repeated writes of the same value are safe. Carried for parity; not yet
+///     threaded through to <c>DriverAttributeInfo</c> in <c>DiscoverAsync</c> (see
+///     Driver.FOCAS.Contracts-002). Defaults to <c>false</c>.
+/// </param>
 public sealed record FocasTagDefinition(
     string Name,
     string DeviceHostAddress,
@@ -144,6 +169,12 @@ public sealed record FocasTagDefinition(
     bool Writable = true,
     bool WriteIdempotent = false);
 
+/// <summary>
+///     Controls periodic connectivity probing. One <c>cnc_rdcncstat</c> call per
+///     configured device per tick; transitions fire <c>OnHostStatusChanged</c>.
+///     Enabled by default so the driver surfaces a fast host-state transition when
+///     a CNC goes offline between data reads.
+/// </summary>
 public sealed class FocasProbeOptions
 {
     /// <summary>Gets or sets a value indicating whether probing is enabled.</summary>