CopilotSdkStreamAdapter default arm leaks [unknown-copilot-sdk-event: ...] placeholders into user-visible chat and persisted history (#1312 follow-up)

[JoshuaRowePhantom]

CopilotSdkStreamAdapter default arm leaks [unknown-copilot-sdk-event: ...] placeholders into user-visible chat and persisted history

Summary

The HTML agent-chat view is polluted with literal placeholder strings of the form [unknown-copilot-sdk-event: GitHub.Copilot.<EventTypeName>], interleaved char-by-char with the real assistant text. The observed placeholder types include AssistantStreamingDeltaEvent (appears many times, one per streaming byte-count ping — this is the source of the character-by-character interleaving), AssistantMessageStartEvent, AssistantTurnStartEvent, UserMessageEvent, SystemMessageEvent, PendingMessagesModifiedEvent, SessionToolsUpdatedEvent, SessionSkillsLoadedEvent, SessionCustomAgentsUpdatedEvent, and SessionUsageInfoEvent.

Root cause: the fix for #1312 added a default: arm to CopilotSdkStreamAdapter.TranslateCopilotSdkSessionEvents that emits a ChatRole.Assistant TextContent reading "[unknown-copilot-sdk-event: <RuntimeTypeName>]" for every unmapped SessionEvent subtype. That "surface it so it isn't invisibly lost" choice was principled, but in practice the SDK emits many unmapped events per turn — most notoriously AssistantStreamingDeltaEvent, a per-chunk streaming progress ping — so the default arm now sprays user-visible placeholder text throughout every response and (via StreamingPersistenceMiddleware) persists that noise into the transcript.

Root cause

The default arm (introduced by #1312)

Phantom.Workspaces.Llm.Core/CopilotSdkStreamAdapter.cs lines 260–282:

default:
    // Fix #1312: never silently drop UNKNOWN event kinds.
    var runtimeTypeName = sessionEvent?.GetType().FullName ?? "<null>";
    var runtimeAgentId = sessionEvent?.AgentId;
    logger.LogWarning(
        "Copilot SDK adapter received an unmapped session event of type {EventType} for AgentId {AgentId}; surfacing as a generic informational update.",
        runtimeTypeName,
        string.IsNullOrEmpty(runtimeAgentId) ? "<root>" : runtimeAgentId);
    var unknown = Tag(
        new TextContent($"[{UnknownCopilotSdkEventContentType}: {runtimeTypeName}]"),
        runtimeAgentId);
    unknown.AdditionalProperties![ContentTypePropertyName] = UnknownCopilotSdkEventContentType;
    yield return new ChatResponseUpdate
    {
        Role = ChatRole.Assistant,
        Contents = [unknown],
    };
    break;

This produces a ChatRole.Assistant update carrying a plain TextContent — indistinguishable from real assistant text as far as StreamingPersistenceMiddleware and the HTML view are concerned. UnknownCopilotSdkEventContentType is applied to AdditionalProperties, but nothing on the render / persistence path filters on it, so the string bleeds straight into the user-visible transcript and into history.

Which SDK event types actually fall through

Verified against C:\dev\microsoft\copilot-sdk\dotnet\src\Generated\SessionEvents.cs. All types below are real classes in the GitHub.Copilot namespace that inherit SessionEvent, and none of them appear in the adapter's switch:

SDK event class (file:line)	Why it fires
AssistantStreamingDeltaEvent (SessionEvents.cs:643)	Streaming progress ping (assistant.streaming_delta) with just TotalResponseSizeBytes. Emitted per-chunk, so appears dozens of times per turn — the source of the char-by-char interleaving.
AssistantMessageStartEvent (SessionEvents.cs:669)	Lifecycle: assistant message opening. Should be silent (or start a message).
AssistantTurnStartEvent (SessionEvents.cs:578)	Lifecycle: turn opening. Should be silent.
UserMessageEvent (SessionEvents.cs:552)	Echo of the user's own message. Should be silent (we already have it) or ignored.
SystemMessageEvent (SessionEvents.cs:956)	System-role bookkeeping. Should be silent or routed to ChatRole.System.
PendingMessagesModifiedEvent (SessionEvents.cs:565)	Bookkeeping. Silent.
SessionToolsUpdatedEvent (SessionEvents.cs:1334)	Session metadata update. Silent.
SessionSkillsLoadedEvent (SessionEvents.cs:1360)	Session metadata update. Silent.
SessionCustomAgentsUpdatedEvent (SessionEvents.cs:1373)	Session metadata update. Silent.
SessionUsageInfoEvent (SessionEvents.cs:500)	Usage info (not the same as the mapped AssistantUsageEvent). Silent, or map to UsageContent.

The AssistantStreamingDeltaEvent vs AssistantMessageDeltaEvent clarification

Both are real, sibling SDK classes:

• AssistantMessageDeltaEvent — event type assistant.message_delta, payload AssistantMessageDeltaData with a DeltaContent string (SessionEvents.cs:682 and :2643). This is the actual streaming assistant text and the adapter's case AssistantMessageDeltaEvent when !string.IsNullOrEmpty(delta.Data?.DeltaContent) at line 123 already handles it correctly.
• AssistantStreamingDeltaEvent — event type assistant.streaming_delta, payload AssistantStreamingDeltaData = { TotalResponseSizeBytes } only (SessionEvents.cs:643 and :2520). This is a byte-count progress ping, has no textual content, and is not handled — it falls through to the fix-Copilot SDK adapter silently drops unhandled SDK session events (no default arm; self-invoke bypasses framework tool loop) #1312 default arm and produces one [unknown-copilot-sdk-event: GitHub.Copilot.AssistantStreamingDeltaEvent] placeholder per streaming chunk.

The two names are easily confused, and #1312's own scope explicitly deferred reconciling the case list against the live SDK event set. This is the concrete manifestation of that deferred work.

The self-invoke bypass makes the adapter the single surface

Per #1312: CopilotSdkChatClient implements ISelfInvokingToolChatClient (CopilotSdkChatClient.cs:34), and AgentFactory.WrapWithMiddleware deliberately skips ToolResultSteeringMiddleware / FunctionInvokingChatClient for it. So whatever CopilotSdkStreamAdapter yields goes straight into the transcript. There is no downstream filter that discards UnknownCopilotSdkEventContentType, so the placeholder TextContent reaches both the HTML chat and StreamingPersistenceMiddleware, permanently corrupting history.

Affected files

File	Role
Phantom.Workspaces.Llm.Core/CopilotSdkStreamAdapter.cs	Primary — houses both the switch case list and the leaky default arm (lines 260–282).
Phantom.Workspaces.Llm.Core/CopilotSdkChatClient.cs	Mirror non-streaming switch at line 543; also calls TranslateCopilotSdkSessionEvents at 660.
Phantom.Workspaces.Llm.Core.Tests/CopilotSdkStreamAdapterTests.cs	Existing _UnknownEventKind_IsSurfacedNotDropped currently asserts the bad behaviour and must be revised.
docs/design/copilot-sdk-session-events.md	Reference doc for which SDK events should be silent/mapped/error; #1312 explicitly deferred reconciling this.

Design / Fix

The fix for #1312 rightly rejected silent-drop; the fix here is to keep observability (logging) while removing the user-visible / persisted placeholder.

1. Do not emit user-visible or persistable TextContent from the default arm. Change the default arm to log at Debug (or keep Warning once, then downgrade repeats — the streaming delta will spam Warning otherwise) but yield nothing to the ChatResponseUpdate stream. Retain the UnknownCopilotSdkEventContentType marker constant so the design intent is discoverable in the log message, but do not put the marker string into TextContent.

2. Explicitly no-op the known-benign lifecycle / metadata events, so they don't even reach the default arm and don't spam logs. Add:

   case AssistantStreamingDeltaEvent:              // per-chunk byte-count progress ping
   case AssistantMessageStartEvent:                // lifecycle: message opening
   case AssistantTurnStartEvent:                   // lifecycle: turn opening
   case AssistantTurnEndEvent:                     // lifecycle: turn closing
   case AssistantIdleEvent:                        // lifecycle: assistant idle
   case AssistantMessageEvent:                     // whole-message duplicate of the deltas
   case UserMessageEvent:                          // echo of caller's own input
   case SystemMessageEvent:                        // system-role bookkeeping (map to System role if we want it in history)
   case PendingMessagesModifiedEvent:              // bookkeeping
   case SessionToolsUpdatedEvent:
   case SessionSkillsLoadedEvent:
   case SessionCustomAgentsUpdatedEvent:
   case SessionUsageInfoEvent:                     // consider mapping to UsageContent instead of dropping
       break;

   Names verified against C:\dev\microsoft\copilot-sdk\dotnet\src\Generated\SessionEvents.cs.

3. Sketch:

   default:
       logger.LogDebug(
           "Copilot SDK adapter received an unmapped session event of type {EventType} for AgentId {AgentId}; no transcript output emitted.",
           sessionEvent?.GetType().FullName ?? "<null>",
           string.IsNullOrEmpty(sessionEvent?.AgentId) ? "<root>" : sessionEvent!.AgentId);
       break;

4. Mirror in the non-streaming path (CopilotSdkChatClient.GetResponseAsync, switch at line 543): its default arm also currently logs a Warning per unmapped event; downgrade to Debug (or per-type sample) for the same reason.

5. Update the design doc docs/design/copilot-sdk-session-events.md with the reconciled list of intentionally-consumed-silent lifecycle events and the mapping table above — this is the Copilot SDK adapter silently drops unhandled SDK session events (no default arm; self-invoke bypasses framework tool loop) #1312 (d) follow-up item that was deferred.

Relationship to #1312

This is the follow-up to #1312. #1312 explicitly deferred two things that this bug forces us to finish:

• (a) Reconciling the adapter's switch against the live SDK event set — needed because AssistantStreamingDeltaEvent, AssistantMessageStartEvent, UserMessageEvent, PendingMessagesModifiedEvent, SessionToolsUpdatedEvent, SessionSkillsLoadedEvent, SessionCustomAgentsUpdatedEvent, SessionUsageInfoEvent, and SystemMessageEvent are all real SDK types that were never added as case arms.
• (b) Choosing a surfacing mechanism that does not produce user-visible transcript pollution. Copilot SDK adapter silently drops unhandled SDK session events (no default arm; self-invoke bypasses framework tool loop) #1312 chose "emit a diagnostic TextContent"; in practice that puts noisy strings in front of the user and into persisted history. The fix is: log-only in the default arm; explicit silent no-op for known-benign lifecycle/metadata events.

Note on a plausible-but-wrong hypothesis worth recording: it looks at first glance like the adapter matches AssistantMessageDeltaEvent when it should have matched AssistantStreamingDeltaEvent. That is not the case — those are two different SDK classes, AssistantMessageDeltaEvent carries the DeltaContent string and IS the streaming text, and the adapter handles it correctly. The real trouble is that AssistantStreamingDeltaEvent (progress bytes only) is a separate, unmapped, high-frequency event and it is what the default arm is spraying into chat.

Expected tests

Added to Phantom.Workspaces.Llm.Core.Tests/CopilotSdkStreamAdapterTests.cs, matching the existing TranslateCopilotSdkSessionEvents_<Subject>_<Scenario>_<Outcome> naming used throughout that file.

Test	Purpose
TranslateCopilotSdkSessionEvents_AssistantStreamingDeltaEvent_ProducesNoVisibleContent	Feed one AssistantStreamingDeltaEvent (progress ping) and assert zero non-idle ChatResponseUpdates are yielded (and specifically no TextContent containing unknown-copilot-sdk-event).
TranslateCopilotSdkSessionEvents_AssistantMessageStartEvent_IsConsumedSilently	Same but for AssistantMessageStartEvent.
TranslateCopilotSdkSessionEvents_AssistantTurnStartEvent_IsConsumedSilently	Same for AssistantTurnStartEvent.
TranslateCopilotSdkSessionEvents_UserMessageEvent_IsConsumedSilently	Same for UserMessageEvent.
TranslateCopilotSdkSessionEvents_PendingMessagesModifiedEvent_IsConsumedSilently	Same for PendingMessagesModifiedEvent.
TranslateCopilotSdkSessionEvents_SessionToolsUpdatedEvent_IsConsumedSilently	Same for SessionToolsUpdatedEvent.
TranslateCopilotSdkSessionEvents_SessionSkillsLoadedEvent_IsConsumedSilently	Same for SessionSkillsLoadedEvent.
TranslateCopilotSdkSessionEvents_SessionCustomAgentsUpdatedEvent_IsConsumedSilently	Same for SessionCustomAgentsUpdatedEvent.
TranslateCopilotSdkSessionEvents_SessionUsageInfoEvent_IsConsumedSilently	Same for SessionUsageInfoEvent.
TranslateCopilotSdkSessionEvents_UnmappedEvent_DoesNotEmitUserVisiblePlaceholderText	Feed a truly unrecognised SessionEvent subclass and assert that no ChatResponseUpdate carries a TextContent whose text contains "unknown-copilot-sdk-event".
TranslateCopilotSdkSessionEvents_UnmappedEvent_LogsDebugWithTypeAndAgentId	Assert that the runtime type name and (agent id or <root> marker) still show up in a Debug-level log entry — observability preserved.
TranslateCopilotSdkSessionEvents_InterleavedStreamingDeltaAndText_ProducesOnlyText	Feed [AssistantStreamingDeltaEvent, AssistantMessageDeltaEvent("Hi"), AssistantStreamingDeltaEvent, AssistantMessageDeltaEvent(" there")] and assert the visible output is exactly "Hi" and " there" — the direct regression test for the screenshot.

Existing test TranslateCopilotSdkSessionEvents_UnknownEventKind_IsSurfacedNotDropped (line 585) currently asserts the bad behaviour and needs to be revised to assert the log-only behaviour instead.

[JoshuaRowePhantom]

Design

Root cause: CopilotSdkStreamAdapter.TranslateCopilotSdkSessionEvents default arm (#1312, lines 260-282) emits a user-visible ChatRole.Assistant TextContent reading [unknown-copilot-sdk-event: ] for every unmapped SessionEvent. High-frequency SDK events like AssistantStreamingDeltaEvent (per-chunk progress ping) fall through and spam the transcript char-by-char. CopilotSdkChatClient.GetResponseAsync mirrors the same warning pattern on the non-streaming path.

Fix:

1. Default arm in CopilotSdkStreamAdapter: replace user-visible emission with LogDebug — retain the runtime type name + AgentId in the log so unmapped events remain diagnosable, but yield nothing to the ChatResponseUpdate stream. Keep the UnknownCopilotSdkEventContentType constant (still referenced from the log message).
2. Explicit silent no-op arms in the streaming adapter switch, before the default:
   • AssistantStreamingDeltaEvent (progress ping)
   • AssistantMessageStartEvent / AssistantMessageEvent
   • AssistantTurnStartEvent / AssistantTurnEndEvent / AssistantIdleEvent
   • UserMessageEvent / SystemMessageEvent / PendingMessagesModifiedEvent
   • SessionToolsUpdatedEvent / SessionSkillsLoadedEvent / SessionCustomAgentsUpdatedEvent / SessionUsageInfoEvent
3. Non-streaming mirror (CopilotSdkChatClient.GetResponseAsync, ~line 559-583): downgrade the LogWarning to LogDebug for the same reason (a chatty AssistantStreamingDeltaEvent in a non-streaming turn would otherwise spam warning logs).
4. Revise existing tests in CopilotSdkStreamAdapterTests.cs: TranslateCopilotSdkSessionEvents_UnknownEventKind_IsSurfacedNotDropped and TranslateCopilotSdkSessionEvents_UnknownEventKindWithAgentId_TagsWithAgentId currently assert the bad behaviour (visible TextContent emitted) — rewrite them to assert log-only behaviour.
5. Add regression tests per the issue's expected-tests table, including the interleaved-streaming-delta regression that reproduces the screenshot.

Alternatives considered / rejected:

• Downstream filter on UnknownCopilotSdkEventContentType in StreamingPersistenceMiddleware / the HTML view: rejected — leaks the noise-emit-then-filter pattern into unrelated layers and doesn't address the ChatResponseUpdate stream itself.
• Silent-drop default arm without logging: rejected — Copilot SDK adapter silently drops unhandled SDK session events (no default arm; self-invoke bypasses framework tool loop) #1312's original intent was to keep unmapped events observable.

Files that will change:

• Phantom.Workspaces.Llm.Core/CopilotSdkStreamAdapter.cs
• Phantom.Workspaces.Llm.Core/CopilotSdkChatClient.cs
• Phantom.Workspaces.Llm.Core.Tests/CopilotSdkStreamAdapterTests.cs

[JoshuaRowePhantom]

Design

Root cause. CopilotSdkStreamAdapter.TranslateCopilotSdkSessionEvents default arm (added by #1312, lines 260-282) emits a ChatRole.Assistant TextContent reading [unknown-copilot-sdk-event: <TypeName>] for every unmapped SessionEvent subtype. High-frequency SDK events like AssistantStreamingDeltaEvent (per-chunk streaming byte-count ping) fall through and pollute the visible transcript and persisted history char-by-char. CopilotSdkChatClient.GetResponseAsync mirrors the same log pattern on the non-streaming path.

Fix.

1. In CopilotSdkStreamAdapter.TranslateCopilotSdkSessionEvents, add explicit no-op case arms for the known-benign SDK lifecycle / metadata event types verified against the current SDK (SessionEvents.cs) — AssistantStreamingDeltaEvent, AssistantMessageStartEvent, AssistantTurnStartEvent, AssistantTurnEndEvent, AssistantIdleEvent, AssistantMessageEvent, AssistantIntentEvent, AssistantReasoningEvent, AssistantToolCallDeltaEvent, UserMessageEvent, UserInputRequestedEvent, UserInputCompletedEvent, SystemMessageEvent, PendingMessagesModifiedEvent, plus the various Session*Event metadata / lifecycle types (SessionInfoEvent, SessionWarningEvent, SessionUsageInfoEvent, SessionUsageCheckpointEvent, SessionToolsUpdatedEvent, SessionSkillsLoadedEvent, SessionCustomAgentsUpdatedEvent, SessionMcpServersLoadedEvent, SessionMcpServerStatusChangedEvent, SessionExtensionsLoadedEvent, SessionExtensionsAttachmentsPushedEvent, SessionStartEvent, SessionResumeEvent, SessionShutdownEvent, SessionTitleChangedEvent, SessionModelChangeEvent, SessionModeChangedEvent, SessionRemoteSteerableChangedEvent, SessionSessionLimitsChangedEvent, SessionPermissionsChangedEvent, SessionPlanChangedEvent, SessionTodosChangedEvent, SessionWorkspaceFileChangedEvent, SessionHandoffEvent, SessionTruncationEvent, SessionSnapshotRewindEvent, SessionContextChangedEvent, SessionCompactionStartEvent, SessionCompactionCompleteEvent, SessionTaskCompleteEvent, SessionBinaryAssetEvent, SessionCustomNotificationEvent, SessionLimitsExhaustedRequestedEvent, SessionLimitsExhaustedCompletedEvent, SessionAutoModeResolvedEvent, SessionBackgroundTasksChangedEvent, SessionScheduleCreatedEvent, SessionScheduleCancelledEvent, SessionScheduleRearmedEvent, SessionAutopilotObjectiveChangedEvent, SessionCanvasOpenedEvent, SessionCanvasClosedEvent, SessionCanvasRegistryChangedEvent, SessionCanvasUnavailableEvent, SessionCanvasRecordedEvent, SessionCanvasRemovedEvent).
2. Change the default arm to log at Debug (per issue) and yield NOTHING to the transcript. Keep the UnknownCopilotSdkEventContentType marker constant available for callers/tests; do NOT emit a TextContent carrying that string.
3. Mirror the same downgrade in CopilotSdkChatClient.GetResponseAsync's subscription default arm: change LogWarning to LogDebug.
4. Revise existing tests that assert the pre-fix "surface as placeholder TextContent" behaviour and add tests for silent-consumption of the benign SDK events and log-only handling of a truly unknown SessionEvent subclass.

Design alternatives considered / rejected.

• Keep the placeholder TextContent but filter it downstream in StreamingPersistenceMiddleware / HTML render path — rejected: the issue owner explicitly prefers handling at the emission site so nothing leaks into any downstream consumer.
• Rate-limit the placeholder — rejected: still visible in the transcript.

Files to change.

• Phantom.Workspaces.Llm.Core/CopilotSdkStreamAdapter.cs
• Phantom.Workspaces.Llm.Core/CopilotSdkChatClient.cs
• Phantom.Workspaces.Llm.Core.Tests/CopilotSdkStreamAdapterTests.cs

[JoshuaRowePhantom]

Implementation notes

Commit: 8ca24b1

Deviations from proposed design

The design listed several benign SDK event types that are marked [Experimental("GHCP001")] on the current SDK (SessionBinaryAssetEvent, SessionAutoModeResolvedEvent, and the SessionCanvas* family). Adding explicit case arms for them would introduce a compile-time dependency on evaluation-only APIs (compiler error GHCP001). They were intentionally omitted from the explicit no-op list and instead fall through to the Debug-logging default arm — equally silent for the transcript, and no experimental API is referenced. A code comment documents this.

Scope changes

None beyond the above. Default arm downgraded to LogDebug and yields no transcript content; non-streaming mirror in CopilotSdkChatClient downgraded LogWarning -> LogDebug; design doc docs/design/copilot-sdk-session-events.md updated with the reconciled silent-consumption list.

Tests

Revised the four existing _UnknownEventKind_* tests (which asserted the old placeholder behaviour) and added: a [Theory] covering silent consumption of every non-experimental benign lifecycle/metadata event, _AssistantStreamingDeltaEvent_ProducesNoVisibleContent, _InterleavedStreamingDeltaAndText_ProducesOnlyText (screenshot regression), _UnmappedEvent_DoesNotEmitUserVisiblePlaceholderText, _UnmappedEvent_LogsDebugWithTypeAndAgentId, and _UnmappedEventRootAgent_LogsRootMarker (using a test-local genuinely-unknown SessionEvent subclass). All 47 CopilotSdkStreamAdapterTests pass.

Files changed

• Phantom.Workspaces.Llm.Core/CopilotSdkStreamAdapter.cs
• Phantom.Workspaces.Llm.Core/CopilotSdkChatClient.cs
• Phantom.Workspaces.Llm.Core.Tests/CopilotSdkStreamAdapterTests.cs
• docs/design/copilot-sdk-session-events.md

[JoshuaRowePhantom]

Verification result

Status: ✅ Verified
Issue: #1323 — CopilotSdkStreamAdapter default arm leaks [unknown-copilot-sdk-event: ...] placeholders into chat/history (#1312 follow-up)
Commit checked: 8ca24b1b Fix #1323: stop leaking unknown-copilot-sdk-event placeholders into chat

Feature coverage

• ✅ Default arm in CopilotSdkStreamAdapter.TranslateCopilotSdkSessionEvents no longer yields a user-visible TextContent; it now logs at Debug (with runtime type name + AgentId/<root> marker) and emits nothing to the ChatResponseUpdate stream.
• ✅ Explicit silent no-op case arms added for the known-benign SDK lifecycle/metadata events (AssistantStreamingDeltaEvent, AssistantMessageStartEvent, AssistantTurnStartEvent, UserMessageEvent, PendingMessagesModifiedEvent, SessionToolsUpdatedEvent, SessionSkillsLoadedEvent, SessionCustomAgentsUpdatedEvent, SessionUsageInfoEvent, and the wider reconciled Session* set) so they never reach the default arm.
• ✅ UnknownCopilotSdkEventContentType marker constant retained (docs/logs only), no longer injected into transcript text.
• ✅ Non-streaming mirror in CopilotSdkChatClient.GetResponseAsync default arm downgraded LogWarning → LogDebug.
• ✅ Design doc docs/design/copilot-sdk-session-events.md reconciled with the silent-consumption list (the deferred Copilot SDK adapter silently drops unhandled SDK session events (no default arm; self-invoke bypasses framework tool loop) #1312(d) item).
• ✅ [Experimental("GHCP001")] SDK event types intentionally omitted from explicit arms (avoids compile-time dependency on evaluation-only APIs) and documented in-code; they fall through to the equally-silent Debug default arm.

Test coverage

• ✅ Every expected test from the issue table exists. Individual per-event silent-consumption cases are covered by the [Theory] BenignLifecycleEvent_IsConsumedSilently data rows (AssistantMessageStart / AssistantTurnStart / UserMessage / PendingMessagesModified / SessionToolsUpdated / SessionSkillsLoaded / SessionCustomAgentsUpdated / SessionUsageInfo).
• ✅ AssistantStreamingDeltaEvent_ProducesNoVisibleContent, InterleavedStreamingDeltaAndText_ProducesOnlyText (screenshot regression), UnmappedEvent_DoesNotEmitUserVisiblePlaceholderText, UnmappedEvent_LogsDebugWithTypeAndAgentId, UnmappedEventRootAgent_LogsRootMarker all present.
• ✅ Pre-fix _UnknownEventKind_* tests that asserted the leaky placeholder behaviour were revised to assert log-only/no-emission behaviour.
• ✅ Branch coverage: default-arm agent-id vs <root> paths, benign silent path, and real-text path all exercised.
• ✅ No skipped/quarantined tests, no unresolved TODOs, no timing-dependent synchronization (uses channel completion), no dispatcher pumping, no synchronous task blocking.

Tests

8ca24b1b Fix #1323: stop leaking unknown-copilot-sdk-event placeholders into chat

[JoshuaRowePhantom]

Checked in

Commit: 880956e
Branch fast-forwarded: features → 880956e