Sub-agents stream `assistant.reasoning_delta` but never emit the `assistant.reasoning` close event documented as always-sent

Start in sdk/index.js around createAgentCallbackBridge (~L4554) and compare it with the main-agent path near ~L4541. Review session-events.schema.json lines 520-522 and the README contract, then determine whether sub-agents should emit a matching assistant.reasoning event or document the assistant.message shape. Done means the chosen behavior is implemented consistently and verified against the described reasoningId event sequence.

Summary

Per the Python SDK README:

Note: assistant.message and assistant.reasoning (final events) are always sent regardless of streaming setting.

And per the schema docstring on AssistantReasoningDeltaData.reasoningId (session-events.schema.json line 520-522):

Reasoning block ID this delta belongs to, matching the corresponding assistant.reasoning event.

In practice, sub-agents stream assistant.reasoning_delta events (with agentId set on the event), but the matching assistant.reasoning close is never emitted. Instead the reasoning content lands in the sub-agent's assistant.message.reasoningText / reasoningOpaque fields. Consumers that rely on the documented per-reasoningId close event end up with permanently-orphaned streaming reasoning blocks for every sub-agent turn.

Environment

• @github/copilot (CLI bundle): 1.0.49
• github-copilot-sdk (Python): 1.0.0b4
• Default provider (not BYOK)
• Reasoning model in use (Claude Opus / Sonnet via the default backend)

What I'd expect (per docs)

For every sub-agent reasoning block: one or more assistant.reasoning_delta events with a given reasoningId, followed by exactly one assistant.reasoning event carrying that same reasoningId and the complete content — symmetric with master-agent reasoning.

What actually happens

Code path (built bundle, sdk/index.js ~ L4554, inside createAgentCallbackBridge):

case "reasoning":
  u.reasoningId && u.deltaContent &&
    this.emitEphemeral("assistant.reasoning_delta",
      { reasoningId: u.reasoningId, deltaContent: u.deltaContent }, r);
// ...later, when the sub-agent's response completes:
this.emit("assistant.message",
  { parentToolCallId: r, messageId: kr(), ..., reasoningOpaque, reasoningText }, r);
// No paired emit("assistant.reasoning", ...) here.

Compare to the main-agent path (sdk/index.js ~ L4541) which does:

this.emit("assistant.message", { ... });
Ho && this.emitEphemeral("assistant.reasoning", { reasoningId: Ho, content: Yi ?? "" });

So the close emit is conditional on the main-agent code path and absent from the sub-agent bridge.

Empirical evidence

Across six debug-mode session JSONL transcripts (~17K reasoning_delta events total) captured from a production run with multiple custom sub-agents (planner, implementer, profiler_agent):

• Every assistant.reasoning_delta carrying a sub-agent's agentId is never followed by a matching assistant.reasoning.
• Sub-agent assistant.message events for those same turns do carry non-empty reasoningText (often several KB) and reasoningOpaque, so the data is being retained — it just travels via the message payload instead of the documented close event.
• The two log files in the same run that contain no sub-agent activity (master-only) have zero orphan reasoning streams. The four files with sub-agent activity have 25 orphan streams collectively. The pattern is deterministic and 1:1 with sub-agent boundaries.

Suggested fixes (either or both)

1. Behavior fix: also emit assistant.reasoning from the sub-agent bridge after endCurrentStreamingMessage, mirroring the main-agent path. This keeps the documented contract (1:1 reasoning_delta → assistant.reasoning) and matches downstream consumer expectations.
2. Documentation fix: if the embed-in-assistant.message shape is intentional for sub-agents, document it explicitly on AssistantReasoningDeltaData (e.g. "for sub-agent reasoning, the closing assistant.message event for the same agentId carries the complete content in reasoningText / reasoningOpaque; no separate assistant.reasoning event is emitted") and in the streaming-events guide referenced by #922.

Related: #922 (CLOSED — "Extended thinking/reasoning events not firing for OpenAI and Anthropic models via Copilot SDK"), #1064 (CLOSED — "BYOK Anthropic provider missing multiple session events ... reasoning ..."). Same family of bug but neither covers this specific main-vs-sub-agent asymmetry on the default provider.

Happy to provide the full JSONL fixtures if helpful.