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

Bắt đầu trong sdk/index.js, quanh createAgentCallbackBridge (~L4554), và so sánh với luồng main-agent gần ~L4541. Xem xét session-events.schema.json ở các dòng 520-522 và contract trong README, sau đó xác định liệu sub-agents có nên phát ra một sự kiện assistant.reasoning tương ứng hay tài liệu hóa dạng của assistant.message. Hoàn tất khi hành vi được chọn đã được triển khai nhất quán và được xác minh dựa trên chuỗi sự kiện reasoningId đã mô tả.

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.