claude-agent-sdk 0.1.61: interleaved subagent Bash tool span dropped (only beta survives)

[AbhiPrasad]

Summary

With claude-agent-sdk==0.1.61, two interleaved-subagent tests fail because only one of the two concurrent subagents' Bash tool spans ends up in the emitted tool-span list. The cassette contains both subagents' tool_use + tool_result events, but our tracing code only produces a single Bash tool span (the second subagent's).

Pinning latest back to claude-agent-sdk==0.1.60 in py/pyproject.toml makes the full test_claude_agent_sdk(latest) session pass (46/46). The 0.1.60 → 0.1.61 bump surfaced a real behavior change in the SDK's event stream that our integration code can't currently handle.

Affected tests

Both live in py/src/braintrust/integrations/claude_agent_sdk/test_claude_agent_sdk.py and share a cassette via _sdk_cassette_name("test_interleaved_subagent_tool_output_preserved", min_version="0.1.11"):

• test_interleaved_subagent_tool_spans_preserve_output
• test_interleaved_subagent_tool_spans_parent_to_correct_llm

Both fail with:

AssertionError: Expected a tool span output containing 'alpha-bash-output'

The bash_spans list contains only one entry, with input.command == "echo beta-bash-output". Alpha's Bash span is missing / has no output.

What the cassette contains (0.1.61 recording)

Events in py/src/braintrust/integrations/claude_agent_sdk/cassettes/latest/test_interleaved_subagent_tool_output_preserved.json (as recorded against claude-agent-sdk==0.1.61):

• One echo alpha-bash-output command block with matching tool_use_id / parent_tool_use_id / subagent markers.
• One echo beta-bash-output command block with matching markers.
• 9 occurrences each of alpha-bash-output and beta-bash-output across events (tool_use + tool_result payloads).

So both subagent Bash invocations are present in the transport stream. The regression is not the model skipping a call; it is our tracing code only emitting a span for one of them.

Reproduction

cd py
# bump matrix key back to 0.1.61 temporarily
# [tool.braintrust.matrix.claude-agent-sdk]
# latest = "claude-agent-sdk==0.1.61"

rm -rf src/braintrust/integrations/claude_agent_sdk/cassettes/latest
mkdir -p src/braintrust/integrations/claude_agent_sdk/cassettes/latest
BRAINTRUST_CLAUDE_AGENT_SDK_RECORD_MODE=all nox -s "test_claude_agent_sdk(latest)"
# -> 44 pass, the two interleaved tests fail during record

nox -s "test_claude_agent_sdk(latest)" -- -k "test_interleaved_subagent_tool_spans"
# -> fails on replay too; assertion on alpha-bash-output

Likely cause

Something changed between claude-agent-sdk==0.1.60 and ==0.1.61 in how interleaved subagent events are emitted (order, parent_tool_use_id wiring, or tool-use-id scoping for concurrent delegated agents). Our integration's span emission / output attachment logic in py/src/braintrust/integrations/claude_agent_sdk/ (likely tracing.py and the subagent bookkeeping) drops or fails to attach output to one of the two concurrent Bash spans.

All non-interleaved concurrent / parallel subagent tests still pass:

• test_concurrent_subagents_produce_parallel_llm_spans_with_correct_parenting
• test_five_parallel_bundled_subagents_preserve_task_parenting
• test_multiple_bundled_subagents_keep_outer_orchestration_separate

So the regression is specific to the interleaved case (both Task tool_uses in flight before either result returns).

Temporary mitigation

Reverted the bump for claude-agent-sdk latest from 0.1.61 back to 0.1.60 in py/pyproject.toml so the daily dependency PR can merge. Once this bug is fixed, bump back to 0.1.61 (or whatever is current) and re-record the cassette.

Next steps

1. Diff claude-agent-sdk 0.1.60 → 0.1.61 release notes / source for event-stream changes, especially around Task tool handling and parent_tool_use_id emission timing.
2. Inspect the 0.1.61 cassette (events 5–14) to see exactly how alpha and beta are interleaved.
3. Fix the tracing code so alpha's Bash span is emitted with its output attached in the interleaved case.
4. Re-record the cassette and bump latest to the current claude-agent-sdk release.

Context

Surfaced during the daily dependency update in commit 95cc341 on branch deps/daily-update-2026-04-17.