fix(langchain): clean up streaming run memo on error

[dhionson2]

Summary

Fixes #1977.

This PR fixes stale run ID retention in LangchainCallbackHandler when a streamed LLM execution terminates with an error.

Root cause

on_llm_new_token() adds the run ID to _updated_completion_start_time_memo after recording the first-token timestamp.

While on_llm_end() removes the run ID in its finally block, on_llm_error() did not perform the same cleanup.

This could cause incremental UUID retention when long-lived callback handlers are reused across failed streaming executions.

Changes

• Add per-run memo cleanup to the finally block of on_llm_error().
• Add a regression test covering a failed streamed run and another active run.
• Verify that cleanup removes only the failed run ID.
• Ensure the remaining active run is properly finalized during test teardown.

The change preserves existing public SDK behavior and LangGraph control-flow handling.

Verification

Regression test

• RED: confirmed the failed run ID remained in the memo.
• GREEN: confirmed the ID is removed after the fix.

Targeted validation

• tests/unit/test_langchain.py: 24 passed.
• Ruff lint and formatting: passed.
• git diff --check: passed.

Full unit suite on Windows

• 883 passed
• 2 skipped
• 3 failed outside the modified test module: two subprocess import failures involving pydantic and one Windows/POSIX path serialization mismatch.

The full suite has not yet been confirmed green on Linux. CI validation is pending.

Scope

This is a minimal, backward-compatible fix limited to the LangChain callback implementation and its regression test.

No dependency, generated API, or public interface changes.

Confidence Score: 5/5

This PR appears safe to merge; no actionable issues were found.

Summary

This PR removes a streamed run’s first-token memo entry when on_llm_error finishes.

• Errored LLM streams clear their own first-token memo.

_{Reviews (1) · Last reviewed commit: "fix(langchain): clean up streaming run m..." · Reviewed by Greptile}

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Codex Review Summary

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Review	Status	Commit	Review trigger
📝 Code Review	✅ Completed 2026-10-11T01:55:45.069099Z	11ac8a0	PR opened

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[chrikrah]

@dhionson2 approving at 11ac8a0. This fixes #1977, and the new test fails when I put back the old on_llm_error.

$ python -m pytest tests/unit/test_langchain.py -q -p no:cacheprovider    # Python 3.14, uv sync --frozen
# head 11ac8a0
24 passed
# head tests, CallbackHandler.py from base bf11ec1
1 failed, 23 passed
FAILED tests/unit/test_langchain.py::test_streamed_llm_error_cleans_only_its_completion_start_time_memo

@hassiebp could you review this one?