fix(langchain): preserve error description on span status

[glogiotatidis]

Span.set_status(...) per the OTel SDK contract silently drops the description argument and emits a warning when the first argument is already a Status object:

Description ... ignored. Use either `Status` or `(StatusCode, Description)`

TraceloopCallbackHandler._handle_error was calling

span.set_status(Status(StatusCode.ERROR), str(error))

so the human-readable error message was never set on the span's status, and every callback error caused a warning to be logged from opentelemetry.sdk.trace.

Pass the description inside Status(...) instead — matching the canonical pattern used elsewhere in the openllmetry monorepo (see PR #3970 for anthropic/groq/mistralai).

Adds tests/test_error_status.py as a regression test that pins the behaviour: the span's status description equals the exception message, and no Description ... ignored warning is emitted.

• I have added tests that cover my changes.
• If adding a new instrumentation or changing an existing one, I've added screenshots from some observability platform showing the change.
• PR name follows conventional commits format: feat(instrumentation): ... or fix(instrumentation): ....
• (If applicable) I have updated the documentation accordingly.

Summary by CodeRabbit

• Bug Fixes

  • Error spans now include the exception message in their status description, ensuring tracing captures full error context and avoids lost descriptions.

• Tests

  • Added a regression test that verifies run-time errors produce spans marked ERROR with the exact exception message in the status description and that no SDK warnings about dropped descriptions are emitted.

[CLAassistant]

All committers have signed the CLA.

[coderabbitai]

No actionable comments were generated in the recent review. 🎉

ℹ️ Recent review info

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Configuration used: defaults

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📥 Commits

Reviewing files that changed from the base of the PR and between ad68e02 and aab61fd.

📒 Files selected for processing (2)

• packages/opentelemetry-instrumentation-langchain/opentelemetry/instrumentation/langchain/callback_handler.py
• packages/opentelemetry-instrumentation-langchain/tests/test_error_status.py

🚧 Files skipped from review as they are similar to previous changes (1)

• packages/opentelemetry-instrumentation-langchain/tests/test_error_status.py

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📝 Walkthrough

Walkthrough

Span status setting in the LangChain OpenTelemetry callback was corrected to embed the exception message in the Status object passed to span.set_status, and a regression test was added to assert exported spans carry the exception message in status.description.

Changes

Error Status Handling Fix

Layer / File(s)	Summary
Core Bug Fix packages/opentelemetry-instrumentation-langchain/opentelemetry/instrumentation/langchain/callback_handler.py	_handle_error now calls span.set_status(Status(StatusCode.ERROR, str(error))), placing the exception message inside the Status object rather than passing it as a separate set_status argument.
Tests / Regression packages/opentelemetry-instrumentation-langchain/tests/test_error_status.py	Adds _explode(_) helper and test_chain_error_sets_status_description that runs a failing RunnableLambda, collects finished spans, asserts at least one span has status.status_code == StatusCode.ERROR, checks each ERROR span's status.description == "boom: something went wrong", and ensures no SDK warning about ignored descriptions was emitted.

Estimated code review effort

🎯 2 (Simple) | ⏱️ ~10 minutes

Poem

A rabbit hops through error traces,
and tucks descriptions in the right places.
No warnings left to make a fuss,
the spans now whisper what went wrong to us. 🐇

🚥 Pre-merge checks | ✅ 4 | ❌ 1

❌ Failed checks (1 warning)

Check name	Status	Explanation	Resolution
Docstring Coverage	⚠️ Warning	Docstring coverage is 66.67% which is insufficient. The required threshold is 80.00%.	Write docstrings for the functions missing them to satisfy the coverage threshold.

✅ Passed checks (4 passed)

Check name	Status	Explanation
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The title clearly and concisely summarizes the main fix: preserving error description on span status in the langchain instrumentation, which is the primary change in the changeset.
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.

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✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests

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[coderabbitai]

Actionable comments posted: 1

🤖 Prompt for all review comments with AI agents

Verify each finding against the current code and only fix it if needed.

Inline comments:
In `@packages/opentelemetry-instrumentation-langchain/tests/test_error_status.py`:
- Around line 38-52: The test is flakey because the session-scoped
InMemorySpanExporter (span_exporter) accumulates spans across tests; to isolate
this test call span_exporter.clear() at the start of the test_error_status
function (before the chain runs and before calling
span_exporter.get_finished_spans()) so get_finished_spans() only returns spans
produced by this test and the subsequent assertions on span.status.description
are reliable.

🪄 Autofix (Beta)

Fix all unresolved CodeRabbit comments on this PR:

• Push a commit to this branch (recommended)
• Create a new PR with the fixes

---

ℹ️ Review info

⚙️ Run configuration

Configuration used: defaults

Review profile: CHILL

Plan: Pro

Run ID: 1d260691-e20a-4e39-80ba-55aea839ba52

📥 Commits

Reviewing files that changed from the base of the PR and between abbcc11 and 47447e0.

📒 Files selected for processing (2)

• packages/opentelemetry-instrumentation-langchain/opentelemetry/instrumentation/langchain/callback_handler.py
• packages/opentelemetry-instrumentation-langchain/tests/test_error_status.py