Bug

When start_as_current_observation(end_on_exit=False) is closed from a different execution context than the one it opened in (normal in async code where the manual close runs on another task), the span-activation context token is detached in a context it was not created in. _start_as_current_otel_span_with_processed_media relied on OTel's start_as_current_span, whose internal context.detach is unguarded, so OTel logs Failed to detach context (ValueError: Token was created in a different Context) on every such close. Langfuse only guarded its own baggage token.

Fixes langfuse/langfuse#13590.

Fix

Activate the span directly (start_span + context.attach) and detach the span-activation token through the existing _detach_context_token_safely helper, which was built for exactly this case and already guards the baggage token. The span is ended explicitly when end_on_exit is set. Exception handling matches OTel's start_as_current_span defaults (record_exception + error status), so error observations are unchanged.

Verification

New unit test tests/unit/test_context_detach.py opens an observation, closes it on a copied asyncio context, and asserts no detach error is logged. Fails on main, passes with this change. Existing tests/unit otel/propagation/app-root suites stay green (221 passed, 2 skipped).

Greptile Summary

The PR replaces OpenTelemetry’s unguarded current-span context manager with explicit span activation and safe token detachment, preserving exception recording and conditional span ending.

• Creates spans with start_span and activates them through the OTel context API.
• Routes baggage and span tokens through the existing safe-detach helper.
• Adds an async cross-context regression test for detach-error logging.

Confidence Score: 4/5

The implementation appears safe to merge, with a non-blocking test-isolation issue that should be addressed to prevent ambient span leakage between tests.

The production lifecycle change preserves the prior span behavior while safely handling cross-context detachment; only the regression test leaves stale ambient context after it returns.

Files Needing Attention: tests/unit/test_context_detach.py

Sequence Diagram

sequenceDiagram
    participant Caller
    participant Langfuse
    participant Context as OTel Context
    participant Span as OTel Span
    Caller->>Langfuse: Enter current observation
    Langfuse->>Span: start_span()
    Langfuse->>Context: attach(span context)
    Langfuse-->>Caller: observation wrapper
    Caller->>Langfuse: Exit from copied async context
    Langfuse->>Context: safely detach baggage token
    Langfuse->>Context: safely detach span token
    Langfuse->>Span: end() when end_on_exit
    Note over Context: Opening context remains unchanged when exit occurs in a copy

### Issue 1
tests/unit/test_context_detach.py:32
**Test leaks the active span**

The observation is entered in the test's root context but exited only in a copied task context, so safe detachment cannot reset the token in the opening context. The test therefore returns with an ended span still current, making later context-sensitive tests order-dependent or assigning new observations the wrong parent.

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For each issue above, determine whether it is valid and should be fixed. If so, fix it directly.

Reviews (1): Last reviewed commit: "fix(client): route span-activation detac..." | Re-trigger Greptile

Context used:

• Knowledge Base — Client Core
• Knowledge Base — OTel Span Processing and Export Pipeline