fix(core): context preservation in shielded async callbacks

[Copilot]

The @shielded decorator in async callback managers was not preserving context variables, breaking OpenTelemetry instrumentation and other context-dependent functionality.

Problem

When using async callbacks with the @shielded decorator (applied to methods like on_llm_end, on_chain_end, etc.), context variables were not being preserved across the shield boundary. This caused issues with:

• OpenTelemetry span context propagation
• Other instrumentation that relies on context variables
• Inconsistent context behavior between sync and async execution

The issue was reproducible with:

from contextvars import copy_context
import asyncio
from langgraph.graph import StateGraph

# Sync case: context remains consistent
print("SYNC")
print(copy_context())  # Same object
graph.invoke({"result": "init"})
print(copy_context())  # Same object

# Async case: context was inconsistent (before fix)
print("ASYNC") 
asyncio.run(graph.ainvoke({"result": "init"}))
print(copy_context())  # Different object than expected

Root Cause

The original shielded decorator implementation:

async def wrapped(*args: Any, **kwargs: Any) -> Any:
    return await asyncio.shield(func(*args, **kwargs))

Used asyncio.shield() directly without preserving the current execution context, causing context variables to be lost.

Solution

Modified the shielded decorator to:

1. Capture the current context using copy_context()
2. Create a task with explicit context using asyncio.create_task(coro, context=ctx) for Python 3.11+
3. Shield the context-aware task
4. Fallback to regular task creation for Python < 3.11

async def wrapped(*args: Any, **kwargs: Any) -> Any:
    # Capture the current context to preserve context variables
    ctx = copy_context()
    coro = func(*args, **kwargs)
    
    try:
        # Create a task with the captured context to preserve context variables
        task = asyncio.create_task(coro, context=ctx)
        return await asyncio.shield(task)
    except TypeError:
        # Python < 3.11 fallback
        task = asyncio.create_task(coro)
        return await asyncio.shield(task)

Testing

• Added comprehensive test test_shielded_callback_context_preservation() that validates context variables are preserved across shielded callback boundaries
• Verified the fix resolves the original LangGraph context consistency issue
• Confirmed all existing callback manager tests still pass
• Validated OpenTelemetry-like instrumentation scenarios work correctly

The fix is minimal, maintains backward compatibility, and ensures proper context preservation for both modern Python versions and older ones.

Fixes #31398.

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1 Skipped Deployment

Project	Deployment	Preview	Comments	Updated (UTC)
langchain	⬜️ Ignored	Preview		Aug 11, 2025 10:33pm

[codspeed-hq]

CodSpeed Performance Report

Merging #32163 will not alter performance

_{Comparing copilot/fix-31398 (2174a80) with master (ca1a3fb)}

⚠️ Unknown Walltime execution environment detected

Using the Walltime instrument on standard Hosted Runners will lead to inconsistent data.

For the most accurate results, we recommend using CodSpeed Macro Runners: bare-metal machines fine-tuned for performance measurement consistency.

Summary

✅ 13 untouched
⏩ 21 skipped¹

Footnotes

1. 21 benchmarks were skipped, so the baseline results were used instead. If they were deleted from the codebase, click here and archive them to remove them from the performance reports. ↩

[codspeed-hq]

CodSpeed Instrumentation Performance Report

Merging #32163 will not alter performance

_{Comparing copilot/fix-31398 (8959773) with master (166c027)}

Summary

✅ 14 untouched benchmarks