[BUG] MCP Client does not propagate contextvars #56

Checks

• I have updated to the lastest minor and patch version of Strands
• I have checked the documentation and this is not expected behavior
• I have searched ./issues and there are no duplicates of my issue

Strands Version

1.21.1

Python Version

3.10

Operating System

MacOS 26.2

Installation Method

pip

Steps to Reproduce

# Factory function runs in background thread
mcp_client = MCPClient(lambda: streamablehttp_client(MCP_URL))

# Set Context Var in Current thread
context_var.set("some-value")

# The factory runs in BACKGROUND thread, so current contextvars do not propagate
# This makes then inaccessible to transports or auth logic that requires them
with mcp_client:
    # Make request requiring context var

Expected Behavior

Contextvar propagated to background thread and is accessible by transports.

Actual Behavior

New thread is created without propagating current contextvars

Additional Context

In MCPClient.start, the background thread is started with

 self._background_thread = threading.Thread(target=self._background_task, args=[], daemon=True)

This issue was fixed in strands-agents#1146 for tool invocations by propagating copying contextvars in the ThreadPoolExecutor.

Possible Solution

This can be fixed by copying context vars in the start method when starting the background thread.

# Copy the current context
ctx = contextvars.copy_context()

# Create the thread with the context
self._background_thread = threading.Thread(
    target=ctx.run,
    args=(self._background_task,),
    daemon=True
)

Related Issues

strands-agents#946, strands-agents#946