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Python: [Bug]: mcp server can not receive self-define header #7161
Description
Description
I define mcp and function_middleware below, but when "tools/call", mcp server can not receive self-define header Authorization:
mcp_server = MCPStreamableHTTPTool(
name="powergridcasemcp",
load_prompts=False,
url="http://127.0.0.1:8000/mcp",
header_provider=header_provider=lambda kwargs: {"Authorization": f"Bearer {kwargs['mcp_api_key']}"},
)
@function_middleware
async def function_inject_kwargs(context, call_next) -> None:
"""Middleware that adds tool-only runtime values before execution."""
context.kwargs.setdefault("mcp_api_key", "123")
await call_next()i debug find:
_mcp.py 3106 token = _mcp_call_headers.set(headers) exec set success
_mcp.py 3074 headers = _mcp_call_headers.get({}) exec get {}
_mcp.py 113 _mcp_call_headers: contextvars.ContextVar[dict[str, str]] = contextvars.ContextVar("_mcp_call_headers")
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Package Versions
agent-framework-core: 1.11.0
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github-actions commentedon Jul 17, 2026
🤖 Automated triage reproduction notes (agent-authored — trust but verify)
Agent analysis
Repro: MCPStreamableHTTPTool.call_tool() in packages/core/agent_framework/_mcp.py sets _mcp_call_headers ContextVar at line 3106, but the _inject_headers httpx event hook at line 3074 reads it in the MCP transport's post_writer task context (spawned during connect via streamable_http_client), which never has the var set. Minimal repro: create an MCPStreamableHTTPTool with a header_provider, connect it, then call call_tool—the headers will be empty in the actual HTTP request because the transport's task group was created before call_tool set the ContextVar. A fix must propagate headers through a mechanism that crosses task boundaries (e.g., attaching headers to the session message metadata or write stream rather than relying on contextvars).
python/packages/core/tests/core/test_mcp_header_contextvar_bug.py1.11.01.11.0robbiebusinessacc commentedon Jul 17, 2026
I'd like to take this one — could you assign it to me?
The reporter's debugging and the triage notes match what I see in the code:
_inject_headersruns inside the transport'spost_writertask, which got its own copy of the context when the connection was opened, so the_mcp_call_headers.set()that happens later incall_tool()is never visible there and the hook reads{}. Since a ContextVar can't cross that task boundary, the headers need to travel another way — either a per-call snapshot on the tool instance that the hook falls back to, or propagation through the request metadata as the triage notes suggest. I'll start with a regression test that fails on current main, and can go with whichever fix shape you prefer.moonbox3 commentedon Jul 20, 2026
Please have at it, Robbie Walmsley (@robbiebusinessacc). Let us know when there's a PR to review. Thank you.