MCPServerStreamableHTTP headers parameter not sent with HTTP requests

[tychodev321]

Initial Checks

• I confirm that I'm using the latest version of Pydantic AI
• I confirm that I searched for my issue in https://github.com/pydantic/pydantic-ai/issues before opening this issue

Description

Issue

When using MCPServerStreamableHTTP with the headers parameter, authentication headers are not being sent with HTTP requests to the MCP server, causing authentication failures.

Code Example

from pydantic_ai.mcp import MCPServerStreamableHTTP

# Headers are not sent to the server
mcp_server = MCPServerStreamableHTTP(
    url="http://localhost:8080/mcp",
    headers={"x-api-key": "my-api-key"}
)

Expected Behavior

Headers should be included in all HTTP requests to the MCP server, similar to how MCPServerTool handles the headers parameter in the documentation.

Question

Is this a bug, or is there a different recommended way to pass authentication headers to MCP servers using MCPServerStreamableHTTP?

Python, Pydantic AI & LLM client version

pydantic-ai version: 1.9.1
Python version: 3.13
httpx version: 0.28.1

[DouweM]

@tychodev321 We pass the headers into the MCP SDK here:

pydantic-ai/pydantic_ai_slim/pydantic_ai/mcp.py

Lines 757 to 758 in 9c31521

	async with transport_client_partial(headers=self.headers) as (read_stream, write_stream, *_):
	yield read_stream, write_stream

In the case of MCPServerStreamableHTTP, that transport_client_partial represents mcp.client.streameable_http.streamablehttp_client:

pydantic-ai/pydantic_ai_slim/pydantic_ai/mcp.py

Lines 867 to 869 in 9c31521

	@property
	def _transport_client(self):
	return streamablehttp_client # pragma: no cover

I just verified the headers are being passed into that function correctly. If it's still not working for you, it'd help to figure out where it's getting dropped, possibly inside the MCP SDK.

[tychodev321]

@DouweM Thank you for looking into it. I did more digging in my stack, and found that the FastMCP Middleware that is authenticating the MCP requests from the Agent was not working as expected. The Headers are making it through for all calls expect initialize, when I printed the headers during mcp initalize I saw empty headers, but it seems like that is now expected behavior with FastMCP.