Problem

When an MCP server is created with FastMCP.from_openapi() and served over Streamable HTTP, the get_http_headers() call in OpenAPITool.run() forwards the incoming MCP transport's Authorization header to the downstream API, overwriting the API key configured on the httpx client.

This means the downstream API receives the MCP client's auth token instead of the intended API key, causing auth failures (503/401/403).

This is a security issue — MCP transport credentials should never leak to downstream third-party APIs.

Root Cause

Two problems in OpenAPITool.run() (components.py):

1. get_http_headers() does not exclude authorization from forwarded headers
2. request.headers.update(mcp_headers) overwrites existing headers, while the client headers block above uses if key not in request.headers (non-overwriting) — inconsistent behavior

# Client headers: non-overwriting ✓
if self._client.headers:
    for key, value in self._client.headers.items():
        if key not in request.headers:
            request.headers[key] = value

# MCP headers: overwrites everything ✗
mcp_headers = get_http_headers()
if mcp_headers:
    request.headers.update(mcp_headers)  # <-- overwrites Authorization!

Fix

1. Add authorization to exclude_headers in get_http_headers() (dependencies.py) — MCP transport credentials should not be forwarded to downstream APIs by default
2. Change MCP header forwarding to use the same non-overwriting pattern as client headers — only set if key not already present

# After fix: consistent non-overwriting behavior
mcp_headers = get_http_headers()
if mcp_headers:
    for key, value in mcp_headers.items():
        if key not in request.headers:
            request.headers[key] = value

Impact

• Security: Prevents MCP client credentials from leaking to third-party APIs
• Correctness: httpx client's configured API key is now respected
• Consistency: Both client headers and MCP headers use the same non-overwriting pattern

Fixes #3260