Fix production 421 "Invalid Host header" on /mcp - #160
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orthocal/asgi.py called mcp.streamable_http_app() with no arguments, so the SDK defaulted host='127.0.0.1' and auto-enabled DNS-rebinding protection restricted to localhost -- fine in dev, but it rejected every real request in production, since Cloud Run's own hostname (forwarded as the Host header by the Firebase Hosting proxy in front of it) isn't on that allowlist. Confirmed via Cloud Run logs showing "Invalid Host header: orthocal-6czswbhara-uc.a.run.app" on every /mcp request since the merge. DNS-rebinding protection defends a server bound to localhost against a malicious webpage's JS reaching it through the browser; it doesn't apply to a public HTTPS API with no localhost-only trust boundary, so disabling it explicitly is the correct fix. Verified locally by replaying the exact Host header from the production error logs. Co-Authored-By: Claude Sonnet 5 <[email protected]> Claude-Session: https://claude.ai/code/session_01Hf6j2xXQXywHVh3HAVRxB3
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Summary
/mcpreturned 421 "Invalid Host header" for every real request in production, confirmed via Cloud Run logs (Invalid Host header: orthocal-6czswbhara-uc.a.run.appon every request since PR Add an MCP server exposing calendar and saint-search tools #158 merged).orthocal/asgi.pycalledmcp.streamable_http_app()with no arguments. The SDK defaultshost='127.0.0.1'in that case, which auto-enables DNS-rebinding protection restricted tolocalhost/127.0.0.1/::1— fine in local dev (where the Host header naturally islocalhost), but it rejects every real production request, since Cloud Run's own hostname (forwarded as the Host header by the Firebase Hosting proxy in front of it) isn't on that allowlist.transport_security=TransportSecuritySettings(enable_dns_rebinding_protection=False). DNS-rebinding protection defends a server bound to localhost against a malicious webpage's JS reaching it through the browser — it doesn't apply to a public HTTPS API with no localhost-only trust boundary, so disabling it here is the correct fix, not a workaround.Test plan
docker compose run --rm tests— 135/135 passingHost: orthocal-6czswbhara-uc.a.run.app) against the dev server — confirmed 200 OK after the fix, reproduced the 421 before it🤖 Generated with Claude Code
https://claude.ai/code/session_01Hf6j2xXQXywHVh3HAVRxB3