FastMCP defaults to an allow-list of localhost variants for the Host
header (DNS-rebinding protection). Any deployment behind a reverse
proxy hitting a non-localhost hostname (e.g. devassistant.traefik.internal)
gets 421 Misdirected Request with:
WARNING mcp.server.transport_security: Invalid Host header: <name>
The protection exists to stop a malicious browser page from rebinding
DNS to attack a localhost MCP server. Our deployment is HTTP transport
behind a reverse proxy with bearer-token auth, which already gates
every request — so the rebinding threat doesn't apply. Disabling
the check lets any Host through; auth still rejects unauthorized
requests at 401.
This also makes the integration test pass without test-only host
hackery — every realistic Host header (Traefik internal hostname,
CDN domain, custom DNS) now reaches FastMCP cleanly.
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
bvandeusenandClaude Opus 4.7
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