fix(qr-server): pass host/port to FastMCP for Docker compatibility #372
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When FastMCP is created with the default host=127.0.0.1, MCP SDK 1.26+ automatically enables DNS rebinding protection that only allows requests from localhost. This breaks Docker where requests come from host.docker.internal, causing 421 Invalid Host header errors. By passing host=HOST (0.0.0.0 for Docker), DNS rebinding protection is not auto-enabled, allowing requests from any host.
@modelcontextprotocol/ext-apps
@modelcontextprotocol/server-basic-react
@modelcontextprotocol/server-basic-vanillajs
@modelcontextprotocol/server-budget-allocator
@modelcontextprotocol/server-cohort-heatmap
@modelcontextprotocol/server-customer-segmentation
@modelcontextprotocol/server-map
@modelcontextprotocol/server-pdf
@modelcontextprotocol/server-scenario-modeler
@modelcontextprotocol/server-shadertoy
@modelcontextprotocol/server-sheet-music
@modelcontextprotocol/server-system-monitor
@modelcontextprotocol/server-threejs
@modelcontextprotocol/server-transcript
@modelcontextprotocol/server-video-resource
@modelcontextprotocol/server-wiki-explorer
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Problem
When using the qr-server in a Docker environment, requests fail with 421 Invalid Host header errors.
Root Cause
MCP SDK 1.26+ introduced automatic DNS rebinding protection. When FastMCP is created with the default host=127.0.0.1, it automatically enables this protection which only allows requests from localhost, 127.0.0.1, and ::1.
In Docker deployments, the backend container sends requests with Host: host.docker.internal, which gets rejected.
Solution
Pass host=HOST and port=PORT to the FastMCP constructor. Since HOST defaults to 0.0.0.0 (for Docker compatibility), this prevents the auto-enabling of DNS rebinding protection, allowing requests from any host.
Testing