Return 401 challenge for unauthenticated /mcp requests - #6
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An /mcp request without a Bearer token built the MCP server with a nil server_context. MCP::ServerContext wraps it and delegates unknown methods via method_missing, so tools/list still returned 200 while any actual tool call failed with an opaque JSON-RPC -32603: undefined method 'imap_config' for an instance of MCP::ServerContext Reject the request up front with a 401 and the WWW-Authenticate resource_metadata pointer, so MCP clients re-run the OAuth flow instead. Extracted the response builder into unauthorized_response, reused by the existing JwtService::Error rescue. Note: bare OPTIONS /mcp preflights are now also rejected. The app has no CORS handling today, so browser clients were already broken; adding one will need to bypass this check. Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
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Problem
An
/mcprequest without aBearertoken returned[nil, nil]fromresolve_mcp_context, so the MCP server was built with anilserver_context. The mcp gem wraps whatever it is given inMCP::ServerContext, which delegates unknown methods to the wrapped object viamethod_missing. Withnilwrapped,tools/liststill succeeded with a 200 — but any actual tool call blew up with an opaque JSON-RPC-32603:Fix
Reject the request up front with a
401and theWWW-Authenticate: Bearer resource_metadata=…pointer, which is what tells an MCP client to re-run the OAuth flow. Extracted the response builder into a privateunauthorized_response(description)helper, reused by the existingJwtService::Errorrescue.Testing
Added a spec for the missing-header case. Verified it fails without the
app.rbchange (returns 200, sincetools/listnever touches the context — the same shape as the original bug). Full suite: 78 examples, 0 failures. Rubocop clean.Note
Bare
OPTIONS /mcppreflights are now also rejected. The app has no CORS handling today, so browser-based clients were already broken; if one is added later it will need to bypass this check.Gemfile.lockcarries an incidentalmail_mcp (1.0.0)→(1.0.1)sync picked up when bundler resolved against the v1.0.1 version file.🤖 Generated with Claude Code