MCP server accepts unauthenticated initialize, so Cursor never runs OAuth and every tool call fails with "Unauthorized" #14

Summary

The MagicPath MCP server at https://api.magicpath.ai/mcp returns HTTP 200 to initialize and tools/list from clients that have no access token. It only sends the 401 + WWW-Authenticate challenge on tools/call. Cursor runs its OAuth flow based on the connection-time 401, so it sees the server as connected and never prompts for login. Every tool call then fails with Unauthorized. Triggering MCP auth manually in Cursor reports "Successfully authenticated" but no browser login opens, and calls still fail.

The plugin README says: "Unauthenticated requests return 401 with a WWW-Authenticate challenge." The server does that for tools/call, but not for initialize.

Environment

• Client: Cursor 3.22.12 (arm64, macOS 26.5)
• Plugin: magicpath v0.7.0 from the Cursor public marketplace (commit 3cc4aeb0ff327877fa31fe3e7dd02193eb6a9e3f)
• Transport: streamable-http, https://api.magicpath.ai/mcp
• Date observed: 2026-10-01 03:28 UTC

Steps to reproduce

1. Install the MagicPath plugin in Cursor.
2. Call any tool, for example get_context. Result: {"error":"Unauthorized"}.
3. Run MCP authentication for the server in Cursor. It reports success right away, with no browser window.
4. Call get_context again. Still Unauthorized.

Reinstalling the plugin and re-running authentication gives the same result.

Direct reproduction without a token

initialize succeeds without credentials:

curl -i -X POST https://api.magicpath.ai/mcp \
  -H 'Content-Type: application/json' \
  -H 'Accept: application/json, text/event-stream' \
  -d '{"jsonrpc":"2.0","id":1,"method":"initialize","params":{"protocolVersion":"2025-06-18","capabilities":{},"clientInfo":{"name":"probe","version":"0"}}}'
# HTTP/2 200 with the full InitializeResult (serverInfo magicpath 1.1.0), no WWW-Authenticate header

tools/call without credentials is the only request that gets the challenge:

curl -i -X POST https://api.magicpath.ai/mcp \
  -H 'Content-Type: application/json' \
  -H 'Accept: application/json, text/event-stream' \
  -H 'MCP-Protocol-Version: 2025-06-18' \
  -d '{"jsonrpc":"2.0","id":2,"method":"tools/call","params":{"name":"get_context","arguments":{}}}'
# HTTP/2 401
# www-authenticate: Bearer resource_metadata="https://api.magicpath.ai/.well-known/oauth-protected-resource/mcp", error="invalid_token", error_description="A valid MagicPath MCP access token is required."

The protected-resource metadata is served correctly. It lists the authorization server https://api.magicpath.ai/ and the scopes magicpath:read and magicpath:write.

Expected

Per the MCP authorization spec and the plugin README, an initialize request without a valid bearer token should get HTTP 401 with the WWW-Authenticate / resource_metadata challenge. Clients would then discover the authorization server and run OAuth before the session starts.

Actual

initialize and tools/list succeed without a token. The challenge only comes on tools/call, after the client has already decided the server is connected. Cursor never starts OAuth, so the plugin can't be used.

Suggested fix

Require a valid bearer token on every request to /mcp, including initialize. Alternatively, return the 401 challenge on initialize whenever no Authorization header is present.