Unauthenticated GET returns 200 without the MCP Accept header, hiding the 401 OAuth challenge

[sumanthratna]

Summary

Since the fix for #1, both protected-resource metadata documents resolve correctly. A related gap remains one layer up: an unauthenticated GET https://mcp.motion.dev/plus sent without MCP accept types returns 200 and a JSON server description, with no WWW-Authenticate header. The 401 OAuth challenge is only reachable when the request carries Accept: application/json, text/event-stream (or is a POST).

Clients whose OAuth discovery probe is a plain GET therefore never see the challenge. Some of them parse that 200 body as RFC 9728 protected-resource metadata and fail confusingly, because the description document has no resource field.

Reproduction

$ curl -sS -o /dev/null -w '%{http_code}\n' https://mcp.motion.dev/plus
200

$ curl -sS -o /dev/null -w '%{http_code}\n' \
    -H 'Accept: application/json, text/event-stream' https://mcp.motion.dev/plus
405

$ curl -sS https://mcp.motion.dev/plus | jq 'keys'
["description","docs","name","protocolVersion","tools","transport"]

No WWW-Authenticate header is present on the 200. The challenge does appear on a POST:

$ curl -sS -o /dev/null -D - -X POST https://mcp.motion.dev/plus \
    -H 'Accept: application/json, text/event-stream' -H 'Content-Type: application/json' \
    -d '{"jsonrpc":"2.0","id":1,"method":"initialize","params":{"protocolVersion":"2025-06-18","capabilities":{},"clientInfo":{"name":"probe","version":"1"}}}'
HTTP/2 401
www-authenticate: Bearer realm="motion", resource_metadata="https://mcp.motion.dev/.well-known/oauth-protected-resource/plus"

The base server behaves the same way (no-Accept=200, with-Accept=405), though it matters less there since it serves tools unauthenticated.

Impact

Codex CLI is a concrete case. codex mcp login motion-plus fails with:

Error: Metadata error: Protected resource metadata missing required resource field

Codex's discovery probe is a plain GET with no Accept header (openai/codex#37830), so it receives the 200 description document and parses it as protected-resource metadata. The error names a metadata document that was never fetched, which sent me a long way down the wrong path before I compared the two responses.

Forcing the accept types makes login succeed on the first attempt:

[mcp_servers.motion-plus]
url = "https://mcp.motion.dev/plus"
http_headers = { "Accept" = "application/json, text/event-stream" }

That is a client-side workaround for a specific client. Any other client that probes with a plain GET hits the same wall.

Expected

An unauthenticated request to a protected MCP endpoint should return 401 with WWW-Authenticate regardless of the Accept header, so auth discovery can find the challenge by the documented path.

If the descriptive JSON is worth keeping, serving it from a separate unprotected URL (or gating it behind an explicit Accept: application/json alone) would preserve it without masking the challenge.

Environment

• Codex CLI 0.149.1, macOS 26.5.1 arm64
• Signed-in Motion account without Motion+ (correcting an earlier version of this issue that said otherwise). The OAuth flow itself completes; entitlement is separate and does not affect anything above, since the reproduction is unauthenticated.
• Verified 2026-08-24, after Motion+ MCP server OAuth fails with invalid_target — /plus resource not registered in auth metadata #1 was closed. Both /.well-known/oauth-protected-resource and /.well-known/oauth-protected-resource/plus return 200 with a valid resource field.
• The 200/405 split reproduced 5 times out of 5.

Happy to test a fix.