[Bug] MCP OAuth discovery ignores path-scoped protected-resource metadata (Plane /http/mcp)

[misunders2d]

Summary

Prime Agent 0.7.4 completes the browser OAuth flow for Plane's hosted MCP, but obtains a token from the wrong authorization-server path. The browser reports Authentication successful, while the MCP endpoint then returns 401 invalid_token.

This is a concrete reproduction of the RFC 9728 discovery problem previously documented in #766. That issue was closed during the move to discussion-first intake, but the behavior remains reproducible in 0.7.4.

Environment

• Prime Agent 0.7.4
• Linux
• Plane hosted Streamable HTTP MCP

Configuration

"mcpServers": {
  "plane": {
    "type": "http",
    "url": "https://mcp.plane.so/http/mcp",
    "oauth": true
  }
}

Reproduction

1. Add the configuration above.
2. Run /mcp login plane.
3. Complete the browser flow. Plane displays Authentication successful.
4. Initialize the MCP connection or list its tools.

Actual result:

HTTP 401
{"error":"invalid_token","error_description":"Authentication failed. The provided bearer token is invalid, expired, or no longer recognized by the server..."}

No credentials or token values are included in this report.

Diagnosis

Plane publishes path-scoped protected-resource metadata correctly:

GET https://mcp.plane.so/.well-known/oauth-protected-resource/http/mcp

It identifies:

{
  "resource": "https://mcp.plane.so/http/mcp",
  "authorization_servers": ["https://mcp.plane.so/http"]
}

The corresponding authorization-server metadata is available at:

https://mcp.plane.so/.well-known/oauth-authorization-server/http

and advertises /http/authorize, /http/token, and /http/register.

Prime's discover() currently reduces the MCP URL to new URL(url).origin and probes only root authorization-server metadata. It therefore uses https://mcp.plane.so/, which issues a token whose audience is the legacy https://mcp.plane.so/sse endpoint. Plane's /http/mcp resource correctly rejects that token.

The same Plane configuration works in another MCP client that follows the protected-resource metadata.

Expected behavior

Prime should follow the MCP authorization discovery flow:

1. Read the resource_metadata pointer from the endpoint's WWW-Authenticate response when available.
2. Otherwise probe the RFC 9728 path-suffixed protected-resource metadata URL.
3. Read authorization_servers from that document.
4. Fetch RFC 8414 metadata for the selected issuer, including issuers with a path.
5. Use the advertised registration, authorization, token, and refresh endpoints.
6. Preserve the current origin-based lookup only as a compatibility fallback.

A regression test using Plane's /http/mcp → /http metadata shape would cover this case.