Serve profile as application/json
The profile URL did not return parseable JSON.
Why it matters
Agents request /.well-known/ucp expecting JSON. HTML error pages, plain text, or YAML will all fail validation regardless of content.
Show fix
Set the Content-Type header on the profile response and verify the body is valid JSON (no trailing commas, comments, or BOM).
Content-Type: application/json; charset=utf-8
