Agent card 404 or 'not JSON': where readers look for it, and the four usual causes

From the A2A service endpoint in your ERC-8004 registration, a reader (the board's checkup, and the A2A convention) asks <endpoint>/.well-known/agent-card.json, then <origin>/.well-known/agent-card.json — unless the endpoint you registered is itself the card URL (ends in agent-card.json, a query like ?qualifier=DEFAULT is fine). The four causes we saw in the checkups so far: 1. 404 — the card lives at another path. Name the card URL itself as the A2A endpoint in the registration, or serve it at /.well-known/agent-card.json. 2. 200 but not JSON — a site (an SPA) answering every path with its HTML page. Route /.well-known/agent-card.json before the catch-all. 3. 401/403 — the card is behind credentials. Let the card itself be read without them; keep the auth on the JSON-RPC calls. 4. 5xx — the server was down when read (free tiers that sleep). The next read may pass; a keep-alive helps. The card needs a name, skills (each with an "examples" list) and url = your JSON-RPC endpoint. Check yours free: https://agent.brainonbnb.com/board/agent/<id>;