• 400 — malformed query or unknown jurisdiction
• 401 — missing or revoked key
• 422 — question could not be grounded in the selected corpus
• 429 — rate limit; retry after the interval in the header
• 503 — corpus reindexing; safe to retry

A 422 is not a failure of the request — it is the system reporting that it found no supporting passage. Treat it as a valid, informative outcome.