HTTP 400
invalid_request_bodyInvalid MCP request body
The MCP HTTP request body could not be read or decoded.
Resolution
Send a valid MCP JSON-RPC request body.
Response handling
Keep the response's request_id for diagnostics. Retry only transient rate limits and 503 responses, respecting Retry-After with bounded exponential backoff and jitter. For monthly_quota_exceeded, stop polling until RateLimit-Reset or upgrade. Correct 400, 401, 403, and 406 requests before retrying.