The server is overloaded or not ready yet
A 503 means OpenAI’s servers are temporarily over capacity — a provider-side issue, not your account. Retry with backoff and add a fallback so your app rides through the spike.
Cross-provider failover is built in. When one model is overloaded, Standard Compute reroutes to another automatically — no 503s bubbling up to your agent.
429 means you exceeded your limit; 503 means the provider itself is overloaded. 429 is about your usage, 503 is about their capacity.