Reliable AI API routing is not one retry loop. It means giving each API key a clear primary group, fallback groups, model capabilities, and request limits, while retaining enough user-facing evidence when a request slows down or fails. A good routing design makes the result explainable and prevents a group switch from silently changing the requested model or billing policy. Start with an explicit key policy Modelflare supports two routing approaches: A regular API key uses one explicit primary group and an ordered list of fallback groups. A Smart API Key evaluates the groups currently available to the account and continues through eligible candidates according to its strategy. Fallback order is not traffic splitting. It is a priority sequence. When the primary group is unavailable, requests continue through the later groups in order. Create separate keys for separate applications or environments. This makes group access, quota, expiration, and usage records easier to reason about than one shared key used everywhere. A fallback must preserve the request contract Before adding a group to a fallback list, verify that it: exposes the requested model; supports the same endpoint and streaming behavior; permits any required Service Tier or provider-specific field; has an acceptable usage multiplier and request-rate limit; is actually unlocked for the account. A fallback group is not a license to substitute an arbitrary model. The requested model and protocol still define the wire contract. Understand group request limits Request-rate limits are applied according to the account and the group that handles the request. Separate API keys make usage easier to analyze, but they do not automatically bypass account or group limits. When the current group reaches its limit, an API key with fallback groups can continue to the next available group. If no fallback is available, the request receives 429 . Client retries should use bounded backoff rather than immediately creating a burst of new requests. Use request-level performance metrics Usage logs expose the product-level metrics needed to understand a request: A slow first response usually means the request waited longer before generation began. A normal first response followed by low output speed points more toward the generation phase. Combine these metrics with status, model, group, and time range to distinguish a one-off delay from a sustained issue.

Tool-call-only responses may not contain visible text, so “first response” is the safer first-effective-output signal for Responses traffic. Use evidence that is safe to retain Modelflare timing metadata does not include prompts, response text, raw request bodies, API keys, emails, or plaintext IP addresses. This keeps the operational record useful without turning latency diagnostics into a second content store. For an incident, capture: request ID and timestamp; requested model and selected group; endpoint and streaming mode; downstream status; status, total response time, and first-response timing; whether the client cancelled before the request completed. That evidence lets support verify group switching, slow model generation, and client cancellation without exposing internal service topology in ordinary logs. Production reliability checklist Give every application its own API key. Keep the primary and fallback order intentional. Verify model and protocol support in every candidate group. Set a client timeout long enough for the real workload. Use bounded retries with jitter for retryable failures. Do not retry authentication, quota, or model-access errors as if they were transient. Test both non-streaming and streaming requests. Monitor 429 , first response, output throughput, and cancellation. Review group pricing before treating a fallback as equivalent. Reliability comes from preserving the request contract while retaining enough evidence when a request fails. Ordered fallback groups reduce dependence on one group, while performance and usage records make remaining failures explainable. Use the AI API Error Guide to interpret authentication, access, rate-limit, cancellation, and 5xx failures. Apply the controls in AI API Key Security before expanding fallback access.