Start with the contract your application already depends on
Changing a base URL is the smallest part of a gateway migration. The real contract includes the SDK version, endpoint, request fields, response fields, streaming event order, error shape, timeout behavior, and the usage data your product stores. Before touching production, capture representative requests from each workload and identify every response field the application reads. The EveryModelAPI documentation describes the intended compatibility surface, but your own traffic is the authoritative migration inventory.
Separate simple text generation from higher-risk capabilities such as tool calls, structured output, images, audio, reasoning controls, and provider-specific extensions. A gateway can be compatible with a common request envelope while still differing at capability edges. This is why migration acceptance criteria should be written per workload rather than reduced to one successful chat-completion test.
Freeze a baseline before introducing the gateway
Create a small, repeatable contract suite against the current provider before testing the gateway. Store sanitized request fixtures and assert the response properties that matter: HTTP status, content type, event sequence, finish reason, tool-call identifiers, structured-output validity, and usage fields. Do not compare generated prose byte for byte; compare the protocol and application invariants around it.
Run the same suite through the proposed gateway and record intentional differences. If a field is normalized or omitted, decide whether the client can safely ignore it or whether the gateway needs an adapter. Keep these decisions in version control so a later gateway upgrade can be evaluated against the same baseline instead of relying on memory.
Keep model identity and routing explicit
A stable model identifier should describe what developers can rely on. If several authorized upstreams serve the same model, routing may change between equivalent endpoints, but the platform should not silently substitute another model family. Review the planned AI model API catalog and decide which model IDs are pinned, which are aliases, and which may use controlled fallback routing.
Define fallback rules before launch. A fallback may be acceptable for a low-risk summarization job and unacceptable for a regulated extraction workflow or a tool-using agent. Record the selected upstream, provider request ID, model version when available, and fallback reason with each request so support and billing investigations can reconstruct what actually happened.
Treat streaming as a separate migration track
A non-streaming response can pass while streaming fails behind a reverse proxy. Verify that server-sent events are forwarded without buffering, heartbeats or data frames arrive before idle timeouts, event order remains intact, and the stream terminates with the shape expected by the SDK. Test through the same CDN and proxy path that production clients will use, not only against a container port on localhost.
Also test the uncomfortable paths: client cancellation, an upstream timeout before the first token, a provider failure after partial output, and a network interruption near the final usage event. The companion guide on streaming AI API responses across providers provides a deeper test matrix for these cases.
Design retries around billable side effects
Retries are safe only when the previous attempt is known not to have created a billable generation or another external side effect. Retry connection failures before a request is accepted, selected rate-limit responses with bounded backoff, and explicitly documented transient provider errors. Do not automatically replay a request merely because the client did not receive the final response.
Give every client request a stable idempotency or correlation identifier and retain an attempt-level record beneath it. If the gateway tries a second channel, the record should show both attempts, their timing, the selected result, and any upstream cost. That distinction prevents a support team from confusing one customer action with two independent purchases.
Migrate authentication without creating shared secrets
Create separate gateway keys for production, staging, batch processing, and developer use. Map each key to the minimum model set, quota, network policy, and expiration behavior required by that workload. Never copy an unrestricted administrator or upstream provider key into application configuration.
Plan rotation before the first cutover. A practical sequence is to issue a second gateway key, deploy it alongside the old one, confirm traffic has moved, and then revoke the first key. The API cost-control guide explains why key separation also improves budget enforcement and incident response.
Reconcile usage and price versions
Provider usage, gateway usage, and the customer-visible charge are related but not interchangeable. Preserve the input and output units reported by the upstream, the normalization rule applied by the gateway, the price version effective at request time, and the resulting charge. If the provider reports usage only at the end of a stream, define how interrupted requests are estimated and later reconciled.
Before accepting money, verify the full lifecycle described on the AI API pricing and usage-controls page: reserve or validate available balance, execute once, finalize usage, release unused reservation, and handle failures without double charging. A dashboard counter alone is not an auditable ledger.
Use a staged cutover with an explicit rollback
Begin with internal traffic, then a non-critical workload, then a small percentage of production requests. Compare success rate, first-token latency, total latency, error categories, tool-call completion, usage deltas, and cost per successful request. Increase traffic only when the previous stage has enough volume to expose meaningful differences.
Keep rollback boring: a configuration flag or deployment setting should return the workload to the original provider without a client release. The migration is complete only after the old credential is revoked, monitoring covers the gateway path, support can trace a request end to end, and the team has documented which native provider capabilities still bypass the compatibility layer.