Responses API and Chat Completions are different application contracts, not interchangeable URLs. Before migrating a coding Agent, check its response parser, tool loop, streaming consumer, and conversation state—not only whether a short prompt returns text.
OpenAI recommends Responses for new projects while continuing to support Chat Completions, according to its migration guide, checked September 29, 2026. A third-party compatible endpoint must be evaluated separately.
Decide whether a migration solves your problem
Migrate when your chosen model, client, or required feature calls for Responses, or when you intentionally want that integration model. A working application does not need a rushed protocol change merely to adopt a newer name.
List what the Agent actually does: plain text, structured output, local function calls, streaming, images, or provider-hosted tools. Treat each as a separate acceptance requirement.
If you are also consolidating providers, use the one-key model migration guide. Avoid changing the protocol, model, endpoint, and prompt simultaneously; a failure would have too many possible causes.
Map the contracts your application reads
These differences come from the official migration guide. They do not imply that a gateway supports every OpenAI feature.
Do not assume the first output item is user-facing text. Likewise, a streaming connection opening successfully is not proof that the consumer handles completion, failure, and tool events correctly.
Test one complete Agent loop
Use fixtures with no external side effects before real tools.
Keep model and task constant while comparing protocols. The goal is application correctness, not a synthetic claim that one endpoint is always smarter or cheaper. Reuse a small coding regression set to check the actual output after transport tests pass.
Preserve a rollback path
Change the adapter behind a controlled configuration choice. Keep the previous path available until your important tasks pass and usage records reconcile. Avoid running both paths on real side-effecting tasks just to compare them.
Decide who owns conversation state. Do not mix a server-managed continuation mechanism with blindly resending the same full history. Confirm the selected endpoint's retention and continuation behavior; compatibility should not be inferred from a shared SDK.
Verify the endpoint you intend to use
The Agent.Space Developer API guide explains model discovery and protocol selection. It is the place to confirm a supported integration shape, not evidence of complete hosted-tool parity.
Open Agent.Space if you want to connect a supported model route. Validate a complete, bounded Agent loop before directing production work to it.