A Codex custom API provider changes where the local client sends model requests. It does not change which files Codex can access, turn a subscription into API credit, or move your project into a hosted Workspace. Configure the provider, verify the running process can read its key, and reconcile one small request before opening a real repository task.
This is a documentation-based setup checklist, checked September 29, 2026—not a production test of every compatible provider. For the product setup itself, start with the Agent.Space Developer API guide.
Prepare the connection, not just a key
Collect the provider's exact endpoint, an available model ID, the required protocol, and a dedicated credential. A model appearing in discovery is not enough: the client and endpoint must support the same request and streaming formats.
For Agent.Space, use the live Codex setup template. Preserve your existing configuration and unrelated settings; do not replace an entire configuration file with a Blog snippet. If you are still choosing a billing route, read the Codex subscription, API, and Workspace comparison first.
Understand the configuration fields
The official Codex configuration guide documents custom provider selection, base URLs, and environment-backed credentials. The following is an illustrative shape, not a ready-to-run account configuration:
Replace the placeholders with your provider's supported values. The value of env_key is the environment variable's name, not the secret. Load the secret through your approved credential mechanism before starting the client. Do not add a real key to the TOML file, shell history, or a troubleshooting screenshot.
An editor started before the environment changed may not inherit the new value. Verify the process you actually launch, not merely a different terminal.
Prove one request took the intended route
Start a new session with a harmless prompt requiring no tools or repository access. Use a dedicated key so the test is distinguishable from background automation.
Record the timestamp, selected model, endpoint, client version, and resulting request ID if available. In the provider's usage history, look for the corresponding request and expected funding source. A fluent answer alone does not prove which account paid.
Then test one bounded tool interaction in a disposable project. Text generation success does not establish tool-call compatibility. Keep the previous configuration available until both checks pass.
Diagnose the first failure before changing more settings
Keep secrets and private code out of diagnostic logs. Change one setting at a time and repeat the same small test; changing the endpoint, model, key, and prompt together destroys the comparison.
Keep model access separate from the workspace
A successful connection supplies model access to your local client. Your local files and tool permissions still need their own review. If you want the hosted workflow instead, see Codex on Agent.Space.
Ready to connect a supported client? Open Agent.Space, use its current setup template, and verify one small request before scaling usage.