One key, one host, two different base URL strings. The wrong one answers 404, and most clients report that as a connection problem rather than a wrong path.
The path segment that bites
Anthropic's SDK often expects a base URL that already ends in a version prefix in some setups, while OpenAI-style clients expect …/v1 and append the rest. On Supesonic, prefer https://api.supesonic.com/v1 for OpenAI-compatible clients. For Anthropic-shaped clients, confirm whether the SDK appends /v1/messages itself — a duplicated or missing segment is the usual 404.
What to check
- Log the final request URL from the client, not only the configured base.
- Compare against the endpoint table in the docs.
- Confirm the key authenticates with a known-good OpenAI-compatible path first.
See API reference and the migration checklist.