AI Foundry 404 DeploymentNotFound: use the responses API for conn/deployment routing

TL;DR: Switch the call to responses.create with the same connection/deployment target and it routes. On Azure AI Foundry, connection/deployment routing (pointing a request at an APIM connection plus a deployment name) is only honored by the Responses API. If you send that targeting to chat.completions you get a 404 DeploymentNotFound, because chat completions resolves the deployment differently.

AI Foundry 404 DeploymentNotFound: use the responses API for conn/deployment routing

Fix

1. Switch the call to responses.create with the same connection/deployment target and it routes. Expected: the request routes instead of erroring.

2. Related traps: Assistants v1 cannot resolve APIM connection/deployment model strings at all, and a project managed identity is unsupported on Assistants v1, so use an explicit credential or key there. Expected: this specific failure stops.

3. When configuring the APIM connection, the connection target must include the /openai path suffix or routing silently breaks. Expected: this specific failure stops.

When to use

You hit exactly this: AI Foundry 404 DeploymentNotFound: use the responses API for conn/deployment routing in AI Foundry 404 DeploymentNotFound.

When not to use

A different error, or the same symptom in a different tool. This page only covers the failure above.

Compatibility

AI Foundry 404 DeploymentNotFound (versions mentioned here: v1).

Root cause

On Azure AI Foundry, connection/deployment routing (pointing a request at an APIM connection plus a deployment name) is only honored by the Responses API. If you send that targeting to chat.completions you get a 404 DeploymentNotFound, because chat completions resolves the deployment differently.