The interactions interface is incompatible with the Vertex AI Express mode #6966

The interactions (NextGen) interface is incompatible with the Vertex AI Express mode (API Key authentication).

When using Express mode, the client initializer enforces a mutual exclusion between the API key and the project/location parameters. Because the project and location are missing, the SDK fails to construct the required Vertex AI resource path (/v1beta1/projects/{project}/locations/{location}/interactions). Instead, it falls back to a default path (/v1beta1/interactions) on the Gemini Developer API server, which does not exist and results in a 404 Not Found error.

To use the interactions interface, you must use the standard GCP Credentials authentication mode by providing a project, location, and valid Google Cloud credentials.