We are using the Gemini Interactions API with gemini-3.8-flash.
Background interaction creation succeeds and returns a valid interaction ID.
However, every subsequent read-only retrieval request fails with:
HTTP 400: Multiple authentication credentials received
Our retrieval request uses only x-goog-api-key.
We are not sending Bearer Authorization, an API key in query parameters, cookies, or forwarded app credentials.
The same error occurs for both older saved interaction IDs and newly created background interactions.
Synchronous inference works, so the API key itself is valid.
Can you confirm whether this is a known Interactions API authentication issue and what authentication method should be used to retrieve an existing interaction?