Gemini Interactions API: `GET /v1beta/interactions/{id}` returns 400 `Multiple authentication credentials received. Please pass only one.` for background interactions (fixed server-side 2026-10-08)

Context

Source(s): https://discuss.ai.google.dev/t/get-v1beta-interactions-id-returns-400-multiple-authentication-credentials-received-for-background-interactions-aq-key-since-oct-3-4/186789 Signal: 255 views, 12 posts; at least four developers confirmed the same failure; staff accepted answer (October 2026). Status: reported working by source, not reproduced by us.

Problem

Creating a background interaction (POST /v1beta/interactions with "background": true) returns 200 with an id and status in_progress, but every GET /v1beta/interactions/{id} on it returns HTTP 400 with the error above, even though only one credential (the x-goog-api-key header) is sent. The same key still works for creating interactions, countTokens and generateContent. AI Studio showed the second message for the same interaction.

Error (redacted / normalized)

{"error":{"message":"Multiple authentication credentials received. Please pass only one.","code":"invalid_request"}}

Agent finished with error: Failed to close Interaction stream: Multiple authentication credentials received. Please pass only one.

Applies when

Gemini API v1beta Interactions, gemini-3.8-flash, background: true, header Api-Revision: 2026-05-20, API key in the x-goog-api-key header only (the original poster used an AQ.-prefix key). Failing from 2026-10-04 to 2026-10-08 (UTC).

Confirm it is this problem

1. Your GET sends one credential only (no Authorization header and no ?key=), and still gets the 400 above. → expect: if you do send two credentials, this card does not apply.
2. The POST that created the interaction succeeded. → expect: only the retrieval call fails.

Not this when / boundary

Your request really does carry two credentials, for example an Authorization: Bearer header plus an API key (our inference from the error text).

Root cause

A server-side problem on the retrieval path; Google staff said on 2026-10-08 that it "has been resolved". The original poster's unchanged retry worked, so no client change was needed (our inference).

Fix

1. Retry polling GET /v1beta/interactions/{id} with the same API key, unchanged (staff). → expect: the original poster replied "It works now!!".
2. If you still get a 400 after 2026-10-08, staff asked affected users to send the interaction ID, the approximate request time and the full error message by direct message on the forum.
3. While it was broken, one user switched to the synchronous (non-background) mode instead (workaround only).

How to verify

The GET returns 200 with the interaction's status and, once finished, its output.

What did not work

none reported; developers said there was no workaround besides synchronous mode.

Risks

Nobody answered in the thread whether background interactions that could not be retrieved were still billed. Check usage before resubmitting long or costly jobs, because resubmitting can mean paying twice (our inference).

Verified scope

Gemini API Interactions background retrieval, 2026-10-04 to 2026-10-08 (UTC).

Becomes obsolete when

Already fixed server-side; keep only as a reference for the error text.