gemini-3.8-flash: 400 'Requests ending with a model turn are not supported' after a model turn

Description

Using Google Gemini 3.8 Flash via the Gemini API (https://aiplatform.googleapis.com/v1beta1/.../models/gemini-3.8-flash:streamGenerateContent?alt=sse), the request fails with 400 INVALID_ARGUMENT:

Requests ending with a model turn are not supported.

isRetryable: false, so the session hard-fails (a session.error event is emitted, session ses_f99077bd3ffekifM3UhsAgnIpk, project 2acd036c175137252d8b8a07a3ea82d0fe0658a3) and the only way to continue is to type a new prompt.

The cause is the projected conversation history being sent with the last contents entry as a model (assistant) turn. Gemini rejects any request whose contents ends with a model role. This surfaces after the model finishes answering / thinking, and the history is reloaded with an assistant-only tail — e.g. on the next prompt, after an interrupted stream, or via a continue/auto-continue path.

Same root cause as the already-open #45359 (reproduced on gemini-3.7-flash); it also reproduces on gemini-3.8-flash.

Plugins

Unknown / not provided in the error payload.

OpenCode version

Unknown from the error payload (please confirm).

Steps to reproduce

1. Configure the Google/Gemini provider and select gemini-3.8-flash.
2. Run a normal session and let the model finish its answer (complete a model turn).
3. On the next prompt, the stream returns 400 Requests ending with a model turn are not supported.
4. Only typing a fresh prompt lets the conversation continue.

Screenshot and/or share link

No response — see the error payload below.

Operating System

Unknown from the error payload.

Terminal

Unknown from the error payload.

Error payload

{
  "directory": "/workspace/xxx",
  "project": "2acd036c175137252d8b8a07a3ea82d0fe0658a3",
  "payload": {
    "id": "evt_067152aee001TL0iqoRHGvPa9n",
    "type": "session.error",
    "properties": {
      "sessionID": "ses_f99077bd3ffekifM3UhsAgnIpk",
      "error": {
        "name": "APIError",
        "data": {
          "message": "Requests ending with a model turn are not supported.",
          "statusCode": 400,
          "isRetryable": false,
          "responseHeaders": {
            "alt-svc": "h3=\":443\"; ma=2592000,h3-29=\":443\"; ma=2592000",
            "content-length": "142",
            "content-type": "text/event-stream",
            "date": "Thu, 03 Sep 2026 11:43:55 GMT",
            "server": "scaffolding on HTTPServer2",
            "vary": "Origin, X-Origin, Referer",
            "x-content-type-options": "nosniff",
            "x-frame-options": "SAMEORIGIN",
            "x-xss-protection": "0"
          },
          "responseBody": "{\n  \"error\": {\n    \"code\": 400,\n    \"message\": \"Requests ending with a model turn are not supported.\",\n    \"status\": \"INVALID_ARGUMENT\"\n  }\n}\n",
          "metadata": {
            "url": "https://aiplatform.googleapis.com/v1beta1/projects/XXXX/locations/global/publishers/google/models/gemini-3.8-flash:streamGenerateContent?alt=sse"
          }
        }
      }
    }
  }
}