[Bug] HTTP 400 Error when creating files using the Claude model (assistant message prefill not supported)

Google Antigravity
1

When using the Claude model, standard chat functionalities work perfectly fine. However, triggering a “create file” action causes an error and fails. Interestingly, if I switch the model to Gemini, the exact same file creation process completes successfully without any issues.

Based on the error logs, it seems the file creation workflow uses the “assistant message prefill” feature (ending the conversation payload with an assistant message) when calling the API, but the currently integrated Claude model does not support this format.

Error Logs:

  • Trajectory ID: `a4ac628c-cb60-4ea2-9518-88202c8e…

  • Error: HTTP 400 Bad Request

  • TraceID: `0x2036d0d8abf22f..

Sherlog / Headers:

{
“Alt-Svc”: [“h3=”:443"; ma=2592000,h3-29=“:443”; ma=2592000"],
“Content-Length”: [“328”],
“Content-Type”: [“text/event-stream”],
“Date”: [“Thu, 28 May 2026 18:19:30 GMT”],
“Server”: [“ESF”],
“Server-Timing”: [“gfet4t7; dur=1319”],
“Vary”: [“Origin”,“X-Origin”,“Referer”],
“X-Cloudaicompanion-Trace-Id”: [“2036d0d8abf22f89”],
“X-Content-Type-Options”: [“nosniff”],
“X-Frame-Options”: [“SAMEORIGIN”],
“X-Xss-Protection”: [“0”]
}

Error Body:

{
“error”: {
“code”: 400,
“message”: “{“type”:“error”,“error”:{“type”:“invalid_request_error”,“message”:“This model does not support assistant message prefill. The conversation must end with a user message.”},“request_id”:“req_vrtx_011CbVRohMJpnjdouz7xHwm7”}”,
“status”: “INVALID_ARGUMENT”
}
}

3

Facing the same issue… Have you found any solution to this issue yet?

4

Yes.I’ve been digging into this recently,and it looks like others are running into the same HTTP 400 issue.

It seems like the bug might be patched in the most recent Antigravity release, and my issue was solved by updating it. So maybe you can try it too.

Since I was searching for fixes, I also collected a few other things people suggested online that might help if the update doesn’t immediately work:

  • Reinstall Antigravity: I did try a full reinstallation myself, but honestly, it didn’t seem to help much in my case.
  • Clear Cache and Re-login: Some users have suggested this error is sometimes caused by the conversation project’s skill/MCP servers. Clearing your application cache and attempting a full re-login is a common fix.