[BUG]: Google Auth Gemini 3 models return 404 "Requested entity was not found" error

[ydemir]

Description

When using Gemini 3 models (gemini-3-pro-preview, gemini-3-flash-preview) via Google OAuth (Antigravity auth), requests fail with:

Requested entity was not found.

[Debug Info]
Requested Model: gemini-3-pro-preview
Effective Model: gemini-3-pro-preview
Status: 404
Endpoint: https://cloudcode-pa.googleapis.com/v1internal:streamGenerateContent?alt=sse

Related Issue

This appears to be related to #4832 (thoughtSignature support), but the error message is different (404 instead of 400).

Environment

• OS: Windows 11
• OpenCode version: 1.0.213
• Plugin: oh-my-opencode + [email protected]
• Auth method: Google OAuth (Antigravity)

Configuration

{
  "google_auth": true,
  "agents": {
    "frontend-ui-ux-engineer": { "model": "google/gemini-3-pro-preview" },
    "document-writer": { "model": "google/gemini-3-flash-preview" }
  }
}

Steps to Reproduce

1. Configure opencode with opencode-antigravity-auth plugin
2. Login with Google OAuth (opencode auth login → Google → Antigravity)
3. Set any agent to use google/gemini-3-pro-preview or google/gemini-3-flash-preview
4. Start opencode and try any command
5. Get 404 error

Expected Behavior

Gemini 3 models should work via Google OAuth just like Gemini 2.5 models do.

Workaround

Using github-copilot/gemini-3-pro-preview works, but this consumes GitHub Copilot quota instead of Google quota.

Additional Context

• google/gemini-2.5-pro and google/gemini-2.5-flash work fine via Google OAuth
• The models appear in /models list under Google provider
• Same Google account works fine with Gemini 2.5 models