[Bug]: Antigravity: GOOGLE_APPLICATION_CREDENTIALS in provider env is saved but never reaches the runtime #30413

Before submitting

• I searched existing issues and did not find a duplicate.
• I included enough detail to reproduce or investigate the problem.

Area

apps/server

Steps to reproduce

1. Antigravity provider, Sign-in method "Agent Platform (Vertex AI)", with a GCP project and location set and AGY_ADC_AUTH in the provider environment.
2. In Settings > Providers > Antigravity > Environment variables, add GOOGLE_APPLICATION_CREDENTIALS = path to a service-account JSON key. Save.
3. Start a new Antigravity thread and send a message.
4. Inspect the spawned agy_acp_server.par process: tr '\0' '\n' </proc/<pid>/environ | grep -E '^(AGY|GOOGLE)_'.

Expected behavior

GOOGLE_APPLICATION_CREDENTIALS reaches the Antigravity runtime, so Vertex calls authenticate as the service account. Standalone agy honours the variable: AGY_ADC_AUTH=true GOOGLE_APPLICATION_CREDENTIALS=key.json agy -p "hi" works on the same machine.

If T3 deliberately strips the variable, the settings UI should say so instead of accepting and saving it.

Actual behavior

The value is saved (a provider-env-…-GOOGLE_APPLICATION_CREDENTIALS.bin secret appears at 19:33:52, and settings.json lists the entry). The runtime spawned at 19:34:55 has only AGY_ADC_AUTH=true and AGY_ACP_FORCE_FILE_STORAGE=1. GOOGLE_APPLICATION_CREDENTIALS is missing, so the runtime falls back to the user ADC file.

Why it matters: in Google Workspace orgs with Google Cloud session control, user ADC expires every N hours with invalid_grant "reauth related error (invalid_rapt)". A service-account key is the standard fix, and this route to it is silently blocked.

Workaround: overwrite ~/.config/gcloud/application_default_credentials.json with the service-account key. That works, but it changes ADC for every tool on the machine.

Impact

Major degradation or frequent failure

Version or commit

T3 Code 0.1.6 (Linux AppImage), managed Antigravity ACP runtime release 2e37012c…, agy 1.4.12

Environment

Linux Mint 22.3 (kernel 6.8), desktop app, Antigravity with authMethod: "agent-platform" on a paid GCP project (credits, not an AI Pro subscription)

Related: #23036 (Agent Platform ADC setup gaps).