[BUG] GCP OAuth2 token fetched on every request instead of being cached

Preflight Checklist

• [x] I have searched existing issues and this hasn't been reported yet
• [x] This is a single bug report (please file separate reports for different bugs)
• [x] I am using the latest version of Claude Code

What's Wrong?

Environment

• Claude Code version: 2.1.118 (Claude Code)
• Backend: Vertex AI
• Environment variables set:

``
CLAUDE_CODE_USE_VERTEX=1
CLOUD_ML_REGION=global
ANTHROPIC_VERTEX_PROJECT_ID=prj-noncore-dev-totc2
VERTEX_REGION_CLAUDE_3_5_HAIKU=global
``

Describe the bug

Claude Code appears to call https://oauth2.googleapis.com/token on every outgoing request to Vertex AI, rather than fetching a token once, caching it, and reusing it for up to its 1-hour TTL.

Expected behavior

GCP Bearer tokens are valid for 1 hour. The token should be fetched once per session (or per expiry), cached in memory, and reused for all subsequent Vertex AI requests within that window — consistent with standard GCP client library behavior.

Actual behavior

A call to oauth2.googleapis.com/token is observed before every call to aiplatform.googleapis.com, e.g.:

POST https://oauth2.googleapis.com/token        ← repeated every request
POST https://aiplatform.googleapis.com/v1/...   ← actual Claude API call
POST https://oauth2.googleapis.com/token        ← repeated every request
POST https://aiplatform.googleapis.com/v1/...   ← actual Claude API call
...

Impact

• Adds latency to every request (an extra round-trip to Google's auth endpoint)
• Unnecessarily chatty — inflates network traffic and token issuance volume
• May cause rate limiting on oauth2.googleapis.com in high-usage scenarios

Workaround

None available for CLI users.

What Should Happen?

GCP Bearer tokens are valid for 1 hour. The token should be fetched once per session (or per expiry), cached in memory, and reused for all subsequent Vertex AI requests within that window — consistent with standard GCP client library behavior.

Error Messages/Logs

Steps to Reproduce

Run claude and send hi.

Claude Model

Sonnet (default)

Is this a regression?

I don't know

Last Working Version

_No response_

Claude Code Version

2.1.118 (Claude Code)

Platform

Google Vertex AI

Operating System

macOS

Terminal/Shell

Terminal.app (macOS)

Additional Information

_No response_