[BUG] WebFetch downloads content successfully but returns 404 errors to assistant when using Vertex AI

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?

When using Claude Code with Google Vertex AI, the WebFetch tool shows successful downloads in the UI (200 OK status) but returns 404 errors in function results to the assistant, preventing the AI from accessing or analyzing the fetched content.

Environment:

• Claude Code Version: 2.0.31
• Platform: macOS (Darwin 24.6.0)
• Model: Claude Sonnet 4.5 (claude-sonnet-4-5@20250929)
• API Provider: Google Vertex AI
• Region: us-central1 / global

Configuration:
CLAUDE_CODE_USE_VERTEX=1
CLOUD_ML_REGION=us-central1
CLAUDE_MODEL=claude-sonnet-4-5@20250929

Actual Behavior:

What the user sees in UI:
Fetch(https://example.com)
⎿ Received 513 bytes (200 OK)

Fetch(https://www.anthropic.com)
⎿ Received 266.4KB (200 OK)

Fetch(https://news.ycombinator.com)
⎿ Received 33.9KB (200 OK)

What the assistant receives in function results:
API Error: 404 {"error":{"code":404,"message":"Requested entity was not found.","status":"NOT_FOUND"}}

What Should Happen?

Expected Behavior:

• WebFetch should download the content AND return it to the assistant in function results
• Assistant should be able to analyze and summarize the fetched content

Error Messages/Logs

API Error: 404 {"error":{"code":404,"message":"Requested entity was not found.","status":"NOT_FOUND"}}

Steps to Reproduce

1. Configure Claude Code to use Vertex AI (set environment variables as shown above)
2. Start a conversation with Claude Code
3. Request the assistant to fetch a website using WebFetch
4. Observe the UI output vs function results

Claude Model

Sonnet (default)

Is this a regression?

I don't know

Last Working Version

_No response_

Claude Code Version

2.0.31

Platform

Google Vertex AI

Operating System

macOS

Terminal/Shell

Terminal.app (macOS)

Additional Information

_No response_