[BUG] [VSCODE] WebFetch should not attempt requests to RFC 1918 private network addresses

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?

Description

When Claude Code uses the WebFetch tool on a URL containing a private/internal IP address (RFC 1918), the request always fails because the tool runs externally and cannot reach the user's local network.

Currently there's no guardrail preventing this — Claude will attempt the fetch, wait for it to timeout or fail, and only then realize it can't access the resource.

Expected behavior

Claude Code should detect private network IPs before attempting the fetch and either:

• Skip the fetch and inform the user that private addresses are unreachable
• Suggest alternatives (e.g., asking the user to paste the content, or using curl via the Bash tool which runs locally)

Affected address ranges

Per RFC 1918 and related:

• 10.0.0.0/8
• 172.16.0.0/12
• 192.168.0.0/16
• 127.0.0.0/8 (loopback)
• ::1, fc00::/7 (IPv6 private/loopback)

Reproduction

1. Have a service running on a local network (e.g. http://192.168.1.100:5009/api/health)
2. Ask Claude Code to fetch or check that URL
3. Claude attempts WebFetch, which fails

What Should Happen?

Suggested fix

Add a pre-flight check in the WebFetch tool that rejects private/loopback IPs with a clear message, so Claude can immediately fall back to local alternatives.

Error Messages/Logs

Steps to Reproduce

1. Have a service running on a local network (e.g. http://192.168.1.100:5009/api/health)
2. Ask Claude Code to fetch or check that URL
3. Claude attempts WebFetch, which fails

Claude Model

Opus

Is this a regression?

No, this never worked

Last Working Version

_No response_

Claude Code Version

8,010,114 Claude Code for VS Code

Platform

Anthropic API

Operating System

Windows

Terminal/Shell

WSL (Windows Subsystem for Linux)

Additional Information

_No response_