[BUG] WebFetchTool returns invalid JSON when invoked via MCP

Environment

• Platform (select one):
• [ ] Anthropic API
• [ ] AWS Bedrock
• [ ] Google Vertex AI
• [x] Other: MCP in Claude Desktop
• Claude CLI version: 0.2.56
• Operating System: Mac OS
• Terminal: N/A (Claude Desktop)

Bug Description

When invoked via MCP, the WebFetchTool returns a line containing getClaudeMdHosts Set(0) {} before sending a line with valid JSON, which causes an issue to pop up in Claude.

<img width="1112" alt="Image" src="https://github.com/user-attachments/assets/65db0a62-5140-4ad4-a741-17d87d59b739" />

I can also reproduce it when calling the MCP tool via the CLI:

<img width="1481" alt="Image" src="https://github.com/user-attachments/assets/0122d788-3215-4e31-84a0-c16b6681dd54" />

Steps to Reproduce

1. In a terminal, run claude mcp serve
2. When the MCP server starts, paste {"jsonrpc":"2.0","id":"toolu_0115GhU2yDntaQ2FkEMwqNf4","method":"tools/call","params":{"name":"WebFetchTool","arguments":{"url": "https://modelcontextprotocol.io/introduction", "prompt": "Summarize the ModelContextProtocol introduction, including the title, description, and any important comments or context."}}} and hit enter
3. Observe that the invalid JSON comes back

Expected Behavior

The getClaudeMdHosts Set(0) {} line should be entirely omitted

Actual Behavior

It is not

Additional Context

Similar repro to #646, but I'm filing a separate issue as I believe these are unrelated.