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How to sandbox an MCP server with one line in .mcp.json with Sandbox Runtime

Wrap a local MCP server in srt so it can only write and connect where you allow.

Short answer: Run npm install -g @anthropic-ai/sandbox-runtime; then in .mcp.json, change the server's command to srt and move the original command into args, for example "command": "srt" with "args": ["npx", "-y", "@modelcontextprotocol/server-filesystem"]; create ~/.srt-settings.json with filesystem rules (denyRead, allowWrite, denyWrite) and network rules (allowedDomains, deniedDomains). All steps.

Who it is forA developer adding a community MCP server to Claude Code who does not fully trust what it might write or call.

The problemA local MCP server runs with your full user permissions, so a bad one can read secrets or write anywhere.

The outcomeThe same MCP server running under OS-level limits, with writes outside allowed paths failing with operation not permitted.

What you need first

  • Node.js and npm to install the CLI
  • macOS, Linux with bubblewrap, or Windows
  • Optional: a settings file at ~/.srt-settings.json; without one srt uses built-in defaults

Steps

  1. Run npm install -g @anthropic-ai/sandbox-runtime
  2. In .mcp.json, change the server's command to srt and move the original command into args, for example "command": "srt" with "args": ["npx", "-y", "@modelcontextprotocol/server-filesystem"]
  3. Create ~/.srt-settings.json with filesystem rules (denyRead, allowWrite, denyWrite) and network rules (allowedDomains, deniedDomains)
  4. Restart your agent and ask the MCP server to write to a denied path to confirm it gets EPERM

Prompt for your agent

Install anthropics/sandbox-runtime and wrap my <MCP server> in srt in .mcp.json. Write ~/.srt-settings.json so it can write only to <allowed path> and reach only <allowed domains>, then prove it by asking it to write to <denied path> and showing me the error.

Caveat. It is a beta research preview, so the README warns that APIs and configuration formats may change.

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Using the Deduped MCP, get the setup steps for anthropics/sandbox-runtime and "Sandbox an MCP server with one line in .mcp.json"

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claude mcp add --transport http deduped https://dedupedai.com/mcp

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