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Cursor + DiffGate

Use DiffGate as a post-edit verification gate inside Cursor's composer / agent loop.

1. Install

pipx install diffgate
diffgate --version    # sanity check

2. Register the MCP server

Open Cursor → Settings → MCP → Add new MCP server. Add this entry (or paste it directly into ~/.cursor/mcp.json):

{
  "mcpServers": {
    "diffgate": {
      "command": "diffgate",
      "args": ["mcp-server", "--stdio"],
      "env": {}
    }
  }
}

Restart Cursor. The composer's MCP indicator should show diffgate · 1 tool (the tool name is verify_edit).

3. Wire it into the agent loop

In Cursor's Rules → Agent Rules, add:

After every file edit, call the `verify_edit` MCP tool with:
  - before_blob: the file contents you read
  - after_blob:  the file contents you wrote
  - claimed_actions: the structured list of edits you just performed
If verify_edit returns passed=false, do NOT report success. Read the
`mismatches` array, fix the discrepancy, and retry the edit.

That's it. The next time the agent claims "renamed foo→bar across module_x.py" but the actual diff is empty, the tool call returns:

{
  "passed": false,
  "mismatches": ["claimed 4 renames of `foo`, observed 0 in module_x.py"],
  "structural_diff": { "renames": [], "adds": [], "deletes": [] }
}

…and Cursor's loop retries with the failure context, instead of returning success and moving on.

4. One-screen example

User prompt:

Rename foo to bar across module_x.py.

Without DiffGate, the agent sometimes returns:

✓ Renamed foo to bar in 4 locations.

…even when module_x.py is byte-identical. With DiffGate wired in, the agent sees passed: false, mismatches: ["claimed 4 renames of foo, observed 0"], reads the structural diff, retries, and only then reports success.

Troubleshooting

Symptom Likely cause Fix
diffgate: command not found in Cursor pipx binaries not on Cursor's PATH Use the absolute path in command:
verify_edit tool missing from indicator MCP server crashed on startup Run diffgate mcp-server --stdio manually, read stderr
All claims pass even on obvious lies Language not in languages config Add the file's language to your config
False positive on rename strict_renames: true requires all-refs hit Set strict_renames: false

For a richer hook example (and the equivalent Claude Code wiring), see claude_code_hook.md.