apply_diff · Cursor MCP Server MCP: Risk & Policy

WHAT IT DOES

What apply_diff does on Cursor MCP Server

AI agents use apply_diff to create or update resources in Cursor MCP Server — usually the action step of a workflow, after the agent has gathered context. Every call changes real data in your Cursor MCP Server environment.

WHY IT NEEDS A POLICY

Why apply_diff needs a policy

apply_diff modifies file content by applying a patch. This is a reversible write operation—the change can be undone via version control or by applying a reverse diff. Severity is high because misapplication of diffs could corrupt source code or introduce bugs across multiple lines, though the effect is bounded to a single file and recoverable through git or backups.

From the tool's definition The tool description states it 'Apply[s] a unified diff to a file', which modifies file contents. The optional backup feature indicates the operation is reversible, placing it in Write rather than Destructive.

Attacks that exploit this kind of access

Recommended rule Ready

Verdict

apply_diff Rate-limited

apply_diff stays usable, but capped — an agent stuck in a loop can't make hundreds of changes a minute. Everything else on the server is denied unless you say otherwise.

View as policy code

policy.json

{
  "version": "1",
  "default": "deny",
  "tools": {
    "apply_diff": {
      "limits": [
        {
          "counter": "apply_diff_rate",
          "window": "minute",
          "max": 30,
          "scope": "grant"
        }
      ]
    }
  }
}

LIMIT THIS TOOL →

Instant setup, no code required.

PolicyLayer is an MCP gateway: every tool call is checked against your rule before it reaches Cursor MCP Server, so you can run agents against it safely.

FAQ

Questions about apply_diff

What does the apply_diff tool do? +

Apply a unified diff to a file. Optionally creates a backup. It is categorised as a Write tool in the Cursor MCP Server MCP Server, which means it can create or modify data. Consider rate limits to prevent runaway writes.

How do I enforce a policy on apply_diff? +

Register the Cursor MCP Server MCP server in PolicyLayer and add a rule for apply_diff: allow, deny, rate-limit, or require approval. Point your MCP client at the PolicyLayer proxy URL and the rule is enforced on every call, before it reaches Cursor MCP Server. Nothing to install.

What risk level is apply_diff? +

apply_diff is a Write tool with medium risk. Write tools should be rate-limited to prevent accidental bulk modifications.

Can I rate-limit apply_diff? +

Yes. Add a rate_limit block to the apply_diff rule in your PolicyLayer policy. For example, setting max: 10 and window: 60 limits the tool to 10 calls per minute. Rate limits are tracked per agent session and reset automatically.

How do I block apply_diff completely? +

Set action: deny in the PolicyLayer policy for apply_diff. The AI agent will receive a policy violation error and cannot call the tool. You can also include a reason field to explain why the tool is blocked.

What MCP server provides apply_diff? +

apply_diff is provided by the Cursor MCP Server MCP server (tariqnasheed/cursor_mcp_server). PolicyLayer sits as a proxy in front of this server to enforce policies before tool calls reach the server.