edit_file_lines · MCP Start App MCP: Risk & Policy

WHAT IT DOES

What edit_file_lines does on MCP Start App

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

WHY IT NEEDS A POLICY

Why edit_file_lines needs a policy

This tool creates or modifies data reversibly by replacing lines within a file. While the modification is targeted (specific lines rather than wholesale file deletion), it does alter existing content and could corrupt configuration files, source code, or other critical data if misused by an agent.

From the tool's definition Tool name 'edit_file_lines' combined with description 'Replace specific lines in a file with new content' indicates modification of existing file content. The description explicitly uses 'Replace', confirming write capability.

Attacks that exploit this kind of access

Recommended rule Ready

Verdict

edit_file_lines Rate-limited

edit_file_lines 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": {
    "edit_file_lines": {
      "limits": [
        {
          "counter": "edit_file_lines_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 MCP Start App, so you can run agents against it safely.

FAQ

Questions about edit_file_lines

What does the edit_file_lines tool do? +

Replace specific lines in a file with new content. It is categorised as a Write tool in the MCP Start App MCP Server, which means it can create or modify data. Consider rate limits to prevent runaway writes.

How do I enforce a policy on edit_file_lines? +

Register the MCP Start App MCP server in PolicyLayer and add a rule for edit_file_lines: 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 MCP Start App. Nothing to install.

What risk level is edit_file_lines? +

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

Can I rate-limit edit_file_lines? +

Yes. Add a rate_limit block to the edit_file_lines 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 edit_file_lines completely? +

Set action: deny in the PolicyLayer policy for edit_file_lines. 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 edit_file_lines? +

edit_file_lines is provided by the MCP Start App MCP server (nexus-aissam/mcp-local). PolicyLayer sits as a proxy in front of this server to enforce policies before tool calls reach the server.