fs_write_file · Polybridge MCP: Risk & Policy

WHAT IT DOES

What fs_write_file does on Polybridge MCP

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

WHY IT NEEDS A POLICY

Why fs_write_file needs a policy

This tool modifies filesystem state by writing content to files. While file overwrites cannot be easily recovered without backups, the action itself is reversible in principle (the file can be rewritten with different content or deleted). It does not permanently destroy data in the sense of a destructive delete operation, nor does it execute arbitrary code or trigger external workflows.

From the tool's definition Tool description states: 'Write content to a file. Creates the file if it does not exist, overwrites if it does.' The verb 'write' and actions of creating/overwriting files are reversible modifications.

Attacks that exploit this kind of access

Recommended rule Ready

Verdict

fs_write_file Rate-limited

fs_write_file 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": {
    "fs_write_file": {
      "limits": [
        {
          "counter": "fs_write_file_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 Polybridge MCP, so you can run agents against it safely.

FAQ

Questions about fs_write_file

What does the fs_write_file tool do? +

Write content to a file. Creates the file if it does not exist, overwrites if it does. It is categorised as a Write tool in the Polybridge MCP MCP Server, which means it can create or modify data. Consider rate limits to prevent runaway writes.

How do I enforce a policy on fs_write_file? +

Register the Polybridge MCP server in PolicyLayer and add a rule for fs_write_file: 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 Polybridge MCP. Nothing to install.

What risk level is fs_write_file? +

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

Can I rate-limit fs_write_file? +

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

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

fs_write_file is provided by the Polybridge MCP server (madjeek-web/polybridge-mcp). PolicyLayer sits as a proxy in front of this server to enforce policies before tool calls reach the server.