fs_write · E MCP: Risk & Policy

WHAT IT DOES

What fs_write does on Emcp

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

WHY IT NEEDS A POLICY

Why fs_write needs a policy

fs_write creates or modifies files on the filesystem. This is a Write operation (reversible modification) rather than Read (fs_read exists separately) or Destructive (fs_rm handles deletion). Severity is high because an AI agent with fs_write access could modify critical configuration files, application code, or data files, causing significant damage—though not catastrophic like Destructive operations.

From the tool's definition Tool named 'fs_write' on a filesystem operations server; sibling tools include fs_read, fs_rm, fs_mkdir, and shell_run, establishing this as a file manipulation tool.

Attacks that exploit this kind of access

Recommended rule Ready

Verdict

fs_write Rate-limited

fs_write 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": {
      "limits": [
        {
          "counter": "fs_write_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 Emcp, so you can run agents against it safely.

FAQ

Questions about fs_write

What does the fs_write tool do? +

fs_write. It is categorised as a Write tool in the Emcp 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? +

Register the E MCP server in PolicyLayer and add a rule for fs_write: 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 Emcp. Nothing to install.

What risk level is fs_write? +

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

Can I rate-limit fs_write? +

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

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

fs_write is provided by the E MCP server (slezica/emcp). PolicyLayer sits as a proxy in front of this server to enforce policies before tool calls reach the server.