write_file · MCP Files MCP: Risk & Policy

WHAT IT DOES

What write_file does on MCP Files

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

WHY IT NEEDS A POLICY

Why write_file needs a policy

write_file creates or modifies file contents, which is a Write operation. It is reversible—files can be edited or restored. The tool respects security controls (extension whitelist, immutable paths, quota limits), reducing risk.

From the tool's definition Tool description states 'Create or update a UTF-8 file' and includes 'append=true to append instead of replacing', indicating reversible modification of file content without deletion.

Attacks that exploit this kind of access

Recommended rule Ready

Verdict

write_file Rate-limited

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": {
    "write_file": {
      "limits": [
        {
          "counter": "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 MCP Files, so you can run agents against it safely.

FAQ

Questions about write_file

What does the write_file tool do? +

Create or update a UTF-8 file under the workspace. Honors extension whitelist, immutable paths, and quota limits; use append=true to append instead of replacing. It is categorised as a Write tool in the MCP Files MCP Server, which means it can create or modify data. Consider rate limits to prevent runaway writes.

How do I enforce a policy on write_file? +

Register the MCP Files MCP server in PolicyLayer and add a rule for 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 MCP Files. Nothing to install.

What risk level is write_file? +

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

Can I rate-limit write_file? +

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

Set action: deny in the PolicyLayer policy for 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 write_file? +

write_file is provided by the MCP Files MCP server (tspspi/mcpfiles). PolicyLayer sits as a proxy in front of this server to enforce policies before tool calls reach the server.