write_file

Q: What does the write_file tool do?

Completely replace file contents. Best for large changes (>20% of file) or when edit_block fails. It is categorised as a Write tool in the Desktop Commander MCP MCP Server, which means it can create or modify data. Consider rate limits to prevent runaway writes.

THE RISK

Medium Risk

Why write_file needs a policy

This tool creates or modifies file data reversibly by completely replacing file contents. While it can overwrite files, the operation itself is reversible (files can be restored from backups or version control). It does not delete or irreversibly destroy data, making it Write rather than Destructive.

From the tool's definition Tool description states 'Completely replace file contents' which is a write/modify operation. The tool is designed for updating files reversibly (not deletion).

Documented attack patterns abuse exactly the kind of access write_file gives an agent:

POLICY

How to control write_file

PolicyLayer is an MCP gateway — it sits between your AI agents and Desktop Commander MCP, and nothing reaches the server without passing your rules. This is the rule we recommend for write_file:

policy.json

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

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.

Create a free account and register Desktop Commander MCP — nothing to install.
Add this policy — paste it, or build it visually.
Point your MCP client (Claude, Cursor, anything) at your gateway URL.

LIMIT THIS TOOL →

Free to start. No card required.

EXPLORE

Related tools and policies

More Desktop Commander MCP tools

Destructive force_terminate Force terminate a running terminal session. Destructive kill_process Terminate a running process by PID. Use with caution as this will Execute unblock_command Remove a command from the blacklist. Once unblocked, the command can be executed normally. Execute execute_command Execute a terminal command with timeout. Command will continue running in background if it Write block_command Add a command to the blacklist. Once blocked, the command cannot be executed until unblock Write create_directory Create a new directory or ensure a directory exists. Can create multiple Write edit_block Apply surgical text replacements to files. Best for small changes (<20% of file size). Write move_file Move or rename files and directories. Can move files between directories

All 19 Desktop Commander MCP tools →

Write tools on other servers

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Go deeper

The MCP Attack Database → Documented attack patterns against MCP deployments — and the policies that stop them.
Argument validation → Checking tool call arguments against schema and policy before they run.
Scoped token → Per-person credentials that grant a defined subset of servers and tools.
Rate Limiting MCP Tool Calls: A Practical Guide →
MCP Security: Why Prompt Guardrails Aren't Enough →

FAQ

Questions about write_file

What does the write_file tool do? +

Completely replace file contents. Best for large changes (>20% of file) or when edit_block fails. It is categorised as a Write tool in the Desktop Commander 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 write_file? +

Register the Desktop Commander 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 Desktop Commander MCP. 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 Desktop Commander MCP server (mrgnss/claudedesktopcommander). PolicyLayer sits as a proxy in front of this server to enforce policies before tool calls reach the server.

Enforce policy on every Desktop Commander MCP tool call.

Start from Desktop Commander MCP, add the rest of your stack, and see everything your agents can call. Then put policy on all of it.

CHECK YOUR STACK →