Writes content to a file, creating directories if necessary.
AI agents use write_file to create or update resources in MCP Tools — usually the action step of a workflow, after the agent has gathered context. Every call changes real data in your MCP Tools environment.
This tool creates or modifies data reversibly through file writes. While not destructive (data can be recovered or overwritten), it is Write-category because it persistently modifies the file system. Severity is high because an LLM agent misusing this could overwrite critical configuration files, application code, or user data, even if the changes are theoretically reversible.
From the tool's definition Tool name is 'write_file' and description states it 'Writes content to a file, creating directories if necessary.' This directly creates or modifies files on the file system.
Documented attack patterns abuse exactly the kind of access write_file gives an agent:
PolicyLayer is an MCP gateway — it sits between your AI agents and MCP Tools, and nothing reaches the server without passing your rules. This is the rule we recommend for write_file:
{
"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.
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Writes content to a file, creating directories if necessary. It is categorised as a Write tool in the MCP Tools MCP Server, which means it can create or modify data. Consider rate limits to prevent runaway writes.
Register the MCP Tools 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 Tools. Nothing to install.
write_file is a Write tool with medium risk. Write tools should be rate-limited to prevent accidental bulk modifications.
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.
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.
write_file is provided by the MCP Tools MCP server (zbigniewtomanek/my-mcp-server). PolicyLayer sits as a proxy in front of this server to enforce policies before tool calls reach the server.
Start from MCP Tools, add the rest of your stack, and see everything your agents can call. Then put policy on all of it.
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6 MCP Tools tools catalogued and risk-classified — across an index of 43,000+ MCP servers.