AI agents use write_file to create or update resources in MCPX MCP Gateway — usually the action step of a workflow, after the agent has gathered context. Every call changes real data in your MCPX MCP Gateway environment.
This tool reversibly creates or modifies data by writing to a file. It is not destructive (no irreversible deletion), not Execute (no code execution), and not Financial. The severity is medium because file writes can corrupt data or overwrite important files if misused by an AI agent, but the effect is theoretically reversible (via backups or undo).
From the tool's definition Tool name and description: 'Write a file' — explicitly creates or modifies file content on the 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 MCPX MCP Gateway, 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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Write a file. It is categorised as a Write tool in the MCPX MCP Gateway MCP Server, which means it can create or modify data. Consider rate limits to prevent runaway writes.
Register the MCPX MCP Gateway 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 MCPX MCP Gateway. 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 MCPX MCP Gateway MCP server (thelunarcompany/lunar). PolicyLayer sits as a proxy in front of this server to enforce policies before tool calls reach the server.
Deterministic rules across all 14 MCPX MCP Gateway tools. Per-identity grants. Full audit log. Live in minutes. Nothing to install.
Free to start. No card required.
14 MCPX MCP Gateway tools catalogued and risk-classified — across an index of 42,500+ MCP servers.