Create or overwrite a file. Bypasses native Write
AI agents use mcx_write to create or update resources in MCX (Modular Code Execution) — usually the action step of a workflow, after the agent has gathered context. Every call changes real data in your MCX (Modular Code Execution) environment.
An AI agent can call mcx_write faster than any human can review — one bad instruction and it creates or modifies resources in MCX (Modular Code Execution) by the hundred, each call as confident as the last.
Documented attack patterns abuse exactly the kind of access mcx_write gives an agent:
PolicyLayer is an MCP gateway — it sits between your AI agents and MCX (Modular Code Execution), and nothing reaches the server without passing your rules. This is the rule we recommend for mcx_write:
{
"version": "1",
"default": "deny",
"tools": {
"mcx_write": {
"limits": [
{
"counter": "mcx_write_rate",
"window": "minute",
"max": 30,
"scope": "grant"
}
]
}
}
}mcx_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.
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Create or overwrite a file. Bypasses native Write. It is categorised as a Write tool in the MCX (Modular Code Execution) MCP Server, which means it can create or modify data. Consider rate limits to prevent runaway writes.
Register the MCX (Modular Code Execution) MCP server in PolicyLayer and add a rule for mcx_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 MCX (Modular Code Execution). Nothing to install.
mcx_write 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 mcx_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.
Set action: deny in the PolicyLayer policy for mcx_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.
mcx_write is provided by the MCX (Modular Code Execution) MCP server (schizoidcock/mcx). PolicyLayer sits as a proxy in front of this server to enforce policies before tool calls reach the server.
Deterministic rules across all 18 MCX (Modular Code Execution) tools. Per-identity grants. Full audit log. Live in minutes. Nothing to install.
Free to start. No card required.
18 MCX (Modular Code Execution) tools catalogued and risk-classified — across an index of 42,500+ MCP servers.