AI agents use undo to create or update resources in Reaper — usually the action step of a workflow, after the agent has gathered context. Every call changes real data in your Reaper environment.
Undo reverses a previous action, modifying the current state of the project. It is a reversible write operation (the undone action can be redone), so it falls under Write rather than Destructive. Misuse could inadvertently revert intentional changes in a music production session.
From the tool's definition Undo the last action in REAPER
Documented attack patterns abuse exactly the kind of access undo gives an agent:
PolicyLayer is an MCP gateway — it sits between your AI agents and Reaper, and nothing reaches the server without passing your rules. This is the rule we recommend for undo:
{
"version": "1",
"default": "deny",
"tools": {
"undo": {
"limits": [
{
"counter": "undo_rate",
"window": "minute",
"max": 30,
"scope": "grant"
}
]
}
}
}undo 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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Undo the last action in REAPER. It is categorised as a Write tool in the Reaper MCP Server, which means it can create or modify data. Consider rate limits to prevent runaway writes.
Register the Reaper MCP server in PolicyLayer and add a rule for undo: 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 Reaper. Nothing to install.
undo 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 undo 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 undo. 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.
undo is provided by the Reaper MCP server (twelvetake-studios/reaper-mcp). PolicyLayer sits as a proxy in front of this server to enforce policies before tool calls reach the server.
Start from Reaper, add the rest of your stack, and see everything your agents can call. Then put policy on all of it.
Free to start. No card required.
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