Stop playback/recording.
AI agents invoke transport_stop to trigger actions in ReaperMCP. What it does depends on the arguments the agent supplies, and its effects often reach beyond the immediate call — builds kicked off, notifications sent, workflows started.
The tool executes a control command in an external music production application. While stopping playback is a reversible action (playback can be restarted), it actively triggers an external operation whose effects manifest in the REAPER software and any connected audio hardware/monitoring. This fits the Execute category—commands that run operations whose effects depend on system state.
From the tool's definition Tool name is 'transport_stop' which performs an action (stopping playback/recording) that affects the state of REAPER and triggers an external operation—the music production software's transport control.
Documented attack patterns abuse exactly the kind of access transport_stop gives an agent:
PolicyLayer is an MCP gateway — it sits between your AI agents and ReaperMCP, and nothing reaches the server without passing your rules. This is the rule we recommend for transport_stop:
{
"version": "1",
"default": "deny",
"tools": {
"transport_stop": {
"limits": [
{
"counter": "transport_stop_rate",
"window": "minute",
"max": 10,
"scope": "grant"
}
]
}
}
}transport_stop stays usable, but rate-capped — a runaway agent can't fire it dozens of times a minute. Everything else on the server is denied unless you say otherwise.
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Stop playback/recording. It is categorised as a Execute tool in the ReaperMCP MCP Server, which means it can trigger actions or run processes. Use rate limits and argument validation.
Register the Reaper MCP server in PolicyLayer and add a rule for transport_stop: 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 ReaperMCP. Nothing to install.
transport_stop is a Execute tool with high risk. Execute tools should be rate-limited and have argument validation enabled.
Yes. Add a rate_limit block to the transport_stop 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 transport_stop. 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.
transport_stop is provided by the Reaper MCP server (xdarkzx/reaper-mcp). PolicyLayer sits as a proxy in front of this server to enforce policies before tool calls reach the server.
Start from ReaperMCP, add the rest of your stack, and see everything your agents can call. Then put policy on all of it.
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138 ReaperMCP tools catalogued and risk-classified — across an index of 43,000+ MCP servers.