AI agents use project_open to create or update resources in ReaperMCP — usually the action step of a workflow, after the agent has gathered context. Every call changes real data in your ReaperMCP environment.
Opening a project file is a state-modifying operation that changes the current working context within REAPER. While it doesn't directly create or delete data on disk, it transitions the application to a new state and may trigger associated processes (loading audio files, plugins, automation). This is reversible (another project can be opened), so it qualifies as Write rather than Execute or Destructive.
From the tool's definition Tool name 'project_open' and description 'Open .rpp project file' indicate loading a project file into REAPER, which modifies the application state and active workspace context.
Documented attack patterns abuse exactly the kind of access project_open 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 project_open:
{
"version": "1",
"default": "deny",
"tools": {
"project_open": {
"limits": [
{
"counter": "project_open_rate",
"window": "minute",
"max": 30,
"scope": "grant"
}
]
}
}
}project_open 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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Open .rpp project file. It is categorised as a Write tool in the ReaperMCP 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 project_open: 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.
project_open 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 project_open 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 project_open. 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.
project_open 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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