thread_select · Rr MCP: Risk & Policy

WHAT IT DOES

What thread_select does on Rr

AI agents invoke thread_select to trigger actions in Rr. 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.

WHY IT NEEDS A POLICY

Why thread_select needs a policy

Switching the active thread in a debugging session is an action that changes the execution context/state of the debugger. It triggers an external operation (thread context switch in the rr debugging session) whose effects depend on the thread argument provided.

From the tool's definition Switch to a different thread for inspection

Attacks that exploit this kind of access

Recommended rule Ready

Verdict

thread_select Rate-limited

thread_select 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.

View as policy code

policy.json

{
  "version": "1",
  "default": "deny",
  "tools": {
    "thread_select": {
      "limits": [
        {
          "counter": "thread_select_rate",
          "window": "minute",
          "max": 10,
          "scope": "grant"
        }
      ]
    }
  }
}

RATE-LIMIT THIS TOOL →

Instant setup, no code required.

PolicyLayer is an MCP gateway: every tool call is checked against your rule before it reaches Rr, so you can run agents against it safely.

FAQ

Questions about thread_select

What does the thread_select tool do? +

Switch to a different thread for inspection. It is categorised as a Execute tool in the Rr MCP Server, which means it can trigger actions or run processes. Use rate limits and argument validation.

How do I enforce a policy on thread_select? +

Register the Rr MCP server in PolicyLayer and add a rule for thread_select: 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 Rr. Nothing to install.

What risk level is thread_select? +

thread_select is a Execute tool with high risk. Execute tools should be rate-limited and have argument validation enabled.

Can I rate-limit thread_select? +

Yes. Add a rate_limit block to the thread_select 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.

How do I block thread_select completely? +

Set action: deny in the PolicyLayer policy for thread_select. 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.

What MCP server provides thread_select? +

thread_select is provided by the Rr MCP server (jnjaeschke/rr-mcp). PolicyLayer sits as a proxy in front of this server to enforce policies before tool calls reach the server.