harness_init · Loom MCP: Risk & Policy

WHAT IT DOES

What harness_init does on Loom

AI agents use harness_init to create or update resources in Loom — usually the action step of a workflow, after the agent has gathered context. Every call changes real data in your Loom environment.

WHY IT NEEDS A POLICY

Why harness_init needs a policy

This tool creates a new harness manifest file in a specified directory. While file creation is reversible (can be deleted), it modifies the filesystem state by introducing a new configuration resource. This is a Write operation rather than Read (no data retrieval) or Execute (no command execution with agent-determined side effects).

From the tool's definition Tool name 'harness_init' and description 'Scaffold a harness manifest at <contextDir>/harnesses/<name>.md from the template' indicates creation of a new file/configuration artifact. The verb 'scaffold' implies generating and writing initial structure.

Attacks that exploit this kind of access

Recommended rule Ready

Verdict

harness_init Rate-limited

harness_init 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.

View as policy code

policy.json

{
  "version": "1",
  "default": "deny",
  "tools": {
    "harness_init": {
      "limits": [
        {
          "counter": "harness_init_rate",
          "window": "minute",
          "max": 30,
          "scope": "grant"
        }
      ]
    }
  }
}

LIMIT THIS TOOL →

Instant setup, no code required.

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

FAQ

Questions about harness_init

What does the harness_init tool do? +

Scaffold a harness manifest at <contextDir>/harnesses/<name>.md from the template. It is categorised as a Write tool in the Loom MCP Server, which means it can create or modify data. Consider rate limits to prevent runaway writes.

How do I enforce a policy on harness_init? +

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

What risk level is harness_init? +

harness_init is a Write tool with medium risk. Write tools should be rate-limited to prevent accidental bulk modifications.

Can I rate-limit harness_init? +

Yes. Add a rate_limit block to the harness_init 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 harness_init completely? +

Set action: deny in the PolicyLayer policy for harness_init. 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 harness_init? +

harness_init is provided by the Loom MCP server (jbarket/loom). PolicyLayer sits as a proxy in front of this server to enforce policies before tool calls reach the server.