squash · Jj MCP: Risk & Policy

WHAT IT DOES

What squash does on Jj

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

WHY IT NEEDS A POLICY

Why squash needs a policy

Squash moves/merges changes between revisions, which modifies repository history. While it alters commit structure, it is a standard VCS operation that can generally be undone (jj is designed with mutability in mind and has an undo feature), placing it in Write rather than Destructive. Misuse could corrupt commit history or lose separation of changes.

From the tool's definition Move changes from one revision into another revision in a Jujutsu (jj) repository

Risk signalsAdmin/system-level operation

Attacks that exploit this kind of access

Recommended rule Ready

Verdict

squash Rate-limited

squash 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": {
    "squash": {
      "limits": [
        {
          "counter": "squash_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 Jj, so you can run agents against it safely.

FAQ

Questions about squash

What does the squash tool do? +

Move changes from one revision into another revision in a Jujutsu (jj) repository. Only supports whole-file changes (no --interactive flag support). Parameters: source (Source revision to squash from, defaults to working copy), destination (Destination revision to squash into), paths (Optional path patterns to squash, whole paths only, no partial file changes), keepEmptied (Optional flag to keep emptied source revision), repoPath (Optional path to repo root or working directory), cwd (Optional working directory to run the command in). It is categorised as a Write tool in the Jj MCP Server, which means it can create or modify data. Consider rate limits to prevent runaway writes.

How do I enforce a policy on squash? +

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

What risk level is squash? +

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

Can I rate-limit squash? +

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

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

squash is provided by the Jj MCP server (jj-mcp-server). PolicyLayer sits as a proxy in front of this server to enforce policies before tool calls reach the server.