hg_update · Mercurial MCP: Risk & Policy

WHAT IT DOES

What hg_update does on Mercurial

AI agents invoke hg_update to trigger actions in Mercurial. 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 hg_update needs a policy

hg update changes the working directory state by switching to a different revision, branch, or topic. This is an Execute-level operation because it triggers an external VCS operation that modifies the working directory state (checking out files, updating tracked state).

From the tool's definition Updates to a different topic, branch, or revision

Attacks that exploit this kind of access

Recommended rule Ready

Verdict

hg_update Rate-limited

hg_update 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": {
    "hg_update": {
      "limits": [
        {
          "counter": "hg_update_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 Mercurial, so you can run agents against it safely.

FAQ

Questions about hg_update

What does the hg_update tool do? +

Updates to a different topic, branch, or revision. It is categorised as a Execute tool in the Mercurial MCP Server, which means it can trigger actions or run processes. Use rate limits and argument validation.

How do I enforce a policy on hg_update? +

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

What risk level is hg_update? +

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

Can I rate-limit hg_update? +

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

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

hg_update is provided by the Mercurial MCP server (metal-shark-sharktech/mcp-server-mercurial). PolicyLayer sits as a proxy in front of this server to enforce policies before tool calls reach the server.