fill_zones · Kicad MCP: Risk & Policy

WHAT IT DOES

What fill_zones does on Kicad

AI agents invoke fill_zones to trigger actions in Kicad. 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 fill_zones needs a policy

This tool triggers an external operation (zone filling) in KiCad's pcbnew application. It modifies the PCB design by filling copper zones, which is a computational operation that applies changes to the board layout. While it modifies data, it's more of an 'execute' action that runs the zone filler algorithm rather than a simple write/create.

From the tool's definition Fill all copper zones on the board using pcbnew's zone filler

Attacks that exploit this kind of access

Recommended rule Ready

Verdict

fill_zones Rate-limited

fill_zones 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": {
    "fill_zones": {
      "limits": [
        {
          "counter": "fill_zones_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 Kicad, so you can run agents against it safely.

FAQ

Questions about fill_zones

What does the fill_zones tool do? +

Fill all copper zones on the board using pcbnew's zone filler. It is categorised as a Execute tool in the Kicad MCP Server, which means it can trigger actions or run processes. Use rate limits and argument validation.

How do I enforce a policy on fill_zones? +

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

What risk level is fill_zones? +

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

Can I rate-limit fill_zones? +

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

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

fill_zones is provided by the Kicad MCP server (productofamerica/mcp-server-kicad). PolicyLayer sits as a proxy in front of this server to enforce policies before tool calls reach the server.