set_brush_size · Paint MCP: Risk & Policy

WHAT IT DOES

What set_brush_size does on Paint MCP

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

WHY IT NEEDS A POLICY

Why set_brush_size needs a policy

This tool changes drawing configuration parameters that influence how subsequent strokes appear on the canvas. It is a reversible modification with no permanent data loss or external side effects—the brush size can be changed again or the canvas can be cleared. It does not execute arbitrary code, delete data, or commit financial transactions.

From the tool's definition Tool modifies canvas state by adjusting brush parameters ('Set the brush/stroke size'), which affects subsequent drawing operations but is reversible and non-destructive.

Attacks that exploit this kind of access

Recommended rule Ready

Verdict

set_brush_size Rate-limited

set_brush_size 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": {
    "set_brush_size": {
      "limits": [
        {
          "counter": "set_brush_size_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 Paint MCP, so you can run agents against it safely.

FAQ

Questions about set_brush_size

What does the set_brush_size tool do? +

Set the brush/stroke size (1-50 pixels). It is categorised as a Write tool in the Paint MCP MCP Server, which means it can create or modify data. Consider rate limits to prevent runaway writes.

How do I enforce a policy on set_brush_size? +

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

What risk level is set_brush_size? +

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

Can I rate-limit set_brush_size? +

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

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

set_brush_size is provided by the Paint MCP server (joeyballentine/paint-mcp). PolicyLayer sits as a proxy in front of this server to enforce policies before tool calls reach the server.