save_canvas · Paint MCP: Risk & Policy

WHAT IT DOES

What save_canvas does on Paint MCP

AI agents use save_canvas 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 save_canvas needs a policy

The tool creates a new file or overwrites an existing one at a user-specified path. This is a reversible write operation (the file can be deleted or replaced). It is not Destructive because file creation/overwrite via a drawing application is typically expected and recoverable.

From the tool's definition Tool description states 'Save the current canvas to a PNG file at the given path' — this creates/writes a file to the filesystem.

Attacks that exploit this kind of access

Recommended rule Ready

Verdict

save_canvas Rate-limited

save_canvas 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": {
    "save_canvas": {
      "limits": [
        {
          "counter": "save_canvas_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 save_canvas

What does the save_canvas tool do? +

Save the current canvas to a PNG file at the given path. 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 save_canvas? +

Register the Paint MCP server in PolicyLayer and add a rule for save_canvas: 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 save_canvas? +

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

Can I rate-limit save_canvas? +

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

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

save_canvas 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.