set_metadata · Calibre MCP: Risk & Policy

WHAT IT DOES

What set_metadata does on Calibre

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

WHY IT NEEDS A POLICY

Why set_metadata needs a policy

The tool modifies metadata fields in a Calibre library, making it a Write operation. It is reversible (metadata can be changed again), so not Destructive. The confidence is reduced slightly because the description is empty, relying on name and context inference. Severity is medium because misuse could corrupt library metadata, but effects are localized to the Calibre database and reversible.

From the tool's definition Tool name 'set_metadata' indicates modification of book metadata in a Calibre library. Sibling tools include 'get_' and 'search_' operations (Read category) and this tool stands alone as a write operation.

Attacks that exploit this kind of access

Recommended rule Ready

Verdict

set_metadata Rate-limited

set_metadata 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_metadata": {
      "limits": [
        {
          "counter": "set_metadata_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 Calibre, so you can run agents against it safely.

FAQ

Questions about set_metadata

What does the set_metadata tool do? +

set_metadata. It is categorised as a Write tool in the Calibre 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_metadata? +

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

What risk level is set_metadata? +

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

Can I rate-limit set_metadata? +

Yes. Add a rate_limit block to the set_metadata 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_metadata completely? +

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

set_metadata is provided by the Calibre MCP server (xmkevinchen/calibre-mcp). PolicyLayer sits as a proxy in front of this server to enforce policies before tool calls reach the server.