check_function_availability

THE RISK

Low Risk

Why check_function_availability needs a policy

This is a purely informational tool that queries the permission model and available functions. It has no side effects, creates no data changes, executes no operations, and poses minimal risk if misused — an AI agent calling it repeatedly would only retrieve redundant permission information. Classified as Read.

From the tool's definition Tool name is 'check_function_availability' and description states it 'Check[s] which MCP functions are available based on account permissions' — this is a query/inspection operation that retrieves metadata about available capabilities without modifying any…

Documented attack patterns abuse exactly the kind of access check_function_availability gives an agent:

POLICY

How to control check_function_availability

PolicyLayer is an MCP gateway — it sits between your AI agents and Fastmail MCP Server, and nothing reaches the server without passing your rules. This is the rule we recommend for check_function_availability:

policy.json

{
  "version": "1",
  "default": "deny",
  "tools": {
    "check_function_availability": {}
  }
}

check_function_availability is read-only, so it stays allowed — but everything else on the server is denied unless you say otherwise.

Create a free account and register Fastmail MCP Server — nothing to install.
Add this policy — paste it, or build it visually.
Point your MCP client (Claude, Cursor, anything) at your gateway URL.

CAP THIS TOOL →

Free to start. No card required.

EXPLORE

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Go deeper

The MCP Attack Database → Documented attack patterns against MCP deployments — and the policies that stop them.
Data exfiltration → How read access becomes an exfiltration channel when chained with untrusted content.
MCP token cost → What connecting this server costs in context-window tokens on every request.
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FAQ

Questions about check_function_availability

What does the check_function_availability tool do? +

Check which MCP functions are available based on account permissions. It is categorised as a Read tool in the Fastmail MCP Server MCP Server, which means it retrieves data without modifying state.

How do I enforce a policy on check_function_availability? +

Register the Fastmail MCP Server MCP server in PolicyLayer and add a rule for check_function_availability: 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 Fastmail MCP Server. Nothing to install.

What risk level is check_function_availability? +

check_function_availability is a Read tool with low risk. Read-only tools are generally safe to allow by default.

Can I rate-limit check_function_availability? +

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

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

check_function_availability is provided by the Fastmail MCP Server MCP server (madllama25/fastmail-mcp). PolicyLayer sits as a proxy in front of this server to enforce policies before tool calls reach the server.

Enforce policy on every Fastmail MCP Server tool call.

Start from Fastmail MCP Server, add the rest of your stack, and see everything your agents can call. Then put policy on all of it.

CHECK YOUR STACK →