files_upload · Pyfastmail MCP: Risk & Policy

WHAT IT DOES

What files_upload does on Pyfastmail

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

WHY IT NEEDS A POLICY

Why files_upload needs a policy

File upload is a Write operation: it creates and stores new data in the user's Fastmail account. It is reversible (files can be deleted), so it does not reach Destructive severity. However, it carries medium severity due to potential for storage quota abuse, uploading malicious content to the user's account, or overwriting existing files if that capability exists.

From the tool's definition Tool description states 'Upload a file to Fastmail Files (WebDAV)' — this creates new file data in the user's storage account.

Attacks that exploit this kind of access

Recommended rule Ready

Verdict

files_upload Rate-limited

files_upload 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": {
    "files_upload": {
      "limits": [
        {
          "counter": "files_upload_rate",
          "window": "minute",
          "max": 30,
          "scope": "grant"
        }
      ]
    }
  }
}

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PolicyLayer is an MCP gateway: every tool call is checked against your rule before it reaches Pyfastmail, so you can run agents against it safely.

FAQ

Questions about files_upload

What does the files_upload tool do? +

Upload a file to Fastmail Files (WebDAV). It is categorised as a Write tool in the Pyfastmail MCP Server, which means it can create or modify data. Consider rate limits to prevent runaway writes.

How do I enforce a policy on files_upload? +

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

What risk level is files_upload? +

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

Can I rate-limit files_upload? +

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

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

files_upload is provided by the Pyfastmail MCP server (pjosols/pyfastmail-mcp). PolicyLayer sits as a proxy in front of this server to enforce policies before tool calls reach the server.