generate_hmac · TinyFn MCP: Risk & Policy

WHAT IT DOES

What generate_hmac does on TinyFn

AI agents invoke generate_hmac to trigger actions in TinyFn. What it does depends on the arguments the agent supplies, and its effects often reach beyond the immediate call — builds kicked off, notifications sent, workflows started.

PARAMETERS

Parameter	Type	Required	Description
`key`	string	Yes	Secret key
`message`	string	Yes	Message to sign
`algorithm`	string	—	Hash algorithm: md5, sha1, sha256, sha512

Parameters from the server's own tool schema.

WHY IT NEEDS A POLICY

Why generate_hmac needs a policy

Generating an HMAC involves executing a cryptographic operation using a secret key. While it doesn't modify persistent data (so not Write/Destructive), it is more than a pure read — it actively computes/produces a new artifact. If misused, an AI agent could generate valid authentication tokens or signatures for unauthorized requests.

From the tool's definition Generate HMAC signature — computes a cryptographic signature using a secret key and input data

Attacks that exploit this kind of access

Recommended rule Ready

Verdict

generate_hmac Rate-limited

generate_hmac stays usable, but rate-capped — a runaway agent can't fire it dozens of times a minute. Everything else on the server is denied unless you say otherwise.

View as policy code

policy.json

{
  "version": "1",
  "default": "deny",
  "tools": {
    "generate_hmac": {
      "limits": [
        {
          "counter": "generate_hmac_rate",
          "window": "minute",
          "max": 10,
          "scope": "grant"
        }
      ]
    }
  }
}

RATE-LIMIT THIS TOOL →

Instant setup, no code required.

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

FAQ

Questions about generate_hmac

What does the generate_hmac tool do? +

Generate HMAC signature. It is categorised as a Execute tool in the TinyFn MCP Server, which means it can trigger actions or run processes. Use rate limits and argument validation.

What parameters does generate_hmac accept? +

generate_hmac accepts 3 parameters: key, message, algorithm. Required: key, message. The full parameter table on this page comes from the server's own tool schema.

How do I enforce a policy on generate_hmac? +

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

What risk level is generate_hmac? +

generate_hmac is a Execute tool with high risk. Execute tools should be rate-limited and have argument validation enabled.

Can I rate-limit generate_hmac? +

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

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

generate_hmac is provided by the TinyFn MCP server (https://api.tinyfn.io/mcp/all/). PolicyLayer sits as a proxy in front of this server to enforce policies before tool calls reach the server.