build_url · TinyFn MCP: Risk & Policy

WHAT IT DOES

What build_url does on TinyFn

AI agents invoke build_url 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
`base`	string	Yes	Base URL
`path`	string	—	Path to append
`params`	string	—	Query params as key=value&key2=value2

Parameters from the server's own tool schema.

WHY IT NEEDS A POLICY

Why build_url needs a policy

build_url triggers real processes with real consequences. An agent gone sideways doesn't fire it once — it starts dozens of builds, sends mass notifications, or burns through compute before anyone looks up.

Risk signalsAccepts file system path (path)

Attacks that exploit this kind of access

Recommended rule Ready

Verdict

build_url Rate-limited

build_url 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": {
    "build_url": {
      "limits": [
        {
          "counter": "build_url_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 build_url

What does the build_url tool do? +

Build a URL from components. 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 build_url accept? +

build_url accepts 3 parameters: base, path, params. Required: base. The full parameter table on this page comes from the server's own tool schema.

How do I enforce a policy on build_url? +

Register the TinyFn MCP server in PolicyLayer and add a rule for build_url: 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 build_url? +

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

Can I rate-limit build_url? +

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

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

build_url 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.