regex_replace · TinyFn MCP: Risk & Policy

WHAT IT DOES

What regex_replace does on TinyFn

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

PARAMETERS

Parameter	Type	Required	Description
`text`	string	Yes	Text to process
`flags`	string	—	Flags: i=ignore case, m=multiline, s=dotall
`pattern`	string	Yes	Regular expression pattern
`replacement`	string	Yes	Replacement string

Parameters from the server's own tool schema.

WHY IT NEEDS A POLICY

Why regex_replace needs a policy

This tool creates or modifies data (text) in a reversible manner. While the modification could theoretically be undone or the original preserved, the primary action is transformation of input strings.

From the tool's definition Tool name 'regex_replace' and description 'Replace pattern matches in text' indicate the tool modifies text content based on pattern matching arguments.

Attacks that exploit this kind of access

Recommended rule Ready

Verdict

regex_replace Rate-limited

regex_replace 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": {
    "regex_replace": {
      "limits": [
        {
          "counter": "regex_replace_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 TinyFn, so you can run agents against it safely.

FAQ

Questions about regex_replace

What does the regex_replace tool do? +

Replace pattern matches in text. It is categorised as a Write tool in the TinyFn MCP Server, which means it can create or modify data. Consider rate limits to prevent runaway writes.

What parameters does regex_replace accept? +

regex_replace accepts 4 parameters: text, flags, pattern, replacement. Required: text, pattern, replacement. The full parameter table on this page comes from the server's own tool schema.

How do I enforce a policy on regex_replace? +

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

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

Can I rate-limit regex_replace? +

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

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

regex_replace 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.