Extensions

An extension hooks into the moments of a run: before the model starts, before a tool runs, after each step. Each hook returns what it wants to change, and Latch applies it.

Why it exists

Every product ends up with a few rules of its own: this tool needs the Team plan, show the user what each answer cost, answer /stats without calling the model, keep an audit log. Without extensions, each one means forking the runtime or wrapping every tool. With them, each is one small file.

The smallest version

import { defineExtension } from "@intentface/latch-core";

export const plans = defineExtension("plans", (ext) => {
  // Refuse a tool. The reason reaches the model, so it can explain instead of retrying.
  ext.on("tool_call", ({ toolName }, run) => {
    if (toolName === "export_data" && planOf(run.principal) === "free")
      return { block: true, reason: "Exporting needs the Team plan." };
  });

  // A slash command, answered without the model.
  ext.registerCommand("stats", {
    description: "Messages in this chat",
    handler: async (_args, { messages }) => `${messages.length} messages so far.`,
  });
});

createRuntime({ …, extensions: [plans] });     // every agent
defineAgent({ …, extensions: [plans] });       // or just one

The rules that keep them safe

• Each moment allows certain changes. A tool_call hook can block a tool; it can't rewrite history. That's what keeps two extensions from fighting.
• What's stored stays true. Changing what the user sees never changes what's saved, unless a hook is explicitly about the saved record.
• A broken extension doesn't break the run. A hook that throws is reported and skipped, unless you mark the extension as critical.

Go deeper

• Every moment and what it can change: Extensions in depth