Fake Model Agent

Source: examples/fake-llm

Overview

This template demonstrates how to use fake() from aixyz/model to build and test an agent with zero API calls and no API key. The fake model’s transform function deterministically maps each user message to a response, making every test fast, repeatable, and CI-safe. The example agent checks whether the user’s input is a palindrome, or reverses it if not — using both the lastMessage and the full prompt history from the transform signature.

Project Structure

fake-llm/
├── aixyz.config.ts         # Agent metadata
├── app/
│   ├── agent.ts            # Agent using fake() model
│   └── agent.test.ts       # Fully deterministic test suite
├── package.json
└── vercel.json

The Fake Model

Import fake from aixyz/model and pass a transform function:

app/agent.ts

import { fake } from "aixyz/model";
import { ToolLoopAgent } from "ai";
import type { Accepts } from "aixyz/accepts";
import type { Capabilities } from "aixyz/app/plugins/a2a";

export const model = fake((lastMessage, prompt) => {
  const reversed = [...lastMessage].reverse().join("");
  const isPalindrome = lastMessage.toLowerCase() === reversed.toLowerCase();
  const turn = prompt.filter((m) => m.role === "user").length;

  if (isPalindrome) {
    return `"${lastMessage}" is a palindrome! (turn ${turn})`;
  }
  return `"${lastMessage}" reversed is "${reversed}" (turn ${turn})`;
});

export const accepts: Accepts = {
  scheme: "free",
};

export const capabilities: Capabilities = {
  streaming: false,
  pushNotifications: false,
};

export default new ToolLoopAgent({
  model,
  instructions: "You analyze text for palindromes and reverse it when asked.",
});

The transform receives:

lastMessage — the text content of the last user turn
prompt — the full conversation history, useful for tracking turn count or extracting prior context

Test Pattern

Because the fake model is deterministic, every test runs without an API key:

app/agent.test.ts

import { describe, expect, test } from "bun:test";
import type { Prompt } from "aixyz/model";
import { model } from "./agent";

describe("palindrome checker (fake model)", () => {
  test("detects a palindrome", async () => {
    const prompt: Prompt = [{ role: "user", content: [{ type: "text", text: "racecar" }] }];
    const result = await model.doGenerate({ prompt });
    expect(result.content[0]).toEqual(
      expect.objectContaining({ type: "text", text: expect.stringContaining("is a palindrome") }),
    );
  });

  test("tracks turn number from prompt context", async () => {
    const prompt: Prompt = [
      { role: "user", content: [{ type: "text", text: "hi" }] },
      { role: "assistant", content: [{ type: "text", text: "..." }] },
      { role: "user", content: [{ type: "text", text: "level" }] },
    ];
    const result = await model.doGenerate({ prompt });
    expect(result.content[0]).toEqual(
      expect.objectContaining({ type: "text", text: expect.stringContaining("turn 2") }),
    );
  });
});

Key Features

No API key — fake() never makes network requests
Fully deterministic — same input always produces the same output
CI-safe — all tests run in any environment
Full prompt context — the transform’s second argument exposes the entire conversation history

Running Tests

cd examples/fake-llm
bun install
bun test

Running the Agent

cd examples/fake-llm
bun run dev

Endpoints available at http://localhost:3000:

Endpoint	Protocol	Description
`/.well-known/agent-card.json`	A2A	Agent discovery card
`/agent`	A2A	JSON-RPC task handler

Templates

Basic

Advanced

Fake Model Agent

Overview

Project Structure

The Fake Model

Test Pattern

Key Features

Running Tests

Running the Agent

​Overview

​Project Structure

​The Fake Model

​Test Pattern

​Key Features

​Running Tests

​Running the Agent

Overview

Project Structure

The Fake Model

Test Pattern

Key Features

Running Tests

Running the Agent