> ## Documentation Index
> Fetch the complete documentation index at: https://aixyz.sh/llms.txt
> Use this file to discover all available pages before exploring further.

# Fake Model Agent

> Fully deterministic agent testing using the aixyz fake model — no API key required

<Info>**Source:** [`examples/fake-llm`](https://github.com/AgentlyHQ/aixyz/tree/main/examples/fake-llm)</Info>

## 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:

```typescript title="app/agent.ts" theme={null}
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:

```typescript title="app/agent.test.ts" theme={null}
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

```bash theme={null}
cd examples/fake-llm
bun install
bun test
```

## Running the Agent

```bash theme={null}
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 |