# Testing Loops with a Scripted Model — Agent Loops, Stop Conditions and Token Budgets

Source: https://www.geekswithgeeks.com/en/agent-loops/test-fake-model

> Replace the real model with a script of replies so loop logic is tested fast and deterministically.

## Test the loop, not the model

Real model calls are slow, costly and not repeatable, so they are bad for unit tests of your loop. Instead inject a **fake model** that returns a prepared sequence of replies (two tool requests, then a final answer). Then you can assert the loop stops correctly, charges the budget and enforces its limits, in milliseconds.

## A scripted model and a test

The fake object has the same shape as the real reply for the fields your loop reads. The first demo ran as shown and counted 3 steps.

```python
class FakeReply:
    def __init__(self, stop_reason, text=""):
        self.stop_reason, self.text = stop_reason, text

script = iter([FakeReply("tool_use"), FakeReply("tool_use"), FakeReply("end_turn", "done")])
steps = 0
for reply in script:
    steps += 1
    if reply.stop_reason != "tool_use":
        break
print(steps)

# pytest style:
# def test_stops_after_step_limit():
#     with pytest.raises(RuntimeError):
#         run(messages, call_model=lambda m: FakeReply("tool_use"), max_steps=3)
```

Output:

```
3
```

## Test the nasty cases

Write tests for a model that never stops calling tools, one that repeats the same call, and one whose tool always errors. These are the situations your guards exist for.

**Quiz:** Why use a fake model in unit tests of the loop?

- [x] It makes tests fast, free and repeatable
- [ ] Real models cannot be called from tests
- [ ] Fake models are smarter
- [ ] It tests the model's quality

*Answer:* It makes tests fast, free and repeatable. Scripted replies make the loop's logic deterministic and cheap to verify.