# Testing Graphs Without a Model — LangGraph Agents & Multi-Agent Systems

Source: https://www.geekswithgeeks.com/en/langgraph-agents/t-test

> Unit-test nodes, routers and whole flows with scripted fakes.

## Most of an agent is ordinary code

Nodes and routers are plain functions, so test them directly with sample states. For whole-graph tests, replace the model node with a **scripted fake** that returns a fixed sequence (as in the tool-loop example) and assert on the final state, the nodes visited and the stop reason. Cover the unhappy paths: a tool error, an invalid tool name, the loop cap, a rejected approval, a thread resumed after an interrupt. Keep a **small suite against the real model** for quality, run separately and less often, since real outputs vary. Snapshot the graph's structure (nodes and edges) so accidental changes to the flow show up in review.

## Make agents dependable

Test nodes and routes without a model, evaluate whole trajectories, and deploy with tracing and guardrails.

![Three habits: test, evaluate, protect.](assets/figures/langgraph-agents/section-7-map.svg) — Figure 7.1 — Test, evaluate and protect.

## A graph test with a scripted model (illustrative)

Uses the same style as the tool-loop example above: no network, deterministic. Not run as a test here.

```python
def test_agent_stops_after_final_answer():
    app = build_app(model_script=[
        {"tool": "add", "args": {"a": 2, "b": 3}},
        {"final": "5"},
    ])
    out = app.invoke({"messages": [{"user": "2+3?"}]}, {"recursion_limit": 10})
    assert out["messages"][-1] == {"final": "5"}
    assert {"tool_result": 5} in out["messages"]

def test_unknown_tool_is_reported_not_executed():
    app = build_app(model_script=[{"tool": "rm", "args": {}}, {"final": "sorry"}])
    out = app.invoke({"messages": [{"user": "delete everything"}]}, {"recursion_limit": 10})
    assert {"tool_result": "error: unknown tool"} in out["messages"]
```

**Quiz:** How do you test a whole graph without calling a real LLM?

- [ ] Call production for every test
- [ ] You cannot
- [x] Replace the model node with a scripted fake
- [ ] Delete the graph

*Answer:* Replace the model node with a scripted fake. A deterministic fake makes tests fast, free and repeatable.