Model-based testing

Model-based testing describes expected behavior as a small model. It generates action sequences and compares the real system with the model after each step. It suits anything with history: sessions, retries, lifecycles.

David Khourshid7 described the approach with state machines in Model-Based Testing in React with State Machines. XState builds it into xstate/graph: createTestModel wraps a machine, and getShortestPaths turns it into test paths. “Path generation enables model-based testing - generating test cases directly from your state machine.” Graph & Paths

Effect 4’s Arbitrary.array generates command sequences. Its documentation uses a state machine as the example, and failed sequences shrink: the generator removes irrelevant commands to reduce the failing sequence. Arbitrary.ts

In rat-stack, the visitor identity model generates visits from three devices. Visits may keep or drop the cookie. Visits also cross day boundaries.

A persistent id survives every visit that keeps its cookie. A daily id stays stable within a day and sets no cookie. It never repeats on a later day. Making the daily hash ignore the day broke the model at once.

Machines in this repo are lifecycles20, so each one is a ready-made test model. See property-based testing18 for single-input rules.

See also: Tests through supplied services22.

Sources

  1. Model-Based Testing in React with State Machines. David Khourshid · CSS-Tricks. Testing article; used for checking action sequences against a model. Accessed 2026-10-01.
  2. Graph & Paths. David Khourshid · Stately docs. Graph docs; used for generated paths from XState machines. Accessed 2026-10-01.
  3. Effect contributors. 2026. Arbitrary. Effect 4.0.0.. GitHub Effect-TS/effect. Arbitrary API; used for generated and shrinking command sequences. Accessed 2026-10-01.
  4. rat-stack/packages/events/test/visitor.model.test.ts at main · joelhooks/rat-stack · GitHub. GitHub joelhooks/rat-stack. Visitor model test; used for identity across cookies, devices, and days. Accessed 2026-10-01.

Last changed 2026-10-07 in f1a00f2. Source on GitHub. Change log.

Linked from

  1. Agent guide. An Effect stack so pure (aspirational) Kit Langton will blush.

    Model-based testing — Describe what a system should do as a small model, generate action sequences, and check the real system against the model.

  2. Agents explore, tests remember. Let agent users explore scenarios; turn a good bug reproduction into a repeatable test.

    Model-based testing generates sequences of actions and checks the system against a model.

  3. Analytics. One event per HTTP request, stored unchanged for queries to select and interpret later.

    The cartridge uses property-based testing and model-based testing.¶

  4. Capture first, shape later. Store events without rejecting them for an evolving event shape, then decide their shape when you query.

    See property-based testing and model-based testing.¶

  5. Change log. What changed in the files served here, newest first.

    Model-based testing — edited: feat(learn): teach cards 43 to 62 with plain lines, diagrams and snippets

  6. Code snippets quote a pinned source. Shipped design brief: pinned Git objects, typed diagnostics, and one build-only snippet model for both views.

    See model-based testing and tests that earn their place.¶

  7. David Khourshid. David Khourshid teaches state machines with XState and argues for model-based and property-based testing.
    Link context

    In the testing lesson of his Frontend Masters course on XState, he describes a test model as “a state machine, and the state machine represents all of your given when then statements in a single state machine.” He adds that model-based testing lets you “test any application whether they’re using state machines or not.” Testing, State Machines in JavaScript with XState, v2¶

  8. Full agent guide. An Effect stack so pure (aspirational) Kit Langton will blush.
  9. Property-based testing. State a rule that must hold for every input, then let a generator hunt for the smallest input that breaks it.

    For sequences of actions, use model-based testing.¶

  10. Rat Stack lore | rat-stack. Short, source-grounded notes on the ideas and decisions behind rat-stack.

    Model-based testing — Describe what a system should do as a small model, generate action sequences, and check the real system against the model.

  11. Source change history (log.md). The generated change log as Markdown.

    Model-based testing — edited: feat(learn): teach cards 43 to 62 with plain lines, diagrams and snippets

  12. test-behaviour. Pick the test shape that proves a behaviour. Use when you ask "how do I test this?", add a test, test a domain rule, a lifecycle, a retry, a session, a UI interaction, a timer, or a race, or review a test that might be tautological.

    Read model-based testing.¶

  13. Tests prove behavior through supplied services. Supply services through Layers, choose properties or models for domain rules, and reject tests that cannot detect broken behavior.

    Model-based testing explains the assertions and records the failed day-boundary mutation.

  14. UI behaviour is a scene. A Foldkit Scene drives update, view, and subscriptions with input and time as plain data, then asserts on what the view shows.

    See also: tests that earn their place, model-based testing, and the test-behaviour skill.¶