xstate

Read this page from the installed package, usually node_modules/xstate/docs/agents.md. docs/index.md identifies the exact version. docs/api-stability.json classifies exports from that source tree. Prefer these files over examples for another major version. Install framework and integration packages from the same release family.

Author v6 machines

• Declare contracts under schemas, using types<T>() for TypeScript-only inference or Standard Schema for runtime schemas. Inference alone does not validate input. See runtime validation.
• Use { target: 'next' } for a static transition. Use one synchronous function for a conditional transition; return undefined before any enqueue call to refuse the event. String transitions and transition arrays are not v6 authoring syntax. See transitions.
• Return a shallow { context: { field: value } } patch. Keep I/O, time reads, randomness and external mutation outside transition calculation. Queue effects with enq, or invoke actor logic for asynchronous work.
• Put transient data in state input when it belongs to the active state. Use typestates to refine root context. State input is removed on exit; root context survives state changes.
• Invoke for work owned by a state's lifetime; spawn for actors with an explicit longer lifetime. Stop, cancellation and service failure need tests.
• Keep target literals visible when visual inspection matters. A graph is not complete when analysis.transitionTargetsComplete is false. Read its diagnostics rather than guessing the missing destinations.
• Before exporting code-authored definitions, inspect getMachineSerializationDiagnostics(machine). Serialization can omit inline actors and runtime schemas. Code expressions need explicit evaluators and dependencies; JSON conversion does not preserve arbitrary closures.

Use machineToJSON, machineConfigToJSON and machineFromJSON for definitions. Use getPersistedSnapshot and the persistence guide for running state. A definition and a checkpoint are different artifacts. Do not restore old snapshots without checking their version and migration requirements.

Local JSON operations

The core CLI's agent commands return one JSON object with schemaVersion, xstateVersion, command, ok and either result or error. Failures exit with status 1. --json is accepted; JSON is also the default for these commands. No command installs packages, imports application modules, executes code expressions or runs external effects. migrate remains a separate source transformation command that delegates to the codemod package.

To migrate a v5 machine file, use the @xstate/codemod loop. Run xstate-codemod baseline <file> before editing, then xstate-codemod migrate <file> --json. Exit code 2 means the report's manualSites need manual changes. Then run xstate-codemod verify --baseline <baseline.json> <file> --json. It exits with 0 when the v6 behavior matches the v5 trace, 3 when a diff is unclassified, 4 when a module fails to load or run, and 1 on a usage error. See the migration guide.

These commands accept JSON definitions, not TypeScript source files. Definitions with @code expressions return EVALUATOR_REQUIRED and their JSON Pointer paths. Definitions requiring custom runtime implementations must use the library with explicit sources instead. The CLI never fills missing implementations with stubs.

For example, save this as machine.json:

{
  "id": "review",
  "initial": "pending",
  "states": {
    "pending": {
      "on": {
        "APPROVE": { "target": "approved" },
        "REJECT": { "target": "rejected" }
      }
    },
    "approved": { "type": "final" },
    "rejected": { "type": "final" }
  }
}

An events file is an array such as [{ "type": "APPROVE" }], or an object { "events": [...], "input": ... } when startup input is required.

A checks file supplies event representatives and at least one assertion:

{
  "events": [{ "type": "APPROVE" }],
  "never": ["#review.rejected"],
  "reachable": ["#review.approved"],
  "limit": 1000
}

never and reachable use existing state IDs prefixed with #. Coverage is relative to the supplied event representatives and stable macrostep snapshots. The example's event array excludes REJECT; it makes no assertion about runs that send it. An event map instead augments the machine's default cases, including unconfigured payload-free events and synthetic actor/timer events; see event cases. The default traversal limit is 10,000. Exceeding a bound fails instead of reporting success. A reachability failure means no supplied representative witnessed the state, not that the state is proven unreachable.

A forbidden-state failure contains error.details.fixture. Save that object and pass it to replay. Its definition hash and XState version prevent accidental replay against another model. Replay reports a property violation again while the same assertion still fails. These CLI fixtures are separate from @xstate/test fixtures. Use model-based testing for PBT, payload shrinking, service outcomes, concurrency testing and comparison with an actual system under test.

Repair and verify

Preserve the failing event sequence before editing. Reproduce it, make the smallest behavior change, then replay it and run relevant tests. Check forbidden behavior and resource cleanup as well as successful outcomes. A corrected type error must also compile without suppressions or broad casts.

For a running agent, expose public event schemas and descriptions through a host adapter. Type-only schemas do not describe runtime parameters. Recheck state revision and host authorization when accepting a command. snapshot.can(event) is advisory, and sending an event does not establish durable commit or completion. Actor completion capability is distinct from idleness or waiting for input.

Keep sampled tests, bounded exploration and proof distinct. State the event representatives, state abstraction, time and schedule assumptions, and limits with the result. Experimental APIs may still change independently of the public beta surface.