XState Typegen

XState Typegen

Generate full type safety for XState machines with typegen (v4) and the setup pattern (v5)

When to Use

• Getting TypeScript errors about missing event types in guards or actions
• Wanting autocompletion for state names, event types, and action names
• Ensuring type safety between machine definition and service implementations
• Setting up a new XState project with strict typing from the start

Instructions

XState v5 (recommended): setup pattern

1. Use setup() to declare all types, actions, guards, actors, and delays before creating the machine.
2. Define types with TypeScript as assertion for context, events, input, and output.
3. All actions, guards, and actors referenced in the machine must be declared in setup() — runtime errors if missing.
4. The machine is fully typed from the setup() declaration — no separate code generation step.

// auth.machine.ts (v5)
import { setup, assign, fromPromise } from 'xstate';

interface AuthContext {
  user: { id: string; name: string } | null;
  error: string | null;
}

type AuthEvent = { type: 'LOGIN'; email: string; password: string } | { type: 'LOGOUT' };

const authMachine = setup({
  types: {} as {
    context: AuthContext;
    events: AuthEvent;
    input: { redirectUrl?: string };
  },
  actors: {
    authenticate: fromPromise(async ({ input }: { input: { email: string; password: string } }) => {
      const res = await fetch('/api/login', {
        method: 'POST',
        body: JSON.stringify(input),
      });
      if (!res.ok) throw new Error('Auth failed');
      return res.json() as Promise<{ id: string; name: string }>;
    }),
  },
  actions: {
    setUser: assign({
      user: (_, params: { id: string; name: string }) => params,
    }),
    clearUser: assign({ user: null, error: null }),
    setError: assign({
      error: (_, params: { message: string }) => params.message,
    }),
  },
  guards: {
    isAuthenticated: ({ context }) => context.user !== null,
  },
}).createMachine({
  id: 'auth',
  initial: 'idle',
  context: ({ input }) => ({
    user: null,
    error: null,
  }),
  states: {
    idle: {
      on: { LOGIN: 'authenticating' },
    },
    authenticating: {
      invoke: {
        src: 'authenticate',
        input: ({ event }) => ({
          email: (event as { email: string }).email,
          password: (event as { password: string }).password,
        }),
        onDone: {
          target: 'authenticated',
          actions: { type: 'setUser', params: ({ event }) => event.output },
        },
        onError: {
          target: 'error',
          actions: {
            type: 'setError',
            params: ({ event }) => ({ message: (event.error as Error).message }),
          },
        },
      },
    },
    authenticated: {
      on: { LOGOUT: { target: 'idle', actions: 'clearUser' } },
    },
    error: {
      on: { LOGIN: 'authenticating' },
    },
  },
});

XState v4: typegen

1. Install @xstate/cli: npm install -D @xstate/cli.
2. Add tsTypes: {} to the machine config to enable typegen.
3. Run xstate typegen "src/**/*.machine.ts" to generate .typegen.ts files.
4. The generated file provides exact types for which events are valid in which states, which services are invoked, and which actions/guards are used.

// v4 with typegen
import { createMachine } from 'xstate';

const machine = createMachine({
  tsTypes: {} as import('./auth.machine.typegen').Typegen0,
  schema: {
    context: {} as AuthContext,
    events: {} as AuthEvent,
    services: {} as { authenticate: { data: User } },
  },
  // ...
});

Details

v5 setup benefits over v4 typegen:

• No code generation step — types flow from setup() directly
• Actions and guards must exist at declaration time — catches typos immediately
• Actor input/output types are inferred from fromPromise, fromCallback, etc.
• No .typegen.ts files to maintain or commit

Typing event narrowing in actions: In v5, action implementations receive the full event union. Narrow when needed:

actions: {
  handleLogin: ({ event }) => {
    if (event.type === 'LOGIN') {
      console.log(event.email); // Typed correctly
    }
  },
},

Common type issues:

• Type 'string' is not assignable to type 'never' in transitions — usually means the event is not listed in the events type
• Missing action/guard in setup() — v5 requires all referenced names to be declared upfront
• Circular type inference — break cycles by explicitly typing context function return

Source

https://stately.ai/docs/typescript

Process

1. Read the instructions and examples in this document.
2. Apply the patterns to your implementation, adapting to your specific context.
3. Verify your implementation against the details and edge cases listed above.

Harness Integration

• Type: knowledge — this skill is a reference document, not a procedural workflow.
• No tools or state — consumed as context by other skills and agents.

Success Criteria

• The patterns described in this document are applied correctly in the implementation.
• Edge cases and anti-patterns listed in this document are avoided.