diff --git a/frontend/src/spa/gameplay-phase-machine.ts b/frontend/src/spa/gameplay-phase-machine.ts
new file mode 100644
index 0000000..a2c24a1
--- /dev/null
+++ b/frontend/src/spa/gameplay-phase-machine.ts
@@ -0,0 +1,58 @@
+import type { SessionDetailResponse } from '../api/types';
+
+export type GameplayPhase = 'lie' | 'guess' | 'reveal' | 'scoreboard';
+
+export type GameplayPhaseEvent =
+  | 'LIES_LOCKED'
+  | 'GUESSES_LOCKED'
+  | 'SCOREBOARD_READY'
+  | 'NEXT_ROUND';
+
+export interface GameplayTransitionResult {
+  phase: GameplayPhase;
+  changed: boolean;
+}
+
+const TRANSITIONS: Record<GameplayPhase, Partial<Record<GameplayPhaseEvent, GameplayPhase>>> = {
+  lie: {
+    LIES_LOCKED: 'guess'
+  },
+  guess: {
+    GUESSES_LOCKED: 'reveal'
+  },
+  reveal: {
+    SCOREBOARD_READY: 'scoreboard'
+  },
+  scoreboard: {
+    NEXT_ROUND: 'lie'
+  }
+};
+
+export function transitionGameplayPhase(phase: GameplayPhase, event: GameplayPhaseEvent): GameplayTransitionResult {
+  const next = TRANSITIONS[phase][event] ?? phase;
+  return {
+    phase: next,
+    changed: next !== phase
+  };
+}
+
+export function allowedGameplayEvents(phase: GameplayPhase): GameplayPhaseEvent[] {
+  return Object.keys(TRANSITIONS[phase]) as GameplayPhaseEvent[];
+}
+
+export function deriveGameplayPhase(session: SessionDetailResponse | null): GameplayPhase | null {
+  const status = session?.session.status;
+  if (!status) {
+    return null;
+  }
+
+  if (status === 'lie' || status === 'guess' || status === 'reveal') {
+    return status;
+  }
+
+  if (status === 'finished') {
+    return 'scoreboard';
+  }
+
+  return null;
+}
diff --git a/frontend/src/spa/vertical-slice.ts b/frontend/src/spa/vertical-slice.ts
index 9e901f3..7a2d6d0 100644
--- a/frontend/src/spa/vertical-slice.ts
+++ b/frontend/src/spa/vertical-slice.ts
@@ -6,6 +6,7 @@ import {
   type SessionContextInput,
   type SessionContextStore as PersistedSessionContextStore
 } from './session-context-store';
+import { deriveGameplayPhase, type GameplayPhase } from './gameplay-phase-machine';
 
 export type AsyncState = 'idle' | 'loading' | 'success' | 'error';
 
@@ -14,6 +15,7 @@ export type SessionContextStore = Pick<PersistedSessionContextStore, 'get' | 'se
 export interface VerticalSliceState {
   sessionCode: string;
   session: SessionDetailResponse | null;
+  gameplayPhase: GameplayPhase | null;
   joinState: AsyncState;
   startRoundState: AsyncState;
   loadingSession: boolean;
@@ -36,6 +38,7 @@ export function createVerticalSliceController(
   const state: VerticalSliceState = {
     sessionCode: persistedContext?.sessionCode ?? '',
     session: null,
+    gameplayPhase: null,
     joinState: 'idle',
     startRoundState: 'idle',
     loadingSession: false,
@@ -63,10 +66,12 @@ export function createVerticalSliceController(
 
     if (!result.ok) {
       state.errorMessage = 'Kunne ikke hente lobby-status.';
+      state.gameplayPhase = null;
       return { ...state };
     }
 
     state.session = result.data;
+    state.gameplayPhase = deriveGameplayPhase(result.data);
     state.sessionCode = normalizeCode(result.data.session.code);
 
     if (persistedContext && state.sessionCode === normalizeCode(persistedContext.sessionCode)) {
diff --git a/frontend/tests/gameplay-phase-machine.test.ts b/frontend/tests/gameplay-phase-machine.test.ts
new file mode 100644
index 0000000..2d71a7b
--- /dev/null
+++ b/frontend/tests/gameplay-phase-machine.test.ts
@@ -0,0 +1,106 @@
+import { describe, expect, it } from 'vitest';
+import {
+  allowedGameplayEvents,
+  deriveGameplayPhase,
+  transitionGameplayPhase,
+  type GameplayPhase
+} from '../src/spa/gameplay-phase-machine';
+
+describe('gameplay phase machine skeleton', () => {
+  it('supports canonical phase progression lie -> guess -> reveal -> scoreboard -> lie', () => {
+    let phase: GameplayPhase = 'lie';
+
+    phase = transitionGameplayPhase(phase, 'LIES_LOCKED').phase;
+    expect(phase).toBe('guess');
+
+    phase = transitionGameplayPhase(phase, 'GUESSES_LOCKED').phase;
+    expect(phase).toBe('reveal');
+
+    phase = transitionGameplayPhase(phase, 'SCOREBOARD_READY').phase;
+    expect(phase).toBe('scoreboard');
+
+    phase = transitionGameplayPhase(phase, 'NEXT_ROUND').phase;
+    expect(phase).toBe('lie');
+  });
+
+  it('keeps state unchanged for invalid transition events', () => {
+    const transition = transitionGameplayPhase('lie', 'NEXT_ROUND');
+    expect(transition.phase).toBe('lie');
+    expect(transition.changed).toBe(false);
+  });
+
+  it('exposes allowed events per phase', () => {
+    expect(allowedGameplayEvents('guess')).toEqual(['GUESSES_LOCKED']);
+    expect(allowedGameplayEvents('scoreboard')).toEqual(['NEXT_ROUND']);
+  });
+
+  it('derives gameplay phase from session detail status', () => {
+    expect(
+      deriveGameplayPhase({
+        session: { code: 'ABCD12', status: 'lie', host_id: 1, current_round: 1, players_count: 3 },
+        players: [],
+        round_question: null,
+        phase_view_model: {
+          status: 'lie',
+          round_number: 1,
+          players_count: 3,
+          constraints: {
+            min_players_to_start: 3,
+            max_players_mvp: 5,
+            min_players_reached: true,
+            max_players_allowed: true
+          },
+          host: {
+            can_start_round: false,
+            can_show_question: true,
+            can_mix_answers: true,
+            can_calculate_scores: false,
+            can_reveal_scoreboard: false,
+            can_start_next_round: false,
+            can_finish_game: false
+          },
+          player: {
+            can_join: false,
+            can_submit_lie: true,
+            can_submit_guess: false,
+            can_view_final_result: false
+          }
+        }
+      })
+    ).toBe('lie');
+
+    expect(
+      deriveGameplayPhase({
+        session: { code: 'ABCD12', status: 'finished', host_id: 1, current_round: 1, players_count: 3 },
+        players: [],
+        round_question: null,
+        phase_view_model: {
+          status: 'finished',
+          round_number: 1,
+          players_count: 3,
+          constraints: {
+            min_players_to_start: 3,
+            max_players_mvp: 5,
+            min_players_reached: true,
+            max_players_allowed: true
+          },
+          host: {
+            can_start_round: false,
+            can_show_question: false,
+            can_mix_answers: false,
+            can_calculate_scores: false,
+            can_reveal_scoreboard: false,
+            can_start_next_round: false,
+            can_finish_game: false
+          },
+          player: {
+            can_join: false,
+            can_submit_lie: false,
+            can_submit_guess: false,
+            can_view_final_result: true
+          }
+        }
+      })
+    ).toBe('scoreboard');
+  });
+});
diff --git a/frontend/tests/vertical-slice.test.ts b/frontend/tests/vertical-slice.test.ts
index 7c4c521..07be788 100644
--- a/frontend/tests/vertical-slice.test.ts
+++ b/frontend/tests/vertical-slice.test.ts
@@ -105,6 +105,7 @@ describe('vertical slice controller: lobby -> join -> start round', () => {
     vi.doUnmock('../src/spa/session-context-store');
     vi.resetModules();
   });
+
   it('tracks loading and success state for join + start flow', async () => {
     const api = makeApiMock();
     const controller = createVerticalSliceController(api);