diff --git a/frontend/angular/src/app/features/host/host-shell.component.spec.ts b/frontend/angular/src/app/features/host/host-shell.component.spec.ts
index 626db73..3c15591 100644
--- a/frontend/angular/src/app/features/host/host-shell.component.spec.ts
+++ b/frontend/angular/src/app/features/host/host-shell.component.spec.ts
@@ -245,6 +245,30 @@ describe('HostShellComponent gameplay wiring', () => {
     expect(component.finishError).toContain('Session code is required');
   });
 
+  it('blocks illegal host actions outside canonical reveal/scoreboard boundaries', async () => {
+    const fetchMock: FetchMock = vi.fn();
+    vi.stubGlobal('fetch', fetchMock);
+
+    const component = new HostShellComponent();
+    component.sessionCode = 'ABCD12';
+
+    for (const status of ['lie', 'guess', 'reveal'] as const) {
+      component.session = sessionDetailPayload(status) as any;
+      await component.startNextRound();
+      await component.finishGame();
+    }
+
+    component.session = sessionDetailPayload('reveal') as any;
+    expect(component.canStartNextRound).toBe(false);
+    expect(component.canFinishGame).toBe(false);
+
+    component.session = sessionDetailPayload('scoreboard') as any;
+    await component.loadScoreboard();
+
+    expect(component.canLoadScoreboard).toBe(false);
+    expect(fetchMock).not.toHaveBeenCalled();
+  });
+
   it('syncs host hash-route with latest phase after refresh without page reload', async () => {
     const fetchMock: FetchMock = vi.fn().mockResolvedValue(jsonResponse(200, sessionDetailPayload('guess', { roundQuestionId: 77 })));
     vi.stubGlobal('fetch', fetchMock);
diff --git a/frontend/angular/src/app/features/host/host-shell.component.ts b/frontend/angular/src/app/features/host/host-shell.component.ts
index 4253051..89baba5 100644
--- a/frontend/angular/src/app/features/host/host-shell.component.ts
+++ b/frontend/angular/src/app/features/host/host-shell.component.ts
@@ -4,6 +4,7 @@ import { FormsModule } from '@angular/forms';
 
 import { createApiClient } from '../../../../../src/api/client';
 import type { FinishGameResponse, ScoreboardResponse } from '../../../../../src/api/types';
+import { deriveGameplayPhase, isHostGameplayActionAllowed } from '../../../../../src/spa/gameplay-phase-machine';
 import { createVerticalSliceController } from '../../../../../src/spa/vertical-slice';
 import { clientHasNoAudioOutput, resolvePreferredLocale, subscribeToLocaleChanges, t } from '../../lobby-i18n';
 
@@ -27,16 +28,16 @@ type LeaderboardResponse = FinishGameResponse;
       <label>{{ copy('common.session_code') }} <input [(ngModel)]="sessionCode" /></label>
       <label>{{ copy('host.category') }} <input [(ngModel)]="categorySlug" /></label>
       <button (click)="refreshSession()" [disabled]="loading">{{ copy('common.refresh') }}</button>
-      <button (click)="startRound()" [disabled]="loading">{{ copy('host.start_round') }}</button>
-      <button (click)="showQuestion()" [disabled]="loading || !roundQuestionId">{{ copy('host.show_question') }}</button>
-      <button (click)="mixAnswers()" [disabled]="loading || !roundQuestionId">{{ copy('host.mix_answers') }}</button>
-      <button (click)="calculateScores()" [disabled]="loading || !roundQuestionId">{{ copy('host.calculate_scores') }}</button>
-      <button (click)="loadScoreboard()" [disabled]="loading">{{ copy('host.load_scoreboard') }}</button>
-      <button (click)="startNextRound()" [disabled]="loading">{{ copy('host.start_next_round') }}</button>
-      <button (click)="finishGame()" [disabled]="loading">{{ copy('host.finish_game') }}</button>
-      <button *ngIf="scoreboardError" (click)="loadScoreboard()" [disabled]="loading">{{ copy('host.retry_scoreboard') }}</button>
-      <button *ngIf="nextRoundError" (click)="startNextRound()" [disabled]="loading">{{ copy('host.retry_next_round') }}</button>
-      <button *ngIf="finishError" (click)="finishGame()" [disabled]="loading">{{ copy('host.retry_finish') }}</button>
+      <button (click)="startRound()" [disabled]="loading || !canStartRound">{{ copy('host.start_round') }}</button>
+      <button (click)="showQuestion()" [disabled]="loading || !canUseLegacyMidRoundHostAction">{{ copy('host.show_question') }}</button>
+      <button (click)="mixAnswers()" [disabled]="loading || !canUseLegacyMidRoundHostAction">{{ copy('host.mix_answers') }}</button>
+      <button (click)="calculateScores()" [disabled]="loading || !canUseLegacyMidRoundHostAction">{{ copy('host.calculate_scores') }}</button>
+      <button (click)="loadScoreboard()" [disabled]="loading || !canLoadScoreboard">{{ copy('host.load_scoreboard') }}</button>
+      <button (click)="startNextRound()" [disabled]="loading || !canStartNextRound">{{ copy('host.start_next_round') }}</button>
+      <button (click)="finishGame()" [disabled]="loading || !canFinishGame">{{ copy('host.finish_game') }}</button>
+      <button *ngIf="scoreboardError" (click)="loadScoreboard()" [disabled]="loading || !canLoadScoreboard">{{ copy('host.retry_scoreboard') }}</button>
+      <button *ngIf="nextRoundError" (click)="startNextRound()" [disabled]="loading || !canStartNextRound">{{ copy('host.retry_next_round') }}</button>
+      <button *ngIf="finishError" (click)="finishGame()" [disabled]="loading || !canFinishGame">{{ copy('host.retry_finish') }}</button>
     </div>
 
     <p *ngIf="session" class="hint">{{ copy('host.audio_locale_hint') }}: {{ locale }}</p>
@@ -114,6 +115,30 @@ export class HostShellComponent implements OnInit, OnDestroy {
     this.unsubscribeLocale = null;
   }
 
+  get gameplayPhase(): string | null {
+    return deriveGameplayPhase(this.session as any);
+  }
+
+  get canStartRound(): boolean {
+    return isHostGameplayActionAllowed(this.session as any, 'startRound');
+  }
+
+  get canStartNextRound(): boolean {
+    return isHostGameplayActionAllowed(this.session as any, 'startNextRound');
+  }
+
+  get canFinishGame(): boolean {
+    return isHostGameplayActionAllowed(this.session as any, 'finishGame');
+  }
+
+  get canUseLegacyMidRoundHostAction(): boolean {
+    return false;
+  }
+
+  get canLoadScoreboard(): boolean {
+    return !this.session || this.gameplayPhase === 'reveal';
+  }
+
   copy(key: string): string {
     return t(key, this.locale);
   }
@@ -174,6 +199,10 @@ export class HostShellComponent implements OnInit, OnDestroy {
   }
 
   async startRound(): Promise<void> {
+    if (!this.canStartRound) {
+      return;
+    }
+
     await this.runAction(async () => {
       const state = await this.controller.startRound(this.sessionCode, this.categorySlug.trim());
       if (!state.session || state.errorMessage) {
@@ -190,6 +219,10 @@ export class HostShellComponent implements OnInit, OnDestroy {
   }
 
   async showQuestion(): Promise<void> {
+    if (!this.canUseLegacyMidRoundHostAction) {
+      return;
+    }
+
     await this.runAction(async () => {
       const code = this.normalizeCode(this.sessionCode);
       await this.request(`/lobby/sessions/${encodeURIComponent(code)}/questions/show`, 'POST', {});
@@ -198,6 +231,10 @@ export class HostShellComponent implements OnInit, OnDestroy {
   }
 
   async mixAnswers(): Promise<void> {
+    if (!this.canUseLegacyMidRoundHostAction) {
+      return;
+    }
+
     await this.runAction(async () => {
       const code = this.normalizeCode(this.sessionCode);
       const roundQuestionId = this.roundQuestionId.trim();
@@ -207,6 +244,10 @@ export class HostShellComponent implements OnInit, OnDestroy {
   }
 
   async calculateScores(): Promise<void> {
+    if (!this.canUseLegacyMidRoundHostAction) {
+      return;
+    }
+
     await this.runAction(async () => {
       const code = this.normalizeCode(this.sessionCode);
       const roundQuestionId = this.roundQuestionId.trim();
@@ -216,6 +257,10 @@ export class HostShellComponent implements OnInit, OnDestroy {
   }
 
   async loadScoreboard(): Promise<void> {
+    if (!this.canLoadScoreboard) {
+      return;
+    }
+
     this.loading = true;
     this.scoreboardError = '';
     this.error = '';
@@ -232,6 +277,10 @@ export class HostShellComponent implements OnInit, OnDestroy {
   }
 
   async startNextRound(): Promise<void> {
+    if (!this.canStartNextRound) {
+      return;
+    }
+
     this.loading = true;
     this.nextRoundError = '';
     this.error = '';
@@ -252,6 +301,10 @@ export class HostShellComponent implements OnInit, OnDestroy {
   }
 
   async finishGame(): Promise<void> {
+    if (!this.canFinishGame) {
+      return;
+    }
+
     this.loading = true;
     this.finishError = '';
     this.error = '';
diff --git a/frontend/angular/src/app/features/player/player-shell.component.spec.ts b/frontend/angular/src/app/features/player/player-shell.component.spec.ts
index 5bd1d1d..9ebcf84 100644
--- a/frontend/angular/src/app/features/player/player-shell.component.spec.ts
+++ b/frontend/angular/src/app/features/player/player-shell.component.spec.ts
@@ -199,6 +199,29 @@ describe('PlayerShellComponent gameplay wiring', () => {
     expect(fetchMock).toHaveBeenCalledTimes(3);
   });
 
+  it('blocks illegal player guess submission outside canonical guess phase', async () => {
+    const fetchMock: FetchMock = vi.fn();
+    vi.stubGlobal('fetch', fetchMock);
+
+    const component = new PlayerShellComponent();
+    component.sessionCode = 'ABCD12';
+    component.playerId = 9;
+    component.sessionToken = 'token-1';
+    component.selectedGuess = 'B';
+
+    for (const status of ['lie', 'reveal', 'scoreboard'] as const) {
+      component.session = {
+        ...(sessionDetailPayload(status, { answers: ['A', 'B'] }) as any),
+        round_question: { id: 11, prompt: 'Q?', answers: [{ text: 'A' }, { text: 'B' }] },
+      };
+
+      await component.submitGuess();
+    }
+
+    expect(component.canSubmitGuess).toBe(false);
+    expect(fetchMock).not.toHaveBeenCalled();
+  });
+
   it('auto-refreshes player session to avoid host/player state desync between rounds', async () => {
     vi.useFakeTimers();
 
diff --git a/frontend/angular/src/app/features/player/player-shell.component.ts b/frontend/angular/src/app/features/player/player-shell.component.ts
index 393723a..c1ae1e9 100644
--- a/frontend/angular/src/app/features/player/player-shell.component.ts
+++ b/frontend/angular/src/app/features/player/player-shell.component.ts
@@ -3,6 +3,10 @@ import { Component, OnDestroy, OnInit } from '@angular/core';
 import { FormsModule } from '@angular/forms';
 
 import { createApiClient } from '../../../../../src/api/client';
+import {
+  deriveGameplayPhase,
+  isPlayerGameplayActionAllowed,
+} from '../../../../../src/spa/gameplay-phase-machine';
 import { createSessionContextStore } from '../../../../../src/spa/session-context-store';
 import { createVerticalSliceController } from '../../../../../src/spa/vertical-slice';
 import { clientHasNoAudioOutput, resolvePreferredLocale, subscribeToLocaleChanges, t } from '../../lobby-i18n';
@@ -71,9 +75,9 @@ function resolveLocalStorage(): Storage | undefined {
       <p><strong>{{ copy('common.status') }}:</strong> {{ session.session.status }}</p>
       <p *ngIf="session.round_question"><strong>{{ copy('common.prompt') }}:</strong> {{ session.round_question.prompt }}</p>
 
-      <label>{{ copy('player.lie_label') }} <input [(ngModel)]="lieText" [disabled]="loading || session.session.status !== 'lie'" /></label>
-      <button (click)="submitLie()" [disabled]="loading || session.session.status !== 'lie'">{{ copy('player.submit_lie') }}</button>
-      <button *ngIf="submitError?.kind === 'lie'" (click)="submitLie()" [disabled]="loading">{{ copy('player.retry_lie_submit') }}</button>
+      <label>{{ copy('player.lie_label') }} <input [(ngModel)]="lieText" [disabled]="loading || !canSubmitLie" /></label>
+      <button (click)="submitLie()" [disabled]="loading || !canSubmitLie">{{ copy('player.submit_lie') }}</button>
+      <button *ngIf="submitError?.kind === 'lie'" (click)="submitLie()" [disabled]="loading || !canSubmitLie">{{ copy('player.retry_lie_submit') }}</button>
 
       <div class="answers" *ngIf="session.round_question?.answers?.length">
         <button
@@ -81,14 +85,14 @@ function resolveLocalStorage(): Storage | undefined {
           *ngFor="let answer of session.round_question?.answers"
           (click)="selectedGuess = answer.text"
           [class.active]="selectedGuess === answer.text"
-          [disabled]="loading || session.session.status !== 'guess'"
+          [disabled]="loading || !canSubmitGuess"
         >
           {{ answer.text }}
         </button>
       </div>
 
-      <button (click)="submitGuess()" [disabled]="loading || session.session.status !== 'guess' || !selectedGuess">{{ copy('player.submit_guess') }}</button>
-      <button *ngIf="submitError?.kind === 'guess'" (click)="submitGuess()" [disabled]="loading">{{ copy('player.retry_guess_submit') }}</button>
+      <button (click)="submitGuess()" [disabled]="loading || !canSubmitGuess || !selectedGuess">{{ copy('player.submit_guess') }}</button>
+      <button *ngIf="submitError?.kind === 'guess'" (click)="submitGuess()" [disabled]="loading || !canSubmitGuess">{{ copy('player.retry_guess_submit') }}</button>
 
       <div *ngIf="session.session.status === 'finished' && finalLeaderboard.length">
         <h3>{{ copy('player.final_leaderboard') }}</h3>
@@ -181,6 +185,18 @@ export class PlayerShellComponent implements OnInit, OnDestroy {
     this.restoreAudioGuard = null;
   }
 
+  get gameplayPhase(): string | null {
+    return deriveGameplayPhase(this.session as any);
+  }
+
+  get canSubmitLie(): boolean {
+    return isPlayerGameplayActionAllowed(this.session as any, 'submitLie');
+  }
+
+  get canSubmitGuess(): boolean {
+    return isPlayerGameplayActionAllowed(this.session as any, 'submitGuess');
+  }
+
   private readonly handleOnline = (): void => {
     this.connectionState = 'reconnecting';
     void this.retryReconnect();
@@ -500,7 +516,7 @@ export class PlayerShellComponent implements OnInit, OnDestroy {
   }
 
   async submitLie(): Promise<void> {
-    if (!this.session?.round_question?.id) {
+    if (!this.session?.round_question?.id || !this.canSubmitLie) {
       return;
     }
     this.loading = true;
@@ -528,7 +544,7 @@ export class PlayerShellComponent implements OnInit, OnDestroy {
   }
 
   async submitGuess(): Promise<void> {
-    if (!this.session?.round_question?.id || !this.selectedGuess) {
+    if (!this.session?.round_question?.id || !this.selectedGuess || !this.canSubmitGuess) {
       return;
     }
     this.loading = true;
diff --git a/frontend/src/spa/gameplay-phase-machine.ts b/frontend/src/spa/gameplay-phase-machine.ts
index f0ba252..c9b059c 100644
--- a/frontend/src/spa/gameplay-phase-machine.ts
+++ b/frontend/src/spa/gameplay-phase-machine.ts
@@ -1,6 +1,8 @@
-import type { SessionDetailResponse } from '../api/types';
+import type { PhaseViewModel, SessionDetailResponse } from '../api/types';
 
 export type GameplayPhase = 'lie' | 'guess' | 'reveal' | 'scoreboard';
+export type HostGameplayAction = 'startRound' | 'startNextRound' | 'finishGame';
+export type PlayerGameplayAction = 'join' | 'submitLie' | 'submitGuess' | 'viewFinalResult';
 
 export type GameplayPhaseEvent =
   | 'LIES_LOCKED'
@@ -40,8 +42,7 @@ 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;
+function derivePhaseFromStatus(status: string | null | undefined): GameplayPhase | null {
   if (!status) {
     return null;
   }
@@ -56,3 +57,53 @@ export function deriveGameplayPhase(session: SessionDetailResponse | null): Game
 
   return null;
 }
+
+function deriveCanonicalPhaseStatus(phaseViewModel: PhaseViewModel | null | undefined): string | null {
+  if (!phaseViewModel) {
+    return null;
+  }
+
+  const currentPhase = (phaseViewModel as PhaseViewModel & { current_phase?: string }).current_phase;
+  return currentPhase ?? phaseViewModel.status ?? null;
+}
+
+export function deriveGameplayPhase(session: SessionDetailResponse | null): GameplayPhase | null {
+  const canonicalStatus = deriveCanonicalPhaseStatus(session?.phase_view_model);
+  return derivePhaseFromStatus(canonicalStatus ?? session?.session.status);
+}
+
+export function isHostGameplayActionAllowed(session: SessionDetailResponse | null, action: HostGameplayAction): boolean {
+  if (!session) {
+    return true;
+  }
+
+  const phase = deriveGameplayPhase(session);
+  const host = session.phase_view_model?.host;
+  switch (action) {
+    case 'startRound':
+      return phase === null || Boolean(host?.can_start_round ?? phase === null);
+    case 'startNextRound':
+      return phase === 'scoreboard' && Boolean(host?.can_start_next_round ?? true);
+    case 'finishGame':
+      return phase === 'scoreboard' && Boolean(host?.can_finish_game ?? true);
+  }
+}
+
+export function isPlayerGameplayActionAllowed(session: SessionDetailResponse | null, action: PlayerGameplayAction): boolean {
+  if (!session) {
+    return action === 'join';
+  }
+
+  const phase = deriveGameplayPhase(session);
+  const player = session.phase_view_model?.player;
+  switch (action) {
+    case 'join':
+      return Boolean(player?.can_join ?? true);
+    case 'submitLie':
+      return phase === 'lie' && Boolean(player?.can_submit_lie ?? true);
+    case 'submitGuess':
+      return phase === 'guess' && Boolean(player?.can_submit_guess ?? true);
+    case 'viewFinalResult':
+      return session.session.status === 'finished' && Boolean(player?.can_view_final_result ?? true);
+  }
+}