diff --git a/frontend/src/api/client.ts b/frontend/src/api/client.ts
index 65ef550..02b3775 100644
--- a/frontend/src/api/client.ts
+++ b/frontend/src/api/client.ts
@@ -5,9 +5,9 @@ import {
   mapJoinSessionResponse,
   mapMixAnswersResponse,
   mapScoreboardResponse,
-  mapStartNextRoundResponse,
   mapSessionDetailResponse,
   mapShowQuestionResponse,
+  mapStartNextRoundResponse,
   mapStartRoundResponse,
   mapSubmitGuessResponse,
   mapSubmitLieResponse
@@ -21,9 +21,9 @@ import type {
   JoinSessionResponse,
   MixAnswersResponse,
   ScoreboardResponse,
-  StartNextRoundResponse,
   SessionDetailResponse,
   ShowQuestionResponse,
+  StartNextRoundResponse,
   StartRoundRequest,
   StartRoundResponse,
   SubmitGuessRequest,
diff --git a/frontend/src/api/mappers.ts b/frontend/src/api/mappers.ts
index 1944ec3..0cab015 100644
--- a/frontend/src/api/mappers.ts
+++ b/frontend/src/api/mappers.ts
@@ -60,6 +60,17 @@ function readBoolean(record: Record<string, unknown>, key: string, path: string)
   return value;
 }
 
+function readNullableNumber(record: Record<string, unknown>, key: string, path: string): number | null {
+  const value = record[key];
+  if (value === undefined || value === null) {
+    return null;
+  }
+  if (!isNumber(value)) {
+    throw new Error(`Invalid API contract: expected number|null at ${path}.${key}`);
+  }
+  return value;
+}
+
 export function mapHealthResponse(payload: unknown): HealthResponse {
   const root = asRecord(payload, 'health');
   return {
@@ -130,23 +141,25 @@ function mapSessionDetail(payload: unknown): SessionDetailResponse {
         };
       }),
       guesses: guessesRaw.map((guess, index) => {
-        const record = asRecord(guess, `session_detail.reveal.guesses[${index}]`);
-        const fooledPlayerId = record.fooled_player_id;
-        if (fooledPlayerId !== null && !isNumber(fooledPlayerId)) {
-          throw new Error(`Invalid API contract: expected number|null at session_detail.reveal.guesses[${index}].fooled_player_id`);
-        }
+        const path = `session_detail.reveal.guesses[${index}]`;
+        const record = asRecord(guess, path);
+        const fooledPlayerId = readNullableNumber(record, 'fooled_player_id', path);
         const fooledPlayerNickname = record.fooled_player_nickname;
-        if (fooledPlayerNickname !== undefined && !isString(fooledPlayerNickname)) {
-          throw new Error(`Invalid API contract: expected string at session_detail.reveal.guesses[${index}].fooled_player_nickname`);
+        if (fooledPlayerId === null) {
+          if (fooledPlayerNickname !== undefined) {
+            throw new Error(`Invalid API contract: expected ${path}.fooled_player_nickname to be omitted when fooled_player_id is null`);
+          }
+        } else if (!isString(fooledPlayerNickname)) {
+          throw new Error(`Invalid API contract: expected string at ${path}.fooled_player_nickname when fooled_player_id is set`);
         }
         return {
-          player_id: readNumber(record, 'player_id', `session_detail.reveal.guesses[${index}]`),
-          nickname: readString(record, 'nickname', `session_detail.reveal.guesses[${index}]`),
-          selected_text: readString(record, 'selected_text', `session_detail.reveal.guesses[${index}]`),
-          is_correct: readBoolean(record, 'is_correct', `session_detail.reveal.guesses[${index}]`),
+          player_id: readNumber(record, 'player_id', path),
+          nickname: readString(record, 'nickname', path),
+          selected_text: readString(record, 'selected_text', path),
+          is_correct: readBoolean(record, 'is_correct', path),
           fooled_player_id: fooledPlayerId,
           ...(fooledPlayerNickname === undefined ? {} : { fooled_player_nickname: fooledPlayerNickname }),
-          created_at: readString(record, 'created_at', `session_detail.reveal.guesses[${index}]`)
+          created_at: readString(record, 'created_at', path)
         };
       })
     };
@@ -388,10 +401,7 @@ export function mapSubmitGuessResponse(payload: unknown): SubmitGuessResponse {
   const root = asRecord(payload, 'submit_guess');
   const guess = asRecord(root.guess, 'submit_guess.guess');
   const window = asRecord(root.window, 'submit_guess.window');
-  const fooledPlayerId = guess.fooled_player_id;
-  if (fooledPlayerId !== null && !isNumber(fooledPlayerId)) {
-    throw new Error('Invalid API contract: expected number|null at submit_guess.guess.fooled_player_id');
-  }
+  const fooledPlayerId = readNullableNumber(guess, 'fooled_player_id', 'submit_guess.guess');
 
   return {
     guess: {
diff --git a/frontend/tests/angular-api-client.test.ts b/frontend/tests/angular-api-client.test.ts
index d0f2aef..275f463 100644
--- a/frontend/tests/angular-api-client.test.ts
+++ b/frontend/tests/angular-api-client.test.ts
@@ -1,6 +1,7 @@
 import { describe, expect, it, vi } from 'vitest';
 
 import { createAngularApiClient, type AngularHttpClientLike } from '../src/api/angular-client';
+import { mapSessionDetailResponse, mapSubmitGuessResponse } from '../src/api/mappers';
 
 describe('createAngularApiClient', () => {
   it('reads health and session detail using Django-compatible endpoints', async () => {
@@ -283,6 +284,309 @@ describe('createAngularApiClient', () => {
     }
   });
 
+  it('normalizes omitted fooled_player_id to null in canonical reveal payloads', async () => {
+    const get = vi.fn<AngularHttpClientLike['get']>(async <T>(url: string) => {
+      if (url === '/lobby/sessions/ABCD12') {
+        return {
+          session: { code: 'ABCD12', status: 'reveal', host_id: 1, current_round: 1, players_count: 2 },
+          players: [
+            { id: 2, nickname: 'Maja', score: 10, is_connected: true },
+            { id: 3, nickname: 'Bo', score: 7, is_connected: true }
+          ],
+          round_question: {
+            id: 77,
+            round_number: 1,
+            prompt: 'Q?',
+            shown_at: '2026-03-01T18:00:00Z',
+            answers: [{ text: 'A' }, { text: 'B' }]
+          },
+          reveal: {
+            round_question_id: 77,
+            round_number: 1,
+            prompt: 'Q?',
+            correct_answer: 'A',
+            lies: [],
+            guesses: [
+              {
+                player_id: 3,
+                nickname: 'Bo',
+                selected_text: 'A',
+                is_correct: true,
+                created_at: '2026-03-01T18:00:15Z'
+              }
+            ]
+          },
+          phase_view_model: {
+            status: 'reveal',
+            round_number: 1,
+            players_count: 2,
+            constraints: {
+              min_players_to_start: 2,
+              max_players_mvp: 8,
+              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: true,
+              can_start_next_round: false,
+              can_finish_game: false
+            },
+            player: {
+              can_join: true,
+              can_submit_lie: false,
+              can_submit_guess: false,
+              can_view_final_result: false
+            }
+          }
+        } as T;
+      }
+      throw { status: 404, error: { error: 'Not found' } };
+    });
+
+    const post = vi.fn<AngularHttpClientLike['post']>(async <T>(url: string, body: unknown) => {
+      if (url === '/lobby/sessions/ABCD12/questions/77/guesses/submit') {
+        expect(body).toEqual({ player_id: 9, session_token: 'tok', selected_text: 'A' });
+        return {
+          guess: {
+            id: 200,
+            player_id: 9,
+            round_question_id: 77,
+            selected_text: 'A',
+            is_correct: false,
+            created_at: '2026-03-01T16:01:00Z'
+          },
+          window: { guess_deadline_at: '2026-03-01T16:01:30Z' }
+        } as T;
+      }
+      throw { status: 404, error: { error: 'Not found' } };
+    });
+
+    const client = createAngularApiClient({ get, post } as AngularHttpClientLike);
+    const session = await client.getSession('abcd12');
+
+    expect(session.ok).toBe(true);
+    if (session.ok) {
+      expect(session.data.reveal?.guesses[0].fooled_player_id).toBeNull();
+      expect(session.data.reveal?.guesses[0]).not.toHaveProperty('fooled_player_nickname');
+    }
+
+    const submitGuess = await client.submitGuess('ABCD12', 77, {
+      player_id: 9,
+      session_token: 'tok',
+      selected_text: 'A'
+    });
+    expect(submitGuess.ok).toBe(true);
+    if (submitGuess.ok) {
+      expect(submitGuess.data.guess.fooled_player_id).toBeNull();
+    }
+  });
+
+  it('maps omitted fooled_player_id to null in submit guess mapper payloads', () => {
+    const mapped = mapSubmitGuessResponse({
+      guess: {
+        id: 200,
+        player_id: 9,
+        round_question_id: 77,
+        selected_text: 'A',
+        is_correct: false,
+        created_at: '2026-03-01T16:01:00Z'
+      },
+      window: { guess_deadline_at: '2026-03-01T16:01:30Z' }
+    });
+
+    expect(mapped.guess.fooled_player_id).toBeNull();
+  });
+
+  it('keeps fooled_player_nickname omitted when canonical reveal payload omits fooled player refs', () => {
+    const mapped = mapSessionDetailResponse({
+      session: { code: 'ABCD12', status: 'reveal', host_id: 1, current_round: 1, players_count: 2 },
+      players: [
+        { id: 2, nickname: 'Maja', score: 10, is_connected: true },
+        { id: 3, nickname: 'Bo', score: 7, is_connected: true }
+      ],
+      round_question: {
+        id: 77,
+        round_number: 1,
+        prompt: 'Q?',
+        shown_at: '2026-03-01T18:00:00Z',
+        answers: [{ text: 'A' }, { text: 'B' }]
+      },
+      reveal: {
+        round_question_id: 77,
+        round_number: 1,
+        prompt: 'Q?',
+        correct_answer: 'A',
+        lies: [],
+        guesses: [
+          {
+            player_id: 3,
+            nickname: 'Bo',
+            selected_text: 'A',
+            is_correct: true,
+            created_at: '2026-03-01T18:00:15Z'
+          }
+        ]
+      },
+      phase_view_model: {
+        status: 'reveal',
+        round_number: 1,
+        players_count: 2,
+        constraints: {
+          min_players_to_start: 2,
+          max_players_mvp: 8,
+          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: true,
+          can_start_next_round: false,
+          can_finish_game: false
+        },
+        player: {
+          can_join: true,
+          can_submit_lie: false,
+          can_submit_guess: false,
+          can_view_final_result: false
+        }
+      }
+    });
+
+    expect(mapped.reveal?.guesses[0].fooled_player_id).toBeNull();
+    expect(mapped.reveal?.guesses[0]).not.toHaveProperty('fooled_player_nickname');
+  });
+
+  it('rejects canonical reveal payloads that include fooled_player_nickname without fooled_player_id', () => {
+    expect(() =>
+      mapSessionDetailResponse({
+        session: { code: 'ABCD12', status: 'reveal', host_id: 1, current_round: 1, players_count: 2 },
+        players: [
+          { id: 2, nickname: 'Maja', score: 10, is_connected: true },
+          { id: 3, nickname: 'Bo', score: 7, is_connected: true }
+        ],
+        round_question: {
+          id: 77,
+          round_number: 1,
+          prompt: 'Q?',
+          shown_at: '2026-03-01T18:00:00Z',
+          answers: [{ text: 'A' }, { text: 'B' }]
+        },
+        reveal: {
+          round_question_id: 77,
+          round_number: 1,
+          prompt: 'Q?',
+          correct_answer: 'A',
+          lies: [],
+          guesses: [
+            {
+              player_id: 3,
+              nickname: 'Bo',
+              selected_text: 'A',
+              is_correct: true,
+              fooled_player_nickname: 'Maja',
+              created_at: '2026-03-01T18:00:15Z'
+            }
+          ]
+        },
+        phase_view_model: {
+          status: 'reveal',
+          round_number: 1,
+          players_count: 2,
+          constraints: {
+            min_players_to_start: 2,
+            max_players_mvp: 8,
+            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: true,
+            can_start_next_round: false,
+            can_finish_game: false
+          },
+          player: {
+            can_join: true,
+            can_submit_lie: false,
+            can_submit_guess: false,
+            can_view_final_result: false
+          }
+        }
+      })
+    ).toThrow('fooled_player_nickname to be omitted when fooled_player_id is null');
+  });
+
+  it('rejects canonical reveal payloads that omit fooled_player_nickname when fooled_player_id is set', () => {
+    expect(() =>
+      mapSessionDetailResponse({
+        session: { code: 'ABCD12', status: 'reveal', host_id: 1, current_round: 1, players_count: 2 },
+        players: [
+          { id: 2, nickname: 'Maja', score: 10, is_connected: true },
+          { id: 3, nickname: 'Bo', score: 7, is_connected: true }
+        ],
+        round_question: {
+          id: 77,
+          round_number: 1,
+          prompt: 'Q?',
+          shown_at: '2026-03-01T18:00:00Z',
+          answers: [{ text: 'A' }, { text: 'B' }]
+        },
+        reveal: {
+          round_question_id: 77,
+          round_number: 1,
+          prompt: 'Q?',
+          correct_answer: 'A',
+          lies: [],
+          guesses: [
+            {
+              player_id: 3,
+              nickname: 'Bo',
+              selected_text: 'B',
+              is_correct: false,
+              fooled_player_id: 2,
+              created_at: '2026-03-01T18:00:15Z'
+            }
+          ]
+        },
+        phase_view_model: {
+          status: 'reveal',
+          round_number: 1,
+          players_count: 2,
+          constraints: {
+            min_players_to_start: 2,
+            max_players_mvp: 8,
+            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: true,
+            can_start_next_round: false,
+            can_finish_game: false
+          },
+          player: {
+            can_join: true,
+            can_submit_lie: false,
+            can_submit_guess: false,
+            can_view_final_result: false
+          }
+        }
+      })
+    ).toThrow('fooled_player_nickname when fooled_player_id is set');
+  });
+
   it('maps host/player gameplay endpoints through typed response mappers', async () => {
     const get = vi.fn<AngularHttpClientLike['get']>(async <T>(url: string) => {
       if (url === '/lobby/sessions/ABCD12/scoreboard') {
diff --git a/lobby/tests.py b/lobby/tests.py
index 4322a32..c537595 100644
--- a/lobby/tests.py
+++ b/lobby/tests.py
@@ -1427,6 +1427,7 @@ class SessionDetailRoundQuestionTests(TestCase):
         self.assertEqual(payload["reveal"]["guesses"][1]["selected_text"], "Edison")
         self.assertTrue(payload["reveal"]["guesses"][1]["is_correct"])
         self.assertIsNone(payload["reveal"]["guesses"][1]["fooled_player_id"])
+        self.assertNotIn("fooled_player_nickname", payload["reveal"]["guesses"][1])
 
     def test_session_detail_includes_canonical_reveal_payload_in_scoreboard_phase(self):
         self.session.status = GameSession.Status.SCOREBOARD