diff --git a/apps/api/src/quests/quest.errors.ts b/apps/api/src/quests/quest.errors.ts
new file mode 100644
index 0000000..081c38f
--- /dev/null
+++ b/apps/api/src/quests/quest.errors.ts
@@ -0,0 +1,81 @@
+import { HttpException, HttpStatus } from '@nestjs/common';
+
+export type QuestErrorCode =
+  | 'QUEST_NOT_FOUND'
+  | 'QUEST_NOT_OFFERED_HERE'
+  | 'QUEST_ALREADY_ACCEPTED'
+  | 'QUEST_NOT_ACTIVE'
+  | 'QUEST_STEP_NOT_HERE'
+  | 'QUEST_OBJECTIVE_INCOMPLETE';
+
+export class QuestDomainError extends HttpException {
+  constructor(
+    public readonly code: QuestErrorCode,
+    status: HttpStatus,
+    message: string,
+  ) {
+    super({ statusCode: status, code, message }, status);
+  }
+}
+
+export function questNotFound(): QuestDomainError {
+  return new QuestDomainError(
+    'QUEST_NOT_FOUND',
+    HttpStatus.NOT_FOUND,
+    'This quest could not be found.',
+  );
+}
+
+export function questNotOfferedHere(): QuestDomainError {
+  return new QuestDomainError(
+    'QUEST_NOT_OFFERED_HERE',
+    HttpStatus.CONFLICT,
+    'This person has nothing to ask of you.',
+  );
+}
+
+/**
+ * Raised when the quest is already on the character's list.
+ *
+ * Also what a lost race resolves to: the unique index on
+ * (character_id, quest_id) is the real guarantee, and this turns the
+ * constraint violation into something the client can explain (AGENTS.md §30).
+ */
+export function questAlreadyAccepted(): QuestDomainError {
+  return new QuestDomainError(
+    'QUEST_ALREADY_ACCEPTED',
+    HttpStatus.CONFLICT,
+    'You have already taken this on.',
+  );
+}
+
+export function questNotActive(): QuestDomainError {
+  return new QuestDomainError(
+    'QUEST_NOT_ACTIVE',
+    HttpStatus.CONFLICT,
+    'You are not on this quest.',
+  );
+}
+
+/**
+ * Raised when this NPC is not what the quest currently needs.
+ *
+ * Covers both "you are talking to the wrong person" and "you came back too
+ * early": the current step is derived, so a turn-in attempt with four pelts
+ * simply is not the step the character is on.
+ */
+export function questStepNotHere(): QuestDomainError {
+  return new QuestDomainError(
+    'QUEST_STEP_NOT_HERE',
+    HttpStatus.CONFLICT,
+    'This is not what the quest needs from you right now.',
+  );
+}
+
+export function questObjectiveIncomplete(): QuestDomainError {
+  return new QuestDomainError(
+    'QUEST_OBJECTIVE_INCOMPLETE',
+    HttpStatus.CONFLICT,
+    'You do not have what this step needs yet.',
+  );
+}
diff --git a/apps/api/src/quests/quest.service.spec.ts b/apps/api/src/quests/quest.service.spec.ts
new file mode 100644
index 0000000..c59a4ed
--- /dev/null
+++ b/apps/api/src/quests/quest.service.spec.ts
@@ -0,0 +1,867 @@
+import { DataSource, EntityManager } from 'typeorm';
+import { Character } from '../characters/entities/character.entity';
+import { CharacterItem } from '../items/entities/character-item.entity';
+import { ItemDefinition } from '../items/entities/item-definition.entity';
+import { LootCategory } from '../items/loot-category.enum';
+import { CharacterLootBag } from '../loot-bags/entities/character-loot-bag.entity';
+import { LootBagDefinition } from '../loot-bags/entities/loot-bag-definition.entity';
+import { CharacterNpcState } from '../npcs/entities/character-npc-state.entity';
+import { NpcDefinition } from '../npcs/entities/npc-definition.entity';
+import { NpcService } from '../npcs/npc.service';
+import { ReputationService } from '../reputation/reputation.service';
+import { CharacterQuest } from './entities/character-quest.entity';
+import { NpcQuestAssignment } from './entities/npc-quest-assignment.entity';
+import { QuestDefinition } from './entities/quest-definition.entity';
+import { QuestObjective } from './entities/quest-objective.entity';
+import {
+  QuestObjectiveState,
+  QuestProgressService,
+  QuestState,
+} from './quest-progress.service';
+import { QuestService } from './quest.service';
+import {
+  CharacterQuestStatus,
+  NpcQuestRole,
+  QuestObjectiveType,
+  QuestStatus,
+} from './quest.types';
+
+const CHARACTER_ID = 'character-1';
+const QUEST_ID = 'quest-1';
+const QUEST_KEY = 'trouble-beyond-the-gate';
+const WARDEN_ID = 'npc-warden';
+const WARDEN_KEY = 'south-gate-warden';
+const BORIN_ID = 'npc-borin';
+const BORIN_KEY = 'borin-quartermaster';
+const REFERRAL_FLAG = 'referred-by-south-gate-warden';
+const PELT_DEFINITION_ID = 'item-pelt';
+const HIDE_BAG_DEFINITION_ID = 'bag-hide';
+
+function objective(over: Partial<QuestObjective>): QuestObjective {
+  return {
+    id: `objective-${over.orderIndex ?? 0}`,
+    questId: QUEST_ID,
+    key: `step-${over.orderIndex ?? 0}`,
+    orderIndex: 0,
+    type: QuestObjectiveType.COLLECT_ITEM,
+    targetKey: 'ash-pelt',
+    requiredQuantity: 5,
+    description: 'Collect Ashen Pelts',
+    npcLine: null,
+    hintText: null,
+    advanceWhenBlocked: false,
+    consumeOnComplete: false,
+    grantsLootBagKey: null,
+    setsFlagKey: null,
+    setsFlagNpcKey: null,
+    enabled: true,
+    ...over,
+  } as QuestObjective;
+}
+
+/** The real five-step chain (Slice 0.9 §3–§8). */
+function chain(): QuestObjective[] {
+  return [
+    objective({
+      orderIndex: 0,
+      key: 'collect-pelts-first',
+      advanceWhenBlocked: true,
+      hintText: 'You cannot carry enough pelts. Return to the South Gate Warden.',
+    }),
+    objective({
+      orderIndex: 1,
+      key: 'report-capacity',
+      type: QuestObjectiveType.TALK_TO_NPC,
+      targetKey: WARDEN_KEY,
+      requiredQuantity: 1,
+      description: 'Return to the South Gate Warden',
+      npcLine: 'Go see Borin in Graufurt.',
+      setsFlagKey: REFERRAL_FLAG,
+      setsFlagNpcKey: BORIN_KEY,
+    }),
+    objective({
+      orderIndex: 2,
+      key: 'collect-bag',
+      type: QuestObjectiveType.TALK_TO_NPC,
+      targetKey: BORIN_KEY,
+      requiredQuantity: 1,
+      description: 'Speak with Borin in Graufurt',
+      npcLine: 'Take this.',
+      grantsLootBagKey: 'basic-hide-bag',
+    }),
+    objective({
+      orderIndex: 3,
+      key: 'collect-pelts',
+      consumeOnComplete: true,
+    }),
+    objective({
+      orderIndex: 4,
+      key: 'turn-in',
+      type: QuestObjectiveType.TALK_TO_NPC,
+      targetKey: WARDEN_KEY,
+      requiredQuantity: 1,
+      description: 'Bring the pelts to the South Gate Warden',
+      npcLine: "Good. That's enough for me.",
+    }),
+  ];
+}
+
+interface Fixture {
+  /** Which NPC the character is standing with; null means unreachable. */
+  standingWith?: 'warden' | 'borin' | null;
+  questEnabled?: boolean;
+  hasOfferAssignment?: boolean;
+  /** Undefined means the quest was never accepted. */
+  status?: CharacterQuestStatus;
+  storedIndex?: number;
+  /** The step the derivation says the character is on. */
+  currentIndex?: number | null;
+  pelts?: number;
+  blockedIndexes?: number[];
+  ownsHideBag?: boolean;
+  borinFlags?: Record<string, boolean | string | number>;
+  silver?: number;
+  rewardReputation?: number;
+  rewardSilver?: number;
+  /** Makes the insert fail the way a lost unique-index race would. */
+  insertRace?: boolean;
+}
+
+function createWorld(fixture: Fixture = {}) {
+  const quest = {
+    id: QUEST_ID,
+    key: QUEST_KEY,
+    title: 'Trouble Beyond the Gate',
+    description: 'Five pelts.',
+    rewardFactionKey: 'border-guard',
+    rewardReputation: fixture.rewardReputation ?? 10,
+    rewardSilver: fixture.rewardSilver ?? 0,
+    enabled: fixture.questEnabled ?? true,
+  } as QuestDefinition;
+
+  const objectives = chain();
+
+  const npcs: Record<string, NpcDefinition> = {
+    warden: { id: WARDEN_ID, key: WARDEN_KEY } as NpcDefinition,
+    borin: { id: BORIN_ID, key: BORIN_KEY } as NpcDefinition,
+  };
+
+  const questRow: CharacterQuest | null =
+    fixture.status === undefined
+      ? null
+      : ({
+          id: 'character-quest-1',
+          characterId: CHARACTER_ID,
+          questId: QUEST_ID,
+          status: fixture.status,
+          currentObjectiveIndex: fixture.storedIndex ?? 0,
+          acceptedAt: new Date('2026-01-01T00:00:00Z'),
+          completedAt: null,
+        } as CharacterQuest);
+
+  const character = {
+    id: CHARACTER_ID,
+    silver: fixture.silver ?? 0,
+  } as Character;
+
+  const peltStack =
+    (fixture.pelts ?? 0) > 0
+      ? ({
+          id: 'character-item-pelt',
+          characterId: CHARACTER_ID,
+          itemDefinitionId: PELT_DEFINITION_ID,
+          quantity: fixture.pelts as number,
+        } as CharacterItem)
+      : null;
+
+  const state = {
+    characterItems: peltStack ? [peltStack] : ([] as CharacterItem[]),
+    lootBags: fixture.ownsHideBag
+      ? [
+          {
+            characterId: CHARACTER_ID,
+            lootBagDefinitionId: HIDE_BAG_DEFINITION_ID,
+            active: true,
+          },
+        ]
+      : ([] as Array<Record<string, unknown>>),
+    npcStates: fixture.borinFlags
+      ? [
+          {
+            characterId: CHARACTER_ID,
+            npcId: BORIN_ID,
+            flags: { ...fixture.borinFlags },
+          },
+        ]
+      : ([] as Array<Record<string, unknown>>),
+    questRows: questRow ? [questRow] : ([] as CharacterQuest[]),
+    savedQuestRows: [] as CharacterQuest[],
+    savedNpcStates: [] as Array<Record<string, unknown>>,
+    savedLootBags: [] as Array<Record<string, unknown>>,
+    removedItems: [] as CharacterItem[],
+    savedCharacters: [] as Character[],
+  };
+
+  const manager = {
+    getRepository: (entity: unknown) => {
+      if (entity === QuestDefinition) {
+        return {
+          findOneBy: async (criteria: { key: string; enabled: boolean }) =>
+            quest.key === criteria.key && quest.enabled === criteria.enabled
+              ? quest
+              : null,
+        };
+      }
+      if (entity === NpcQuestAssignment) {
+        return {
+          findOneBy: async (criteria: { npcId: string; role: NpcQuestRole }) =>
+            (fixture.hasOfferAssignment ?? true) &&
+            criteria.npcId === WARDEN_ID &&
+            criteria.role === NpcQuestRole.OFFER
+              ? { id: 'assignment-1' }
+              : null,
+        };
+      }
+      if (entity === CharacterQuest) {
+        return {
+          findOne: async () => state.questRows[0] ?? null,
+          create: (value: Record<string, unknown>) => value,
+          save: async (value: CharacterQuest) => {
+            state.savedQuestRows.push(value);
+            if (fixture.insertRace && state.questRows.length === 0) {
+              // What Postgres raises when the unique index refuses a second
+              // row for the same (character, quest).
+              throw Object.assign(new Error('duplicate key'), {
+                driverError: { code: '23505' },
+              });
+            }
+            return value;
+          },
+        };
+      }
+      if (entity === NpcDefinition) {
+        return {
+          findOneBy: async (criteria: { key: string }) =>
+            Object.values(npcs).find((npc) => npc.key === criteria.key) ?? null,
+        };
+      }
+      if (entity === CharacterNpcState) {
+        return {
+          findOneBy: async (criteria: { npcId: string }) =>
+            state.npcStates.find((row) => row.npcId === criteria.npcId) ?? null,
+          create: (value: Record<string, unknown>) => value,
+          save: async (value: Record<string, unknown>) => {
+            state.savedNpcStates.push(value);
+            return value;
+          },
+        };
+      }
+      if (entity === LootBagDefinition) {
+        return {
+          findOneBy: async (criteria: { key: string }) =>
+            criteria.key === 'basic-hide-bag'
+              ? {
+                  id: HIDE_BAG_DEFINITION_ID,
+                  key: 'basic-hide-bag',
+                  name: 'Basic Hide Bag',
+                  lootCategory: LootCategory.HIDE,
+                  capacity: 5,
+                }
+              : null,
+        };
+      }
+      if (entity === CharacterLootBag) {
+        return {
+          findOne: async () => state.lootBags[0] ?? null,
+          create: (value: Record<string, unknown>) => value,
+          save: async (value: Record<string, unknown>) => {
+            state.savedLootBags.push(value);
+            state.lootBags.push(value);
+            return value;
+          },
+        };
+      }
+      if (entity === ItemDefinition) {
+        return {
+          findOneBy: async (criteria: { key: string }) =>
+            criteria.key === 'ash-pelt'
+              ? { id: PELT_DEFINITION_ID, key: 'ash-pelt' }
+              : null,
+        };
+      }
+      if (entity === CharacterItem) {
+        return {
+          findOne: async () => state.characterItems[0] ?? null,
+          save: async (value: CharacterItem) => value,
+          remove: async (value: CharacterItem) => {
+            state.removedItems.push(value);
+            state.characterItems = state.characterItems.filter(
+              (item) => item !== value,
+            );
+            return value;
+          },
+        };
+      }
+      if (entity === Character) {
+        return {
+          findOne: async () => character,
+          save: async (value: Character) => {
+            state.savedCharacters.push(value);
+            return value;
+          },
+        };
+      }
+      throw new Error('Unexpected repository');
+    },
+  } as unknown as EntityManager;
+
+  const dataSource = {
+    getRepository: manager.getRepository.bind(manager),
+    transaction: async <T>(run: (m: EntityManager) => Promise<T>) => run(manager),
+  } as unknown as DataSource;
+
+  const objectiveStates: QuestObjectiveState[] = objectives.map(
+    (candidate, index) => ({
+      objective: candidate,
+      current:
+        candidate.type === QuestObjectiveType.COLLECT_ITEM
+          ? (fixture.pelts ?? 0)
+          : 0,
+      required: candidate.requiredQuantity,
+      blocked: (fixture.blockedIndexes ?? []).includes(index),
+      satisfied:
+        candidate.type === QuestObjectiveType.COLLECT_ITEM &&
+        (fixture.pelts ?? 0) >= candidate.requiredQuantity,
+    }),
+  );
+
+  const status: QuestStatus =
+    fixture.status === undefined
+      ? 'AVAILABLE'
+      : fixture.status === CharacterQuestStatus.COMPLETED
+        ? 'COMPLETED'
+        : 'ACTIVE';
+
+  const questState: QuestState = {
+    quest,
+    objectives: objectiveStates,
+    row: questRow,
+    status,
+    currentIndex:
+      fixture.currentIndex === undefined
+        ? status === 'ACTIVE'
+          ? (fixture.storedIndex ?? 0)
+          : null
+        : fixture.currentIndex,
+  };
+
+  const progress = {
+    getQuestStates: jest.fn().mockResolvedValue([questState]),
+    getQuestState: jest.fn().mockResolvedValue(questState),
+    getNpcQuestStates: jest.fn().mockResolvedValue([]),
+  } as unknown as QuestProgressService;
+
+  const npcService = {
+    requireReachableNpc: jest.fn(async (_characterId: string, key: string) => {
+      // Not `??`: the fixture uses an explicit null to mean "nowhere near
+      // anyone", which `??` would quietly turn back into the warden.
+      const standing =
+        fixture.standingWith === undefined ? 'warden' : fixture.standingWith;
+      if (standing === null) {
+        throw new Error('NPC_UNAVAILABLE');
+      }
+      if (npcs[standing].key !== key) {
+        throw new Error('NPC_UNAVAILABLE');
+      }
+      return npcs[standing];
+    }),
+  } as unknown as NpcService;
+
+  const reputation = {
+    grantReputation: jest.fn().mockResolvedValue({
+      factionKey: 'border-guard',
+      previousReputation: 0,
+      newReputation: 10,
+      previousRank: 'NEUTRAL',
+      newRank: 'NEUTRAL',
+      rankChanged: false,
+    }),
+  } as unknown as ReputationService;
+
+  return {
+    service: new QuestService(dataSource, progress, npcService, reputation),
+    state,
+    character,
+    reputation,
+    questState,
+  };
+}
+
+describe('QuestService.acceptQuest', () => {
+  it('records the quest as active at the first step', async () => {
+    const { service, state } = createWorld({ status: undefined });
+
+    const result = await service.acceptQuest(
+      CHARACTER_ID,
+      WARDEN_KEY,
+      QUEST_KEY,
+    );
+
+    expect(state.savedQuestRows[0]).toMatchObject({
+      characterId: CHARACTER_ID,
+      questId: QUEST_ID,
+      status: CharacterQuestStatus.ACTIVE,
+      currentObjectiveIndex: 0,
+    });
+    // The offer line is dialogue content; the accept itself says nothing.
+    expect(result.npcLine).toBeNull();
+    expect(result.grantedBag).toBeNull();
+  });
+
+  it('refuses an NPC that does not offer the quest', async () => {
+    const { service } = createWorld({
+      status: undefined,
+      hasOfferAssignment: false,
+    });
+
+    await expect(
+      service.acceptQuest(CHARACTER_ID, WARDEN_KEY, QUEST_KEY),
+    ).rejects.toMatchObject({ response: { code: 'QUEST_NOT_OFFERED_HERE' } });
+  });
+
+  it('refuses an unknown or disabled quest', async () => {
+    const { service } = createWorld({ status: undefined, questEnabled: false });
+
+    await expect(
+      service.acceptQuest(CHARACTER_ID, WARDEN_KEY, QUEST_KEY),
+    ).rejects.toMatchObject({ response: { code: 'QUEST_NOT_FOUND' } });
+  });
+
+  it('accepts the quest only once', async () => {
+    const { service } = createWorld({ status: CharacterQuestStatus.ACTIVE });
+
+    await expect(
+      service.acceptQuest(CHARACTER_ID, WARDEN_KEY, QUEST_KEY),
+    ).rejects.toMatchObject({ response: { code: 'QUEST_ALREADY_ACCEPTED' } });
+  });
+
+  it('turns a lost unique-index race into the same refusal', async () => {
+    // Two clicks that both pass the existence check: the database refuses the
+    // second, and the player must not see a 500 for it (AGENTS.md §30).
+    const { service } = createWorld({ status: undefined, insertRace: true });
+
+    await expect(
+      service.acceptQuest(CHARACTER_ID, WARDEN_KEY, QUEST_KEY),
+    ).rejects.toMatchObject({ response: { code: 'QUEST_ALREADY_ACCEPTED' } });
+  });
+
+  it('never accepts a quest the character cannot reach', async () => {
+    const { service } = createWorld({ status: undefined, standingWith: null });
+
+    await expect(
+      service.acceptQuest(CHARACTER_ID, WARDEN_KEY, QUEST_KEY),
+    ).rejects.toThrow('NPC_UNAVAILABLE');
+  });
+});
+
+describe('QuestService.advanceQuest', () => {
+  it('sets the referral flag on Borin, not on the warden', async () => {
+    const { service, state } = createWorld({
+      status: CharacterQuestStatus.ACTIVE,
+      storedIndex: 0,
+      currentIndex: 1,
+      pelts: 1,
+      blockedIndexes: [0, 3],
+    });
+
+    const result = await service.advanceQuest(
+      CHARACTER_ID,
+      WARDEN_KEY,
+      QUEST_KEY,
+    );
+
+    // Slice 0.9 §6: the 0.8.5 bypass is evaluated with Borin in context, so
+    // the flag has to live on his row (NPC spec §7).
+    expect(state.savedNpcStates[0]).toMatchObject({
+      npcId: BORIN_ID,
+      flags: { [REFERRAL_FLAG]: true },
+    });
+    expect(result.npcLine).toBe('Go see Borin in Graufurt.');
+  });
+
+  it('keeps the flags an NPC row already carried', async () => {
+    const { service, state } = createWorld({
+      status: CharacterQuestStatus.ACTIVE,
+      storedIndex: 0,
+      currentIndex: 1,
+      pelts: 1,
+      borinFlags: { met: true },
+    });
+
+    await service.advanceQuest(CHARACTER_ID, WARDEN_KEY, QUEST_KEY);
+
+    // `met` drives Borin's first-meeting line. Replacing the map instead of
+    // merging into it would make him greet the player as a stranger again.
+    expect(state.savedNpcStates[0].flags).toEqual({
+      met: true,
+      [REFERRAL_FLAG]: true,
+    });
+  });
+
+  it('advances the stored floor past the step it performed', async () => {
+    const { service, state } = createWorld({
+      status: CharacterQuestStatus.ACTIVE,
+      storedIndex: 0,
+      currentIndex: 1,
+      pelts: 1,
+    });
+
+    await service.advanceQuest(CHARACTER_ID, WARDEN_KEY, QUEST_KEY);
+
+    expect(state.savedQuestRows[0].currentObjectiveIndex).toBe(2);
+  });
+
+  it('hands the Basic Hide Bag over on the step at Borin', async () => {
+    const { service, state } = createWorld({
+      standingWith: 'borin',
+      status: CharacterQuestStatus.ACTIVE,
+      storedIndex: 2,
+      currentIndex: 2,
+      pelts: 1,
+    });
+
+    const result = await service.advanceQuest(
+      CHARACTER_ID,
+      BORIN_KEY,
+      QUEST_KEY,
+    );
+
+    expect(state.savedLootBags[0]).toMatchObject({
+      lootBagDefinitionId: HIDE_BAG_DEFINITION_ID,
+      active: true,
+    });
+    expect(result.grantedBag).toEqual({
+      key: 'basic-hide-bag',
+      name: 'Basic Hide Bag',
+      lootCategory: LootCategory.HIDE,
+      capacity: 5,
+    });
+  });
+
+  it('does not hand over a bag the character already carries', async () => {
+    // Slice 0.9 §11: the grant is idempotent, so development data or a repeat
+    // click cannot produce a second bag or an error.
+    const { service, state } = createWorld({
+      standingWith: 'borin',
+      status: CharacterQuestStatus.ACTIVE,
+      storedIndex: 2,
+      currentIndex: 2,
+      ownsHideBag: true,
+    });
+
+    const result = await service.advanceQuest(
+      CHARACTER_ID,
+      BORIN_KEY,
+      QUEST_KEY,
+    );
+
+    expect(state.savedLootBags).toHaveLength(0);
+    expect(result.grantedBag).toBeNull();
+    // The step still counts: the chain must not stall on an already-owned bag.
+    expect(state.savedQuestRows[0].currentObjectiveIndex).toBe(3);
+  });
+
+  it('refuses a step at the wrong NPC', async () => {
+    const { service } = createWorld({
+      standingWith: 'borin',
+      status: CharacterQuestStatus.ACTIVE,
+      storedIndex: 0,
+      currentIndex: 1,
+    });
+
+    await expect(
+      service.advanceQuest(CHARACTER_ID, BORIN_KEY, QUEST_KEY),
+    ).rejects.toMatchObject({ response: { code: 'QUEST_STEP_NOT_HERE' } });
+  });
+
+  it('refuses the turn-in while the pelts are still short', async () => {
+    const { service } = createWorld({
+      status: CharacterQuestStatus.ACTIVE,
+      storedIndex: 3,
+      currentIndex: 3,
+      pelts: 4,
+    });
+
+    await expect(
+      service.advanceQuest(CHARACTER_ID, WARDEN_KEY, QUEST_KEY),
+    ).rejects.toMatchObject({ response: { code: 'QUEST_STEP_NOT_HERE' } });
+  });
+
+  it('refuses to advance a quest that was never accepted', async () => {
+    const { service } = createWorld({ status: undefined });
+
+    await expect(
+      service.advanceQuest(CHARACTER_ID, WARDEN_KEY, QUEST_KEY),
+    ).rejects.toMatchObject({ response: { code: 'QUEST_NOT_ACTIVE' } });
+  });
+
+  it('refuses to advance a completed quest', async () => {
+    const { service } = createWorld({
+      status: CharacterQuestStatus.COMPLETED,
+      storedIndex: 5,
+    });
+
+    await expect(
+      service.advanceQuest(CHARACTER_ID, WARDEN_KEY, QUEST_KEY),
+    ).rejects.toMatchObject({ response: { code: 'QUEST_NOT_ACTIVE' } });
+  });
+
+  it('consumes exactly the pelts the quest asked for', async () => {
+    const { service, state } = createWorld({
+      status: CharacterQuestStatus.ACTIVE,
+      storedIndex: 3,
+      currentIndex: 4,
+      pelts: 7,
+    });
+
+    const result = await service.advanceQuest(
+      CHARACTER_ID,
+      WARDEN_KEY,
+      QUEST_KEY,
+    );
+
+    // Two collect steps ask for five each; only the second consumes, so the
+    // turn-in takes five and leaves the surplus (§8).
+    expect(result.consumedItems).toEqual([{ itemKey: 'ash-pelt', quantity: 5 }]);
+    expect(state.characterItems[0].quantity).toBe(2);
+    expect(state.removedItems).toHaveLength(0);
+  });
+
+  it('removes the item row when the last pelt is consumed', async () => {
+    const { service, state } = createWorld({
+      status: CharacterQuestStatus.ACTIVE,
+      storedIndex: 3,
+      currentIndex: 4,
+      pelts: 5,
+    });
+
+    await service.advanceQuest(CHARACTER_ID, WARDEN_KEY, QUEST_KEY);
+
+    expect(state.removedItems).toHaveLength(1);
+  });
+
+  it('refuses the turn-in when the pelts vanished between read and write', async () => {
+    // The derived step said "turn-in", but the transaction is where ownership
+    // is actually decided.
+    const { service } = createWorld({
+      status: CharacterQuestStatus.ACTIVE,
+      storedIndex: 3,
+      currentIndex: 4,
+      pelts: 0,
+    });
+
+    await expect(
+      service.advanceQuest(CHARACTER_ID, WARDEN_KEY, QUEST_KEY),
+    ).rejects.toMatchObject({
+      response: { code: 'QUEST_OBJECTIVE_INCOMPLETE' },
+    });
+  });
+
+  it('completes the quest and stamps the time', async () => {
+    const { service, state } = createWorld({
+      status: CharacterQuestStatus.ACTIVE,
+      storedIndex: 3,
+      currentIndex: 4,
+      pelts: 5,
+    });
+
+    await service.advanceQuest(CHARACTER_ID, WARDEN_KEY, QUEST_KEY);
+
+    const saved = state.savedQuestRows[0];
+    expect(saved.status).toBe(CharacterQuestStatus.COMPLETED);
+    expect(saved.completedAt).toBeInstanceOf(Date);
+    expect(saved.currentObjectiveIndex).toBe(5);
+  });
+
+  it('pays the regional reputation reward on completion', async () => {
+    const { service, reputation } = createWorld({
+      status: CharacterQuestStatus.ACTIVE,
+      storedIndex: 3,
+      currentIndex: 4,
+      pelts: 5,
+    });
+
+    const result = await service.advanceQuest(
+      CHARACTER_ID,
+      WARDEN_KEY,
+      QUEST_KEY,
+    );
+
+    expect(reputation.grantReputation).toHaveBeenCalledWith(
+      CHARACTER_ID,
+      'border-guard',
+      10,
+      expect.anything(),
+    );
+    expect(result.rewards).toEqual({
+      factionKey: 'border-guard',
+      reputation: 10,
+      silver: 0,
+    });
+  });
+
+  it('pays no silver, because the content asks for none', async () => {
+    // Slice 0.9 decision D3. The branch exists so retuning is a seed change,
+    // but the seeded quest deliberately pays nothing.
+    const { service, state } = createWorld({
+      status: CharacterQuestStatus.ACTIVE,
+      storedIndex: 3,
+      currentIndex: 4,
+      pelts: 5,
+    });
+
+    await service.advanceQuest(CHARACTER_ID, WARDEN_KEY, QUEST_KEY);
+
+    expect(state.savedCharacters).toHaveLength(0);
+  });
+
+  it('pays silver when content asks for it', async () => {
+    const { service, state, character } = createWorld({
+      status: CharacterQuestStatus.ACTIVE,
+      storedIndex: 3,
+      currentIndex: 4,
+      pelts: 5,
+      rewardSilver: 25,
+      silver: 4,
+    });
+
+    const result = await service.advanceQuest(
+      CHARACTER_ID,
+      WARDEN_KEY,
+      QUEST_KEY,
+    );
+
+    expect(character.silver).toBe(29);
+    expect(state.savedCharacters).toHaveLength(1);
+    expect(result.rewards?.silver).toBe(25);
+  });
+
+  it('pays nothing on a step that is not the last one', async () => {
+    const { service, reputation, state } = createWorld({
+      status: CharacterQuestStatus.ACTIVE,
+      storedIndex: 0,
+      currentIndex: 1,
+      pelts: 1,
+    });
+
+    const result = await service.advanceQuest(
+      CHARACTER_ID,
+      WARDEN_KEY,
+      QUEST_KEY,
+    );
+
+    expect(reputation.grantReputation).not.toHaveBeenCalled();
+    expect(result.rewards).toBeNull();
+    expect(result.consumedItems).toEqual([]);
+    expect(state.savedQuestRows[0].status).toBe(CharacterQuestStatus.ACTIVE);
+  });
+});
+
+describe('QuestService.getQuestLog', () => {
+  it('renders collect progress against what the step needs', async () => {
+    const { service } = createWorld({
+      status: CharacterQuestStatus.ACTIVE,
+      storedIndex: 0,
+      currentIndex: 0,
+      pelts: 1,
+    });
+
+    const [quest] = await service.getQuestLog(CHARACTER_ID);
+
+    expect(quest).toMatchObject({
+      key: QUEST_KEY,
+      title: 'Trouble Beyond the Gate',
+      status: 'ACTIVE',
+      currentObjectiveKey: 'collect-pelts-first',
+    });
+    expect(quest.objectives[0]).toMatchObject({
+      description: 'Collect Ashen Pelts',
+      current: 1,
+      required: 5,
+      completed: false,
+    });
+  });
+
+  it('shows the hint while the active step is blocked', async () => {
+    const { service } = createWorld({
+      status: CharacterQuestStatus.ACTIVE,
+      storedIndex: 0,
+      currentIndex: 0,
+      pelts: 1,
+      blockedIndexes: [0],
+    });
+
+    const [quest] = await service.getQuestLog(CHARACTER_ID);
+
+    // Slice 0.9 §4/§12: the line that keeps "1 / 5" from reading as a wall.
+    expect(quest.hint).toBe(
+      'You cannot carry enough pelts. Return to the South Gate Warden.',
+    );
+  });
+
+  it('shows no hint for a step that is merely unfinished', async () => {
+    const { service } = createWorld({
+      status: CharacterQuestStatus.ACTIVE,
+      storedIndex: 0,
+      currentIndex: 0,
+      pelts: 1,
+    });
+
+    const [quest] = await service.getQuestLog(CHARACTER_ID);
+
+    expect(quest.hint).toBeNull();
+  });
+
+  it('marks every step behind the current one as done', async () => {
+    const { service } = createWorld({
+      status: CharacterQuestStatus.ACTIVE,
+      storedIndex: 3,
+      currentIndex: 3,
+      pelts: 1,
+    });
+
+    const [quest] = await service.getQuestLog(CHARACTER_ID);
+
+    expect(quest.objectives.map((entry) => entry.completed)).toEqual([
+      true,
+      true,
+      true,
+      false,
+      false,
+    ]);
+  });
+
+  it('marks a completed quest as done throughout and points at no step', async () => {
+    const { service } = createWorld({
+      status: CharacterQuestStatus.COMPLETED,
+      storedIndex: 5,
+    });
+
+    const [quest] = await service.getQuestLog(CHARACTER_ID);
+
+    expect(quest.status).toBe('COMPLETED');
+    expect(quest.currentObjectiveKey).toBeNull();
+    expect(quest.hint).toBeNull();
+    expect(quest.objectives.every((entry) => entry.completed)).toBe(true);
+  });
+
+  it('reports an unstarted quest as available with nothing done', async () => {
+    const { service } = createWorld({ status: undefined });
+
+    const [quest] = await service.getQuestLog(CHARACTER_ID);
+
+    expect(quest.status).toBe('AVAILABLE');
+    expect(quest.currentObjectiveKey).toBeNull();
+    expect(quest.objectives.every((entry) => !entry.completed)).toBe(true);
+  });
+});
diff --git a/apps/api/src/quests/quest.service.ts b/apps/api/src/quests/quest.service.ts
new file mode 100644
index 0000000..fec2bf6
--- /dev/null
+++ b/apps/api/src/quests/quest.service.ts
@@ -0,0 +1,489 @@
+import { Injectable } from '@nestjs/common';
+import { DataSource, EntityManager } from 'typeorm';
+import { Character } from '../characters/entities/character.entity';
+import { CharacterItem } from '../items/entities/character-item.entity';
+import { ItemDefinition } from '../items/entities/item-definition.entity';
+import { CharacterLootBag } from '../loot-bags/entities/character-loot-bag.entity';
+import { LootBagDefinition } from '../loot-bags/entities/loot-bag-definition.entity';
+import { CharacterNpcState } from '../npcs/entities/character-npc-state.entity';
+import { NpcDefinition } from '../npcs/entities/npc-definition.entity';
+import { NpcService } from '../npcs/npc.service';
+import { ReputationService } from '../reputation/reputation.service';
+import { CharacterQuest } from './entities/character-quest.entity';
+import { NpcQuestAssignment } from './entities/npc-quest-assignment.entity';
+import { QuestDefinition } from './entities/quest-definition.entity';
+import { QuestObjective } from './entities/quest-objective.entity';
+import { QuestProgressService, QuestState } from './quest-progress.service';
+import {
+  questAlreadyAccepted,
+  questNotActive,
+  questNotFound,
+  questNotOfferedHere,
+  questObjectiveIncomplete,
+  questStepNotHere,
+} from './quest.errors';
+import {
+  CharacterQuestStatus,
+  GrantedLootBagDto,
+  NpcQuestRole,
+  QuestConsumedItemDto,
+  QuestDto,
+  QuestInteractionResultDto,
+  QuestObjectiveType,
+  QuestRewardDto,
+} from './quest.types';
+
+/** Postgres' unique-violation SQLSTATE. */
+const UNIQUE_VIOLATION = '23505';
+
+/**
+ * Runs a quest chain (Playable Slice 0.9 §10).
+ *
+ * A step-based state machine and nothing more: §10 asks for exactly that and
+ * warns off a branching narrative engine. What a step *does* is content on the
+ * objective -- set a flag, hand a bag over, consume what it asked for -- so
+ * this service applies effects it finds rather than switching on quest keys
+ * (AGENTS.md §9).
+ *
+ * Steps are triggered by their own endpoints rather than by dialogue actions
+ * (decision D4). NPC spec §40 sketched `START_QUEST` / `COMPLETE_QUEST` as
+ * dialogue actions, but there is no dialogue-response endpoint to carry them,
+ * and building one would be the narrative engine §10 rules out. The beat's line
+ * travels back on the step result instead.
+ *
+ * Every softlock case in §11 is handled by *not* storing progress: the active
+ * step is derived from what the character owns right now
+ * (`QuestProgressService`), so selling the pelts reopens the objective, owning
+ * some before accepting already counts, and a bag obtained some other way just
+ * lets the first hunt finish.
+ */
+@Injectable()
+export class QuestService {
+  constructor(
+    private readonly dataSource: DataSource,
+    private readonly progress: QuestProgressService,
+    private readonly npcs: NpcService,
+    private readonly reputation: ReputationService,
+  ) {}
+
+  /** Every enabled quest and where this character stands with it (spec §12). */
+  async getQuestLog(characterId: string): Promise<QuestDto[]> {
+    const states = await this.progress.getQuestStates(characterId);
+    return states.map((state) => toQuestDto(state));
+  }
+
+  /**
+   * Takes a quest on, once (spec §13).
+   *
+   * The NPC has to actually offer it: an `OFFER` assignment is what makes a
+   * person a quest giver, not a flag on the NPC row (NPC spec §14).
+   */
+  async acceptQuest(
+    characterId: string,
+    npcKey: string,
+    questKey: string,
+  ): Promise<QuestInteractionResultDto> {
+    // The character's own location decides reachability, never the request.
+    const npc = await this.npcs.requireReachableNpc(characterId, npcKey);
+
+    await this.dataSource.transaction(async (manager) => {
+      const quest = await this.requireQuest(manager, questKey);
+
+      const assignment = await manager
+        .getRepository(NpcQuestAssignment)
+        .findOneBy({
+          npcId: npc.id,
+          questId: quest.id,
+          role: NpcQuestRole.OFFER,
+          enabled: true,
+        });
+      if (!assignment) {
+        throw questNotOfferedHere();
+      }
+
+      const rows = manager.getRepository(CharacterQuest);
+      const existing = await rows.findOne({
+        where: { characterId, questId: quest.id },
+      });
+      if (existing) {
+        throw questAlreadyAccepted();
+      }
+
+      try {
+        await rows.save(
+          rows.create({
+            characterId,
+            questId: quest.id,
+            status: CharacterQuestStatus.ACTIVE,
+            currentObjectiveIndex: 0,
+            acceptedAt: new Date(),
+            completedAt: null,
+          }),
+        );
+      } catch (error) {
+        // Two clicks can both clear the check above. The unique index is the
+        // real guarantee; this is what stops the loser seeing a 500.
+        if (!isUniqueViolation(error)) {
+          throw error;
+        }
+        throw questAlreadyAccepted();
+      }
+    });
+
+    const state = await this.progress.getQuestState(characterId, questKey);
+    if (!state) {
+      throw questNotFound();
+    }
+
+    // No line of its own: the warden's offer is a dialogue node, and the NPC
+    // screen re-reads dialogue after the quest state changes.
+    return {
+      quest: toQuestDto(state),
+      npcLine: null,
+      grantedBag: null,
+      consumedItems: [],
+      rewards: null,
+    };
+  }
+
+  /**
+   * Performs the talk step the character is currently on (spec §5, §6, §8).
+   *
+   * One endpoint for all three beats, because the difference between them is
+   * content: the referral sets a flag, Borin's step grants a bag, and the last
+   * step consumes and completes. Everything runs in one transaction under a
+   * write lock on the quest row, so two clicks cannot both consume the pelts
+   * (AGENTS.md §29, §30).
+   */
+  async advanceQuest(
+    characterId: string,
+    npcKey: string,
+    questKey: string,
+  ): Promise<QuestInteractionResultDto> {
+    const npc = await this.npcs.requireReachableNpc(characterId, npcKey);
+
+    return this.dataSource.transaction(async (manager) => {
+      const quest = await this.requireQuest(manager, questKey);
+
+      const rows = manager.getRepository(CharacterQuest);
+      const row = await rows.findOne({
+        where: { characterId, questId: quest.id },
+        lock: { mode: 'pessimistic_write' },
+      });
+      if (!row || row.status !== CharacterQuestStatus.ACTIVE) {
+        throw questNotActive();
+      }
+
+      const state = await this.progress.getQuestState(
+        characterId,
+        questKey,
+        manager,
+      );
+      if (!state || state.currentIndex === null) {
+        throw questNotActive();
+      }
+
+      const currentIndex = state.currentIndex;
+      const step = state.objectives[currentIndex]?.objective;
+      // Also what refuses an early turn-in: with the pelts still short, the
+      // derived step is the hunt, not the hand-over.
+      if (
+        !step ||
+        step.type !== QuestObjectiveType.TALK_TO_NPC ||
+        step.targetKey !== npc.key
+      ) {
+        throw questStepNotHere();
+      }
+
+      row.currentObjectiveIndex = currentIndex + 1;
+
+      const grantedBag = await this.grantLootBag(manager, characterId, step);
+      await this.setStepFlag(manager, characterId, step);
+
+      let consumedItems: QuestConsumedItemDto[] = [];
+      let rewards: QuestRewardDto | null = null;
+
+      if (row.currentObjectiveIndex >= state.objectives.length) {
+        consumedItems = await this.consumeQuestItems(
+          manager,
+          characterId,
+          state,
+        );
+        rewards = await this.grantRewards(manager, characterId, quest);
+        row.status = CharacterQuestStatus.COMPLETED;
+        row.completedAt = new Date();
+      }
+
+      await rows.save(row);
+
+      const refreshed = await this.progress.getQuestState(
+        characterId,
+        questKey,
+        manager,
+      );
+
+      return {
+        quest: toQuestDto(refreshed ?? state),
+        npcLine: step.npcLine,
+        grantedBag,
+        consumedItems,
+        rewards,
+      };
+    });
+  }
+
+  private async requireQuest(
+    manager: EntityManager,
+    questKey: string,
+  ): Promise<QuestDefinition> {
+    const quest = await manager
+      .getRepository(QuestDefinition)
+      .findOneBy({ key: questKey, enabled: true });
+    if (!quest) {
+      throw questNotFound();
+    }
+    return quest;
+  }
+
+  /**
+   * Hands the step's bag over, at most once (spec §11, decision D1).
+   *
+   * A bag the character already holds is not an error: development data, a
+   * repeated click and a re-run of the same step must all leave the chain
+   * moving. Only the roomiest active bag per category counts anyway
+   * (Slice 0.7.5 §6), so a second copy would grant nothing.
+   */
+  private async grantLootBag(
+    manager: EntityManager,
+    characterId: string,
+    step: QuestObjective,
+  ): Promise<GrantedLootBagDto | null> {
+    if (!step.grantsLootBagKey) {
+      return null;
+    }
+
+    const definition = await manager
+      .getRepository(LootBagDefinition)
+      .findOneBy({ key: step.grantsLootBagKey });
+    if (!definition) {
+      // Content names a bag that does not exist. Refusing the whole step would
+      // strand the player on a content bug they cannot fix; the step still
+      // counts and the missing bag is visible in its absence.
+      return null;
+    }
+
+    const bags = manager.getRepository(CharacterLootBag);
+    const existing = await bags.findOne({
+      where: { characterId, lootBagDefinitionId: definition.id },
+    });
+    if (existing) {
+      return null;
+    }
+
+    await bags.save(
+      bags.create({
+        characterId,
+        lootBagDefinitionId: definition.id,
+        active: true,
+      }),
+    );
+
+    return {
+      key: definition.key,
+      name: definition.name,
+      lootCategory: definition.lootCategory,
+      capacity: definition.capacity,
+    };
+  }
+
+  /**
+   * Writes the step's dialogue flag onto whichever NPC content names (spec §5).
+   *
+   * The warden's referral belongs on Borin's row, because a flag is per-NPC
+   * player state (NPC spec §7) and Borin is the NPC in context when the Hide
+   * Bag offer's 0.8.5 bypass is evaluated (spec §6). Merged into the existing
+   * flags rather than replacing them -- `met` drives Borin's first-meeting
+   * line, and dropping it would make him greet the player as a stranger again.
+   */
+  private async setStepFlag(
+    manager: EntityManager,
+    characterId: string,
+    step: QuestObjective,
+  ): Promise<void> {
+    if (!step.setsFlagKey || !step.setsFlagNpcKey) {
+      return;
+    }
+
+    const target = await manager
+      .getRepository(NpcDefinition)
+      .findOneBy({ key: step.setsFlagNpcKey });
+    if (!target) {
+      return;
+    }
+
+    const states = manager.getRepository(CharacterNpcState);
+    const existing = await states.findOneBy({ characterId, npcId: target.id });
+
+    if (existing) {
+      existing.flags = { ...existing.flags, [step.setsFlagKey]: true };
+      await states.save(existing);
+      return;
+    }
+
+    await states.save(
+      states.create({
+        characterId,
+        npcId: target.id,
+        flags: { [step.setsFlagKey]: true },
+      }),
+    );
+  }
+
+  /**
+   * Takes the goods the quest asked for (spec §8).
+   *
+   * Only steps marked `consumeOnComplete` are taken: this chain asks for five
+   * pelts twice, and consuming both would quietly demand ten. Quantities are
+   * re-checked here rather than trusted from the derivation, because the
+   * derivation ran before the lock and the player may have traded in between.
+   */
+  private async consumeQuestItems(
+    manager: EntityManager,
+    characterId: string,
+    state: QuestState,
+  ): Promise<QuestConsumedItemDto[]> {
+    const consumed: QuestConsumedItemDto[] = [];
+
+    for (const entry of state.objectives) {
+      const step = entry.objective;
+      if (
+        step.type !== QuestObjectiveType.COLLECT_ITEM ||
+        !step.consumeOnComplete
+      ) {
+        continue;
+      }
+
+      const definition = await manager
+        .getRepository(ItemDefinition)
+        .findOneBy({ key: step.targetKey });
+      if (!definition) {
+        throw questObjectiveIncomplete();
+      }
+
+      const characterItems = manager.getRepository(CharacterItem);
+      const owned = await characterItems.findOne({
+        where: { characterId, itemDefinitionId: definition.id },
+        lock: { mode: 'pessimistic_write' },
+      });
+      if (!owned || owned.quantity < step.requiredQuantity) {
+        throw questObjectiveIncomplete();
+      }
+
+      if (owned.quantity === step.requiredQuantity) {
+        await characterItems.remove(owned);
+      } else {
+        owned.quantity -= step.requiredQuantity;
+        await characterItems.save(owned);
+      }
+
+      consumed.push({
+        itemKey: step.targetKey,
+        quantity: step.requiredQuantity,
+      });
+    }
+
+    return consumed;
+  }
+
+  /**
+   * Pays the quest out, once (spec §9).
+   *
+   * Reputation and Silver only. There is no XP anywhere in the project any
+   * more (Slice 0.7 V2 §7), and no renown milestone on this quest by decision
+   * D2 -- one would reach World Renown 3 and open a Slice 0.8.5 offer that is
+   * meant to stay out of reach until 0.11. The seeded values pay 10 reputation
+   * and no Silver (D3); both branches exist so retuning stays a content change.
+   */
+  private async grantRewards(
+    manager: EntityManager,
+    characterId: string,
+    quest: QuestDefinition,
+  ): Promise<QuestRewardDto> {
+    if (quest.rewardFactionKey && quest.rewardReputation > 0) {
+      await this.reputation.grantReputation(
+        characterId,
+        quest.rewardFactionKey,
+        quest.rewardReputation,
+        manager,
+      );
+    }
+
+    if (quest.rewardSilver > 0) {
+      const characters = manager.getRepository(Character);
+      const character = await characters.findOne({
+        where: { id: characterId },
+        lock: { mode: 'pessimistic_write' },
+      });
+      if (character) {
+        character.silver += quest.rewardSilver;
+        await characters.save(character);
+      }
+    }
+
+    return {
+      factionKey: quest.rewardFactionKey,
+      reputation: quest.rewardReputation,
+      silver: quest.rewardSilver,
+    };
+  }
+}
+
+/**
+ * The quest as the player reads it (spec §12).
+ *
+ * `completed` is positional rather than stored: everything behind the derived
+ * step is done, and everything from it onward is not. A finished quest reads as
+ * done throughout, which is what keeps the journal honest after the goods were
+ * consumed.
+ */
+export function toQuestDto(state: QuestState): QuestDto {
+  const currentIndex = state.currentIndex;
+  const currentEntry =
+    currentIndex === null ? undefined : state.objectives[currentIndex];
+
+  return {
+    key: state.quest.key,
+    title: state.quest.title,
+    description: state.quest.description,
+    status: state.status,
+    objectives: state.objectives.map((entry, index) => ({
+      key: entry.objective.key,
+      description: entry.objective.description,
+      type: entry.objective.type,
+      targetKey: entry.objective.targetKey,
+      required: entry.required,
+      current: entry.current,
+      completed:
+        state.status === 'COMPLETED' ||
+        (currentIndex !== null && index < currentIndex),
+    })),
+    currentObjectiveKey: currentEntry?.objective.key ?? null,
+    // Only while the step genuinely cannot progress. A step that is merely
+    // unfinished needs no explanation, and saying one anyway would train the
+    // player to ignore the line that matters.
+    hint:
+      currentEntry && currentEntry.blocked && !currentEntry.satisfied
+        ? currentEntry.objective.hintText
+        : null,
+  };
+}
+
+function isUniqueViolation(error: unknown): boolean {
+  const code = (
+    error as { driverError?: { code?: string }; code?: string } | null
+  )?.driverError?.code;
+  return (
+    code === UNIQUE_VIOLATION ||
+    (error as { code?: string } | null)?.code === UNIQUE_VIOLATION
+  );
+}