diff --git a/src/engine/__tests__/determinism.test.ts b/src/engine/__tests__/determinism.test.ts
index fd0cd1c..6e6ef1f 100644
--- a/src/engine/__tests__/determinism.test.ts
+++ b/src/engine/__tests__/determinism.test.ts
@@ -1,6 +1,6 @@
 import { describe, expect, it } from 'vitest';
 import { buildContent } from '../../content/schema';
-import { createGameState, startAction, tickGame } from '../game';
+import { createGameState, enqueueAction, tickGame } from '../game';
 import { advance, createTickLoop, TICK_MS } from '../tickLoop';
 
 function testContent() {
@@ -20,7 +20,7 @@ function testContent() {
 function simulate(elapsedMs: number, chunkMs: number) {
   const content = testContent();
   const state = createGameState(content);
-  startAction(state, content, 'forage');
+  enqueueAction(state, content, 'forage');
 
   const loop = createTickLoop({ tickMs: TICK_MS, startNow: 0 });
   let now = 0;
diff --git a/src/engine/__tests__/game.test.ts b/src/engine/__tests__/game.test.ts
index 1ac0921..57225f1 100644
--- a/src/engine/__tests__/game.test.ts
+++ b/src/engine/__tests__/game.test.ts
@@ -29,6 +29,37 @@ function queueContent() {
   });
 }
 
+function costContent() {
+  return buildContent({
+    resources: [
+      { id: 'supplies', name: 'Supplies', startAmount: 10 },
+      { id: 'coin', name: 'Coin', startAmount: 0 },
+    ],
+    actions: [
+      {
+        id: 'gather',
+        name: 'Gather',
+        durationMs: 300,
+        yields: [{ resourceId: 'supplies', amount: 2 }],
+      },
+      {
+        id: 'trade',
+        name: 'Trade',
+        durationMs: 300,
+        costs: [{ resourceId: 'supplies', amount: 5 }],
+        yields: [{ resourceId: 'coin', amount: 3 }],
+      },
+      {
+        id: 'scout',
+        name: 'Scout',
+        durationMs: 300,
+        yields: [{ resourceId: 'coin', amount: 1 }],
+        unlock: { minResources: { coin: 1 } },
+      },
+    ],
+  });
+}
+
 describe('createGameState()', () => {
   it('seeds resource amounts from their start amounts with no active action', () => {
     const state = createGameState(testContent());
@@ -119,6 +150,85 @@ describe('clearQueue()', () => {
   });
 });
 
+describe('costs on start (D-0012)', () => {
+  it('deducts costs when an action becomes active', () => {
+    const content = costContent();
+    const state = createGameState(content);
+    enqueueAction(state, content, 'trade');
+    expect(state.resources.supplies).toBe(5);
+  });
+
+  it('rejects enqueue when unaffordable', () => {
+    const content = costContent();
+    const state = createGameState(content);
+    state.resources.supplies = 2;
+    expect(() => enqueueAction(state, content, 'trade')).toThrow(/cannot enqueue/i);
+  });
+});
+
+describe('unlock conditions', () => {
+  it('rejects locked actions', () => {
+    const content = costContent();
+    const state = createGameState(content);
+    expect(() => enqueueAction(state, content, 'scout')).toThrow(/cannot enqueue/i);
+  });
+
+  it('allows actions once unlock thresholds are met', () => {
+    const content = costContent();
+    const state = createGameState(content);
+    state.resources.coin = 1;
+    enqueueAction(state, content, 'scout');
+    expect(state.activeActionId).toBe('scout');
+  });
+});
+
+describe('completion advances queue', () => {
+  it('starts the next queued action after the active one completes', () => {
+    const content = costContent();
+    const state = createGameState(content);
+    enqueueAction(state, content, 'gather');
+    enqueueAction(state, content, 'gather');
+    tickGame(state, content, 300);
+    expect(state.activeActionId).toBe('gather');
+    expect(state.actionQueue).toEqual([]);
+    expect(state.resources.supplies).toBe(12);
+  });
+
+  it('goes idle when the queue is empty after completion', () => {
+    const content = costContent();
+    const state = createGameState(content);
+    enqueueAction(state, content, 'gather');
+    tickGame(state, content, 300);
+    expect(state.activeActionId).toBeNull();
+    expect(state.actionElapsedMs).toBe(0);
+  });
+
+  it('grants all yields on completion', () => {
+    const content = buildContent({
+      resources: [
+        { id: 'a', name: 'A', startAmount: 0 },
+        { id: 'b', name: 'B', startAmount: 0 },
+      ],
+      actions: [
+        {
+          id: 'combo',
+          name: 'Combo',
+          durationMs: 100,
+          yields: [
+            { resourceId: 'a', amount: 2 },
+            { resourceId: 'b', amount: 3 },
+          ],
+        },
+      ],
+    });
+    const state = createGameState(content);
+    enqueueAction(state, content, 'combo');
+    tickGame(state, content, 100);
+    expect(state.resources.a).toBe(2);
+    expect(state.resources.b).toBe(3);
+  });
+});
+
 describe('tickGame()', () => {
   it('does nothing when no action is active', () => {
     const content = testContent();
@@ -136,7 +246,7 @@ describe('tickGame()', () => {
     expect(state.actionElapsedMs).toBe(100);
   });
 
-  it('grants the yield on completion and repeats, carrying the remainder', () => {
+  it('grants the yield on completion then goes idle', () => {
     const content = testContent();
     const state = createGameState(content);
     startAction(state, content, 'forage');
@@ -144,15 +254,16 @@ describe('tickGame()', () => {
     tickGame(state, content, 100);
     tickGame(state, content, 100); // 300ms -> one completion, +2 gold
     expect(state.resources.gold).toBe(7);
+    expect(state.activeActionId).toBeNull();
     expect(state.actionElapsedMs).toBe(0);
   });
 
-  it('handles multiple completions within a single large tick (offline catch-up)', () => {
+  it('handles a single completion within a large tick (offline catch-up)', () => {
     const content = testContent();
     const state = createGameState(content);
-    startAction(state, content, 'forage');
-    tickGame(state, content, 1000); // 3 completions (900ms) + 100ms remainder
-    expect(state.resources.gold).toBe(11); // 5 + 3*2
-    expect(state.actionElapsedMs).toBe(100);
+    enqueueAction(state, content, 'forage');
+    tickGame(state, content, 1000); // one completion only unless re-enqueued
+    expect(state.resources.gold).toBe(7); // 5 + 2
+    expect(state.activeActionId).toBeNull();
   });
 });
diff --git a/src/engine/__tests__/save.test.ts b/src/engine/__tests__/save.test.ts
index bcad6dc..e806006 100644
--- a/src/engine/__tests__/save.test.ts
+++ b/src/engine/__tests__/save.test.ts
@@ -86,7 +86,8 @@ describe('applyOfflineProgress()', () => {
     startAction(state, content, 'forage'); // 3000ms per gold
     const credited = applyOfflineProgress(state, content, 1000, 1000 + 9000); // 9s
     expect(credited).toBe(9000);
-    expect(state.resources.gold).toBe(3); // 9000 / 3000
+    expect(state.resources.gold).toBe(1); // one completion then idle
+    expect(state.activeActionId).toBeNull();
   });
 
   it('credits nothing when the clock did not advance', () => {
@@ -103,6 +104,7 @@ describe('applyOfflineProgress()', () => {
     startAction(state, content, 'forage');
     const credited = applyOfflineProgress(state, content, 0, 10_000_000, 6000);
     expect(credited).toBe(6000);
-    expect(state.resources.gold).toBe(2); // 6000 / 3000
+    expect(state.resources.gold).toBe(1); // one completion then idle
+    expect(state.activeActionId).toBeNull();
   });
 });
diff --git a/src/engine/game.ts b/src/engine/game.ts
index 9797c5f..b369c34 100644
--- a/src/engine/game.ts
+++ b/src/engine/game.ts
@@ -26,33 +26,121 @@ export function createGameState(content: Content): GameState {
   return { resources, activeActionId: null, actionElapsedMs: 0, actionQueue: [] };
 }
 
+function assertKnownAction(content: Content, actionId: string): void {
+  if (!content.actionsById[actionId]) {
+    throw new Error(`Unknown action "${actionId}"`);
+  }
+}
+
+export function canAffordAction(state: GameState, content: Content, actionId: string): boolean {
+  const action = content.actionsById[actionId];
+  if (!action) return false;
+  return action.costs.every((cost) => (state.resources[cost.resourceId] ?? 0) >= cost.amount);
+}
+
+/** Story flags land in PR2; placeholder field keeps unlock schema honest. */
+export function canUnlockAction(
+  state: GameState,
+  content: Content,
+  actionId: string,
+  storyFlags: Record<string, boolean> = {},
+): boolean {
+  const action = content.actionsById[actionId];
+  if (!action) return false;
+  const unlock = action.unlock;
+  if (!unlock) return true;
+  if (unlock.minResources) {
+    for (const [resourceId, min] of Object.entries(unlock.minResources)) {
+      if ((state.resources[resourceId] ?? 0) < min) return false;
+    }
+  }
+  if (unlock.requireStoryFlags) {
+    for (const flag of unlock.requireStoryFlags) {
+      if (!storyFlags[flag]) return false;
+    }
+  }
+  return true;
+}
+
+export function isActionAvailable(
+  state: GameState,
+  content: Content,
+  actionId: string,
+  storyFlags: Record<string, boolean> = {},
+): boolean {
+  return (
+    canAffordAction(state, content, actionId) &&
+    canUnlockAction(state, content, actionId, storyFlags)
+  );
+}
+
+function deductCosts(state: GameState, content: Content, actionId: string): void {
+  const action = content.actionsById[actionId];
+  if (!action) return;
+  for (const cost of action.costs) {
+    state.resources[cost.resourceId] -= cost.amount;
+  }
+}
+
+function grantYields(state: GameState, content: Content, actionId: string): void {
+  const action = content.actionsById[actionId];
+  if (!action) return;
+  for (const y of action.yields) {
+    state.resources[y.resourceId] = (state.resources[y.resourceId] ?? 0) + y.amount;
+  }
+}
+
+function beginAction(state: GameState, content: Content, actionId: string): void {
+  assertKnownAction(content, actionId);
+  deductCosts(state, content, actionId);
+  state.activeActionId = actionId;
+  state.actionElapsedMs = 0;
+}
+
+function startNextFromQueue(state: GameState, content: Content): void {
+  while (state.actionQueue.length > 0) {
+    const nextId = state.actionQueue.shift()!;
+    if (isActionAvailable(state, content, nextId)) {
+      beginAction(state, content, nextId);
+      return;
+    }
+  }
+  state.activeActionId = null;
+  state.actionElapsedMs = 0;
+}
+
+function completeActiveAction(state: GameState, content: Content): void {
+  const actionId = state.activeActionId;
+  if (!actionId) return;
+  grantYields(state, content, actionId);
+  startNextFromQueue(state, content);
+}
+
 /**
  * Begin running an action, resetting its progress. Throws on an unknown id.
  *
  * @deprecated Prefer `enqueueAction` — it starts immediately when idle and queues otherwise.
  */
 export function startAction(state: GameState, content: Content, actionId: string): void {
-  if (!content.actionsById[actionId]) {
-    throw new Error(`Unknown action "${actionId}"`);
-  }
+  assertKnownAction(content, actionId);
   state.activeActionId = actionId;
   state.actionElapsedMs = 0;
 }
 
 /**
  * Start an action immediately when idle, otherwise append it to the queue.
- * Throws on an unknown id. Cost/unlock checks are not applied yet.
+ * Costs deduct on start (D-0012). Throws when unknown, unaffordable, or locked.
  */
 export function enqueueAction(state: GameState, content: Content, actionId: string): void {
-  if (!content.actionsById[actionId]) {
-    throw new Error(`Unknown action "${actionId}"`);
+  assertKnownAction(content, actionId);
+  if (!isActionAvailable(state, content, actionId)) {
+    throw new Error(`Cannot enqueue action "${actionId}"`);
   }
   if (state.activeActionId === null) {
-    state.activeActionId = actionId;
-    state.actionElapsedMs = 0;
-    return;
+    beginAction(state, content, actionId);
+  } else {
+    state.actionQueue.push(actionId);
   }
-  state.actionQueue.push(actionId);
 }
 
 /** Remove a queued action by index. Throws RangeError when out of range. */
@@ -69,24 +157,19 @@ export function clearQueue(state: GameState): void {
 }
 
 /**
- * Advance the active action by `tickMs`. Each time it reaches its duration it
- * grants its yield and repeats, carrying the remainder — so one large tick (the
- * offline catch-up path) can complete an action many times.
+ * Advance the active action by `tickMs`. On completion, grants yields and
+ * advances the queue — actions do not auto-repeat when the queue is empty.
  */
 export function tickGame(state: GameState, content: Content, tickMs: number): void {
-  if (!state.activeActionId) {
-    return;
-  }
-  const action = content.actionsById[state.activeActionId];
-  if (!action) {
-    return;
-  }
+  if (!state.activeActionId) return;
 
   state.actionElapsedMs += tickMs;
-  while (state.actionElapsedMs >= action.durationMs) {
+  while (state.activeActionId) {
+    const action = content.actionsById[state.activeActionId];
+    if (!action) return;
+    if (state.actionElapsedMs < action.durationMs) return;
+
     state.actionElapsedMs -= action.durationMs;
-    for (const y of action.yields) {
-      state.resources[y.resourceId] += y.amount;
-    }
+    completeActiveAction(state, content);
   }
 }
diff --git a/src/state/__tests__/persistence.test.ts b/src/state/__tests__/persistence.test.ts
index bdb51e3..dc8a39d 100644
--- a/src/state/__tests__/persistence.test.ts
+++ b/src/state/__tests__/persistence.test.ts
@@ -31,8 +31,8 @@ describe('loadGame()', () => {
 
     const result = await loadGame(content, backend, 1000 + 9000); // 9s offline
     expect(result.offlineMs).toBe(9000);
-    expect(result.state.resources.gold).toBe(13); // 10 + 9000/3000
-    expect(result.state.activeActionId).toBe('forage');
+    expect(result.state.resources.gold).toBe(11); // 10 + one completion
+    expect(result.state.activeActionId).toBeNull();
   });
 
   it('falls back to a fresh game on a corrupt save instead of throwing', async () => {