docs/architecture.md

@@ -8,7 +8,8 @@ Idlegame is split into four layers.
 
 - `tickLoop.ts`: fixed-timestep accumulator. Large elapsed deltas, including
   offline catch-up, drain through the same tick path as active play.
-- `game.ts`: core game state, active action progress, and resource accrual.
+- `game.ts`: core game state, action queue, costs/unlocks on start, completion-driven
+  queue advancement (actions do not auto-repeat when the queue is empty).
 - `save.ts`: versioned save schema, serialized export/import strings, and
   offline elapsed calculation.
 - `num.ts`: branded numeric boundary and human-readable formatting.

src/content/__tests__/definitions.test.ts

@@ -0,0 +1,26 @@
+import { describe, expect, it } from 'vitest';
+import { createGameState, enqueueAction, tickGame } from '../../engine/game';
+import { content } from '../index';
+
+describe('M1 stub content pack', () => {
+  it('defines two resources and four to five actions with costs and unlocks', () => {
+    expect(content.resources).toHaveLength(2);
+    expect(content.actions.length).toBeGreaterThanOrEqual(4);
+    expect(content.actions.length).toBeLessThanOrEqual(5);
+    const withCosts = content.actions.filter((a) => a.costs.length > 0);
+    const withUnlocks = content.actions.filter((a) => a.unlock !== undefined);
+    expect(withCosts.length).toBeGreaterThanOrEqual(2);
+    expect(withUnlocks.length).toBeGreaterThanOrEqual(1);
+  });
+
+  it('can simulate a costed action without throwing', () => {
+    const state = createGameState(content);
+    const trade = content.actions.find((a) => a.costs.length > 0);
+    if (!trade) {
+      throw new Error('expected at least one costed action');
+    }
+    enqueueAction(state, content, trade.id);
+    tickGame(state, content, trade.durationMs);
+    expect(state.activeActionId).toBeNull();
+  });
+});

src/content/__tests__/schema.test.ts

@@ -3,7 +3,12 @@ import { buildContent } from '../schema';
 
 const validResources = [{ id: 'gold', name: 'Gold' }];
 const validActions = [
-  { id: 'forage', name: 'Forage', durationMs: 3000, yields: { resourceId: 'gold', amount: 1 } },
+  {
+    id: 'forage',
+    name: 'Forage',
+    durationMs: 3000,
+    yields: [{ resourceId: 'gold', amount: 1 }],
+  },
 ];
 
 describe('buildContent()', () => {
@@ -24,7 +29,7 @@ describe('buildContent()', () => {
         id: 'forage',
         name: 'Forage',
         durationMs: 3000,
-        yields: { resourceId: 'ghost', amount: 1 },
+        yields: [{ resourceId: 'ghost', amount: 1 }],
       },
     ];
     expect(() => buildContent({ resources: validResources, actions })).toThrow(/unknown resource/i);
@@ -40,8 +45,82 @@ describe('buildContent()', () => {
 
   it('rejects a structurally invalid definition', () => {
     const actions = [
-      { id: 'forage', name: 'Forage', durationMs: -1, yields: { resourceId: 'gold', amount: 1 } },
+      {
+        id: 'forage',
+        name: 'Forage',
+        durationMs: -1,
+        yields: [{ resourceId: 'gold', amount: 1 }],
+      },
     ];
     expect(() => buildContent({ resources: validResources, actions })).toThrow();
   });
 });
+
+describe('costs and multi-yield', () => {
+  it('accepts optional costs and multiple yields', () => {
+    const resources = [
+      { id: 'gold', name: 'Gold' },
+      { id: 'wood', name: 'Wood' },
+    ];
+    const actions = [
+      {
+        id: 'craft',
+        name: 'Craft',
+        durationMs: 1000,
+        costs: [{ resourceId: 'wood', amount: 2 }],
+        yields: [
+          { resourceId: 'gold', amount: 1 },
+          { resourceId: 'wood', amount: 1 },
+        ],
+      },
+    ];
+    const content = buildContent({ resources, actions });
+    expect(content.actionsById.craft.costs).toEqual([{ resourceId: 'wood', amount: 2 }]);
+    expect(content.actionsById.craft.yields).toHaveLength(2);
+  });
+
+  it('rejects cost referencing unknown resource', () => {
+    const actions = [
+      {
+        id: 'craft',
+        name: 'Craft',
+        durationMs: 1000,
+        costs: [{ resourceId: 'ghost', amount: 1 }],
+        yields: [{ resourceId: 'gold', amount: 1 }],
+      },
+    ];
+    expect(() => buildContent({ resources: validResources, actions })).toThrow(/unknown resource/i);
+  });
+
+  it('defaults costs to empty array', () => {
+    const content = buildContent({ resources: validResources, actions: validActions });
+    expect(content.actionsById.forage.costs).toEqual([]);
+  });
+});
+
+describe('unlock conditions', () => {
+  it('accepts optional minResources and requireStoryFlags', () => {
+    const actions = [
+      {
+        id: 'forage',
+        name: 'Forage',
+        durationMs: 3000,
+        yields: [{ resourceId: 'gold', amount: 1 }],
+        unlock: {
+          minResources: { gold: 10 },
+          requireStoryFlags: ['intro_complete'],
+        },
+      },
+    ];
+    const content = buildContent({ resources: validResources, actions });
+    expect(content.actionsById.forage.unlock).toEqual({
+      minResources: { gold: 10 },
+      requireStoryFlags: ['intro_complete'],
+    });
+  });
+
+  it('defaults unlock to undefined when omitted', () => {
+    const content = buildContent({ resources: validResources, actions: validActions });
+    expect(content.actionsById.forage.unlock).toBeUndefined();
+  });
+});

src/content/definitions.ts

@@ -1,17 +1,45 @@
-/**
- * Walking-skeleton content: one resource, one timed action.
- *
- * M1 expands this into the real resource/action set and the story graph. Kept as
- * plain data so it stays diffable and authorable without touching engine code.
- */
-
-export const resourceDefs = [{ id: 'gold', name: 'Gold', startAmount: 0 }];
+export const resourceDefs = [
+  { id: 'supplies', name: 'Supplies', startAmount: 10 },
+  { id: 'coin', name: 'Coin', startAmount: 0 },
+];
 
 export const actionDefs = [
   {
-    id: 'forage',
-    name: 'Forage for coin',
+    id: 'gather_supplies',
+    name: 'Gather supplies',
     durationMs: 3000,
-    yields: { resourceId: 'gold', amount: 1 },
+    yields: [{ resourceId: 'supplies', amount: 2 }],
+  },
+  {
+    id: 'scout_path',
+    name: 'Scout the path',
+    durationMs: 5000,
+    costs: [{ resourceId: 'supplies', amount: 2 }],
+    yields: [{ resourceId: 'coin', amount: 1 }],
+  },
+  {
+    id: 'trade_supplies',
+    name: 'Trade at camp',
+    durationMs: 4000,
+    costs: [{ resourceId: 'supplies', amount: 3 }],
+    yields: [{ resourceId: 'coin', amount: 2 }],
+    unlock: { minResources: { coin: 1 } },
+  },
+  {
+    id: 'fortify_camp',
+    name: 'Fortify camp',
+    durationMs: 8000,
+    costs: [
+      { resourceId: 'supplies', amount: 5 },
+      { resourceId: 'coin', amount: 2 },
+    ],
+    yields: [{ resourceId: 'supplies', amount: 4 }],
+    unlock: { minResources: { supplies: 8 } },
+  },
+  {
+    id: 'rest',
+    name: 'Rest briefly',
+    durationMs: 2000,
+    yields: [{ resourceId: 'supplies', amount: 1 }],
   },
 ];

src/content/schema.ts

@@ -15,17 +15,28 @@ export const resourceDefSchema = z.object({
   startAmount: z.number().nonnegative().default(0),
 });
 
+export const resourceAmountSchema = z.object({
+  resourceId: z.string().min(1),
+  amount: z.number().positive(),
+});
+
+export const unlockDefSchema = z.object({
+  minResources: z.record(z.string(), z.number().nonnegative()).optional(),
+  requireStoryFlags: z.array(z.string().min(1)).optional(),
+});
+
 export const actionDefSchema = z.object({
   id: z.string().min(1),
   name: z.string().min(1),
   durationMs: z.number().positive(),
-  yields: z.object({
-    resourceId: z.string().min(1),
-    amount: z.number().positive(),
-  }),
+  costs: z.array(resourceAmountSchema).default([]),
+  yields: z.array(resourceAmountSchema).min(1),
+  unlock: unlockDefSchema.optional(),
 });
 
 export type ResourceDef = z.infer<typeof resourceDefSchema>;
+export type ResourceAmount = z.infer<typeof resourceAmountSchema>;
+export type UnlockDef = z.infer<typeof unlockDefSchema>;
 export type ActionDef = z.infer<typeof actionDefSchema>;
 
 export interface Content {
@@ -55,10 +66,15 @@ export function buildContent(input: { resources: unknown[]; actions: unknown[] }
   const actionsById = indexById(actions, 'action');
 
   for (const action of actions) {
-    if (!resourcesById[action.yields.resourceId]) {
-      throw new Error(
-        `Action "${action.id}" yields unknown resource "${action.yields.resourceId}"`,
-      );
+    for (const y of action.yields) {
+      if (!resourcesById[y.resourceId]) {
+        throw new Error(`Action "${action.id}" yields unknown resource "${y.resourceId}"`);
+      }
+    }
+    for (const c of action.costs) {
+      if (!resourcesById[c.resourceId]) {
+        throw new Error(`Action "${action.id}" cost references unknown resource "${c.resourceId}"`);
+      }
     }
   }
 

src/engine/__tests__/determinism.test.ts

@@ -0,0 +1,44 @@
+import { describe, expect, it } from 'vitest';
+import { buildContent } from '../../content/schema';
+import { createGameState, enqueueAction, tickGame } from '../game';
+import { advance, createTickLoop, TICK_MS } from '../tickLoop';
+
+function testContent() {
+  return buildContent({
+    resources: [{ id: 'gold', name: 'Gold', startAmount: 0 }],
+    actions: [
+      {
+        id: 'forage',
+        name: 'Forage',
+        durationMs: 300,
+        yields: [{ resourceId: 'gold', amount: 1 }],
+      },
+    ],
+  });
+}
+
+function simulate(elapsedMs: number, chunkMs: number) {
+  const content = testContent();
+  const state = createGameState(content);
+  enqueueAction(state, content, 'forage');
+
+  const loop = createTickLoop({ tickMs: TICK_MS, startNow: 0 });
+  let now = 0;
+  while (now < elapsedMs) {
+    const next = Math.min(now + chunkMs, elapsedMs);
+    advance(loop, next, () => {
+      tickGame(state, content, TICK_MS);
+    });
+    now = next;
+  }
+  return { tickCount: loop.tickCount, gold: state.resources.gold };
+}
+
+describe('determinism integration', () => {
+  it('produces identical tick counts and game state regardless of advance chunking', () => {
+    const whole = simulate(5000, 5000);
+    const chunked = simulate(5000, 37);
+    expect(chunked.tickCount).toBe(whole.tickCount);
+    expect(chunked.gold).toBe(whole.gold);
+  });
+});

src/engine/__tests__/game.test.ts

@@ -1,12 +1,67 @@
 import { describe, expect, it } from 'vitest';
 import { buildContent } from '../../content/schema';
-import { createGameState, startAction, tickGame } from '../game';
+import {
+  cancelQueuedAction,
+  canUnlockAction,
+  clearQueue,
+  createGameState,
+  enqueueAction,
+  startAction,
+  tickGame,
+} from '../game';
 
 function testContent() {
   return buildContent({
     resources: [{ id: 'gold', name: 'Gold', startAmount: 5 }],
     actions: [
-      { id: 'forage', name: 'Forage', durationMs: 300, yields: { resourceId: 'gold', amount: 2 } },
+      {
+        id: 'forage',
+        name: 'Forage',
+        durationMs: 300,
+        yields: [{ resourceId: 'gold', amount: 2 }],
+      },
+    ],
+  });
+}
+
+function queueContent() {
+  return buildContent({
+    resources: [{ id: 'gold', name: 'Gold', startAmount: 0 }],
+    actions: [
+      { id: 'a', name: 'A', durationMs: 1000, yields: [{ resourceId: 'gold', amount: 1 }] },
+      { id: 'b', name: 'B', durationMs: 1000, yields: [{ resourceId: 'gold', amount: 1 }] },
+      { id: 'c', name: 'C', durationMs: 1000, yields: [{ resourceId: 'gold', amount: 1 }] },
+    ],
+  });
+}
+
+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 } },
+      },
     ],
   });
 }
@@ -17,6 +72,7 @@ describe('createGameState()', () => {
     expect(state.resources.gold).toBe(5);
     expect(state.activeActionId).toBeNull();
     expect(state.actionElapsedMs).toBe(0);
+    expect(state.actionQueue).toEqual([]);
   });
 });
 
@@ -37,6 +93,201 @@ describe('startAction()', () => {
   });
 });
 
+describe('enqueueAction()', () => {
+  it('starts immediately when idle', () => {
+    const content = queueContent();
+    const state = createGameState(content);
+    enqueueAction(state, content, 'a');
+    expect(state.activeActionId).toBe('a');
+    expect(state.actionElapsedMs).toBe(0);
+    expect(state.actionQueue).toEqual([]);
+  });
+
+  it('queues when another action is active', () => {
+    const content = queueContent();
+    const state = createGameState(content);
+    enqueueAction(state, content, 'a');
+    enqueueAction(state, content, 'b');
+    enqueueAction(state, content, 'c');
+    expect(state.activeActionId).toBe('a');
+    expect(state.actionQueue).toEqual(['b', 'c']);
+  });
+
+  it('throws on an unknown action id', () => {
+    const content = queueContent();
+    const state = createGameState(content);
+    expect(() => enqueueAction(state, content, 'nope')).toThrow(/unknown action/i);
+  });
+});
+
+describe('cancelQueuedAction()', () => {
+  it('removes a queued action by index', () => {
+    const content = queueContent();
+    const state = createGameState(content);
+    enqueueAction(state, content, 'a');
+    enqueueAction(state, content, 'b');
+    enqueueAction(state, content, 'c');
+    cancelQueuedAction(state, 0);
+    expect(state.activeActionId).toBe('a');
+    expect(state.actionQueue).toEqual(['c']);
+  });
+
+  it('throws RangeError when index is out of range', () => {
+    const content = queueContent();
+    const state = createGameState(content);
+    enqueueAction(state, content, 'a');
+    enqueueAction(state, content, 'b');
+    expect(() => cancelQueuedAction(state, 5)).toThrow(RangeError);
+  });
+});
+
+describe('clearQueue()', () => {
+  it('empties the queue without stopping the active action', () => {
+    const content = queueContent();
+    const state = createGameState(content);
+    enqueueAction(state, content, 'a');
+    enqueueAction(state, content, 'b');
+    enqueueAction(state, content, 'c');
+    state.actionElapsedMs = 250;
+    clearQueue(state);
+    expect(state.activeActionId).toBe('a');
+    expect(state.actionElapsedMs).toBe(250);
+    expect(state.actionQueue).toEqual([]);
+  });
+});
+
+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');
+  });
+
+  it('rejects actions until required story flags are set', () => {
+    const content = buildContent({
+      resources: [{ id: 'gold', name: 'Gold', startAmount: 0 }],
+      actions: [
+        {
+          id: 'secret',
+          name: 'Secret',
+          durationMs: 100,
+          yields: [{ resourceId: 'gold', amount: 1 }],
+          unlock: { requireStoryFlags: ['path_scouted'] },
+        },
+      ],
+    });
+    const state = createGameState(content);
+    expect(() => enqueueAction(state, content, 'secret')).toThrow(/cannot enqueue/i);
+    expect(canUnlockAction(state, content, 'secret', { path_scouted: true })).toBe(true);
+  });
+});
+
+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('skips queued actions that are unaffordable when their turn arrives', () => {
+    const content = buildContent({
+      resources: [{ id: 'supplies', name: 'Supplies', startAmount: 0 }],
+      actions: [
+        {
+          id: 'cheap',
+          name: 'Cheap',
+          durationMs: 100,
+          yields: [{ resourceId: 'supplies', amount: 1 }],
+        },
+        {
+          id: 'dear',
+          name: 'Dear',
+          durationMs: 100,
+          costs: [{ resourceId: 'supplies', amount: 6 }],
+          yields: [{ resourceId: 'supplies', amount: 1 }],
+        },
+        {
+          id: 'free',
+          name: 'Free',
+          durationMs: 100,
+          yields: [{ resourceId: 'supplies', amount: 1 }],
+        },
+      ],
+    });
+    const state = createGameState(content);
+    state.activeActionId = 'cheap';
+    state.actionElapsedMs = 99;
+    state.actionQueue = ['dear', 'free'];
+    state.resources.supplies = 3;
+    tickGame(state, content, 1);
+    expect(state.activeActionId).toBe('free');
+    expect(state.actionQueue).toEqual([]);
+  });
+
+  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();
@@ -54,7 +305,7 @@ 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');
@@ -62,15 +313,16 @@ 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();
   });
 });

src/engine/__tests__/purity.test.ts

@@ -0,0 +1,26 @@
+import { readdir, readFile } from 'node:fs/promises';
+import { join } from 'node:path';
+import { describe, expect, it } from 'vitest';
+
+const ENGINE_DIR = join(import.meta.dirname, '..');
+const FORBIDDEN = [/from\s+['"]react/, /from\s+['"]react-dom/, /from\s+['"]zustand/];
+
+async function engineSourceFiles(): Promise<string[]> {
+  const entries = await readdir(ENGINE_DIR, { withFileTypes: true });
+  return entries
+    .filter((e) => e.isFile() && e.name.endsWith('.ts') && !e.name.endsWith('.test.ts'))
+    .map((e) => join(ENGINE_DIR, e.name));
+}
+
+describe('engine purity', () => {
+  it('does not import React, react-dom, or Zustand', async () => {
+    const files = await engineSourceFiles();
+    expect(files.length).toBeGreaterThan(0);
+    for (const file of files) {
+      const source = await readFile(file, 'utf8');
+      for (const pattern of FORBIDDEN) {
+        expect(source, `${file} must stay free of ${pattern}`).not.toMatch(pattern);
+      }
+    }
+  });
+});

src/engine/__tests__/save.test.ts

@@ -15,7 +15,12 @@ function testContent() {
   return buildContent({
     resources: [{ id: 'gold', name: 'Gold', startAmount: 0 }],
     actions: [
-      { id: 'forage', name: 'Forage', durationMs: 3000, yields: { resourceId: 'gold', amount: 1 } },
+      {
+        id: 'forage',
+        name: 'Forage',
+        durationMs: 3000,
+        yields: [{ resourceId: 'gold', amount: 1 }],
+      },
     ],
   });
 }
@@ -44,6 +49,15 @@ describe('createSave()', () => {
     state.resources.gold = 999;
     expect(save.state.resources.gold).toBe(12);
   });
+
+  it('snapshots actionQueue in save payload', () => {
+    const state = sampleState();
+    state.actionQueue = ['forage', 'forage'];
+    const save = createSave(state, 1700);
+    expect(save.state.actionQueue).toEqual(['forage', 'forage']);
+    state.actionQueue.push('forage');
+    expect(save.state.actionQueue).toEqual(['forage', 'forage']);
+  });
 });
 
 describe('serialize / deserialize round-trip', () => {
@@ -86,7 +100,8 @@ 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 +118,7 @@ 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();
   });
 });

src/engine/game.ts

@@ -14,6 +14,8 @@ export interface GameState {
   activeActionId: string | null;
   /** Progress of the active action, in milliseconds. */
   actionElapsedMs: number;
+  /** Action ids waiting to run after the active action finishes. */
+  actionQueue: string[];
 }
 
 export function createGameState(content: Content): GameState {
@@ -21,35 +23,156 @@ export function createGameState(content: Content): GameState {
   for (const resource of content.resources) {
     resources[resource.id] = resource.startAmount;
   }
-  return { resources, activeActionId: null, actionElapsedMs: 0 };
+  return { resources, activeActionId: null, actionElapsedMs: 0, actionQueue: [] };
 }
 
-/** Begin running an action, resetting its progress. Throws on an unknown id. */
-export function startAction(state: GameState, content: Content, actionId: string): void {
+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 (!nextId) {
+      break;
+    }
+    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 {
+  assertKnownAction(content, actionId);
   state.activeActionId = actionId;
   state.actionElapsedMs = 0;
 }
 
 /**
- * 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.
+ * Start an action immediately when idle, otherwise append it to the queue.
+ * Costs deduct on start (D-0012). Throws when unknown, unaffordable, or locked.
  */
-export function tickGame(state: GameState, content: Content, tickMs: number): void {
-  if (!state.activeActionId) {
-    return;
+export function enqueueAction(state: GameState, content: Content, actionId: string): void {
+  assertKnownAction(content, actionId);
+  if (!isActionAvailable(state, content, actionId)) {
+    throw new Error(`Cannot enqueue action "${actionId}"`);
   }
-  const action = content.actionsById[state.activeActionId];
-  if (!action) {
-    return;
+  if (state.activeActionId === null) {
+    beginAction(state, content, actionId);
+  } else {
+    state.actionQueue.push(actionId);
   }
+}
 
+/** Remove a queued action by index. Throws RangeError when out of range. */
+export function cancelQueuedAction(state: GameState, index: number): void {
+  if (index < 0 || index >= state.actionQueue.length) {
+    throw new RangeError(`Queue index ${index} is out of range`);
+  }
+  state.actionQueue.splice(index, 1);
+}
+
+/** Clear all queued actions without stopping the active action. */
+export function clearQueue(state: GameState): void {
+  state.actionQueue.length = 0;
+}
+
+/**
+ * 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;
+
   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;
-    state.resources[action.yields.resourceId] += action.yields.amount;
+    completeActiveAction(state, content);
   }
 }

src/engine/save.ts

@@ -13,6 +13,10 @@ import { TICK_MS } from './tickLoop';
  * into a validated, versioned, compressed string and back. Saves carry a
  * `version` so future migrations have a hook; for now a version mismatch is
  * rejected cleanly rather than silently coerced.
+ *
+ * Offline credit: `applyOfflineProgress` floors elapsed ms to whole ticks
+ * (same `TICK_MS` as the live loop) and runs `tickGame` that many times.
+ * Elapsed beyond `DEFAULT_MAX_OFFLINE_MS` is not credited.
  */
 
 export const SAVE_VERSION = 1;
@@ -24,6 +28,7 @@ export const gameStateSchema = z.object({
   resources: z.record(z.string(), z.number()),
   activeActionId: z.string().nullable(),
   actionElapsedMs: z.number().nonnegative(),
+  actionQueue: z.array(z.string()).default([]),
 });
 
 export const saveSchema = z.object({
@@ -43,6 +48,7 @@ export function createSave(state: GameState, now: number): SaveData {
       resources: { ...state.resources },
       activeActionId: state.activeActionId,
       actionElapsedMs: state.actionElapsedMs,
+      actionQueue: [...state.actionQueue],
     },
   };
 }

src/engine/tickLoop.ts

@@ -6,11 +6,13 @@
  * keeping the sub-tick remainder so results are independent of how the caller
  * chunks calls (determinism — see the test suite).
  *
- * Offline catch-up is the *same* code path: a large `now` delta simply owes many
- * ticks. `maxTicks` caps how many run per call so a huge delta can't lock the
- * thread; the remainder stays in the accumulator and drains on the next call,
- * so no ticks are ever lost. Clamping the *credited* offline window lives in the
- * save layer (T3.3), not here.
+ * Offline catch-up uses this same path: a large `now` delta owes many ticks.
+ * `maxTicks` caps work per `advance()` call; the accumulator retains the
+ * remainder so no ticks are lost across calls.
+ *
+ * The save layer (`save.ts`) separately clamps how much *wall-clock* elapsed
+ * time is credited on load via `DEFAULT_MAX_OFFLINE_MS`. Tick loop batching
+ * and offline credit clamping are independent concerns.
  */
 
 export const TICK_HZ = 10;

src/state/__tests__/persistence.test.ts

@@ -1,6 +1,6 @@
 import { describe, expect, it } from 'vitest';
 import { buildContent } from '../../content/schema';
-import { createGameState, startAction } from '../../engine/game';
+import { createGameState, enqueueAction, startAction } from '../../engine/game';
 import { createSave, serializeSave } from '../../engine/save';
 import { createMemoryBackend, loadGame, saveGame } from '../persistence';
 
@@ -8,7 +8,22 @@ function testContent() {
   return buildContent({
     resources: [{ id: 'gold', name: 'Gold', startAmount: 0 }],
     actions: [
-      { id: 'forage', name: 'Forage', durationMs: 3000, yields: { resourceId: 'gold', amount: 1 } },
+      {
+        id: 'forage',
+        name: 'Forage',
+        durationMs: 3000,
+        yields: [{ resourceId: 'gold', amount: 1 }],
+      },
+    ],
+  });
+}
+
+function queueTestContent() {
+  return buildContent({
+    resources: [{ id: 'gold', name: 'Gold', startAmount: 0 }],
+    actions: [
+      { id: 'a', name: 'A', durationMs: 3000, yields: [{ resourceId: 'gold', amount: 1 }] },
+      { id: 'b', name: 'B', durationMs: 3000, yields: [{ resourceId: 'gold', amount: 1 }] },
     ],
   });
 }
@@ -31,8 +46,8 @@ 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 () => {
@@ -42,11 +57,29 @@ describe('loadGame()', () => {
     expect(result.offlineMs).toBe(0);
   });
 
+  it('restores actionQueue on load', async () => {
+    const content = queueTestContent();
+    const state = createGameState(content);
+    enqueueAction(state, content, 'a');
+    enqueueAction(state, content, 'b');
+    const backend = createMemoryBackend();
+    await saveGame(state, backend, 1000);
+
+    const result = await loadGame(content, backend, 1000);
+    expect(result.state.activeActionId).toBe('a');
+    expect(result.state.actionQueue).toEqual(['b']);
+  });
+
   it('drops an active action that no longer exists in content', async () => {
     const content = testContent();
     const stale = serializeSave(
       createSave(
-        { resources: { gold: 1 }, activeActionId: 'ghost-action', actionElapsedMs: 0 },
+        {
+          resources: { gold: 1 },
+          activeActionId: 'ghost-action',
+          actionElapsedMs: 0,
+          actionQueue: [],
+        },
         1000,
       ),
     );

src/state/__tests__/viewModel.test.ts

@@ -1,13 +1,18 @@
 import { describe, expect, it } from 'vitest';
 import { buildContent } from '../../content/schema';
-import { createGameState, startAction } from '../../engine/game';
+import { createGameState, enqueueAction, startAction } from '../../engine/game';
 import { formatOfflineDuration, toView } from '../viewModel';
 
 function testContent() {
   return buildContent({
     resources: [{ id: 'gold', name: 'Gold', startAmount: 4 }],
     actions: [
-      { id: 'forage', name: 'Forage', durationMs: 200, yields: { resourceId: 'gold', amount: 1 } },
+      {
+        id: 'forage',
+        name: 'Forage',
+        durationMs: 200,
+        yields: [{ resourceId: 'gold', amount: 1 }],
+      },
     ],
   });
 }
@@ -44,6 +49,22 @@ describe('toView()', () => {
     state.actionElapsedMs = 999;
     expect(toView(state, content).actionProgress).toBe(1);
   });
+
+  it('includes queued action ids in order', () => {
+    const content = buildContent({
+      resources: [{ id: 'gold', name: 'Gold' }],
+      actions: [
+        { id: 'a', name: 'Alpha', durationMs: 1000, yields: [{ resourceId: 'gold', amount: 1 }] },
+        { id: 'b', name: 'Bravo', durationMs: 1000, yields: [{ resourceId: 'gold', amount: 1 }] },
+      ],
+    });
+    const state = createGameState(content);
+    enqueueAction(state, content, 'a');
+    enqueueAction(state, content, 'b');
+    const view = toView(state, content);
+    expect(view.queuedActionIds).toEqual(['b']);
+    expect(view.queuedActionNames).toEqual(['Bravo']);
+  });
 });
 
 describe('formatOfflineDuration()', () => {

src/state/persistence.ts

@@ -102,6 +102,7 @@ export async function loadGame(
       resources: { ...base.resources, ...save.state.resources },
       activeActionId,
       actionElapsedMs: save.state.actionElapsedMs,
+      actionQueue: [...(save.state.actionQueue ?? [])],
     };
     savedAt = save.savedAt;
   } catch {

src/state/runtime.ts

@@ -1,5 +1,5 @@
 import { content } from '../content';
-import { startAction as engineStartAction, type GameState, tickGame } from '../engine/game';
+import { enqueueAction as engineEnqueueAction, type GameState, tickGame } from '../engine/game';
 import { advance, createTickLoop, TICK_MS, type TickLoop } from '../engine/tickLoop';
 import { createDefaultBackend, loadGame, type SaveBackend, saveGame } from './persistence';
 import { useGameStore } from './store';
@@ -58,15 +58,21 @@ class GameRuntime {
     }
   }
 
-  startAction(actionId: string): void {
+  enqueueAction(actionId: string): void {
     const state = this.state;
     if (!state) {
       return;
     }
-    engineStartAction(state, content, actionId);
-    const action = content.actionsById[actionId];
-    if (action) {
-      useGameStore.getState().appendLog(`Started: ${action.name}.`);
+    try {
+      engineEnqueueAction(state, content, actionId);
+      const action = content.actionsById[actionId];
+      if (action) {
+        const verb = state.actionQueue.includes(actionId) ? 'Queued' : 'Started';
+        useGameStore.getState().appendLog(`${verb}: ${action.name}.`);
+      }
+    } catch (err) {
+      const msg = err instanceof Error ? err.message : 'Cannot start action';
+      useGameStore.getState().appendLog(msg);
     }
     this.publish();
   }

src/state/store.ts

@@ -20,6 +20,8 @@ export const useGameStore = create<GameStoreState>((set) => ({
   activeActionId: null,
   actionName: null,
   actionProgress: 0,
+  queuedActionIds: [],
+  queuedActionNames: [],
   log: [],
   setView: (view) => set(view),
   appendLog: (line) => set((state) => ({ log: [...state.log, line].slice(-MAX_LOG_LINES) })),

src/state/viewModel.ts

@@ -18,6 +18,8 @@ export interface GameView {
   actionName: string | null;
   /** Progress of the active action, clamped to 0..1. */
   actionProgress: number;
+  queuedActionIds: string[];
+  queuedActionNames: string[];
 }
 
 export function toView(state: GameState, content: Content): GameView {
@@ -29,12 +31,16 @@ export function toView(state: GameState, content: Content): GameView {
 
   const action = state.activeActionId ? content.actionsById[state.activeActionId] : undefined;
   const actionProgress = action ? Math.min(1, state.actionElapsedMs / action.durationMs) : 0;
+  const queuedActionIds = [...state.actionQueue];
+  const queuedActionNames = queuedActionIds.map((id) => content.actionsById[id]?.name ?? id);
 
   return {
     resources,
     activeActionId: state.activeActionId,
     actionName: action ? action.name : null,
     actionProgress,
+    queuedActionIds,
+    queuedActionNames,
   };
 }
 

src/ui/ActionPanel.tsx

@@ -16,7 +16,7 @@ export function ActionPanel() {
           <button
             type="button"
             key={action.id}
-            onClick={() => gameRuntime.startAction(action.id)}
+            onClick={() => gameRuntime.enqueueAction(action.id)}
             className="relative overflow-hidden rounded-lg border border-slate-700 bg-slate-800/70 px-4 py-3 text-left transition-colors hover:border-amber-500/60 hover:bg-slate-800"
           >
             {isActive ? (