diff --git a/index.html b/index.html
index d4bb033..a57a88e 100644
--- a/index.html
+++ b/index.html
@@ -83,6 +83,12 @@
       </div>
       <input id="pack-file" class="hidden" type="file" accept=".zip" />
       <input id="pack-folder" class="hidden" type="file" webkitdirectory />
+      <div class="pack-controls">
+        <button id="replay-save" class="ghost compact" type="button">Save Replay</button>
+        <button id="replay-load" class="ghost compact" type="button">Load Replay</button>
+        <div id="replay-status" class="pack-status">Replay: none</div>
+      </div>
+      <input id="replay-file" class="hidden" type="file" accept=".json" />
 
       <label id="control-mode-field" class="field hidden">
         <span>Control Mode</span>
diff --git a/src/game.ts b/src/game.ts
index bc3a9e9..43fbb57 100644
--- a/src/game.ts
+++ b/src/game.ts
@@ -17,6 +17,7 @@ import { intersectsMovingSpheres, tfPhysballToAnimGroupSpace } from './collision
 import { MatrixStack, sqrt, toS16 } from './math.js';
 import { GameplayCamera } from './camera.js';
 import { dequantizeStick, quantizeStick, type QuantizedStick } from './determinism.js';
+import { createReplayData, type ReplayData } from './replay.js';
 import {
   checkBallEnteredGoal,
   createBallState,
@@ -338,6 +339,9 @@ export class Game {
   public inputRecord: QuantizedStick[] | null;
   public fixedTickMode: boolean;
   public fixedTicksPerUpdate: number;
+  public autoRecordInputs: boolean;
+  public inputStartTick: number;
+  public replayInputStartTick: number | null;
 
   constructor({
     hud,
@@ -433,6 +437,9 @@ export class Game {
     this.inputRecord = null;
     this.fixedTickMode = false;
     this.fixedTicksPerUpdate = 1;
+    this.autoRecordInputs = true;
+    this.inputStartTick = 0;
+    this.replayInputStartTick = null;
   }
 
   setGameSource(source: GameSource) {
@@ -451,6 +458,19 @@ export class Game {
     this.fixedTicksPerUpdate = Math.max(1, ticksPerUpdate | 0);
   }
 
+  setReplayMode(enabled: boolean, useFixedTicks = true) {
+    this.autoRecordInputs = !enabled;
+    this.setFixedTickMode(enabled && useFixedTicks, 1);
+    if (enabled) {
+      this.inputRecord = null;
+      this.replayInputStartTick = 0;
+    } else {
+      this.inputFeed = null;
+      this.inputFeedIndex = 0;
+      this.replayInputStartTick = null;
+    }
+  }
+
   startInputRecording() {
     this.inputRecord = [];
   }
@@ -461,6 +481,21 @@ export class Game {
     return record;
   }
 
+  exportReplay(note?: string, hashes?: number[]): ReplayData | null {
+    if (!this.stage || !this.inputRecord) {
+      return null;
+    }
+    return createReplayData(
+      this.gameSource,
+      this.stage.stageId,
+      this.inputRecord.length,
+      this.inputStartTick,
+      this.inputRecord.slice(),
+      hashes,
+      note,
+    );
+  }
+
   init() {
     this.input = new Input();
     this.world = new World();
@@ -1228,6 +1263,7 @@ export class Game {
       this.stageRuntime = new StageRuntime(stage);
       this.simTick = 0;
       this.inputFeedIndex = 0;
+      this.inputStartTick = 0;
       this.animGroupTransforms = this.stageRuntime.animGroups.map((group) => group.transform);
       this.interpolatedAnimGroupTransforms = null;
       this.bananaGroups = new Array(this.stage.animGroupCount);
@@ -1267,6 +1303,9 @@ export class Game {
       this.hurryUpAnnouncerPlayed = false;
       this.timeOverAnnouncerPlayed = false;
       this.pendingAdvance = false;
+      if (this.autoRecordInputs) {
+        this.startInputRecording();
+      }
 
       this.resetBallForStage({ withIntro: true });
 
@@ -1753,20 +1792,32 @@ export class Game {
 
   private readDeterministicStick(inputEnabled: boolean) {
     let frame = null;
-    if (this.inputFeed && this.inputFeedIndex < this.inputFeed.length) {
-      frame = this.inputFeed[this.inputFeedIndex];
+    const canConsumeReplay = this.replayInputStartTick === null || this.simTick >= this.replayInputStartTick;
+    if (this.inputFeed && canConsumeReplay) {
+      frame = this.inputFeed[this.inputFeedIndex] ?? { x: 0, y: 0 };
       this.inputFeedIndex += 1;
     }
+
     if (!inputEnabled) {
       return { x: 0, y: 0 };
     }
+
+    if (this.inputRecord && this.autoRecordInputs) {
+      if (this.inputRecord.length === 0) {
+        this.inputStartTick = this.simTick;
+      }
+      if (!frame) {
+        const raw = this.input?.getStick?.() ?? { x: 0, y: 0 };
+        frame = quantizeStick(raw);
+      }
+      this.inputRecord.push({ x: frame.x, y: frame.y });
+    }
+
     if (!frame) {
       const raw = this.input?.getStick?.() ?? { x: 0, y: 0 };
       frame = quantizeStick(raw);
-      if (this.inputRecord) {
-        this.inputRecord.push({ x: frame.x, y: frame.y });
-      }
     }
+
     return dequantizeStick(frame);
   }
 
diff --git a/src/main.ts b/src/main.ts
index b67004e..2386252 100644
--- a/src/main.ts
+++ b/src/main.ts
@@ -33,6 +33,7 @@ import { StageId, STAGE_INFO_MAP } from './noclip/SuperMonkeyBall/StageInfo.js';
 import type { StageData } from './noclip/SuperMonkeyBall/World.js';
 import { convertSmb2StageDef, getMb2wsStageInfo, getSmb2StageInfo } from './smb2_render.js';
 import { HudRenderer } from './hud.js';
+import type { ReplayData } from './replay.js';
 import {
   fetchPackSlice,
   getActivePack,
@@ -213,6 +214,10 @@ const packLoadFolderButton = document.getElementById('pack-load-folder') as HTML
 const packStatus = document.getElementById('pack-status') as HTMLElement | null;
 const packFileInput = document.getElementById('pack-file') as HTMLInputElement | null;
 const packFolderInput = document.getElementById('pack-folder') as HTMLInputElement | null;
+const replaySaveButton = document.getElementById('replay-save') as HTMLButtonElement | null;
+const replayLoadButton = document.getElementById('replay-load') as HTMLButtonElement | null;
+const replayFileInput = document.getElementById('replay-file') as HTMLInputElement | null;
+const replayStatus = document.getElementById('replay-status') as HTMLElement | null;
 const smb1Fields = document.getElementById('smb1-fields') as HTMLElement;
 const smb2Fields = document.getElementById('smb2-fields') as HTMLElement;
 const smb2ModeSelect = document.getElementById('smb2-mode') as HTMLSelectElement;
@@ -868,6 +873,7 @@ async function startStage(
     hudStatus.textContent = '';
   }
 
+  game.setReplayMode(false);
   game.setGameSource(activeGameSource);
   game.stageBasePath = getStageBasePath(activeGameSource);
   currentSmb2LikeMode =
@@ -876,6 +882,55 @@ async function startStage(
   await game.start(difficulty);
 }
 
+function setReplayStatus(text: string) {
+  if (replayStatus) {
+    replayStatus.textContent = text;
+  }
+}
+
+async function startReplay(replay: ReplayData) {
+  setOverlayVisible(false);
+  resumeButton.disabled = true;
+  if (hudStatus) {
+    hudStatus.textContent = '';
+  }
+  game.setReplayMode(true, true);
+  game.setGameSource(replay.gameSource);
+  game.stageBasePath = getStageBasePath(replay.gameSource);
+  currentSmb2LikeMode = null;
+  game.course = null;
+  void audio.resume();
+  await game.loadStage(replay.stageId);
+  if (game.ball && game.stage) {
+    const startTick = Math.max(0, replay.inputStartTick ?? 0);
+    game.introTotalFrames = startTick;
+    game.introTimerFrames = startTick;
+    game.cameraController?.initForStage(game.ball, game.ball.startRotY, game.stageRuntime);
+  }
+  game.replayInputStartTick = Math.max(0, replay.inputStartTick ?? 0);
+  game.setInputFeed(replay.inputs);
+  game.paused = false;
+  while (game.simTick < game.replayInputStartTick) {
+    game.update(game.fixedStep);
+  }
+  game.setFixedTickMode(false, 1);
+  setReplayStatus(`Replay loaded (stage ${replay.stageId})`);
+}
+
+function downloadReplay(replay: ReplayData) {
+  const label = String(replay.stageId).padStart(3, '0');
+  const filename = `replay_${replay.gameSource}_st${label}.json`;
+  const blob = new Blob([JSON.stringify(replay, null, 2)], { type: 'application/json' });
+  const url = URL.createObjectURL(blob);
+  const link = document.createElement('a');
+  link.href = url;
+  link.download = filename;
+  document.body.appendChild(link);
+  link.click();
+  link.remove();
+  setTimeout(() => URL.revokeObjectURL(url), 1000);
+}
+
 function bindVolumeControl(
   input: HTMLInputElement | null,
   output: HTMLOutputElement | null,
@@ -1403,6 +1458,43 @@ packFolderInput?.addEventListener('change', async () => {
   }
 });
 
+replaySaveButton?.addEventListener('click', () => {
+  if (!game || !game.stage) {
+    setReplayStatus('Replay: no stage active');
+    return;
+  }
+  const replay = game.exportReplay();
+  if (!replay) {
+    setReplayStatus('Replay: no inputs recorded');
+    return;
+  }
+  downloadReplay(replay);
+  setReplayStatus(`Replay saved (stage ${replay.stageId})`);
+});
+
+replayLoadButton?.addEventListener('click', () => {
+  replayFileInput?.click();
+});
+
+replayFileInput?.addEventListener('change', async () => {
+  const file = replayFileInput.files?.[0];
+  replayFileInput.value = '';
+  if (!file) {
+    return;
+  }
+  try {
+    const text = await file.text();
+    const replay = JSON.parse(text) as ReplayData;
+    if (!replay || replay.version !== 1 || !Array.isArray(replay.inputs)) {
+      throw new Error('Invalid replay');
+    }
+    await startReplay(replay);
+  } catch (error) {
+    console.error(error);
+    setReplayStatus('Replay: failed to load');
+  }
+});
+
 smb2ModeSelect?.addEventListener('change', () => {
   updateSmb2ModeFields();
 });
diff --git a/src/replay.ts b/src/replay.ts
new file mode 100644
index 0000000..96853cf
--- /dev/null
+++ b/src/replay.ts
@@ -0,0 +1,34 @@
+import type { GameSource } from './constants.js';
+import type { QuantizedStick } from './determinism.js';
+
+export type ReplayData = {
+  version: 1;
+  gameSource: GameSource;
+  stageId: number;
+  ticks: number;
+  inputStartTick: number;
+  inputs: QuantizedStick[];
+  hashes?: number[];
+  note?: string;
+};
+
+export function createReplayData(
+  gameSource: GameSource,
+  stageId: number,
+  ticks: number,
+  inputStartTick: number,
+  inputs: QuantizedStick[],
+  hashes?: number[],
+  note?: string,
+): ReplayData {
+  return {
+    version: 1,
+    gameSource,
+    stageId,
+    ticks,
+    inputStartTick,
+    inputs,
+    hashes,
+    note,
+  };
+}