diff --git a/playground/js/shapeseq/session.js b/playground/js/shapeseq/session.js
new file mode 100644
index 0000000..1fad8f7
--- /dev/null
+++ b/playground/js/shapeseq/session.js
@@ -0,0 +1,116 @@
+/**
+ * ShapeSeq session state serialization.
+ *
+ * Captures and restores the complete ShapeSeq state:
+ * - Chain configuration (which primitives, in what order)
+ * - Primitive states (e.g., PitchWalker position)
+ * - Generator combine mode
+ * - Projection config (3 knobs)
+ * - Voice mode (mono/poly)
+ * - Sequence MLP weights (if in dual mode)
+ * - Freeze state (mode, frozen params, live flags, seeds)
+ * - Clock config (BPM, step count)
+ *
+ * @module shapeseq/session
+ */
+
+import { PRIMITIVE_REGISTRY } from './primitives.js';
+
+/**
+ * Serialize ShapeSeq engine state to a plain object (JSON-safe).
+ * @param {import('./sequencer.js').ShapeSeqEngine} engine
+ * @returns {Object}
+ */
+export function serializeShapeSeqState(engine) {
+  const chain = engine.getChain();
+  const prims = chain.getPrimitives();
+
+  return {
+    version: 1,
+    chain: {
+      primitives: prims.map(p => ({
+        name: p.name,
+      })),
+      states: chain.getState(),
+      generatorCombineMode: chain.generatorCombineMode,
+    },
+    projection: engine.getProjection(),
+    voiceMode: engine.voiceMode,
+    clock: {
+      bpm: engine.getClock().bpm,
+    },
+    // Sequence MLP weights (for dual mode restoration)
+    sequenceWeights: engine.getSequenceIML()?.getWeights?.() ?? null,
+    // Freeze state
+    freeze: serializeFreezeState(engine.freezeManager),
+  };
+}
+
+/**
+ * Serialize freeze manager state to a JSON-safe object.
+ * Returns null when not frozen.
+ *
+ * @param {import('./freeze.js').FreezeManager} fm
+ * @returns {Object|null}
+ */
+function serializeFreezeState(fm) {
+  if (!fm.isFrozen) return null;
+  return {
+    mode: fm.freezeMode,
+    frozenParams: fm.getFrozenParams() ? Array.from(fm.getFrozenParams()) : null,
+    liveFlags: fm.getLiveFlags() ? Array.from(fm.getLiveFlags()) : null,
+    seeds: fm.getFrozenSeeds(),
+    states: fm.getFrozenStates(),
+    masterSeed: fm.getMasterSeed(),
+    // For pattern mode:
+    frozenPattern: fm.getFrozenPattern() ?? null,
+  };
+}
+
+/**
+ * Restore ShapeSeq engine state from a serialized object.
+ * @param {import('./sequencer.js').ShapeSeqEngine} engine
+ * @param {Object} state - from serializeShapeSeqState
+ */
+export function restoreShapeSeqState(engine, state) {
+  if (!state || state.version !== 1) return;
+
+  const chain = engine.getChain();
+
+  // Rebuild chain from primitive names
+  if (state.chain?.primitives) {
+    // Clear existing
+    const existing = chain.getPrimitives();
+    for (let i = existing.length - 1; i >= 0; i--) {
+      chain.removePrimitive(i);
+    }
+    // Add from state
+    for (const p of state.chain.primitives) {
+      const Ctor = PRIMITIVE_REGISTRY[p.name];
+      if (Ctor) chain.addPrimitive(new Ctor());
+    }
+    // Restore states
+    if (state.chain.states) chain.setState(state.chain.states);
+    if (state.chain.generatorCombineMode) {
+      chain.generatorCombineMode = state.chain.generatorCombineMode;
+    }
+  }
+
+  // Projection
+  if (state.projection) engine.setProjection(state.projection);
+
+  // Voice mode
+  if (state.voiceMode) engine.setVoiceMode(state.voiceMode);
+
+  // Clock
+  if (state.clock?.bpm) engine.setTempo(state.clock.bpm);
+
+  // Sequence MLP weights
+  if (state.sequenceWeights) {
+    const iml = engine.getSequenceIML();
+    if (iml?.setWeights) iml.setWeights(state.sequenceWeights);
+  }
+
+  // Freeze state restoration is complex — skip for now, just ensure unfrozen on load
+  // TODO: restore freeze state properly
+}
diff --git a/playground/js/shapeseq/tests/session.test.js b/playground/js/shapeseq/tests/session.test.js
new file mode 100644
index 0000000..277e449
--- /dev/null
+++ b/playground/js/shapeseq/tests/session.test.js
@@ -0,0 +1,394 @@
+import { describe, it } from 'node:test';
+import { strict as assert } from 'node:assert';
+import { serializeShapeSeqState, restoreShapeSeqState } from '../session.js';
+import { Chain } from '../chain.js';
+import { FreezeManager } from '../freeze.js';
+import {
+  EuclideanRhythm,
+  ProbabilityGate,
+  PitchWalker,
+  IntervalLock,
+  VelocityShaper,
+  PRIMITIVE_REGISTRY,
+} from '../primitives.js';
+import { createProjection } from '../projection.js';
+
+// ---------------------------------------------------------------------------
+// Helpers — minimal engine stub
+// ---------------------------------------------------------------------------
+
+/**
+ * Build a fake ShapeSeqEngine with the same accessor shape the session
+ * module expects, but without AudioContext/EventBus/C15 dependencies.
+ */
+function makeFakeEngine(opts = {}) {
+  const chain = new Chain();
+  const prims = opts.primitives ?? [
+    new EuclideanRhythm(),
+    new ProbabilityGate(),
+    new PitchWalker(),
+    new IntervalLock(),
+    new VelocityShaper(),
+  ];
+  for (const p of prims) chain.addPrimitive(p);
+  if (opts.combineMode) chain.generatorCombineMode = opts.combineMode;
+
+  const projection = opts.projection ?? createProjection();
+  const fm = new FreezeManager();
+
+  let voiceMode = opts.voiceMode ?? 'mono';
+  const bpm = opts.bpm ?? 120;
+  let seqWeights = opts.sequenceWeights ?? null;
+
+  return {
+    getChain: () => chain,
+    getClock: () => ({ bpm }),
+    getSequenceIML: () => seqWeights
+      ? { getWeights: () => seqWeights, setWeights: (w) => { seqWeights = w; } }
+      : null,
+    getProjection: () => projection,
+    get voiceMode() { return voiceMode; },
+    setVoiceMode: (m) => { voiceMode = m; },
+    setProjection: (p) => { /* stub */ },
+    setTempo: (t) => { /* stub */ },
+    get freezeManager() { return fm; },
+  };
+}
+
+// ---------------------------------------------------------------------------
+// serializeShapeSeqState
+// ---------------------------------------------------------------------------
+
+describe('serializeShapeSeqState', () => {
+  it('produces a version-1 object with all expected top-level keys', () => {
+    const engine = makeFakeEngine();
+    const state = serializeShapeSeqState(engine);
+
+    assert.strictEqual(state.version, 1);
+    assert.ok('chain' in state);
+    assert.ok('projection' in state);
+    assert.ok('voiceMode' in state);
+    assert.ok('clock' in state);
+    assert.ok('sequenceWeights' in state);
+    assert.ok('freeze' in state);
+  });
+
+  it('serializes chain primitives by name', () => {
+    const engine = makeFakeEngine({
+      primitives: [new EuclideanRhythm(), new PitchWalker()],
+    });
+    const state = serializeShapeSeqState(engine);
+
+    assert.strictEqual(state.chain.primitives.length, 2);
+    assert.strictEqual(state.chain.primitives[0].name, 'EuclideanRhythm');
+    assert.strictEqual(state.chain.primitives[1].name, 'PitchWalker');
+  });
+
+  it('captures generator combine mode', () => {
+    const engine = makeFakeEngine({ combineMode: 'multiplicative' });
+    const state = serializeShapeSeqState(engine);
+
+    assert.strictEqual(state.chain.generatorCombineMode, 'multiplicative');
+  });
+
+  it('captures primitive states array', () => {
+    const engine = makeFakeEngine({
+      primitives: [new PitchWalker()],
+    });
+    const state = serializeShapeSeqState(engine);
+
+    assert.ok(Array.isArray(state.chain.states));
+    assert.strictEqual(state.chain.states.length, 1);
+    // PitchWalker default state has position
+    assert.strictEqual(typeof state.chain.states[0].position, 'number');
+  });
+
+  it('captures projection config', () => {
+    const proj = createProjection({
+      velocityCurve: 'exponential',
+      gateThreshold: 0.25,
+      pitchRange: { low: 36, high: 84 },
+    });
+    const engine = makeFakeEngine({ projection: proj });
+    const state = serializeShapeSeqState(engine);
+
+    assert.strictEqual(state.projection.velocityCurve, 'exponential');
+    assert.strictEqual(state.projection.gateThreshold, 0.25);
+    assert.strictEqual(state.projection.pitchRange.low, 36);
+    assert.strictEqual(state.projection.pitchRange.high, 84);
+  });
+
+  it('captures voice mode', () => {
+    const engine = makeFakeEngine({ voiceMode: 'poly' });
+    const state = serializeShapeSeqState(engine);
+
+    assert.strictEqual(state.voiceMode, 'poly');
+  });
+
+  it('captures clock BPM', () => {
+    const engine = makeFakeEngine({ bpm: 140 });
+    const state = serializeShapeSeqState(engine);
+
+    assert.strictEqual(state.clock.bpm, 140);
+  });
+
+  it('captures sequence MLP weights when available', () => {
+    const weights = [[1, 2], [3, 4]];
+    const engine = makeFakeEngine({ sequenceWeights: weights });
+    const state = serializeShapeSeqState(engine);
+
+    assert.deepStrictEqual(state.sequenceWeights, weights);
+  });
+
+  it('sets sequenceWeights to null when IML is unavailable', () => {
+    const engine = makeFakeEngine(); // no sequenceWeights
+    const state = serializeShapeSeqState(engine);
+
+    assert.strictEqual(state.sequenceWeights, null);
+  });
+
+  it('freeze is null when not frozen', () => {
+    const engine = makeFakeEngine();
+    const state = serializeShapeSeqState(engine);
+
+    assert.strictEqual(state.freeze, null);
+  });
+
+  it('freeze state is serialized when frozen in algorithm mode', () => {
+    const engine = makeFakeEngine({
+      primitives: [new EuclideanRhythm(), new ProbabilityGate()],
+    });
+    const chain = engine.getChain();
+    const paramCount = chain.totalParamCount;
+    const params = new Float32Array(paramCount);
+    for (let i = 0; i < paramCount; i++) params[i] = i / paramCount;
+
+    engine.freezeManager.freeze(chain, params, 42);
+
+    const state = serializeShapeSeqState(engine);
+
+    assert.ok(state.freeze !== null);
+    assert.strictEqual(state.freeze.mode, 'algorithm');
+    assert.ok(Array.isArray(state.freeze.frozenParams));
+    assert.strictEqual(state.freeze.frozenParams.length, paramCount);
+    assert.ok(Array.isArray(state.freeze.liveFlags));
+    assert.strictEqual(state.freeze.masterSeed, 42);
+  });
+});
+
+// ---------------------------------------------------------------------------
+// restoreShapeSeqState
+// ---------------------------------------------------------------------------
+
+describe('restoreShapeSeqState', () => {
+  it('rebuilds chain from primitive names', () => {
+    const engine = makeFakeEngine({ primitives: [] });
+    assert.strictEqual(engine.getChain().getPrimitives().length, 0);
+
+    const state = {
+      version: 1,
+      chain: {
+        primitives: [
+          { name: 'PitchWalker' },
+          { name: 'EuclideanRhythm' },
+          { name: 'VelocityShaper' },
+        ],
+        states: null,
+        generatorCombineMode: 'additive',
+      },
+      projection: null,
+      voiceMode: null,
+      clock: null,
+      sequenceWeights: null,
+      freeze: null,
+    };
+
+    restoreShapeSeqState(engine, state);
+
+    const prims = engine.getChain().getPrimitives();
+    assert.strictEqual(prims.length, 3);
+    assert.strictEqual(prims[0].name, 'PitchWalker');
+    assert.strictEqual(prims[1].name, 'EuclideanRhythm');
+    assert.strictEqual(prims[2].name, 'VelocityShaper');
+  });
+
+  it('restores generator combine mode', () => {
+    const engine = makeFakeEngine();
+    const state = {
+      version: 1,
+      chain: {
+        primitives: [{ name: 'EuclideanRhythm' }],
+        states: null,
+        generatorCombineMode: 'multiplicative',
+      },
+      projection: null,
+      voiceMode: null,
+      clock: null,
+      sequenceWeights: null,
+      freeze: null,
+    };
+
+    restoreShapeSeqState(engine, state);
+    assert.strictEqual(engine.getChain().generatorCombineMode, 'multiplicative');
+  });
+
+  it('restores projection config', () => {
+    let capturedProjection = null;
+    const engine = makeFakeEngine();
+    engine.setProjection = (p) => { capturedProjection = p; };
+
+    const proj = {
+      velocityCurve: 'sCurve',
+      gateThreshold: 0.4,
+      pitchRange: { low: 24, high: 96 },
+    };
+
+    restoreShapeSeqState(engine, {
+      version: 1,
+      chain: null,
+      projection: proj,
+      voiceMode: null,
+      clock: null,
+      sequenceWeights: null,
+      freeze: null,
+    });
+
+    assert.deepStrictEqual(capturedProjection, proj);
+  });
+
+  it('restores voice mode', () => {
+    let capturedMode = null;
+    const engine = makeFakeEngine();
+    engine.setVoiceMode = (m) => { capturedMode = m; };
+
+    restoreShapeSeqState(engine, {
+      version: 1,
+      chain: null,
+      projection: null,
+      voiceMode: 'poly',
+      clock: null,
+      sequenceWeights: null,
+      freeze: null,
+    });
+
+    assert.strictEqual(capturedMode, 'poly');
+  });
+
+  it('restores clock BPM', () => {
+    let capturedBPM = null;
+    const engine = makeFakeEngine();
+    engine.setTempo = (t) => { capturedBPM = t; };
+
+    restoreShapeSeqState(engine, {
+      version: 1,
+      chain: null,
+      projection: null,
+      voiceMode: null,
+      clock: { bpm: 90 },
+      sequenceWeights: null,
+      freeze: null,
+    });
+
+    assert.strictEqual(capturedBPM, 90);
+  });
+
+  it('restores sequence MLP weights when IML available', () => {
+    const weights = [[5, 6], [7, 8]];
+    const engine = makeFakeEngine({ sequenceWeights: [[0, 0]] });
+
+    restoreShapeSeqState(engine, {
+      version: 1,
+      chain: null,
+      projection: null,
+      voiceMode: null,
+      clock: null,
+      sequenceWeights: weights,
+      freeze: null,
+    });
+
+    // The stub's setWeights should have been called
+    assert.deepStrictEqual(engine.getSequenceIML().getWeights(), weights);
+  });
+
+  it('skips unknown primitive names gracefully', () => {
+    const engine = makeFakeEngine({ primitives: [] });
+
+    const state = {
+      version: 1,
+      chain: {
+        primitives: [
+          { name: 'EuclideanRhythm' },
+          { name: 'NonExistentPrimitive' },
+          { name: 'PitchWalker' },
+        ],
+        states: null,
+        generatorCombineMode: 'additive',
+      },
+      projection: null,
+      voiceMode: null,
+      clock: null,
+      sequenceWeights: null,
+      freeze: null,
+    };
+
+    // Should not throw
+    restoreShapeSeqState(engine, state);
+
+    // Unknown primitive skipped, only valid ones added
+    const prims = engine.getChain().getPrimitives();
+    assert.strictEqual(prims.length, 2);
+    assert.strictEqual(prims[0].name, 'EuclideanRhythm');
+    assert.strictEqual(prims[1].name, 'PitchWalker');
+  });
+
+  it('handles null state without throwing', () => {
+    const engine = makeFakeEngine();
+    // Should not throw
+    restoreShapeSeqState(engine, null);
+    restoreShapeSeqState(engine, undefined);
+  });
+
+  it('handles wrong version without throwing', () => {
+    const engine = makeFakeEngine();
+    restoreShapeSeqState(engine, { version: 99 });
+    // Chain should be untouched
+    assert.strictEqual(engine.getChain().getPrimitives().length, 5);
+  });
+
+  it('handles state with missing optional fields', () => {
+    const engine = makeFakeEngine({ primitives: [] });
+
+    // Minimal valid state — only version and chain
+    restoreShapeSeqState(engine, {
+      version: 1,
+      chain: {
+        primitives: [{ name: 'EuclideanRhythm' }],
+      },
+    });
+
+    assert.strictEqual(engine.getChain().getPrimitives().length, 1);
+  });
+
+  it('restores primitive states alongside chain rebuild', () => {
+    const engine = makeFakeEngine({ primitives: [] });
+
+    const state = {
+      version: 1,
+      chain: {
+        primitives: [{ name: 'PitchWalker' }],
+        states: [{ position: 0.75 }],
+        generatorCombineMode: 'additive',
+      },
+      projection: null,
+      voiceMode: null,
+      clock: null,
+      sequenceWeights: null,
+      freeze: null,
+    };
+
+    restoreShapeSeqState(engine, state);
+
+    const chainStates = engine.getChain().getState();
+    assert.strictEqual(chainStates[0].position, 0.75);
+  });
+});
diff --git a/playground/js/ui/session-presets.js b/playground/js/ui/session-presets.js
index 99e005f..e50c531 100644
--- a/playground/js/ui/session-presets.js
+++ b/playground/js/ui/session-presets.js
@@ -128,6 +128,38 @@ function decodeFromURL(urlParams) {
 export class SessionPresetManager {
   constructor() {
     this._presets = this._loadFromStorage();
+
+    /**
+     * Optional reference to an active ShapeSeq engine.
+     * When set, save/load will include ShapeSeq state.
+     * @private @type {import('../shapeseq/sequencer.js').ShapeSeqEngine|null}
+     */
+    this._shapeSeqEngine = null;
+
+    /**
+     * Lazy-loaded ShapeSeq session helpers.
+     * @private @type {{ serializeShapeSeqState: Function, restoreShapeSeqState: Function }|null}
+     */
+    this._shapeSeqSession = null;
+  }
+
+  /**
+   * Register a ShapeSeq engine so that save/load includes its state.
+   * Call with null to unregister.
+   *
+   * @param {import('../shapeseq/sequencer.js').ShapeSeqEngine|null} engine
+   */
+  setShapeSeqEngine(engine) {
+    this._shapeSeqEngine = engine;
+    if (engine && !this._shapeSeqSession) {
+      // Lazy-load the session helpers to avoid hard dependency
+      import('../shapeseq/session.js').then(mod => {
+        this._shapeSeqSession = mod;
+      }).catch(() => {
+        // ShapeSeq module not available — ignore silently
+        this._shapeSeqSession = null;
+      });
+    }
   }
 
   /**
@@ -143,7 +175,7 @@ export class SessionPresetManager {
    * @returns {object} the captured preset
    */
   capture(name, state) {
-    return {
+    const preset = {
       name,
       controlSurface: state.controlSurface || null,
       synthPresetId: state.synthPresetId || null,
@@ -152,6 +184,13 @@ export class SessionPresetManager {
       outputPipeline: state.outputPipeline || null,
       timestamp: Date.now(),
     };
+
+    // Include ShapeSeq state when engine is registered
+    if (this._shapeSeqEngine && this._shapeSeqSession) {
+      preset.shapeseq = this._shapeSeqSession.serializeShapeSeqState(this._shapeSeqEngine);
+    }
+
+    return preset;
   }
 
   /**
@@ -169,11 +208,20 @@ export class SessionPresetManager {
 
   /**
    * Load a named session preset.
+   *
+   * If the preset contains a `shapeseq` key and a ShapeSeq engine is
+   * registered, the ShapeSeq state is automatically restored. Presets
+   * without a `shapeseq` key load fine (backward compatible).
+   *
    * @param {string} name
    * @returns {object|null}
    */
   load(name) {
-    return this._presets[name] || null;
+    const preset = this._presets[name] || null;
+    if (preset && preset.shapeseq && this._shapeSeqEngine && this._shapeSeqSession) {
+      this._shapeSeqSession.restoreShapeSeqState(this._shapeSeqEngine, preset.shapeseq);
+    }
+    return preset;
   }
 
   /**