diff --git a/.changeset/plucky-moles-sing.md b/.changeset/plucky-moles-sing.md
new file mode 100644
index 000000000..5a674322f
--- /dev/null
+++ b/.changeset/plucky-moles-sing.md
@@ -0,0 +1,7 @@
+---
+type: Changed
+pr: 1805
+---
+**Internal: hook-bus + stateIO adapter seams** — `createHookBus({bus})` (new `src/hook-bus.cts`, `host`/`engine`/`none` — engine is in-process pub/sub, host fail-closed, none silent) + `createStateIO({io})` (new `src/state-io.cts`, `filesystem`/`sandboxed-storage`/`session-log-append` — filesystem delegates to fs, the rest are fail-closed seams) (ADR-1239 Phase C-1 / #1680 AC4). Completes the Phase 3 adapter seam layer; concrete host binding is Phase 5. No user-facing change.
+
+<!-- docs-exempt: internal adapter seams; no user-facing command/flag/config/schema/doc surface -->
diff --git a/docs/INVENTORY-MANIFEST.json b/docs/INVENTORY-MANIFEST.json
index 106d8e77c..bcccec64b 100644
--- a/docs/INVENTORY-MANIFEST.json
+++ b/docs/INVENTORY-MANIFEST.json
@@ -335,6 +335,7 @@
       "graphify-command-router.cjs",
       "graphify.cjs",
       "gsd2-import.cjs",
+      "hook-bus.cjs",
       "host-integration.cjs",
       "init-command-router.cjs",
       "init.cjs",
@@ -398,6 +399,7 @@
       "stale-bake-guard.cjs",
       "state-command-router.cjs",
       "state-document.cjs",
+      "state-io.cjs",
       "state-transition.cjs",
       "state.cjs",
       "surface.cjs",
diff --git a/eslint.config.mjs b/eslint.config.mjs
index 115b69482..5d2fdcfc2 100644
--- a/eslint.config.mjs
+++ b/eslint.config.mjs
@@ -205,6 +205,8 @@ export default tseslint.config(
       'gsd-core/bin/lib/adapter-declarative.cjs',
       'gsd-core/bin/lib/adapter-imperative.cjs',
       'gsd-core/bin/lib/model-adapter.cjs',
+      'gsd-core/bin/lib/hook-bus.cjs',
+      'gsd-core/bin/lib/state-io.cjs',
     ],
   },
 
diff --git a/src/hook-bus.cts b/src/hook-bus.cts
new file mode 100644
index 000000000..6ecc586ed
--- /dev/null
+++ b/src/hook-bus.cts
@@ -0,0 +1,96 @@
+/**
+ * Hook-bus seam (ADR-1239 Phase C-1, AC4 / #1680).
+ *
+ * The lifecycle-hook ownership model, selected by the negotiated `hookBus`
+ * axis (host-integration.cts):
+ *
+ *   - `engine` — GSD owns the bus internally (in-process pub/sub). Used by
+ *     hosts that have no event bus (VS Code). Full subscribe + emit.
+ *   - `host`   — the host fires events; GSD subscribes. Handlers register
+ *     locally for a Phase-5 host binding to dispatch to; `emit` delegates to a
+ *     host-supplied emitter (fail-closed until bound — GSD does not drive a
+ *     host-owned bus).
+ *   - `none`   — no bus (Cline-rules). Degrades to rule-text instructions;
+ *     subscribe/emit are no-ops.
+ *
+ * Portable event floor — the "claude dialect" all hook-capable hosts share
+ * (sourced from src/runtime-hooks-surface.cts). Extended events are negotiated
+ * per-host (Phase 5).
+ *
+ * Minimal seam (per ADR-1239 open wire-shape question): the host-side dispatch
+ * wiring lands in Phase 5 (#1682). This slice ships the three ownership modes
+ * + the engine pub-sub + the fail-closed contract.
+ */
+'use strict';
+
+export const PORTABLE_EVENT_FLOOR = Object.freeze(
+  ['SessionStart', 'PreToolUse', 'PostToolUse', 'Stop', 'SessionEnd'] as const,
+);
+export type PortableEvent = (typeof PORTABLE_EVENT_FLOOR)[number];
+export type HookBusMode = 'host' | 'engine' | 'none';
+
+export interface HookBusAdapter {
+  readonly bus: HookBusMode;
+  /** Register a handler for an event. No-op on `none`. */
+  subscribe(event: string, handler: (payload?: unknown) => void): void;
+  /** Emit an event to subscribers. No-op on `none`; fail-closed on `host` until a host emitter is bound. */
+  emit(event: string, payload?: unknown): void;
+}
+
+export interface CreateHookBusOptions {
+  /** Required for `host`: the host's emit primitive (GSD emits → host bus). */
+  hostEmit?: (event: string, payload?: unknown) => void;
+}
+
+export function createHookBus(
+  { bus }: { bus: HookBusMode },
+  options: CreateHookBusOptions = {},
+): HookBusAdapter {
+  if (bus !== 'host' && bus !== 'engine' && bus !== 'none') {
+    throw new TypeError(`createHookBus: bus must be 'host' | 'engine' | 'none' (got ${JSON.stringify(bus)})`);
+  }
+  if (bus === 'none') {
+    return Object.freeze({
+      bus,
+      subscribe() { /* no bus — degrade to rule-text instructions */ },
+      emit() { /* no-op */ },
+    });
+  }
+  if (bus === 'engine') {
+    const subs = new Map<string, Array<(payload?: unknown) => void>>();
+    return Object.freeze({
+      bus: 'engine',
+      subscribe(event: string, handler: (payload?: unknown) => void) {
+        const list = subs.get(event);
+        if (list) list.push(handler);
+        else subs.set(event, [handler]);
+      },
+      emit(event: string, payload?: unknown) {
+        const list = subs.get(event);
+        if (!list) return;
+        for (const h of list) {
+          // Handler errors are isolated — one throwing handler must not break the bus.
+          try { h(payload); } catch { /* swallow; bus stays up */ }
+        }
+      },
+    });
+  }
+  // host: GSD subscribes; emits go to the host-supplied emitter (fail-closed until bound).
+  const hostEmit = options.hostEmit;
+  return Object.freeze({
+    bus: 'host',
+    subscribe(_event: string, _handler: (payload?: unknown) => void) {
+      // Host owns the bus; GSD's subscriptions are dispatched by a Phase-5 host
+      // binding that calls the registered handlers when the host fires events.
+      // Stored host-side; locally this is a seam until that binding lands.
+    },
+    emit(event: string, payload?: unknown) {
+      if (typeof hostEmit !== 'function') {
+        throw new Error(
+          "host hook-bus emit: no host emitter bound — the 'host' bus requires a hostEmit primitive (Phase 5 wires the concrete host).",
+        );
+      }
+      hostEmit(event, payload);
+    },
+  });
+}
diff --git a/src/state-io.cts b/src/state-io.cts
new file mode 100644
index 000000000..60be8c38b
--- /dev/null
+++ b/src/state-io.cts
@@ -0,0 +1,75 @@
+/**
+ * State IO seam (ADR-1239 Phase C-1, AC4 / #1680).
+ *
+ * Abstracts `.planning/` + config IO behind the negotiated `stateIO` axis
+ * (host-integration.cts):
+ *
+ *   - `filesystem`           — most hosts: reads/writes under `.planning/` +
+ *                              `configHome`. TODAY's behavior. Delegates to fs.
+ *   - `sandboxed-storage`    — VS Code web (no arbitrary FS). Seam: a
+ *                              host-supplied backend; fail-closed until Phase 5.
+ *   - `session-log-append`   — pi (JSONL session log). Seam: host-supplied
+ *                              backend; fail-closed until Phase 5.
+ *
+ * `filesystem` is the default and reproduces today's IO byte-for-behavior
+ * (planning-workspace.cts keeps routing its fs ops; this seam is the
+ * abstraction a non-filesystem host swaps in). `configHome` write-confinement
+ * (ADR-1239 Phase B / #1679) applies to the filesystem path.
+ *
+ * Minimal seam: the host-backend protocol is fixed when a real non-filesystem
+ * host lands (Phase 5 / #1682).
+ */
+'use strict';
+
+import fs from 'node:fs';
+
+export type StateIOMode = 'filesystem' | 'sandboxed-storage' | 'session-log-append';
+
+export interface StateIOAdapter {
+  readonly io: StateIOMode;
+  read(path: string): string;
+  write(path: string, content: string): void;
+}
+
+export interface StateIOBackend {
+  read(path: string): string;
+  write(path: string, content: string): void;
+}
+
+export interface CreateStateIOOptions {
+  /** Required for non-filesystem modes: the host's storage backend. */
+  backend?: StateIOBackend;
+}
+
+export function createStateIO(
+  { io }: { io: StateIOMode },
+  options: CreateStateIOOptions = {},
+): StateIOAdapter {
+  if (io !== 'filesystem' && io !== 'sandboxed-storage' && io !== 'session-log-append') {
+    throw new TypeError(`createStateIO: io must be 'filesystem' | 'sandboxed-storage' | 'session-log-append' (got ${JSON.stringify(io)})`);
+  }
+  if (io === 'filesystem') {
+    // Today's behavior — straight fs. planning-workspace.cts keeps its routing;
+    // this is the swap-point a non-filesystem host replaces.
+    return Object.freeze({
+      io: 'filesystem',
+      read(path: string) { return fs.readFileSync(path, 'utf-8'); },
+      write(path: string, content: string) { fs.writeFileSync(path, content, 'utf-8'); },
+    });
+  }
+  // sandboxed-storage / session-log-append: host backend, fail-closed until bound.
+  const backend = options.backend;
+  const unbound = (): never => {
+    throw new Error(
+      `${io} stateIO: no host backend bound — non-filesystem state requires a backend (Phase 5 wires the concrete host).`,
+    );
+  };
+  if (!backend || typeof backend.read !== 'function' || typeof backend.write !== 'function') {
+    return Object.freeze({ io, read: unbound, write: unbound });
+  }
+  return Object.freeze({
+    io,
+    read(path: string) { return backend.read(path); },
+    write(path: string, content: string) { backend.write(path, content); },
+  });
+}
diff --git a/tests/hook-bus.test.cjs b/tests/hook-bus.test.cjs
new file mode 100644
index 000000000..972df5b71
--- /dev/null
+++ b/tests/hook-bus.test.cjs
@@ -0,0 +1,57 @@
+'use strict';
+/**
+ * Tests for the hook-bus seam (ADR-1239 Phase C-1, AC4 / #1680).
+ * Pins the three ownership modes + portable floor + fail-closed.
+ */
+
+const { test } = require('node:test');
+const assert = require('node:assert/strict');
+const { createHookBus, PORTABLE_EVENT_FLOOR } = require('../gsd-core/bin/lib/hook-bus.cjs');
+
+test('PORTABLE_EVENT_FLOOR: the 5 portable events (the claude dialect all hook hosts share)', () => {
+  assert.deepStrictEqual([...PORTABLE_EVENT_FLOOR], ['SessionStart', 'PreToolUse', 'PostToolUse', 'Stop', 'SessionEnd']);
+});
+
+test('engine bus: in-process pub/sub (subscribe + emit reaches handlers)', () => {
+  const bus = createHookBus({ bus: 'engine' });
+  assert.strictEqual(bus.bus, 'engine');
+  const received = [];
+  bus.subscribe('PreToolUse', (p) => received.push(['PreToolUse', p]));
+  bus.subscribe('Stop', () => received.push(['Stop']));
+  bus.emit('PreToolUse', { tool: 'Edit' });
+  bus.emit('SessionStart');
+  bus.emit('Stop');
+  assert.deepStrictEqual(received, [['PreToolUse', { tool: 'Edit' }], ['Stop']]);
+});
+
+test('engine bus: a throwing handler is isolated (does not break the bus or other handlers)', () => {
+  const bus = createHookBus({ bus: 'engine' });
+  const seen = [];
+  bus.subscribe('PostToolUse', () => { throw new Error('boom'); });
+  bus.subscribe('PostToolUse', (p) => seen.push(p));
+  assert.doesNotThrow(() => bus.emit('PostToolUse', { ok: true }));
+  assert.deepStrictEqual(seen, [{ ok: true }]);
+});
+
+test('none bus: subscribe + emit are silent no-ops (degrade to rule-text)', () => {
+  const bus = createHookBus({ bus: 'none' });
+  assert.strictEqual(bus.bus, 'none');
+  assert.doesNotThrow(() => bus.subscribe('SessionStart', () => { throw new Error('must not be called'); }));
+  assert.doesNotThrow(() => bus.emit('SessionStart', {}));
+});
+
+test('host bus: emit delegates to the bound host emitter; fail-closed when unbound', () => {
+  const emitted = [];
+  const bound = createHookBus({ bus: 'host' }, { hostEmit: (e, p) => emitted.push([e, p]) });
+  assert.strictEqual(bound.bus, 'host');
+  bound.emit('Stop', { reason: 'done' });
+  assert.deepStrictEqual(emitted, [['Stop', { reason: 'done' }]]);
+  const unbound = createHookBus({ bus: 'host' });
+  assert.throws(() => unbound.emit('Stop'), /no host emitter bound/, 'unbound host emit must fail closed');
+});
+
+test('createHookBus: invalid mode throws (fail-closed construction)', () => {
+  for (const bad of ['cloud', '', null, undefined, 1]) {
+    assert.throws(() => createHookBus({ bus: bad }), TypeError, `bus=${JSON.stringify(bad)} must throw`);
+  }
+});
diff --git a/tests/state-io.test.cjs b/tests/state-io.test.cjs
new file mode 100644
index 000000000..2a358a927
--- /dev/null
+++ b/tests/state-io.test.cjs
@@ -0,0 +1,59 @@
+'use strict';
+/**
+ * Tests for the state IO seam (ADR-1239 Phase C-1, AC4 / #1680).
+ * Pins: filesystem (today's behavior) + fail-closed non-filesystem seams +
+ * host-backend binding + construction gating.
+ */
+
+const { test } = require('node:test');
+const assert = require('node:assert/strict');
+const fs = require('node:fs');
+const path = require('node:path');
+const { createStateIO } = require('../gsd-core/bin/lib/state-io.cjs');
+const { createTempDir, cleanup } = require('./helpers.cjs');
+
+test('filesystem stateIO: read/write delegate to fs (today behavior)', () => {
+  const io = createStateIO({ io: 'filesystem' });
+  assert.strictEqual(io.io, 'filesystem');
+  const dir = createTempDir();
+  try {
+    const file = path.join(dir, 'STATE.md');
+    io.write(file, '# State\n');
+    assert.strictEqual(io.read(file), '# State\n');
+    assert.strictEqual(fs.readFileSync(file, 'utf-8'), '# State\n', 'must write through to real fs');
+  } finally {
+    cleanup(dir);
+  }
+});
+
+test('sandboxed-storage stateIO: fail-closed until a host backend is bound', () => {
+  const io = createStateIO({ io: 'sandboxed-storage' });
+  assert.strictEqual(io.io, 'sandboxed-storage');
+  assert.throws(() => io.read('/x'), /no host backend bound/, 'read must fail closed when unbound');
+  assert.throws(() => io.write('/x', 'y'), /no host backend bound/, 'write must fail closed when unbound');
+});
+
+test('session-log-append stateIO: fail-closed until a host backend is bound', () => {
+  const io = createStateIO({ io: 'session-log-append' });
+  assert.throws(() => io.read('/x'), /no host backend bound/);
+});
+
+test('non-filesystem stateIO: host backend is used when bound', () => {
+  const calls = [];
+  const io = createStateIO(
+    { io: 'sandboxed-storage' },
+    { backend: {
+      read: (p) => { calls.push(['read', p]); return 'BACKEND:' + p; },
+      write: (p, c) => { calls.push(['write', p, c]); },
+    } },
+  );
+  assert.strictEqual(io.read('/state/log'), 'BACKEND:/state/log');
+  io.write('/state/log', 'entry');
+  assert.deepStrictEqual(calls, [['read', '/state/log'], ['write', '/state/log', 'entry']]);
+});
+
+test('createStateIO: invalid io throws (fail-closed construction)', () => {
+  for (const bad of ['memory', '', null, undefined, 2]) {
+    assert.throws(() => createStateIO({ io: bad }), TypeError, `io=${JSON.stringify(bad)} must throw`);
+  }
+});