diff --git a/tests/soak/core/deferred.ts b/tests/soak/core/deferred.ts
new file mode 100644
index 0000000..4e235ad
--- /dev/null
+++ b/tests/soak/core/deferred.ts
@@ -0,0 +1,20 @@
+/**
+ * Promise deferred primitive — lets external code resolve or reject
+ * a promise. Used by RoomCoordinator for host→joiners handoff.
+ */
+
+export interface Deferred<T> {
+  promise: Promise<T>;
+  resolve(value: T): void;
+  reject(error: unknown): void;
+}
+
+export function deferred<T>(): Deferred<T> {
+  let resolve!: (value: T) => void;
+  let reject!: (error: unknown) => void;
+  const promise = new Promise<T>((res, rej) => {
+    resolve = res;
+    reject = rej;
+  });
+  return { promise, resolve, reject };
+}
diff --git a/tests/soak/core/types.ts b/tests/soak/core/types.ts
new file mode 100644
index 0000000..6a13b8b
--- /dev/null
+++ b/tests/soak/core/types.ts
@@ -0,0 +1,185 @@
+/**
+ * Core type definitions for the soak harness.
+ *
+ * Contracts here are consumed by runner.ts, SessionPool, scenarios,
+ * and the dashboard. Keep this file small and stable.
+ */
+
+import type { BrowserContext, Page } from 'playwright-core';
+
+// =============================================================================
+// GolfBot structural interface
+// =============================================================================
+
+/**
+ * Structural interface for the real `GolfBot` class from
+ * `tests/e2e/bot/golf-bot.ts`. We can't type-import the real class
+ * because (a) it lives outside this package's `rootDir`, and (b) it
+ * imports `@playwright/test` which isn't in this package's deps.
+ *
+ * Instead we declare the narrow public contract the soak harness
+ * actually calls. `SessionPool` constructs the real class at runtime
+ * via a dynamic require and casts it to this interface. When golf-bot
+ * gains new methods the harness wants, add them here — TypeScript will
+ * flag drift at the first call site.
+ */
+
+export type GamePhase =
+  | 'lobby'
+  | 'waiting_for_flip'
+  | 'playing'
+  | 'round_over'
+  | 'game_over'
+  | 'unknown';
+
+export interface StartGameOptions {
+  holes?: number;
+  decks?: number;
+  initialFlips?: number;
+  flipMode?: 'never' | 'always' | 'endgame';
+  knockPenalty?: boolean;
+  jokerMode?: 'none' | 'standard' | 'lucky-swing' | 'eagle-eye';
+}
+
+export interface TurnResult {
+  success: boolean;
+  action: string;
+  details?: Record<string, unknown>;
+  error?: string;
+}
+
+export interface GolfBot {
+  readonly page: Page;
+  goto(url?: string): Promise<void>;
+  createGame(playerName: string): Promise<string>;
+  joinGame(roomCode: string, playerName: string): Promise<void>;
+  addCPU(profileName?: string): Promise<void>;
+  startGame(options?: StartGameOptions): Promise<void>;
+  isMyTurn(): Promise<boolean>;
+  waitForMyTurn(timeout?: number): Promise<boolean>;
+  getGamePhase(): Promise<GamePhase>;
+  getGameState(): Promise<Record<string, unknown>>;
+  playTurn(): Promise<TurnResult>;
+  completeInitialFlips(): Promise<void>;
+  isFrozen(timeout?: number): Promise<boolean>;
+  takeScreenshot(label: string): Promise<Buffer>;
+  getConsoleErrors?(): string[];
+}
+
+// =============================================================================
+// Accounts & sessions
+// =============================================================================
+
+export interface Account {
+  /** Stable key used in logs, e.g. "soak_00". */
+  key: string;
+  username: string;
+  password: string;
+  /** JWT returned from /api/auth/login, may be refreshed by SessionPool. */
+  token: string;
+}
+
+export interface Session {
+  account: Account;
+  context: BrowserContext;
+  page: Page;
+  bot: GolfBot;
+  /** Convenience mirror of account.key. */
+  key: string;
+}
+
+// =============================================================================
+// Scenarios
+// =============================================================================
+
+export interface ScenarioNeeds {
+  /** Total number of authenticated sessions the scenario requires. */
+  accounts: number;
+  /** How many rooms to partition sessions into (default: 1). */
+  rooms?: number;
+  /** CPUs to add per room (default: 0). */
+  cpusPerRoom?: number;
+}
+
+/** Free-form per-scenario config merged with CLI flags. */
+export type ScenarioConfig = Record<string, unknown>;
+
+export interface ScenarioError {
+  room: string;
+  reason: string;
+  detail?: string;
+  timestamp: number;
+}
+
+export interface ScenarioResult {
+  gamesCompleted: number;
+  errors: ScenarioError[];
+  durationMs: number;
+  customMetrics?: Record<string, number>;
+}
+
+export interface ScenarioContext {
+  /** Merged config: CLI flags → env → scenario defaults → runner defaults. */
+  config: ScenarioConfig;
+  /** Pre-authenticated sessions; ordered. */
+  sessions: Session[];
+  coordinator: RoomCoordinatorApi;
+  dashboard: DashboardReporter;
+  logger: Logger;
+  signal: AbortSignal;
+  /** Reset the per-room watchdog. Call at each progress point. */
+  heartbeat(roomId: string): void;
+}
+
+export interface Scenario {
+  name: string;
+  description: string;
+  defaultConfig: ScenarioConfig;
+  needs: ScenarioNeeds;
+  run(ctx: ScenarioContext): Promise<ScenarioResult>;
+}
+
+// =============================================================================
+// Room coordination
+// =============================================================================
+
+export interface RoomCoordinatorApi {
+  announce(roomId: string, code: string): void;
+  await(roomId: string, timeoutMs?: number): Promise<string>;
+}
+
+// =============================================================================
+// Dashboard reporter
+// =============================================================================
+
+export interface RoomState {
+  phase?: string;
+  currentPlayer?: string;
+  hole?: number;
+  totalHoles?: number;
+  game?: number;
+  totalGames?: number;
+  moves?: number;
+  players?: Array<{ key: string; score: number | null; isActive: boolean }>;
+  message?: string;
+}
+
+export interface DashboardReporter {
+  update(roomId: string, state: Partial<RoomState>): void;
+  log(level: 'info' | 'warn' | 'error', msg: string, meta?: object): void;
+  incrementMetric(name: string, by?: number): void;
+}
+
+// =============================================================================
+// Logger
+// =============================================================================
+
+export type LogLevel = 'debug' | 'info' | 'warn' | 'error';
+
+export interface Logger {
+  debug(msg: string, meta?: object): void;
+  info(msg: string, meta?: object): void;
+  warn(msg: string, meta?: object): void;
+  error(msg: string, meta?: object): void;
+  child(meta: object): Logger;
+}
diff --git a/tests/soak/tests/deferred.test.ts b/tests/soak/tests/deferred.test.ts
new file mode 100644
index 0000000..c0977fa
--- /dev/null
+++ b/tests/soak/tests/deferred.test.ts
@@ -0,0 +1,24 @@
+import { describe, it, expect } from 'vitest';
+import { deferred } from '../core/deferred';
+
+describe('deferred', () => {
+  it('resolves with the given value', async () => {
+    const d = deferred<string>();
+    d.resolve('hello');
+    await expect(d.promise).resolves.toBe('hello');
+  });
+
+  it('rejects with the given error', async () => {
+    const d = deferred<string>();
+    const err = new Error('boom');
+    d.reject(err);
+    await expect(d.promise).rejects.toBe(err);
+  });
+
+  it('ignores second resolve calls', async () => {
+    const d = deferred<number>();
+    d.resolve(1);
+    d.resolve(2);
+    await expect(d.promise).resolves.toBe(1);
+  });
+});