diff --git a/packages/tsunami/AGENTS.md b/packages/tsunami/AGENTS.md
new file mode 100644
index 00000000..a6983e99
--- /dev/null
+++ b/packages/tsunami/AGENTS.md
@@ -0,0 +1,29 @@
+# tsunami — implementation guide
+
+Error model, decided in FCE-3580. Follow it exactly; do not invent new error
+mechanisms.
+
+## The rule
+
+**Both surfaces, one error object.** A failing imperative method rejects with
+a typed `FishjamError` subclass, and the same object is written to the
+matching `ClientState` field. Success on the same resource clears the field.
+
+| Path                                | Contract                                                                                                                                                                                 |
+| ----------------------------------- | ---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- |
+| Device acquisition                  | Rejects with a `DeviceError` subclass; same object in `cameraError` / `microphoneError`. Exception: `initializeDevices` _resolves_ with a result (degraded init is a supported outcome). |
+| `addTrack`, room refuses track kind | Rejects with `TrackTypeError` — never a synchronous throw. Keep-local recovery is the adapter's policy.                                                                                  |
+| `connect()`                         | Rejects with `AuthError` / `JoinError` / `SocketError` / `ConnectionError` (never `undefined`, never hangs) and sets `peerStatus: "error"` — one shared event list for both surfaces.    |
+
+## Rules
+
+1. Per-failure classes in domain files (`devices/errors.ts`, …): extend
+   `FishjamError`, explicit `this.name` literal (minification-safe), set
+   `recoverability`, original failure as `cause`. No `{ name }` objects,
+   codes, or reason unions.
+2. Land error classes with the code that first throws them (YAGNI).
+3. Misuse errors (`ClientDisposedError`) are the only sync throws; never in state.
+4. Platform error shapes stay behind `IDeviceManager` — DOMException
+   classification belongs in `WebDeviceManager`, never in core.
+5. Every error path ships both tests: typed rejection + state transition.
+6. `AbortError` is cancellation, not failure — never mapped or stored.
diff --git a/packages/tsunami/src/ClientResourceScope.ts b/packages/tsunami/src/ClientResourceScope.ts
index a65288aa..8d7624bb 100644
--- a/packages/tsunami/src/ClientResourceScope.ts
+++ b/packages/tsunami/src/ClientResourceScope.ts
@@ -1,4 +1,4 @@
-import { ClientDisposedError } from "./errors";
+import { ClientDisposedError } from "./errors/lifecycleErrors";
 
 type Cleanup = () => void;
 
diff --git a/packages/tsunami/src/FishjamClient.test.ts b/packages/tsunami/src/FishjamClient.test.ts
index af791373..f6edf2ad 100644
--- a/packages/tsunami/src/FishjamClient.test.ts
+++ b/packages/tsunami/src/FishjamClient.test.ts
@@ -2,7 +2,7 @@ import { type ConnectConfig, FishjamClient as TsClient, type GenericMetadata } f
 import { afterEach, beforeEach, describe, expect, it, vi } from "vitest";
 
 import { ClientResourceScope } from "./ClientResourceScope";
-import { ClientDisposedError } from "./errors";
+import { ClientDisposedError } from "./errors/lifecycleErrors";
 import { FishjamClient } from "./FishjamClient";
 
 const mediaStreamConstructor = vi.fn();
diff --git a/packages/tsunami/src/errors.ts b/packages/tsunami/src/errors.ts
deleted file mode 100644
index c1ae586e..00000000
--- a/packages/tsunami/src/errors.ts
+++ /dev/null
@@ -1,10 +0,0 @@
-/**
- * Thrown when an operation is attempted on a client that has reached the end
- * of its lifecycle.
- */
-export class ClientDisposedError extends Error {
-  public constructor() {
-    super("FishjamClient has been disposed and cannot be used again");
-    this.name = "ClientDisposedError";
-  }
-}
diff --git a/packages/tsunami/src/errors/FishjamError.ts b/packages/tsunami/src/errors/FishjamError.ts
new file mode 100644
index 00000000..7fb693a8
--- /dev/null
+++ b/packages/tsunami/src/errors/FishjamError.ts
@@ -0,0 +1,24 @@
+/** Possible values of {@link FishjamError.recoverability}. */
+export type ErrorRecoverability = "retry" | "user_action" | "fatal";
+
+/**
+ * Base class of every error thrown by the SDK.
+ *
+ *
+ * When the SDK wraps a platform failure (for example a `DOMException` from the
+ * browser), the original error is available on `cause`.
+ */
+export abstract class FishjamError extends Error {
+  /**
+   * Hints how your application can respond to this error:
+   * - `"retry"` — the same operation may succeed if attempted again.
+   * - `"user_action"` — ask the user to fix something first, for example grant
+   *   a permission, free up the device, or sign in again.
+   * - `"fatal"` — the operation cannot succeed; do not retry.
+   */
+  public abstract readonly recoverability: ErrorRecoverability;
+
+  protected constructor(message: string, options?: { cause?: unknown }) {
+    super(message, options);
+  }
+}
diff --git a/packages/tsunami/src/errors/lifecycleErrors.test.ts b/packages/tsunami/src/errors/lifecycleErrors.test.ts
new file mode 100644
index 00000000..58ad1c39
--- /dev/null
+++ b/packages/tsunami/src/errors/lifecycleErrors.test.ts
@@ -0,0 +1,18 @@
+import { describe, expect, it } from "vitest";
+
+import { FishjamError } from "./FishjamError";
+import { ClientDisposedError } from "./lifecycleErrors";
+
+describe("ClientDisposedError", () => {
+  it("keeps the message the lifecycle machinery and its tests rely on", () => {
+    expect(new ClientDisposedError().message).toBe("FishjamClient has been disposed and cannot be used again");
+  });
+
+  it("is a fatal FishjamError with a stable name", () => {
+    const error = new ClientDisposedError();
+
+    expect(error).toBeInstanceOf(FishjamError);
+    expect(error.name).toBe("ClientDisposedError");
+    expect(error.recoverability).toBe("fatal");
+  });
+});
diff --git a/packages/tsunami/src/errors/lifecycleErrors.ts b/packages/tsunami/src/errors/lifecycleErrors.ts
new file mode 100644
index 00000000..0842fffd
--- /dev/null
+++ b/packages/tsunami/src/errors/lifecycleErrors.ts
@@ -0,0 +1,17 @@
+import { type ErrorRecoverability, FishjamError } from "./FishjamError";
+
+/**
+ * Thrown when a method is called on a client after `dispose()`.
+ *
+ * A disposed client is permanently unusable — create a new `FishjamClient`
+ * instance instead of retrying. You may also see this error as the rejection
+ * of an operation that was still in flight when the client was disposed.
+ */
+export class ClientDisposedError extends FishjamError {
+  public readonly recoverability: ErrorRecoverability = "fatal";
+
+  public constructor() {
+    super("FishjamClient has been disposed and cannot be used again");
+    this.name = "ClientDisposedError";
+  }
+}
diff --git a/packages/tsunami/src/index.ts b/packages/tsunami/src/index.ts
index 1b258723..d5222e1d 100644
--- a/packages/tsunami/src/index.ts
+++ b/packages/tsunami/src/index.ts
@@ -6,7 +6,8 @@
 export type { DeviceItem, DeviceType, IDeviceManager, IDevicePersistence } from "./devices/deviceManager";
 export { LocalStorageDevicePersistence } from "./devices/LocalStorageDevicePersistence";
 export { WebDeviceManager, type WebDeviceManagerOptions } from "./devices/WebDeviceManager";
-export { ClientDisposedError } from "./errors";
+export { type ErrorRecoverability, FishjamError } from "./errors/FishjamError";
+export { ClientDisposedError } from "./errors/lifecycleErrors";
 export { FishjamClient } from "./FishjamClient";
 export * from "@fishjam-cloud/ts-client";
 
diff --git a/tsconfig.base.json b/tsconfig.base.json
index 0fb4e06b..8f474196 100644
--- a/tsconfig.base.json
+++ b/tsconfig.base.json
@@ -11,7 +11,7 @@
     "skipLibCheck": true,
     "strict": true,
     "target": "ESNext",
-    "lib": ["es2020", "DOM", "DOM.Iterable"],
+    "lib": ["es2022", "DOM", "DOM.Iterable"],
     "jsx": "react-jsx"
   }
 }