API Reference

Complete API documentation for WS-Kit.

Core Exports

@ws-kit/zod

import { z, message, rpc, createRouter } from "@ws-kit/zod";

• z - Re-exported Zod instance (canonical import source)
• message() - Create type-safe message schemas
• rpc() - Create request-response (RPC) schemas
• createRouter() - Create a type-safe WebSocket router

@ws-kit/valibot

import { v, message, rpc, createRouter } from "@ws-kit/valibot";

• v - Re-exported Valibot instance (canonical import source)
• message() - Create type-safe message schemas
• rpc() - Create request-response (RPC) schemas
• createRouter() - Create a type-safe WebSocket router

@ws-kit/client/zod and @ws-kit/client/valibot

import { wsClient } from "@ws-kit/client/zod";
// or
import { wsClient } from "@ws-kit/client/valibot";

• wsClient() - Create a type-safe WebSocket client

@ws-kit/bun

import { serve, createBunHandler } from "@ws-kit/bun";

• serve() - High-level server function
• createBunHandler() - Low-level handler for custom setups

Platform & Pub/Sub Adapters

WS-Kit provides pluggable adapters for different deployment platforms and pub/sub implementations.

Built-in adapters:

// Memory adapter (default, included in @ws-kit/core)
import { MemoryPubSub } from "@ws-kit/core";

// Bun platform adapter (included in @ws-kit/bun)
import { createBunHandler } from "@ws-kit/bun";

// Cloudflare Durable Objects adapter
import { createDurableObjectHandler } from "@ws-kit/cloudflare";

Optional add-ons:

// Redis rate limiter and pub/sub for distributed deployments
import { redisPubSub, redisRateLimiter } from "@ws-kit/redis";

// In-memory pub/sub for local deployments
import { memoryPubSub } from "@ws-kit/memory";

// Middleware helpers for composing adapters and handlers
import {} from /* middleware utilities */ "@ws-kit/middleware";

For complete adapter documentation including configuration, guarantees, and platform-specific limits, see Adapter Responsibilities.

---

Schema Creation

message()

Create a type-safe message schema. Supports both fire-and-forget messages and RPC-style request-response patterns.

Signatures:

// Simple message (no payload)
function message<T extends string>(type: T): MessageSchema<T>;

// With payload (Zod object or raw shape)
function message<
  T extends string,
  P extends ZodObject<ZodRawShape> | ZodRawShape,
>(type: T, payload: P): MessageSchema<T, P>;

// With payload and custom metadata
function message<
  T extends string,
  P extends ZodObject<ZodRawShape> | ZodRawShape,
  M extends ZodRawShape,
>(type: T, payload: P, meta: M): MessageSchema<T, P, M>;

// Unified API with config object (payload, response, and/or meta)
function message<T extends string>(
  type: T,
  config: { payload?: P; response?: R; meta?: M },
): MessageSchema<T>;

Parameters:

• type - Unique message type identifier (string literal). Must not start with $ws: (reserved for internal control messages).
• payload - Zod object, raw shape, or config object for payload validation (optional)
• meta - Custom metadata schema fields (optional, cannot use reserved keys: clientId, receivedAt)

Returns: MessageSchema with full TypeScript inference

Strict Validation: All message schemas automatically reject unknown keys at root, meta, and payload levels. This prevents security issues and ensures wire format cleanliness.

Reserved Message Type Prefix: Message types starting with $ws: are reserved for internal control messages and will throw an error at schema creation time:

// ❌ Throws: Reserved prefix "$ws:" not allowed
const BadMessage = message("$ws:custom", { data: z.string() });

Examples:

import { z, message } from "@ws-kit/zod";

// Simple message (no payload)
const PingMessage = message("PING");

// With payload (raw shape)
const ChatMessage = message("CHAT_MESSAGE", {
  text: z.string(),
  roomId: z.string(),
});

// With Zod object
const ChatMessage = message(
  "CHAT_MESSAGE",
  z.object({
    text: z.string(),
    roomId: z.string(),
  }),
);

// With custom metadata
const TrackedMessage = message(
  "TRACKED_ACTION",
  { action: z.string() },
  { traceId: z.string() },
);

// ✅ RECOMMENDED: Unified config-based syntax (RPC/request-response)
const GetUser = message("GET_USER", {
  payload: { id: z.string() },
  response: { user: UserSchema },
});

// RPC with metadata
const TrackedAction = message("TRACKED_ACTION", {
  payload: { action: z.string() },
  response: { result: z.string() },
  meta: { traceId: z.string() },
});

// RPC with response only (no request payload)
const GetTime = message("GET_TIME", {
  response: { timestamp: z.number() },
});

Reserved Meta Keys: Cannot use clientId or receivedAt in custom metadata - these are server-controlled fields injected after validation.

rpc()

Create a request-response (RPC) schema that binds request and response types.

Note: The rpc() function is maintained for backward compatibility. For new code, prefer the unified message() API with config object (see examples above). Both approaches work identically at runtime.

Signature:

function rpc<
  ReqT extends string,
  ReqP extends ZodType | ValibotSchema | undefined,
  ResT extends string,
  ResP extends ZodType | ValibotSchema | undefined,
>(
  requestType: ReqT,
  requestPayload: ReqP,
  responseType: ResT,
  responsePayload: ResP,
): RpcSchema<ReqT, ReqP, ResT, ResP>;

Reserved Prefix Validation: Both message() and rpc() will throw an error at schema creation time if the message type starts with $ws:, as this prefix is reserved for internal control messages.

Parameters:

• requestType - Message type for the request
• requestPayload - Validation schema for request payload (use undefined for no payload)
• responseType - Message type for the response
• responsePayload - Validation schema for response payload (use undefined for no payload)

Returns: RPC schema with .response property for type inference

Examples:

import { z, rpc } from "@ws-kit/zod";

// Simple RPC
const Ping = rpc("PING", { text: z.string() }, "PONG", { reply: z.string() });

// RPC with no payloads
const Heartbeat = rpc("HEARTBEAT", undefined, "HEARTBEAT_ACK", undefined);

// Complex RPC
const GetUser = rpc("GET_USER", { userId: z.string() }, "USER_RESPONSE", {
  user: UserSchema,
});

---

Router API

createRouter()

Create a type-safe WebSocket router.

Signature:

function createRouter<TData extends WebSocketData = WebSocketData>(
  options?: WebSocketRouterOptions<ZodAdapter, TData>,
): WebSocketRouter<ZodAdapter, TData>;

Type Parameters:

• TData - Custom connection data type (extends { clientId: string })

Options:

interface WebSocketRouterOptions<V, TData> {
  // Core adapters
  validator?: V; // Validator adapter (auto-configured by createRouter)
  platform?: PlatformAdapter; // Platform adapter (optional)
  pubsub?: PubSub; // PubSub implementation (lazy: MemoryPubSub on first use)

  // Lifecycle hooks
  hooks?: {
    onOpen?: OpenHandler<TData>;
    onClose?: CloseHandler<TData>;
    onAuth?: AuthHandler<TData>;
    onError?: ErrorHandler<TData>;
  };

  // Heartbeat configuration (opt-in — NOT initialized by default)
  // Only enable when explicitly configured in options.heartbeat
  heartbeat?: {
    intervalMs?: number; // Default: 30000 (milliseconds between heartbeats)
    timeoutMs?: number; // Default: 5000 (milliseconds to wait for heartbeat response)
    onStaleConnection?: (clientId: string, ws: ServerWebSocket<TData>) => void;
  };

  // Limits and configuration
  limits?: {
    maxPayloadBytes?: number; // Default: 1000000 (1MB)
  };
  socketBufferLimitBytes?: number; // Default: 1000000 (backpressure threshold)
  rpcTimeoutMs?: number; // Default: 30000
  dropProgressOnBackpressure?: boolean; // Default: true
  maxInflightRpcsPerSocket?: number; // Default: 1000
  rpcIdleTimeoutMs?: number; // Default: rpcTimeoutMs + 10000

  // Error handling
  autoSendErrorOnThrow?: boolean; // Default: true
  exposeErrorDetails?: boolean; // Default: false
  warnIncompleteRpc?: boolean; // Default: true (dev mode only)
}

Example:

import { createRouter } from "@ws-kit/zod";

type AppData = { userId?: string; roles?: string[] };

const router = createRouter<AppData>({
  heartbeat: {
    intervalMs: 30000,
    timeoutMs: 5000,
  },
  limits: {
    maxPayloadBytes: 1_000_000,
  },
  autoSendErrorOnThrow: true,
  exposeErrorDetails: false,
});

Router Methods

on(schema, handler)

Register a handler for fire-and-forget messages or pub/sub events.

on<Schema extends MessageSchemaType>(
  schema: Schema,
  handler: EventHandler<Schema, TData>
): this;

Parameters:

• schema - Message schema created with message()
• handler - Event handler function (receives EventMessageContext)

Returns: Router instance for chaining

Example:

router.on(ChatMessage, (ctx) => {
  // ctx.payload is fully typed
  console.log(`Message from ${ctx.data.userId}: ${ctx.payload.text}`);

  // Publish to room subscribers
  ctx.publish(`room:${ctx.payload.roomId}`, ChatMessage, ctx.payload);
});

rpc(schema, handler)

Register a handler for request-response (RPC) messages.

rpc<Schema extends MessageSchemaType>(
  schema: Schema,
  handler: RpcHandler<Schema, TData>
): this;

IMPORTANT: The schema must have a .response field to be registered with router.rpc(). Use the rpc() helper function or message() with config object to create RPC schemas. For fire-and-forget messaging without a response, use router.on() instead.

Parameters:

• schema - RPC message schema created with rpc() or message() with response field
• handler - RPC handler function (receives RpcMessageContext)

Returns: Router instance for chaining

Throws: Error if schema does not have a .response field

Example:

// Using config-based message (recommended)
const GetUser = message("GET_USER", {
  payload: { userId: z.string() },
  response: { user: UserSchema },
});

// Using legacy rpc() helper (still supported)
const GetUser = rpc("GET_USER", { userId: z.string() }, "USER_RESPONSE", {
  user: UserSchema,
});

router.rpc(GetUser, async (ctx) => {
  const user = await db.users.findById(ctx.payload.userId);

  if (!user) {
    ctx.error("NOT_FOUND", "User not found");
    return;
  }

  // Terminal response (one-shot, correlation tracked)
  ctx.reply(GetUser.response, { user });
});

off(schema)

Unregister a handler for a specific message type.

off<Schema extends MessageSchemaType>(schema: Schema): this;

Parameters:

• schema - Message schema to unregister

Returns: Router instance for chaining

use(middleware) — Global Middleware

Register middleware that runs for all messages.

use(middleware: Middleware<TContext>): this;

Parameters:

• middleware - Middleware function with signature (ctx, next) => Promise<void>

Returns: Router instance for chaining

Context Type:

Middleware receives MinimalContext<TContext>:

type Middleware<TContext extends ConnectionData = ConnectionData> = (
  ctx: MinimalContext<TContext>,
  next: () => Promise<void>,
) => Promise<void>;

Where MinimalContext<TContext> includes only connection data (ctx.data, ctx.type, ctx.error(), etc.), not the message payload.

Examples:

// Global authentication middleware
router.use((ctx, next) => {
  if (!ctx.data?.userId && ctx.type !== "LOGIN") {
    ctx.error("UNAUTHENTICATED", "Not authenticated");
    return;
  }
  return next();
});

route(schema) - Per-Route Middleware Registration

Register per-route middleware using the builder pattern.

route<S extends MessageSchema>(schema: S): RouteBuilder<S, TContext>;

Returns a RouteBuilder for fluent registration:

interface RouteBuilder<
  S extends MessageSchema,
  TContext extends ConnectionData = ConnectionData,
> {
  use(middleware: Middleware<TContext>): this;
  on(handler: MessageHandler<S, TContext>): this;
}

Examples:

// Per-route rate limiting
router
  .route(SendMessage)
  .use((ctx, next) => {
    if (isRateLimited(ctx.data?.userId)) {
      ctx.error("RESOURCE_EXHAUSTED", "Too many messages");
      return;
    }
    return next();
  })
  .on((ctx) => {
    // Handle message
  });

onOpen(handler), onClose(handler), onAuth(handler), onError(handler)

Register lifecycle hooks.

onOpen(handler: OpenHandler<TData>): this;
onClose(handler: CloseHandler<TData>): this;
onAuth(handler: AuthHandler<TData>): this;
onError(handler: ErrorHandler<TData>): this;

Handler Signatures:

type OpenHandler<TData> = (
  ctx: OpenHandlerContext<TData>,
) => void | Promise<void>;
type CloseHandler<TData> = (
  ctx: CloseHandlerContext<TData>,
) => void | Promise<void>;
type AuthHandler<TData> = (
  ctx: MessageContext<any, TData>,
) => boolean | Promise<boolean>;
type ErrorHandler<TData> = (
  error: WsKitError,
  ctx: MessageContext<any, TData>,
) => boolean | void;

Examples:

router.onOpen((ctx) => {
  console.log(`Client ${ctx.clientId} connected`);
  ctx.send(WelcomeMessage, { text: "Welcome!" });
});

router.onClose((ctx) => {
  console.log(`Client ${ctx.clientId} disconnected: ${ctx.code}`);
});

router.onAuth((ctx) => {
  // Return false to reject connection
  return ctx.data?.userId !== undefined;
});

router.onError((error, ctx) => {
  console.error("Error:", error.code, error.message, error.details);
  // Return false to suppress automatic error response
  return false; // Suppress auto-send
});

onLimitExceeded(handler)

Register a handler for limit exceeded events (payload size, rate limiting, etc.).

onLimitExceeded(handler: LimitExceededHandler<TData>): this;

Handler Signature:

type LimitExceededHandler<TData> = (
  info: LimitExceededInfo<TData>,
) => void | Promise<void>;

type LimitExceededInfo<TData> =
  | {
      type: "payload";
      clientId: string;
      ws: ServerWebSocket<TData>;
      observed: number; // Actual payload size (bytes)
      limit: number; // Configured max payload size
    }
  | {
      type: "rate";
      clientId: string;
      ws: ServerWebSocket<TData>;
      observed: number; // Attempted cost (tokens)
      limit: number; // Rate limit capacity
      retryAfterMs: number | null; // null = impossible (cost > capacity)
    };

Returns: Router instance for chaining

Example:

router.onLimitExceeded((info) => {
  if (info.type === "payload") {
    console.warn(
      `Client ${info.clientId} exceeded payload limit: ` +
        `${info.observed} > ${info.limit} bytes`,
    );
  } else if (info.type === "rate") {
    console.warn(
      `Client ${info.clientId} exceeded rate limit ` +
        `(attempted: ${info.observed}, capacity: ${info.limit})`,
    );
  }
});

merge(router)

Merge routes from another router.

merge(router: WebSocketRouter<V, TData>): this;

IMPORTANT: Merging routers with different validators (e.g., Zod + Valibot) is type-safe but will cause runtime validation errors. Use createRouter() from the same package (@ws-kit/zod or @ws-kit/valibot) for all routers in a merge chain to avoid inconsistent validation.

Parameters:

• router - Another router to merge (must use same validator)

Returns: Router instance for chaining

Example:

import { createRouter } from "@ws-kit/zod"; // Use same package

const authRouter = createRouter();
authRouter.on(LoginMessage, handleLogin);

const chatRouter = createRouter();
chatRouter.on(MessageMessage, handleMessage);

const mainRouter = createRouter().merge(authRouter).merge(chatRouter);

publish(topic, schema, payload, options?)

Publish a typed message to a topic (broadcasts to all subscribers).

publish(
  topic: string,
  schema: MessageSchemaType,
  payload: unknown,
  options?: PublishOptions
): Promise<PublishResult>;

Parameters:

• topic - Topic name (e.g., "room:123", "user:456")
• schema - Message schema for validation
• payload - Message payload (validated against schema)
• options - Publish options (optional)

Options:

interface PublishOptions {
  excludeSelf?: boolean; // Throws error if true (not yet implemented)
  partitionKey?: string; // For sharding (future use)
}

Note on excludeSelf: This option will raise an error if set to true. The feature is not yet implemented and requires pubsub adapter support. Use workarounds like dedicated topics per connection or checking message origin in subscriber handlers. See the PublishOptions type definition for details.

Returns: Promise<PublishResult> with subscriber match count and capability info

Example:

// Inside a handler
router.on(UserCreated, async (ctx) => {
  const result = await ctx.publish(
    `org:${ctx.payload.orgId}:users`,
    UserListInvalidated,
    { orgId: ctx.payload.orgId },
  );
  if (result.ok && result.matchedLocal !== undefined) {
    console.log(`Notified ${result.matchedLocal} subscribers`);
  }
});

// Outside handlers (cron, queue, lifecycle)
const result = await router.publish("system:announcements", Announcement, {
  text: "Server maintenance at 02:00 UTC",
});
if (result.ok) {
  console.log("Announcement published");
}

reset()

Clear all registered handlers, middleware, and state.

reset(): this;

Returns: Router instance for chaining

Note: Useful for resetting the router in tests without creating a new instance. Preserves configuration (validator, platform, limits). Does NOT reset:

• Heartbeat states for active connections
• RPC state (managed by RpcManager, survives router reset)

---

Context API

Context objects are passed to message handlers and provide type-safe access to the WebSocket connection, payload, and various utility methods.

Event Message Context

Context for fire-and-forget messages (via router.on()).

interface EventMessageContext<TSchema, TData> {
  // Connection and message info
  ws: ServerWebSocket<TData>; // WebSocket connection
  type: string; // Message type
  payload?: unknown; // Validated payload (only if schema defines it)
  meta: MessageMeta; // Message metadata
  receivedAt: number; // Server receive timestamp (ms)

  // Type information
  isRpc: false; // Always false for events
  timeRemaining(): number; // Returns Infinity for events

  // Sending messages
  send: SendFunction; // Send message to this connection
  error(code: string, message: string, details?: Record<string, unknown>): void;

  // Connection data management
  assignData(partial: Partial<TData>): void;

  // Pub/sub
  subscribe(topic: string): void;
  unsubscribe(topic: string): void;
  publish(
    topic: string,
    schema: MessageSchemaType,
    payload: unknown,
    options?: PublishOptions,
  ): Promise<PublishResult>;
}

Note: For messages without a payload (e.g., message("PING")), ctx.payload will be undefined.

RPC Message Context (RPC-Only Contexts)

Context for request-response messages (via router.rpc()).

interface RpcMessageContext<TSchema, TData> {
  // Connection and message info
  ws: ServerWebSocket<TData>; // WebSocket connection
  type: string; // Message type
  payload?: unknown; // Validated payload (only if schema defines it)
  meta: MessageMeta; // Message metadata (includes correlationId)
  receivedAt: number; // Server receive timestamp (ms)

  // Type information
  isRpc: true; // Always true for RPC
  deadline: number; // Request deadline (ms since epoch)
  timeRemaining(): number; // Milliseconds until deadline

  // Sending messages
  send: SendFunction; // Send side-effect messages
  error(code: string, message: string, details?: Record<string, unknown>): void;

  // RPC-specific methods
  reply(
    responseSchema: MessageSchemaType,
    data: unknown,
    options?: Record<string, unknown>,
  ): void;
  progress(data?: unknown): void;
  onCancel(cb: () => void): () => void;
  abortSignal: AbortSignal;

  // Connection data management
  assignData(partial: Partial<TData>): void;

  // Pub/sub
  subscribe(topic: string): void;
  unsubscribe(topic: string): void;
  publish(
    topic: string,
    schema: MessageSchemaType,
    payload: unknown,
    options?: PublishOptions,
  ): Promise<PublishResult>;
}

RPC Requirements:

• Must call ctx.reply() or ctx.error() for terminal response
• Can call ctx.progress() multiple times before terminal response
• correlationId is auto-synthesized if missing (belt-and-suspenders approach)
• One-shot guarantee: multiple reply() calls are guarded (only first succeeds)

Context Methods

ctx.send(schema, payload, options?)

Send a validated message to the current client.

send(schema: MessageSchemaType, payload: unknown, options?: SendOptions): void;

Parameters:

• schema - Message schema
• payload - Message payload (validated against schema)
• options - Optional metadata (timestamp auto-added, can include correlationId)

Auto-injected metadata:

• timestamp - Producer timestamp (milliseconds since epoch)
• Does NOT inject clientId or receivedAt (outbound messages use client time)

Example:

router.on(PingMessage, (ctx) => {
  ctx.send(PongMessage, { reply: "pong" });
});

Note: For RPC handlers, use ctx.reply() for terminal responses instead of ctx.send() to ensure one-shot guarantee and proper correlation tracking.

ctx.error(code, message?, details?, options?)

Send a type-safe error response with optional retry semantics.

error(
  code: string | ErrorCode,
  message?: string,
  details?: Record<string, unknown>,
  options?: {
    retryable?: boolean;    // Override auto-inferred retry behavior
    retryAfterMs?: number;  // Backoff hint for transient errors
  }
): void;

Parameters:

• code - Standard error code (see Error Codes section). Auto-infers retry semantics unless explicitly overridden.
• message - Optional human-readable error description
• details - Optional error context. Must not contain sensitive data; server sanitizes before sending to clients.
• options - Optional retry semantics (overrides code-based inference):
  • retryable - Explicitly set whether error is retryable (optional; client infers from code if omitted)
  • retryAfterMs - Backoff interval hint for transient errors (only valid for retryable errors)

Examples:

Non-retryable error with context details:

router.on(JoinRoom, (ctx) => {
  if (!roomExists(ctx.payload.roomId)) {
    // Client will auto-infer NOT_FOUND as non-retryable
    ctx.error("NOT_FOUND", "Room not found", { roomId: ctx.payload.roomId });
    return;
  }
});

Terminal error (explicit non-retryable):

router.on(DeleteResource, (ctx) => {
  if (!userOwnsResource(ctx)) {
    // Explicit retryable: false (though inferred anyway)
    ctx.error("PERMISSION_DENIED", "You do not own this resource", undefined, {
      retryable: false,
    });
    return;
  }
});

Transient error with backoff hint:

router.on(ProcessPayment, (ctx) => {
  try {
    processPayment(ctx.payload);
  } catch (err) {
    if (isRateLimited(err)) {
      // Client infers RESOURCE_EXHAUSTED as retryable, uses retryAfterMs
      ctx.error("RESOURCE_EXHAUSTED", "Rate limited", undefined, {
        retryAfterMs: 5000, // Client will wait 5s before retrying
      });
    } else if (isTemporaryFailure(err)) {
      // Default: infer retryable from code
      ctx.error("UNAVAILABLE", "Processing service unavailable");
    } else {
      ctx.error("INTERNAL", "Payment processing failed");
    }
  }
});

ctx.reply(responseSchema, data, options?) (RPC only)

Send a terminal reply for an RPC request.

reply(responseSchema: MessageSchemaType, data: unknown, options?: Record<string, unknown>): void;

Parameters:

• responseSchema - Response message schema
• data - Response data (validated)
• options - Optional metadata

Example:

router.rpc(GetUser, async (ctx) => {
  const user = await db.users.findById(ctx.payload.userId);
  ctx.reply(GetUser.response, { user });
});

ctx.progress(data?) (RPC only)

Send a non-terminal progress update.

progress(data?: unknown): void;

Parameters:

• data - Optional progress data

Example:

router.rpc(LongQuery, async (ctx) => {
  for (const batch of largeBatches) {
    ctx.progress({ processed: batch.count, total: largeBatches.length });
    await processBatch(batch);
  }
  ctx.reply(LongQuery.response, { result: finalResult });
});

ctx.onCancel(callback) (RPC only)

Register a callback for when the RPC is cancelled.

onCancel(cb: () => void): () => void;

Parameters:

• cb - Callback invoked on cancellation (client abort or disconnect)

Returns: Unregister function

Example:

router.rpc(LongOperation, async (ctx) => {
  const timer = setTimeout(() => doWork(), 1000);

  ctx.onCancel(() => {
    clearTimeout(timer);
    console.log("Operation cancelled");
  });

  const result = await doWork();
  ctx.reply(LongOperation.response, { result });
});

ctx.abortSignal (RPC only)

Standard AbortSignal that fires when the RPC is cancelled.

abortSignal: AbortSignal;

Example:

router.rpc(FetchData, async (ctx) => {
  const response = await fetch(url, { signal: ctx.abortSignal });
  const data = await response.json();
  ctx.reply(FetchData.response, { data });
});

ctx.topics.subscribe(topic) and ctx.topics.unsubscribe(topic)

Subscribe/unsubscribe to pub/sub topics.

subscribe(topic: string): Promise<void>;
unsubscribe(topic: string): Promise<void>;

Example:

router.on(JoinRoom, async (ctx) => {
  await ctx.topics.subscribe(`room:${ctx.payload.roomId}`);
  ctx.data.currentRoom = ctx.payload.roomId;
});

router.on(LeaveRoom, async (ctx) => {
  await ctx.topics.unsubscribe(`room:${ctx.data.currentRoom}`);
});

ctx.publish(topic, schema, payload, options?)

Publish a message to a topic (convenience method, delegates to router.publish()).

publish(topic: string, schema: MessageSchemaType, payload: unknown, options?: PublishOptions): Promise<PublishResult>;

Example:

router.on(SendMessage, async (ctx) => {
  const result = await ctx.publish(`room:${ctx.payload.roomId}`, ChatMessage, {
    text: ctx.payload.text,
    roomId: ctx.payload.roomId,
  });
  if (result.ok) {
    console.log(`Published to ${result.matchedLocal ?? "?"} subscribers`);
  }
});

ctx.getData(key)

Access connection data fields with type safety.

getData<K extends keyof TData>(key: K): TData[K];

Parameters:

• key - Property name from your custom connection data type

Returns: Value of the property, or undefined if not set

Example:

type AppData = { userId?: string; roles?: string[] };
const router = createRouter<AppData>();

router.on(SecureMessage, (ctx) => {
  const userId = ctx.getData("userId"); // Type: string | undefined
  const roles = ctx.getData("roles"); // Type: string[] | undefined

  if (!userId) {
    ctx.error("UNAUTHENTICATED", "Not authenticated");
    return;
  }
});

ctx.assignData(partial)

Merge partial data into connection data.

assignData(partial: Partial<TData>): void;

Example:

router.on(LoginMessage, (ctx) => {
  const user = authenticate(ctx.payload);
  ctx.assignData({ userId: user.id, roles: user.roles });
});

---

Client API

wsClient()

Create a type-safe WebSocket client.

Signature:

function wsClient<TRouter = unknown>(options: ClientOptions): WebSocketClient;

Options:

interface ClientOptions {
  url: string | URL; // WebSocket URL
  protocols?: string | string[]; // WebSocket subprotocols

  // Reconnection
  reconnect?: {
    enabled?: boolean; // Default: true
    maxAttempts?: number; // Default: Infinity
    initialDelayMs?: number; // Default: 300
    maxDelayMs?: number; // Default: 10000
    jitter?: "full" | "none"; // Default: "full" (randomize delay to prevent thundering herd)
  };

  // Queue configuration
  queue?: "drop-oldest" | "drop-newest" | "off"; // Default: "drop-newest"
  queueSize?: number; // Default: 1000 (pending messages while offline)

  // Behavior
  autoConnect?: boolean; // Default: false
  pendingRequestsLimit?: number; // Default: 1000

  // Authentication
  auth?: {
    attach?: "query" | "protocol"; // Default: "query"
    queryParam?: string; // Default: "access_token"
    protocolPrefix?: string; // Default: "bearer."
    protocolPosition?: "prepend" | "append"; // Default: "append"
    getToken?: () => string | Promise<string> | null | Promise<null>;
  };

  // Advanced
  wsFactory?: (url: string | URL, protocols?: string | string[]) => WebSocket; // For dependency injection in tests
}

Returns: WebSocketClient instance

Example:

import { wsClient } from "@ws-kit/client/zod";

const client = wsClient({
  url: "ws://localhost:3000",
  reconnect: {
    enabled: true,
    maxAttempts: 10,
  },
  auth: {
    attach: "query",
    getToken: () => localStorage.getItem("token"),
  },
});

Client Methods

client.connect()

Manually connect to the server.

connect(): Promise<void>;

Returns: Promise that resolves when connected

client.close(options?)

Close the connection.

close(options?: { code?: number; reason?: string }): Promise<void>;

Parameters:

• options.code - WebSocket close code (default: 1000)
• options.reason - Close reason string

Returns: Promise that resolves when closed

client.on(schema, handler)

Register a message handler.

on<S extends MessageSchema>(schema: S, handler: (payload: InferPayload<S>) => void): () => void;

Parameters:

• schema - Message schema
• handler - Message handler function

Returns: Unregister function

Example:

const RoomUpdated = message("ROOM_UPDATED", {
  roomId: z.string(),
  users: z.number(),
});

client.on(RoomUpdated, (payload) => {
  console.log(`Room ${payload.roomId} has ${payload.users} users`);
});

client.send(schema, payload, options?)

Send a fire-and-forget message.

send<S extends MessageSchema>(
  schema: S,
  payload: InferPayload<S>,
  options?: { meta?: Record<string, unknown>; correlationId?: string }
): boolean;

Parameters:

• schema - Message schema
• payload - Message payload
• options - Optional metadata

Returns: true if sent immediately, false if queued

Example:

const success = client.send(ChatMessage, {
  text: "Hello!",
  roomId: "general",
});

client.request(schema, payload, options?)

Send a request and wait for response (RPC).

RPC-style overload (recommended):

request<S extends RpcSchema>(
  schema: S,
  payload?: InferPayload<S>,
  options?: RequestOptions
): Promise<InferMessage<S["response"]>>;

Traditional style overload (explicit response schema):

request<S extends MessageSchema, R extends MessageSchema>(
  schema: S,
  payload: InferPayload<S>,
  reply: R,
  options?: RequestOptions
): Promise<InferMessage<R>>;

Options:

interface RequestOptions {
  timeoutMs?: number; // Default: 30000
  meta?: Record<string, unknown>; // Custom metadata (correlationId auto-generated if not provided)
  correlationId?: string; // Explicit correlation ID (auto-generated if not provided)
  signal?: AbortSignal; // Cancellation signal
}

Returns: Promise resolving to the full response message object {type, meta, payload} with the response type and validated payload

Behavior:

• Auto-generates correlationId if not provided
• Strips reserved meta keys (clientId, receivedAt) from user meta
• Auto-connects if autoConnect: true and never attempted
• Queues request if disconnected (unless queue: "off")
• Rejects with ValidationError if outbound validation fails
• Rejects with timeout error if no response within timeoutMs

Examples:

RPC-style with auto-detected response (recommended):

const GetUser = message("GET_USER", {
  payload: { id: z.string() },
  response: { user: UserSchema },
});

const response = await client.request(GetUser, { id: "123" });
// response.payload is fully typed: { user: UserSchema }
console.log(response.payload.user.name);

// Or using legacy rpc() helper
const Ping = rpc("PING", { text: z.string() }, "PONG", { reply: z.string() });
const response = await client.request(Ping, { text: "hello" });
console.log(response.payload.reply); // Fully typed

Traditional style with explicit response schema:

const response = await client.request(
  PingMessage,
  { text: "hello" },
  PongMessage,
  { timeoutMs: 5000 },
);
console.log(response.payload.reply);

With custom metadata and auto-correlation:

const response = await client.request(
  GetUser,
  { id: "123" },
  {
    timeoutMs: 10000,
    meta: { source: "ui" },
    // correlationId auto-generated if not provided
  },
);

With AbortSignal for cancellation:

const controller = new AbortController();
const promise = client.request(
  GetUser,
  { id: "123" },
  {
    signal: controller.signal,
  },
);
setTimeout(() => controller.abort(), 1000); // Cancel after 1s

client.onState(callback)

Register a state change listener.

onState(callback: (state: ClientState) => void): () => void;

States:

• "closed" - Connection is closed (initial state)
• "connecting" - Connection attempt in progress
• "open" - Connected and ready
• "closing" - Close initiated, waiting for close event
• "reconnecting" - Reconnect attempt scheduled or in progress

Returns: Unregister function

client.onceOpen()

Wait for the next "open" state.

onceOpen(): Promise<void>;

Returns: Promise that resolves when connected

client.onUnhandled(callback)

Register a handler for unhandled messages.

onUnhandled(callback: (msg: AnyInboundMessage) => void): () => void;

Returns: Unregister function

client.onError(callback)

Register an error handler.

onError(callback: (error: Error, context: ErrorContext) => void): () => void;

Error Context:

interface ErrorContext {
  type: "parse" | "validation" | "overflow" | "unknown";
  details?: unknown;
}

Returns: Unregister function

Client Properties

client.state: ClientState;           // Current connection state
client.isConnected: boolean;         // True if state === "open"
client.protocol: string;             // Selected WebSocket subprotocol

---

Platform Adapters

Bun Adapter

serve(router, options)

High-level server function for Bun.

function serve<TData extends { clientId: string }>(
  router: WebSocketRouter<any, TData>,
  options?: ServeOptions<TData>,
): Promise<void>;

Options:

interface ServeOptions<TData> {
  port?: number; // Default: 3000
  authenticate?: (
    req: Request,
  ) => TData | Promise<TData> | undefined | Promise<undefined>;
  onError?: (error: Error, ctx: MessageContext) => void;
  onBroadcast?: (message: unknown, topic: string) => void;
  onUpgrade?: (req: Request, ws: ServerWebSocket<TData>) => void;
  onOpen?: (ws: ServerWebSocket<TData>) => void;
  onClose?: (ws: ServerWebSocket<TData>, code: number, reason: string) => void;
  context?: Record<string, unknown>;
  clientIdHeader?: string;
}

Example:

import { serve } from "@ws-kit/bun";
import { createRouter } from "@ws-kit/zod";

const router = createRouter();

serve(router, {
  port: 3000,
  authenticate(req) {
    const token = req.headers.get("authorization")?.replace("Bearer ", "");
    return token ? { userId: "123", roles: ["admin"] } : undefined;
  },
});

createBunHandler(router, options)

Low-level handler creation for custom setups.

function createBunHandler<TData extends { clientId: string }>(
  router: WebSocketRouter<any, TData>,
  options?: BunHandlerOptions<TData>,
): { fetch: FetchHandler; websocket: WebSocketHandler<TData> };

Returns: Object with fetch and websocket handlers for Bun.serve()

Example:

import { createBunHandler } from "@ws-kit/bun";

const { fetch, websocket } = createBunHandler(router, {
  authenticate(req) {
    // Custom auth logic
  },
});

Bun.serve({
  port: 3000,
  fetch,
  websocket,
});

---

Error Handling

Standard Error Codes

13 standard error codes aligned with gRPC conventions (from ErrorCode enum):

Terminal errors (do NOT retry):

• UNAUTHENTICATED - Not authenticated / auth token missing, expired, or invalid
• PERMISSION_DENIED - Permission denied / authenticated but lacks rights (authZ)
• INVALID_ARGUMENT - Invalid argument / input validation or semantic violation
• FAILED_PRECONDITION - Failed precondition / state requirement not met
• NOT_FOUND - Not found / target resource absent
• ALREADY_EXISTS - Already exists / uniqueness or idempotency replay violation
• ABORTED - Aborted / concurrency conflict (race condition)
• CANCELLED - Cancelled / call cancelled (client disconnect, timeout abort)

Transient errors (retry with backoff):

• DEADLINE_EXCEEDED - Deadline exceeded / RPC timed out
• RESOURCE_EXHAUSTED - Resource exhausted / rate limit, quota, or buffer overflow
• UNAVAILABLE - Unavailable / transient infrastructure error

Server/evolution:

• UNIMPLEMENTED - Unimplemented / feature not supported or deployed
• INTERNAL - Internal / unexpected server error (bug)

Client Inference Rules for Retry Semantics

The client automatically infers whether to retry based on the error code. If the server omits the retryable field, the client uses these rules:

Error Code	Retryable	Notes
DEADLINE_EXCEEDED	✅ Yes	Transient timeout
RESOURCE_EXHAUSTED	✅ Yes	Rate limit or quota, may have retryAfterMs
UNAVAILABLE	✅ Yes	Transient infrastructure issue
ABORTED	✅ Yes	Concurrency conflict, safe to retry
UNAUTHENTICATED	❌ No	Fix auth, then retry
PERMISSION_DENIED	❌ No	Insufficient privileges, don't retry
INVALID_ARGUMENT	❌ No	Fix input, then retry
FAILED_PRECONDITION	❌ No	Server state issue, don't retry
NOT_FOUND	❌ No	Resource absent, don't retry
ALREADY_EXISTS	❌ No	Duplicate, don't retry
UNIMPLEMENTED	❌ No	Feature not available, don't retry
INTERNAL	❌ No	Server bug, don't retry without investigation
CANCELLED	❌ No	Request cancelled, don't auto-retry

Server Behavior: If the server explicitly sends retryable: true or retryable: false, the client uses the server's value instead of inferring from the code.

retryAfterMs Validation Rules

Error Code	retryAfterMs Status	When Used	Example
RESOURCE_EXHAUSTED	Should include	Rate limiting, quota	{ retryAfterMs: 5000 }
DEADLINE_EXCEEDED	Optional	Long operations	Omit or use default backoff
UNAVAILABLE	Optional	Service degradation	Omit or suggest delay
Terminal errors	Forbidden	N/A	Must NOT include
Other transient	Optional	Custom delays	Omit for default

Validation: Server rejects retryAfterMs for terminal errors (codes marked "do NOT retry" above).

Error Response Methods

Send a type-safe error response using ctx.error() with automatic retry inference:

error(
  code: ErrorCode,
  message?: string,
  details?: Record<string, unknown>,
  options?: { retryable?: boolean; retryAfterMs?: number }
): void;

Clients automatically infer whether errors are retryable based on the error code. Use retryAfterMs to provide backoff hints for transient errors.

Non-retryable error:

router.on(JoinRoom, (ctx) => {
  if (!roomExists(ctx.payload.roomId)) {
    ctx.error("NOT_FOUND", "Room not found", { roomId: ctx.payload.roomId });
    return;
  }
});

Transient error with backoff:

router.on(QueryData, (ctx) => {
  if (isSaturated()) {
    ctx.error("RESOURCE_EXHAUSTED", "Server busy", undefined, {
      retryable: true,
      retryAfterMs: 1000,
    });
  }
});

WsKitError

Standardized error object for structured error handling. Follows WHATWG Error standard with cause for error chaining.

class WsKitError extends Error {
  // Public properties
  name: "WsKitError";
  code: string;
  override message: string;
  details: Record<string, unknown>;
  override cause?: unknown; // WHATWG standard: original error

  // Convenience accessor
  get originalError(): Error | undefined;

  // Static factory methods
  static from(
    code: string,
    message: string,
    details?: Record<string, unknown>,
  ): WsKitError;

  static wrap(
    error: unknown,
    code: string,
    message?: string,
    details?: Record<string, unknown>,
  ): WsKitError;

  static isWsKitError(value: unknown): value is WsKitError;

  // Serialization methods
  toJSON(): {
    code: string;
    message: string;
    details: Record<string, unknown>;
    stack: string | undefined;
    cause?:
      | { name: string; message: string; stack: string | undefined }
      | string;
  };

  toPayload(): ErrorPayload;
}

Static Methods:

• from() - Create a new WsKitError
• wrap() - Wrap an existing error (preserves as cause). If already a WsKitError, returns as-is
• isWsKitError() - Type guard to check if a value is a WsKitError

Instance Methods:

• toJSON() - Serialize to plain object for structured logging (includes cause and stack)
• toPayload() - Create error payload for client transmission (excludes cause and stack)

Example:

router.on(CreateUser, async (ctx) => {
  try {
    const user = await db.users.create(ctx.payload);
    ctx.send(UserCreated, user);
  } catch (err) {
    throw WsKitError.wrap(err, "INTERNAL", "Failed to create user", {
      email: ctx.payload.email,
    });
  }
});

Error Handler Example:

router.onError((error, ctx) => {
  // Error is always a WsKitError (wrapped if needed)
  logger.error({
    code: error.code,
    message: error.message,
    details: error.details,
    cause: error.originalError, // Access wrapped error
    clientId: ctx.data?.clientId,
  });
});

---

Type Utilities

Infer Types from Schemas

import type { InferPayload, InferMessage } from "@ws-kit/zod";

// Infer payload type
type ChatPayload = InferPayload<typeof ChatMessage>;

// Infer full message type
type ChatMsg = InferMessage<typeof ChatMessage>;

Connection Data Type (Declaration Merging)

Declare default connection data type once using TypeScript declaration merging:

// types/connection-data.d.ts
declare module "@ws-kit/core" {
  interface ConnectionData {
    userId?: string;
    roles?: string[];
    tenant?: string;
  }
}

// Now in any module (no generic needed):
const router = createRouter(); // Automatically uses ConnectionData

router.on(SecureMessage, (ctx) => {
  // ctx.data is properly typed with userId, roles, tenant
  const userId = ctx.data?.userId;
});

---

Message Metadata

All messages include a meta object with standard fields:

interface MessageMeta {
  // Server-controlled (injected after validation)
  clientId: string; // Client identifier (UUID v7)
  receivedAt: number; // Server ingress timestamp (ms)

  // Client-provided (validated, not trusted for server logic)
  timestamp?: number; // Client timestamp (for UI display)
  correlationId?: string; // Request/response correlation
  timeoutMs?: number; // RPC timeout

  // Custom metadata (via schema)
  [key: string]: unknown;
}

Which timestamp to use:

• Server logic (rate limiting, ordering, TTL): Use ctx.receivedAt (authoritative, server time)
• UI display (relative timestamps): Use ctx.meta.timestamp (client time, for display only)

Reserved meta keys: clientId and receivedAt are server-controlled and injected after validation. Never trust client-provided values for these fields.

---

WebSocket Connection Data

Every connection has a data object with custom application data:

interface WebSocketData<T = unknown> {
  clientId: string; // Auto-generated UUID v7 (always present)
} & T

Access via ctx.data:

router.on(SecureMessage, (ctx) => {
  const userId = ctx.data?.userId;
  const clientId = ctx.clientId; // Always present
});

Modify via ctx.assignData():

router.on(LoginMessage, (ctx) => {
  ctx.assignData({ userId: "123", roles: ["admin"] });
});

---

Reserved Message Type Prefix

Message types starting with $ws: are reserved for internal control messages and cannot be registered with router.on() or router.rpc().

Reserved types:

• $ws:rpc-progress - RPC progress updates (sent by ctx.progress())
• $ws:abort - RPC abort signal (sent by client to cancel RPC)

Enforcement:

• Design-time: message() and rpc() throw if message type uses reserved prefix
• Runtime: Messages with reserved prefix are handled internally, never dispatched to user handlers

Example (will throw):

// ❌ Throws: Reserved prefix "$ws:" not allowed
const BadMessage = message("$ws:custom", { data: z.string() });
const BadRpc = rpc("$ws:query", payload, "$ws:response", response);

---

Best Practices

Type Safety

// ✅ Use message() for full type inference
const ChatMessage = message("CHAT_MESSAGE", {
  text: z.string(),
  roomId: z.string(),
});

router.on(ChatMessage, (ctx) => {
  // ctx.payload.text is typed as string
  console.log(ctx.payload.text);
});

// ✅ Use rpc() for request-response patterns
const GetUser = rpc("GET_USER", { userId: z.string() }, "USER_RESPONSE", {
  user: UserSchema,
});

router.rpc(GetUser, async (ctx) => {
  const user = await db.users.findById(ctx.payload.userId);
  ctx.reply(GetUser.response, { user }); // Fully typed
});

Error Handling Patterns

// ✅ Use standard error codes
ctx.error("NOT_FOUND", "User not found", { userId });

// ✅ Wrap exceptions with WsKitError
try {
  await db.users.create(data);
} catch (err) {
  throw WsKitError.wrap(err, "INTERNAL", "Database error");
}

// ✅ Register error handlers for observability
router.onError((error, ctx) => {
  logger.error({
    code: error.code,
    message: error.message,
    details: error.details,
    clientId: ctx.data?.clientId,
  });
});

Middleware Patterns

// ✅ Global authentication middleware
router.use((ctx, next) => {
  if (!ctx.data?.userId && ctx.type !== "LOGIN") {
    ctx.error("UNAUTHENTICATED", "Not authenticated");
    return;
  }
  return next();
});

// ✅ Per-route rate limiting (builder pattern)
const rateLimiter = new Map<string, number>();
router
  .route(SendMessage)
  .use((ctx, next) => {
    const userId = ctx.data?.userId || "anon";
    const count = (rateLimiter.get(userId) || 0) + 1;
    if (count > 10) {
      ctx.error("RESOURCE_EXHAUSTED", "Too many messages");
      return;
    }
    rateLimiter.set(userId, count);
    return next();
  })
  .on((ctx) => {
    // Handle message
  });

Broadcasting

// ✅ Use ctx.publish() for ergonomics
router.on(SendMessage, (ctx) => {
  ctx.publish(`room:${ctx.payload.roomId}`, ChatMessage, ctx.payload);
});

// ✅ Use router.publish() outside handlers
await router.publish("system:announcements", Announcement, {
  text: "Server maintenance at 02:00 UTC",
});

Client Patterns

// ✅ Use RPC for request-response
const response = await client.request(GetUser, { userId: "123" });

// ✅ Use send() for fire-and-forget
client.send(ChatMessage, { text: "Hello!", roomId: "general" });

// ✅ Handle connection states
client.onState((state) => {
  if (state === "open") {
    console.log("Connected!");
  } else if (state === "reconnecting") {
    console.log("Reconnecting...");
  }
});

// ✅ Use AbortSignal for cancellation
const controller = new AbortController();
const promise = client.request(
  LongQuery,
  { query: "..." },
  {
    signal: controller.signal,
  },
);
setTimeout(() => controller.abort(), 5000);