When multiple messages arrive on the same thread while a handler is still processing, the SDK needs a strategy. By default, the incoming message is dropped. The concurrency option on ChatConfig lets you choose what happens instead.

Drop (default)

The original behavior. If a handler is already running on a thread, the new message is discarded and a LockError is thrown. No queuing, no retries.

const bot = new Chat({
  concurrency: "drop",
  // ...
});

Queue

Messages that arrive while a handler is running are enqueued. When the current handler finishes, only the latest queued message is dispatched. All intermediate messages are provided as context.skipped, giving your handler full visibility into what happened while it was busy.

const bot = new Chat({
  concurrency: "queue",
  // ...
});

bot.onNewMention(async (thread, message, context) => {
  if (context && context.skipped.length > 0) {
    await thread.post(
      `You sent ${context.totalSinceLastHandler} messages while I was thinking. Responding to your latest.`
    );
  }

  const response = await generateAIResponse(message.text);
  await thread.post(response);
});

Flow:

A arrives  → acquire lock → process A
B arrives  → lock busy → enqueue B
C arrives  → lock busy → enqueue C
D arrives  → lock busy → enqueue D
A done     → drain: [B, C, D] → handler(D, { skipped: [B, C] })
D done     → queue empty → release lock

Burst

Waits for debounceMs before the first handler on an idle thread, then drains the collected burst like queue. The latest message is dispatched, and earlier messages in the burst are available as context.skipped.

Use this for assistant-style bots where users often send one logical turn as several short messages inside a small window and you want one response with full context. Compared to debounce, burst keeps the earlier messages in context.skipped instead of dropping them, and it flushes queued messages that arrive while the handler is running.

Choose debounce when the latest message replaces earlier ones, like rapid corrections. Choose burst when earlier messages still matter, like "hey", "quick question", and then the actual question. The tradeoff is that even a lone message waits for debounceMs before the handler runs.

const bot = new Chat({
  concurrency: { strategy: "burst", debounceMs: 1000 },
  // ...
});

bot.onNewMention(async (thread, message, context) => {
  const turn = [...(context?.skipped ?? []), message]
    .map((m) => m.text)
    .join("\n\n");

  const response = await generateAIResponse(turn);
  await thread.post(response);
});

Flow:

A arrives  → acquire lock → enqueue A → sleep(debounceMs)
B arrives  → lock busy → enqueue B
C arrives  → lock busy → enqueue C
             ... debounceMs elapses ...
           → drain: [A, B, C] → handler(C, { skipped: [A, B] })
D arrives while C is running → lock busy → enqueue D
E arrives while C is running → lock busy → enqueue E
C done     → drain: [D, E] → handler(E, { skipped: [D] })
E done     → queue empty → release lock

Debounce

The first message waits for debounceMs. Messages that arrive during that window replace the pending message, so only the final message in the burst window is processed.

This is particularly useful for platforms like WhatsApp and Telegram where users tend to send a flurry of short messages in quick succession instead of composing a single message - "hey", "quick question", "how do I reset my password?" arriving as three separate webhooks within a few seconds. Without debounce, the bot would respond to "hey" before the actual question even arrives. With debounce, the SDK waits briefly and processes only the final message in the window.

const bot = new Chat({
  concurrency: { strategy: "debounce", debounceMs: 1500 },
  // ...
});

Flow:

A arrives  → acquire lock → store A as pending → sleep(debounceMs)
B arrives  → lock busy → overwrite pending with B (A dropped)
C arrives  → lock busy → overwrite pending with C (B dropped)
             ... debounceMs elapses with no new message ...
           → process C → release lock

Debounce also works well for rapid corrections ("wait, I meant...") and multi-part messages on any platform.

Concurrent

No locking at all. Every message is processed immediately in its own handler invocation. Use this for stateless handlers where thread ordering doesn't matter.

const bot = new Chat({
  concurrency: "concurrent",
  // ...
});

For fine-grained control, pass a ConcurrencyConfig object instead of a strategy string:

const bot = new Chat({
  concurrency: {
    strategy: "queue",
    maxQueueSize: 20,          // Max queued messages per thread (default: 10)
    onQueueFull: "drop-oldest", // or "drop-newest" (default: "drop-oldest")
    queueEntryTtlMs: 60_000,  // Discard stale entries after 60s (default: 90s)
  },
  // ...
});

All options

MessageContext

All handler types (onNewMention, onSubscribedMessage, onNewMessage) accept an optional MessageContext as their last parameter. It is populated when using the queue strategy for queued messages, and when using burst for a collapsed turn. A lone burst message receives skipped: [] and totalSinceLastHandler: 1.

interface MessageContext {
  /** Messages that arrived while the previous handler was running, in chronological order. */
  skipped: Message[];
  /** Total messages received since last handler ran (skipped.length + 1). */
  totalSinceLastHandler: number;
}

Existing handlers that don't use context are unaffected — the parameter is optional.

Example: Pass all messages to an LLM

bot.onSubscribedMessage(async (thread, message, context) => {
  // Combine skipped messages with the current one for full context
  const allMessages = [...(context?.skipped ?? []), message];

  const response = await generateAIResponse(
    allMessages.map((m) => m.text).join("\n\n")
  );
  await thread.post(response);
});

By default, locks are scoped to the thread — messages in different threads are processed independently. For platforms like WhatsApp and Telegram where conversations happen at the channel level rather than in threads, the lock scope defaults to "channel".

You can override this globally:

const bot = new Chat({
  concurrency: "queue",
  lockScope: "channel", // or "thread" (default)
  // ...
});

Or resolve it dynamically per message:

const bot = new Chat({
  concurrency: "queue",
  lockScope: ({ isDM, adapter }) => {
    // Use channel scope for DMs, thread scope for group channels
    return isDM ? "channel" : "thread";
  },
  // ...
});

The queue, debounce, and burst strategies require three additional methods on your state adapter:

All built-in state adapters (@chat-adapter/state-memory, @chat-adapter/state-redis, @chat-adapter/state-ioredis) implement these methods. The Redis adapters use Lua scripts for atomicity.

All strategies emit structured log events at info level:

• The default strategy is drop — existing behavior is unchanged.
• The deprecated onLockConflict option continues to work but should be replaced with concurrency.
• Handler signatures are backward-compatible; the new context parameter is optional.
• Deduplication always runs regardless of strategy.