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# Memory.copyThread()

The `.copyThread()` method copies an existing thread and its messages to a new thread. It behaves like [`cloneThread()`](https://mastra.ai/reference/memory/cloneThread), including working memory, observational memory, and semantic-recall embeddings, but it doesn't return the copied messages. On `@mastra/libsql` and `@mastra/pg` the rows are copied inside the database, so the copy itself never loads message content into the Node.js process. See [Memory usage](#memory-usage) for the cases where content is still read.

Use `copyThread()` when you only need the new thread ID, such as forking a conversation for a subagent. Use `cloneThread()` when you need the copied messages in the response.

## Usage example

```typescript
import { Memory } from '@mastra/memory'
import { LibSQLStore } from '@mastra/libsql'

const memory = new Memory({
  storage: new LibSQLStore({ id: 'memory-store', url: 'file:./memory.db' }),
})

const { thread, messageIdMap } = await memory.copyThread({
  sourceThreadId: 'original-thread-123',
})
```

## Parameters

`copyThread()` accepts the same parameters as `cloneThread()`.

**sourceThreadId** (`string`): The ID of the thread to copy

**newThreadId** (`string`): Optional custom ID for the new thread. If not provided, one will be generated.

**resourceId** (`string`): Optional resource ID for the new thread. Defaults to the source thread's resourceId.

**title** (`string`): Optional title for the new thread. If omitted, the copy uses Clone of ${sourceThread.title} when the source thread has a title. Otherwise, the title is empty.

**metadata** (`Record<string, unknown>`): Optional metadata to merge with the source thread's metadata. Clone metadata is automatically added.

**options** (`CloneOptions`): Optional filtering options. See cloneThread() for the full shape.

**options.messageLimit** (`number`): Maximum number of messages to copy. When set, copies the most recent N messages.

**options.messageFilter** (`MessageFilter`): Filter criteria for selecting which messages to copy, by date range or message IDs.

## Returns

**thread** (`StorageThreadType`): The newly created thread with clone metadata.

**messageIdMap** (`Record<string, string>`): A mapping from source message IDs to their corresponding copied message IDs.

## Memory usage

`copyThread()` is designed for large threads:

- `@mastra/libsql` and `@mastra/pg` copy messages with `INSERT … SELECT` statements, so the copy step doesn't read message content into the process.
- Other storage adapters copy messages through the adapter's existing `cloneThread()` implementation, which may read and re-write each message. `copyThread()` still discards the payloads before returning.
- When semantic recall is enabled, the copied messages are read back to generate embeddings. This happens in batches of 100 by destination message ID, so only one batch is held in memory at a time.

`cloneThread()` calls `copyThread()` and then reads the new thread's messages back to populate `clonedMessages`.

## Related

- [cloneThread](https://mastra.ai/reference/memory/cloneThread)
- [Clone Utility Methods](https://mastra.ai/reference/memory/clone-utilities)