Mastra•5mo ago•

8 replies

using `writer.custom()` for UI framework integration

Hey team!

I'm trying to implement tool streaming with custom events following the docs here:
https://mastra.ai/en/docs/streaming/tool-streaming#using-the-writer-argument

The docs show using

writer.custom()

writer.custom()

for UI framework integration:

await writer?.custom({
  type: "data-tool-progress",
  status: "pending"
});

await writer?.custom({
  type: "data-tool-progress",
  status: "pending"
});

However, I'm getting

TypeError: writer?.custom is not a function

TypeError: writer?.custom is not a function

at runtime.

Environment:
-

@mastra/[email protected]

@mastra/[email protected]

(all Mastra packages on 0.21.x)
- Verified with

npm list @mastra/core

npm list @mastra/core

- all dependencies properly deduplicated

What I found:
Looking at

node_modules/@mastra/core/dist/tools/stream.d.ts

node_modules/@mastra/core/dist/tools/stream.d.ts

, the

ToolStream

ToolStream

class only has a

write()

write()

method, not

custom()

custom()

export declare class ToolStream<T> extends WritableStream<T> {
    write(data: any): Promise<void>;
    // ❌ No custom() method
}

export declare class ToolStream<T> extends WritableStream<T> {
    write(data: any): Promise<void>;
    // ❌ No custom() method
}

Questions:
1. Is

writer.custom()

writer.custom()

available in 0.21.1, or is this a docs issue?
2. If it's coming in a future release, when can we expect it? (0.21.2, 0.22.0?)

Thanks!

Learn how to use tool streaming in Mastra, including handling tool calls, tool results, and tool execution events during streaming.