C1 Response

Server-side helpers for constructing and streaming C1 Response.

Unlike standard LLM responses, which are plain text or markdown, a C1 Response is a string that is structured payload to contain rich content. It uses an XML-like structure to package multiple types of content into a single string.

It is generally stored as assistant message content in the database.

<!-- A C1 Response can contain multiple content types -->
<thinking>...</thinking><content>...</content><artifact>...</artifact>

Node.js SDK (@thesysai/genui-sdk/server)

This SDK provides a function to create a Response Builder object that manages the creation and streaming of the C1 Response.

import { makeC1Response } from "@thesysai/genui-sdk/server";
import { transformStream } from "@crayonai/stream";

export async function POST(req: NextRequest) {
  const c1Response = makeC1Response();
  // ... use c1Response methods ...
  return new NextResponse(c1Response.responseStream, {
    headers: { "Content-Type": "text/event-stream" },
  });
}

Main Function

makeC1Response() · C1ResponseBuilder

Creates and returns an instance of the Response Builder object.

Response Builder Methods

The object returned by makeC1Response() has the following methods and properties:

writeContent · (content: string) => void

Pipes the raw string response from a C1 API call (e.g., from the Generative UI or Artifacts endpoint) into the response stream.

writeCustomMarkdown · (markdown: string) => void

Adds a custom markdown response to the stream. This will be wrapped in <custom_markdown> tags.

writeThinkItem · (item: ThinkItem) => void

Adds a thinking state entry to the <thinking> part of the response stream.

Show ThinkItem

title · string

The title of the thinking state.

description · string · optional

A longer description of the current process.

end · () => void

Signals that the stream is complete and closes it. This should be called in the onEnd callback of your stream processing.

responseStream · ReadableStream

The ReadableStream object that should be returned from your API route.

---

Python SDK (thesys_genui_sdk)

The thesys_genui_sdk package provides the C1Response class, which acts as the Response Builder. Please refer to the Python SDK Reference for more details.

A Practical Example: Adding Thinking States

A common use case for the Response Builder is to add real-time thinking state indicators to the stream before the main content from the LLM arrives. This significantly improves the user’s perception of responsiveness.

For a complete, step-by-step example of how to use these helpers, see the Thinking States guide.