Server Streaming

OpenHarness integrates with AI SDK 5’s data stream protocol, so you can stream agent sessions directly to useChat-based React and Vue UIs.

`toResponse()`

Both Session and Conversation provide two methods for streaming to the client:

toUIMessageStream(input) — returns a ReadableStream<UIMessageChunk> that maps session events to AI SDK 5 typed chunks
toResponse(input, options) — wraps the stream in an HTTP Response with SSE headers, ready to return from any route handler

Next.js Example

app/api/chat/route.ts

import {
  Agent, Conversation, toRunner, apply,
  withTurnTracking, withCompaction, withRetry, withPersistence,
  extractUserInput, type SessionStore,
} from "@openharness/core";

const store: SessionStore = {
  async load(id) { return db.get(id); },
  async save(id, messages) { await db.set(id, messages); },
};

const conversations = new Map<string, Conversation>();

function getOrCreateConversation(id?: string): Conversation {
  const convId = id ?? crypto.randomUUID();
  let conv = conversations.get(convId);
  if (!conv) {
    const runner = apply(
      toRunner(agent),
      withTurnTracking(),
      withCompaction({ contextWindow: 128_000, model: agent.model }),
      withRetry(),
      withPersistence({ store, sessionId: convId }),
    );
    conv = new Conversation({ runner, sessionId: convId, store });
    conversations.set(convId, conv);
  }
  return conv;
}

export async function POST(req: Request) {
  const { id, messages } = await req.json();
  const conv = getOrCreateConversation(id);
  const input = await extractUserInput(messages);
  return conv.toResponse(input, { signal: req.signal });
}

Resumable Streams

The direct toResponse(input, { signal: req.signal }) pattern is enough for simple request-bound chat. For resumable chat, the server must decouple the active run from the HTTP response:

POST starts or attaches to a server-owned run for the chat id.
Client disconnect removes only that subscriber; it should not abort the run.
GET ${endpoint}/${id}/stream returns the active stream, or 204 when no stream is running.
Cancellation should be a separate server-side action.

React and Vue clients can enable reconnects with resume: true and customize the GET endpoint with prepareReconnectToStreamRequest.

Custom Data Parts

The stream emits all standard AI SDK 5 chunk types (text-delta, reasoning-delta, tool-input-available, tool-output-available, start, finish, etc.) plus custom OpenHarness data parts for subagent activity, compaction, retry, and turn lifecycle:

Data part	Description
`data-oh:subagent.start`	A subagent task was spawned
`data-oh:subagent.done`	A subagent task completed
`data-oh:subagent.error`	A subagent task failed
`data-oh:compaction.start`	Compaction started
`data-oh:compaction.done`	Compaction finished
`data-oh:retry`	Retrying after transient error
`data-oh:turn.start`	Turn started
`data-oh:turn.done`	Turn finished
`data-oh:session.compacting`	Session is compacting

Using Data Part Types

If you’re building custom UI components that consume the stream directly, the core package exports typed data part types and guards:

import {
  type OHDataPart,
  isSubagentEvent,
  isCompactionEvent,
} from "@openharness/core";

Getting Started

Core Concepts

Tools

Advanced

UI Integration

Resources

`toResponse()`

Next.js Example

Resumable Streams

Custom Data Parts

Using Data Part Types

​toResponse()

​Next.js Example

​Resumable Streams

​Custom Data Parts

​Using Data Part Types

`toResponse()`

Next.js Example

Resumable Streams

Custom Data Parts

Using Data Part Types