Streaming Output from Claude Agent SDK Agents

Overview

By default the Agent SDK yields complete AssistantMessage objects after each response. To receive incremental updates as text and tool calls are generated, enable partial message streaming. This powers chat UIs and progress indicators.

Enable streaming

Set includePartialMessages (TS) / include_partial_messages (Python) to true. The SDK then also yields StreamEvent messages (TypeScript: SDKPartialAssistantMessage with type: "stream_event") carrying raw API events.

for await (const message of query({
  prompt: "List the files in my project",
  options: { includePartialMessages: true, allowedTools: ["Bash", "Read"] },
})) {
  if (message.type === "stream_event") {
    const event = message.event;
    if (event.type === "content_block_delta" && event.delta.type === "text_delta") {
      process.stdout.write(event.delta.text);
    }
  }
}

Read the deltas

• Text: content_block_delta events where delta.type is text_delta carry text chunks.
• Tool calls: content_block_start (tool begins), content_block_delta with input_json_delta (accumulate partial_json), and content_block_stop (call complete). Parse the accumulated JSON once complete.

Message flow

With partial messages on, you receive message_start, content_block_start, content_block_delta chunks, content_block_stop, message_delta, message_stop, then the complete AssistantMessage, and finally a ResultMessage. Without it, you receive the complete messages but no StreamEvents.

Build a streaming UI

Track an inTool flag from content_block_start/content_block_stop to show a status like [Using Read...] while a tool runs, and stream text only when not in a tool. Print a completion marker on ResultMessage.

Known limitation

Structured output does not stream as deltas; the JSON appears only in the final ResultMessage.structured_output.

Source

• Stream responses in real-time: https://code.claude.com/docs/en/agent-sdk/streaming-output