Claude API Streaming in Node.js / TypeScript

Stream Claude API responses in Node.js using @anthropic-ai/sdk. Print tokens as they arrive with the streaming helper or raw event stream.

The @anthropic-ai/sdk streaming helper handles SSE parsing automatically.

Simple streaming

import Anthropic from "@anthropic-ai/sdk";

const client = new Anthropic();

const stream = await client.messages.stream({
  model: "claude-sonnet-4-6",
  max_tokens: 1024,
  messages: [{ role: "user", content: "Write a function that parses CSV in JavaScript." }]
});

for await (const chunk of stream) {
  if (chunk.type === "content_block_delta" && chunk.delta.type === "text_delta") {
    process.stdout.write(chunk.delta.text);
  }
}
console.log();

Using the text stream helper

const stream = client.messages.stream({
  model: "claude-sonnet-4-6",
  max_tokens: 1024,
  messages: [{ role: "user", content: "Explain closures in JavaScript." }]
});

stream.on("text", (text) => process.stdout.write(text));
const finalMsg = await stream.finalMessage();
console.log(`
Used ${finalMsg.usage.output_tokens} output tokens.`);

Express.js streaming endpoint

import express from "express";
import Anthropic from "@anthropic-ai/sdk";

const app = express();
app.use(express.json());
const client = new Anthropic();

app.post("/stream", async (req, res) => {
  res.setHeader("Content-Type", "text/plain; charset=utf-8");
  res.setHeader("Transfer-Encoding", "chunked");

  const stream = await client.messages.stream({
    model: "claude-sonnet-4-6",
    max_tokens: 1024,
    messages: [{ role: "user", content: req.body.prompt }]
  });

  for await (const chunk of stream) {
    if (chunk.type === "content_block_delta" && chunk.delta.type === "text_delta") {
      res.write(chunk.delta.text);
    }
  }
  res.end();
});

app.listen(3000);

See the Python streaming example for server-side Python usage. For tool use with streaming, see the Node.js tool use example.

Frequently asked questions

Free tools

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