Claude API with Express.js

How to call the Claude API from an Express.js backend in 2026. Covers minimal handler, streaming SSE to browser, conversation history with sessions, and rate-limit middleware.

Express.js is the most widely used Node.js web framework. This guide shows how to wire Claude into an Express backend — from a minimal single-endpoint handler through streaming SSE, conversation history, and production-ready rate limiting.

Installation

npm install express @anthropic-ai/sdk express-session

Minimal Express handler

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

const app = express();
app.use(express.json());

const client = new Anthropic(); // reads ANTHROPIC_API_KEY from env

app.post("/api/chat", async (req, res) => {
  const { message } = req.body;

  const response = await client.messages.create({
    model: "claude-haiku-4-5-20251001",
    max_tokens: 1024,
    messages: [{ role: "user", content: message }],
  });

  res.json({ reply: response.content[0].text });
});

app.listen(3000, () => console.log("Listening on :3000"));

Streaming SSE to browser

app.post("/api/chat/stream", async (req, res) => {
  const { message } = req.body;

  res.setHeader("Content-Type", "text/event-stream");
  res.setHeader("Cache-Control", "no-cache");
  res.setHeader("X-Accel-Buffering", "no"); // disable Nginx buffering

  const stream = client.messages.stream({
    model: "claude-haiku-4-5-20251001",
    max_tokens: 1024,
    messages: [{ role: "user", content: message }],
  });

  for await (const event of stream) {
    if (event.type === "content_block_delta" && event.delta.type === "text_delta") {
      res.write(`data: ${JSON.stringify({ text: event.delta.text })}

`);
    }
  }
  res.write("data: [DONE]

");
  res.end();
});

Conversation history with express-session

import session from "express-session";

app.use(session({
  secret: process.env.SESSION_SECRET,
  resave: false,
  saveUninitialized: true,
  cookie: { maxAge: 3600000 }, // 1h session
}));

app.post("/api/chat/history", async (req, res) => {
  const { message } = req.body;
  if (!req.session.history) req.session.history = [];

  req.session.history.push({ role: "user", content: message });

  // cap at 20 turns to stay within context
  const trimmed = req.session.history.slice(-20);

  const response = await client.messages.create({
    model: "claude-haiku-4-5-20251001",
    max_tokens: 1024,
    messages: trimmed,
  });

  const reply = response.content[0].text;
  req.session.history.push({ role: "assistant", content: reply });

  res.json({ reply, turns: req.session.history.length / 2 });
});

// DELETE /api/chat/history resets context
app.delete("/api/chat/history", (req, res) => {
  req.session.history = [];
  res.json({ ok: true });
});

Rate-limit middleware (per-user token bucket)

import rateLimit from "express-rate-limit";

const chatLimiter = rateLimit({
  windowMs: 60_000,  // 1 minute
  max: 20,           // 20 requests per IP per minute
  standardHeaders: true,
  legacyHeaders: false,
  handler: (req, res) => res.status(429).json({
    error: "Too many requests. Please wait.",
    retryAfter: Math.ceil(req.rateLimit.resetTime / 1000),
  }),
});

app.post("/api/chat", chatLimiter, async (req, res) => { /* ... */ });

Express vs Next.js API routes vs plain Node.js http

Estimate API costs before going to production with the Claude API Cost Calculator. For the Next.js integration pattern (App Router + Server Components), see the Next.js example.

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