* [Reproduction](#reproduction)

npm run build:wasm

<a id="reproduction"></a>

When reporting a bug, a high-quality reproduction helps maintainers diagnose and fix the issue quickly. Follow the guidelines below to create a minimal, standalone reproduction script.

- **Standalone**: The script must be self-contained and run without external dependencies beyond `undici` and Node.js built-in modules.

- **Minimal**: Strip everything unrelated to the bug — no production configs, no frameworks, no unnecessary middleware.

- **Reproducible**: Run the script and confirm the bug occurs before submitting.

- **Specific**: Include the exact `undici` API call that triggers the issue (e.g., `Client`, `Pool`, `Agent`, `fetch`, `request`, `stream`, `pipeline`).

- **Run with Node's test runner**: Use `node --test` (Node 20+) or `node test/your-repro.js`.

Use `createServer` from `node:http` (or `node:https` for TLS-related bugs) to run a local server inside the same script. This avoids external service dependencies and keeps the reproduction fully self-contained.

const { test } = require('node:test')

const { createServer } = require('node:http')

const { once } = require('node:events')

const { Client } = require('undici')

test('short description of the bug', { timeout: 60000 }, async (t) => {

t.plan(1)

// 1. Start a local HTTP server that reproduces the scenario

const server = createServer({ joinDuplicateHeaders: true }, async (req, res) => {

// Simulate the bug-triggering response

res.statusCode = 200

res.setHeader('content-length', 100)

res.end('hello'.repeat(20))

})

t.after(() => { server.closeAllConnections?.(); server.close() })

server.listen(0)

await once(server, 'listening')

// 2. Create the undici client pointing to the local server

const client = new Client(`http://localhost:${server.address().port}`)

t.after(() => client.close())

// 3. Perform the request that triggers the bug

const { body } = await client.request({ path: '/', method: 'GET' })

let responseBody = ''

for await (const chunk of body) {

responseBody += chunk

}

t.assert.strictEqual(responseBody, 'hello'.repeat(20))

})

Save the script as `test/repro-XXXX.js` and run it:

```bash

node --test test/repro-XXXX.js

For bugs involving `fetch`, use the same standalone server pattern:

```javascript

const { test } = require('node:test')

const { createServer } = require('node:http')

const { once } = require('node:events')

const { fetch, Agent } = require('undici')

test('short description of the fetch bug', { timeout: 60000 }, async (t) => {

t.plan(1)

const server = createServer({ joinDuplicateHeaders: true }, async (req, res) => {

res.statusCode = 200

res.setHeader('content-type', 'application/json')

res.end(JSON.stringify({ ok: true }))

})

t.after(() => server.close())

server.listen(0)

await once(server, 'listening')

const agent = new Agent({ keepAliveTimeout: 1 })

t.after(() => agent.close())

const response = await fetch(`http://localhost:${server.address().port}/`, {

dispatcher: agent

})

await response.body.cancel()

t.assert.strictEqual(response.status, 200)

})

```javascript

const { test } = require('node:test')

const { createServer } = require('node:http')

const { once } = require('node:events')

const { Pool } = require('undici')

test('Pool bug reproduction', { timeout: 60000 }, async (t) => {

t.plan(1)

const server = createServer({ joinDuplicateHeaders: true }, (req, res) => {

res.end('ok')

})

t.after(() => server.close())

server.listen(0)

await once(server, 'listening')

const pool = new Pool(`http://localhost:${server.address().port}`)

t.after(() => pool.close())

const data = await pool.request({ path: '/', method: 'GET' })

await data.body.dump()

t.assert.strictEqual(data.statusCode, 200)

})

If you are submitting a bug fix via a pull request, include a test file `test/issue-XXXX.js` that reproduces the bug and passes with the fix.