docs: add reproduction guide and update bug report template

.github/ISSUE_TEMPLATE/bug-report.md

@@ -11,23 +11,65 @@ assignees: ''
 
 <!-- A clear and concise description of what the bug is. -->
 
-## Reproducible By
+## Reproduction
+
+<!-- 
+IMPORTANT: Provide a standalone reproduction script.
+
+See the Reproduction guide in CONTRIBUTING.md for detailed instructions:
+https://github.com/nodejs/undici/blob/main/CONTRIBUTING.md#reproduction
+
+Guidelines:
+- The script must be self-contained (uses only undici and Node.js built-in modules).
+- Use createServer from node:http to run a local server inside the script.
+- Run `node --test test/repro-XXXX.js` locally to confirm the bug is reproducible.
+- Attach the script inline or as a GitHub Gist link below.
+-->
+
+**Standalone reproduction script:**
+
+```javascript
+'use strict'
+
+// Paste your standalone reproduction script here.
+// Example structure:
+//
+// const { test, after } = require('node:test')
+// const { createServer } = require('node:http')
+// const { once } = require('node:events')
+// const { Client } = require('undici')
+//
+// test('bug reproduction', { timeout: 60000 }, async (t) => {
+//   const { tspl } = require('@matteo.collina/tspl')
+//   t = tspl(t, { plan: 1 })
+//
+//   const server = createServer(...)
+//   ...
+// })
+```
+
+**Steps to reproduce (if not using the script above):**
 
 <!-- A step by step list on how the bug can be reproduced for examination. -->
 
 ## Expected Behavior
 
 <!-- A clear and concise description of what you expected to happen. -->
 
+## Actual Behavior
+
+<!-- What actually happens instead. Include any error messages. -->
+
 ## Logs & Screenshots
 
-<!-- If applicable, add screenshots to help explain your problem, or 
+<!-- If applicable, add screenshots to help explain your problem, or
 alternatively add your console logs here. -->
 
 ## Environment
 
-<!-- This is just your OS and environment information [e.g. Ubuntu 18.04 LTS,
-Node v14.14.0] -->
+- OS: <!-- e.g. Ubuntu 24.04 LTS -->
+- Node.js version: <!-- e.g. v22.14.0 -->
+- undici version: <!-- e.g. 7.x.x (run `node -e 'require("undici/package.json").version'`) -->
 
 ### Additional context
 

CONTRIBUTING.md

@@ -10,6 +10,7 @@
   * [Building for externally shared node builtins](#external-builds)
   *  [Benchmarks](#benchmarks)
   *  [Documentation](#documentation)
+  *  [Reproduction](#reproduction)
 * [Developer's Certificate of Origin 1.1](#developers-certificate-of-origin)
   * [Moderation Policy](#moderation-policy)
 
@@ -198,6 +199,157 @@ cd docs && npm i && npm run serve
 
 The documentation will be available at `http://localhost:3000`.
 
+<a id="reproduction"></a>
+### Reproduction
+
+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.
+
+#### What Makes a Good Reproduction
+
+- **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`.
+
+#### Standalone Server Pattern
+
+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.
+
+```javascript
+'use strict'
+
+const { test, after } = require('node:test')
+const { createServer } = require('node:http')
+const { once } = require('node:events')
+const { Client, errors } = require('undici')
+
+// https://github.com/nodejs/undici/issues/XXXX
+
+test('short description of the bug', { timeout: 60000 }, async (t) => {
+  const { tspl } = require('@matteo.collina/tspl')
+  t = tspl(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))
+  })
+  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:${server.address().port}`)
+  after(() => client.close())
+
+  // 3. Perform the request that triggers the bug
+  client.request({ path: '/', method: 'GET' }, (err, data) => {
+    t.ifError(err)
+
+    let body = ''
+    data.body.on('data', (chunk) => { body += chunk })
+    data.body.on('end', () => {
+      t.strictEqual(body, 'hello'.repeat(20))
+    })
+  })
+})
+```
+
+Save the script as `test/repro-XXXX.js` and run it:
+
+```bash
+node --test test/repro-XXXX.js
+```
+
+#### Fetch-Based Reproduction
+
+For bugs involving `fetch`, use the same standalone server pattern:
+
+```javascript
+'use strict'
+
+const { test, after } = require('node:test')
+const { createServer } = require('node:http')
+const { once } = require('node:events')
+const { fetch, Agent, setGlobalDispatcher } = require('undici')
+
+// https://github.com/nodejs/undici/issues/XXXX
+
+test('short description of the fetch bug', { timeout: 60000 }, async (t) => {
+  const { tspl } = require('@matteo.collina/tspl')
+  t = tspl(t, { plan: 1 })
+
+  const server = createServer({ joinDuplicateHeaders: true }, async (req, res) => {
+    res.statusCode = 200
+    res.setHeader('content-type', 'application/json')
+    res.end(JSON.stringify({ ok: true }))
+  })
+  after(() => server.close())
+
+  server.listen(0)
+  await once(server, 'listening')
+
+  const agent = new Agent({ keepAliveTimeout: 1 })
+  after(() => agent.close())
+
+  const response = await fetch(`http://localhost:${server.address().port}/`, {
+    dispatcher: agent
+  })
+
+  t.strictEqual(response.status, 200)
+})
+```
+
+#### Using Pool or Agent
+
+For bugs involving connection pooling or agent behavior:
+
+```javascript
+'use strict'
+
+const { test, after } = require('node:test')
+const { createServer } = require('node:http')
+const { once } = require('node:events')
+const { Pool } = require('undici')
+
+// https://github.com/nodejs/undici/issues/XXXX
+
+test('Pool bug reproduction', { timeout: 60000 }, async (t) => {
+  const { tspl } = require('@matteo.collina/tspl')
+  t = tspl(t, { plan: 1 })
+
+  const server = createServer({ joinDuplicateHeaders: true }, (req, res) => {
+    res.end('ok')
+  })
+  after(() => server.close())
+
+  server.listen(0)
+  await once(server, 'listening')
+
+  const pool = new Pool(`http://localhost:${server.address().port}`)
+  after(() => pool.close())
+
+  const data = await pool.request({ path: '/', method: 'GET' })
+  t.strictEqual(data.statusCode, 200)
+})
+```
+
+#### Adding the Reproduction to the Repository
+
+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.
+
+#### Checklist for Submitting a Bug Report
+
+1. [ ] Write a standalone reproduction script.
+2. [ ] Run it locally to confirm the bug is reproducible.
+3. [ ] Attach the script (or a gist link) in the bug report.
+4. [ ] Include the exact undici version and Node.js version.
+5. [ ] Describe the expected vs. actual behavior.
+
 <a id="developers-certificate-of-origin"></a>
 ## Developer's Certificate of Origin 1.1