fix: preserve fetch multipart body on MockAgent fallback

lib/mock/mock-utils.js

@@ -424,7 +424,9 @@ function buildMockDispatch () {
             throw new MockNotMatchedError(`${error.message}: subsequent request to origin ${origin} was not allowed (net.connect disabled)${interceptsMessage}`)
           }
           if (checkNetConnect(netConnect, origin)) {
-            originalDispatch.call(this, opts, handler)
+            originalDispatch.call(this, '__mockAgentBodyForDispatch' in opts
+              ? { ...opts, body: opts.__mockAgentBodyForDispatch }
+              : opts, handler)
           } else {
             throw new MockNotMatchedError(`${error.message}: subsequent request to origin ${origin} was not allowed (net.connect is not enabled for this origin)${interceptsMessage}`)
           }

lib/web/fetch/index.js

@@ -2184,6 +2184,8 @@ async function httpNetworkFetch (
           origin: url.origin,
           method: request.method,
           body: agent.isMockActive ? request.body && (request.body.source || request.body.stream) : body,
+          // Preserve the serialized fetch body for MockAgent net-connect fallthroughs.
+          __mockAgentBodyForDispatch: body,
           headers: request.headersList.entries,
           maxRedirections: 0,
           upgrade: request.mode === 'websocket' ? 'websocket' : undefined,

test/fetch/issue-5241.js

@@ -0,0 +1,73 @@
+'use strict'
+
+const { createServer } = require('node:http')
+const { once } = require('node:events')
+const { test } = require('node:test')
+const {
+  fetch,
+  FormData,
+  MockAgent,
+  getGlobalDispatcher,
+  setGlobalDispatcher
+} = require('../..')
+
+// https://github.com/nodejs/undici/issues/5241
+test('MockAgent net connect keeps multipart boundaries in sync with fetch headers', async (t) => {
+  let requestBody = null
+  let contentType = null
+
+  const server = createServer((req, res) => {
+    const chunks = []
+
+    req.on('data', chunk => {
+      chunks.push(chunk)
+    })
+
+    req.on('end', () => {
+      requestBody = Buffer.concat(chunks).toString('utf8')
+      contentType = req.headers['content-type']
+
+      res.writeHead(200, {
+        'content-type': 'application/json'
+      })
+      res.end('{}')
+    })
+  })
+
+  t.after(() => {
+    server.closeAllConnections?.()
+    server.close()
+  })
+
+  await once(server.listen(0), 'listening')
+
+  const previousDispatcher = getGlobalDispatcher()
+  const mockAgent = new MockAgent()
+  const origin = `http://localhost:${server.address().port}`
+
+  mockAgent.enableNetConnect(`localhost:${server.address().port}`)
+  setGlobalDispatcher(mockAgent)
+
+  t.after(async () => {
+    setGlobalDispatcher(previousDispatcher)
+    await mockAgent.close()
+  })
+
+  const form = new FormData()
+  form.append('file', new Blob([Buffer.from('hello world')], { type: 'text/plain' }), 'document.txt')
+
+  const response = await fetch(origin, {
+    method: 'POST',
+    body: form
+  })
+
+  t.assert.strictEqual(response.status, 200)
+  t.assert.deepStrictEqual(await response.json(), {})
+
+  t.assert.ok(contentType.startsWith('multipart/form-data; boundary='))
+
+  const boundary = contentType.slice(contentType.indexOf('boundary=') + 'boundary='.length)
+
+  t.assert.strictEqual(requestBody.split('\r\n', 1)[0], `--${boundary}`)
+  t.assert.ok(requestBody.includes(`\r\n--${boundary}--\r\n`))
+})