diff --git a/lib/dispatcher/dispatcher1-wrapper.js b/lib/dispatcher/dispatcher1-wrapper.js
index f5813288cb3..7795e314673 100644
--- a/lib/dispatcher/dispatcher1-wrapper.js
+++ b/lib/dispatcher/dispatcher1-wrapper.js
@@ -60,6 +60,56 @@ class LegacyHandlerWrapper {
   }
 }
 
+// Legacy consumers (e.g. Node's bundled fetch) may send an identical
+// comma-repeated content-length ("58, 58") that the current core rejects.
+// RFC 9110 allows collapsing identical repeats; conflicting values still
+// fail downstream. See https://github.com/nodejs/undici/issues/5500
+function collapseRepeatedContentLength (value) {
+  if (typeof value !== 'string' || !value.includes(',')) {
+    return value
+  }
+
+  const parts = value.split(',')
+  const first = parts[0].trim()
+
+  if (first === '') {
+    return value
+  }
+
+  for (let i = 1; i < parts.length; i++) {
+    if (parts[i].trim() !== first) {
+      return value
+    }
+  }
+
+  return first
+}
+
+function normalizeLegacyHeaders (headers) {
+  if (Array.isArray(headers)) {
+    for (let i = 0; i + 1 < headers.length; i += 2) {
+      if (typeof headers[i] === 'string' && headers[i].toLowerCase() === 'content-length') {
+        const collapsed = collapseRepeatedContentLength(headers[i + 1])
+        if (collapsed !== headers[i + 1]) {
+          headers = headers.slice()
+          headers[i + 1] = collapsed
+        }
+      }
+    }
+  } else if (headers && typeof headers === 'object') {
+    for (const key of Object.keys(headers)) {
+      if (key.toLowerCase() === 'content-length') {
+        const collapsed = collapseRepeatedContentLength(headers[key])
+        if (collapsed !== headers[key]) {
+          headers = { ...headers, [key]: collapsed }
+        }
+      }
+    }
+  }
+
+  return headers
+}
+
 class Dispatcher1Wrapper extends Dispatcher {
   #dispatcher
 
@@ -92,6 +142,11 @@ class Dispatcher1Wrapper extends Dispatcher {
       opts = { ...opts, allowH2: false }
     }
 
+    const headers = normalizeLegacyHeaders(opts.headers)
+    if (headers !== opts.headers) {
+      opts = { ...opts, headers }
+    }
+
     return this.#dispatcher.dispatch(opts, Dispatcher1Wrapper.wrapHandler(handler))
   }
 
diff --git a/test/dispatcher1-wrapper-content-length.js b/test/dispatcher1-wrapper-content-length.js
new file mode 100644
index 00000000000..bec910e99ac
--- /dev/null
+++ b/test/dispatcher1-wrapper-content-length.js
@@ -0,0 +1,117 @@
+'use strict'
+
+const { test, after } = require('node:test')
+const assert = require('node:assert')
+const { createServer } = require('node:http')
+const { once } = require('node:events')
+const { Agent, Dispatcher1Wrapper } = require('..')
+
+// See https://github.com/nodejs/undici/issues/5500
+
+function legacyDispatch (wrapper, opts) {
+  return new Promise((resolve, reject) => {
+    let body = ''
+    wrapper.dispatch(opts, {
+      onConnect () {},
+      onHeaders (statusCode) {
+        this.statusCode = statusCode
+        return true
+      },
+      onData (chunk) {
+        body += chunk
+        return true
+      },
+      onComplete () {
+        resolve({ statusCode: this.statusCode, body })
+      },
+      onError (err) {
+        reject(err)
+      }
+    })
+  })
+}
+
+async function startServer (t) {
+  const server = createServer((req, res) => {
+    let length = 0
+    req.on('data', (chunk) => { length += chunk.length })
+    req.on('end', () => {
+      res.end(`${length}:${req.headers['content-length']}`)
+    })
+  })
+  server.listen(0)
+  await once(server, 'listening')
+  return server
+}
+
+test('collapses identical repeated content-length from a legacy consumer', async (t) => {
+  const server = await startServer(t)
+  const agent = new Agent()
+  const wrapper = new Dispatcher1Wrapper(agent)
+  after(() => { server.close(); return agent.close() })
+
+  const { statusCode, body } = await legacyDispatch(wrapper, {
+    origin: `http://127.0.0.1:${server.address().port}`,
+    path: '/',
+    method: 'POST',
+    headers: { 'Content-Length': '13, 13', 'content-type': 'text/plain' },
+    body: 'update=INSERT'
+  })
+
+  assert.strictEqual(statusCode, 200)
+  assert.strictEqual(body, '13:13')
+})
+
+test('collapses identical repeated content-length in flat array headers', async (t) => {
+  const server = await startServer(t)
+  const agent = new Agent()
+  const wrapper = new Dispatcher1Wrapper(agent)
+  after(() => { server.close(); return agent.close() })
+
+  const { statusCode, body } = await legacyDispatch(wrapper, {
+    origin: `http://127.0.0.1:${server.address().port}`,
+    path: '/',
+    method: 'POST',
+    headers: ['content-length', '13 , 13', 'content-type', 'text/plain'],
+    body: 'update=INSERT'
+  })
+
+  assert.strictEqual(statusCode, 200)
+  assert.strictEqual(body, '13:13')
+})
+
+test('still rejects conflicting repeated content-length', async (t) => {
+  const server = await startServer(t)
+  const agent = new Agent()
+  const wrapper = new Dispatcher1Wrapper(agent)
+  after(() => { server.close(); return agent.close() })
+
+  await assert.rejects(
+    legacyDispatch(wrapper, {
+      origin: `http://127.0.0.1:${server.address().port}`,
+      path: '/',
+      method: 'POST',
+      headers: { 'content-length': '10, 13' },
+      body: 'update=INSERT'
+    }),
+    { code: 'UND_ERR_INVALID_ARG', message: 'invalid content-length header' }
+  )
+})
+
+test('does not mutate the caller-provided headers object', async (t) => {
+  const server = await startServer(t)
+  const agent = new Agent()
+  const wrapper = new Dispatcher1Wrapper(agent)
+  after(() => { server.close(); return agent.close() })
+
+  const headers = { 'content-length': '13, 13' }
+  await legacyDispatch(wrapper, {
+    origin: `http://127.0.0.1:${server.address().port}`,
+    path: '/',
+    method: 'POST',
+    headers,
+    body: 'update=INSERT'
+  })
+
+  assert.strictEqual(headers['content-length'], '13, 13')
+})
diff --git a/test/fixtures/global-fetch-content-length.js b/test/fixtures/global-fetch-content-length.js
new file mode 100644
index 00000000000..42e030bcf78
--- /dev/null
+++ b/test/fixtures/global-fetch-content-length.js
@@ -0,0 +1,38 @@
+'use strict'
+
+// Exercising Node's *global* fetch/Headers against the dispatcher bridge
+// is the point. See https://github.com/nodejs/undici/issues/5500
+/* eslint-disable no-restricted-globals */
+
+require('../..')
+
+const http = require('node:http')
+const { once } = require('node:events')
+
+async function main () {
+  const server = http.createServer((req, res) => {
+    let length = 0
+    req.on('data', (chunk) => { length += chunk.length })
+    req.on('end', () => res.end(String(length)))
+  })
+  server.listen(0)
+  await once(server, 'listening')
+
+  try {
+    const body = 'update=INSERT'
+    const headers = new Headers()
+    headers.append('Content-Type', 'application/x-www-form-urlencoded')
+    headers.append('Content-Length', String(body.length))
+
+    const url = 'http://127.0.0.1:' + server.address().port
+    const res = await fetch(url, { method: 'POST', headers, body })
+    process.stdout.write(await res.text())
+  } finally {
+    server.close()
+  }
+}
+
+main().catch((err) => {
+  console.error(err?.cause?.stack || err?.stack || err)
+  process.exitCode = 1
+})
diff --git a/test/node-test/global-dispatcher-version.js b/test/node-test/global-dispatcher-version.js
index 8fa688043e6..bfbfa24f05e 100644
--- a/test/node-test/global-dispatcher-version.js
+++ b/test/node-test/global-dispatcher-version.js
@@ -98,6 +98,18 @@ test('setGlobalDispatcher mirrors a v1-compatible dispatcher that Node.js global
   assert.strictEqual(payload.mirroredV2, true)
 })
 
+test('requiring undici does not break Node.js global fetch with a request-set Content-Length', () => {
+  // See https://github.com/nodejs/undici/issues/5500
+  const result = spawnSync(
+    process.execPath,
+    [join(__dirname, '../fixtures/global-fetch-content-length.js')],
+    { cwd, encoding: 'utf8' }
+  )
+
+  assert.strictEqual(result.status, 0, result.stderr)
+  assert.strictEqual(result.stdout, '13')
+})
+
 test('Dispatcher1Wrapper bridges legacy handlers to a new Agent', () => {
   const script = `
     const { Agent, Dispatcher1Wrapper } = require('./index.js')