diff --git a/README.md b/README.md
index 169d0cab13d..fe58899affc 100644
--- a/README.md
+++ b/README.md
@@ -86,6 +86,24 @@ Using [benchmark-http2.js](https://github.com/nodejs/undici/blob/main/benchmarks
 
 Node.js includes a built-in `fetch()` implementation powered by undici starting from Node.js v18. However, there are important differences between using the built-in fetch and installing undici as a separate module.
 
+### WebAssembly requirement
+
+Undici's HTTP/1 parser requires WebAssembly. There is no alternative parser when
+WebAssembly is unavailable, for example in Node.js configurations where
+`--jitless` disables it. The existence of `fetch` does not imply that it can make
+HTTP/1 requests in that environment.
+
+When the HTTP/1 parser is needed without WebAssembly, Client and dispatcher
+requests fail with an `Error` whose code is `ERR_WEBASSEMBLY_NOT_SUPPORTED`.
+`fetch` rejects with a `TypeError` carrying that error in `error.cause`. Importing
+Undici, accessing `fetch`, and using `Headers`, `FormData`, `Request`, or `Response`
+do not themselves require the HTTP/1 parser.
+
+The error is defined locally in Undici and does not depend on Node.js providing
+that error code. For built-in fetch, this behavior depends on the bundled Undici
+version. Node.js receives upstream fixes through its normal vendored Undici
+update, rather than changes to the generated bundle alone.
+
 ### Built-in Fetch (Node.js v18+)
 
 Node.js's built-in fetch is powered by a bundled version of undici:
diff --git a/docs/docs/api/Errors.md b/docs/docs/api/Errors.md
index c00729009f9..57ad0afa883 100644
--- a/docs/docs/api/Errors.md
+++ b/docs/docs/api/Errors.md
@@ -18,7 +18,7 @@ if (err instanceof errors.ConnectTimeoutError) {
 }
 ```
 
-All errors, except [`HTTPParserError`][], extend [`UndiciError`][]. Each error
+All errors, except [`HTTPParserError`][] and `WebAssemblyNotSupportedError`, extend [`UndiciError`][]. Each error
 carries a stable `code` string (for example `UND_ERR_CONNECT_TIMEOUT`) and a
 `name`.
 
@@ -52,6 +52,35 @@ on a well-known symbol rather than the prototype chain.
 * `name` {string} Always `'UndiciError'`.
 * `code` {string} Always `'UND_ERR'`.
 
+## Class: `WebAssemblyNotSupportedError`
+
+* Extends: {Error}
+
+WebAssembly is unavailable when Undici initializes its HTTP/1 parser. Undici does
+not provide an alternative parser for environments without WebAssembly, including
+Node.js configurations where `--jitless` disables it. The check is performed when
+the parser is needed, not when importing Undici or accessing `fetch`.
+
+* `name` {string} Always `'Error'`.
+* `code` {string} Always `'ERR_WEBASSEMBLY_NOT_SUPPORTED'`.
+* `message` {string} By default `'WebAssembly is not supported in this environment, but is required for HTTP/1 parsing'`.
+
+Client and dispatcher requests report this error directly. `fetch` rejects with
+a `TypeError` whose `cause` is this error:
+
+```js
+try {
+  await fetch('http://localhost:3000')
+} catch (error) {
+  if (error.cause?.code === 'ERR_WEBASSEMBLY_NOT_SUPPORTED') {
+    // HTTP/1 parsing requires WebAssembly in this environment.
+  }
+}
+```
+
+`Headers`, `FormData`, `Request`, `Response`, and other operations that do not need
+the HTTP/1 parser remain usable without WebAssembly.
+
 ## Class: `ConnectTimeoutError`
 
 <!-- YAML
diff --git a/lib/core/errors.js b/lib/core/errors.js
index ca2ac09e846..cf5026546a6 100644
--- a/lib/core/errors.js
+++ b/lib/core/errors.js
@@ -306,6 +306,22 @@ class BalancedPoolMissingUpstreamError extends UndiciError {
   }
 }
 
+const kWebAssemblyNotSupportedError = Symbol.for('undici.error.ERR_WEBASSEMBLY_NOT_SUPPORTED')
+class WebAssemblyNotSupportedError extends Error {
+  constructor (message) {
+    super(message || 'WebAssembly is not supported in this environment, but is required for HTTP/1 parsing')
+    this.code = 'ERR_WEBASSEMBLY_NOT_SUPPORTED'
+  }
+
+  static [Symbol.hasInstance] (instance) {
+    return instance && instance[kWebAssemblyNotSupportedError] === true
+  }
+
+  get [kWebAssemblyNotSupportedError] () {
+    return true
+  }
+}
+
 const kHTTPParserError = Symbol.for('undici.error.UND_ERR_HTTP_PARSER')
 class HTTPParserError extends Error {
   constructor (message, code, data) {
@@ -487,6 +503,7 @@ class MessageSizeExceededError extends UndiciError {
 
 module.exports = {
   AbortError,
+  WebAssemblyNotSupportedError,
   HTTPParserError,
   UndiciError,
   HeadersTimeoutError,
diff --git a/lib/dispatcher/client-h1.js b/lib/dispatcher/client-h1.js
index 4f2aa524011..1d3a6cbb820 100644
--- a/lib/dispatcher/client-h1.js
+++ b/lib/dispatcher/client-h1.js
@@ -17,6 +17,7 @@ const {
   InformationalError,
   BodyTimeoutError,
   HTTPParserError,
+  WebAssemblyNotSupportedError,
   ResponseExceededMaxSizeError
 } = require('../core/errors.js')
 const {
@@ -68,6 +69,11 @@ const kTypeOfService = Symbol('kTypeOfService')
 let extractBody
 
 function lazyllhttp () {
+  // Check only when HTTP/1 needs the parser, so other APIs remain usable without WASM.
+  if (typeof WebAssembly === 'undefined') {
+    throw new WebAssemblyNotSupportedError()
+  }
+
   const llhttpWasmData = process.env.JEST_WORKER_ID ? require('../llhttp/llhttp-wasm.js') : undefined
 
   let mod
diff --git a/lib/dispatcher/parser-h1.js b/lib/dispatcher/parser-h1.js
index 456c44a3cc8..cabd023faaf 100644
--- a/lib/dispatcher/parser-h1.js
+++ b/lib/dispatcher/parser-h1.js
@@ -60,15 +60,6 @@ if (process.env.UNDICI_NO_WASM_SIMD === '1') {
   useWasmSIMD = true
 }
 
-if (useWasmSIMD) {
-  try {
-    miloPackage = require('../milo/src/simd/index.js')
-  } catch {}
-}
-
-miloPackage = miloPackage || require('../milo/src/no-simd/index.js')
-const { setup } = miloPackage
-
 const USE_NATIVE_TIMER = 0
 const USE_FAST_TIMER = 1
 const TIMEOUT_HEADERS = 2 | USE_FAST_TIMER
@@ -110,7 +101,15 @@ function getMilo () {
     return miloInstance
   }
 
-  miloInstance = setup()
+  // Milo modules compile WASM on load; defer loading until the parser is needed.
+  if (useWasmSIMD) {
+    try {
+      miloPackage = require('../milo/src/simd/index.js')
+    } catch {}
+  }
+
+  miloPackage = miloPackage || require('../milo/src/no-simd/index.js')
+  miloInstance = miloPackage.setup()
   return miloInstance
 }
 
diff --git a/test/client-wasm-unavailable.js b/test/client-wasm-unavailable.js
new file mode 100644
index 00000000000..6c963b4cc16
--- /dev/null
+++ b/test/client-wasm-unavailable.js
@@ -0,0 +1,31 @@
+'use strict'
+
+const assert = require('node:assert/strict')
+const { spawnSync } = require('node:child_process')
+const { join } = require('node:path')
+const { test } = require('node:test')
+
+// Each process starts with an empty parser cache and must exit without open sockets.
+for (const mode of ['deleted', 'undefined', 'jitless', 'normal', 'no-simd', 'fallback', 'milo', 'milo-unavailable']) {
+  test(`HTTP/1 WebAssembly support: ${mode}`, (t) => {
+    const args = mode === 'jitless' ? ['--jitless'] : []
+    const result = spawnSync(process.execPath, [
+      ...args,
+      join(__dirname, 'fixtures/wasm-support.js'),
+      mode
+    ], {
+      env: { ...process.env, UNDICI_NO_WASM_SIMD: '0', UNDICI_USE_MILO: '0' },
+      timeout: 10000,
+      encoding: 'utf8'
+    })
+
+    assert.ifError(result.error)
+    assert.equal(result.status, 0, result.stderr)
+    assert.equal(result.signal, null)
+    if (result.stdout.trim() === 'WebAssembly available under --jitless') {
+      t.skip('--jitless does not disable WebAssembly in this runtime')
+      return
+    }
+    assert.equal(result.stdout.trim(), 'ok')
+  })
+}
diff --git a/test/errors.js b/test/errors.js
index e34b406a905..35e436acd1c 100644
--- a/test/errors.js
+++ b/test/errors.js
@@ -13,6 +13,7 @@ const createScenario = (ErrorClass, defaultMessage, name, code) => ({
 })
 
 const scenarios = [
+  createScenario(errors.WebAssemblyNotSupportedError, 'WebAssembly is not supported in this environment, but is required for HTTP/1 parsing', 'Error', 'ERR_WEBASSEMBLY_NOT_SUPPORTED'),
   createScenario(errors.UndiciError, '', 'UndiciError', 'UND_ERR'),
   createScenario(errors.ConnectTimeoutError, 'Connect Timeout Error', 'ConnectTimeoutError', 'UND_ERR_CONNECT_TIMEOUT'),
   createScenario(errors.HeadersTimeoutError, 'Headers Timeout Error', 'HeadersTimeoutError', 'UND_ERR_HEADERS_TIMEOUT'),
diff --git a/test/fixtures/wasm-support.js b/test/fixtures/wasm-support.js
new file mode 100644
index 00000000000..8bc8c095954
--- /dev/null
+++ b/test/fixtures/wasm-support.js
@@ -0,0 +1,138 @@
+'use strict'
+
+const assert = require('node:assert/strict')
+const { once } = require('node:events')
+const { createServer } = require('node:http')
+
+const mode = process.argv[2]
+if (mode === 'deleted') {
+  delete globalThis.WebAssembly
+} else if (mode === 'undefined') {
+  globalThis.WebAssembly = undefined
+} else if (mode === 'no-simd') {
+  process.env.UNDICI_NO_WASM_SIMD = '1'
+} else if (mode === 'milo') {
+  process.env.UNDICI_USE_MILO = '1'
+} else if (mode === 'milo-unavailable') {
+  process.env.UNDICI_USE_MILO = '1'
+  delete globalThis.WebAssembly
+}
+
+let compilations = 0
+if (mode === 'fallback') {
+  const Module = WebAssembly.Module
+  // Reject the SIMD attempt while allowing the generic module to compile.
+  WebAssembly.Module = function (bytes) {
+    if (++compilations === 1) {
+      throw new WebAssembly.CompileError('SIMD unavailable for this test')
+    }
+    return new Module(bytes)
+  }
+}
+
+function checkError (error) {
+  assert.ok(error instanceof Error)
+  assert.ok(!(error instanceof TypeError))
+  assert.equal(error.name, 'Error')
+  assert.equal(error.code, 'ERR_WEBASSEMBLY_NOT_SUPPORTED')
+  assert.equal(error.message, 'WebAssembly is not supported in this environment, but is required for HTTP/1 parsing')
+  assert.equal(error.cause, undefined)
+  return true
+}
+
+async function checkIndependentAPIs (undici) {
+  assert.equal(typeof undici.fetch, 'function')
+  const headers = new undici.Headers({ 'x-test': 'value' })
+  assert.equal(headers.get('x-test'), 'value')
+  const form = new undici.FormData()
+  form.append('field', 'value')
+  assert.equal(form.get('field'), 'value')
+  const request = new undici.Request('http://localhost', { method: 'POST', body: form })
+  assert.equal((await request.formData()).get('field'), 'value')
+  const response = new undici.Response('usable')
+  assert.equal(await response.text(), 'usable')
+  // A fetch operation without the HTTP/1 parser remains possible.
+  assert.equal(await (await undici.fetch('data:text/plain,usable')).text(), 'usable')
+}
+
+async function main () {
+  if (mode === 'jitless' && typeof WebAssembly !== 'undefined') {
+    console.log('WebAssembly available under --jitless')
+    return
+  }
+
+  // Import after removing WASM to catch accidental eager compilation.
+  const undici = require('../..')
+  const nodeFetch = require('../../index-fetch')
+  await checkIndependentAPIs(undici)
+  await checkIndependentAPIs(nodeFetch)
+
+  const server = createServer((req, res) => res.end('ok'))
+  server.listen(0, '127.0.0.1')
+  await once(server, 'listening')
+  const origin = `http://127.0.0.1:${server.address().port}`
+  const client = new undici.Client(origin)
+  const agent = new undici.Agent()
+
+  try {
+    if (typeof WebAssembly === 'undefined') {
+      // Reusing the client must reject again rather than leave its queue stalled.
+      for (let i = 0; i < 2; i++) {
+        await assert.rejects(client.request({ path: '/', method: 'GET' }), checkError)
+      }
+      await assert.rejects(new Promise((resolve, reject) => {
+        client.dispatch({ path: '/', method: 'GET' }, {
+          onRequestStart () {},
+          onResponseStart () { reject(new Error('Unexpected response')) },
+          onResponseData () {},
+          onResponseEnd () { resolve() },
+          onResponseError (controller, error) { reject(error) }
+        })
+      }), checkError)
+      await assert.rejects(undici.request(origin, { dispatcher: agent }), checkError)
+      for (const { fetch } of [undici, nodeFetch]) {
+        await assert.rejects(fetch(origin, { dispatcher: agent }), error => {
+          assert.ok(error instanceof TypeError)
+          assert.equal(error.message, 'fetch failed')
+          return checkError(error.cause)
+        })
+      }
+      await checkIndependentAPIs(undici)
+      // Node's native HTTP parser is independent of Undici's WASM parser.
+      const { get } = require('node:http')
+      await new Promise((resolve, reject) => {
+        get(origin, res => {
+          res.resume()
+          res.on('end', resolve)
+          res.on('error', reject)
+        }).on('error', reject)
+      })
+    } else {
+      const response = await client.request({ path: '/', method: 'GET' })
+      assert.equal(response.statusCode, 200)
+      assert.equal(await response.body.text(), 'ok')
+      const fetched = await undici.fetch(origin, { dispatcher: agent })
+      assert.equal(fetched.status, 200)
+      assert.equal(await fetched.text(), 'ok')
+      if (mode === 'fallback') {
+        assert.equal(compilations, 2)
+      }
+    }
+  } finally {
+    await client.destroy()
+    await agent.destroy()
+    await new Promise((resolve, reject) => server.close(error => {
+      if (error) {
+        reject(error)
+      } else {
+        resolve()
+      }
+    }))
+  }
+  console.log('ok')
+}
+
+main().catch(error => {
+  console.error(error)
+  process.exitCode = 1
+})
diff --git a/test/types/errors.test-d.ts b/test/types/errors.test-d.ts
index a8ad7fcabfe..35fffc9583d 100644
--- a/test/types/errors.test-d.ts
+++ b/test/types/errors.test-d.ts
@@ -7,6 +7,11 @@ expectAssignable<errors.UndiciError>(new errors.UndiciError())
 expectAssignable<string>(new errors.UndiciError().name)
 expectAssignable<string>(new errors.UndiciError().code)
 
+expectAssignable<Error>(new errors.WebAssemblyNotSupportedError())
+expectAssignable<errors.WebAssemblyNotSupportedError>(new errors.WebAssemblyNotSupportedError())
+expectAssignable<'Error'>(new errors.WebAssemblyNotSupportedError().name)
+expectAssignable<'ERR_WEBASSEMBLY_NOT_SUPPORTED'>(new errors.WebAssemblyNotSupportedError().code)
+
 expectAssignable<errors.UndiciError>(new errors.ConnectTimeoutError())
 expectAssignable<errors.ConnectTimeoutError>(new errors.ConnectTimeoutError())
 expectAssignable<'ConnectTimeoutError'>(new errors.ConnectTimeoutError().name)
diff --git a/types/errors.d.ts b/types/errors.d.ts
index 644a0302d68..90246b31648 100644
--- a/types/errors.d.ts
+++ b/types/errors.d.ts
@@ -121,6 +121,13 @@ declare namespace Errors {
     code: string
   }
 
+  /** WebAssembly is unavailable when initializing the HTTP/1 parser. */
+  export class WebAssemblyNotSupportedError extends Error {
+    constructor (message?: string)
+    name: 'Error'
+    code: 'ERR_WEBASSEMBLY_NOT_SUPPORTED'
+  }
+
   /** The response exceed the length allowed. */
   export class ResponseExceededMaxSizeError extends UndiciError {
     name: 'ResponseExceededMaxSizeError'