diff --git a/okhttp/build.gradle.kts b/okhttp/build.gradle.kts
index cccc3bfb5..26e8a479c 100644
--- a/okhttp/build.gradle.kts
+++ b/okhttp/build.gradle.kts
@@ -97,6 +97,8 @@ kotlin {
         implementation(projects.okhttpSse)
         implementation(projects.okhttpCoroutines)
         implementation(libs.kotlinx.coroutines.core)
+        implementation(libs.squareup.moshi)
+        implementation(libs.squareup.moshi.kotlin)
         implementation(libs.squareup.okio.fakefilesystem)
         implementation(libs.conscrypt.openjdk)
         implementation(libs.junit)
diff --git a/okhttp/src/jvmTest/java/okhttp3/WebPlatformToAsciiData.kt b/okhttp/src/jvmTest/java/okhttp3/WebPlatformToAsciiData.kt
new file mode 100644
index 000000000..0d7e13461
--- /dev/null
+++ b/okhttp/src/jvmTest/java/okhttp3/WebPlatformToAsciiData.kt
@@ -0,0 +1,50 @@
+/*
+ * Copyright (C) 2023 Block, Inc.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+package okhttp3
+
+import com.squareup.moshi.Moshi
+import com.squareup.moshi.adapter
+import com.squareup.moshi.kotlin.reflect.KotlinJsonAdapterFactory
+import okio.FileSystem
+import okio.Path.Companion.toPath
+
+/**
+ * A test from the [Web Platform To ASCII](https://github.com/web-platform-tests/wpt/blob/master/url/resources/toascii.json).
+ *
+ * Each test is a line of the file `toascii.json`.
+ */
+class WebPlatformToAsciiData {
+  var input: String? = null
+  var output: String? = null
+  var comment: String? = null
+
+  override fun toString() = "input=$input output=$output"
+
+  companion object {
+    fun load(): List<WebPlatformToAsciiData> {
+      val moshi = Moshi.Builder()
+        .add(KotlinJsonAdapterFactory())
+        .build()
+
+      @OptIn(ExperimentalStdlibApi::class)
+      val adapter = moshi.adapter<List<WebPlatformToAsciiData>>()
+
+      return FileSystem.RESOURCES.read("/web-platform-test-toascii.json".toPath()) {
+        adapter.fromJson(this)!!
+      }
+    }
+  }
+}
diff --git a/okhttp/src/jvmTest/java/okhttp3/WebPlatformToAsciiTest.kt b/okhttp/src/jvmTest/java/okhttp3/WebPlatformToAsciiTest.kt
new file mode 100644
index 000000000..620651135
--- /dev/null
+++ b/okhttp/src/jvmTest/java/okhttp3/WebPlatformToAsciiTest.kt
@@ -0,0 +1,93 @@
+/*
+ * Copyright (C) 2023 Block, Inc.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+package okhttp3
+
+import assertk.assertThat
+import assertk.assertions.isEqualTo
+import assertk.assertions.isNotNull
+import okhttp3.HttpUrl.Companion.toHttpUrlOrNull
+import org.junit.jupiter.api.DynamicTest
+import org.junit.jupiter.api.TestFactory
+
+/** Runs the web platform ToAscii tests. */
+class WebPlatformToAsciiTest {
+  val knownFailures = setOf(
+    // OkHttp rejects empty labels.
+    "x..xn--zca",
+    "x..ß",
+
+    // OkHttp rejects labels longer than 63 code points, the web platform tests don't.
+    "x01234567890123456789012345678901234567890123456789012345678901x.xn--zca",
+    "x01234567890123456789012345678901234567890123456789012345678901x.ß",
+    "x01234567890123456789012345678901234567890123456789012345678901x",
+    "x01234567890123456789012345678901234567890123456789012345678901†",
+
+    // OkHttp rejects domain names longer than 253 code points, the web platform tests don't.
+    "01234567890123456789012345678901234567890123456789.01234567890123456789012345678901234567890123456789.01234567890123456789012345678901234567890123456789.01234567890123456789012345678901234567890123456789.0123456789012345678901234567890123456789012345678.x",
+    "01234567890123456789012345678901234567890123456789.01234567890123456789012345678901234567890123456789.01234567890123456789012345678901234567890123456789.01234567890123456789012345678901234567890123456789.0123456789012345678901234567890123456789012345678.xn--zca",
+    "01234567890123456789012345678901234567890123456789.01234567890123456789012345678901234567890123456789.01234567890123456789012345678901234567890123456789.01234567890123456789012345678901234567890123456789.0123456789012345678901234567890123456789012345678.ß",
+
+    // OkHttp incorrectly does transitional processing, so it maps 'ß' to 'ss'
+    "-x.ß",
+    "ab--c.ß",
+    "x-.ß",
+    "xn--a.ß",
+    "xn--zca.ß",
+    "ශ්‍රී",
+
+    // OkHttp does not reject invalid Punycode.
+    "xn--",
+    "xn--a",
+    "xn--a.xn--zca",
+    "xn--a-yoc",
+    "xn--ls8h=",
+
+    // OkHttp doesn't reject U+FFFD encoded in Punycode.
+    "xn--zn7c.com",
+
+    // OkHttp doesn't reject a U+200D. https://www.rfc-editor.org/rfc/rfc5892.html#appendix-A.2
+    "xn--1ug.example",
+
+    // OkHttp returns `xn--mgba3gch31f`, not `xn--mgba3gch31f060k`.
+    "نامه‌ای",
+  )
+
+  @TestFactory
+  fun testFactory(): List<DynamicTest> {
+    val list = WebPlatformToAsciiData.load()
+    return list.map { entry ->
+      DynamicTest.dynamicTest(entry.input!!) {
+        var failure: AssertionError? = null
+        try {
+          testToAscii(entry.input!!, entry.output, entry.comment)
+        } catch (e: AssertionError) {
+          failure = e
+        }
+
+        if (entry.input in knownFailures) {
+          assertThat(failure).isNotNull()
+        } else {
+          if (failure != null) throw failure
+        }
+      }
+    }
+  }
+
+  private fun testToAscii(input: String, output: String?, comment: String?) {
+    val url = "https://$input/".toHttpUrlOrNull()
+    assertThat(url?.host, name = comment ?: input).isEqualTo(output)
+  }
+}
diff --git a/okhttp/src/jvmTest/java/okhttp3/WebPlatformUrlTestData.kt b/okhttp/src/jvmTest/java/okhttp3/WebPlatformUrlTestData.kt
index 458d3f91d..47d72651c 100644
--- a/okhttp/src/jvmTest/java/okhttp3/WebPlatformUrlTestData.kt
+++ b/okhttp/src/jvmTest/java/okhttp3/WebPlatformUrlTestData.kt
@@ -20,7 +20,7 @@ import okio.Buffer
 import okio.BufferedSource
 
 /**
- * A test from the [Web Platform URL test suite](https://github.com/w3c/web-platform-tests/tree/master/url).
+ * A test from the [Web Platform URL test suite](https://github.com/web-platform-tests/wpt/blob/master/url/resources/urltestdata.json).
  *
  * Each test is a line of the file `urltestdata.txt`. The format is informally specified by its
  * JavaScript parser `urltestparser.js` with which this class attempts to be compatible.
diff --git a/okhttp/src/jvmTest/resources/web-platform-test-toascii.json b/okhttp/src/jvmTest/resources/web-platform-test-toascii.json
new file mode 100644
index 000000000..152d9dc87
--- /dev/null
+++ b/okhttp/src/jvmTest/resources/web-platform-test-toascii.json
@@ -0,0 +1,192 @@
+[
+  {
+    "comment": "Label with hyphens in 3rd and 4th position",
+    "input": "aa--",
+    "output": "aa--"
+  },
+  {
+    "input": "a†--",
+    "output": "xn--a---kp0a"
+  },
+  {
+    "input": "ab--c",
+    "output": "ab--c"
+  },
+  {
+    "comment": "Label with leading hyphen",
+    "input": "-x",
+    "output": "-x"
+  },
+  {
+    "input": "-†",
+    "output": "xn----xhn"
+  },
+  {
+    "input": "-x.xn--zca",
+    "output": "-x.xn--zca"
+  },
+  {
+    "input": "-x.ß",
+    "output": "-x.xn--zca"
+  },
+  {
+    "comment": "Label with trailing hyphen",
+    "input": "x-.xn--zca",
+    "output": "x-.xn--zca"
+  },
+  {
+    "input": "x-.ß",
+    "output": "x-.xn--zca"
+  },
+  {
+    "comment": "Empty labels",
+    "input": "x..xn--zca",
+    "output": "x..xn--zca"
+  },
+  {
+    "input": "x..ß",
+    "output": "x..xn--zca"
+  },
+  {
+    "comment": "Invalid Punycode",
+    "input": "xn--a",
+    "output": null
+  },
+  {
+    "input": "xn--a.xn--zca",
+    "output": null
+  },
+  {
+    "input": "xn--a.ß",
+    "output": null
+  },
+  {
+    "input": "xn--ls8h=",
+    "output": null
+  },
+  {
+    "comment": "Invalid Punycode (contains non-ASCII character)",
+    "input": "xn--tešla",
+    "output": null
+  },
+  {
+    "comment": "Valid Punycode",
+    "input": "xn--zca.xn--zca",
+    "output": "xn--zca.xn--zca"
+  },
+  {
+    "comment": "Mixed",
+    "input": "xn--zca.ß",
+    "output": "xn--zca.xn--zca"
+  },
+  {
+    "input": "ab--c.xn--zca",
+    "output": "ab--c.xn--zca"
+  },
+  {
+    "input": "ab--c.ß",
+    "output": "ab--c.xn--zca"
+  },
+  {
+    "comment": "CheckJoiners is true",
+    "input": "\u200D.example",
+    "output": null
+  },
+  {
+    "input": "xn--1ug.example",
+    "output": null
+  },
+  {
+    "comment": "CheckBidi is true",
+    "input": "يa",
+    "output": null
+  },
+  {
+    "input": "xn--a-yoc",
+    "output": null
+  },
+  {
+    "comment": "processing_option is Nontransitional_Processing",
+    "input": "ශ්‍රී",
+    "output": "xn--10cl1a0b660p"
+  },
+  {
+    "input": "نامه‌ای",
+    "output": "xn--mgba3gch31f060k"
+  },
+  {
+    "comment": "U+FFFD",
+    "input": "\uFFFD.com",
+    "output": null
+  },
+  {
+    "comment": "U+FFFD character encoded in Punycode",
+    "input": "xn--zn7c.com",
+    "output": null
+  },
+  {
+    "comment": "Label longer than 63 code points",
+    "input": "x01234567890123456789012345678901234567890123456789012345678901x",
+    "output": "x01234567890123456789012345678901234567890123456789012345678901x"
+  },
+  {
+    "input": "x01234567890123456789012345678901234567890123456789012345678901†",
+    "output": "xn--x01234567890123456789012345678901234567890123456789012345678901-6963b"
+  },
+  {
+    "input": "x01234567890123456789012345678901234567890123456789012345678901x.xn--zca",
+    "output": "x01234567890123456789012345678901234567890123456789012345678901x.xn--zca"
+  },
+  {
+    "input": "x01234567890123456789012345678901234567890123456789012345678901x.ß",
+    "output": "x01234567890123456789012345678901234567890123456789012345678901x.xn--zca"
+  },
+  {
+    "comment": "Domain excluding TLD longer than 253 code points",
+    "input": "01234567890123456789012345678901234567890123456789.01234567890123456789012345678901234567890123456789.01234567890123456789012345678901234567890123456789.01234567890123456789012345678901234567890123456789.0123456789012345678901234567890123456789012345678.x",
+    "output": "01234567890123456789012345678901234567890123456789.01234567890123456789012345678901234567890123456789.01234567890123456789012345678901234567890123456789.01234567890123456789012345678901234567890123456789.0123456789012345678901234567890123456789012345678.x"
+  },
+  {
+    "input": "01234567890123456789012345678901234567890123456789.01234567890123456789012345678901234567890123456789.01234567890123456789012345678901234567890123456789.01234567890123456789012345678901234567890123456789.0123456789012345678901234567890123456789012345678.xn--zca",
+    "output": "01234567890123456789012345678901234567890123456789.01234567890123456789012345678901234567890123456789.01234567890123456789012345678901234567890123456789.01234567890123456789012345678901234567890123456789.0123456789012345678901234567890123456789012345678.xn--zca"
+  },
+  {
+    "input": "01234567890123456789012345678901234567890123456789.01234567890123456789012345678901234567890123456789.01234567890123456789012345678901234567890123456789.01234567890123456789012345678901234567890123456789.0123456789012345678901234567890123456789012345678.ß",
+    "output": "01234567890123456789012345678901234567890123456789.01234567890123456789012345678901234567890123456789.01234567890123456789012345678901234567890123456789.01234567890123456789012345678901234567890123456789.0123456789012345678901234567890123456789012345678.xn--zca"
+  },
+  {
+    "comment": "IDNA ignored code points",
+    "input": "a\u00ADb",
+    "output": "ab"
+  },
+  {
+    "input": "a%C2%ADb",
+    "output": "ab"
+  },
+  {
+    "comment": "Empty host after domain to ASCII",
+    "input": "\u00AD",
+    "output": null
+  },
+  {
+    "input": "%C2%AD",
+    "output": null
+  },
+  {
+    "input": "xn--",
+    "output": null
+  },
+  {
+    "comment": "Interesting UseSTD3ASCIIRules=false cases",
+    "input": "≠",
+    "output": "xn--1ch"
+  },
+  {
+    "input": "≮",
+    "output": "xn--gdh"
+  },
+  {
+    "input": "≯",
+    "output": "xn--hdh"
+  }
+]