diff --git a/src/jsc/bindings/NodeURL.cpp b/src/jsc/bindings/NodeURL.cpp
index 09af0674a4e..92c733cf3f9 100644
--- a/src/jsc/bindings/NodeURL.cpp
+++ b/src/jsc/bindings/NodeURL.cpp
@@ -4,6 +4,210 @@
 
 namespace Bun {
 
+// RFC 3492 Punycode encoder without ICU's ENCODE_MAX_CODE_UNITS (=1000) cap.
+// Used as a fallback when a single label is too long for u_strToPunycode.
+// Appends the encoding of `label` (UTF-16) to `out`, not including the "xn--" prefix.
+static bool punycodeEncodeLabel(std::span<const char16_t> label, Vector<char16_t>& out)
+{
+    constexpr char32_t base = 36, tMin = 1, tMax = 26, skew = 38, damp = 700;
+    constexpr char32_t initialBias = 72, initialN = 0x80;
+    auto adapt = [](char32_t delta, char32_t numPoints, bool firstTime) {
+        delta = firstTime ? delta / damp : delta / 2;
+        delta += delta / numPoints;
+        char32_t k = 0;
+        while (delta > ((base - tMin) * tMax) / 2) {
+            delta /= base - tMin;
+            k += base;
+        }
+        return k + (base - tMin + 1) * delta / (delta + skew);
+    };
+    auto encodeDigit = [](char32_t d) -> char16_t {
+        return static_cast<char16_t>(d + (d < 26 ? 'a' : ('0' - 26)));
+    };
+
+    Vector<char32_t, 256> codePoints;
+    codePoints.reserveCapacity(label.size());
+    for (size_t i = 0; i < label.size(); ++i) {
+        char16_t c = label[i];
+        if (U16_IS_LEAD(c) && i + 1 < label.size() && U16_IS_TRAIL(label[i + 1])) {
+            codePoints.append(U16_GET_SUPPLEMENTARY(c, label[i + 1]));
+            ++i;
+        } else if (U16_IS_SURROGATE(c)) {
+            return false;
+        } else
+            codePoints.append(c);
+    }
+
+    char32_t n = initialN, bias = initialBias;
+    size_t handled = 0;
+    for (auto cp : codePoints) {
+        if (cp < 0x80) {
+            out.append(static_cast<char16_t>(cp));
+            ++handled;
+        }
+    }
+    size_t basic = handled;
+    if (basic)
+        out.append('-');
+
+    WTF::CheckedUint32 delta = 0;
+    while (handled < codePoints.size()) {
+        char32_t m = std::numeric_limits<char32_t>::max();
+        for (auto cp : codePoints) {
+            if (cp >= n && cp < m)
+                m = cp;
+        }
+        delta += WTF::CheckedUint32(m - n) * WTF::CheckedUint32(static_cast<uint32_t>(handled + 1));
+        if (delta.hasOverflowed())
+            return false;
+        n = m;
+        for (auto cp : codePoints) {
+            if (cp < n) {
+                delta += 1;
+                if (delta.hasOverflowed())
+                    return false;
+            } else if (cp == n) {
+                char32_t q = delta.value();
+                for (char32_t k = base;; k += base) {
+                    char32_t t = k <= bias ? tMin : (k >= bias + tMax ? tMax : k - bias);
+                    if (q < t)
+                        break;
+                    out.append(encodeDigit(t + (q - t) % (base - t)));
+                    q = (q - t) / (base - t);
+                }
+                out.append(encodeDigit(q));
+                bias = adapt(delta.value(), static_cast<char32_t>(handled + 1), handled == basic);
+                delta = 0;
+                ++handled;
+            }
+        }
+        delta += 1;
+        ++n;
+    }
+    return true;
+}
+
+// Fallback for uidna_nameToASCII when a label exceeds ICU's ENCODE_MAX_CODE_UNITS:
+// run UTS #46 ToUnicode (which does mapping/NFC/validation but no Punycode encode
+// for non-ACE labels) then Punycode-encode each non-ASCII label ourselves.
+static bool nameToASCIIFallback(const UIDNA& transcoder, const char16_t* src, int32_t srcLength, Vector<char16_t>& ascii, UIDNAInfo& processingDetails)
+{
+    Vector<char16_t> unicode;
+    UIDNAInfo unicodeInfo = UIDNA_INFO_INITIALIZER;
+    UErrorCode error = U_ZERO_ERROR;
+    int32_t len = uidna_nameToUnicode(&transcoder, src, srcLength, nullptr, 0, &unicodeInfo, &error);
+    if (error != U_BUFFER_OVERFLOW_ERROR || len <= 0)
+        return false;
+    if (!unicode.tryGrow(static_cast<size_t>(len)))
+        return false;
+    error = U_ZERO_ERROR;
+    unicodeInfo = UIDNA_INFO_INITIALIZER;
+    len = uidna_nameToUnicode(&transcoder, src, srcLength, unicode.mutableSpan().data(), static_cast<int32_t>(unicode.size()), &unicodeInfo, &error);
+    if (U_FAILURE(error))
+        return false;
+    unicode.shrink(static_cast<size_t>(len));
+    processingDetails.errors = unicodeInfo.errors;
+    if (unicodeInfo.errors & ~WTF::URLParser::allowedNameToASCIIErrors)
+        return false;
+
+    ascii.reserveCapacity(unicode.size() + 16);
+    size_t labelStart = 0;
+    auto emitLabel = [&](size_t start, size_t end) -> bool {
+        auto label = unicode.subspan(start, end - start);
+        bool allAscii = true;
+        for (auto c : label) {
+            if (c >= 0x80) {
+                allAscii = false;
+                break;
+            }
+        }
+        if (allAscii) {
+            ascii.append(label);
+            if (label.size() > 63)
+                processingDetails.errors |= UIDNA_ERROR_LABEL_TOO_LONG;
+            return true;
+        }
+        size_t before = ascii.size();
+        ascii.append('x');
+        ascii.append('n');
+        ascii.append('-');
+        ascii.append('-');
+        if (!punycodeEncodeLabel(label, ascii))
+            return false;
+        if (ascii.size() - before > 63)
+            processingDetails.errors |= UIDNA_ERROR_LABEL_TOO_LONG;
+        return true;
+    };
+    for (size_t i = 0; i < unicode.size(); ++i) {
+        if (unicode[i] == '.') {
+            if (!emitLabel(labelStart, i))
+                return false;
+            ascii.append('.');
+            labelStart = i + 1;
+        }
+    }
+    if (!emitLabel(labelStart, unicode.size()))
+        return false;
+    if (ascii.size() > 253 && (ascii.size() > 254 || ascii.last() != '.'))
+        processingDetails.errors |= UIDNA_ERROR_DOMAIN_NAME_TOO_LONG;
+    return true;
+}
+
+static bool domainNameToASCII(StringView domain, Vector<char16_t>& ascii, UIDNAInfo& processingDetails)
+{
+    auto source = domain.upconvertedCharacters();
+    const auto& transcoder = WTF::URLParser::internationalDomainNameTranscoder();
+    UErrorCode error = U_ZERO_ERROR;
+    processingDetails = UIDNA_INFO_INITIALIZER;
+    std::array<char16_t, WTF::URLParser::hostnameBufferLength> buffer;
+    int32_t len = uidna_nameToASCII(&transcoder, source, domain.length(), buffer.data(), buffer.size(), &processingDetails, &error);
+    if (error == U_BUFFER_OVERFLOW_ERROR && len > 0) {
+        if (!ascii.tryGrow(static_cast<size_t>(len)))
+            return false;
+        error = U_ZERO_ERROR;
+        processingDetails = UIDNA_INFO_INITIALIZER;
+        len = uidna_nameToASCII(&transcoder, source, domain.length(), ascii.mutableSpan().data(), static_cast<int32_t>(ascii.size()), &processingDetails, &error);
+        if (U_SUCCESS(error) && len > 0) {
+            ascii.shrink(static_cast<size_t>(len));
+            return true;
+        }
+        ascii.clear();
+    }
+    if (error == U_INPUT_TOO_LONG_ERROR)
+        return nameToASCIIFallback(transcoder, source, domain.length(), ascii, processingDetails);
+    if (!U_SUCCESS(error) || len <= 0)
+        return false;
+    ascii.append(std::span { buffer }.first(static_cast<size_t>(len)));
+    return true;
+}
+
+static bool domainNameToUnicode(StringView domain, Vector<char16_t>& out, UIDNAInfo& processingDetails)
+{
+    auto source = domain.upconvertedCharacters();
+    const auto& transcoder = WTF::URLParser::internationalDomainNameTranscoder();
+    UErrorCode error = U_ZERO_ERROR;
+    processingDetails = UIDNA_INFO_INITIALIZER;
+    std::array<char16_t, WTF::URLParser::hostnameBufferLength> buffer;
+    int32_t len = uidna_nameToUnicode(&transcoder, source, domain.length(), buffer.data(), buffer.size(), &processingDetails, &error);
+    if (error == U_BUFFER_OVERFLOW_ERROR && len > 0) {
+        if (!out.tryGrow(static_cast<size_t>(len)))
+            return false;
+        error = U_ZERO_ERROR;
+        processingDetails = UIDNA_INFO_INITIALIZER;
+        len = uidna_nameToUnicode(&transcoder, source, domain.length(), out.mutableSpan().data(), static_cast<int32_t>(out.size()), &processingDetails, &error);
+        if (U_SUCCESS(error) && len > 0) {
+            out.shrink(static_cast<size_t>(len));
+            return true;
+        }
+        out.clear();
+        return false;
+    }
+    if (!U_SUCCESS(error) || len <= 0)
+        return false;
+    out.append(std::span { buffer }.first(static_cast<size_t>(len)));
+    return true;
+}
+
 JSC_DEFINE_HOST_FUNCTION(jsDomainToASCII, (JSC::JSGlobalObject * globalObject, JSC::CallFrame* callFrame))
 {
     auto& vm = JSC::getVM(globalObject);
@@ -52,21 +256,16 @@ JSC_DEFINE_HOST_FUNCTION(jsDomainToASCII, (JSC::JSGlobalObject * globalObject, J
 
     if (domain.containsOnlyASCII())
         return JSC::JSValue::encode(arg0);
-    if (domain.is8Bit())
-        domain.convertTo16Bit();
 
-    constexpr static int allowedNameToASCIIErrors = UIDNA_ERROR_EMPTY_LABEL | UIDNA_ERROR_LABEL_TOO_LONG | UIDNA_ERROR_DOMAIN_NAME_TOO_LONG | UIDNA_ERROR_LEADING_HYPHEN | UIDNA_ERROR_TRAILING_HYPHEN | UIDNA_ERROR_HYPHEN_3_4;
-    constexpr static size_t hostnameBufferLength = 2048;
-
-    auto encoder = &WTF::URLParser::internationalDomainNameTranscoder();
-    char16_t hostnameBuffer[hostnameBufferLength];
-    UErrorCode error = U_ZERO_ERROR;
+    Vector<char16_t> hostnameBuffer;
     UIDNAInfo processingDetails = UIDNA_INFO_INITIALIZER;
-    const auto span = domain.span16();
-    int32_t numCharactersConverted = uidna_nameToASCII(encoder, span.data(), span.size(), hostnameBuffer, hostnameBufferLength, &processingDetails, &error);
-
-    if (U_SUCCESS(error) && !(processingDetails.errors & ~allowedNameToASCIIErrors) && numCharactersConverted) {
-        return JSC::JSValue::encode(JSC::jsString(vm, WTF::String(std::span { hostnameBuffer, static_cast<unsigned int>(numCharactersConverted) })));
+    if (domainNameToASCII(domain, hostnameBuffer, processingDetails)
+        && !(processingDetails.errors & ~WTF::URLParser::allowedNameToASCIIErrors) && !hostnameBuffer.isEmpty()) {
+        Vector<Latin1Character> ascii;
+        ascii.reserveInitialCapacity(hostnameBuffer.size());
+        for (auto c : hostnameBuffer)
+            ascii.append(static_cast<Latin1Character>(c));
+        return JSC::JSValue::encode(JSC::jsString(vm, WTF::String(ascii.span())));
     }
     return JSC::JSValue::encode(jsEmptyString(vm));
 }
@@ -121,22 +320,13 @@ JSC_DEFINE_HOST_FUNCTION(jsDomainToUnicode, (JSC::JSGlobalObject * globalObject,
         // this function is only for undoing punycode so its okay if utf-16 text makes it out unchanged.
         return JSC::JSValue::encode(arg0);
 
-    domain.convertTo16Bit();
-
     constexpr static int allowedNameToUnicodeErrors = UIDNA_ERROR_EMPTY_LABEL | UIDNA_ERROR_LABEL_TOO_LONG | UIDNA_ERROR_DOMAIN_NAME_TOO_LONG | UIDNA_ERROR_LEADING_HYPHEN | UIDNA_ERROR_TRAILING_HYPHEN | UIDNA_ERROR_HYPHEN_3_4;
-    constexpr static int hostnameBufferLength = 2048;
 
-    auto encoder = &WTF::URLParser::internationalDomainNameTranscoder();
-    char16_t hostnameBuffer[hostnameBufferLength];
-    UErrorCode error = U_ZERO_ERROR;
+    Vector<char16_t> hostnameBuffer;
     UIDNAInfo processingDetails = UIDNA_INFO_INITIALIZER;
-
-    const auto span = domain.span16();
-
-    int32_t numCharactersConverted = uidna_nameToUnicode(encoder, span.data(), span.size(), hostnameBuffer, hostnameBufferLength, &processingDetails, &error);
-
-    if (U_SUCCESS(error) && !(processingDetails.errors & ~allowedNameToUnicodeErrors) && numCharactersConverted) {
-        return JSC::JSValue::encode(JSC::jsString(vm, WTF::String(std::span { hostnameBuffer, static_cast<unsigned int>(numCharactersConverted) })));
+    if (domainNameToUnicode(domain, hostnameBuffer, processingDetails)
+        && !(processingDetails.errors & ~allowedNameToUnicodeErrors) && !hostnameBuffer.isEmpty()) {
+        return JSC::JSValue::encode(JSC::jsString(vm, WTF::String(hostnameBuffer.span())));
     }
     return JSC::JSValue::encode(jsEmptyString(vm));
 }
diff --git a/test/js/node/url/url-domain-ascii-unicode.test.js b/test/js/node/url/url-domain-ascii-unicode.test.js
index d46b30f8c2e..0431dc9b3cf 100644
--- a/test/js/node/url/url-domain-ascii-unicode.test.js
+++ b/test/js/node/url/url-domain-ascii-unicode.test.js
@@ -1,4 +1,5 @@
 import { describe, expect, test } from "bun:test";
+import punycode from "node:punycode";
 import url from "node:url";
 
 const pairs = [
@@ -99,3 +100,53 @@ describe("url.domainToUnicode", () => {
     });
   }
 });
+
+// UTS #46 ToASCII runs with VerifyDnsLength=false so host length must not reject.
+// These cases cover ICU's ENCODE_MAX_CODE_UNITS (1000 UTF-16 units per label)
+// and the former fixed 2048-unit output buffer.
+describe("url.domainToASCII/domainToUnicode with long internationalised hosts", () => {
+  const a = n => Buffer.alloc(n, "a").toString();
+
+  const longLabelCases = [
+    ["label just past ICU's 1000-unit cap", "\u00fc" + a(1000)],
+    ["1500-unit label", "\u00fc" + a(1499)],
+    ["long label followed by ASCII labels", "\u00fc" + a(1200) + ".example.com"],
+    ["long label with supplementary-plane code point", "\u{1F600}" + a(1100)],
+    ["long label of only BMP non-ASCII", "\u03b1\u03b2\u03b3\u03b4".repeat(300)],
+    ["several long labels with output over 2048 units", `${"\u00fc" + a(1000)}.`.repeat(3) + "com"],
+  ];
+  for (const [name, input] of longLabelCases) {
+    test(`ToASCII encodes a ${name}`, () => {
+      const got = url.domainToASCII(input);
+      // Reference: the pure-JS RFC 3492 encoder in node:punycode has no length
+      // cap. UTS #46 mapping for these inputs reduces to NFC + lowercasing.
+      const mapped = input.normalize("NFC").toLowerCase();
+      expect(got).toBe(punycode.toASCII(mapped));
+      expect(got.startsWith("xn--")).toBe(true);
+      expect(url.domainToUnicode(got)).toBe(mapped);
+    });
+  }
+
+  test("ToASCII handles output longer than 2048 units (many labels)", () => {
+    const oneLabel = "\u00fc" + a(250);
+    const oneAce = punycode.toASCII(oneLabel);
+    const input = (oneLabel + ".").repeat(9) + "com";
+    const got = url.domainToASCII(input);
+    expect(got).toBe((oneAce + ".").repeat(9) + "com");
+    expect(got.length).toBe(2334);
+    expect(url.domainToUnicode(got)).toBe(input);
+  });
+
+  test("ToUnicode handles output longer than 2048 units", () => {
+    const oneLabel = "\u00fc" + a(250);
+    const oneAce = punycode.toASCII(oneLabel);
+    const input = (oneAce + ".").repeat(9) + "com";
+    const got = url.domainToUnicode(input);
+    expect(got).toBe((oneLabel + ".").repeat(9) + "com");
+    expect(url.domainToASCII(got)).toBe(input);
+  });
+
+  test("ToASCII still rejects disallowed code points in a long label", () => {
+    expect(url.domainToASCII("\u0378" + a(1100))).toBe("");
+  });
+});