diff --git a/scripts/zero-cost-compare.mjs b/scripts/zero-cost-compare.mjs
index 40f7287fb..accae34d8 100644
--- a/scripts/zero-cost-compare.mjs
+++ b/scripts/zero-cost-compare.mjs
@@ -393,6 +393,38 @@ export function dynamicTierSymbols(names) {
  * program without a call into Node.js may define any of these. */
 export const NODE_ISLAND_PREFIXES = ["scr_node_island", "scr_loop_set_node_island"];
 
+/** The namespace ICU's own symbols live in after its version-suffixed
+ * renaming: `icu_78`, which Itanium mangling spells `6icu_78`. */
+export const ICU_CPP_NAMESPACE = "6icu_78";
+
+/** The ICU archive: no program that answers its Intl forms without
+ * ICU — that is, every program whose IR has no `icu.*` libCall — may define
+ * a symbol from it. The test is exact because ICU builds with symbol
+ * renaming on: every C API symbol carries the `_78` version suffix
+ * (`ubrk_open_78`), its data symbol is `icudt78_dat`, and every C++ symbol
+ * is an Itanium mangling that names namespace `icu_78`, which the mangling
+ * spells `6icu_78` wherever the name occurs — as the function's own
+ * namespace (`_ZN6icu_78...`), in its parameter types, in vtables, typeinfo
+ * and guard variables, or in a libc++ instantiation over an ICU type. No
+ * symbol outside the archive carries either mark.
+ *
+ * scr_intl.c's own functions are deliberately not tested by name. The
+ * engine-free Intl paths share their prefix (`scr_intl_num_format_en_us` in
+ * scr_lib.c, `scr_intl_segmenter_new` in scr_grapheme.c), and they must stay
+ * linkable without ICU (I4). Any scr_intl.c function that does link brings an
+ * ICU symbol with it, which the test catches. The C++ standard library
+ * merged into the archive cannot leak on its own either: an archive
+ * contributes a member only when something references it, and only ICU's
+ * objects reference libc++. */
+export function icuSymbols(names) {
+  return [...new Set(names)]
+    .filter(
+      (name) =>
+        name.endsWith("_78") || name.includes(ICU_CPP_NAMESPACE) || name.startsWith("icudt78"),
+    )
+    .sort();
+}
+
 /** The Node island symbols among `names`, sorted. */
 export function nodeIslandSymbols(names) {
   return [...new Set(names)]
diff --git a/scripts/zero-cost-gate.mjs b/scripts/zero-cost-gate.mjs
index 5b5f51a21..aef7a4e60 100644
--- a/scripts/zero-cost-gate.mjs
+++ b/scripts/zero-cost-gate.mjs
@@ -48,6 +48,7 @@ import {
   linkedRuntimeFiles,
   llvmFunctionHashes,
   median,
+  icuSymbols,
   nodeIslandSymbols,
   nmOutput,
   parseNm,
@@ -379,6 +380,10 @@ function dynamicTierCheck(program, baseline, candidate, artifacts, allowed) {
   const nodeIsland = nodeIslandSymbols(definedFunctions(candidate.exe));
   if (nodeIsland.length > 0 && !allowed)
     failures.push(`links Node island symbols: ${nodeIsland.slice(0, 5).join(", ")}`);
+  // Intl over ICU is linked only into programs whose IR lowers to it. A
+  // program that legitimately calls it names the pattern in --allow.
+  const icu = icuSymbols(definedFunctions(candidate.exe));
+  if (icu.length > 0 && !allowed) failures.push(`links ICU symbols: ${icu.slice(0, 5).join(", ")}`);
   if (typed && after.length > 0)
     failures.push(
       `fully typed program links dynamic-tier symbols: ${after.slice(0, 5).join(", ")}`,
diff --git a/tests/harness/zero-cost-icu-symbols.test.ts b/tests/harness/zero-cost-icu-symbols.test.ts
new file mode 100644
index 000000000..ee9fac2a1
--- /dev/null
+++ b/tests/harness/zero-cost-icu-symbols.test.ts
@@ -0,0 +1,56 @@
+// The symbol classifier the zero-cost gate uses to prove that a program which
+// does not use Intl links no part of the ICU archive. The samples
+// are the shapes `nm` printed for the archive-linked binaries: ICU's C API
+// with its `_78` version suffix, its data symbol, and the several Itanium
+// manglings of names in namespace icu_78 — own namespace, parameters only,
+// internal linkage, guard variables, vtables and typeinfo.
+import { describe, expect, it } from "vitest";
+
+import { icuSymbols } from "../../scripts/zero-cost-compare.mjs";
+
+const ICU_SYMBOLS = [
+  "ubrk_open_78",
+  "unum_open_78",
+  "u_strToUTF8_78",
+  "icudt78_dat",
+  "_ZN6icu_7813UnicodeString6appendEPKDsi",
+  "_ZNK6icu_7824DateTimePatternGenerator17getDateTimeFormatE16UDateFormatStyleR10UErrorCode",
+  "_Z24ulocimp_toLanguageTag_78PKcRN6icu_788ByteSinkEbR10UErrorCode",
+  "_ZL10createPathPKciS0_iS0_RN6icu_7810CharStringEP10UErrorCode",
+  "_ZGVZN6icu_7823IslamicUmalquraCalendar19handleComputeFieldsEiR10UErrorCodeE14kUmalquraStart",
+  "_ZTWN6icu_7810CharStringE",
+  "_ZTVN6icu_7813UnicodeStringE",
+  "_ZTIN6icu_7813UnicodeStringE",
+  "_ZTSN6icu_7813UnicodeStringE",
+  // A libc++ instantiation over an ICU type exists only inside the archive.
+  "_ZNSt3__112basic_stringIcNS_11char_traitsIcEENS_9allocatorIcEEE6appendERN6icu_7810CharStringE",
+];
+
+// The engine-free Intl paths share the `scr_intl_` prefix and must stay
+// linkable without ICU (grapheme segmentation, the en-US number fast path),
+// so the classifier does not key on it.
+const NON_ICU_SYMBOLS = [
+  "main",
+  "scr_intl_num_format_en_us",
+  "scr_intl_segmenter_new",
+  "scr_intl_segmenter_refusing",
+  "scr_dyn_call",
+  "scr_string_concat",
+];
+
+describe("icuSymbols", () => {
+  it("classifies every ICU archive symbol", () => {
+    expect(icuSymbols(ICU_SYMBOLS)).toEqual([...ICU_SYMBOLS].sort());
+  });
+
+  it("leaves the engine-free runtime alone", () => {
+    expect(icuSymbols(NON_ICU_SYMBOLS)).toEqual([]);
+  });
+
+  it("deduplicates and sorts", () => {
+    expect(icuSymbols(["ubrk_close_78", "main", "ubrk_close_78", "icudt78_dat"])).toEqual([
+      "icudt78_dat",
+      "ubrk_close_78",
+    ]);
+  });
+});