diff --git a/packages/runtime/src/scr_cycle.c b/packages/runtime/src/scr_cycle.c
index c5ea9e26e..2aba7406b 100644
--- a/packages/runtime/src/scr_cycle.c
+++ b/packages/runtime/src/scr_cycle.c
@@ -266,8 +266,10 @@ SCR_TL size_t scr_cyc_live = 0;
 SCR_TL size_t scr_cyc_old_freed = 0;
 #endif
 
-/* Descriptor copies by registry id. Id 0 stays empty (an unregistered type). */
-ScrCycType scr_cyc_types[SCR_CYC_MAX_TYPES];
+/* Descriptor copies by registry id. Id 0 stays empty (an unregistered type).
+ * Explicitly initialized so library localization can demote it: a tentative
+ * definition is a COMMON symbol on Mach-O, and COMMON symbols stay global. */
+ScrCycType scr_cyc_types[SCR_CYC_MAX_TYPES] = {0};
 static uint32_t scr_cyc_ntypes = 1;
 
 /* Assign `type` its id on its first allocation; any thread may get there
diff --git a/packages/runtime/src/scr_string.c b/packages/runtime/src/scr_string.c
index 71016cfdc..3010721f8 100644
--- a/packages/runtime/src/scr_string.c
+++ b/packages/runtime/src/scr_string.c
@@ -96,8 +96,10 @@ typedef struct ScrSidx {
 /* Keep short-string cursor traffic out of the sparse cache. A short-lived
  * one-byte receiver can be far more common than a large indexed one; sharing
  * the round-robin slots would otherwise rebuild a warm index every few calls.
+ * Explicitly initialized so library localization can demote it: a tentative
+ * definition is a COMMON symbol on Mach-O, and COMMON symbols stay global.
  */
-SCR_TL ScrStrScan scr_str_scan;
+SCR_TL ScrStrScan scr_str_scan = {0};
 static SCR_TL ScrSidx scr_sidx_sparse_tab[SCR_SIDX_N];
 static SCR_TL ScrSidx scr_sidx_cursor_tab[SCR_SIDX_N];
 static SCR_TL unsigned scr_sidx_sparse_clock;
@@ -936,7 +938,8 @@ static inline uint32_t scr_utf8_decode(const char *p, size_t *adv) {
 
 /* ── WTF-8 lone surrogates (scr_runtime.h) ────────────────────────── */
 
-bool scr_wtf8_lone;
+/* Explicitly initialized for library localization (see scr_str_scan). */
+bool scr_wtf8_lone = false;
 
 void scr_wtf8_note(void) {
   if (!scr_wtf8_any()) __atomic_store_n(&scr_wtf8_lone, true, __ATOMIC_RELAXED);
diff --git a/packages/runtime/test/common-symbols.test.ts b/packages/runtime/test/common-symbols.test.ts
new file mode 100644
index 000000000..1244f258d
--- /dev/null
+++ b/packages/runtime/test/common-symbols.test.ts
@@ -0,0 +1,58 @@
+import { execFile } from "node:child_process";
+import { mkdir, readdir, readFile } from "node:fs/promises";
+import { availableParallelism } from "node:os";
+import { join } from "node:path";
+import { promisify } from "node:util";
+import { expect, test } from "vitest";
+
+const execFileAsync = promisify(execFile);
+const testDir = import.meta.dirname;
+const srcDir = join(testDir, "../src");
+const vendorDir = join(testDir, "../vendor");
+const buildDir = join(testDir, "build", "common-symbols");
+
+// Library localization (abi.localize_runtime) keeps COMMON symbols global,
+// so a runtime global written as a tentative definition (`T x;` at file
+// scope with no initializer) is exported from every localized library
+// archive on toolchains that emit tentative definitions as COMMON symbols,
+// such as the macOS runtime build. Linux clang defaults to -fno-common, where
+// the same declaration is an ordinary definition, so the library-multi
+// symbol checks only catch it on macOS. This test compiles every runtime
+// source with -fcommon, in the plain and worker configurations, and expects
+// no COMMON symbols: every external runtime global needs an initializer.
+test("runtime globals are not tentative definitions", async () => {
+  await mkdir(buildDir, { recursive: true });
+  const sources = (await readdir(srcDir))
+    .filter((name) => name.endsWith(".c") && !name.endsWith(".test.c"))
+    .sort();
+  const jobs = sources.flatMap((name) =>
+    [[], ["-DSCR_WORKERS"]].map((defines) => ({ name, defines })),
+  );
+  const common: string[] = [];
+  let next = 0;
+  const worker = async (): Promise<void> => {
+    while (next < jobs.length) {
+      const { name, defines } = jobs[next++]!;
+      const output = join(buildDir, `${name}${defines.join("")}.ll`);
+      await execFileAsync("clang", [
+        "-std=c11",
+        "-O0",
+        "-fcommon",
+        "-S",
+        "-emit-llvm",
+        ...(process.platform === "linux" ? ["-D_GNU_SOURCE"] : []),
+        ...defines,
+        `-I${join(vendorDir, "quickjs-ng")}`,
+        `-I${join(vendorDir, "mbedtls", "include")}`,
+        "-o",
+        output,
+        join(srcDir, name),
+      ]);
+      for (const match of (await readFile(output, "utf8")).matchAll(/^@(\S+) = common /gm)) {
+        common.push(`${name}${defines.length > 0 ? ` (${defines.join(" ")})` : ""}: ${match[1]}`);
+      }
+    }
+  };
+  await Promise.all(Array.from({ length: Math.min(8, availableParallelism()) }, worker));
+  expect(common.sort()).toEqual([]);
+});