diff --git a/next.config.ts b/next.config.ts
index 2edfcfba3..672f10a91 100644
--- a/next.config.ts
+++ b/next.config.ts
@@ -1,8 +1,31 @@
+import { createHash } from "node:crypto";
+import { readdirSync, readFileSync } from "node:fs";
+import { join } from "node:path";
+
 import type { NextConfig } from "next";
 import bundleAnalyzer from "@next/bundle-analyzer";
 
 import { version as PKG_VERSION } from "./package.json";
 
+/**
+ * The message catalogs' content hash, the cache key of the boot script
+ * (`/i18n/<locale>.js?v=…`, `src/app/layout.tsx`). The service worker serves
+ * that URL cache-first, so a key that does not move with the catalog keeps
+ * an old catalog alive: keyed by the package version, every build of one
+ * release (a beta, a redeploy) shared one URL, and a key added since the
+ * first of them read as its raw name.
+ */
+function catalogVersion(): string {
+  const dir = join(process.cwd(), "messages");
+  const hash = createHash("sha256");
+  for (const file of readdirSync(dir)
+    .filter((f) => f.endsWith(".json"))
+    .sort()) {
+    hash.update(file).update(readFileSync(join(dir, file)));
+  }
+  return hash.digest("hex").slice(0, 16);
+}
+
 const nextConfig: NextConfig = {
   output: "standalone",
   // `next build` type-checks the whole tsconfig program since 16.3, which
@@ -20,6 +43,7 @@ const nextConfig: NextConfig = {
   // had to discover "pull-to-refresh" themselves after every release.
   env: {
     NEXT_PUBLIC_APP_VERSION: PKG_VERSION,
+    NEXT_PUBLIC_I18N_CATALOG_VERSION: catalogVersion(),
   },
   // v1.4.33 IW2 — strip `console.*` calls from the production bundle
   // (keep `console.error` + `console.warn` so the GlitchTip reporter
diff --git a/public/sw.js b/public/sw.js
index ce689494f..603d4a6af 100644
--- a/public/sw.js
+++ b/public/sw.js
@@ -230,8 +230,9 @@ self.addEventListener("fetch", (event) => {
     return;
   }
 
-  // Locale-catalog boot script — cache-first. The URL is versioned
-  // (`/i18n/<locale>?v=<build>`) and served immutable, so cache-first is
+  // Locale-catalog boot script — cache-first. The URL is versioned by the
+  // catalogs' content hash (`/i18n/<locale>?v=<hash>`, `next.config.ts`),
+  // so a changed catalog is a new URL and cache-first is
   // correct and keeps the catalog available on an offline relaunch (the
   // cached shell HTML references the same versioned URL it was rendered
   // with). Carries no user data — same public strings as the repository's
diff --git a/src/__tests__/compose-env-whitelist-guard.test.ts b/src/__tests__/compose-env-whitelist-guard.test.ts
index 79f28af68..027cb5262 100644
--- a/src/__tests__/compose-env-whitelist-guard.test.ts
+++ b/src/__tests__/compose-env-whitelist-guard.test.ts
@@ -102,6 +102,8 @@ const NOT_OPERATOR_CONFIGURABLE: Readonly<Record<string, string>> = {
   // --- Baked at build time by the Dockerfile's ARG/ENV pairs. ---
   NEXT_PUBLIC_APP_VERSION:
     "Dockerfile build arg; a runtime value cannot change what is compiled in",
+  NEXT_PUBLIC_I18N_CATALOG_VERSION:
+    "computed by next.config.ts from the message catalogs at build time",
   NEXT_PUBLIC_APP_BUILD_SHA:
     "Dockerfile build arg, stamped from the release commit",
   NEXT_PUBLIC_APP_BUILT_AT: "Dockerfile build arg, stamped at image build",
diff --git a/src/__tests__/i18n-catalog-cache-key.test.ts b/src/__tests__/i18n-catalog-cache-key.test.ts
new file mode 100644
index 000000000..6888e79f6
--- /dev/null
+++ b/src/__tests__/i18n-catalog-cache-key.test.ts
@@ -0,0 +1,31 @@
+/**
+ * The locale catalog's boot script is cached by its URL: the service worker
+ * serves `/i18n/<locale>.js?v=…` cache-first. Keyed by the package version,
+ * every build of one release shared one URL, so a beta updated within a
+ * release kept its first catalog and every key added since read as its raw
+ * name ("timeline.zoom.range"). The key is the catalogs' content hash now.
+ */
+import { readFileSync } from "node:fs";
+import { join } from "node:path";
+
+import { describe, expect, it } from "vitest";
+
+const read = (rel: string) => readFileSync(join(process.cwd(), rel), "utf8");
+
+describe("the locale catalog's cache key", () => {
+  it("moves with the catalogs, not with the release", () => {
+    const layout = read("src/app/layout.tsx");
+    const src =
+      /src=\{`\/i18n\/\$\{initialLocale\}\.js\?v=\$\{([^}]*)\}`\}/.exec(layout);
+    expect(src?.[1]).toContain("NEXT_PUBLIC_I18N_CATALOG_VERSION");
+    expect(src?.[1]).not.toContain("NEXT_PUBLIC_APP_VERSION");
+  });
+
+  it("is computed from the message catalogs at build time", () => {
+    const config = read("next.config.ts");
+    expect(config).toMatch(
+      /NEXT_PUBLIC_I18N_CATALOG_VERSION:\s*catalogVersion\(\)/,
+    );
+    expect(config).toMatch(/join\(process\.cwd\(\), "messages"\)/);
+  });
+});
diff --git a/src/app/layout.tsx b/src/app/layout.tsx
index 57e112094..e935efd77 100644
--- a/src/app/layout.tsx
+++ b/src/app/layout.tsx
@@ -131,7 +131,9 @@ export default async function RootLayout({
           id="healthlog-i18n-boot"
           strategy="beforeInteractive"
           nonce={nonce}
-          src={`/i18n/${initialLocale}.js?v=${process.env.NEXT_PUBLIC_APP_VERSION || "dev"}`}
+          // Keyed by the catalogs' content (`next.config.ts`), not the
+          // release: the service worker serves this URL cache-first.
+          src={`/i18n/${initialLocale}.js?v=${process.env.NEXT_PUBLIC_I18N_CATALOG_VERSION || "dev"}`}
         />
       </head>
       <body className={`${inter.variable} font-sans antialiased`}>