diff --git a/packages/cli/script/build.ts b/packages/cli/script/build.ts
index e81731d7a..d72433ea2 100644
--- a/packages/cli/script/build.ts
+++ b/packages/cli/script/build.ts
@@ -37,6 +37,7 @@ import { gzip } from "node:zlib";
 import { build as esbuild } from "esbuild";
 import { uploadSourcemaps } from "../src/lib/api/sourcemaps.js";
 import { injectDebugId, PLACEHOLDER_DEBUG_ID } from "./debug-id.js";
+import { zodEnglishLocalesPlugin } from "./lib/zod-locales-plugin.js";
 import { textImportPlugin } from "./text-import-plugin.js";
 
 const gzipAsync = promisify(gzip);
@@ -160,6 +161,7 @@ async function bundleJs(): Promise<boolean> {
       },
       plugins: [
         textImportPlugin,
+        zodEnglishLocalesPlugin,
         // Transform _require() → require() so esbuild resolves lazy relative
         // requires at bundle time. In tsx dev mode, _require is a file-local
         // createRequire(import.meta.url) that resolves relative to the file.
diff --git a/packages/cli/script/bundle.ts b/packages/cli/script/bundle.ts
index d6624cfca..fff32d363 100644
--- a/packages/cli/script/bundle.ts
+++ b/packages/cli/script/bundle.ts
@@ -6,6 +6,7 @@ import { build, type Plugin } from "esbuild";
 import pkg from "../package.json";
 import { uploadSourcemaps } from "../src/lib/api/sourcemaps.js";
 import { injectDebugId, PLACEHOLDER_DEBUG_ID } from "./debug-id.js";
+import { zodEnglishLocalesPlugin } from "./lib/zod-locales-plugin.js";
 import { buildCoreDeclarations, SDK_TYPES_PATH } from "./sdk-declarations.js";
 import { textImportPlugin } from "./text-import-plugin.js";
 
@@ -253,6 +254,7 @@ const zlibNamespaceShimPlugin: Plugin = {
 
 const plugins: Plugin[] = [
   sentrySourcemapPlugin,
+  zodEnglishLocalesPlugin,
   textImportPlugin,
   requireAliasPlugin,
 ];
diff --git a/packages/cli/script/lib/zod-locales-plugin.ts b/packages/cli/script/lib/zod-locales-plugin.ts
new file mode 100644
index 000000000..21ee6ba15
--- /dev/null
+++ b/packages/cli/script/lib/zod-locales-plugin.ts
@@ -0,0 +1,56 @@
+/**
+ * esbuild plugin that bundles only Zod's English error messages.
+ *
+ * Zod 4's `core`, `classic`, and `mini` entry points all re-export
+ * `locales/index.js`, which statically imports every locale (about 260 KB
+ * minified). Zod only uses English unless code calls
+ * `z.config(z.locales.xx())`, and nothing we bundle does. This plugin swaps
+ * that index for one that exports only `en`, so `z.locales.en` still works
+ * and error messages are unchanged. `classic/schemas.js` imports `en.js`
+ * directly to set the default locale, so that path is not touched.
+ *
+ * Shared by the CLI bundles (`script/bundle.ts`, `script/build.ts`), which
+ * inline `@sentry/mcp-core` for `sentry mcp`.
+ */
+
+import { existsSync } from "node:fs";
+import { dirname, join } from "node:path";
+import type { Plugin } from "esbuild";
+
+const ZOD_LOCALES_NS = "zod-en-locales";
+const LOCALES_INDEX_FILTER = /[\\/]locales[\\/]index\.c?js$/;
+const ANY_FILTER = /.*/;
+/** Matches a file inside an installed `zod` package (npm or pnpm layout). */
+const ZOD_PACKAGE_RE = /[\\/]node_modules[\\/]zod[\\/]/;
+
+export const zodEnglishLocalesPlugin: Plugin = {
+  name: "zod-english-locales",
+  setup(build) {
+    build.onResolve({ filter: LOCALES_INDEX_FILTER }, (args) => {
+      if (!ZOD_PACKAGE_RE.test(`${args.resolveDir}/`)) {
+        return;
+      }
+      // `zod/locales/index.js` re-exports `../v4/locales/index.js`, so both
+      // resolve here. Keep the importer's module format: `.cjs` for require.
+      const indexPath = join(args.resolveDir, args.path);
+      const enPath = join(
+        dirname(indexPath),
+        indexPath.endsWith(".cjs") ? "en.cjs" : "en.js",
+      );
+      if (!existsSync(enPath)) {
+        // Fail loudly if Zod changes its layout, rather than shipping a
+        // locales namespace without English.
+        return {
+          errors: [{ text: `Zod English locale not found at ${enPath}` }],
+        };
+      }
+      return { path: enPath, namespace: ZOD_LOCALES_NS };
+    });
+
+    build.onLoad({ filter: ANY_FILTER, namespace: ZOD_LOCALES_NS }, (args) => ({
+      contents: `export { default as en } from ${JSON.stringify(args.path)};`,
+      resolveDir: dirname(args.path),
+      loader: "js",
+    }));
+  },
+};
diff --git a/packages/cli/test/script/zod-locales-plugin.test.ts b/packages/cli/test/script/zod-locales-plugin.test.ts
new file mode 100644
index 000000000..ab61757f2
--- /dev/null
+++ b/packages/cli/test/script/zod-locales-plugin.test.ts
@@ -0,0 +1,119 @@
+/**
+ * Tests for the esbuild plugin that bundles only Zod's English locale.
+ *
+ * Builds a small entry against the real `zod` package (resolved from
+ * `@sentry/mcp-core`, which depends on it) with and without the plugin, then
+ * runs both bundles and compares their error messages.
+ */
+
+import { createRequire } from "node:module";
+import { dirname, join } from "node:path";
+import { fileURLToPath } from "node:url";
+import { runInNewContext } from "node:vm";
+import { build } from "esbuild";
+import { describe, expect, test } from "vitest";
+import { zodEnglishLocalesPlugin } from "../../script/lib/zod-locales-plugin.js";
+
+const MCP_CORE_DIR = join(
+  dirname(fileURLToPath(import.meta.url)),
+  "../../../mcp-core",
+);
+
+/** `import` resolves Zod's ESM build and `require` its CJS build. */
+const IMPORTS = {
+  esm: `import { z } from "zod";`,
+  cjs: `const { z } = require("zod");`,
+};
+
+const BODY = `
+const issues = [
+  z.string().safeParse(1),
+  z.object({ n: z.number().min(5) }).safeParse({ n: 1 }),
+  z.enum(["a", "b"]).safeParse("c"),
+].flatMap((r) => r.error.issues.map((i) => i.message));
+module.exports = { issues, locales: Object.keys(z.locales) };
+`;
+
+type BundleResult = {
+  bytes: number;
+  issues: string[];
+  locales: string[];
+};
+
+async function bundleZod(
+  format: "cjs" | "esm",
+  withPlugin: boolean,
+): Promise<BundleResult> {
+  const result = await build({
+    stdin: {
+      contents: IMPORTS[format] + BODY,
+      resolveDir: MCP_CORE_DIR,
+      loader: "js",
+    },
+    bundle: true,
+    write: false,
+    platform: "node",
+    format: "cjs",
+    logLevel: "silent",
+    plugins: withPlugin ? [zodEnglishLocalesPlugin] : [],
+  });
+  const code = result.outputFiles[0]?.text ?? "";
+  const sandbox = {
+    module: { exports: {} },
+    require: createRequire(import.meta.url),
+  };
+  runInNewContext(code, sandbox);
+  const { issues, locales } = sandbox.module.exports as Omit<
+    BundleResult,
+    "bytes"
+  >;
+  return { bytes: code.length, issues, locales };
+}
+
+describe("zodEnglishLocalesPlugin", () => {
+  test.each(["esm", "cjs"] as const)(
+    "keeps English messages and drops other locales (%s)",
+    async (format) => {
+      const full = await bundleZod(format, false);
+      const english = await bundleZod(format, true);
+
+      expect(english.issues).toEqual(full.issues);
+      expect(english.issues[0]).toBe(
+        "Invalid input: expected string, received number",
+      );
+      expect(full.locales).toContain("fr");
+      expect(english.locales).toEqual(["en"]);
+      // All other locales together are well over 100 KB unminified.
+      expect(full.bytes - english.bytes).toBeGreaterThan(100_000);
+    },
+  );
+
+  test("leaves non-Zod locales/index.js imports alone", async () => {
+    const result = await build({
+      stdin: {
+        contents: `import { en } from "./locales/index.js"; module.exports = en;`,
+        resolveDir: "/nonexistent",
+        loader: "js",
+      },
+      bundle: true,
+      write: false,
+      logLevel: "silent",
+      plugins: [
+        zodEnglishLocalesPlugin,
+        {
+          name: "fake-locales",
+          setup(b) {
+            b.onResolve({ filter: /locales/ }, () => ({
+              path: "fake",
+              namespace: "fake",
+            }));
+            b.onLoad({ filter: /.*/, namespace: "fake" }, () => ({
+              contents: `export const en = "fake";`,
+            }));
+          },
+        },
+      ],
+    });
+    expect(result.outputFiles[0]?.text).toContain('"fake"');
+  });
+});