diff --git a/packages/kernel-utils/CHANGELOG.md b/packages/kernel-utils/CHANGELOG.md
index 56c5e767b0..88f885e05e 100644
--- a/packages/kernel-utils/CHANGELOG.md
+++ b/packages/kernel-utils/CHANGELOG.md
@@ -13,6 +13,7 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 - Add an optional `required` field to `MethodSchema` (mirroring `required` on object `JsonSchema`) naming which arguments are required, and a `{ required }` option on `methodArgsToStruct` that validates unlisted arguments as optional, so a method's argument schema can faithfully represent the optional trailing arguments its guard already allows ([#958](https://github.com/MetaMask/ocap-kernel/pull/958))
 - Add `getLibp2pRelayHome()` to the `./nodejs` exports, returning the libp2p relay's bookkeeping directory (default `~/.libp2p-relay`, overridable via `$LIBP2P_RELAY_HOME`) — kept separate from `$OCAP_HOME` so one relay can serve daemons with different OCAP_HOMEs ([#952](https://github.com/MetaMask/ocap-kernel/pull/952))
 - `startRelay()` accepts an optional `publicIp` that is fed to libp2p's `appendAnnounce`, so a relay running on a NAT-backed host can announce its publicly-reachable IPv4 alongside its bound NIC addresses ([#952](https://github.com/MetaMask/ocap-kernel/pull/952))
+- Add `replaceNodeEnvPlugin` to the `./vite-plugins` exports, a Rolldown transform plugin that inlines `process.env.NODE_ENV` as a string literal (configurable via `{ value }`, defaulting to `'production'`) in any module referencing it — used by `bundleVat` in place of a build-config `define`, so vats that pull in libraries like immer resolve the reference at bundle time ([#967](https://github.com/MetaMask/ocap-kernel/pull/967))
 
 ## [0.5.0]
 
diff --git a/packages/kernel-utils/src/vite-plugins/bundle-vat.ts b/packages/kernel-utils/src/vite-plugins/bundle-vat.ts
index 49e2c5c3fc..6ed2ff6de1 100644
--- a/packages/kernel-utils/src/vite-plugins/bundle-vat.ts
+++ b/packages/kernel-utils/src/vite-plugins/bundle-vat.ts
@@ -7,6 +7,7 @@ import { build } from 'vite';
 
 import type { VatBundle } from '../vat-bundle.ts';
 import { exportMetadataPlugin } from './export-metadata-plugin.ts';
+import { replaceNodeEnvPlugin } from './replace-node-env-plugin.ts';
 import { stripCommentsPlugin } from './strip-comments-plugin.ts';
 
 export type { VatBundle } from '../vat-bundle.ts';
@@ -88,12 +89,6 @@ export async function bundleVat(sourcePath: string): Promise<VatBundle> {
     result = await build({
       configFile: false,
       logLevel: 'silent',
-      // TODO: Remove this define block and add a process shim to VatSupervisor
-      // workerEndowments instead. This injects into ALL bundles but is only needed
-      // for libraries like immer that check process.env.NODE_ENV.
-      define: {
-        'process.env.NODE_ENV': JSON.stringify('production'),
-      },
       build: {
         write: false,
         minify: false,
@@ -109,6 +104,7 @@ export async function bundleVat(sourcePath: string): Promise<VatBundle> {
           },
           plugins: [
             removeDynamicImportsPlugin(),
+            replaceNodeEnvPlugin(),
             stripCommentsPlugin(),
             metadataPlugin,
           ],
diff --git a/packages/kernel-utils/src/vite-plugins/index.ts b/packages/kernel-utils/src/vite-plugins/index.ts
index e4e15fc068..ce52fb000f 100644
--- a/packages/kernel-utils/src/vite-plugins/index.ts
+++ b/packages/kernel-utils/src/vite-plugins/index.ts
@@ -4,6 +4,8 @@ import { bundleVat } from './bundle-vat.ts';
 
 export { bundleVat, removeDynamicImportsPlugin } from './bundle-vat.ts';
 export type { VatBundle } from './bundle-vat.ts';
+export { replaceNodeEnvPlugin } from './replace-node-env-plugin.ts';
+export type { ReplaceNodeEnvPluginOptions } from './replace-node-env-plugin.ts';
 
 type VatEntry = {
   /** Absolute path to the vat source file */
diff --git a/packages/kernel-utils/src/vite-plugins/replace-node-env-plugin.test.ts b/packages/kernel-utils/src/vite-plugins/replace-node-env-plugin.test.ts
new file mode 100644
index 0000000000..b10a581bcf
--- /dev/null
+++ b/packages/kernel-utils/src/vite-plugins/replace-node-env-plugin.test.ts
@@ -0,0 +1,71 @@
+import { describe, it, expect } from 'vitest';
+
+import { replaceNodeEnvPlugin } from './replace-node-env-plugin.ts';
+
+type TransformFn = (
+  code: string,
+  id: string,
+) => { code: string; map: null } | null;
+
+describe('replaceNodeEnvPlugin', () => {
+  const plugin = replaceNodeEnvPlugin();
+  const transform = plugin.transform as TransformFn;
+
+  it.each([
+    ['bare reference', `process.env.NODE_ENV`, `"production"`],
+    [
+      'reference in a conditional',
+      `if (process.env.NODE_ENV !== 'production') { warn(); }`,
+      `if ("production" !== 'production') { warn(); }`,
+    ],
+    [
+      'multiple references in one file',
+      `const a = process.env.NODE_ENV;\nconst b = process.env.NODE_ENV;`,
+      `const a = "production";\nconst b = "production";`,
+    ],
+    [
+      'reference surrounded by other code',
+      `const x = 1;\nexport const mode = process.env.NODE_ENV;\nconst y = 2;`,
+      `const x = 1;\nexport const mode = "production";\nconst y = 2;`,
+    ],
+  ])(
+    'replaces %s with the "production" string literal',
+    (_name, code, expected) => {
+      const result = transform(code, 'test.ts');
+      expect(result).toStrictEqual({ code: expected, map: null });
+    },
+  );
+
+  it('injects a quoted string literal, not the bare word', () => {
+    const result = transform(`const mode = process.env.NODE_ENV;`, 'test.ts');
+    expect(result?.code).toContain(`"production"`);
+    expect(result?.code).not.toContain(`process.env.NODE_ENV`);
+    expect(result?.code).not.toContain(`= production;`);
+  });
+
+  it.each([
+    ['no process reference', 'const x = 1;'],
+    ['unrelated process.env key', `const port = process.env.PORT;`],
+    ['empty code', ''],
+  ])('returns null for %s', (_name, code) => {
+    expect(transform(code, 'test.ts')).toBeNull();
+  });
+
+  it('returns null when the substring matches but the word boundary does not', () => {
+    const code = `const x = process.env.NODE_ENVIRONMENT;`;
+    expect(transform(code, 'test.ts')).toBeNull();
+  });
+
+  it('inlines a configured value instead of the default', () => {
+    const devTransform = replaceNodeEnvPlugin({ value: 'development' })
+      .transform as TransformFn;
+    const result = devTransform(
+      `const mode = process.env.NODE_ENV;`,
+      'test.ts',
+    );
+    expect(result).toStrictEqual({
+      code: `const mode = "development";`,
+      map: null,
+    });
+  });
+});
diff --git a/packages/kernel-utils/src/vite-plugins/replace-node-env-plugin.ts b/packages/kernel-utils/src/vite-plugins/replace-node-env-plugin.ts
new file mode 100644
index 0000000000..5245c1caff
--- /dev/null
+++ b/packages/kernel-utils/src/vite-plugins/replace-node-env-plugin.ts
@@ -0,0 +1,51 @@
+import type { Plugin as RolldownPlugin } from 'rolldown';
+
+export type ReplaceNodeEnvPluginOptions = {
+  /**
+   * The value to inline for `process.env.NODE_ENV`. Defaults to `'production'`.
+   */
+  value?: string;
+};
+
+/**
+ * A Rolldown plugin that inlines `process.env.NODE_ENV` as a string literal in
+ * any module that references it.
+ *
+ * This replaces the former build-config `define` (issue #812). Libraries such
+ * as immer branch on `process.env.NODE_ENV`, and vats have no `process` global
+ * at runtime, so the reference must be resolved at bundle time. The plugin
+ * auto-detects the need: modules without the reference are left untouched.
+ *
+ * This is a textual replacement (the same class of approach as the sibling
+ * {@link removeDynamicImportsPlugin}) and handles the dotted
+ * `process.env.NODE_ENV` form, consistent with the `define` it replaces.
+ *
+ * @param options - Plugin options.
+ * @param options.value - The value to inline for `process.env.NODE_ENV`.
+ * Defaults to `'production'`.
+ * @returns A Rolldown plugin.
+ */
+export function replaceNodeEnvPlugin({
+  value = 'production',
+}: ReplaceNodeEnvPluginOptions = {}): RolldownPlugin {
+  const replacement = JSON.stringify(value);
+  return {
+    name: 'ocap-kernel:replace-node-env',
+    transform(code) {
+      if (!code.includes('process.env.NODE_ENV')) {
+        return null;
+      }
+
+      const transformed = code.replace(
+        /\bprocess\.env\.NODE_ENV\b/gu,
+        replacement,
+      );
+
+      if (transformed === code) {
+        return null;
+      }
+
+      return { code: transformed, map: null };
+    },
+  };
+}