diff --git a/apps/oxlint/.gitignore b/apps/oxlint/.gitignore
index 3db66a0047980..7d6c1bcaaad85 100644
--- a/apps/oxlint/.gitignore
+++ b/apps/oxlint/.gitignore
@@ -1,3 +1,4 @@
 /node_modules/
 /dist/
+/debug/
 *.node
diff --git a/apps/oxlint/package.json b/apps/oxlint/package.json
index 78db6a130730f..2c6a191332398 100644
--- a/apps/oxlint/package.json
+++ b/apps/oxlint/package.json
@@ -47,6 +47,6 @@
     ]
   },
   "imports": {
-    "#oxlint": "./dist/index.js"
+    "#oxlint": "./debug/index.js"
   }
 }
diff --git a/apps/oxlint/scripts/build.ts b/apps/oxlint/scripts/build.ts
index 7c356f92ac956..6b0182fc66437 100755
--- a/apps/oxlint/scripts/build.ts
+++ b/apps/oxlint/scripts/build.ts
@@ -3,7 +3,8 @@ import { copyFileSync, readdirSync, readFileSync, writeFileSync } from 'node:fs'
 import { join } from 'node:path';
 
 const oxlintDirPath = join(import.meta.dirname, '..'),
-  distDirPath = join(oxlintDirPath, 'dist');
+  distDirPath = join(oxlintDirPath, 'dist'),
+  debugDirPath = join(oxlintDirPath, 'debug');
 
 // Modify `bindings.js` to use correct package names
 console.log('Modifying bindings.js...');
@@ -26,6 +27,7 @@ const srcDirPath = join(oxlintDirPath, 'src-js');
 for (const filename of readdirSync(srcDirPath)) {
   if (!filename.endsWith('.node')) continue;
   copyFileSync(join(srcDirPath, filename), join(distDirPath, filename));
+  copyFileSync(join(srcDirPath, filename), join(debugDirPath, filename));
 }
 
 console.log('Build complete!');
diff --git a/apps/oxlint/src-js/globals.d.ts b/apps/oxlint/src-js/globals.d.ts
new file mode 100644
index 0000000000000..ee7e100c44946
--- /dev/null
+++ b/apps/oxlint/src-js/globals.d.ts
@@ -0,0 +1,2 @@
+// Global constant defined at build time by TSDown
+declare const DEBUG: boolean;
diff --git a/apps/oxlint/src-js/plugins/utils.ts b/apps/oxlint/src-js/plugins/utils.ts
index c5467bed9ce0e..e496510acab5f 100644
--- a/apps/oxlint/src-js/plugins/utils.ts
+++ b/apps/oxlint/src-js/plugins/utils.ts
@@ -45,13 +45,20 @@ export function assertIs<T>(value: unknown): asserts value is T {}
 /**
  * Assert a value is not `null` or `undefined`.
  *
- * Has no runtime effect - only for guiding the type-checker.
- * Minification removes this function and all calls to it, so it has zero runtime cost.
+ * In release builds, is a no-op. Only does runtime checks in debug builds.
+ * Minification removes this function and all calls to it in release builds, so it has zero runtime cost.
  *
  * @param value - Value
  */
 // oxlint-disable-next-line no-unused-vars
-export function assertIsNonNull<T>(value: T | null | undefined): asserts value is T {}
+export function assertIsNonNull<T>(value: T | null | undefined): asserts value is T {
+  if (!DEBUG) return;
+
+  if (value === null || value === undefined) {
+    // oxlint-disable-next-line typescript/restrict-template-expressions
+    throw new Error(`Expected non-null value, got ${value}`);
+  }
+}
 
 /**
  * Utility type to make specified properties of a type nullable.
diff --git a/apps/oxlint/test/e2e.test.ts b/apps/oxlint/test/e2e.test.ts
index a74a20cd5d01f..56c976c0896c2 100644
--- a/apps/oxlint/test/e2e.test.ts
+++ b/apps/oxlint/test/e2e.test.ts
@@ -4,7 +4,7 @@ import { PACKAGE_ROOT_PATH, getFixtures, testFixtureWithCommand } from './utils.
 
 import type { Fixture } from './utils.ts';
 
-const CLI_PATH = pathJoin(PACKAGE_ROOT_PATH, 'dist/cli.js');
+const CLI_PATH = pathJoin(PACKAGE_ROOT_PATH, 'debug/cli.js');
 
 // Use current NodeJS executable, rather than `node`, to avoid problems with a Node version manager
 // installed on system resulting in using wrong NodeJS version
diff --git a/apps/oxlint/tsdown.config.ts b/apps/oxlint/tsdown.config.ts
index 03d95794d3cf9..9ceaf9e292b42 100644
--- a/apps/oxlint/tsdown.config.ts
+++ b/apps/oxlint/tsdown.config.ts
@@ -21,10 +21,11 @@ const commonConfig: UserConfig = {
     mangle: false,
     codegen: { removeWhitespace: false },
   },
+  define: { DEBUG: 'false' },
 };
 
 // Only generate `.d.ts` file for main export, not for CLI
-export default defineConfig([
+const configs = defineConfig([
   {
     entry: 'src-js/cli.ts',
     ...commonConfig,
@@ -37,3 +38,13 @@ export default defineConfig([
     attw: true,
   },
 ]);
+
+// Create separate debug build with debug assertions enabled
+const debugConfigs = configs.map((config) => ({
+  ...config,
+  outDir: 'debug',
+  define: { DEBUG: 'true' },
+}));
+configs.push(...debugConfigs);
+
+export default configs;
diff --git a/apps/oxlint/vitest.config.ts b/apps/oxlint/vitest.config.ts
index 9fef8e402cca0..b51b6d82f4ac8 100644
--- a/apps/oxlint/vitest.config.ts
+++ b/apps/oxlint/vitest.config.ts
@@ -4,4 +4,7 @@ export default defineConfig({
   test: {
     exclude: [...configDefaults.exclude, 'fixtures/**'],
   },
+  define: {
+    DEBUG: 'true',
+  },
 });