diff --git a/eslint.config.js b/eslint.config.js
index 3e817f70d..5953aa0fd 100644
--- a/eslint.config.js
+++ b/eslint.config.js
@@ -3,6 +3,7 @@ import js from '@eslint/js';
 import { includeIgnoreFile } from '@eslint/compat';
 import svelte from 'eslint-plugin-svelte';
 import globals from 'globals';
+import { builtinModules } from 'node:module';
 import { fileURLToPath } from 'node:url';
 import ts from 'typescript-eslint';
 
@@ -25,5 +26,34 @@ export default ts.config(
         ...globals.node
       }
     }
+  },
+  // @svelte-vitals/core is runtime-agnostic by contract (design §8, see packages/core/src/index.ts):
+  // no node: imports, no I/O, no runtime-specific globals. Enforce it here so a violation
+  // fails `pnpm lint` / CI instead of relying on review. I/O is injected via the Runtime
+  // interface. The bare-builtin ban list is generated from node:module's builtinModules
+  // (plus a `/*` variant for subpaths like `path/posix`) so it can never go stale.
+  {
+    files: ['packages/core/src/**/*.ts'],
+    rules: {
+      'no-restricted-imports': [
+        'error',
+        {
+          patterns: [
+            {
+              group: ['node:*', ...builtinModules, ...builtinModules.map((m) => `${m}/*`)],
+              message:
+                '@svelte-vitals/core is runtime-agnostic (design §8): no Node builtins here — inject I/O through the Runtime interface (src/runtime.ts).'
+            }
+          ]
+        }
+      ],
+      'no-restricted-globals': [
+        'error',
+        { name: 'process', message: 'core is runtime-agnostic (design §8): no runtime-specific globals.' },
+        { name: '__dirname', message: 'core is runtime-agnostic (design §8): no runtime-specific globals.' },
+        { name: '__filename', message: 'core is runtime-agnostic (design §8): no runtime-specific globals.' },
+        { name: 'Buffer', message: 'core is runtime-agnostic (design §8): no runtime-specific globals.' }
+      ]
+    }
   }
 );