diff --git a/eslint-rules/no-relative-cross-package-imports.js b/eslint-rules/no-relative-cross-package-imports.js
new file mode 100644
index 00000000..ab3ed91c
--- /dev/null
+++ b/eslint-rules/no-relative-cross-package-imports.js
@@ -0,0 +1,159 @@
+/**
+ * @license
+ * Copyright 2025 Google LLC
+ * SPDX-License-Identifier: Apache-2.0
+ */
+
+/**
+ * @fileoverview Disallows relative imports between specified monorepo packages.
+ */
+'use strict';
+
+import path from 'node:path';
+import fs from 'node:fs';
+
+/**
+ * Finds the package name by searching for the nearest `package.json` file
+ * in the directory hierarchy, starting from the given file's directory
+ * and moving upwards until the specified root directory is reached.
+ * It reads the `package.json` and extracts the `name` property.
+ *
+ * @requires module:path Node.js path module
+ * @requires module:fs Node.js fs module
+ *
+ * @param {string} filePath - The path (absolute or relative) to a file within the potential package structure.
+ * The search starts from the directory containing this file.
+ * @param {string} root - The absolute path to the root directory of the project/monorepo.
+ * The upward search stops when this directory is reached.
+ * @returns {string | undefined | null} The value of the `name` field from the first `package.json` found.
+ * Returns `undefined` if the `name` field doesn't exist in the found `package.json`.
+ * Returns `null` if no `package.json` is found before reaching the `root` directory.
+ * @throws {Error} Can throw an error if `fs.readFileSync` fails (e.g., permissions) or if `JSON.parse` fails on invalid JSON content.
+ */
+function findPackageName(filePath, root) {
+  let currentDir = path.dirname(path.resolve(filePath));
+  while (currentDir !== root) {
+    const parentDir = path.dirname(currentDir);
+    const packageJsonPath = path.join(currentDir, 'package.json');
+    if (fs.existsSync(packageJsonPath)) {
+      const pkg = JSON.parse(fs.readFileSync(packageJsonPath, 'utf-8'));
+      return pkg.name;
+    }
+
+    // Move up one level
+    currentDir = parentDir;
+    // Safety break if we somehow reached the root directly in the loop condition (less likely with path.resolve)
+    if (path.dirname(currentDir) === currentDir) break;
+  }
+
+  return null; // Not found within the expected structure
+}
+
+export default {
+  meta: {
+    type: 'problem',
+    docs: {
+      description: 'Disallow relative imports between packages.',
+      category: 'Best Practices',
+      recommended: 'error',
+    },
+    fixable: 'code',
+    schema: [
+      {
+        type: 'object',
+        properties: {
+          root: {
+            type: 'string',
+            description:
+              'Absolute path to the root of all relevant packages to consider.',
+          },
+        },
+        required: ['root'],
+        additionalProperties: false,
+      },
+    ],
+    messages: {
+      noRelativePathsForCrossPackageImport:
+        "Relative import '{{importedPath}}' crosses package boundary from '{{importingPackage}}' to '{{importedPackage}}'. Use a direct package import ('{{importedPackage}}') instead.",
+      relativeImportIsInvalidPackage:
+        "Relative import '{{importedPath}}' does not reference a valid package. All source must be in a package directory.",
+    },
+  },
+
+  create(context) {
+    const options = context.options[0] || {};
+    const allPackagesRoot = options.root;
+
+    const currentFilePath = context.filename;
+    if (
+      !currentFilePath ||
+      currentFilePath === '<input>' ||
+      currentFilePath === '<text>'
+    ) {
+      // Skip if filename is not available (e.g., linting raw text)
+      return {};
+    }
+
+    const currentPackage = findPackageName(currentFilePath, allPackagesRoot);
+
+    // If the current file isn't inside a package structure, don't apply the rule
+    if (!currentPackage) {
+      return {};
+    }
+
+    return {
+      ImportDeclaration(node) {
+        const importingPackage = currentPackage;
+        const importedPath = node.source.value;
+
+        // Only interested in relative paths
+        if (
+          !importedPath ||
+          typeof importedPath !== 'string' ||
+          !importedPath.startsWith('.')
+        ) {
+          return;
+        }
+
+        // Resolve the absolute path of the imported module
+        const absoluteImportPath = path.resolve(
+          path.dirname(currentFilePath),
+          importedPath,
+        );
+
+        // Find the package information for the imported file
+        const importedPackage = findPackageName(
+          absoluteImportPath,
+          allPackagesRoot,
+        );
+
+        // If the imported file isn't in a recognized package, report issue
+        if (!importedPackage) {
+          context.report({
+            node: node.source,
+            messageId: 'relativeImportIsInvalidPackage',
+            data: { importedPath: importedPath },
+          });
+          return;
+        }
+
+        // The core check: Are the source and target packages different?
+        if (currentPackage !== importedPackage) {
+          // We found a relative import crossing package boundaries
+          context.report({
+            node: node.source, // Report the error on the source string literal
+            messageId: 'noRelativePathsForCrossPackageImport',
+            data: {
+              importedPath,
+              importedPackage,
+              importingPackage,
+            },
+            fix(fixer) {
+              return fixer.replaceText(node.source, `'${importedPackage}'`);
+            },
+          });
+        }
+      },
+    };
+  },
+};
diff --git a/eslint.config.js b/eslint.config.js
index a846d22d..16577628 100644
--- a/eslint.config.js
+++ b/eslint.config.js
@@ -13,6 +13,17 @@ import prettierConfig from 'eslint-config-prettier';
 import importPlugin from 'eslint-plugin-import';
 import globals from 'globals';
 import licenseHeader from 'eslint-plugin-license-header';
+import noRelativeCrossPackageImports from './eslint-rules/no-relative-cross-package-imports.js';
+import path from 'node:path'; // Use node: prefix for built-ins
+import url from 'node:url';
+
+// --- ESM way to get __dirname ---
+const __filename = url.fileURLToPath(import.meta.url);
+const __dirname = path.dirname(__filename);
+// --- ---
+
+// Determine the monorepo root (assuming eslint.config.js is at the root)
+const projectRoot = __dirname;
 
 export default tseslint.config(
   {
@@ -22,6 +33,7 @@ export default tseslint.config(
       'eslint.config.js',
       'packages/cli/dist/**',
       'packages/server/dist/**',
+      'eslint-rules/*',
     ],
   },
   eslint.configs.recommended,
@@ -163,4 +175,24 @@ export default tseslint.config(
   },
   // Prettier config must be last
   prettierConfig,
+  // Custom eslint rules for this repo
+  {
+    files: ['packages/**/*.{js,jsx,ts,tsx}'],
+    plugins: {
+      custom: {
+        rules: {
+          'no-relative-cross-package-imports': noRelativeCrossPackageImports,
+        },
+      },
+    },
+    rules: {
+      // Enable and configure your custom rule
+      'custom/no-relative-cross-package-imports': [
+        'error',
+        {
+          root: path.join(projectRoot, 'packages'),
+        },
+      ],
+    },
+  },
 );
diff --git a/package.json b/package.json
index f7011298..c73fa3e5 100644
--- a/package.json
+++ b/package.json
@@ -12,7 +12,7 @@
     "test": "npm run test --workspaces",
     "start": "scripts/start.sh",
     "debug": "scripts/debug.sh",
-    "fix": "eslint . --fix",
+    "lint:fix": "eslint . --fix",
     "lint": "eslint . --ext .ts,.tsx",
     "typecheck": "tsc --noEmit --jsx react",
     "format": "prettier --write .",