diff --git a/.github/workflows/checks.yml b/.github/workflows/checks.yml
index bab356e4d2..21833f795e 100644
--- a/.github/workflows/checks.yml
+++ b/.github/workflows/checks.yml
@@ -49,6 +49,14 @@ jobs:
         run: bun scripts/lint/no-duplicate-deps.ts
       - name: Check package.json ordering
         run: bun scripts/format/sort-package-json.ts --check
+      # TODO: remove continue-on-error once the existing typeof/cast backlog is cleared.
+      # Pre-push hook already blocks new violations — these report on the backlog.
+      - name: Custom lint rules (typeof guards, raw regex, process.env)
+        run: bun lint:custom
+        continue-on-error: true
+      - name: Check unsafe type casts
+        run: bun check:casts:strict
+        continue-on-error: true
       - name: Check types
         run: bun check-types
       - name: Run Expo Doctor
diff --git a/biome.json b/biome.json
index a5d0642202..e775361783 100644
--- a/biome.json
+++ b/biome.json
@@ -47,7 +47,7 @@
         "noUnusedImports": "error"
       },
       "performance": {
-        "useTopLevelRegex": "warn"
+        "useTopLevelRegex": "error"
       },
       "style": {
         "noNonNullAssertion": "error"
diff --git a/lefthook.yml b/lefthook.yml
index 2f99ab00e3..3a0b583cb0 100644
--- a/lefthook.yml
+++ b/lefthook.yml
@@ -6,3 +6,12 @@ pre-commit:
     lint:
       run: bun check {staged_files}
       fail_text: "Linting failed! \nRun `bun lint` to fix. \nCommit with `--no-verify` to bypass check."
+
+pre-push:
+  skip:
+    - run: test "${CI:-false}" = "true"
+    - run: test "${GITHUB_ACTIONS:-false}" = "true"
+  commands:
+    custom-checks:
+      run: bun scripts/check-all.ts
+      fail_text: "Custom checks failed! \nRun `bun check:all` to see details."
diff --git a/package.json b/package.json
index c3a138c8d6..ff3c96f1b3 100644
--- a/package.json
+++ b/package.json
@@ -11,6 +11,7 @@
     "api": "bun run --cwd packages/api dev",
     "bump": "bun .github/scripts/bump.ts",
     "check": "biome check --no-errors-on-unmatched",
+    "check:all": "bun scripts/check-all.ts",
     "check:casts": "bun run --cwd packages/checks check:casts",
     "check:casts:strict": "bun run --cwd packages/checks check:casts:strict",
     "check:catalog": "bun scripts/lint/no-duplicate-deps.ts",
@@ -33,7 +34,7 @@
     "ios": "cd apps/expo && bun ios",
     "lefthook": "lefthook install",
     "lint": "biome check --write",
-    "lint:custom": "bun run scripts/lint/no-raw-typeof.ts && bun run scripts/lint/no-raw-regex.ts && bun run packages/env/scripts/no-raw-process-env.ts",
+    "lint:custom": "bun run scripts/lint/no-raw-typeof.ts && bun run scripts/lint/no-raw-regex.ts && bun run packages/env/scripts/no-raw-process-env.ts && bun run scripts/lint/no-duplicate-guards.ts",
     "lint:strict": "biome check && bun run lint:custom",
     "lint-unsafe": "biome check --write --unsafe",
     "mcp": "bun run --cwd packages/mcp dev",
diff --git a/scripts/check-all.ts b/scripts/check-all.ts
index 3080eef20b..6829f1a093 100644
--- a/scripts/check-all.ts
+++ b/scripts/check-all.ts
@@ -5,9 +5,12 @@
 // Runs the following checks in parallel and prints a unified summary table:
 //   - scripts/lint/no-raw-regex.ts
 //   - scripts/lint/no-raw-typeof.ts
+//   - packages/env/scripts/no-raw-process-env.ts
 //   - scripts/lint/no-circular-deps.ts
 //   - scripts/lint/no-duplicate-deps.ts  (skipped if file doesn't exist)
+//   - scripts/lint/no-duplicate-guards.ts
 //   - packages/checks/src/check-magic-strings.ts
+//   - packages/checks/src/check-type-casts.ts --strict
 //   - scripts/lint/check-react-doctor.ts
 //   - scripts/format/sort-package-json.ts --check
 //
@@ -58,6 +61,10 @@ const ALL_CHECKS: CheckDef[] = [
     name: 'no-raw-typeof',
     script: join(ROOT, 'scripts', 'lint', 'no-raw-typeof.ts'),
   },
+  {
+    name: 'no-raw-process-env',
+    script: join(ROOT, 'packages', 'env', 'scripts', 'no-raw-process-env.ts'),
+  },
   {
     name: 'no-circular-deps',
     script: join(ROOT, 'scripts', 'lint', 'no-circular-deps.ts'),
@@ -66,6 +73,15 @@ const ALL_CHECKS: CheckDef[] = [
     name: 'no-duplicate-deps',
     script: join(ROOT, 'scripts', 'lint', 'no-duplicate-deps.ts'),
   },
+  {
+    name: 'no-duplicate-guards',
+    script: join(ROOT, 'scripts', 'lint', 'no-duplicate-guards.ts'),
+  },
+  {
+    name: 'check-type-casts',
+    script: join(ROOT, 'packages', 'checks', 'src', 'check-type-casts.ts'),
+    args: ['--strict'],
+  },
   {
     name: 'check-magic-strings',
     script: join(ROOT, 'packages', 'checks', 'src', 'check-magic-strings.ts'),
diff --git a/scripts/lint/no-duplicate-guards.ts b/scripts/lint/no-duplicate-guards.ts
new file mode 100644
index 0000000000..3c9f365d48
--- /dev/null
+++ b/scripts/lint/no-duplicate-guards.ts
@@ -0,0 +1,184 @@
+#!/usr/bin/env bun
+//
+// no-duplicate-guards.ts — flags re-implementations of guards that are already
+// exported from @packrat/guards.
+//
+// The guards package (packages/guards/) is the single source of truth for all
+// type narrowing and assertion helpers. Duplicating them in app code leads to
+// subtle behavioural divergence and breaks the "use guards, not casts" policy.
+//
+// Flags:
+//   - assertDefined / assertNonNull / assertPresent / assertIsString /
+//     assertIsNumber / assertIsBoolean / assertAllDefined
+//   - isString / isNumber / isBoolean / isFunction / isArray / isObject /
+//     isDate / isDefined / isPresent (re-implementations, not re-exports)
+//   - makeEnumGuard / makeTypeGuard / assertError / assertNever
+//
+// A "re-implementation" is any function declaration or arrow-function
+// assignment whose name matches one of the guard names above, found outside
+// packages/guards/ and packages/checks/ (the check scripts themselves).
+//
+// Exit code:
+//   0 — no violations
+//   1 — violations found
+//
+// Wired into check-all.ts and lint:custom.
+
+import { readdirSync, readFileSync, statSync } from 'node:fs';
+import { join } from 'node:path';
+
+const ROOT = join(import.meta.dir, '..', '..');
+
+const SCAN_ROOTS = ['apps', 'packages'];
+
+// Names exported from @packrat/guards that should not be re-implemented elsewhere.
+const GUARD_NAMES = new Set([
+  // assertions.ts
+  'assertDefined',
+  'assertNonNull',
+  'assertPresent',
+  'assertIsString',
+  'assertIsNumber',
+  'assertIsBoolean',
+  'assertAllDefined',
+  // re-exported from ts-extras — flag if home-grown
+  'assertError',
+  'assertNever',
+  'isDefined',
+  'isPresent',
+  // re-exported from radash — flag if home-grown
+  'isString',
+  'isNumber',
+  'isBoolean',
+  'isFunction',
+  'isArray',
+  'isObject',
+  'isDate',
+  'isFloat',
+  'isInt',
+  'isSymbol',
+  'isPrimitive',
+  'isPromise',
+  // custom guards/parsers
+  'makeEnumGuard',
+  'makeTypeGuard',
+]);
+
+// Excluded source roots (the canonical definitions live here).
+const EXCLUDED_ROOTS = ['packages/guards', 'packages/checks'];
+
+const EXCLUDED_DIRS = new Set(['node_modules', 'dist', 'build', '.next', '.expo', 'drizzle']);
+
+// Matches:
+//   export function assertDefined(...)
+//   function assertDefined(...)
+//   const assertDefined = (...)
+//   export const assertDefined = (...)
+//   export const assertDefined: (...) =>
+const IMPL_PATTERN =
+  /(?:export\s+)?(?:function\s+|const\s+|let\s+)([A-Za-z][A-Za-z0-9_]*)\s*(?:[=(:<])/g;
+
+interface Violation {
+  file: string;
+  line: number;
+  name: string;
+  source: string;
+}
+
+function isTargetFile(name: string): boolean {
+  return (
+    /\.(ts|tsx|cts|mts)$/.test(name) && !/\.(test|spec|stories|d)\.(ts|tsx|cts|mts)$/.test(name)
+  );
+}
+
+function isExcluded(relPath: string): boolean {
+  return EXCLUDED_ROOTS.some((p) => relPath === p || relPath.startsWith(p + '/'));
+}
+
+function walkDir(dir: string, relPath: string, violations: Violation[]): void {
+  if (isExcluded(relPath)) return;
+
+  let entries: string[];
+  try {
+    entries = readdirSync(dir);
+  } catch {
+    return;
+  }
+
+  for (const entry of entries) {
+    if (EXCLUDED_DIRS.has(entry)) continue;
+
+    const fullPath = join(dir, entry);
+    const entryRel = `${relPath}/${entry}`;
+
+    let isDir = false;
+    try {
+      isDir = statSync(fullPath).isDirectory();
+    } catch {
+      continue;
+    }
+
+    if (isDir) {
+      walkDir(fullPath, entryRel, violations);
+    } else if (isTargetFile(entry)) {
+      let content: string;
+      try {
+        content = readFileSync(fullPath, 'utf8');
+      } catch {
+        continue;
+      }
+
+      const lines = content.split('\n');
+      for (let i = 0; i < lines.length; i++) {
+        const line = lines[i] ?? '';
+        const trimmed = line.trimStart();
+
+        // Skip comment lines and import/export-from lines
+        if (
+          trimmed.startsWith('//') ||
+          trimmed.startsWith('*') ||
+          trimmed.startsWith('/*') ||
+          /^\s*export\s*\{/.test(line) ||
+          /^\s*(import|export)\s+.*\s+from\s+['"]/.test(line)
+        ) {
+          continue;
+        }
+
+        IMPL_PATTERN.lastIndex = 0;
+        for (let m = IMPL_PATTERN.exec(line); m !== null; m = IMPL_PATTERN.exec(line)) {
+          const name = m[1];
+          if (name && GUARD_NAMES.has(name)) {
+            violations.push({ file: entryRel, line: i + 1, name, source: line.trimEnd() });
+          }
+        }
+      }
+    }
+  }
+}
+
+const violations: Violation[] = [];
+for (const root of SCAN_ROOTS) {
+  walkDir(join(ROOT, root), root, violations);
+}
+
+if (violations.length === 0) {
+  console.log('No duplicate guard implementations found.');
+  process.exit(0);
+}
+
+console.log(
+  `Found ${violations.length} guard re-implementation(s) outside @packrat/guards — import from '@packrat/guards' instead:\n`,
+);
+
+let lastFile = '';
+for (const v of violations) {
+  if (v.file !== lastFile) {
+    console.log(`  ${v.file}`);
+    lastFile = v.file;
+  }
+  console.log(`    line ${v.line}: ${v.name}`);
+  console.log(`      ${v.source}`);
+}
+
+console.log("\nFix: remove the local copy and import from '@packrat/guards'.");
+process.exit(1);