diff --git a/package-lock.json b/package-lock.json
index c58adfd..1ef4883 100644
--- a/package-lock.json
+++ b/package-lock.json
@@ -10,7 +10,7 @@
       "license": "PROPRIETARY",
       "dependencies": {
         "openai": "^6.48.0",
-        "zod": "^3.23.0"
+        "zod": "^4.4.3"
       },
       "devDependencies": {
         "@eslint/js": "^9.39.5",
@@ -2923,9 +2923,9 @@
       }
     },
     "node_modules/zod": {
-      "version": "3.25.76",
-      "resolved": "https://registry.npmjs.org/zod/-/zod-3.25.76.tgz",
-      "integrity": "sha512-gzUt/qt81nXsFGKIFcC3YnfEAx5NkunCfnDlvuBSSFS02bcXu4Lmea0AFIUwbLWxWPx3d9p8S5QoaujKcNQxcQ==",
+      "version": "4.4.3",
+      "resolved": "https://registry.npmjs.org/zod/-/zod-4.4.3.tgz",
+      "integrity": "sha512-ytENFjIJFl2UwYglde2jchW2Hwm4GJFLDiSXWdTrJQBIN9Fcyp7n4DhxJEiWNAJMV1/BqWfW/kkg71UDcHJyTQ==",
       "license": "MIT",
       "funding": {
         "url": "https://github.com/sponsors/colinhacks"
diff --git a/package.json b/package.json
index 69abea9..0091d17 100644
--- a/package.json
+++ b/package.json
@@ -34,7 +34,7 @@
   },
   "dependencies": {
     "openai": "^6.48.0",
-    "zod": "^3.23.0"
+    "zod": "^4.4.3"
   },
   "devDependencies": {
     "@eslint/js": "^9.39.5",
diff --git a/tests/zod4-migration.test.ts b/tests/zod4-migration.test.ts
new file mode 100644
index 0000000..4e41627
--- /dev/null
+++ b/tests/zod4-migration.test.ts
@@ -0,0 +1,24 @@
+import { describe, expect, it } from 'vitest';
+import { parseRouterConfig, parseTaskDescriptor, RouterConfigSchema, TaskDescriptorSchema } from '../src/schemas.js';
+import { TaskComplexity, TaskType } from '../src/types.js';
+
+describe('Zod 4 migration compatibility', () => {
+  it('preserves legacy unknown-field stripping', () => {
+    const parsed = parseTaskDescriptor({ type: TaskType.CLASSIFICATION, complexity: TaskComplexity.LOW, futureField: true });
+    expect(parsed).not.toHaveProperty('futureField');
+  });
+
+  it('keeps optional legacy fields optional', () => {
+    expect(TaskDescriptorSchema.safeParse({ type: TaskType.SCORING, complexity: TaskComplexity.TRIVIAL }).success).toBe(true);
+    expect(RouterConfigSchema.safeParse({ perplexityApiKey: 'p', openrouterApiKey: 'o' }).success).toBe(true);
+  });
+
+  it('does not widen enum validation', () => {
+    expect(TaskDescriptorSchema.safeParse({ type: 'classification-ish', complexity: TaskComplexity.LOW }).success).toBe(false);
+  });
+
+  it('keeps validation errors JSON-safe', () => {
+    try { parseRouterConfig({ perplexityApiKey: '', openrouterApiKey: '' }); }
+    catch (error) { expect(() => JSON.stringify(error)).not.toThrow(); }
+  });
+});