diff --git a/mcp-server/package.json b/mcp-server/package.json
index 6ef6263..67dd7d5 100644
--- a/mcp-server/package.json
+++ b/mcp-server/package.json
@@ -23,9 +23,9 @@
   },
   "devDependencies": {
     "@eslint/js": "^9.39.1",
-    "@vitest/ui": "^3.2.4",
+    "@vitest/ui": "^4.0.14",
     "typescript": "^5.9.3",
     "typescript-eslint": "^8.48.0",
-    "vitest": "^3.2.4"
+    "vitest": "^4.0.14"
   }
 }
diff --git a/mcp-server/src/integration/integration.test.ts b/mcp-server/src/integration/integration.test.ts
index 087f6b1..c1beeec 100644
--- a/mcp-server/src/integration/integration.test.ts
+++ b/mcp-server/src/integration/integration.test.ts
@@ -19,28 +19,36 @@ vi.mock('../utils/http-client.js', () => ({
 }))
 
 // Mock only the game classes from shared library
+// In vitest v4, mocks used as constructors must use 'function' syntax
 vi.mock('@turn-based-mcp/shared', async (importOriginal) => {
   const actual = await importOriginal() as any
   return {
     ...actual,
-    TicTacToeGame: vi.fn(() => ({
-      getValidMoves: vi.fn(() => [{ row: 0, col: 0 }])
-    })),
-    RockPaperScissorsGame: vi.fn(() => ({}))
+    TicTacToeGame: vi.fn(function() {
+      return {
+        getValidMoves: vi.fn(() => [{ row: 0, col: 0 }])
+      }
+    }),
+    RockPaperScissorsGame: vi.fn(function() { return {} })
   }
 })
 
 // Mock AI modules
+// In vitest v4, mocks used as constructors must use 'function' syntax
 vi.mock('../ai/tic-tac-toe-ai.js', () => ({
-  TicTacToeAI: vi.fn(() => ({
-    makeMove: vi.fn(() => ({ row: 0, col: 0 }))
-  }))
+  TicTacToeAI: vi.fn(function() {
+    return {
+      makeMove: vi.fn(() => ({ row: 0, col: 0 }))
+    }
+  })
 }))
 
 vi.mock('../ai/rock-paper-scissors-ai.js', () => ({
-  RockPaperScissorsAI: vi.fn(() => ({
-    makeChoice: vi.fn(() => 'rock')
-  }))
+  RockPaperScissorsAI: vi.fn(function() {
+    return {
+      makeChoice: vi.fn(() => 'rock')
+    }
+  })
 }))
 
 describe('MCP Server Integration', () => {
diff --git a/package-lock.json b/package-lock.json
index 7036f96..1d6652f 100644
--- a/package-lock.json
+++ b/package-lock.json
@@ -17,11 +17,11 @@
         "@testing-library/jest-dom": "^6.9.1",
         "@testing-library/react": "^16.3.0",
         "@testing-library/user-event": "^14.6.1",
-        "@vitest/ui": "^3.2.4",
+        "@vitest/ui": "^4.0.14",
         "eslint": "^9.39.1",
         "typescript": "^5.9.3",
         "typescript-eslint": "^8.48.0",
-        "vitest": "^3.2.4"
+        "vitest": "^4.0.14"
       },
       "engines": {
         "node": ">=18.0.0"
@@ -36,10 +36,10 @@
       },
       "devDependencies": {
         "@eslint/js": "^9.39.1",
-        "@vitest/ui": "^3.2.4",
+        "@vitest/ui": "^4.0.14",
         "typescript": "^5.9.3",
         "typescript-eslint": "^8.48.0",
-        "vitest": "^3.2.4"
+        "vitest": "^4.0.14"
       }
     },
     "node_modules/@adobe/css-tools": {
@@ -2582,6 +2582,13 @@
       "dev": true,
       "license": "MIT"
     },
+    "node_modules/@standard-schema/spec": {
+      "version": "1.0.0",
+      "resolved": "https://registry.npmjs.org/@standard-schema/spec/-/spec-1.0.0.tgz",
+      "integrity": "sha512-m2bOd0f2RT9k8QJx1JN85cZYyH1RqFBdlwtkSlf4tBDYLCiiZnv1fIIwacK6cqwXavOydf0NPToMQgpKq+dVlA==",
+      "dev": true,
+      "license": "MIT"
+    },
     "node_modules/@swc/helpers": {
       "version": "0.5.15",
       "resolved": "https://registry.npmjs.org/@swc/helpers/-/helpers-0.5.15.tgz",
@@ -3854,77 +3861,41 @@
         }
       }
     },
-    "node_modules/@vitest/coverage-v8/node_modules/@vitest/pretty-format": {
-      "version": "4.0.14",
-      "resolved": "https://registry.npmjs.org/@vitest/pretty-format/-/pretty-format-4.0.14.tgz",
-      "integrity": "sha512-SOYPgujB6TITcJxgd3wmsLl+wZv+fy3av2PpiPpsWPZ6J1ySUYfScfpIt2Yv56ShJXR2MOA6q2KjKHN4EpdyRQ==",
-      "dev": true,
-      "license": "MIT",
-      "dependencies": {
-        "tinyrainbow": "^3.0.3"
-      },
-      "funding": {
-        "url": "https://opencollective.com/vitest"
-      }
-    },
-    "node_modules/@vitest/coverage-v8/node_modules/@vitest/utils": {
-      "version": "4.0.14",
-      "resolved": "https://registry.npmjs.org/@vitest/utils/-/utils-4.0.14.tgz",
-      "integrity": "sha512-hLqXZKAWNg8pI+SQXyXxWCTOpA3MvsqcbVeNgSi8x/CSN2wi26dSzn1wrOhmCmFjEvN9p8/kLFRHa6PI8jHazw==",
-      "dev": true,
-      "license": "MIT",
-      "dependencies": {
-        "@vitest/pretty-format": "4.0.14",
-        "tinyrainbow": "^3.0.3"
-      },
-      "funding": {
-        "url": "https://opencollective.com/vitest"
-      }
-    },
-    "node_modules/@vitest/coverage-v8/node_modules/tinyrainbow": {
-      "version": "3.0.3",
-      "resolved": "https://registry.npmjs.org/tinyrainbow/-/tinyrainbow-3.0.3.tgz",
-      "integrity": "sha512-PSkbLUoxOFRzJYjjxHJt9xro7D+iilgMX/C9lawzVuYiIdcihh9DXmVibBe8lmcFrRi/VzlPjBxbN7rH24q8/Q==",
-      "dev": true,
-      "license": "MIT",
-      "engines": {
-        "node": ">=14.0.0"
-      }
-    },
     "node_modules/@vitest/expect": {
-      "version": "3.2.4",
-      "resolved": "https://registry.npmjs.org/@vitest/expect/-/expect-3.2.4.tgz",
-      "integrity": "sha512-Io0yyORnB6sikFlt8QW5K7slY4OjqNX9jmJQ02QDda8lyM6B5oNgVWoSoKPac8/kgnCUzuHQKrSLtu/uOqqrig==",
+      "version": "4.0.14",
+      "resolved": "https://registry.npmjs.org/@vitest/expect/-/expect-4.0.14.tgz",
+      "integrity": "sha512-RHk63V3zvRiYOWAV0rGEBRO820ce17hz7cI2kDmEdfQsBjT2luEKB5tCOc91u1oSQoUOZkSv3ZyzkdkSLD7lKw==",
       "dev": true,
       "license": "MIT",
       "dependencies": {
+        "@standard-schema/spec": "^1.0.0",
         "@types/chai": "^5.2.2",
-        "@vitest/spy": "3.2.4",
-        "@vitest/utils": "3.2.4",
-        "chai": "^5.2.0",
-        "tinyrainbow": "^2.0.0"
+        "@vitest/spy": "4.0.14",
+        "@vitest/utils": "4.0.14",
+        "chai": "^6.2.1",
+        "tinyrainbow": "^3.0.3"
       },
       "funding": {
         "url": "https://opencollective.com/vitest"
       }
     },
     "node_modules/@vitest/mocker": {
-      "version": "3.2.4",
-      "resolved": "https://registry.npmjs.org/@vitest/mocker/-/mocker-3.2.4.tgz",
-      "integrity": "sha512-46ryTE9RZO/rfDd7pEqFl7etuyzekzEhUbTW3BvmeO/BcCMEgq59BKhek3dXDWgAj4oMK6OZi+vRr1wPW6qjEQ==",
+      "version": "4.0.14",
+      "resolved": "https://registry.npmjs.org/@vitest/mocker/-/mocker-4.0.14.tgz",
+      "integrity": "sha512-RzS5NujlCzeRPF1MK7MXLiEFpkIXeMdQ+rN3Kk3tDI9j0mtbr7Nmuq67tpkOJQpgyClbOltCXMjLZicJHsH5Cg==",
       "dev": true,
       "license": "MIT",
       "dependencies": {
-        "@vitest/spy": "3.2.4",
+        "@vitest/spy": "4.0.14",
         "estree-walker": "^3.0.3",
-        "magic-string": "^0.30.17"
+        "magic-string": "^0.30.21"
       },
       "funding": {
         "url": "https://opencollective.com/vitest"
       },
       "peerDependencies": {
         "msw": "^2.4.9",
-        "vite": "^5.0.0 || ^6.0.0 || ^7.0.0-0"
+        "vite": "^6.0.0 || ^7.0.0-0"
       },
       "peerDependenciesMeta": {
         "msw": {
@@ -3936,42 +3907,41 @@
       }
     },
     "node_modules/@vitest/pretty-format": {
-      "version": "3.2.4",
-      "resolved": "https://registry.npmjs.org/@vitest/pretty-format/-/pretty-format-3.2.4.tgz",
-      "integrity": "sha512-IVNZik8IVRJRTr9fxlitMKeJeXFFFN0JaB9PHPGQ8NKQbGpfjlTx9zO4RefN8gp7eqjNy8nyK3NZmBzOPeIxtA==",
+      "version": "4.0.14",
+      "resolved": "https://registry.npmjs.org/@vitest/pretty-format/-/pretty-format-4.0.14.tgz",
+      "integrity": "sha512-SOYPgujB6TITcJxgd3wmsLl+wZv+fy3av2PpiPpsWPZ6J1ySUYfScfpIt2Yv56ShJXR2MOA6q2KjKHN4EpdyRQ==",
       "dev": true,
       "license": "MIT",
       "dependencies": {
-        "tinyrainbow": "^2.0.0"
+        "tinyrainbow": "^3.0.3"
       },
       "funding": {
         "url": "https://opencollective.com/vitest"
       }
     },
     "node_modules/@vitest/runner": {
-      "version": "3.2.4",
-      "resolved": "https://registry.npmjs.org/@vitest/runner/-/runner-3.2.4.tgz",
-      "integrity": "sha512-oukfKT9Mk41LreEW09vt45f8wx7DordoWUZMYdY/cyAk7w5TWkTRCNZYF7sX7n2wB7jyGAl74OxgwhPgKaqDMQ==",
+      "version": "4.0.14",
+      "resolved": "https://registry.npmjs.org/@vitest/runner/-/runner-4.0.14.tgz",
+      "integrity": "sha512-BsAIk3FAqxICqREbX8SetIteT8PiaUL/tgJjmhxJhCsigmzzH8xeadtp7LRnTpCVzvf0ib9BgAfKJHuhNllKLw==",
       "dev": true,
       "license": "MIT",
       "dependencies": {
-        "@vitest/utils": "3.2.4",
-        "pathe": "^2.0.3",
-        "strip-literal": "^3.0.0"
+        "@vitest/utils": "4.0.14",
+        "pathe": "^2.0.3"
       },
       "funding": {
         "url": "https://opencollective.com/vitest"
       }
     },
     "node_modules/@vitest/snapshot": {
-      "version": "3.2.4",
-      "resolved": "https://registry.npmjs.org/@vitest/snapshot/-/snapshot-3.2.4.tgz",
-      "integrity": "sha512-dEYtS7qQP2CjU27QBC5oUOxLE/v5eLkGqPE0ZKEIDGMs4vKWe7IjgLOeauHsR0D5YuuycGRO5oSRXnwnmA78fQ==",
+      "version": "4.0.14",
+      "resolved": "https://registry.npmjs.org/@vitest/snapshot/-/snapshot-4.0.14.tgz",
+      "integrity": "sha512-aQVBfT1PMzDSA16Y3Fp45a0q8nKexx6N5Amw3MX55BeTeZpoC08fGqEZqVmPcqN0ueZsuUQ9rriPMhZ3Mu19Ag==",
       "dev": true,
       "license": "MIT",
       "dependencies": {
-        "@vitest/pretty-format": "3.2.4",
-        "magic-string": "^0.30.17",
+        "@vitest/pretty-format": "4.0.14",
+        "magic-string": "^0.30.21",
         "pathe": "^2.0.3"
       },
       "funding": {
@@ -3979,50 +3949,46 @@
       }
     },
     "node_modules/@vitest/spy": {
-      "version": "3.2.4",
-      "resolved": "https://registry.npmjs.org/@vitest/spy/-/spy-3.2.4.tgz",
-      "integrity": "sha512-vAfasCOe6AIK70iP5UD11Ac4siNUNJ9i/9PZ3NKx07sG6sUxeag1LWdNrMWeKKYBLlzuK+Gn65Yd5nyL6ds+nw==",
+      "version": "4.0.14",
+      "resolved": "https://registry.npmjs.org/@vitest/spy/-/spy-4.0.14.tgz",
+      "integrity": "sha512-JmAZT1UtZooO0tpY3GRyiC/8W7dCs05UOq9rfsUUgEZEdq+DuHLmWhPsrTt0TiW7WYeL/hXpaE07AZ2RCk44hg==",
       "dev": true,
       "license": "MIT",
-      "dependencies": {
-        "tinyspy": "^4.0.3"
-      },
       "funding": {
         "url": "https://opencollective.com/vitest"
       }
     },
     "node_modules/@vitest/ui": {
-      "version": "3.2.4",
-      "resolved": "https://registry.npmjs.org/@vitest/ui/-/ui-3.2.4.tgz",
-      "integrity": "sha512-hGISOaP18plkzbWEcP/QvtRW1xDXF2+96HbEX6byqQhAUbiS5oH6/9JwW+QsQCIYON2bI6QZBF+2PvOmrRZ9wA==",
+      "version": "4.0.14",
+      "resolved": "https://registry.npmjs.org/@vitest/ui/-/ui-4.0.14.tgz",
+      "integrity": "sha512-fvDz8o7SQpFLoSBo6Cudv+fE85/fPCkwTnLAN85M+Jv7k59w2mSIjT9Q5px7XwGrmYqqKBEYxh/09IBGd1E7AQ==",
       "dev": true,
       "license": "MIT",
       "dependencies": {
-        "@vitest/utils": "3.2.4",
+        "@vitest/utils": "4.0.14",
         "fflate": "^0.8.2",
         "flatted": "^3.3.3",
         "pathe": "^2.0.3",
-        "sirv": "^3.0.1",
-        "tinyglobby": "^0.2.14",
-        "tinyrainbow": "^2.0.0"
+        "sirv": "^3.0.2",
+        "tinyglobby": "^0.2.15",
+        "tinyrainbow": "^3.0.3"
       },
       "funding": {
         "url": "https://opencollective.com/vitest"
       },
       "peerDependencies": {
-        "vitest": "3.2.4"
+        "vitest": "4.0.14"
       }
     },
     "node_modules/@vitest/utils": {
-      "version": "3.2.4",
-      "resolved": "https://registry.npmjs.org/@vitest/utils/-/utils-3.2.4.tgz",
-      "integrity": "sha512-fB2V0JFrQSMsCo9HiSq3Ezpdv4iYaXRG1Sx8edX3MwxfyNn83mKiGzOcH+Fkxt4MHxr3y42fQi1oeAInqgX2QA==",
+      "version": "4.0.14",
+      "resolved": "https://registry.npmjs.org/@vitest/utils/-/utils-4.0.14.tgz",
+      "integrity": "sha512-hLqXZKAWNg8pI+SQXyXxWCTOpA3MvsqcbVeNgSi8x/CSN2wi26dSzn1wrOhmCmFjEvN9p8/kLFRHa6PI8jHazw==",
       "dev": true,
       "license": "MIT",
       "dependencies": {
-        "@vitest/pretty-format": "3.2.4",
-        "loupe": "^3.1.4",
-        "tinyrainbow": "^2.0.0"
+        "@vitest/pretty-format": "4.0.14",
+        "tinyrainbow": "^3.0.3"
       },
       "funding": {
         "url": "https://opencollective.com/vitest"
@@ -4654,16 +4620,6 @@
         "node": ">= 0.8"
       }
     },
-    "node_modules/cac": {
-      "version": "6.7.14",
-      "resolved": "https://registry.npmjs.org/cac/-/cac-6.7.14.tgz",
-      "integrity": "sha512-b6Ilus+c3RrdDk+JhLKUAQfzzgLEPy6wcXqS7f/xe1EETvsDP6GORG7SFuOs6cID5YkqchW/LXZbX5bc8j7ZcQ==",
-      "dev": true,
-      "license": "MIT",
-      "engines": {
-        "node": ">=8"
-      }
-    },
     "node_modules/cacache": {
       "version": "15.3.0",
       "resolved": "https://registry.npmjs.org/cacache/-/cacache-15.3.0.tgz",
@@ -4828,18 +4784,11 @@
       }
     },
     "node_modules/chai": {
-      "version": "5.3.3",
-      "resolved": "https://registry.npmjs.org/chai/-/chai-5.3.3.tgz",
-      "integrity": "sha512-4zNhdJD/iOjSH0A05ea+Ke6MU5mmpQcbQsSOkgdaUMJ9zTlDTD/GYlwohmIE2u0gaxHYiVHEn1Fw9mZ/ktJWgw==",
+      "version": "6.2.1",
+      "resolved": "https://registry.npmjs.org/chai/-/chai-6.2.1.tgz",
+      "integrity": "sha512-p4Z49OGG5W/WBCPSS/dH3jQ73kD6tiMmUM+bckNK6Jr5JHMG3k9bg/BvKR8lKmtVBKmOiuVaV2ws8s9oSbwysg==",
       "dev": true,
       "license": "MIT",
-      "dependencies": {
-        "assertion-error": "^2.0.1",
-        "check-error": "^2.1.1",
-        "deep-eql": "^5.0.1",
-        "loupe": "^3.1.0",
-        "pathval": "^2.0.0"
-      },
       "engines": {
         "node": ">=18"
       }
@@ -4861,16 +4810,6 @@
         "url": "https://github.com/chalk/chalk?sponsor=1"
       }
     },
-    "node_modules/check-error": {
-      "version": "2.1.1",
-      "resolved": "https://registry.npmjs.org/check-error/-/check-error-2.1.1.tgz",
-      "integrity": "sha512-OAlb+T7V4Op9OwdkjmguYRqncdlx5JiofwOAUkmTF+jNdHwzTaTs4sRAGpzLF3oOz5xAyDGrPgeIDFQmDOTiJw==",
-      "dev": true,
-      "license": "MIT",
-      "engines": {
-        "node": ">= 16"
-      }
-    },
     "node_modules/chownr": {
       "version": "2.0.0",
       "resolved": "https://registry.npmjs.org/chownr/-/chownr-2.0.0.tgz",
@@ -5198,16 +5137,6 @@
         "url": "https://github.com/sponsors/sindresorhus"
       }
     },
-    "node_modules/deep-eql": {
-      "version": "5.0.2",
-      "resolved": "https://registry.npmjs.org/deep-eql/-/deep-eql-5.0.2.tgz",
-      "integrity": "sha512-h5k/5U50IJJFpzfL6nO9jaaumfjO/f2NjK/oYB2Djzm4p9L+3T9qWpZqZ2hAbLPuuYq9wrU08WQyBTL5GbPk5Q==",
-      "dev": true,
-      "license": "MIT",
-      "engines": {
-        "node": ">=6"
-      }
-    },
     "node_modules/deep-extend": {
       "version": "0.6.0",
       "resolved": "https://registry.npmjs.org/deep-extend/-/deep-extend-0.6.0.tgz",
@@ -8411,13 +8340,6 @@
         "loose-envify": "cli.js"
       }
     },
-    "node_modules/loupe": {
-      "version": "3.2.1",
-      "resolved": "https://registry.npmjs.org/loupe/-/loupe-3.2.1.tgz",
-      "integrity": "sha512-CdzqowRJCeLU72bHvWqwRBBlLcMEtIvGrlvef74kMnV2AolS9Y8xUv1I0U/MNAWMhBlKIoyuEgoJ0t/bbwHbLQ==",
-      "dev": true,
-      "license": "MIT"
-    },
     "node_modules/lru-cache": {
       "version": "5.1.1",
       "resolved": "https://registry.npmjs.org/lru-cache/-/lru-cache-5.1.1.tgz",
@@ -9430,16 +9352,6 @@
       "dev": true,
       "license": "MIT"
     },
-    "node_modules/pathval": {
-      "version": "2.0.1",
-      "resolved": "https://registry.npmjs.org/pathval/-/pathval-2.0.1.tgz",
-      "integrity": "sha512-//nshmD55c46FuFw26xV/xFAaB5HF9Xdap7HJBBnrKdAd6/GxDBaNA1870O79+9ueg61cZLSVc+OaFlfmObYVQ==",
-      "dev": true,
-      "license": "MIT",
-      "engines": {
-        "node": ">= 14.16"
-      }
-    },
     "node_modules/picocolors": {
       "version": "1.1.1",
       "resolved": "https://registry.npmjs.org/picocolors/-/picocolors-1.1.1.tgz",
@@ -10905,26 +10817,6 @@
         "url": "https://github.com/sponsors/sindresorhus"
       }
     },
-    "node_modules/strip-literal": {
-      "version": "3.1.0",
-      "resolved": "https://registry.npmjs.org/strip-literal/-/strip-literal-3.1.0.tgz",
-      "integrity": "sha512-8r3mkIM/2+PpjHoOtiAW8Rg3jJLHaV7xPwG+YRGrv6FP0wwk/toTpATxWYOW0BKdWwl82VT2tFYi5DlROa0Mxg==",
-      "dev": true,
-      "license": "MIT",
-      "dependencies": {
-        "js-tokens": "^9.0.1"
-      },
-      "funding": {
-        "url": "https://github.com/sponsors/antfu"
-      }
-    },
-    "node_modules/strip-literal/node_modules/js-tokens": {
-      "version": "9.0.1",
-      "resolved": "https://registry.npmjs.org/js-tokens/-/js-tokens-9.0.1.tgz",
-      "integrity": "sha512-mxa9E9ITFOt0ban3j6L5MpjwegGz6lBQmM1IJkWeBZGcMxto50+eWdjC/52xDbS2vy0k7vIMK0Fe2wfL9OQSpQ==",
-      "dev": true,
-      "license": "MIT"
-    },
     "node_modules/styled-jsx": {
       "version": "5.1.6",
       "resolved": "https://registry.npmjs.org/styled-jsx/-/styled-jsx-5.1.6.tgz",
@@ -11156,30 +11048,10 @@
         "url": "https://github.com/sponsors/SuperchupuDev"
       }
     },
-    "node_modules/tinypool": {
-      "version": "1.1.1",
-      "resolved": "https://registry.npmjs.org/tinypool/-/tinypool-1.1.1.tgz",
-      "integrity": "sha512-Zba82s87IFq9A9XmjiX5uZA/ARWDrB03OHlq+Vw1fSdt0I+4/Kutwy8BP4Y/y/aORMo61FQ0vIb5j44vSo5Pkg==",
-      "dev": true,
-      "license": "MIT",
-      "engines": {
-        "node": "^18.0.0 || >=20.0.0"
-      }
-    },
     "node_modules/tinyrainbow": {
-      "version": "2.0.0",
-      "resolved": "https://registry.npmjs.org/tinyrainbow/-/tinyrainbow-2.0.0.tgz",
-      "integrity": "sha512-op4nsTR47R6p0vMUUoYl/a+ljLFVtlfaXkLQmqfLR1qHma1h/ysYk4hEXZ880bf2CYgTskvTa/e196Vd5dDQXw==",
-      "dev": true,
-      "license": "MIT",
-      "engines": {
-        "node": ">=14.0.0"
-      }
-    },
-    "node_modules/tinyspy": {
-      "version": "4.0.4",
-      "resolved": "https://registry.npmjs.org/tinyspy/-/tinyspy-4.0.4.tgz",
-      "integrity": "sha512-azl+t0z7pw/z958Gy9svOTuzqIk6xq+NSheJzn5MMWtWTFywIacg2wUlzKFGtt3cthx0r2SxMK0yzJOR0IES7Q==",
+      "version": "3.0.3",
+      "resolved": "https://registry.npmjs.org/tinyrainbow/-/tinyrainbow-3.0.3.tgz",
+      "integrity": "sha512-PSkbLUoxOFRzJYjjxHJt9xro7D+iilgMX/C9lawzVuYiIdcihh9DXmVibBe8lmcFrRi/VzlPjBxbN7rH24q8/Q==",
       "dev": true,
       "license": "MIT",
       "engines": {
@@ -11780,75 +11652,51 @@
         }
       }
     },
-    "node_modules/vite-node": {
-      "version": "3.2.4",
-      "resolved": "https://registry.npmjs.org/vite-node/-/vite-node-3.2.4.tgz",
-      "integrity": "sha512-EbKSKh+bh1E1IFxeO0pg1n4dvoOTt0UDiXMd/qn++r98+jPO1xtJilvXldeuQ8giIB5IkpjCgMleHMNEsGH6pg==",
-      "dev": true,
-      "license": "MIT",
-      "dependencies": {
-        "cac": "^6.7.14",
-        "debug": "^4.4.1",
-        "es-module-lexer": "^1.7.0",
-        "pathe": "^2.0.3",
-        "vite": "^5.0.0 || ^6.0.0 || ^7.0.0-0"
-      },
-      "bin": {
-        "vite-node": "vite-node.mjs"
-      },
-      "engines": {
-        "node": "^18.0.0 || ^20.0.0 || >=22.0.0"
-      },
-      "funding": {
-        "url": "https://opencollective.com/vitest"
-      }
-    },
     "node_modules/vitest": {
-      "version": "3.2.4",
-      "resolved": "https://registry.npmjs.org/vitest/-/vitest-3.2.4.tgz",
-      "integrity": "sha512-LUCP5ev3GURDysTWiP47wRRUpLKMOfPh+yKTx3kVIEiu5KOMeqzpnYNsKyOoVrULivR8tLcks4+lga33Whn90A==",
+      "version": "4.0.14",
+      "resolved": "https://registry.npmjs.org/vitest/-/vitest-4.0.14.tgz",
+      "integrity": "sha512-d9B2J9Cm9dN9+6nxMnnNJKJCtcyKfnHj15N6YNJfaFHRLua/d3sRKU9RuKmO9mB0XdFtUizlxfz/VPbd3OxGhw==",
       "dev": true,
       "license": "MIT",
       "dependencies": {
-        "@types/chai": "^5.2.2",
-        "@vitest/expect": "3.2.4",
-        "@vitest/mocker": "3.2.4",
-        "@vitest/pretty-format": "^3.2.4",
-        "@vitest/runner": "3.2.4",
-        "@vitest/snapshot": "3.2.4",
-        "@vitest/spy": "3.2.4",
-        "@vitest/utils": "3.2.4",
-        "chai": "^5.2.0",
-        "debug": "^4.4.1",
-        "expect-type": "^1.2.1",
-        "magic-string": "^0.30.17",
+        "@vitest/expect": "4.0.14",
+        "@vitest/mocker": "4.0.14",
+        "@vitest/pretty-format": "4.0.14",
+        "@vitest/runner": "4.0.14",
+        "@vitest/snapshot": "4.0.14",
+        "@vitest/spy": "4.0.14",
+        "@vitest/utils": "4.0.14",
+        "es-module-lexer": "^1.7.0",
+        "expect-type": "^1.2.2",
+        "magic-string": "^0.30.21",
+        "obug": "^2.1.1",
         "pathe": "^2.0.3",
-        "picomatch": "^4.0.2",
-        "std-env": "^3.9.0",
+        "picomatch": "^4.0.3",
+        "std-env": "^3.10.0",
         "tinybench": "^2.9.0",
         "tinyexec": "^0.3.2",
-        "tinyglobby": "^0.2.14",
-        "tinypool": "^1.1.1",
-        "tinyrainbow": "^2.0.0",
-        "vite": "^5.0.0 || ^6.0.0 || ^7.0.0-0",
-        "vite-node": "3.2.4",
+        "tinyglobby": "^0.2.15",
+        "tinyrainbow": "^3.0.3",
+        "vite": "^6.0.0 || ^7.0.0",
         "why-is-node-running": "^2.3.0"
       },
       "bin": {
         "vitest": "vitest.mjs"
       },
       "engines": {
-        "node": "^18.0.0 || ^20.0.0 || >=22.0.0"
+        "node": "^20.0.0 || ^22.0.0 || >=24.0.0"
       },
       "funding": {
         "url": "https://opencollective.com/vitest"
       },
       "peerDependencies": {
         "@edge-runtime/vm": "*",
-        "@types/debug": "^4.1.12",
-        "@types/node": "^18.0.0 || ^20.0.0 || >=22.0.0",
-        "@vitest/browser": "3.2.4",
-        "@vitest/ui": "3.2.4",
+        "@opentelemetry/api": "^1.9.0",
+        "@types/node": "^20.0.0 || ^22.0.0 || >=24.0.0",
+        "@vitest/browser-playwright": "4.0.14",
+        "@vitest/browser-preview": "4.0.14",
+        "@vitest/browser-webdriverio": "4.0.14",
+        "@vitest/ui": "4.0.14",
         "happy-dom": "*",
         "jsdom": "*"
       },
@@ -11856,13 +11704,19 @@
         "@edge-runtime/vm": {
           "optional": true
         },
-        "@types/debug": {
+        "@opentelemetry/api": {
           "optional": true
         },
         "@types/node": {
           "optional": true
         },
-        "@vitest/browser": {
+        "@vitest/browser-playwright": {
+          "optional": true
+        },
+        "@vitest/browser-preview": {
+          "optional": true
+        },
+        "@vitest/browser-webdriverio": {
           "optional": true
         },
         "@vitest/ui": {
@@ -12224,12 +12078,12 @@
         "@jest/types": "30.2.0",
         "@types/jest": "^30.0.0",
         "@vitest/coverage-v8": "^4.0.14",
-        "@vitest/ui": "^3.2.4",
+        "@vitest/ui": "^4.0.14",
         "eslint": "^9.39.1",
         "fix-esm-import-path": "^1.10.3",
         "typescript": "^5.9.3",
         "typescript-eslint": "^8.48.0",
-        "vitest": "^3.2.4"
+        "vitest": "^4.0.14"
       }
     },
     "web": {
@@ -12257,7 +12111,7 @@
         "@types/react": "^19.2.7",
         "@types/react-dom": "^19.2.3",
         "@vitejs/plugin-react": "^5.1.1",
-        "@vitest/ui": "^3.2.4",
+        "@vitest/ui": "^4.0.14",
         "eslint": "^9.39.1",
         "eslint-config-next": "^16.0.5",
         "eslint-plugin-react": "^7.37.5",
@@ -12268,7 +12122,7 @@
         "typescript": "^5.9.3",
         "typescript-eslint": "^8.48.0",
         "undici": "^7.16.0",
-        "vitest": "^3.2.4"
+        "vitest": "^4.0.14"
       }
     }
   }
diff --git a/package.json b/package.json
index 3f2d391..fcbaf3a 100644
--- a/package.json
+++ b/package.json
@@ -29,11 +29,11 @@
     "@testing-library/jest-dom": "^6.9.1",
     "@testing-library/react": "^16.3.0",
     "@testing-library/user-event": "^14.6.1",
-    "@vitest/ui": "^3.2.4",
+    "@vitest/ui": "^4.0.14",
     "eslint": "^9.39.1",
     "typescript": "^5.9.3",
     "typescript-eslint": "^8.48.0",
-    "vitest": "^3.2.4"
+    "vitest": "^4.0.14"
   },
   "engines": {
     "node": ">=18.0.0"
diff --git a/shared/package.json b/shared/package.json
index bc86754..bf91b13 100644
--- a/shared/package.json
+++ b/shared/package.json
@@ -36,12 +36,12 @@
     "@jest/types": "30.2.0",
     "@types/jest": "^30.0.0",
     "@vitest/coverage-v8": "^4.0.14",
-    "@vitest/ui": "^3.2.4",
+    "@vitest/ui": "^4.0.14",
     "eslint": "^9.39.1",
     "fix-esm-import-path": "^1.10.3",
     "typescript": "^5.9.3",
     "typescript-eslint": "^8.48.0",
-    "vitest": "^3.2.4"
+    "vitest": "^4.0.14"
   },
   "dependencies": {
     "sqlite3": "^5.1.7"
diff --git a/shared/src/storage/sqlite-storage.test.ts b/shared/src/storage/sqlite-storage.test.ts
index 2c5cdd4..9dc70f6 100644
--- a/shared/src/storage/sqlite-storage.test.ts
+++ b/shared/src/storage/sqlite-storage.test.ts
@@ -2,28 +2,31 @@ import { vi } from 'vitest'
 import * as sqliteStorage from './sqlite-storage';
 
 // Mock sqlite3 to prevent actual database operations
+// In vitest v4, mocks used as constructors must use 'function' or 'class' syntax
 vi.mock('sqlite3', () => ({
   default: {
-    Database: vi.fn().mockImplementation(() => ({
-      run: vi.fn(function(sql, params, callback) {
-        if (callback) {
-          // Mock the 'this' context with lastID and changes
-          callback.call({ lastID: 1, changes: 1 });
-        }
-      }),
-      get: vi.fn((sql, params, callback) => {
-        if (callback) callback(null, null);
-      }),
-      all: vi.fn((sql, params, callback) => {
-        if (callback) callback(null, []);
-      }),
-      close: vi.fn((callback) => {
-        if (callback) callback();
-      }),
-      serialize: vi.fn((fn) => {
-        if (fn) fn();
-      })
-    }))
+    Database: vi.fn(function() {
+      return {
+        run: vi.fn(function(sql: string, params: unknown, callback?: (this: { lastID: number; changes: number }) => void) {
+          if (callback) {
+            // Mock the 'this' context with lastID and changes
+            callback.call({ lastID: 1, changes: 1 });
+          }
+        }),
+        get: vi.fn((sql: string, params: unknown, callback?: (err: Error | null, row: unknown) => void) => {
+          if (callback) callback(null, null);
+        }),
+        all: vi.fn((sql: string, params: unknown, callback?: (err: Error | null, rows: unknown[]) => void) => {
+          if (callback) callback(null, []);
+        }),
+        close: vi.fn((callback?: () => void) => {
+          if (callback) callback();
+        }),
+        serialize: vi.fn((fn?: () => void) => {
+          if (fn) fn();
+        })
+      };
+    })
   }
 }));
 
diff --git a/web/package.json b/web/package.json
index 06980ae..c9f57ab 100644
--- a/web/package.json
+++ b/web/package.json
@@ -38,7 +38,7 @@
     "@types/react": "^19.2.7",
     "@types/react-dom": "^19.2.3",
     "@vitejs/plugin-react": "^5.1.1",
-    "@vitest/ui": "^3.2.4",
+    "@vitest/ui": "^4.0.14",
     "eslint": "^9.39.1",
     "eslint-config-next": "^16.0.5",
     "eslint-plugin-react": "^7.37.5",
@@ -49,6 +49,6 @@
     "typescript": "^5.9.3",
     "typescript-eslint": "^8.48.0",
     "undici": "^7.16.0",
-    "vitest": "^3.2.4"
+    "vitest": "^4.0.14"
   }
 }
diff --git a/web/src/app/api/games/rock-paper-scissors/[id]/move/route.test.ts b/web/src/app/api/games/rock-paper-scissors/[id]/move/route.test.ts
index 90819a8..9327c52 100644
--- a/web/src/app/api/games/rock-paper-scissors/[id]/move/route.test.ts
+++ b/web/src/app/api/games/rock-paper-scissors/[id]/move/route.test.ts
@@ -3,6 +3,7 @@ import { NextRequest } from 'next/server';
 import type { GameSession, RPSGameState, RPSMove } from '@turn-based-mcp/shared';
 
 // Mock dependencies BEFORE importing the route - use factory functions for proper setup
+// In vitest v4, mocks used as constructors must use 'function' syntax
 vi.mock('@turn-based-mcp/shared', () => {
   const mockGame = {
     getInitialState: vi.fn(),
@@ -14,7 +15,7 @@ vi.mock('@turn-based-mcp/shared', () => {
   
   return {
     ...vi.importActual('@turn-based-mcp/shared'),
-    RockPaperScissorsGame: vi.fn(() => mockGame),
+    RockPaperScissorsGame: vi.fn(function() { return mockGame; }),
     __mockGameInstance: mockGame
   };
 });
diff --git a/web/src/app/api/games/rock-paper-scissors/route.test.ts b/web/src/app/api/games/rock-paper-scissors/route.test.ts
index c0a1151..3548772 100644
--- a/web/src/app/api/games/rock-paper-scissors/route.test.ts
+++ b/web/src/app/api/games/rock-paper-scissors/route.test.ts
@@ -13,9 +13,10 @@ const mockGame = vi.hoisted(() => ({
 }));
 
 // Mock dependencies BEFORE importing the route
+// In vitest v4, mocks used as constructors must use 'function' syntax
 vi.mock('@turn-based-mcp/shared', () => ({
   ...vi.importActual('@turn-based-mcp/shared'),
-  RockPaperScissorsGame: vi.fn().mockImplementation(() => mockGame)
+  RockPaperScissorsGame: vi.fn(function() { return mockGame; })
 }));
 
 vi.mock('../../../../lib/game-storage', () => ({
diff --git a/web/src/app/api/games/tic-tac-toe/[id]/move/route.test.ts b/web/src/app/api/games/tic-tac-toe/[id]/move/route.test.ts
index b3e0327..01b8df3 100644
--- a/web/src/app/api/games/tic-tac-toe/[id]/move/route.test.ts
+++ b/web/src/app/api/games/tic-tac-toe/[id]/move/route.test.ts
@@ -3,6 +3,7 @@ import { NextRequest } from 'next/server';
 import type { GameSession, TicTacToeGameState, TicTacToeMove } from '@turn-based-mcp/shared';
 
 // Mock dependencies BEFORE importing the route - use factory functions for proper setup
+// In vitest v4, mocks used as constructors must use 'function' syntax
 vi.mock('@turn-based-mcp/shared', () => {
   const mockGame = {
     getInitialState: vi.fn(),
@@ -14,7 +15,7 @@ vi.mock('@turn-based-mcp/shared', () => {
   
   return {
     ...vi.importActual('@turn-based-mcp/shared'),
-    TicTacToeGame: vi.fn(() => mockGame),
+    TicTacToeGame: vi.fn(function() { return mockGame; }),
     getTicTacToeGame: vi.fn(),
     setTicTacToeGame: vi.fn(),
     __mockGameInstance: mockGame
diff --git a/web/src/app/api/games/tic-tac-toe/route.test.ts b/web/src/app/api/games/tic-tac-toe/route.test.ts
index b28bd43..ab073bf 100644
--- a/web/src/app/api/games/tic-tac-toe/route.test.ts
+++ b/web/src/app/api/games/tic-tac-toe/route.test.ts
@@ -13,9 +13,10 @@ const mockGame = vi.hoisted(() => ({
 }));
 
 // Mock dependencies BEFORE importing the route
+// In vitest v4, mocks used as constructors must use 'function' syntax
 vi.mock('@turn-based-mcp/shared', () => ({
   ...vi.importActual('@turn-based-mcp/shared'),
-  TicTacToeGame: vi.fn().mockImplementation(() => mockGame)
+  TicTacToeGame: vi.fn(function() { return mockGame; })
 }));
 
 vi.mock('../../../../lib/game-storage', () => ({