diff --git a/package-lock.json b/package-lock.json
index c590151..5afcfd5 100644
--- a/package-lock.json
+++ b/package-lock.json
@@ -12,6 +12,10 @@
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@@ -1760,6 +1764,17 @@
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@@ -1773,6 +1788,13 @@
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@@ -2597,6 +2804,16 @@
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@@ -3146,6 +3372,16 @@
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@@ -3468,6 +3704,52 @@
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+      "version": "3.2.0",
+      "resolved": "https://registry.npmjs.org/istanbul-reports/-/istanbul-reports-3.2.0.tgz",
+      "integrity": "sha512-HGYWWS/ehqTV3xN10i23tkPkpH46MLCIMFNCaaKNavAXTF1RkqxawEPtnjnGZ6XKSInBKkiOA5BKS+aZiY3AvA==",
+      "dev": true,
+      "license": "BSD-3-Clause",
+      "dependencies": {
+        "html-escaper": "^2.0.0",
+        "istanbul-lib-report": "^3.0.0"
+      },
+      "engines": {
+        "node": ">=8"
+      }
+    },
+    "node_modules/istanbul-reports/node_modules/html-escaper": {
+      "version": "2.0.2",
+      "resolved": "https://registry.npmjs.org/html-escaper/-/html-escaper-2.0.2.tgz",
+      "integrity": "sha512-H2iMtd0I4Mt5eYiapRdIDjp+XzelXQ0tFE4JS7YFwFevXXMmOp9myNrUvCg0D6ws8iqkRPBfKHgbwig1SmlLfg==",
+      "dev": true,
+      "license": "MIT"
+    },
     "node_modules/jiti": {
       "version": "2.7.0",
       "resolved": "https://registry.npmjs.org/jiti/-/jiti-2.7.0.tgz",
@@ -3873,6 +4155,22 @@
         "source-map-js": "^1.2.0"
       }
     },
+    "node_modules/make-dir": {
+      "version": "4.0.0",
+      "resolved": "https://registry.npmjs.org/make-dir/-/make-dir-4.0.0.tgz",
+      "integrity": "sha512-hXdUTZYIVOt1Ex//jAQi+wTZZpUpwBj/0QsOzqegb3rGMMeJiSEu5xLHnYfBrRV4RH2+OCSOO95Is/7x1WJ4bw==",
+      "dev": true,
+      "license": "MIT",
+      "dependencies": {
+        "semver": "^7.5.3"
+      },
+      "engines": {
+        "node": ">=10"
+      },
+      "funding": {
+        "url": "https://github.com/sponsors/sindresorhus"
+      }
+    },
     "node_modules/markdown-table": {
       "version": "3.0.4",
       "resolved": "https://registry.npmjs.org/markdown-table/-/markdown-table-3.0.4.tgz",
@@ -4726,6 +5024,17 @@
       "resolved": "https://registry.npmjs.org/node-releases/-/node-releases-2.0.38.tgz",
       "integrity": "sha512-3qT/88Y3FbH/Kx4szpQQ4HzUbVrHPKTLVpVocKiLfoYvw9XSGOX2FmD2d6DrXbVYyAQTF2HeF6My8jmzx7/CRw=="
     },
+    "node_modules/obug": {
+      "version": "2.1.1",
+      "resolved": "https://registry.npmjs.org/obug/-/obug-2.1.1.tgz",
+      "integrity": "sha512-uTqF9MuPraAQ+IsnPf366RG4cP9RtUi7MLO1N3KEc+wb0a6yKpeL0lmk2IB1jY5KHPAlTc6T/JRdC/YqxHNwkQ==",
+      "dev": true,
+      "funding": [
+        "https://github.com/sponsors/sxzz",
+        "https://opencollective.com/debug"
+      ],
+      "license": "MIT"
+    },
     "node_modules/onetime": {
       "version": "7.0.0",
       "resolved": "https://registry.npmjs.org/onetime/-/onetime-7.0.0.tgz",
@@ -4881,6 +5190,13 @@
         "node": ">=8"
       }
     },
+    "node_modules/pathe": {
+      "version": "2.0.3",
+      "resolved": "https://registry.npmjs.org/pathe/-/pathe-2.0.3.tgz",
+      "integrity": "sha512-WUjGcAqP1gQacoQe+OBJsFA7Ld4DyXuUIjZ5cc75cLHvJ7dtNsTugphxIADwspS+AraAUePCKrSVtPLFj/F88w==",
+      "dev": true,
+      "license": "MIT"
+    },
     "node_modules/picocolors": {
       "version": "1.1.1",
       "resolved": "https://registry.npmjs.org/picocolors/-/picocolors-1.1.1.tgz",
@@ -5252,7 +5568,6 @@
       "version": "1.0.0-rc.18",
       "resolved": "https://registry.npmjs.org/rolldown/-/rolldown-1.0.0-rc.18.tgz",
       "integrity": "sha512-phmyKBpuBdRYDf4hgyynGAYn/rDDe+iZXKVJ7WX5b1zQzpLkP5oJRPGsfJuHdzPMlyyEO/4sPW6yfSx2gf7lVg==",
-      "peer": true,
       "dependencies": {
         "@oxc-project/types": "=0.128.0",
         "@rolldown/pluginutils": "1.0.0-rc.18"
@@ -5437,6 +5752,13 @@
         "@types/hast": "^3.0.4"
       }
     },
+    "node_modules/siginfo": {
+      "version": "2.0.0",
+      "resolved": "https://registry.npmjs.org/siginfo/-/siginfo-2.0.0.tgz",
+      "integrity": "sha512-ybx0WO1/8bSBLEWXZvEd7gMW3Sn3JFlW3TvX1nREbDLRNQNaeNN8WK0meBwPdAaOI7TtRRRJn/Es1zhrrCHu7g==",
+      "dev": true,
+      "license": "ISC"
+    },
     "node_modules/signal-exit": {
       "version": "4.1.0",
       "resolved": "https://registry.npmjs.org/signal-exit/-/signal-exit-4.1.0.tgz",
@@ -5509,6 +5831,20 @@
       "resolved": "https://registry.npmjs.org/sprintf-js/-/sprintf-js-1.0.3.tgz",
       "integrity": "sha512-D9cPgkvLlV3t3IzL0D0YLvGA9Ahk4PcvVwUbN0dSGr1aP0Nrt4AEnTUbuGvquEC0mA64Gqt1fzirlRs5ibXx8g=="
     },
+    "node_modules/stackback": {
+      "version": "0.0.2",
+      "resolved": "https://registry.npmjs.org/stackback/-/stackback-0.0.2.tgz",
+      "integrity": "sha512-1XMJE5fQo1jGH6Y/7ebnwPOBEkIEnT4QF32d5R1+VXdXveM0IBMJt8zfaxX1P3QhVwrYe+576+jkANtSS2mBbw==",
+      "dev": true,
+      "license": "MIT"
+    },
+    "node_modules/std-env": {
+      "version": "4.1.0",
+      "resolved": "https://registry.npmjs.org/std-env/-/std-env-4.1.0.tgz",
+      "integrity": "sha512-Rq7ybcX2RuC55r9oaPVEW7/xu3tj8u4GeBYHBWCychFtzMIr86A7e3PPEBPT37sHStKX3+TiX/Fr/ACmJLVlLQ==",
+      "dev": true,
+      "license": "MIT"
+    },
     "node_modules/stdin-discarder": {
       "version": "0.2.2",
       "resolved": "https://registry.npmjs.org/stdin-discarder/-/stdin-discarder-0.2.2.tgz",
@@ -5585,6 +5921,19 @@
         "node": ">=0.10.0"
       }
     },
+    "node_modules/supports-color": {
+      "version": "7.2.0",
+      "resolved": "https://registry.npmjs.org/supports-color/-/supports-color-7.2.0.tgz",
+      "integrity": "sha512-qpCAvRl9stuOHveKsn7HncJRvv501qIacKzQlO/+Lwxc9+0q2wLyv4Dfvt80/DPn2pqOBsJdDiogXGR9+OvwRw==",
+      "dev": true,
+      "license": "MIT",
+      "dependencies": {
+        "has-flag": "^4.0.0"
+      },
+      "engines": {
+        "node": ">=8"
+      }
+    },
     "node_modules/tailwindcss": {
       "version": "4.3.0",
       "resolved": "https://registry.npmjs.org/tailwindcss/-/tailwindcss-4.3.0.tgz",
@@ -5602,6 +5951,13 @@
         "url": "https://opencollective.com/webpack"
       }
     },
+    "node_modules/tinybench": {
+      "version": "2.9.0",
+      "resolved": "https://registry.npmjs.org/tinybench/-/tinybench-2.9.0.tgz",
+      "integrity": "sha512-0+DUvqWMValLmha6lr4kD8iAMK1HzV0/aKnCtWb9v9641TnP/MFb7Pc2bxoxQjTXAErryXVgUOfv2YqNllqGeg==",
+      "dev": true,
+      "license": "MIT"
+    },
     "node_modules/tinyexec": {
       "version": "0.3.2",
       "resolved": "https://registry.npmjs.org/tinyexec/-/tinyexec-0.3.2.tgz",
@@ -5611,7 +5967,6 @@
       "version": "0.2.16",
       "resolved": "https://registry.npmjs.org/tinyglobby/-/tinyglobby-0.2.16.tgz",
       "integrity": "sha512-pn99VhoACYR8nFHhxqix+uvsbXineAasWm5ojXoN8xEwK5Kd3/TrhNn1wByuD52UxWRLy8pu+kRMniEi6Eq9Zg==",
-      "peer": true,
       "dependencies": {
         "fdir": "^6.5.0",
         "picomatch": "^4.0.4"
@@ -5623,6 +5978,16 @@
         "url": "https://github.com/sponsors/SuperchupuDev"
       }
     },
+    "node_modules/tinyrainbow": {
+      "version": "3.1.0",
+      "resolved": "https://registry.npmjs.org/tinyrainbow/-/tinyrainbow-3.1.0.tgz",
+      "integrity": "sha512-Bf+ILmBgretUrdJxzXM0SgXLZ3XfiaUuOj/IKQHuTXip+05Xn+uyEYdVg0kYDipTBcLrCVyUzAPz7QmArb0mmw==",
+      "dev": true,
+      "license": "MIT",
+      "engines": {
+        "node": ">=14.0.0"
+      }
+    },
     "node_modules/to-regex-range": {
       "version": "5.0.1",
       "resolved": "https://registry.npmjs.org/to-regex-range/-/to-regex-range-5.0.1.tgz",
@@ -5911,7 +6276,6 @@
       "version": "8.0.11",
       "resolved": "https://registry.npmjs.org/vite/-/vite-8.0.11.tgz",
       "integrity": "sha512-Jz1mxtUBR5xTT65VOdJZUUeoyLtqljmFkiUXhPTLZka3RDc9vpi/xXkyrnsdRcm2lIi3l3GPMnAidTsEGIj3Ow==",
-      "peer": true,
       "dependencies": {
         "lightningcss": "^1.32.0",
         "picomatch": "^4.0.4",
@@ -5997,6 +6361,113 @@
         }
       }
     },
+    "node_modules/vitest": {
+      "version": "4.1.5",
+      "resolved": "https://registry.npmjs.org/vitest/-/vitest-4.1.5.tgz",
+      "integrity": "sha512-9Xx1v3/ih3m9hN+SbfkUyy0JAs72ap3r7joc87XL6jwF0jGg6mFBvQ1SrwaX+h8BlkX6Hz9shdd1uo6AF+ZGpg==",
+      "dev": true,
+      "license": "MIT",
+      "dependencies": {
+        "@vitest/expect": "4.1.5",
+        "@vitest/mocker": "4.1.5",
+        "@vitest/pretty-format": "4.1.5",
+        "@vitest/runner": "4.1.5",
+        "@vitest/snapshot": "4.1.5",
+        "@vitest/spy": "4.1.5",
+        "@vitest/utils": "4.1.5",
+        "es-module-lexer": "^2.0.0",
+        "expect-type": "^1.3.0",
+        "magic-string": "^0.30.21",
+        "obug": "^2.1.1",
+        "pathe": "^2.0.3",
+        "picomatch": "^4.0.3",
+        "std-env": "^4.0.0-rc.1",
+        "tinybench": "^2.9.0",
+        "tinyexec": "^1.0.2",
+        "tinyglobby": "^0.2.15",
+        "tinyrainbow": "^3.1.0",
+        "vite": "^6.0.0 || ^7.0.0 || ^8.0.0",
+        "why-is-node-running": "^2.3.0"
+      },
+      "bin": {
+        "vitest": "vitest.mjs"
+      },
+      "engines": {
+        "node": "^20.0.0 || ^22.0.0 || >=24.0.0"
+      },
+      "funding": {
+        "url": "https://opencollective.com/vitest"
+      },
+      "peerDependencies": {
+        "@edge-runtime/vm": "*",
+        "@opentelemetry/api": "^1.9.0",
+        "@types/node": "^20.0.0 || ^22.0.0 || >=24.0.0",
+        "@vitest/browser-playwright": "4.1.5",
+        "@vitest/browser-preview": "4.1.5",
+        "@vitest/browser-webdriverio": "4.1.5",
+        "@vitest/coverage-istanbul": "4.1.5",
+        "@vitest/coverage-v8": "4.1.5",
+        "@vitest/ui": "4.1.5",
+        "happy-dom": "*",
+        "jsdom": "*",
+        "vite": "^6.0.0 || ^7.0.0 || ^8.0.0"
+      },
+      "peerDependenciesMeta": {
+        "@edge-runtime/vm": {
+          "optional": true
+        },
+        "@opentelemetry/api": {
+          "optional": true
+        },
+        "@types/node": {
+          "optional": true
+        },
+        "@vitest/browser-playwright": {
+          "optional": true
+        },
+        "@vitest/browser-preview": {
+          "optional": true
+        },
+        "@vitest/browser-webdriverio": {
+          "optional": true
+        },
+        "@vitest/coverage-istanbul": {
+          "optional": true
+        },
+        "@vitest/coverage-v8": {
+          "optional": true
+        },
+        "@vitest/ui": {
+          "optional": true
+        },
+        "happy-dom": {
+          "optional": true
+        },
+        "jsdom": {
+          "optional": true
+        },
+        "vite": {
+          "optional": false
+        }
+      }
+    },
+    "node_modules/vitest/node_modules/es-module-lexer": {
+      "version": "2.1.0",
+      "resolved": "https://registry.npmjs.org/es-module-lexer/-/es-module-lexer-2.1.0.tgz",
+      "integrity": "sha512-n27zTYMjYu1aj4MjCWzSP7G9r75utsaoc8m61weK+W8JMBGGQybd43GstCXZ3WNmSFtGT9wi59qQTW6mhTR5LQ==",
+      "dev": true,
+      "license": "MIT"
+    },
+    "node_modules/vitest/node_modules/tinyexec": {
+      "version": "1.1.2",
+      "resolved": "https://registry.npmjs.org/tinyexec/-/tinyexec-1.1.2.tgz",
+      "integrity": "sha512-dAqSqE/RabpBKI8+h26GfLq6Vb3JVXs30XYQjdMjaj/c2tS8IYYMbIzP599KtRj7c57/wYApb3QjgRgXmrCukA==",
+      "dev": true,
+      "license": "MIT",
+      "engines": {
+        "node": ">=18"
+      }
+    },
     "node_modules/web-namespaces": {
       "version": "2.0.1",
       "resolved": "https://registry.npmjs.org/web-namespaces/-/web-namespaces-2.0.1.tgz",
@@ -6025,6 +6496,23 @@
         "node": ">=4"
       }
     },
+    "node_modules/why-is-node-running": {
+      "version": "2.3.0",
+      "resolved": "https://registry.npmjs.org/why-is-node-running/-/why-is-node-running-2.3.0.tgz",
+      "integrity": "sha512-hUrmaWBdVDcxvYqnyh09zunKzROWjbZTiNy8dBEjkS7ehEDQibXJ7XvlmtbwuTclUiIyN+CyXQD4Vmko8fNm8w==",
+      "dev": true,
+      "license": "MIT",
+      "dependencies": {
+        "siginfo": "^2.0.0",
+        "stackback": "0.0.2"
+      },
+      "bin": {
+        "why-is-node-running": "cli.js"
+      },
+      "engines": {
+        "node": ">=8"
+      }
+    },
     "node_modules/widest-line": {
       "version": "5.0.0",
       "resolved": "https://registry.npmjs.org/widest-line/-/widest-line-5.0.0.tgz",
diff --git a/package.json b/package.json
index 8414a65..c1293fb 100644
--- a/package.json
+++ b/package.json
@@ -5,12 +5,19 @@
   "scripts": {
     "dev": "astro dev",
     "build": "astro build",
-    "preview": "astro preview"
+    "preview": "astro preview",
+    "test": "vitest run",
+    "test:watch": "vitest",
+    "test:coverage": "vitest run --coverage"
   },
   "dependencies": {
     "@astrojs/sitemap": "^3.2.1",
     "@tailwindcss/vite": "^4.0.0",
     "astro": "^4.16.0",
     "tailwindcss": "^4.0.0"
+  },
+  "devDependencies": {
+    "@vitest/coverage-v8": "^4.1.5",
+    "vitest": "^4.1.5"
   }
 }
diff --git a/src/lib/dice-engine.test.ts b/src/lib/dice-engine.test.ts
new file mode 100644
index 0000000..d78a886
--- /dev/null
+++ b/src/lib/dice-engine.test.ts
@@ -0,0 +1,286 @@
+import { describe, it, expect, vi, beforeEach, afterEach } from "vitest";
+import {
+  parseDiceNotation,
+  rollDice,
+  rollAdvantage,
+  buildNotation,
+  isAdvancedNotation,
+} from "./dice-engine";
+
+describe("parseDiceNotation", () => {
+  it('parses basic notation "d6"', () => {
+    const parsed = parseDiceNotation("d6");
+    expect(parsed).not.toBeNull();
+    expect(parsed!.count).toBe(1);
+    expect(parsed!.sides).toBe(6);
+    expect(parsed!.modifier).toBe(0);
+    expect(parsed!.keepDrop).toBeNull();
+    expect(parsed!.explode.active).toBe(false);
+    expect(parsed!.reroll.active).toBe(false);
+  });
+
+  it('parses "2d20+3"', () => {
+    const parsed = parseDiceNotation("2d20+3");
+    expect(parsed).not.toBeNull();
+    expect(parsed!.count).toBe(2);
+    expect(parsed!.sides).toBe(20);
+    expect(parsed!.modifier).toBe(3);
+  });
+
+  it('parses "3d8-1"', () => {
+    const parsed = parseDiceNotation("3d8-1");
+    expect(parsed!.count).toBe(3);
+    expect(parsed!.sides).toBe(8);
+    expect(parsed!.modifier).toBe(-1);
+  });
+
+  it('parses "4d6kh3"', () => {
+    const parsed = parseDiceNotation("4d6kh3");
+    expect(parsed!.keepDrop).toEqual({ type: "kh", count: 3 });
+  });
+
+  it('parses "5d6dl2"', () => {
+    const parsed = parseDiceNotation("5d6dl2");
+    expect(parsed!.keepDrop).toEqual({ type: "dl", count: 2 });
+  });
+
+  it('parses exploding "3d6!"', () => {
+    const parsed = parseDiceNotation("3d6!");
+    expect(parsed!.explode.active).toBe(true);
+    expect(parsed!.explode.penetrating).toBe(false);
+    expect(parsed!.explode.threshold).toBe(6);
+  });
+
+  it('parses penetrating explode "2d10!p"', () => {
+    const parsed = parseDiceNotation("2d10!p");
+    expect(parsed!.explode.active).toBe(true);
+    expect(parsed!.explode.penetrating).toBe(true);
+  });
+
+  it('parses explode with custom threshold "2d6!>4"', () => {
+    const parsed = parseDiceNotation("2d6!>4");
+    expect(parsed!.explode.active).toBe(true);
+    expect(parsed!.explode.threshold).toBe(4);
+  });
+
+  it('parses reroll "2d6r1"', () => {
+    const parsed = parseDiceNotation("2d6r1");
+    expect(parsed!.reroll.active).toBe(true);
+    expect(parsed!.reroll.once).toBe(false);
+    expect(parsed!.reroll.values).toContain(1);
+  });
+
+  it('parses reroll once "2d6ro1"', () => {
+    const parsed = parseDiceNotation("2d6ro1");
+    expect(parsed!.reroll.active).toBe(true);
+    expect(parsed!.reroll.once).toBe(true);
+  });
+
+  it('parses reroll less-than "2d6r<3"', () => {
+    const parsed = parseDiceNotation("2d6r<3");
+    expect(parsed!.reroll.active).toBe(true);
+    expect(parsed!.reroll.values).toContain(1);
+    expect(parsed!.reroll.values).toContain(2);
+    expect(parsed!.reroll.values).not.toContain(3);
+  });
+
+  it("ignores whitespace", () => {
+    const parsed = parseDiceNotation("  2 d 20 + 5  ");
+    expect(parsed!.count).toBe(2);
+    expect(parsed!.sides).toBe(20);
+    expect(parsed!.modifier).toBe(5);
+  });
+
+  it("returns null for invalid notation", () => {
+    expect(parseDiceNotation("")).toBeNull();
+    expect(parseDiceNotation("abc")).toBeNull();
+    expect(parseDiceNotation("2d6+1+2")).toBeNull();
+    expect(parseDiceNotation("0d6")).toBeNull();
+    expect(parseDiceNotation("21d6")).toBeNull();
+    expect(parseDiceNotation("2d0")).toBeNull();
+    expect(parseDiceNotation("2d6kh0")).toBeNull();
+    expect(parseDiceNotation("2d6kh2")).toBeNull();
+  });
+});
+
+describe("rollDice", () => {
+  let mathRandomSpy: ReturnType<typeof vi.spyOn>;
+
+  afterEach(() => {
+    mathRandomSpy?.mockRestore();
+  });
+
+  it("rolls basic dice", () => {
+    mathRandomSpy = vi.spyOn(Math, "random").mockReturnValue(0.5);
+    const parsed = parseDiceNotation("3d6")!;
+    const result = rollDice(parsed);
+
+    expect(result.dice).toHaveLength(3);
+    expect(result.kept).toHaveLength(3);
+    expect(result.dropped).toHaveLength(0);
+    expect(result.total).toBe(12); // each die: floor(0.5*6)+1 = 4, 3*4 = 12
+  });
+
+  it("applies modifier", () => {
+    mathRandomSpy = vi.spyOn(Math, "random").mockReturnValue(0.5);
+    const parsed = parseDiceNotation("1d20+5")!;
+    const result = rollDice(parsed);
+
+    expect(result.total).toBe(16); // 11 + 5
+  });
+
+  it("keeps highest rolls (kh)", () => {
+    mathRandomSpy = vi
+      .spyOn(Math, "random")
+      .mockReturnValueOnce(0.1) // 1
+      .mockReturnValueOnce(0.9) // 6
+      .mockReturnValueOnce(0.5); // 4
+
+    const parsed = parseDiceNotation("3d6kh2")!;
+    const result = rollDice(parsed);
+
+    expect(result.kept).toHaveLength(2);
+    expect(result.dropped).toHaveLength(1);
+    expect(result.dropped[0].dropped).toBe(true);
+    expect(result.total).toBe(10); // 6 + 4
+  });
+
+  it("drops highest rolls (dh)", () => {
+    mathRandomSpy = vi
+      .spyOn(Math, "random")
+      .mockReturnValueOnce(0.1) // 1
+      .mockReturnValueOnce(0.9) // 6
+      .mockReturnValueOnce(0.5); // 4
+
+    const parsed = parseDiceNotation("3d6dh1")!;
+    const result = rollDice(parsed);
+
+    expect(result.kept).toHaveLength(2);
+    expect(result.dropped).toHaveLength(1);
+    expect(result.dropped[0].value).toBe(6);
+    expect(result.total).toBe(5); // 1 + 4
+  });
+
+  it("handles exploding dice", () => {
+    mathRandomSpy = vi
+      .spyOn(Math, "random")
+      .mockReturnValueOnce(0.99) // first roll: 6 (max, triggers explode)
+      .mockReturnValueOnce(0.99) // explosion 1: 6 (triggers again)
+      .mockReturnValueOnce(0.1); // explosion 2: 1 (stops)
+
+    const parsed = parseDiceNotation("1d6!")!;
+    const result = rollDice(parsed);
+
+    expect(result.dice[0].exploded).toBe(true);
+    expect(result.dice[0].explosions).toEqual([6, 1]);
+    expect(result.total).toBe(13); // 6 + 6 + 1
+  });
+
+  it("handles penetrating explode", () => {
+    mathRandomSpy = vi
+      .spyOn(Math, "random")
+      .mockReturnValueOnce(0.99) // first roll: 6 (explode)
+      .mockReturnValueOnce(0.99); // explosion: 6 -> penetrating -> 5, then 5 < 6 stops
+
+    const parsed = parseDiceNotation("1d6!p")!;
+    const result = rollDice(parsed);
+
+    expect(result.dice[0].explosions).toEqual([5]);
+    expect(result.total).toBe(11); // 6 + 5
+  });
+
+  it("handles reroll", () => {
+    mathRandomSpy = vi
+      .spyOn(Math, "random")
+      .mockReturnValueOnce(0.0) // first roll: 1 (reroll)
+      .mockReturnValueOnce(0.5); // reroll: 4
+
+    const parsed = parseDiceNotation("1d6r1")!;
+    const result = rollDice(parsed);
+
+    expect(result.dice[0].rerolledFrom).toBe(1);
+    expect(result.dice[0].value).toBe(4);
+  });
+
+  it("handles reroll once (ro)", () => {
+    mathRandomSpy = vi
+      .spyOn(Math, "random")
+      .mockReturnValueOnce(0.0) // first: 1 (reroll once)
+      .mockReturnValueOnce(0.0); // second: 1 (stays, no more rerolls)
+
+    const parsed = parseDiceNotation("1d6ro1")!;
+    const result = rollDice(parsed);
+
+    expect(result.dice[0].rerolledFrom).toBe(1);
+    expect(result.dice[0].value).toBe(1);
+  });
+});
+
+describe("rollAdvantage", () => {
+  let mathRandomSpy: ReturnType<typeof vi.spyOn>;
+
+  afterEach(() => {
+    mathRandomSpy?.mockRestore();
+  });
+
+  it("returns highest for advantage", () => {
+    mathRandomSpy = vi
+      .spyOn(Math, "random")
+      .mockReturnValueOnce(0.1) // 3
+      .mockReturnValueOnce(0.9); // 19
+
+    const result = rollAdvantage(20, 0, true);
+    expect(result.total).toBe(19);
+    expect(result.advantageRolls).toEqual([3, 19]);
+  });
+
+  it("returns lowest for disadvantage", () => {
+    mathRandomSpy = vi
+      .spyOn(Math, "random")
+      .mockReturnValueOnce(0.9) // 19
+      .mockReturnValueOnce(0.1); // 3
+
+    const result = rollAdvantage(20, 2, false);
+    expect(result.total).toBe(5); // 3 + 2
+    expect(result.advantageRolls).toEqual([19, 3]);
+  });
+});
+
+describe("buildNotation", () => {
+  it("builds basic notation", () => {
+    expect(buildNotation(1, 6, 0)).toBe("1d6");
+    expect(buildNotation(2, 20, 0)).toBe("2d20");
+  });
+
+  it("adds positive modifier", () => {
+    expect(buildNotation(2, 6, 3)).toBe("2d6+3");
+  });
+
+  it("adds negative modifier", () => {
+    expect(buildNotation(2, 6, -2)).toBe("2d6-2");
+  });
+});
+
+describe("isAdvancedNotation", () => {
+  it("returns false for basic notation", () => {
+    expect(isAdvancedNotation("2d6")).toBe(false);
+    expect(isAdvancedNotation("1d20+5")).toBe(false);
+    expect(isAdvancedNotation("3d8-1")).toBe(false);
+  });
+
+  it("returns true for keep/drop", () => {
+    expect(isAdvancedNotation("4d6kh3")).toBe(true);
+    expect(isAdvancedNotation("5d6dl2")).toBe(true);
+  });
+
+  it("returns true for explode", () => {
+    expect(isAdvancedNotation("3d6!")).toBe(true);
+    expect(isAdvancedNotation("2d10!p")).toBe(true);
+  });
+
+  it("returns true for reroll", () => {
+    expect(isAdvancedNotation("2d6r1")).toBe(true);
+    expect(isAdvancedNotation("2d6ro1")).toBe(true);
+    expect(isAdvancedNotation("2d6r<3")).toBe(true);
+  });
+});
diff --git a/src/lib/dice-engine.ts b/src/lib/dice-engine.ts
index ade5f30..72d4076 100644
--- a/src/lib/dice-engine.ts
+++ b/src/lib/dice-engine.ts
@@ -2,7 +2,7 @@
 // Supports: XdY±Z, kh/kl/dh/dl, ! exploding, r reroll, advantage/disadvantage
 // Caps: explode chain ≤100, reroll recursion ≤1000
 
-type KeepDrop = { type: 'kh' | 'kl' | 'dh' | 'dl'; count: number } | null;
+type KeepDrop = { type: "kh" | "kl" | "dh" | "dl"; count: number } | null;
 
 type Explode = {
   active: boolean;
@@ -14,7 +14,7 @@ type Reroll = {
   active: boolean;
   values: Set<number>;
   once: boolean; // "ro" = once only; "r" = recursive
-  operator: 'eq' | 'lt' | 'gt';
+  operator: "eq" | "lt" | "gt";
 };
 
 export interface Parsed {
@@ -32,7 +32,7 @@ export interface DieResult {
   value: number;
   original: number;
   exploded: boolean;
-  explosions: number[];   // chained explosion rolls
+  explosions: number[]; // chained explosion rolls
   rerolledFrom: number | null;
   dropped: boolean;
 }
@@ -52,7 +52,7 @@ const REROLL_CAP = 1000;
 
 /** Remove all whitespace and lowercase for parsing */
 function normalize(raw: string): string {
-  return raw.replace(/\s+/g, '').toLowerCase();
+  return raw.replace(/\s+/g, "").toLowerCase();
 }
 
 /**
@@ -67,7 +67,7 @@ export function parseDiceNotation(raw: string): Parsed | null {
   const main = s.match(/^(\d*)d(\d+)(.*)$/);
   if (!main) return null;
 
-  const count = main[1] === '' ? 1 : parseInt(main[1], 10);
+  const count = main[1] === "" ? 1 : parseInt(main[1], 10);
   const sides = parseInt(main[2], 10);
   const rest = main[3];
 
@@ -81,7 +81,10 @@ export function parseDiceNotation(raw: string): Parsed | null {
   if (kdMatch) {
     const countKD = parseInt(kdMatch[2], 10);
     if (countKD < 1 || countKD >= count) return null;
-    keepDrop = { type: kdMatch[1] as 'kh' | 'kl' | 'dh' | 'dl', count: countKD };
+    keepDrop = {
+      type: kdMatch[1] as "kh" | "kl" | "dh" | "dl",
+      count: countKD,
+    };
     remaining = remaining.slice(kdMatch[0].length);
   }
 
@@ -90,8 +93,8 @@ export function parseDiceNotation(raw: string): Parsed | null {
   const expMatch = remaining.match(/^(!p|!)(>?)(\d*)/);
   if (expMatch) {
     explode.active = true;
-    explode.penetrating = expMatch[1] === '!p';
-    if (expMatch[2] === '>' && expMatch[3]) {
+    explode.penetrating = expMatch[1] === "!p";
+    if (expMatch[2] === ">" && expMatch[3]) {
       explode.threshold = parseInt(expMatch[3], 10);
       if (explode.threshold < 2 || explode.threshold > sides) return null;
     } else {
@@ -101,21 +104,26 @@ export function parseDiceNotation(raw: string): Parsed | null {
   }
 
   // Reroll: rN, roN, r<N, ro<N
-  let reroll: Reroll = { active: false, values: new Set(), once: false, operator: 'eq' };
+  let reroll: Reroll = {
+    active: false,
+    values: new Set(),
+    once: false,
+    operator: "eq",
+  };
   const rrMatch = remaining.match(/^(ro|r)([<>]?)(\d+)/);
   if (rrMatch) {
     reroll.active = true;
-    reroll.once = rrMatch[1] === 'ro';
-    const op = rrMatch[2] as '' | '<' | '>';
+    reroll.once = rrMatch[1] === "ro";
+    const op = rrMatch[2] as "" | "<" | ">";
     const val = parseInt(rrMatch[3], 10);
-    if (op === '<') {
-      reroll.operator = 'lt';
+    if (op === "<") {
+      reroll.operator = "lt";
       for (let i = 1; i < val && i < sides; i++) reroll.values.add(i);
-    } else if (op === '>') {
-      reroll.operator = 'gt';
+    } else if (op === ">") {
+      reroll.operator = "gt";
       for (let i = val + 1; i <= sides; i++) reroll.values.add(i);
     } else {
-      reroll.operator = 'eq';
+      reroll.operator = "eq";
       reroll.values.add(val);
     }
     remaining = remaining.slice(rrMatch[0].length);
@@ -133,7 +141,16 @@ export function parseDiceNotation(raw: string): Parsed | null {
   // If anything remains unparsed, it's invalid
   if (remaining.length > 0) return null;
 
-  return { count, sides, modifier, keepDrop, explode, reroll, advantage: false, disadvantage: false };
+  return {
+    count,
+    sides,
+    modifier,
+    keepDrop,
+    explode,
+    reroll,
+    advantage: false,
+    disadvantage: false,
+  };
 }
 
 /** Roll a single die with optional reroll and explode logic */
@@ -194,34 +211,47 @@ export function rollDice(parsed: Parsed): RollResult {
 
   if (parsed.keepDrop) {
     const sorted = [...dice].map((d, i) => ({ die: d, idx: i }));
-    if (parsed.keepDrop.type === 'kh') {
+    if (parsed.keepDrop.type === "kh") {
       sorted.sort((a, b) => b.die.value - a.die.value);
-      const keepIndices = new Set(sorted.slice(0, parsed.keepDrop.count).map(x => x.idx));
+      const keepIndices = new Set(
+        sorted.slice(0, parsed.keepDrop.count).map((x) => x.idx),
+      );
       kept = dice.filter((_, i) => keepIndices.has(i));
       dropped = dice.filter((_, i) => !keepIndices.has(i));
-    } else if (parsed.keepDrop.type === 'kl') {
+    } else if (parsed.keepDrop.type === "kl") {
       sorted.sort((a, b) => a.die.value - b.die.value);
-      const keepIndices = new Set(sorted.slice(0, parsed.keepDrop.count).map(x => x.idx));
+      const keepIndices = new Set(
+        sorted.slice(0, parsed.keepDrop.count).map((x) => x.idx),
+      );
       kept = dice.filter((_, i) => keepIndices.has(i));
       dropped = dice.filter((_, i) => !keepIndices.has(i));
-    } else if (parsed.keepDrop.type === 'dh') {
+    } else if (parsed.keepDrop.type === "dh") {
       sorted.sort((a, b) => b.die.value - a.die.value);
-      const dropIndices = new Set(sorted.slice(0, parsed.keepDrop.count).map(x => x.idx));
+      const dropIndices = new Set(
+        sorted.slice(0, parsed.keepDrop.count).map((x) => x.idx),
+      );
       kept = dice.filter((_, i) => !dropIndices.has(i));
       dropped = dice.filter((_, i) => dropIndices.has(i));
-    } else if (parsed.keepDrop.type === 'dl') {
+    } else if (parsed.keepDrop.type === "dl") {
       sorted.sort((a, b) => a.die.value - b.die.value);
-      const dropIndices = new Set(sorted.slice(0, parsed.keepDrop.count).map(x => x.idx));
+      const dropIndices = new Set(
+        sorted.slice(0, parsed.keepDrop.count).map((x) => x.idx),
+      );
       kept = dice.filter((_, i) => !dropIndices.has(i));
       dropped = dice.filter((_, i) => dropIndices.has(i));
     }
   }
 
-  dropped.forEach(d => { d.dropped = true; });
+  dropped.forEach((d) => {
+    d.dropped = true;
+  });
 
   // Calculate total
   let total = kept.reduce((sum, d) => sum + d.value, 0);
-  total += kept.reduce((sum, d) => sum + d.explosions.reduce((s, v) => s + v, 0), 0);
+  total += kept.reduce(
+    (sum, d) => sum + d.explosions.reduce((s, v) => s + v, 0),
+    0,
+  );
   total += parsed.modifier;
 
   return {
@@ -230,30 +260,56 @@ export function rollDice(parsed: Parsed): RollResult {
     dropped,
     modifier: parsed.modifier,
     total,
-    notation: '', // filled by caller
+    notation: "", // filled by caller
     advantageRolls: null,
   };
 }
 
 /** Roll with advantage or disadvantage (2dX keep highest/lowest) */
-export function rollAdvantage(sides: number, modifier: number, advantage: boolean): RollResult {
+export function rollAdvantage(
+  sides: number,
+  modifier: number,
+  advantage: boolean,
+): RollResult {
   const r1 = Math.floor(Math.random() * sides) + 1;
   const r2 = Math.floor(Math.random() * sides) + 1;
   const keptVal = advantage ? Math.max(r1, r2) : Math.min(r1, r2);
 
   return {
-    dice: [{ value: keptVal, original: keptVal, exploded: false, explosions: [], rerolledFrom: null, dropped: false }],
-    kept: [{ value: keptVal, original: keptVal, exploded: false, explosions: [], rerolledFrom: null, dropped: false }],
+    dice: [
+      {
+        value: keptVal,
+        original: keptVal,
+        exploded: false,
+        explosions: [],
+        rerolledFrom: null,
+        dropped: false,
+      },
+    ],
+    kept: [
+      {
+        value: keptVal,
+        original: keptVal,
+        exploded: false,
+        explosions: [],
+        rerolledFrom: null,
+        dropped: false,
+      },
+    ],
     dropped: [],
     modifier,
     total: keptVal + modifier,
-    notation: '',
+    notation: "",
     advantageRolls: [r1, r2],
   };
 }
 
 /** Build notation string from parsed object (basic only, for simple sync) */
-export function buildNotation(count: number, sides: number, modifier: number): string {
+export function buildNotation(
+  count: number,
+  sides: number,
+  modifier: number,
+): string {
   let s = `${count}d${sides}`;
   if (modifier > 0) s += `+${modifier}`;
   else if (modifier < 0) s += modifier;
@@ -263,5 +319,5 @@ export function buildNotation(count: number, sides: number, modifier: number): s
 /** Check if notation contains advanced features beyond basic XdY±Z */
 export function isAdvancedNotation(raw: string): boolean {
   const s = normalize(raw);
-  return /(kh|kl|dh|dl|!|[<>]?r\d)/.test(s);
+  return /(kh|kl|dh|dl|!|ro?[<>]?\d)/.test(s);
 }
diff --git a/vitest.config.ts b/vitest.config.ts
new file mode 100644
index 0000000..7dd1325
--- /dev/null
+++ b/vitest.config.ts
@@ -0,0 +1,9 @@
+import { defineConfig } from 'vitest/config';
+
+export default defineConfig({
+  test: {
+    globals: true,
+    environment: 'node',
+    include: ['src/**/*.test.ts'],
+  },
+});