diff --git a/packages/shared/package.json b/packages/shared/package.json
index a15ef6d..b3b3ac7 100644
--- a/packages/shared/package.json
+++ b/packages/shared/package.json
@@ -7,9 +7,11 @@
   "types": "./src/index.ts",
   "scripts": {
     "lint": "eslint src/",
-    "typecheck": "tsc --noEmit"
+    "typecheck": "tsc --noEmit",
+    "test": "vitest run"
   },
   "devDependencies": {
-    "typescript": "^5.4.0"
+    "typescript": "^5.4.0",
+    "vitest": "^2.0.0"
   }
 }
diff --git a/packages/shared/src/platforms.test.ts b/packages/shared/src/platforms.test.ts
new file mode 100644
index 0000000..d1ff86b
--- /dev/null
+++ b/packages/shared/src/platforms.test.ts
@@ -0,0 +1,76 @@
+import { describe, it, expect } from 'vitest';
+import {
+  PLATFORMS,
+  getAllPlatforms,
+  getPlatform,
+  getProfileUrl,
+  getWebViewUrl,
+  getDeepLinkUrl,
+} from './platforms';
+
+// ─── StackOverflow Platform Tests ───
+
+describe('stackoverflow platform', () => {
+  it('should exist in PLATFORMS registry', () => {
+    expect(PLATFORMS.stackoverflow).toBeDefined();
+  });
+
+  it('should have correct platform definition', () => {
+    const so = PLATFORMS.stackoverflow;
+    expect(so.id).toBe('stackoverflow');
+    expect(so.name).toBe('Stack Overflow');
+    expect(so.icon).toBe('stackoverflow');
+    expect(so.color).toBe('#F58025');
+    expect(so.urlPattern).toBe('https://stackoverflow.com/users/{username}');
+    expect(so.followStrategy).toBe('link');
+    expect(so.deepLinkPattern).toBeNull();
+    expect(so.webViewUrlPattern).toBeNull();
+    expect(so.oauthScopes).toEqual([]);
+    expect(so.usesFullUrl).toBe(false);
+    expect(so.usernamePlaceholder).toBeTruthy();
+  });
+
+  it('should be included in getAllPlatforms()', () => {
+    const all = getAllPlatforms();
+    const so = all.find((p) => p.id === 'stackoverflow');
+    expect(so).toBeDefined();
+    expect(so!.name).toBe('Stack Overflow');
+  });
+
+  it('should be retrievable via getPlatform()', () => {
+    const so = getPlatform('stackoverflow');
+    expect(so).toBeDefined();
+    expect(so!.id).toBe('stackoverflow');
+  });
+});
+
+// ─── getProfileUrl Tests for StackOverflow ───
+
+describe('getProfileUrl – stackoverflow', () => {
+  it('should generate correct URL with user ID and display name', () => {
+    const url = getProfileUrl('stackoverflow', '1234/user');
+    expect(url).toBe('https://stackoverflow.com/users/1234/user');
+  });
+
+  it('should generate correct URL with numeric ID only', () => {
+    const url = getProfileUrl('stackoverflow', '56789');
+    expect(url).toBe('https://stackoverflow.com/users/56789');
+  });
+
+  it('should return empty string for unknown platform', () => {
+    const url = getProfileUrl('nonexistent', '1234');
+    expect(url).toBe('');
+  });
+});
+
+// ─── getWebViewUrl / getDeepLinkUrl for StackOverflow ───
+
+describe('getWebViewUrl / getDeepLinkUrl – stackoverflow', () => {
+  it('should return null for webViewUrl (not supported)', () => {
+    expect(getWebViewUrl('stackoverflow', '1234/user')).toBeNull();
+  });
+
+  it('should return null for deepLinkUrl (not supported)', () => {
+    expect(getDeepLinkUrl('stackoverflow', '1234/user')).toBeNull();
+  });
+});
diff --git a/packages/shared/src/platforms.ts b/packages/shared/src/platforms.ts
index 52b07aa..a218957 100644
--- a/packages/shared/src/platforms.ts
+++ b/packages/shared/src/platforms.ts
@@ -175,6 +175,19 @@ export const PLATFORMS: Record<string, PlatformDef> = {
     usernamePlaceholder: 'e.g. hacker',
     usesFullUrl: false,
   },
+  stackoverflow: {
+    id: 'stackoverflow',
+    name: 'Stack Overflow',
+    icon: 'stackoverflow',
+    color: '#F58025',
+    urlPattern: 'https://stackoverflow.com/users/{username}',
+    deepLinkPattern: null,
+    webViewUrlPattern: null,
+    followStrategy: 'link',
+    oauthScopes: [],
+    usernamePlaceholder: 'e.g. 1234/username',
+    usesFullUrl: false,
+  },
   discord: {
     id: 'discord',
     name: 'Discord',
diff --git a/pnpm-lock.yaml b/pnpm-lock.yaml
index 2637abe..f7b0958 100644
--- a/pnpm-lock.yaml
+++ b/pnpm-lock.yaml
@@ -221,6 +221,9 @@ importers:
       typescript:
         specifier: ^5.4.0
         version: 5.9.3
+      vitest:
+        specifier: ^2.0.0
+        version: 2.1.9(@types/node@22.19.15)(terser@5.46.0)
 
 packages: