diff --git a/.github/CONTRIBUTING.md b/.github/CONTRIBUTING.md
index ec6b88a..2b6442d 100644
--- a/.github/CONTRIBUTING.md
+++ b/.github/CONTRIBUTING.md
@@ -24,11 +24,33 @@ Pre-commit hooks via Husky will run Biome automatically on staged files.
 
 For a better experience, install the [Biome VS Code extension](https://marketplace.visualstudio.com/items?itemName=biomejs.biome) for real-time linting and format-on-save.
 
+## Testing
+
+Run tests before submitting PRs:
+
+```bash
+npm run test        # Unit tests (Vitest)
+npm run test:e2e    # E2E tests (Playwright)
+```
+
+E2E tests use mocked API responses - no AI provider needed. Tests are in `tests/e2e/`.
+
+To run a specific test file:
+```bash
+npx playwright test tests/e2e/diagram-generation.spec.ts
+```
+
+To run tests with UI mode:
+```bash
+npx playwright test --ui
+```
+
 ## Pull Requests
 
 1. Create a feature branch
 2. Make changes and ensure `npm run check` passes
-3. Submit PR against `main` with a clear description
+3. Run tests with `npm run test:e2e`
+4. Submit PR against `main` with a clear description
 
 ## Issues