diff --git a/tests/core.test.ts b/tests/core.test.ts
new file mode 100644
index 0000000..ce1e474
--- /dev/null
+++ b/tests/core.test.ts
@@ -0,0 +1,70 @@
+import * as path from 'path';
+import { parseEnvFile } from '../src/core/parser';
+import { validateEnv, parseSchemaFromEnv } from '../src/core/validator';
+
+describe('Parser', () => {
+  const fixturesDir = path.join(__dirname, 'fixtures');
+
+  test('parses .env file correctly', () => {
+    const envPath = path.join(fixturesDir, '.env');
+    const result = parseEnvFile(envPath);
+
+    expect(result.flat).toBeDefined();
+    expect(result.flat.PORT).toBe('3000');
+    expect(result.flat.NODE_ENV).toBe('development');
+    expect(result.flat.DEBUG).toBe('true');
+  });
+
+  test('creates nested structure for underscore-prefixed variables', () => {
+    const envPath = path.join(fixturesDir, '.env');
+    const result = parseEnvFile(envPath);
+
+    expect(result.nested.DB).toBeDefined();
+    expect((result.nested.DB as Record<string, unknown>).HOST).toBe('localhost');
+    expect((result.nested.DB as Record<string, unknown>).PORT).toBe('5432');
+    expect((result.nested.DB as Record<string, unknown>).USER).toBe('admin');
+  });
+
+  test('throws error for non-existent file', () => {
+    expect(() => parseEnvFile('/non/existent/file.env')).toThrow('not found');
+  });
+});
+
+describe('Validator', () => {
+  const fixturesDir = path.join(__dirname, 'fixtures');
+
+  test('validates correct .env against schema', () => {
+    const envPath = path.join(fixturesDir, '.env');
+    const parsed = parseEnvFile(envPath);
+    const schemaPath = path.join(fixturesDir, 'schema.json');
+    const schema = JSON.parse(require('fs').readFileSync(schemaPath, 'utf-8'));
+
+    const result = validateEnv(parsed.flat, schema);
+
+    expect(result.valid).toBe(true);
+    expect(result.errors).toHaveLength(0);
+  });
+
+  test('fails validation for invalid values', () => {
+    const envPath = path.join(fixturesDir, 'invalid.env');
+    const parsed = parseEnvFile(envPath);
+    const schemaPath = path.join(fixturesDir, 'schema.json');
+    const schema = JSON.parse(require('fs').readFileSync(schemaPath, 'utf-8'));
+
+    const result = validateEnv(parsed.flat, schema);
+
+    expect(result.valid).toBe(false);
+    expect(result.errors.length).toBeGreaterThan(0);
+  });
+
+  test('parses schema from .env file', () => {
+    const envPath = path.join(fixturesDir, '.env');
+    const parsed = parseEnvFile(envPath);
+    const schema = parseSchemaFromEnv(parsed.flat);
+
+    expect(schema.variables).toBeDefined();
+    expect(schema.variables.MAX_RETRIES.type).toBe('int');
+    expect(schema.variables.NODE_ENV.type).toBe('string');
+    expect(schema.variables.DEBUG.type).toBe('boolean');
+  });
+});