diff --git a/frontend/src/lib/services/evaluators.ts b/frontend/src/lib/services/evaluators.ts
index d182ceb..b52e6a6 100644
--- a/frontend/src/lib/services/evaluators.ts
+++ b/frontend/src/lib/services/evaluators.ts
@@ -1,4 +1,4 @@
-import { checkKeyText, validateNodes } from '$services/exercise';
+import { checkKeyText, missingKeyTextKeys, validateNodes } from '$services/exercise';
 import type { LessonContent } from '$types/lesson';
 import type { CircuitJSApi } from '$types/circuitjs';
 
@@ -52,6 +52,12 @@ export function evaluateCircuitSubmission(
 	return { pass: allPassed, output: finalOutput };
 }
 
+export interface LanguageEvalResult {
+	isCorrect: boolean;
+	reason?: 'output-mismatch' | 'keytext-missing';
+	missingKeys?: string[];
+}
+
 export function processLanguageEvaluation(
 	compileOutput: string,
 	code: string,
@@ -60,7 +66,7 @@ export function processLanguageEvaluation(
 	cCode: string,
 	pythonCode: string,
 	data: LessonContent
-): { isCorrect: boolean } {
+): LanguageEvalResult {
 	if (!data.expected_output && !data.expected_output_python) return { isCorrect: true };
 
 	const currentCCode = (currentLanguage === 'c') ? code : cCode;
@@ -71,6 +77,15 @@ export function processLanguageEvaluation(
 		? data.expected_output_python 
 		: data.expected_output;
 
-	const isCorrect = compileOutput.trim() === expected.trim() && checkKeyText(mergedCode, data.key_text ?? '');
-	return { isCorrect };
+	const outputMatches = compileOutput.trim() === (expected ?? '').trim();
+	if (!outputMatches) {
+		return { isCorrect: false, reason: 'output-mismatch' };
+	}
+
+	const missingKeys = missingKeyTextKeys(mergedCode, data.key_text ?? '');
+	if (missingKeys.length > 0) {
+		return { isCorrect: false, reason: 'keytext-missing', missingKeys };
+	}
+
+	return { isCorrect: true };
 }
diff --git a/frontend/src/lib/services/exercise.test.ts b/frontend/src/lib/services/exercise.test.ts
new file mode 100644
index 0000000..8230b3d
--- /dev/null
+++ b/frontend/src/lib/services/exercise.test.ts
@@ -0,0 +1,44 @@
+import { describe, it, expect } from 'vitest';
+import { checkKeyText, missingKeyTextKeys } from './exercise';
+import { processLanguageEvaluation } from './evaluators';
+import type { LessonContent } from '$types/lesson';
+
+describe('missingKeyTextKeys', () => {
+	it('mengembalikan key yang hilang (bukan hanya boolean)', () => {
+		expect(missingKeyTextKeys('int main() {}', 'int\nfloat\n%.2lf')).toEqual(['float', '%.2lf']);
+	});
+
+	it('kompatibel dengan checkKeyText', () => {
+		expect(checkKeyText('int x; float y;', 'int\nfloat')).toBe(true);
+		expect(checkKeyText('int x;', 'int\nfloat')).toBe(false);
+	});
+
+	it('key_text kosong tidak menghasilkan key hilang', () => {
+		expect(missingKeyTextKeys('apa saja', '  \n')).toEqual([]);
+	});
+});
+
+describe('processLanguageEvaluation feedback', () => {
+	const data = {
+		expected_output: 'Halo',
+		key_text: 'printf\n%.2f',
+	} as unknown as LessonContent;
+
+	it('output beda → reason output-mismatch', () => {
+		const result = processLanguageEvaluation('Halo dunia', 'printf("%.2f", 1);', 'c', 'c', '', '', data);
+		expect(result.isCorrect).toBe(false);
+		expect(result.reason).toBe('output-mismatch');
+	});
+
+	it('output sama tapi key_text kurang → reason keytext-missing + daftar key', () => {
+		const result = processLanguageEvaluation('Halo', 'printf("Halo");', 'c', 'c', '', '', data);
+		expect(result.isCorrect).toBe(false);
+		expect(result.reason).toBe('keytext-missing');
+		expect(result.missingKeys).toEqual(['%.2f']);
+	});
+
+	it('output sama dan key lengkap → correct tanpa reason', () => {
+		const result = processLanguageEvaluation('Halo', 'printf("%.2f", 1);', 'c', 'c', '', '', data);
+		expect(result).toEqual({ isCorrect: true });
+	});
+});
diff --git a/frontend/src/lib/services/exercise.ts b/frontend/src/lib/services/exercise.ts
index 7ff83fe..ebec854 100644
--- a/frontend/src/lib/services/exercise.ts
+++ b/frontend/src/lib/services/exercise.ts
@@ -4,11 +4,16 @@
 
 import type { CircuitJSApi } from '$types/circuitjs';
 
+/** Return key_text keywords (one per line) that are missing from the code. */
+export function missingKeyTextKeys(code: string, keyText: string): string[] {
+	if (!keyText.trim()) return [];
+	const keys = keyText.split('\n').map(k => k.trim()).filter(k => k.length > 0);
+	return keys.filter(key => !code.includes(key));
+}
+
 /** Check if code contains all required key_text keywords (one per line). */
 export function checkKeyText(code: string, keyText: string): boolean {
-	if (!keyText.trim()) return true;
-	const keys = keyText.split('\n').map(k => k.trim()).filter(k => k.length > 0);
-	return keys.every(key => code.includes(key));
+	return missingKeyTextKeys(code, keyText).length === 0;
 }
 
 export interface NodeResult {
diff --git a/frontend/src/routes/lesson/[slug]/lesson.svelte.ts b/frontend/src/routes/lesson/[slug]/lesson.svelte.ts
index 2ba8982..b09336f 100644
--- a/frontend/src/routes/lesson/[slug]/lesson.svelte.ts
+++ b/frontend/src/routes/lesson/[slug]/lesson.svelte.ts
@@ -865,8 +865,8 @@ export class LessonManager {
 			out.output = res.output;
 			out.success = true;
 			if (this.data.expected_output || this.data.expected_output_python) {
-				const { isCorrect } = processLanguageEvaluation(res.output, code, lang, this.currentLanguage, this.cCode, this.pythonCode, this.data);
-				if (isCorrect) {
+				const evaluation = processLanguageEvaluation(res.output, code, lang, this.currentLanguage, this.cCode, this.pythonCode, this.data);
+				if (evaluation.isCorrect) {
 					if (lang === 'c') this.cPassed = true;
 					else if (lang === 'python') this.pythonPassed = true;
 					if (!this.checkAllPassed()) {
@@ -883,6 +883,14 @@ export class LessonManager {
 						}
 						setTimeout(() => { this.showCelebration = false; this.activeTab = 'editor'; }, 3000);
 					}
+				} else {
+					out.success = false;
+					if (evaluation.reason === 'keytext-missing') {
+						const missing = (evaluation.missingKeys ?? []).map(k => `\`${k.replace(/\n/g, '\\n')}\``).join(', ');
+						out.output += `\n❌ Output sudah sesuai, tapi kode belum memakai konsep materi. Kata kunci wajib yang belum ada: ${missing}`;
+					} else if (evaluation.reason === 'output-mismatch') {
+						out.output += '\n❌ Output belum sesuai yang diharapkan — periksa format keluaran programmu.';
+					}
 				}
 			}
 		} catch (err: any) {