diff --git a/simulasi-mk.html b/simulasi-mk.html
index 9dbf771..72c3a58 100644
--- a/simulasi-mk.html
+++ b/simulasi-mk.html
@@ -20,6 +20,11 @@
         .course-card:hover { transform: translateY(-2px); box-shadow: 0 4px 6px -1px rgba(0, 0, 0, 0.1); z-index: 10; }
         .course-card.dragging { opacity: 0.5; border: 2px dashed #a8a29e; background-color: #f5f5f4; }
         
+        /* Validation Styles */
+        .course-card.violation { border: 2px solid #ef4444; background-color: #fef2f2; }
+        .course-card.violation .cat-bar { background-color: #ef4444 !important; }
+        .violation-badge { position: absolute; right: 0.5rem; bottom: 0.5rem; font-size: 0.65rem; color: #dc2626; font-weight: bold; display: flex; align-items: center; gap: 2px; }
+
         /* Grade Badges */
         .grade-badge { position: absolute; right: 0.5rem; top: 0.5rem; font-size: 0.7rem; font-weight: bold; padding: 0.1rem 0.4rem; border-radius: 0.25rem; }
         .grade-A { background: #dcfce7; color: #166534; border: 1px solid #bbf7d0; }
@@ -68,7 +73,7 @@
         /* Layout Fixes */
         .main-container {
             width: 100%;
-            max-width: 1280px; /* Sedikit lebih kecil dari 1400px agar lebih rapi */
+            max-width: 1280px;
             margin-left: auto;
             margin-right: auto;
             padding-left: 1rem;
@@ -126,7 +131,7 @@
             <h1 class="text-3xl md:text-4xl font-bold text-slate-900 mb-2">Simulasi Studi Berbasis IPS</h1>
             <p class="text-slate-600 max-w-2xl mx-auto text-sm md:text-base">
                 Simulasi nyata: <strong>IPS Semester lalu menentukan jatah SKS semester depan.</strong><br>
-                Jika nilai <strong>E</strong>, gunakan opsi <em>"Ulang di Tahun Depan"</em> (Klik Kanan) untuk memperbaiki.
+                Sistem <strong>Prasyarat Aktif</strong>: Anda tidak bisa mengambil mata kuliah lanjut jika prasyarat belum lulus.
             </p>
             
             <div class="mt-6 flex flex-wrap justify-center gap-3 text-xs font-semibold opacity-80">
@@ -193,13 +198,10 @@
 
     <script>
         // --- UTILS: Fix for HTTP Environments (Coolify) ---
-        // crypto.randomUUID() only works in Secure Contexts (HTTPS).
-        // This fallback ensures the app works on HTTP or local IPs.
         function generateUUID() {
             if (typeof crypto !== 'undefined' && crypto.randomUUID) {
                 return crypto.randomUUID();
             }
-            // Fallback for non-secure contexts
             return 'xxxxxxxx-xxxx-4xxx-yxxx-xxxxxxxxxxxx'.replace(/[xy]/g, function(c) {
                 var r = Math.random() * 16 | 0, v = c == 'x' ? r : (r & 0x3 | 0x8);
                 return v.toString(16);
@@ -219,6 +221,20 @@
             RES:  { color: 'bg-red-500' }
         };
 
+        // --- Prerequisite Rules ---
+        // M7: Logika Komputasional
+        // M14: Dasar Pemrograman
+        // M19: Struktur Data
+        // M20: Jaringan Komputer
+        // M28: Pemrograman Jaringan
+        // M31: Manajemen Jaringan
+        const prerequisites = {
+            "M14": { reqId: "M7", reqName: "Logika Komputasional" },
+            "M19": { reqId: "M14", reqName: "Dasar Pemrograman" },
+            "M28": { reqId: "M20", reqName: "Jaringan Komputer" },
+            "M31": { reqId: "M28", reqName: "Pemrograman Jaringan" }
+        };
+
         const baseCurriculum = {
             1: [
                 { id: "M1", name: "Pend. Pancasila", sks: 2, cat: "MKWU" },
@@ -327,7 +343,7 @@
         // --- State Management ---
         let currentPlan = {};
         let currentTrackId = 'general';
-        let contextTarget = null; // { semId, index }
+        let contextTarget = null; 
 
         // --- Core Logic ---
 
@@ -339,7 +355,6 @@
             for(let i=1; i<=8; i++) {
                 currentPlan[i] = [];
                 if(baseCurriculum[i]) {
-                    // Use generateUUID instead of crypto.randomUUID
                     currentPlan[i] = baseCurriculum[i].map(c => ({...c, grade: 4, uuid: generateUUID()}));
                 }
             }
@@ -355,15 +370,37 @@
             renderAll();
         }
 
+        // --- Prerequisite Check Function ---
+        function checkPrerequisite(courseId, targetSem) {
+            const rule = prerequisites[courseId];
+            if (!rule) return { allowed: true };
+
+            let passed = false;
+            // Scan only semesters BEFORE the target semester
+            for (let i = 1; i < targetSem; i++) {
+                if (!currentPlan[i]) continue;
+                // Find instance of prerequisite course that is Passed (Grade > 0)
+                const found = currentPlan[i].find(c => c.id === rule.reqId && c.grade > 0);
+                if (found) {
+                    passed = true;
+                    break;
+                }
+            }
+
+            if (!passed) {
+                return { allowed: false, msg: `Syarat: Lulus ${rule.reqName} di semester sebelumnya.` };
+            }
+            return { allowed: true };
+        }
+
         function calculateSemStats(semId) {
             const courses = currentPlan[semId] || [];
             let totalSKS = 0;
             let totalPoints = 0;
-            let loadSKS = 0; // Termasuk yang E
+            let loadSKS = 0; 
 
             courses.forEach(c => {
                 loadSKS += c.sks;
-                // Hanya hitung IP dari sks yang diambil dan bukan yang ditandai mengulang (retake)
                 if(c.grade > 0) {
                     totalSKS += c.sks;
                     totalPoints += (c.sks * c.grade);
@@ -386,6 +423,16 @@
             if (!grid) return;
             grid.innerHTML = '';
 
+            // First Pass: Check Violations (Prerequisites)
+            // Need to run this before rendering to mark cards
+            for(let i=1; i<=8; i++) {
+                currentPlan[i].forEach(course => {
+                    const check = checkPrerequisite(course.id, i);
+                    course.violation = !check.allowed;
+                    course.violationMsg = check.msg;
+                });
+            }
+
             for(let i=1; i<=8; i++) {
                 let maxSKS = 20;
                 let prevIPS = 0;
@@ -459,8 +506,13 @@
             const catData = categories[c.cat] || { color: 'bg-gray-400' };
             const isFailed = c.grade === 0;
             const isRetake = c.isRetake === true;
+            const isViolation = c.violation === true;
+            
+            // Add 'violation' class if prereq failed
+            card.className = `course-card bg-white p-2 pl-4 rounded shadow-sm border border-slate-200 relative 
+                ${isFailed ? 'border-red-300 bg-red-50' : ''} 
+                ${isViolation ? 'violation' : ''}`;
             
-            card.className = `course-card bg-white p-2 pl-4 rounded shadow-sm border border-slate-200 relative ${isFailed ? 'border-red-300 bg-red-50' : ''}`;
             card.draggable = true;
             card.setAttribute('ondragstart', `dragSemester(event, ${semId}, ${index})`);
             card.oncontextmenu = (e) => { e.preventDefault(); showContextMenu(e, semId, index); };
@@ -484,10 +536,61 @@
                     </div>
                 </div>
                 <div class="grade-badge ${gradeClass}">${gradeLabel}</div>
+                ${isViolation ? `<div class="violation-badge"><i class="fas fa-exclamation-triangle"></i> Prasyarat Invalid</div>` : ''}
             `;
             return card;
         }
 
+        // --- Drag & Drop ---
+        let dragSource = null;
+
+        function dragCatalog(ev, id, name, sks, cat) {
+            dragSource = { type: 'catalog', course: { id, name, sks: parseInt(sks), cat, grade: 4, uuid: generateUUID() } };
+            ev.dataTransfer.effectAllowed = "copy";
+        }
+
+        function dragSemester(ev, semId, index) {
+            dragSource = { type: 'semester', semId, index };
+            ev.dataTransfer.effectAllowed = "move";
+            ev.target.classList.add('dragging');
+        }
+
+        function allowDrop(ev) { ev.preventDefault(); }
+
+        function drop(ev, targetSem) {
+            ev.preventDefault();
+            document.querySelectorAll('.course-card').forEach(el => el.classList.remove('dragging'));
+
+            if(!dragSource) return;
+
+            let courseToAdd = null;
+            if(dragSource.type === 'catalog') {
+                courseToAdd = dragSource.course;
+            } else {
+                courseToAdd = currentPlan[dragSource.semId][dragSource.index];
+            }
+
+            // --- Prerequisite Check Before Dropping ---
+            const check = checkPrerequisite(courseToAdd.id, targetSem);
+            if (!check.allowed) {
+                showToast("❌ " + check.msg);
+                return; // Block Action
+            }
+
+            if(dragSource.type === 'catalog') {
+                currentPlan[targetSem].push(dragSource.course);
+            } else {
+                const { semId, index } = dragSource;
+                if(semId === targetSem) return;
+                const item = currentPlan[semId][index];
+                currentPlan[semId].splice(index, 1);
+                currentPlan[targetSem].push(item);
+            }
+
+            renderAll();
+            dragSource = null;
+        }
+
         // --- Context Menu Logic ---
         function showContextMenu(e, semId, index) {
             contextTarget = { semId, index };
@@ -495,7 +598,6 @@
             const course = currentPlan[semId][index];
             const btnRetake = document.getElementById('btn-retake');
 
-            // Show Retake button ONLY if Grade is E (0)
             if (course.grade === 0) {
                 btnRetake.style.display = 'block';
             } else {
@@ -503,12 +605,9 @@
             }
 
             menu.style.display = 'block';
-            
             let x = e.pageX;
             let y = e.pageY;
-            // Prevent menu going offscreen
             if(x + 200 > window.innerWidth) x = window.innerWidth - 210;
-            
             menu.style.left = `${x}px`;
             menu.style.top = `${y}px`;
         }
@@ -522,12 +621,11 @@
                 const { semId, index } = contextTarget;
                 currentPlan[semId][index].grade = grade;
                 
-                // Jika E, beri notifikasi untuk opsi ulang
                 if(grade === 0) {
                     showToast("Mata kuliah Gagal (E). Klik kanan > 'Ulang di Tahun Depan' untuk memperbaiki.");
                 }
                 
-                renderAll();
+                renderAll(); // Will trigger re-validation of downstream courses
             }
             hideContextMenu();
         }
@@ -537,7 +635,6 @@
                 const { semId, index } = contextTarget;
                 const course = currentPlan[semId][index];
                 
-                // Target: 2 semester berikutnya
                 const targetSem = parseInt(semId) + 2;
 
                 if (targetSem > 8) {
@@ -547,15 +644,21 @@
                     if (exists) {
                         showToast("Mata kuliah ini sudah ada di Semester " + targetSem + ".");
                     } else {
-                        const newCourse = {
-                            ...course,
-                            grade: 4, 
-                            uuid: generateUUID(), // Secure UUID
-                            isRetake: true
-                        };
-                        currentPlan[targetSem].push(newCourse);
-                        showToast(`Mata kuliah ditambahkan ke Semester ${targetSem}.`);
-                        renderAll();
+                        // Check Prereq again for the target sem (just in case)
+                        const check = checkPrerequisite(course.id, targetSem);
+                        if (!check.allowed) {
+                            showToast("❌ Tidak bisa mengulang: " + check.msg);
+                        } else {
+                            const newCourse = {
+                                ...course,
+                                grade: 4, 
+                                uuid: generateUUID(),
+                                isRetake: true
+                            };
+                            currentPlan[targetSem].push(newCourse);
+                            showToast(`Mata kuliah ditambahkan ke Semester ${targetSem}.`);
+                            renderAll();
+                        }
                     }
                 }
             }
@@ -584,43 +687,6 @@
             }
         });
 
-        // --- Drag & Drop ---
-        let dragSource = null;
-
-        function dragCatalog(ev, id, name, sks, cat) {
-            // Use generateUUID
-            dragSource = { type: 'catalog', course: { id, name, sks: parseInt(sks), cat, grade: 4, uuid: generateUUID() } };
-            ev.dataTransfer.effectAllowed = "copy";
-        }
-
-        function dragSemester(ev, semId, index) {
-            dragSource = { type: 'semester', semId, index };
-            ev.dataTransfer.effectAllowed = "move";
-            ev.target.classList.add('dragging');
-        }
-
-        function allowDrop(ev) { ev.preventDefault(); }
-
-        function drop(ev, targetSem) {
-            ev.preventDefault();
-            document.querySelectorAll('.course-card').forEach(el => el.classList.remove('dragging'));
-
-            if(!dragSource) return;
-
-            if(dragSource.type === 'catalog') {
-                currentPlan[targetSem].push(dragSource.course);
-            } else {
-                const { semId, index } = dragSource;
-                if(semId === targetSem) return;
-                const item = currentPlan[semId][index];
-                currentPlan[semId].splice(index, 1);
-                currentPlan[targetSem].push(item);
-            }
-
-            renderAll();
-            dragSource = null;
-        }
-
         // --- Sidebar & Catalog ---
         function toggleCatalog() { document.getElementById('catalog-sidebar').classList.toggle('closed'); }