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docs/engine-reference/unreal/breaking-changes.md

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+# Unreal Engine 5.7 — Breaking Changes
+
+**Last verified:** 2026-02-13
+
+This document tracks breaking API changes and behavioral differences between Unreal Engine 5.3
+(likely in model training) and Unreal Engine 5.7 (current version). Organized by risk level.
+
+## HIGH RISK — Will Break Existing Code
+
+### Substrate Material System (Production-Ready in 5.7)
+**Versions:** UE 5.5+ (experimental), 5.7 (production-ready)
+
+Substrate replaces the legacy material system with a modular, physically accurate framework.
+
+```cpp
+// ❌ OLD: Legacy material nodes (still work but deprecated)
+// Standard material graph with Base Color, Metallic, Roughness, etc.
+
+// ✅ NEW: Substrate material layers
+// Use Substrate nodes: Substrate Slab, Substrate Blend, etc.
+// Modular material authoring with true physical accuracy
+```
+
+**Migration:** Enable Substrate in `Project Settings > Engine > Substrate` and rebuild materials using Substrate nodes.
+
+---
+
+### PCG (Procedural Content Generation) API Overhaul
+**Versions:** UE 5.7 (production-ready)
+
+PCG framework reached production-ready status with major API changes.
+
+```cpp
+// ❌ OLD: Experimental PCG API (pre-5.7)
+// Old node types, unstable API
+
+// ✅ NEW: Production PCG API (5.7+)
+// Use FPCGContext, IPCGElement, new node types
+// Stable API, production-ready workflow
+```
+
+**Migration:** Follow PCG migration guide in 5.7 docs. Expect significant refactoring for experimental PCG code.
+
+---
+
+### Megalights Rendering System
+**Versions:** UE 5.5+
+
+New lighting system supports millions of dynamic lights.
+
+```cpp
+// ❌ OLD: Limited dynamic lights (clustered forward shading)
+// Max ~100-200 dynamic lights before performance degrades
+
+// ✅ NEW: Megalights (5.5+)
+// Millions of dynamic lights with minimal performance cost
+// Enable: Project Settings > Engine > Rendering > Megalights
+```
+
+**Migration:** No code changes needed, but lighting behavior may differ. Test scenes after enabling.
+
+---
+
+## MEDIUM RISK — Behavioral Changes
+
+### Enhanced Input System (Now Default)
+**Versions:** UE 5.1+ (recommended), 5.7 (default)
+
+Enhanced Input is now the default input system.
+
+```cpp
+// ❌ OLD: Legacy input bindings (deprecated)
+InputComponent->BindAction("Jump", IE_Pressed, this, &ACharacter::Jump);
+
+// ✅ NEW: Enhanced Input
+SetupPlayerInputComponent(UInputComponent* PlayerInputComponent) {
+    UEnhancedInputComponent* EIC = Cast<UEnhancedInputComponent>(PlayerInputComponent);
+    EIC->BindAction(JumpAction, ETriggerEvent::Started, this, &ACharacter::Jump);
+}
+```
+
+**Migration:** Replace legacy input bindings with Enhanced Input actions.
+
+---
+
+### Nanite Default Enabled
+**Versions:** UE 5.0+ (optional), 5.7 (encouraged)
+
+Nanite virtualized geometry is now the recommended workflow for static meshes.
+
+```cpp
+// Enable Nanite on static mesh:
+// Static Mesh Editor > Details > Nanite Settings > Enable Nanite Support
+```
+
+**Migration:** Convert high-poly meshes to Nanite. Test performance on target platforms.
+
+---
+
+## LOW RISK — Deprecations (Still Functional)
+
+### Legacy Material System
+**Status:** Deprecated but supported
+**Replacement:** Substrate Material System
+
+Legacy materials still work, but Substrate is recommended for new projects.
+
+---
+
+### Old World Partition (UE4 Style)
+**Status:** Deprecated
+**Replacement:** World Partition (UE5+)
+
+Use UE5's World Partition system for large worlds.
+
+---
+
+## Platform-Specific Breaking Changes
+
+### Windows
+- **UE 5.7**: DirectX 12 is now default (was DX11 in older versions)
+- Update shaders for DX12 compatibility
+
+### macOS
+- **UE 5.5+**: Metal 3 required (minimum macOS 13)
+
+### Mobile
+- **UE 5.7**: Minimum Android API level raised to 26 (Android 8.0)
+- Minimum iOS deployment target raised to iOS 14
+
+---
+
+## Migration Checklist
+
+When upgrading from UE 5.3 to UE 5.7:
+
+- [ ] Review Substrate materials (convert if ready for new system)
+- [ ] Audit PCG usage (update to production API if using experimental)
+- [ ] Test Megalights performance (enable and benchmark)
+- [ ] Migrate legacy input to Enhanced Input
+- [ ] Convert high-poly meshes to Nanite
+- [ ] Update shaders for DX12 (Windows) or Metal 3 (macOS)
+- [ ] Verify minimum platform versions (Android 8.0, iOS 14)
+- [ ] Test Lumen and Nanite performance on target hardware
+
+---
+
+**Sources:**
+- https://dev.epicgames.com/documentation/en-us/unreal-engine/unreal-engine-5-7-release-notes
+- https://dev.epicgames.com/documentation/en-us/unreal-engine/upgrading-projects-to-newer-versions-of-unreal-engine