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Unreal Engine 5.9 — Reference

Living C4 · L4 Updated 2026-11-11

Engine decision (Chris, 2026-11-10): Unreal Engine 5.9 — not UEFN, so the game is not locked to the Fortnite ecosystem. UE 5.9 is in the repo as source-level reference: agents read Epic’s documentation and (with org access) the engine source on demand. Never vendor Epic source or documentation text into this repo — the UE EULA governs it; link instead.

Version status

The 5.9 MegaJam Preview (Epic Games Launcher, ~Nov 9–10, 2026) is the first Unreal Engine release built primarily with agentic workflows: assembled fast, shorter QA than usual, parts still experimental. The full 5.9 release is estimated around December 2026. Pin the exact launcher build for the jam; do not upgrade mid-jam.

Verified documentation (loaded and checked 2026-11-10)

Need	Link
UE 5.9 documentation home	https://dev.epicgames.com/documentation/unreal-engine/unreal-engine-5-9-documentation
UE 5.9 Preview release notes	https://dev.epicgames.com/documentation/unreal-engine/unreal-engine-5-9-release-notes
Programming with C++ in Unreal Engine	https://dev.epicgames.com/documentation/unreal-engine/programming-with-cplusplus-in-unreal-engine
Unreal MCP in Unreal Editor (Experimental)	https://dev.epicgames.com/documentation/unreal-engine/unreal-mcp-in-unreal-editor
Build operations — cooking, packaging, Shipping builds	https://dev.epicgames.com/documentation/unreal-engine/build-operations-cooking-packaging-deploying-and-running-projects-in-unreal-engine
C++ API reference	https://dev.epicgames.com/documentation/unreal-engine/API?application_version=5.8 — 5.8 is the newest published index; no 5.9 API index exists yet. For 5.9-changed APIs, use the engine source (`EpicGames/UnrealEngine`, branch `5.9`, org members only).

What 5.9 changes for agents

Unreal MCP: 21 → 41 toolsets, launches with the editor, Blueprints creatable via text, plus a Fab toolset (search/install assets). The AI Assistant and MCP plugins are enabled by default. Setup for this project: enable ModelContextProtocol and AllToolsets in the .uproject, set bAutoStartServer in Saved/Config/<Platform>Editor/EditorPerProjectUserSettings.ini, and generate .mcp.json with ModelContextProtocol.GenerateClientConfig.
Unreal Assistant — in-editor panel for Claude Code / Codex / Cursor / OpenCode. This repo enables Epic’s official skills plugin for Claude Code (.claude/settings.json → unreal-engine-skills-for-claude-code from Anthropic’s official marketplace): the unreal-mcp, create-toolset and unreal-skill skills.
Unreal Canvas — generative images/meshes/video from prompts or scenes (own API keys). Treat output as draft assets; log in the Provenance Log.
Web platform (experimental) — Wasm + WebGPU; requires a C++ project; no Nanite, virtual shadow maps or hardware ray tracing on web.
Expanded Python APIs — audio, MetaSounds, modeling, physics, Niagara, materials, PCG, networking, UI. Prefer Python for editor automation; C++ when the stubs don’t cover it.

The full agent doc set (alignment contract, MCP playbook, setup runbook, authoring standards) lives in the game-design repo under docs/.