Development

Prerequisites

Node.js 18+
Unreal Engine 5.4–5.7 (for live testing)
A UE project to test against

Setup

git clone https://github.com/db-lyon/ue-mcp.git
cd ue-mcp
npm install

Resolving an Issue with Claude Code

From inside your ue-mcp clone:

npx ue-mcp resolve 16

This:

Fetches issue #16 from db-lyon/ue-mcp via gh.
Creates a resolve/16 branch from origin/main in your current checkout.
Pipes a generated prompt (issue body + repo conventions) into claude --print --dangerously-skip-permissions.
Claude reads code, implements the fix, runs npx tsc --noEmit, and commits.
The script then pushes the branch and opens a PR against db-lyon/ue-mcp.

Requires gh and claude CLIs, write access to the repo (or a fork to push to), and you must run it from inside a ue-mcp clone — it operates on the working tree, not a temp clone.

Building

npx tsc                # TypeScript -> dist/ (what the server ships as)
npm run build          # UE C++ plugin build (requires editor closed)

npx tsc emits the TypeScript server into dist/. npm run build is the C++ plugin build that runs Unreal's Build.bat against the test project and requires the editor to be closed first. Use npm run up:build to chain stop-build-start during plugin iteration.

Running

# Build and run
npm run up:build

# Run (assumes already built)
npm run up

# Dev mode (tsx, no build step)
npm run dev

# Direct
node dist/index.js C:/path/to/MyGame.uproject

# Interactive setup (also available via npx ue-mcp init)
node dist/index.js init C:/path/to/MyGame.uproject

Project Structure

src/
├── index.ts              # Entry point, tool registration, MCP server
├── tools.ts              # ALL_TOOLS registry (consumed by index.ts and tests)
├── types.ts              # ToolDef, ActionSpec, categoryTool() factory
├── bridge.ts             # EditorBridge - WebSocket JSON-RPC client
├── project.ts            # ProjectContext - paths, INI, C++ parsing
├── deployer.ts           # Plugin deployment
├── editor-control.ts     # Editor process management
├── instructions.ts       # AI-facing server instructions
├── github-app.ts         # GitHub App auth for feedback submission
├── init.ts / update.ts / resolve.ts / hook-handler.ts  # CLI subcommands
├── flow/                 # Flow engine (registry, loader, task factory, HTTP)
└── tools/                # 19 tool category implementations
    ├── project.ts
    ├── asset.ts
    ├── blueprint.ts
    ├── level.ts
    ├── material.ts
    ├── animation.ts
    ├── landscape.ts
    ├── pcg.ts
    ├── foliage.ts
    ├── niagara.ts
    ├── audio.ts
    ├── widget.ts
    ├── editor.ts
    ├── reflection.ts
    ├── gameplay.ts
    ├── gas.ts
    ├── networking.ts
    ├── demo.ts
    └── feedback.ts

plugin/ue_mcp_bridge/     # C++ bridge plugin (deployed to UE projects)
└── Source/UE_MCP_Bridge/
    ├── UE_MCP_Bridge.Build.cs
    └── Private/
        ├── BridgeServer.cpp/.h
        ├── HandlerRegistry.cpp/.h
        ├── GameThreadExecutor.cpp/.h
        └── Handlers/          # 22 C++ handler groups

tests/smoke/               # Smoke tests (require live editor)
tests/unit/                # Pure-TypeScript unit tests (no editor needed)
scripts/                   # Build and run scripts
docs/                      # Documentation (MkDocs Material)

Testing

Unit Tests

Pure-TypeScript tests under tests/unit/. No editor required.

npm run test:unit

These also run in CI on every PR.

Smoke Tests

Smoke tests run against a live editor and verify tool functionality end-to-end.

# Specific suite
npm run test:level
npm run test:blueprint
npm run test:material
# ... 16 suites total — see scripts in package.json

# All suites (Vitest)
npm test

# Full smoke test runner — exercises every registered handler
npm run test:smoke

Smoke tests require the test project

The smoke runner targets tests/ue_mcp/ue_mcp.uproject only. Real mutations execute against the connected editor (creating blueprints, deleting assets, modifying the level). Never run smoke tests against a real project. The runner aborts if it detects a connection to anything else.

Prerequisites

Editor running with the test project
Bridge connected (project(action="get_status") returns bridgeConnected: true)

Test Suites

Suite	What It Tests
`level`	Actor CRUD, selection, components, volumes, lights
`asset`	Asset listing, search, CRUD, import
`blueprint`	BP reading, creation, graph editing, compilation
`material`	Material creation, parameters, instances
`editor`	Console, PIE, viewport, undo/redo
`reflection`	Class/struct/enum reflection, gameplay tags
`animation`	Anim BP, montages, skeletons
`landscape`	Landscape info, sculpting, painting
`gameplay`	Physics, collision, navigation, AI
`audio`	Sound listing, playback
`niagara`	Niagara system inspection and authoring
`pcg`	PCG graph listing and authoring
`foliage`	Foliage types
`widget`	Widget blueprint creation, tree manipulation, slot properties
`networking`	Replication config
`gas`	GAS component inspection

Adding a New Tool

TypeScript Side

Create src/tools/myfeature.ts:

import { categoryTool, bp, type ToolDef } from '../types.js';
import { z } from 'zod';

export const myfeatureTool: ToolDef = categoryTool(
  'myfeature',
  'Description of this tool category',
  {
    my_action: bp('my_cpp_handler_method'),
    local_action: {
      handler: async (ctx, params) => {
        // local implementation
        return { result: 'done' };
      },
    },
  },
  '- my_action: Does something. Params: foo, bar\n- local_action: Does something locally.',
  {
    foo: z.string().optional().describe('Description of foo'),
  },
);

C++ Side

Create handler files in plugin/ue_mcp_bridge/Source/UE_MCP_Bridge/Private/Handlers/
Register handlers in BridgeServer.cpp
Each handler receives TSharedPtr[FJsonObject] params and returns TSharedPtr[FJsonValue]

C++ Plugin Development

The plugin source lives in plugin/ue_mcp_bridge/. When you modify C++ handler code:

Edit the source in plugin/ue_mcp_bridge/
The deployer copies the plugin to the target project on server start
In the editor, use Live Coding (Ctrl+Alt+F11) or editor(action="hot_reload") to reload

For a full editor restart: editor(action="restart_editor")

Dependencies

Runtime

@modelcontextprotocol/sdk — MCP protocol implementation
ws — WebSocket client
zod — Schema validation

Dev

typescript — Type checking
tsx — TypeScript execution (dev mode)
vitest — Test runner

Development

On this page