codesys-mcp-persistent

MCP server for CODESYS with a persistent UI instance and file-based IPC.

Unlike headless-only approaches that spawn a new CODESYS process per command, this server launches CODESYS with its UI visible and keeps it running. MCP tool calls are sent to the same instance via a file-based IPC watcher, so changes appear in real-time and the user can interact with the IDE alongside AI-driven automation.

Features

• Persistent mode — CODESYS UI stays open; commands execute in the running instance
• Headless fallback — automatic fallback to --noUI spawn-per-command if persistent mode fails
• File-based IPC — proven approach using atomic file writes and a Python watcher script
• Command serialization — async mutex ensures one command at a time
• Health monitoring — detects CODESYS crashes and reports state
• 28 MCP tools — project management, POU authoring, structured compiler diagnostics, runtime monitoring, library management
• Drop-in replacement — same MCP tool names and parameters as @codesys/mcp-toolkit

Installation

npm install -g codesys-mcp-persistent

Or install from the repository:

git clone https://github.com/luke-harriman/Codesys-MCP.git
cd Codesys-MCP
npm install
npm run build
npm link

Requirements: Node.js 18+, Windows, CODESYS 3.5 SP19 or SP21 installed.

Quick Start

Add to your .mcp.json (Claude Code configuration):

{
  "mcpServers": {
    "codesys": {
      "command": "codesys-mcp-persistent",
      "args": [
        "--codesys-path", "C:\\Program Files\\CODESYS 3.5.21.0\\CODESYS\\Common\\CODESYS.exe",
        "--codesys-profile", "CODESYS V3.5 SP21 Patch 3",
        "--mode", "persistent"
      ]
    }
  }
}

Or run directly:

codesys-mcp-persistent \
  --codesys-path "C:\Program Files\CODESYS 3.5.21.0\CODESYS\Common\CODESYS.exe" \
  --codesys-profile "CODESYS V3.5 SP21 Patch 3"

CLI Reference

| Flag | Description | Default | |------|-------------|---------| | -p, --codesys-path <path> | Path to CODESYS executable | $CODESYS_PATH or auto-detected | | -f, --codesys-profile <name> | CODESYS profile name | $CODESYS_PROFILE or CODESYS V3.5 SP21 | | -w, --workspace <dir> | Workspace directory for relative paths | Current directory | | -m, --mode <mode> | persistent (UI) or headless (--noUI) | persistent | | --no-auto-launch | Don't launch CODESYS on startup | Auto-launch enabled | | --fallback-headless | Fall back to headless if persistent fails | true | | --keep-alive | Keep CODESYS running after server stops | false | | --timeout <ms> | Default command timeout | 60000 | | --detect | List installed CODESYS versions and exit | — | | --verbose | Enable verbose logging | — | | --debug | Enable debug logging | — | | -V, --version | Show version number | — | | -h, --help | Show help | — |

Environment variables CODESYS_PATH and CODESYS_PROFILE are used as defaults when the corresponding flags are not provided.

MCP Tools

Management Tools

| Tool | Description | |------|-------------| | launch_codesys | Manually launch CODESYS (use with --no-auto-launch) | | shutdown_codesys | Shut down the persistent CODESYS instance | | get_codesys_status | Get current state, PID, execution mode |

Project Tools

| Tool | Description | |------|-------------| | open_project | Open an existing CODESYS project file | | create_project | Create a new project from the standard template | | save_project | Save the currently open project | | compile_project | Build the primary application with structured error output (120s timeout) | | get_compile_messages | Retrieve last compiler messages without triggering a new build |

POU / Code Authoring Tools

| Tool | Description | |------|-------------| | create_pou | Create a Program, Function Block, or Function | | set_pou_code | Set declaration and/or implementation code | | create_property | Create a property within a Function Block | | create_method | Create a method within a Function Block | | create_dut | Create a Data Unit Type (Structure, Enumeration, Union, Alias) | | create_gvl | Create a Global Variable List with optional initial declaration | | create_folder | Create an organizational folder in the project tree | | delete_object | Delete any project object (POU, DUT, GVL, folder, etc.) | | rename_object | Rename any project object | | get_all_pou_code | Bulk read all declaration and implementation code in the project (120s timeout) |

Online / Runtime Tools

| Tool | Description | |------|-------------| | connect_to_device | Login to the PLC runtime (requires configured device/gateway) | | disconnect_from_device | Logout from the PLC runtime | | get_application_state | Check if the PLC application is running, stopped, or in exception | | read_variable | Read a live variable value from the running PLC (e.g., PLC_PRG.bMotorRunning) | | write_variable | Write/force a variable value on the running PLC | | download_to_device | Download compiled application to PLC (attempts online change first, 120s timeout) | | start_stop_application | Start or stop the PLC application |

Library Management Tools

| Tool | Description | |------|-------------| | list_project_libraries | List all libraries referenced in the project with version info | | add_library | Add a library reference to the project |

MCP Resources

| Resource URI | Description | |--------------|-------------| | codesys://project/status | CODESYS scripting status and open project info | | codesys://project/{path}/structure | Project tree structure | | codesys://project/{path}/pou/{pou}/code | POU declaration and implementation code |

Execution Modes

Persistent Mode (default)

1. Server launches CODESYS.exe with --runscript=watcher.py (no --noUI)
2. CODESYS UI opens — user can see and interact with the IDE
3. The watcher script starts a .NET background thread that polls a commands/ directory, then returns control to CODESYS so the UI stays fully responsive
4. When a tool is called, the server writes a .py script + .command.json to commands/
5. The background thread detects the command and marshals execution onto the CODESYS UI thread via system.execute_on_primary_thread()
6. Results are written atomically to results/
7. Changes made by tools appear in the CODESYS UI in real-time
8. The UI remains interactive between commands — only briefly paused during synchronous API calls (compile, open)

Headless Mode

Falls back to the original approach: each tool call spawns a new CODESYS process with --noUI, runs the script, and exits. No UI is shown. Used when:

• --mode headless is specified
• Persistent mode fails to launch and --fallback-headless is enabled
• CODESYS is launched with --no-auto-launch and launch_codesys hasn't been called yet

Detect Installed Versions

codesys-mcp-persistent --detect

Scans Program Files and Program Files (x86) for CODESYS installations.

Troubleshooting

CODESYS not found Verify the path with --detect. The executable is typically at: C:\Program Files\CODESYS 3.5.XX.X\CODESYS\Common\CODESYS.exe

Project file locked Another CODESYS instance may have the project open. Close it first or use persistent mode so there's only one instance.

Watcher timeout (persistent mode) If the watcher doesn't signal ready within 60 seconds, check:

• CODESYS path and profile are correct
• No modal dialogs are blocking CODESYS startup
• Try --verbose for detailed logging

UI briefly pauses during commands (persistent mode) The watcher uses a background thread that marshals work onto the UI thread, so the UI stays responsive between commands. During synchronous CODESYS API calls (compile, project open), the UI may briefly pause — this is expected and normal. If a command hangs, check the CODESYS messages window for modal dialogs or errors.

Command timeout Default is 60s (120s for compile and download). Increase with --timeout <ms>. Check CODESYS messages window for errors.

Online/runtime tools fail The online tools (connect_to_device, read_variable, etc.) require:

• A device/gateway configured in the CODESYS project
• The project to be compiled successfully before connecting
• A reachable PLC or CODESYS SoftPLC runtime

Development

# Install dependencies
npm install

# Build (compiles TypeScript + copies Python scripts)
npm run build

# Run all tests
npm test

# Type check only
npm run typecheck

# Run tests in watch mode
npm run test:watch

Project Structure

src/
  bin.ts              CLI entry point
  server.ts           MCP tool/resource registration (28 tools, 3 resources)
  launcher.ts         CODESYS process management
  ipc.ts              File-based IPC transport
  headless.ts         Headless fallback executor
  script-manager.ts   Python template loading + interpolation
  types.ts            Shared TypeScript types
  logger.ts           Structured stderr logging
  scripts/            Python scripts (watcher + 2 helpers + 28 tool scripts)
tests/
  unit/               Unit tests (IPC, script manager, launcher)
  integration/        Integration tests (script pipeline, manual CODESYS tests)
  mock_watcher.py     Standalone watcher for testing without CODESYS

License

MIT