About this fork

This is a fork of luke-harriman/Codesys-MCP maintained at phobicdotno/Codesys-MCP on branch sp21-plus-migration-notes.

Why fork. Upstream's watcher relies on system.execute_on_primary_thread() to marshal work from a background thread back to the CODESYS UI thread. That API was removed in CODESYS V3.5 SP21+, so on SP21 / SP22 every tool call returned the same Marshal error: The functionality 'system.execute_on_primary_thread(...)' is no longer supported and the server was effectively unusable on current CODESYS releases.

What's fixed in this fork:

• SP21+/SP22 compatibility — the watcher was rewritten as single-threaded on the primary thread, yielding to the IDE via system.delay(). No background thread, no marshaling. Works on SP19, SP21, and SP22+. Full rationale in docs/MIGRATION-SP21-PLUS.md.
• Cancel-link hardening — the watcher now catches KeyboardInterrupt (which is not a subclass of Exception in Python) at three layers, so clicking "Click here to CANCEL this operation" in CODESYS no longer pops the modal traceback dialog or kills the watcher. Bumped to WATCHER_VERSION 0.4.2.

Verified state of every tool is recorded in docs/SMOKE-TEST-2026-04-25.md: 17 of 28 invocations pass, 8 fail, 3 are partial. The failures are upstream bugs unrelated to the SP21+ fix (e.g. create_folder keyword mismatch, JSON long serialization in compile_project, online-API drift in connect_to_device / write_variable) and are tracked there for follow-up PRs.

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