Codesys-MCP-SP21+

This is a fork. It is not the upstream luke-harriman/Codesys-MCP.

• Upstream: luke-harriman/Codesys-MCP
• npm: codesys-mcp-sp21-plus (published by phobic)
• Maintainer: Karstein Kvistad

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. Several other upstream tools were also broken by unrelated script-engine API drift. This fork fixes all of that and adds a release pipeline on top.

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.

---

Quick Start

1. Install globally from npm:

npm install -g codesys-mcp-sp21-plus

2. Generate your .mcp.json snippet — --print-config scans your installed CODESYS versions and emits a ready-to-paste block per install:

codesys-mcp-sp21-plus --print-config           # one entry per detected install
codesys-mcp-sp21-plus --print-config --sp 21   # only the SP21 entry, named "codesys"

Output looks like this on a machine with two installs:

// Auto-generated by `codesys-mcp-sp21-plus --print-config` on 2026-04-27.
// Detected 2 CODESYS installations. Add the entries you want; remove the rest.
//
// Multiple entries can be active at the same time -- different CODESYS
// installs (e.g. SP21 + SP22) spawn separate processes and run side by side.
// The only hard rule: don't open the SAME .project file from two CODESYS
// instances simultaneously -- file-lock contention pops a "project is
// currently in use" modal that blocks all script execution.

{
  "mcpServers": {
    "codesys-sp21-patch5": {
      "command": "codesys-mcp-sp21-plus",
      "args": [
        "--codesys-path", "C:\\Program Files\\CODESYS 3.5.21.50\\CODESYS\\Common\\CODESYS.exe",
        "--codesys-profile", "CODESYS V3.5 SP21 Patch 5",
        "--mode", "persistent"
      ]
    },
    "codesys-sp22-patch1": { ... }
  }
}

3. Paste into your MCP config:

• Project-scoped (recommended, shareable via git): <your-project-root>/.mcp.json. Create it if it doesn't exist; merge the mcpServers entries into the existing object if it does.
• User-scoped (applies to every Claude Code session): %USERPROFILE%/.claude.json, or use claude mcp add codesys codesys-mcp-sp21-plus -- --codesys-path ... --codesys-profile ....

The snippet is JSON with // comment headers — strip the comments before parsing if your tooling is strict.

4. Restart Claude Code so it re-reads the MCP config.

See Installation for source-install / upgrade / multi-install setups.

---

Live source-control diff (--auto-mirror)

When --auto-mirror is added to the server args, every successful modifying tool call (set_pou_code, create_pou, rename_object, add_library, ...) is followed by an automatic mirror_export. The textual <projectDir>/mcp-mirror/ tree is refreshed on disk in lock-step with the binary .project, so an external editor watching the folder sees the change immediately. The first refresh on a given project also fires code --add <mirrorDir> once, which appends the mirror folder to your active VSCode window so the diff shows up in the Source Control panel.

Multiple projects in one folder: when two or more .project files share a parent dir, each project gets its own mirror at <projectDir>/<projectname>_mcp_mirror/ instead (e.g. ProjectA.project -> ProjectA_mcp_mirror/, ProjectB.project -> ProjectB_mcp_mirror/) so the exports don't clobber each other. Existing setups with a mcp-mirror/ directory keep using it as-is, regardless of how many .project siblings are present, so v* tag history stays intact.

Enable it by adding the flag to the relevant entry's args in .mcp.json:

"args": [
  "--codesys-path", "...",
  "--codesys-profile", "...",
  "--mode", "persistent",
  "--auto-mirror"
]

Recommended one-time setup so the Source Control panel has a baseline to diff against:

cd <projectDir>/mcp-mirror
git init && git add -A && git commit -m "baseline"

After that, watch VSCode's Source Control panel as Claude edits — every tool call shows up as a real diff. The VSCode hook is best-effort: if the code CLI shim isn't on the standard path, the mirror still refreshes silently and the response carries an (auto-mirror: refreshed) hint instead of opening a window.

---

What's new in this fork

Compatibility fixes (the headline)

• 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 IronPython 2.7) at three layers, so clicking "Click here to CANCEL this operation" in CODESYS no longer pops the modal traceback dialog or kills the watcher. WATCHER_VERSION 0.4.2.

Upstream tool fixes

• create_folder — upstream passed name= as a kwarg the API doesn't accept; fixed to use positional foldername= with an SV_POU fallback for SP21+, then walks children to detect success since the API returns void.
• compile_project / get_compile_messages — upstream choked on Python long values that json.dumps can't serialize on IronPython 2.7. Coerced to int before dumping.
• connect_to_device — upstream used the wrong LoginMode signature. Fixed, plus the online tools now auto-login if you haven't already, instead of silently returning empty results.
• ensure_project_open — fixed the cross-project switch path so opening a second project no longer leaves the watcher pinned to the first.
• set_pou_code — upstream wiped the other half of the POU when only declaration or only implementation was passed. Now an omitted field is left intact.
• add_library — pre-resolves via library_manager.find_library and prefers the managed-library overload. Refuses to save if the resulting reference is an unresolvable placeholder, which would otherwise brick the next project open.
• list_project_libraries — switched to the ScriptLibManObjectContainer API (the previous one no longer exists), and now also captures IDE version, devices, and per-Application compiler version.

New tools (not in upstream)

• mirror_export — walks the project tree and writes one .st file per code-bearing object into <projectDir>/mcp-mirror/, preserving the project tree. Read-only; foundation for source-controlled CODESYS projects.
• bump_project_version — bumps one part of the 4-part Project Information.Version (major / minor / revision / build / auto) and maintains a _MCP_PROJECT_VERSION GVL inside the project so the running PLC carries its source version. auto mode classifies via mirror diff vs the latest v* git tag (deletion/rename → major; addition → minor; modification → revision; first-run → seed at 1.0.0.0). Auto-maintains Changelog.md alongside the bump.
• release_project_version — one-shot release pipeline: mirror_export → classify → bump_project_version → regenerate library.md/pou-dump.md/README.md/Changelog.md → git add controlled paths → git commit → git tag v<new> → git push --follow-tags. Tag annotation embeds dual SHAs (project-sha256 + mirror-sha256) so the binary-changed-without-source-diff case still gets a build-bump with provenance.
• read_running_version_online — reads _MCP_PROJECT_VERSION.sVersion from the running PLC over the CODESYS online protocol (port 11740 / gateway). Returns the live value plus a sanity check against the X.Y.Z.W shape.

Reliability fixes

• launcher refuses to spawn a 2nd instance of the same CODESYS install (would conflict on the project file lock). Different installs (SP21 + SP22) coexist fine. Filters by --codesys-path, not just by image name, so multi-install setups work.
• shutdown_codesys kills orphan CODESYS.exe of the configured install when the launcher has no tracked PID (e.g. after a crashed parent). Other installs are left alone.

Verification

The verified state of every tool is recorded in docs/function-test-2026-04-25.md (and the 2026-04-28 re-verification in docs/function-test-2026-04-28.md). Open issues (mostly online-API drift) are tracked in docs/open-bugs-cross-reference.md.

---

Installation

This is a Node.js MCP server published to npm as codesys-mcp-sp21-plus. It is this fork — not the upstream luke-harriman/Codesys-MCP and not a Python package. There is no pip install; the .py files under src/scripts/ are CODESYS IronPython templates bundled inside the npm package itself.

Requirements: Node.js 18+, Windows, CODESYS 3.5 SP19, SP21 (3.5.21.x), or SP22 (3.5.22.x) installed.

Install from npm (recommended)

npm install -g codesys-mcp-sp21-plus

That single command:

• downloads the published tarball from https://www.npmjs.com/package/codesys-mcp-sp21-plus
• installs it globally (-g) so the codesys-mcp-sp21-plus binary is on your PATH (typically %APPDATA%\npm\ on Windows)
• pulls in its 4 dependencies (@modelcontextprotocol/sdk, commander, uuid, zod) automatically

Verify the install:

codesys-mcp-sp21-plus --version
codesys-mcp-sp21-plus --detect       # lists installed CODESYS versions

Then wire it into .mcp.json per Quick Start and start Claude Code.

To upgrade later:

npm install -g codesys-mcp-sp21-plus@latest

Install from source (development / unreleased changes)

If you want to track the fork's main branch directly, contribute fixes, or pin to a specific commit instead of the published version:

git clone https://github.com/phobicdotno/Codesys-MCP-SP21-plus.git
cd Codesys-MCP-SP21-plus
npm install
npm run build
npm link

npm link registers dist/bin.js as the global codesys-mcp-sp21-plus binary, so the same .mcp.json snippet works. Edits to src/ take effect after npm run build; Python script edits hot-reload from dist/scripts/ without a rebuild.

To update later: git pull && npm install && npm run build.

If you'd rather avoid touching the global node_modules, skip npm link and reference the local checkout directly in .mcp.json:

{
  "mcpServers": {
    "codesys": {
      "command": "node",
      "args": [
        "C:\\Users\\<you>\\Codesys-MCP-SP21-plus\\dist\\bin.js",
        "--codesys-path", "C:\\Program Files\\CODESYS 3.5.22.10\\CODESYS\\Common\\CODESYS.exe",
        "--codesys-profile", "CODESYS V3.5 SP22 Patch 1",
        "--mode", "persistent"
      ]
    }
  }
}

Run without .mcp.json

You can also invoke the binary directly from a shell (useful for one-off testing or wrapping in another launcher):

codesys-mcp-sp21-plus \
  --codesys-path "C:\Program Files\CODESYS 3.5.22.10\CODESYS\Common\CODESYS.exe" \
  --codesys-profile "CODESYS V3.5 SP22 Patch 1"

Multiple CODESYS installations

The MCP server is bound to a single --codesys-path / --codesys-profile at startup. launch_codesys takes no parameters — it just starts whichever CODESYS the server was configured against. If you have several CODESYS versions installed and want to drive them all from the same Claude Code session, register one MCP server entry per install with a distinct name.

Both blocks below live in the same .mcp.json. Claude can call either by name (codesys-21 / codesys-22) and the two run as independent processes with independent CODESYS instances:

{
  "mcpServers": {
    "codesys-21": {
      "command": "codesys-mcp-sp21-plus",
      "args": [
        "--codesys-path", "C:\\Program Files\\CODESYS 3.5.21.50\\CODESYS\\Common\\CODESYS.exe",
        "--codesys-profile", "CODESYS V3.5 SP21 Patch 5",
        "--mode", "persistent"
      ]
    },
    "codesys-22": {
      "command": "codesys-mcp-sp21-plus",
      "args": [
        "--codesys-path", "C:\\Program Files\\CODESYS 3.5.22.10\\CODESYS\\Common\\CODESYS.exe",
        "--codesys-profile", "CODESYS V3.5 SP22 Patch 1",
        "--mode", "persistent"
      ]
    }
  }
}

Notes:

• The version numbers (3.5.21.50, 3.5.22.10) match the install directory names under C:\Program Files\ — these are the actual install IDs CODESYS uses, not the marketing names. The marketing name lives in --codesys-profile (e.g., CODESYS V3.5 SP21 Patch 5, CODESYS V3.5 SP22 Patch 1).
• Run codesys-mcp-sp21-plus --detect once to print every CODESYS install the server can see, with its profile name; copy the values from there into .mcp.json rather than guessing.
• Each server entry spawns its own CODESYS process when first invoked. Don't call launch_codesys on both at the same time pointing at projects that overlap — two CODESYS instances racing on the same .project file pop a "project is currently in use" modal that blocks every subsequent script.
• Adding or removing an entry requires a Claude Code restart (the MCP client only reads .mcp.json at startup).

If you have a specific .project file in mind and don't want to eyeball which install opens it, point --for-project at the file and --print-config will narrow the snippet to just the matching install (or warn and fall back to same-SP-different-patch if no exact match exists). The match is driven by the project's saved projectinspectiondata.auxiliary profile, so it works without launching CODESYS:

codesys-mcp-sp21-plus --print-config --for-project "C:\path\to\MyMachine.project"

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 (--noUI) if persistent launch fails	false
--keep-alive	Keep CODESYS running after server stops	false
--timeout <ms>	Default command timeout	60000
--detect	List installed CODESYS versions and exit	—
--print-config	Print a ready-to-paste .mcp.json snippet for every detected install and exit	—
--sp <number>	With --print-config: emit only the entry for CODESYS V3.5 SP<n>	—
--for-project <path>	With --print-config: pick only the install(s) matching the .project file at <path> (exact SP+patch, or fall back to same-SP-different-patch). Mutually exclusive with --sp.	—
--name <name>	With --print-config --sp <n>: override the MCP server entry name	—
--inspect <path>	Read a CODESYS .project offline (no CODESYS needed) and print its profile name/version + mandatory libraries; uses the unzip CLI from Git for Windows / Linux+Mac	—
--ssh-version <host>	SSH to a CODESYS Control Linux PLC and print the running project version (extracted from the boot-application binary). Bypasses CODESYS entirely. Requires SSH key auth + passwordless sudo for strings.	—
--ssh-user <name>	With --ssh-version: SSH user	karstein
--ssh-boot-app <path>	With --ssh-version: path to the boot application on the PLC	/var/opt/codesys/PlcLogic/Application/Application.app
--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.

--ssh-version — read the running PLC's project version over SSH

For CODESYS Control Linux PLCs (Raspberry Pi, IPC, etc.) the running project version can be read straight off the boot-application binary, without CODESYS being installed or the .project file being unlocked:

codesys-mcp-sp21-plus --ssh-version 192.168.1.83
codesys-mcp-sp21-plus --ssh-version myplc.lan --ssh-user pi

Requires SSH key auth + passwordless sudo for /usr/bin/strings on the PLC. If your key isn't installed yet, the error message includes a one-line PowerShell recipe; full setup instructions live at ssh-key-windows.md.

phobiCS-tui

This package ships a small ink TUI for browsing CODESYS-exported ST. After installing, run:

phobiCS-tui                          # auto-discovers mcp-mirror/ from cwd
phobiCS-tui <projectDir>             # explicit project directory
phobiCS-tui approve <a.st> <b.st>    # diff prompt; exit 0 = accept, 1 = reject, 2 = error

Browser-mode keys: j/k (or ↓/↑) move the cursor, l/Enter/→ expand a device, h/← collapse, / filter POUs by name (Enter commits, Esc clears), o open the highlighted POU in $EDITOR (or VS Code), d diff against the same-named POU in another device, r re-scan mcp-mirror/, ? toggle the help overlay, q quits.

Approve-mode keys: y accept, n/q/Esc reject, v toggle unified ↔ side-by-side diff.

The browser writes the current selection to %LOCALAPPDATA%/codesys-mcp/tui-state.json (Windows) or $XDG_STATE_HOME/codesys-mcp/tui-state.json (Linux/Mac, defaulting to ~/.local/state/...). The MCP tool get_user_selection reads it so an agent can ground its actions in what the user is looking at. The header shows mirror staleness when the on-disk export is older than 10 s, and a yellow resize warning appears below 80×20.

The Viewer applies ST syntax highlighting (cyan keywords, magenta types, gray comments, yellow strings).

Approve mode is opt-in for the MCP server's modifying tools — start the server with --approve-edits to wire it in. The v0.2 followup gates all 9 modifying tools: create_pou, create_property, create_method, create_dut, create_gvl, create_folder, delete_object, rename_object, add_library — plus the original set_pou_code. Each operation pops a y/n diff prompt; create/delete render as all-green/all-red one-sided diffs, rename as a del+add of the leaf name, and set_pou_code as a real diff against the existing mirror file. Off by default.

Inline live values (--live-values)

When the server is started with --live-values and the runtime is online, the Viewer overlays each declared variable's live value next to its declaration:

3      counter  : INT := 0;       ◀ live: 47
4      bRunning : BOOL;           ◀ live: TRUE

The server writes a 500 ms snapshot to tui-live-values.json (next to tui-state.json); the TUI polls that file and renders an overlay only when the snapshot's pou_name matches the user's current selection and the file is fresh (≤ 5 s). v0.3 covers top-level vars on the displayed POU; sub-property paths and ARRAY/STRUCT pretty-printing are deferred. Off by default.

MCP Tools

41 tools across the categories below. Tools marked NEW were added in this fork; tools marked FIXED existed upstream but were broken before this fork.

Management Tools

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

Project Tools

Tool	Description
open_project	Open an existing CODESYS project file (cross-project switch FIXED; SP-mismatch pre-flight NEW)
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) — JSON long FIXED
get_compile_messages	Retrieve last compiler messages without triggering a new build — JSON long FIXED

open_project runs an offline pre-flight (projectinspectiondata.auxiliary ZIP+XML — no CODESYS) that compares the project's saved profile against this server's --codesys-profile. Exact match proceeds silently; same-SP-different-patch proceeds with a one-line warning (CODESYS will pop its patch-difference dialog); SP mismatch refuses without opening so the project isn't dragged through a downgrade/upgrade conversion. The refusal includes a routing hint: pick a different MCP server entry or generate one with codesys-mcp-sp21-plus --print-config --for-project "<projectFilePath>". If the inspection itself fails (file missing, malformed .project, non-standard profile name), pre-flight falls through silently and the existing CODESYS open path produces its native error.

POU / Code Authoring Tools

Tool	Description
create_pou	Create a Program, Function Block, or Function
set_pou_code	Set declaration and/or implementation code (omitted-field wipe FIXED)
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 (FIXED)
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 — LoginMode signature + auto-login FIXED; NEW deviceUser/devicePassword args (or CODESYS_DEVICE_USER/CODESYS_DEVICE_PASSWORD env) pre-register credentials via ScriptOnline.set_default_credentials so the modal "Device User Login" dialog is suppressed
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); same deviceUser/devicePassword credential-injection support as connect_to_device so the Device User Login dialog can be suppressed on every download too
start_stop_application	Start or stop the PLC application
restart_runtime_ssh	NEW — SSH into a Linux PLC and restart codesyscontrol via password-fed sudo -S. After issuing systemctl restart, polls ss -tln for the runtime port (default 11740) until it actually comes up — works around systemctl is-active reporting "active" after the binary has died from license-demo expiry. Defaults match the codesys-pi.local Pi

Library Management Tools

Tool	Description
list_project_libraries	List all libraries referenced in the project with version info, plus IDE version, devices, and per-Application compiler version (FIXED — switched to ScriptLibManObjectContainer)
add_library	Add a library reference. Pre-resolves via library_manager.find_library and prefers the managed-library overload; refuses to save if the resulting reference is an unresolvable placeholder (hardened)

Symbol Configuration Tools (NEW)

Wraps ScriptSymbolConfigObject (CODESYS 3.5.10.0+). The Symbol Configuration object controls which IEC variables / FBs / methods are exposed to OPC UA, web visualisations, and other external clients. Reference: helpme-codesys.com/en/ScriptingEngine/ScriptSymbolConfigObject.html and the SP22 stub Stubs/scriptengine/ScriptSymbolConfigObject.pyi.

Tool	Description
find_symbol_config	NEW — Locate the Symbol Configuration object(s) in the project tree (one per Application typically). Read-only
list_all_signatures	NEW — Every POU / FunctionBlock / Method / Function the symbol config could potentially export. compile=true forces an application.build() first
list_all_datatypes	NEW — Every DUT / struct / enum / alias / union (same compile semantics)
list_configured_symbols	NEW — Only those signatures + datatypes actually configured for export, with each variable's configured_access / maximal_access / effective_access
get_symbol_config_settings	NEW — Read every knob: content_feature_flags (OPC UA / IncludeComments / IncludeAttributes / IncludeExecutables / etc.), attribute filter, comment filter, direct I/O access (+ obstacles), client-side layout calculator
create_symbol_config	NEW — application.create_symbol_config(...) under a chosen Application. Idempotent: no-ops with success if a symbol config already exists anywhere in the tree
set_symbol_config_settings	NEW — Partial-update of any subset of the 6 knobs. Refuses to enable direct I/O if check_effective_direct_io_access() reports obstacles
set_symbol_access	NEW — Per-variable configured_access setter (None / ReadOnly / WriteOnly / ReadWrite). Locates the signature by FQN; works on not-yet-configured variables too
set_signature_access_bulk	NEW — Set every variable in one signature to the same access in one call
export_symbol_xsd	NEW — Write the schema bytes from get_symbol_configuration_xsd() to a file (UTF-8). Useful for downstream XML validation in CI

Version Anchor + Release Pipeline (NEW)

These tools maintain a _MCP_PROJECT_VERSION GVL inside the project so the running PLC carries its source version at a known address, and orchestrate the end-to-end release flow (mirror → classify → bump → regen .md → git commit + tag + push).

Tool	Description
bump_project_version	NEW — Bump one part of the 4-part Project Information.Version (major / minor / revision / build / auto) and maintain _MCP_PROJECT_VERSION.sVersion. auto mode classifies via mirror diff vs latest v* git tag
release_project_version	NEW — One-shot release pipeline: mirror_export → classify → bump_project_version → regenerate .md docs → git add → git commit → git tag v<new> → git push --follow-tags. Dual-SHA tag annotation
read_running_version_online	NEW — Reads _MCP_PROJECT_VERSION.sVersion from the running PLC over the CODESYS online protocol (port 11740 / gateway). Caveat: requires some IEC code to reference the variable so the optimizer doesn't strip it from the online symbol table — see the tool's error message for the one-line fix.

Source Mirror (NEW)

Tool	Description
mirror_export	NEW — Walks the project tree and writes one .st file per code-bearing object into <projectDir>/mcp-mirror/, preserving the project tree as nested directories. Read-only. Foundation for the release pipeline classifier

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, SP21+ rewrite)

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 runs single-threaded on the primary thread, polling a commands/ directory and yielding to the IDE via system.delay() between polls (this fork — upstream used a background thread + system.execute_on_primary_thread() which was removed in SP21)
4. When a tool is called, the server writes a .py script + .command.json to commands/
5. The watcher detects the command, executes it directly on the primary thread, and writes results atomically to results/
6. Changes made by tools appear in the CODESYS UI in real-time
7. 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 explicitly opted in (off by default)
• CODESYS is launched with --no-auto-launch and launch_codesys hasn't been called yet

Detect Installed Versions

codesys-mcp-sp21-plus --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. The launcher will refuse to spawn a second CODESYS.exe.

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 executes commands on the primary thread and yields between polls, 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 (41 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 + helpers + 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

Credits

• Upstream project: luke-harriman/Codesys-MCP — original architecture, the persistent-watcher concept, and the bulk of the upstream tool set
• This fork: phobicdotno/Codesys-MCP-SP21-plus — Karstein Kvistad. SP21+/SP22 watcher rewrite, upstream-tool fixes, version-anchor + release pipeline, source-mirror export

License

MIT