lgx-mcp

Version 1.0.1. Verified on FactoryTalk Logix Echo only; not verified on a physical controller. See docs/verified-on.md.

MCP server for Allen-Bradley/Logix PLCs. Static .L5X analysis, live tag access over CIP (EtherNet/IP), and write permissions derived from your own project file — never typed into a config. Open a project, and lgx-mcp reads your tag names, UDT member descriptions, safety-tag prefixes, and rung neutral-text to build a write-gate policy: every tag is classified as granted or denied, and every decision carries a provenance string naming the exact L5X element that justified it.

“Open this project and tell me what an agent would be allowed to write.”

“Read MainRoutine and check whether every rung that sets Motor_Run also checks Safety_OK.”

What it is

The SDK lane. With your own licensed Logix Designer and Logix Designer SDK, an agent can propose online rung edits in Logix Designer while you watch. An edit moves through a pending-edit lifecycle: propose, then pending on the controller, then test, then assemble. Before anything reaches the controller, lgx-mcp can run a build check on an offline copy of your project, so undefined tags show up first. The engineer decides when to test and when to assemble: by default the agent only proposes. The lane is optional, Windows-only, and off unless you build and provision it; see Logix Designer SDK lane. It is verified on Echo only.

The rest of lgx-mcp needs no Rockwell software:

• Static analysis — open a .L5X file (or, in the SDK lane, an .ACD project) and ask about tags, data types, routines, cross-references, and diffs between two project versions, with no controller required.
• Derived write-gates — the open project is the single source of truth for what may be written: Cmd_* and Sim_* tags are writable; logic-written, Safety_-prefixed and undesignated tags are denied. Every decision traces to a specific L5X element. A partial export derives no gate; see the safety model.
• Live tags over CIP — read and poll tags against a real controller using libplctag, license-free.
• Static interlock proof — check whether every rung that writes a given output tag also carries a named condition contact, and get back the exact rung(s) that do not.

What’s verified, and on what

The capabilities listed in docs/verified-on.md were verified against FactoryTalk Logix Echo — Rockwell’s software emulator — and none of them has been verified against a physical controller. Rockwell’s own documentation is explicit that emulated controller behavior can differ from physical hardware. Re-verify anything you rely on against your own physical controller before depending on it.

Requirements

• Static analysis + CIP tag access: Windows, macOS, or Linux; the .NET