AutoCAD Plant 3D MCP Server — native P&ID

MCP (Model Context Protocol) server that lets Claude — in Claude Code, Claude Desktop or any MCP client — build, check and renumber real AutoCAD Plant 3D P&IDs: equipment, process and signal lines, line groups, off-page connectors, tags and labels created through the Plant 3D .NET API (so they pass validation and carry connectivity), plus read-only checks straight on the project database. It also keeps the general AutoCAD tools (drawing, entities, layers, blocks, annotation, screenshots) and the read-only Plant 3D 3D-piping queries.

By Eduardo Mariola — Chemical Engineering, UNESP (Brazil).

Why this exists

My plant design course took two semesters: a benzyl alcohol plant, from patent search to process simulation, economic evaluation and drawings. The P&IDs had to be delivered in AutoCAD Plant 3D: 11 sheets and about 2,400 engineering items.

Drawing them through generic AutoCAD automation does not work — Plant 3D's validation rejects raw geometry and nothing gets connected. So the work went through the Plant 3D API instead, with a .NET plugin, and through a lot of trial and error that ended up in about 150 automation scripts and a set of hard-won rules: in what order a sheet must be built, where exactly a line must end to connect, why property writes silently fail, how to label, how not to crash AutoCAD. The visual standard — one sheet per control volume, numbering by area, orthogonal lines, aligned equipment — came from studying finished projects of previous years and my own finished P&ID.

This repository consolidates that work, done in Claude Code sessions on my own machine, into two MCP tools (pid_native, pid_qa), a plugin with 27 new commands and a field guide (docs/pid-native.md).

Em português. Os P&IDs da planta de álcool benzílico (11 folhas, ~2.400 itens) tinham de sair no Plant 3D, e desenhar por automação genérica do AutoCAD não serve: a validação recusa e nada fica conectado. O caminho foi a API do Plant 3D, um plugin .NET e uns 150 scripts, além das regras aprendidas no erro (ordem de montagem, onde a linha tem de terminar, por que a gravação de propriedade falha em silêncio, rótulos, quedas do AutoCAD). O padrão visual veio de projetos prontos de anos anteriores e do meu P&ID finalizado. Este repositório junta tudo isso.

Related MCP server: AutoCAD MCP Server

Features

• Native P&ID authoring (pid_native): insert equipment, instruments and valves by style; process and signal lines with real connections (and automatic nozzles); line groups; off-page connectors; tag properties; tag labels; register drawings in the project — always in the drawing you name, never in "whatever is active"

• Session control: start Plant 3D, open a drawing, NETLOAD the plugin and dismiss the dialogs that silently block COM (unsigned DLL, Drawing Recovery, Error Report)

• P&ID checks (pid_qa), read-only on ProcessPower.dcf: duplicate and unregistered tags, orphan objects, line list from line groups, control loops from the signal-line graph, connectivity (free line ends, unpaired off-page connectors), picklists

• Sheet audit from the live drawing: diagonal segments, labels over lines or symbols, overlapping symbols, crossings, glued parallel runs, alignment, raw AutoCAD entities, open line ends and the equipment each one leads to

• Orthogonal router for new lines (straight, L, Z, then a grid search with elbow cost)

• Renumbering plans for valves, line groups and instrument loops, in two passes and grouped by sheet, ready for set_props

• AutoCAD tools: drawing, entity, layer, block, annotation, view/screenshot (File IPC with AutoCAD, or headless DXF with ezdxf)

• Plant 3D 3D-piping queries (plant3d): line list, BOM, valves, instruments, welds, bolts and gaskets, pipe length, spec validation, equipment, exports

Requirements

• Windows 10/11

• AutoCAD Plant 3D 2027 (2026 works by building the plugin for .NET 8, see below)

• Python 3.10 or higher, with uv (or pip)

• .NET 10 SDK to build the plugin (dotnet --version)

Installation

1. Clone this repository:

git clone https://github.com/Eduardo-Mariola/autocad-plant3d-mcp.git
cd autocad-plant3d-mcp

2. Install the Python server:

uv sync

3. Build the Plant 3D plugin:

cd plant3d-plugin
dotnet build -c Release

For AutoCAD Plant 3D 2026: dotnet build -c Release -p:AcadDir="C:\Program Files\Autodesk\AutoCAD 2026" -p:TargetFramework=net8.0-windows

The DLL lands in plant3d-plugin\bin\Release\PlantMcpDispatch.dll; pid_native start loads it.

Configuration

Claude Code

claude mcp add --scope user autocad -- "C:\path\to\autocad-plant3d-mcp\.venv\Scripts\python.exe" -m autocad_mcp

Claude Desktop

Add to %APPDATA%\Claude\claude_desktop_config.json:

{
  "mcpServers": {
    "autocad": {
      "command": "C:\\path\\to\\autocad-plant3d-mcp\\.venv\\Scripts\\python.exe",
      "args": ["-m", "autocad_mcp"],
      "env": { "AUTOCAD_MCP_BACKEND": "auto" }
    }
  }
}

Environment variables:

Important notes:

• Use the full absolute path to the virtual environment's Python

• Restart Claude Desktop completely after editing the file

• Keep the Plant 3D window maximized while tools run: a minimized window rejects COM calls

Usage

Running the server

uv run python -m autocad_mcp

Available tools

pid_native — native P&ID authoring (operations)

• Session: start, netload, open_drawing, dismiss_dialogs, quit

• Project: ping, project_info, open_project, add_drawing, remove_drawing, save

• Reading: dwg_assets, asset_info, opc_status, check_annotation, locate, unisolate, pnid_probe

• Building: add_asset, add_control_valve, new_line_group, add_line, add_line_in_group, assign_groups, add_offpage, opc_connect, set_props, set_vertices, move_entity, erase, own_asset, clone_blocks, add_annotation, call (any plugin command)

pid_qa — checks, routing and plans (read-only)

• Database: summary, duplicate_tags, orphans, line_groups, control_loops, connectivity, picklists, sheet_rowids

• Sheet: audit_sheet, label_collisions, open_ends, route

• Plans: plan_valve_numbers, plan_line_numbers, plan_instrument_loops

General AutoCAD and Plant 3D tools

• drawing, entity, layer, block, annotation, view, system — AutoCAD through File IPC (or headless DXF)

• plant3d — read-only 3D-piping queries (list_lines, bom, list_valves, weld_list, pipe_length, validate_specs, list_equipment, export, …)

• specgen — Plant 3D specs/catalogs from a piping-class spreadsheet

• pnid — line list extracted from P&IDs in PDF

• pid — schematic P&ID symbols for AutoCAD LT (CTO library)

Example data

dwg_assets answer (shortened):

{
  "drawing": "P&ID - VC4.dwg",
  "assets": [
    {"rowid": 25422, "class_name": "HorizontalCentrifugalPump", "tag": "P-401B",
     "style": "Horizontal Centrifugal Pump Style", "handle": "11B9",
     "position": [3.296, 4.607], "rotation": 0, "min": [3.081, 4.350], "max": [3.938, 4.950]}
  ],
  "lines": [
    {"rowid": 25530, "class_name": "Primary", "style": "Primary Style",
     "handle": "11F2", "vertices": [[3.938, 4.607], [6.500, 4.607]]}
  ]
}

ProcessPower.dcf tables used by pid_qa:

PnPDrawings            PnPID, "Dwg Name"
PnPDataLinks           RowId, DwgId, RowClassName, DwgHandleLow
PnPRowRelations        ROWID, RELID, RelationshipTypeName   (PipeLineGroupRelationship, LineStartAsset, ...)
Instrumentation        PnPID, Tag, Area, Type, LoopNumber
HandValves             PnPID, Tag, Code, Number
PipeLineGroup          PnPID, LineNumber, Service, NominalSize, NominalSpec
PipeLines / SignalLines  process and signal segments
LineStartAsset / LineEndAsset / LineInlineAsset / LineNozzle / LineOffPageConnector
ConnectorsRelationship off-page connector pairs

Plugin request and answer (C:/temp):

autocad_mcp_plant_cmd_<id>.json     {"request_id", "command", "params", "ts"}
autocad_mcp_plant_result_<id>.json  {"request_id", "ok", "payload", "error"}

Usage examples

Start Plant 3D with the plugin

pid_native(operation="start", data={"drawing": "C:/Projects/Plant/PID DWG/P&ID - VC10.dwg"})
pid_native(operation="project_info", data={"doc": "VC10.dwg"})

Build part of a sheet, in the right order

# 1. equipment
pid_native(operation="add_asset", data={"doc": "VC10.dwg", "style": "Dome Roof Tank Style",
           "items": [{"at": [6.0, 8.0]}, {"at": [12.0, 8.0]}]})
# 3. process line ending exactly on the outline; nozzles created where missing
pid_native(operation="add_line", data={"doc": "VC10.dwg", "style": "Primary Style",
           "nozzle": True, "items": [{"vertices": [[3.0, 6.2], [6.0, 6.2]]}]})
# 4. inline valve on the existing line
pid_native(operation="add_asset", data={"doc": "VC10.dwg", "style": "Gate Valve Style",
           "items": [{"at": [4.5, 6.2], "scale": 0.25}]})
pid_native(operation="save", data={"doc": "VC10.dwg"})

Check the project and one sheet

pid_qa(operation="duplicate_tags", data={"project": "C:/Projects/Plant"})
pid_qa(operation="connectivity", data={"project": "C:/Projects/Plant", "sheet": "VC10"})
pid_qa(operation="audit_sheet", data={"doc": "VC10.dwg"})
pid_qa(operation="open_ends", data={"doc": "VC10.dwg", "project": "C:/Projects/Plant"})
pid_qa(operation="route", data={"doc": "VC10.dwg", "from": [3.2, 14.0], "to": [9.8, 6.5]})

Renumber hand valves by area, safely

plan = pid_qa(operation="plan_valve_numbers", data={"project": "C:/Projects/Plant",
              "areas": {"VC1": 1, "VC2": 2, "VC3": 3}})
# for each sheet: pass1 items, save, pass2 items, save
pid_native(operation="set_props", data={"doc": "VC3.dwg", "items": [...]})
pid_native(operation="save", data={"doc": "VC3.dwg"})

Lessons learned

The short version of docs/pid-native.md:

• Build in order: equipment → off-page connectors → process lines → inline components → bubbles → signal lines → tags → labels.

• A line connects only if its end is exactly on the component outline (or a nozzle). The API never connects line to line: branches need a tee.

• Writing a property needs the owning sheet open and targeted, an unconditional save, and a real change of value (two passes) — otherwise it reports success and persists nothing.

• Never move or scale symbols outside the API: their connection points stay behind.

• Save every few items and back up .dwg and .dcf together: the database is written live, the drawing only on save.

Development

uv sync --extra dev
uv run pytest tests/pid_native      # native P&ID tests (synthetic project database, no AutoCAD)
uv run pytest                       # whole suite

Project layout:

src/autocad_mcp/
  pid_native/          native P&ID: ipc, session, dcf, geometry, routing, numbering, tools
  server.py            MCP tools
  plant3d_query.py     read-only 3D-piping queries
  backends/            File IPC (AutoCAD) and ezdxf (headless)
plant3d-plugin/src/    .NET plugin (MCPPLANTDISPATCH): PidOps, OpcOps, UtilOps, AnnotOps, ...
docs/pid-native.md     field guide

Roadmap, in Portuguese: FERRAMENTAS_PROPOSTAS.md — what is implemented and what is still proposed.

Credits

Built on the MIT-licensed autocadplant3d-mcp (itself based on autocad-mcp); their notice is kept in LICENSE.

License

MIT