The Tool Registry

The Tool Registry is the part of the Revit add-in that discovers, loads, and invokes C# tools at runtime. It reads tool packages from the filesystem, compiles a catalogue of every available tool, and hands that catalogue to the MCP bridge so Claude can call them.

This page explains how packages are structured, what the schema files look like, and where to put tools depending on whether they are built-in or written by you.

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How Discovery Works

When Revit starts, ToolRegistry scans two folder trees and merges the results into a single tool catalogue:

1. {PluginDir}\Tools\Packages\          ← built-in tools (ship with the plugin)
2. %LOCALAPPDATA%\RevitClaudeConnector\
       {RevitMajor}\Tools\Packages\     ← custom / user-installed tools

Both roots are scanned in order. If a tool name appears in both, the custom tool wins — this lets you override a built-in tool without modifying the plugin itself.

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Package Folder Structure

Every tool lives inside a package — a named folder that groups related tools, their compiled DLL, and their schema files.

Tools\
└── Packages\
    └── MyPackage\                 ← package folder (any name)
        ├── manifest.json          ← declares which tools are in this package
        ├── runners\
        │   └── Tools.dll          ← compiled C# assembly containing the tools
        └── schemas\
            └── my_tool.schema.json  ← MCP schema for a single tool (optional)

There is no versions/ subfolder and no current.txt file. The package folder is the active version. The manifest.json may record a version string as metadata, but the folder layout does not depend on it.

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manifest.json

manifest.json sits at the root of every package and lists every tool the package provides.

{
  "version": "1.0.0",
  "tools": [
    {
      "name": "get_elements_by_category",
      "runner": {
        "assembly": "runners/Tools.dll",
        "type": "Tools.GetElementsByCategory",
        "method": "Execute"
      },
      "schema": "schemas/get_elements_by_category.schema.json"
    }
  ]
}

Field	Required	Description
version	No	Informational version string — not used for folder routing
tools[].name	Yes	Unique tool name used by Claude to call the tool
tools[].runner.assembly	Yes	Path to the DLL, relative to the package folder
tools[].runner.type	Yes	Fully-qualified C# class name inside the DLL
tools[].runner.method	No	Method name to call (default: Execute)
tools[].schema	No	Path to the MCP schema file, relative to the package folder

Forward slashes in paths are normalised to the platform separator automatically.

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Tool Schema Files

Each *.schema.json file describes one tool to Claude: what the tool does and what arguments it accepts. The format follows the MCP tool schema convention.

{
  "name": "get_elements_by_category",
  "description": "Returns all element IDs in the active model that belong to the given Revit category name.",
  "input_schema": {
    "type": "object",
    "properties": {
      "categoryName": {
        "type": "string",
        "description": "The built-in Revit category name, e.g. \"Walls\" or \"Doors\"."
      }
    },
    "required": ["categoryName"]
  }
}

Field	Description
name	Canonical tool name (defaults to tools[].name from the manifest if omitted)
description	What the tool does — Claude reads this to decide when to call it. Write clearly.
input_schema	JSON Schema object describing the arguments the tool accepts

Description quality matters

Claude uses the description field to decide which tool fits the user's request. A vague description like "Gets elements" is much less useful than "Returns all element IDs whose Revit category matches the given name".

If a tool has no schema file, the registry registers it with a permissive schema that accepts any object. Claude can still call it, but without a description it will rarely know when to do so.

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The Tool Entry Point

Every tool class must expose a public Execute method (or the name you set in method) with this signature:

public string Execute(UIApplication uiapp, UIDocument uidoc, string requestJson)

• uiapp — the Revit UIApplication; always provided.
• uidoc — the active UIDocument; may be null if no document is open.
• requestJson — the arguments from Claude, serialised as a JSON string.
• Return value — a JSON string. Claude receives this as the tool's result.

The method can be instance or static. If it is an instance method, the registry creates a new instance of the class for each call via Activator.CreateInstance.