Testing with MCP Inspector

Before packaging your server into a .mcpb and installing it in Claude Desktop, you should test it in isolation. MCP Inspector is an official tool from Anthropic that gives you a visual interface to send MCP messages to your server and see the raw responses — exactly what Claude sees when it talks to the server.

This page shows you how to set it up and use it to verify that your Revit connector is working correctly.

Why Test with MCP Inspector?

When something doesn't work inside Claude Desktop, the failure could be in:

The MCP server (index.js)
The connection to the Revit add-in (HTTP bridge)
The tool itself (C# logic)
Claude's interpretation of the tool description

MCP Inspector lets you eliminate the first two immediately. If tools/list returns tools and tools/call returns sensible results in the Inspector, then the MCP server and HTTP bridge are working. Any remaining problem is in Claude Desktop or the C# tool.

Without Inspector:   Claude Desktop → MCP Server → Revit  (3 things to debug)
With Inspector:      Inspector      → MCP Server → Revit  (isolates the MCP layer)

Prerequisites

Node.js ≥ 18 installed
The Revit add-in running inside Revit (so the HTTP bridge on 127.0.0.1:5578 is live)
The server/index.js source file accessible

Launching MCP Inspector

MCP Inspector ships as an npm package and can be run directly with npx — no install needed.

From the repository root, point it at your server entry point:

bash

npx @modelcontextprotocol/inspector node src/Revit.Claude.Connector.Mcp.Server/server/index.js

You will see output like:

Starting MCP inspector...
Proxy server listening on port 6277
Visit http://localhost:6174 to use the inspector

Open http://localhost:6174 in your browser. You will see the Inspector UI with a connection panel on the left and a message area on the right.

Running with a custom base URL

If your Revit add-in is listening on a different port, pass it as an argument:

bash

npx @modelcontextprotocol/inspector node src/Revit.Claude.Connector.Mcp.Server/server/index.js -- --base=http://127.0.0.1:9000

The -- separator is required — everything after it is passed to your server, not to the Inspector.

The Inspector UI

The interface has three main panels:

┌─────────────────────────┬──────────────────────────────────────────┐
│  Connection             │  Messages                                │
│  ─────────────          │  ──────────                              │
│  Status: Connected      │  ← jsonrpc responses appear here        │
│                         │                                          │
│  [Initialize]           │                                          │
│  [List Tools]           │                                          │
│  [Call Tool ▾]          │                                          │
│                         │                                          │
└─────────────────────────┴──────────────────────────────────────────┘

Connection panel — shows whether the Inspector is connected to your server and provides action buttons.
Messages panel — shows the raw JSON-RPC request/response pairs, exactly as exchanged over stdio.

Step 1 — Initialize

Click Connect (or Send Initialize). The Inspector sends:

json

{
  "jsonrpc": "2.0",
  "id": 1,
  "method": "initialize",
  "params": {
    "protocolVersion": "2025-06-18",
    "capabilities": {},
    "clientInfo": { "name": "mcp-inspector", "version": "0.1.0" }
  }
}

Your server should respond with:

json

{
  "jsonrpc": "2.0",
  "id": 1,
  "result": {
    "protocolVersion": "2025-06-18",
    "capabilities": { "tools": {} },
    "serverInfo": { "name": "revit-bridge-js", "version": "1.0.0" },
    "sessionId": "a3f29c..."
  }
}

What to check:

capabilities.tools is present — confirms the server declared tool support.
serverInfo.name is "revit-bridge-js" — confirms the right server started.

If you see a connection error instead, the server process failed to start. Check the terminal where you ran npx for error output.

Step 2 — List Tools

Click List Tools. The Inspector sends:

json

{ "jsonrpc": "2.0", "id": 2, "method": "tools/list", "params": {} }

If Revit is running with the add-in loaded, you will get back the full array of available tools:

json

{
  "jsonrpc": "2.0",
  "id": 2,
  "result": {
    "tools": [
      {
        "name": "GetAllUsedFamilies",
        "description": "Returns all families loaded in the active Revit model.",
        "inputSchema": {
          "type": "object",
          "properties": {},
          "required": []
        }
      },
      {
        "name": "GetElementsByCategory",
        "description": "Returns elements belonging to a given Revit category.",
        "inputSchema": {
          "type": "object",
          "properties": {
            "categoryName": {
              "type": "string",
              "description": "The Revit category name, e.g. 'Walls' or 'Doors'."
            }
          },
          "required": ["categoryName"]
        }
      }
    ]
  }
}

What to check:

The tool list is not empty.
Each tool has a name, description, and inputSchema.
The description fields are clear — remember, Claude reads these to decide which tool to call.

If the list is empty: Revit is reachable but returned an empty array. The add-in may not have loaded any tools from disk yet. Check that the tools folder path is correct in the add-in settings.

If you get an HTTP error: The server started but could not reach 127.0.0.1:5578. Revit may not be running, or the add-in failed to start. Revit shows a startup dialog ("RevitClaudeBridge started on 127.0.0.1:5578") when the bridge is active — check whether that appeared.

Step 3 — Call a Tool

Select a tool from the dropdown (populated automatically after tools/list), fill in any required arguments, and click Call Tool.

For example, calling GetElementsByCategory with categoryName = "Walls":

Inspector sends:

json

{
  "jsonrpc": "2.0",
  "id": 3,
  "method": "tools/call",
  "params": {
    "name": "GetElementsByCategory",
    "arguments": { "categoryName": "Walls" }
  }
}

Expected response:

json

{
  "jsonrpc": "2.0",
  "id": 3,
  "result": {
    "content": [
      {
        "type": "text",
        "text": "{\"elements\": [{\"id\": 123456, \"name\": \"Basic Wall\", ...}]}"
      }
    ]
  }
}

What to check:

result.content[0].type is "text" — the correct content type for this server.
result.content[0].text is valid JSON from the Revit add-in.
The data looks correct for the tool you called.

If you get { "error": "No active document" }: Revit is running and the bridge is up, but no model is open. Open a Revit project file and try again.

If you get { "error": "Unknown tool" }: The tool name in the call doesn't match any loaded tool. Double-check the name in the tools/list response.

Reading the Raw Message Log

The Messages panel shows every exchange in chronological order. This is the most useful debugging feature — you can see exactly what the server received and what it sent back.

Each row shows:

Direction (→ outgoing to server, ← incoming from server)
Timestamp
The full JSON, expandable

When filing a bug report or asking for help, copy the raw messages from here — they contain all the information needed to diagnose the problem.

Testing Without Revit

You can run MCP Inspector against the server even when Revit is not running. The server will start and respond to initialize normally. When you call tools/list, it will attempt to reach 127.0.0.1:5578, fail with a connection refused error, and return an MCP error response:

json

{
  "jsonrpc": "2.0",
  "id": 2,
  "error": {
    "code": -32000,
    "message": "connect ECONNREFUSED 127.0.0.1:5578"
  }
}

This confirms the MCP server itself is healthy — the error is expected and comes from the HTTP layer, not from index.js. Once you start Revit, the same tools/list call will return results without restarting the Inspector.

Quick Reference

What you want to verify	Inspector action	Expected result
Server starts at all	Run `npx @modelcontextprotocol/inspector ...`	No crash in terminal
MCP handshake works	Initialize	`capabilities.tools` in response
Revit bridge is reachable	List Tools	Non-empty tools array
A specific tool runs	Call Tool	JSON result in `content[0].text`
Error handling works	Call Tool with bad args	MCP error with descriptive message

Next Step

Once tools/list returns your tools and tools/call returns sensible results, you are ready to package the server for distribution.

→ Building the .mcpb Extension

Testing with MCP Inspector ​

Why Test with MCP Inspector? ​

Prerequisites ​

Launching MCP Inspector ​

The Inspector UI ​

Step 1 — Initialize ​

Step 2 — List Tools ​

Step 3 — Call a Tool ​

Reading the Raw Message Log ​

Testing Without Revit ​

Quick Reference ​

Next Step ​

Testing with MCP Inspector

Why Test with MCP Inspector?

Prerequisites

Launching MCP Inspector

The Inspector UI

Step 1 — Initialize

Step 2 — List Tools

Step 3 — Call a Tool

Reading the Raw Message Log

Testing Without Revit

Quick Reference

Next Step