OmniSharp MCP Server

A Model Context Protocol (MCP) server that provides C# language intelligence to Claude Code CLI by wrapping the OmniSharp language server.

What It Does

This MCP server gives Claude Code full C# IDE capabilities:

Find Symbols - Search for classes, methods, properties by name
Go to Definition - Jump to where a symbol is defined
Find References - Find all usages of a symbol across the codebase
Find Implementations - Find classes implementing an interface
Get Type Info - Get type information and documentation
Get Diagnostics - Get compiler errors and warnings
Code Completion - Get autocomplete suggestions
Signature Help - Get method parameter hints
Rename Preview - Preview what a rename would change
Decompiled Source - View decompiled .NET framework code

Prerequisites

.NET SDK 9.0+ installed
Claude Code CLI installed
A C# solution file (.sln)

Installation

1. Clone and Build

git clone https://github.com/your-repo/omnisharp-mcp.git
cd omnisharp-mcp
dotnet publish src/OmniSharpMCP/OmniSharpMCP.csproj -c Release -o publish

2. Configure Claude Code

Create and add the MCP server to your Claude Code configuration.

Which is needed Per-project

A. Create .mcp.json in your project root:

{
  "mcpServers": {
    "csharp": {
      "type": "stdio",
      "command": "dotnet",
      "args": ["/path/to/omnisharp-mcp/publish/OmniSharpMCP.dll"],
      "env": {
        "OMNISHARP_SOLUTION": "/path/to/your/project.sln"
      }
    }
  }
}

B. Edit the .claude/mcp.json file args and env values.

Example edited mcp.json file in my project:

{
  "mcpServers": {
    "csharp": {
      "type": "stdio",
      "command": "dotnet",
      "args": ["/Users/omersomekhbeachbum/dev/omnisharp-mcp/publish/OmniSharpMCP.dll"],
      "env": {
        "OMNISHARP_SOLUTION": "/Users/omersomekhbeachbum/dev/rummystars-client/rummystars-client.sln"
      }
    }
  }
}

3. Enable the MCP Server

Create (if not already created) and edit ~/.claude/settings.local.json:

{
  "enabledMcpjsonServers": ["csharp"],
  "enableAllProjectMcpServers": true
}

4. Restart Claude Code

The mcp will be able to start whenever all claude instances are closed.

Start claude again:

claude

Verify the server is connected:

/mcp

You should see csharp in the list of connected servers.

Enable claude to use omnisharp-mcp tool

This is done out of the box! No additional config here is needed. Whenever claude instance is running - he has all the tools, and you dont need to specify for his what tools to use. Based on the prompt the claude instance will decide what tools will serve him the best to get to the wanted result.

First Run

On first run, the MCP server will:

Start immediately (so Claude Code can connect)
Download OmniSharp HTTP server (~50MB) to ~/.omnisharp-mcp/omnisharp/
Start OmniSharp and load your solution in the background

Note: The first startup takes 2-3 minutes while OmniSharp loads your solution. Tools will return errors during this time. Subsequent startups are faster if OmniSharp is already running.

Warming Up OmniSharp (Optional)

To avoid waiting for OmniSharp to load, you can pre-start it:

./warmup-omnisharp.sh

This script:

Starts OmniSharp if not already running
Waits until it's fully loaded
Keeps it running for instant tool availability

Auto-Start on macOS Login

Create ~/Library/LaunchAgents/com.omnisharp.mcp.plist:

<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE plist PUBLIC "-//Apple//DTD PLIST 1.0//EN" "http://www.apple.com/DTDs/PropertyList-1.0.dtd">
<plist version="1.0">
<dict>
    <key>Label</key>
    <string>com.omnisharp.mcp</string>
    <key>ProgramArguments</key>
    <array>
        <string>/usr/local/share/dotnet/dotnet</string>
        <string>/Users/YOUR_USERNAME/.omnisharp-mcp/omnisharp/OmniSharp.dll</string>
        <string>-s</string>
        <string>/path/to/your/project.sln</string>
        <string>-p</string>
        <string>2050</string>
    </array>
    <key>EnvironmentVariables</key>
    <dict>
        <key>DOTNET_ROLL_FORWARD</key>
        <string>LatestMajor</string>
    </dict>
    <key>RunAtLoad</key>
    <true/>
    <key>KeepAlive</key>
    <true/>
    <key>StandardOutPath</key>
    <string>/tmp/omnisharp.log</string>
    <key>StandardErrorPath</key>
    <string>/tmp/omnisharp.log</string>
</dict>
</plist>

Load it:

launchctl load ~/Library/LaunchAgents/com.omnisharp.mcp.plist

Available Tools

Once connected, Claude Code has access to these tools:

| Tool | Description | |------|-------------| | find_symbols | Search for symbols by name pattern | | go_to_definition | Navigate to symbol definition | | find_references | Find all references to a symbol | | find_implementations | Find interface implementations | | get_type_info | Get type and documentation info | | get_diagnostics | Get compiler errors/warnings for a file | | get_completions | Get code completion suggestions | | get_signature_help | Get method signature help | | get_file_members | Get outline of a file | | get_workspace_info | Get solution/project information | | preview_rename | Preview rename changes | | get_decompiled_source | Get decompiled metadata source |

Architecture

Claude Code CLI
      |
      | (stdio - JSON-RPC)
      v
OmniSharp MCP Server (this project)
      |
      | (HTTP - REST API)
      v
OmniSharp HTTP Server (port 2050)
      |
      | (Roslyn)
      v
Your C# Solution

The MCP server acts as a bridge:

Receives tool calls from Claude Code via stdio
Translates them to HTTP requests to OmniSharp
Returns formatted responses back to Claude

Configuration

Environment Variables

| Variable | Description | Default | |----------|-------------|---------| | OMNISHARP_SOLUTION | Path to your .sln file | Required | | OMNISHARP_PORT | OmniSharp HTTP port | 2050 | | OMNISHARP_PATH | Custom OmniSharp DLL path | Auto-download |

Command Line Arguments

dotnet OmniSharpMCP.dll --solution /path/to/solution.sln --port 2050

Troubleshooting

MCP server fails to connect

Check /mcp in Claude Code to see server status
Verify the path in mcp.json is correct
Ensure settings.local.json has "csharp" in enabledMcpjsonServers

Tools return errors

OmniSharp may still be loading. Check status:

curl -X POST http://localhost:2050/checkreadystatus -d '{}'

If it returns {"Ready":false}, wait for OmniSharp to finish loading.

OmniSharp won't start

Check the log:

tail -f /tmp/omnisharp.log

Common issues:

Solution file not found
.NET SDK not installed
Port 2050 already in use

Symbol search returns empty

OmniSharp needs time to index. Large solutions (like Unity projects) can take 2-3 minutes.

Reset OmniSharp

# Kill existing processes
pkill -f "omnisharp/OmniSharp.dll"

# Delete downloaded OmniSharp (will re-download on next start)
rm -rf ~/.omnisharp-mcp/omnisharp

Development

Building from Source

cd omnisharp-mcp
dotnet build src/OmniSharpMCP/OmniSharpMCP.csproj

Running Locally

OMNISHARP_SOLUTION="/path/to/solution.sln" dotnet run --project src/OmniSharpMCP/OmniSharpMCP.csproj

Testing OmniSharp Directly

# Find symbols
curl -X POST http://localhost:2050/findsymbols \
  -H "Content-Type: application/json" \
  -d '{"Filter": "MyClass"}'

# Check ready status
curl -X POST http://localhost:2050/checkreadystatus -d '{}'

License

MIT

MCP Servers