Roslyn CodeLens MCP

This is a Roslyn-based MCP server that provides semantic code intelligence for .NET codebases.

MCP Server

The .mcp.json configures the roslyn-codelens MCP server to analyze this solution. It runs via dotnet run and connects over stdio.

Skill

The plugins/roslyn-codelens/skills/roslyn-codelens/SKILL.md skill teaches Claude when and how to use the 22 code intelligence tools. Use the MCP tools instead of Grep/Glob for any .NET semantic queries (finding implementations, callers, references, diagnostics, etc.).

Project Structure

• src/RoslynCodeLens/ — MCP server (entry point: Program.cs)
• tests/RoslynCodeLens.Tests/ — Unit tests (xUnit)
• benchmarks/RoslynCodeLens.Benchmarks/ — BenchmarkDotNet performance tests
• plugins/roslyn-codelens/ — Claude Code skill definition

Development

• Build: dotnet build
• Test: dotnet test
• Benchmarks: dotnet run --project benchmarks/RoslynCodeLens.Benchmarks -c Release

Roslyn CodeLens MCP Server

A Roslyn-based MCP server that gives AI agents deep semantic understanding of .NET codebases — type hierarchies, call graphs, DI registrations, diagnostics, refactoring, and more.

Hosted deployment

A hosted deployment is available on Fronteir AI.

Features

• find_implementations — Find all classes/structs implementing an interface or extending a class
• find_callers — Find every call site for a method, property, or constructor
• get_type_hierarchy — Walk base classes, interfaces, and derived types
• get_di_registrations — Scan for DI service registrations
• get_project_dependencies — Get the project reference graph
• get_symbol_context — One-shot context dump for any type
• find_reflection_usage — Detect dynamic/reflection-based usage
• find_references — Find all references to any symbol (types, methods, properties, fields, events)
• go_to_definition — Find the source file and line where a symbol is defined
• get_diagnostics — List compiler errors, warnings, and Roslyn analyzer diagnostics
• get_code_fixes — Get available code fixes with structured text edits for any diagnostic
• search_symbols — Fuzzy workspace symbol search by name
• get_nuget_dependencies — List NuGet package references per project
• find_attribute_usages — Find types and members decorated with a specific attribute
• find_circular_dependencies — Detect cycles in project or namespace dependency graphs
• get_complexity_metrics — Cyclomatic complexity analysis per method
• find_naming_violations — Check .NET naming convention compliance
• find_large_classes — Find oversized types by member or line count
• find_unused_symbols — Dead code detection via reference analysis
• get_source_generators — List source generators and their output per project
• get_generated_code — Inspect generated source code from source generators
• get_code_actions — Discover available refactorings and fixes at any position (extract method, rename, inline variable, and more)
• apply_code_action — Execute any Roslyn refactoring by title, with preview mode (returns a diff before writing to disk)
• list_solutions — List all loaded solutions and which one is currently active
• set_active_solution — Switch the active solution by partial name (all subsequent tools operate on it)
• rebuild_solution — Force a full reload of the analyzed solution
• analyze_data_flow — Variable read/write/capture analysis within a statement range (declared, read, written, always assigned, captured, flows in/out)
• analyze_control_flow — Branch/loop reachability analysis within a statement range (start/end reachability, return statements, exit points)
• analyze_change_impact — Show all files, projects, and call sites affected by changing a symbol — combines find_references and find_callers
• get_type_overview — Compound tool: type context + hierarchy + file diagnostics in one call
• analyze_method — Compound tool: method signature + callers + outgoing calls in one call
• get_file_overview — Compound tool: types defined in a file + file-scoped diagnostics in one call

Quick Start

VS Code / Visual Studio (via dnx)

Add to your MCP settings (.vscode/mcp.json or VS settings):

{
  "servers": {
    "roslyn-codelens": {
      "type": "stdio",
      "command": "dnx",
      "args": ["RoslynCodeLens.Mcp", "--yes"]
    }
  }
}

Claude Code Plugin

claude install gh:MarcelRoozekrans/roslyn-codelens-mcp

.NET Global Tool

dotnet tool install -g RoslynCodeLens.Mcp

Then add to your MCP client config:

{
  "mcpServers": {
    "roslyn-codelens": {
      "command": "roslyn-codelens-mcp",
      "args": [],
      "transport": "stdio"
    }
  }
}

Usage

The server automatically discovers .sln files by walking up from the current directory. You can also pass one or more solution paths directly:

# Single solution
roslyn-codelens-mcp /path/to/MySolution.sln

# Multiple solutions — switch between them with set_active_solution
roslyn-codelens-mcp /path/to/A.sln /path/to/B.sln

When multiple solutions are loaded, use list_solutions to see what's available and set_active_solution("B") to switch context. The first path is active by default.

Performance

All type lookups use pre-built reverse inheritance maps, member indexes, and attribute indexes for O(1) access. Benchmarked on an i9-12900HK with .NET 10.0.4:

Hot Reload

The server watches .cs, .csproj, .props, and .targets files for changes. When a change is detected, affected projects are lazily re-compiled on the next tool query — only stale projects and their downstream dependents are re-compiled, not the full solution.

Location-returning tools include an IsGenerated flag to distinguish source-generator output from hand-written code.

Requirements

• .NET 10 SDK
• A .NET solution with compilable projects

Development

dotnet build
dotnet test
dotnet run --project benchmarks/RoslynCodeLens.Benchmarks -c Release

License

MIT