Roslyn Lens

RoslynLens

[](https://github.com/jfmeyers/roslyn-lens) [](https://www.nuget.org/packages/RoslynLens/) [](https://sonarcloud.io/summary/newcode?id=jfmeyersroslyn-lens) [](LICENSE)

A token-efficient MCP (Model Context Protocol) server for .NET codebase navigation, powered by Roslyn semantic analysis. Designed for use with Claude Code.

Instead of reading entire .cs files (500-2000+ tokens each), Claude Code queries this MCP server and receives focused, semantic results (30-150 tokens). This dramatically reduces token consumption when working with large .NET solutions.

Features

34 Navigation & Analysis Tools

| Tool | Purpose | | ---- | ------- | | find_symbol | Locate type/method definitions by name (supports glob patterns) | | find_references | Find all usages of a symbol | | find_implementations | Find interface implementors and derived classes | | find_callers | Find direct callers of a method | | find_overrides | Find overrides of virtual/abstract methods | | find_dead_code | Detect unused types, methods, and properties (with filters) | | find_isolated_symbols | Detect degree-0 types: no incoming refs and no solution-type deps | | find_god_nodes | Identify types/methods with disproportionately high in-degree | | find_surprising_dependencies | Rank unexpected cross-namespace edges by surprise score | | get_communities | Partition namespaces into cohesive communities (label propagation) | | get_type_hierarchy | Show inheritance chain, interfaces, and derived types | | get_public_api | Get public surface without reading the full file | | get_symbol_detail | Full signature, parameters, return type, and XML docs | | get_project_graph | Solution dependency tree (with filtering for large solutions) | | get_dependency_graph | Method call chain visualization | | get_diagnostics | Compiler warnings and errors (opt-in Roslynator analyzers) | | get_test_coverage_map | Heuristic test coverage mapping | | get_complexity_metrics | Cyclomatic, cognitive complexity, nesting depth, LOC | | detect_antipatterns | 18 anti-pattern detectors | | detect_circular_dependencies | Project and type-level cycle detection | | detect_duplicates | Structurally similar code detection via AST fingerprinting | | get_module_depends_on | [DependsOn] attribute graph (modular monoliths) | | validate_conventions | Convention violation checker | | analyze_method | Compound: signature + callers + dependencies + complexity | | get_type_overview | Compound: public API + hierarchy + implementations + diagnostics | | get_file_overview | Compound: types + diagnostics + anti-patterns for a file | | analyze_data_flow | Variable assignments, reads, writes, captured variables | | analyze_control_flow | Reachability, return points, exit points | | find_symbols_batch | Resolve multiple symbol names in one call | | get_public_api_batch | Get public API of multiple types in one call | | get_symbol_detail_batch | Get details of multiple symbols in one call | | resolve_external_source | Resolve NuGet/framework source via SourceLink or decompilation | | list_solutions | List all discovered .sln/.slnx files with active flag | | switch_solution | Switch workspace to a different discovered solution at runtime |

18 Anti-Pattern Detectors

General .NET detectors:

| ID | Detector | Description | | -- | -------- | ----------- | | AP001 | AsyncVoidDetector | async void methods (except event handlers) | | AP002 | SyncOverAsyncDetector | .Result, .Wait(), .GetAwaiter().GetResult() | | AP003 | HttpClientInstantiationDetector | new HttpClient() instead of IHttpClientFactory | | AP004 | DateTimeDirectUseDetector | DateTime.Now/UtcNow instead of TimeProvider | | AP005 | BroadCatchDetector | catch (Exception) without re-throw | | AP006 | LoggingInterpolationDetector | String interpolation in log calls | | AP007 | PragmaWithoutRestoreDetector | #pragma warning disable without matching restore | | AP008 | MissingCancellationTokenDetector | Async methods missing CancellationToken parameter | | AP009 | EfCoreNoTrackingDetector | EF Core queries without .AsNoTracking() |

Domain-specific detectors:

| ID | Detector | Description | | -- | -------- | ----------- | | GR-GUID | GuidNewGuidDetector | Guid.NewGuid() instead of IGuidGenerator | | GR-SECRET | HardcodedSecretDetector | Hardcoded passwords, connection strings, API keys | | GR-SYNC-EF | SynchronousSaveChangesDetector | SaveChanges() instead of SaveChangesAsync() | | GR-BADREQ | TypedResultsBadRequestDetector | TypedResults.BadRequest() instead of Problem() | | GR-REGEX | NewRegexDetector | new Regex() instead of [GeneratedRegex] | | GR-SLEEP | ThreadSleepDetector | Thread.Sleep() in production code | | GR-CONSOLE | ConsoleWriteDetector | Console.Write/WriteLine instead of ILogger | | GR-CFGAWAIT | MissingConfigureAwaitDetector | Missing ConfigureAwait(false) in library code | | GR-DTO | DtoSuffixDetector | Classes/records with *Dto suffix |

Optional Roslynator analyzers

Beyond the curated detectors above, get_diagnostics can optionally run the bundled Roslynator analyzers (500+ rules) by passing includeAnalyzers: true. They ship with the tool and load on demand, so the default path stays compiler-only and fast. See [docs/tools/project.md](docs/tools/project.md) for details.

Requirements

• .NET 10 SDK or later (to run the tool itself)
• A .NET solution (.sln or .slnx)

RoslynLens runs on .NET 10, but analyzes any C# project regardless of its target framework (.NET Framework 4.x, .NET Core, .NET 5–10). If your solution's global.json pins a specific SDK, that SDK must also be installed so MSBuild can restore and load it.

Installation

dotnet tool install --global RoslynLens

Claude Desktop (one-click bundle)

RoslynLens ships an MCP Bundle manifest in [mcpb/](mcpb/manifest.json). Pack it and drag the .mcpb into Claude Desktop → Settings → Extensions — no manual config editing. The bundle picks your solution path and tuning through a settings UI and launches the server via dnx (so the .NET 10 SDK is still required):

npx @anthropic-ai/mcpb pack mcpb roslyn-lens.mcpb

See [docs/getting-started/installation.md](docs/getting-started/installation.md).

Docker

A container image bundles the .NET SDK (required because MSBuildWorkspace evaluates MSBuild at runtime). Mount the repository to analyse at /workspace and keep stdin open (-i) so the server can speak MCP over stdio:

docker build -t roslyn-lens .
docker run --rm -i -v "$PWD":/workspace roslyn-lens

Usage

With Claude Code (recommended)

Add to your Claude Code MCP configuration:

# User-scoped (available in all projects)
claude mcp add --scope user --transport stdio roslyn-lens -- roslyn-lens

# Project-scoped
claude mcp add --transport stdio roslyn-lens -- roslyn-lens

Or via .mcp.json in your project root:

{
  "mcpServers": {
    "roslyn-lens": {
      "type": "stdio",
      "command": "roslyn-lens",
      "args": []
    }
  }
}

Solution Discovery

The server automatically finds the nearest .sln or .slnx file using BFS from the current directory (max 3 levels up). You can also specify a solution explicitly:

roslyn-lens --solution /path/to/MySolution.slnx

When multiple solutions are discovered, the server auto-selects the shallowest one and logs a warning. Use list_solutions and switch_solution to change at runtime without restarting.

Standalone

roslyn-lens

The server communicates over stdio using the MCP protocol.

Building from Source

git clone https://github.com/jfmeyers/roslyn-lens.git
cd roslyn-lens
dotnet build
dotnet test

Pack and install locally

dotnet pack -c Release -o ./nupkgs
dotnet tool install --global --add-source ./nupkgs RoslynLens

Token efficiency

RoslynLens exists to let an agent navigate a codebase by structure instead of re-reading source. A reproducible benchmark quantifies the saving: for every single-type source file it compares the full file against the real get_public_api response, counting both with the GPT-4o (o200k_base) tokenizer whose vocabulary ships embedded in the package — so the run is fully offline.

dotnet run --project benchmarks/RoslynLens.TokenBenchmark -c Release

Self-hosted on this repository it reports a 77% pooled / 73% median token reduction. Results and a per-type table are written to [docs/BENCHMARKS.md](docs/BENCHMARKS.md).

Configuration

Environment variables for runtime tuning:

| Variable | Purpose | Default | | -------- | ------- | ------- | | ROSLYN_LENS_TIMEOUT_SECONDS | Operation timeout | 30 | | ROSLYN_LENS_MAX_RESULTS | Maximum results per query | 100 | | ROSLYN_LENS_CACHE_SIZE | LRU compilation cache size | 20 | | ROSLYN_LENS_LOG_LEVEL | Log verbosity (Trace/Debug/Information/Warning/Error) | Information |

Architecture

src/RoslynLens/
├── Program.cs                  # Host + MCP stdio transport
├── SolutionDiscovery.cs        # BFS .sln/.slnx auto-discovery
├── WorkspaceManager.cs         # MSBuildWorkspace, LRU compilation cache
├── WorkspaceInitializer.cs     # Background solution loading
├── SymbolResolver.cs           # Cross-project symbol resolution + fuzzy FQN
├── RoslynLensConfig.cs          # Environment variable configuration
├── WorkspaceState.cs           # Workspace loading state enum
├── ComplexityAnalyzer.cs       # Cyclomatic/cognitive complexity metrics
├── DuplicateCodeDetector.cs    # AST fingerprinting for duplicate detection
├── ExternalSourceResolver.cs   # SourceLink + decompilation for NuGet deps
├── Tools/                      # 34 MCP tool implementations
├── Analyzers/                  # 18 anti-pattern detectors
└── Responses/                  # Token-optimized DTOs

Key design decisions:

• Lazy compilation: Solutions with 50+ projects only compile

on-demand (LRU cache of 50 compilations)

• File watcher: .cs changes trigger incremental text updates;

.csproj changes trigger full reload

• Background loading: Solution loads asynchronously; tools return

"loading" status until ready

• Logs to stderr: All logging goes to stderr to keep stdout clean

for JSON-RPC

Acknowledgments

Inspired by CWM.RoslynNavigator by Mukesh Murugan (MIT License). Adapted with additional detectors, auto-discovery, and global tool distribution.

Documentation

Full documentation is available in the [docs/](docs/README.md) directory and online at **** — an Astro site ([docs-website/](docs-website/)) generated from those same markdown files and deployed to GitHub Pages.

Author

JF Meyers — GitHub

License

[Apache License 2.0](LICENSE)

Source & license

This open-source MCP server is cataloged on AgentStack and links to its original source — we do not rehost the code.

• Author: jfmeyers
• Source: jfmeyers/roslyn-lens
• License: Apache-2.0
• Homepage: https://jfmeyers.github.io/roslyn-lens/

Install and usage instructions live in the source repository linked above.