Roslyn CodeLens MCP Server

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Roslyn-based semantic .NET/C# intelligence for AI agents

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Category: Dev Tools #20 of 438
Stars: ★ 65 Popular
Transport: stdio (local process)
Runtime: .NET 10+ · Node.js · Docker
Credentials: No credential needed
License: MIT
Last commit: 4 days ago
Tools: 67

68FMRS · C

A very comprehensive Roslyn semantic-analysis MCP server with 67 tools covering navigation, refactoring, test intelligence, quality auditing, and IL inspection. Performance is benchmarked, and a VS-style trust model guards analyzer execution. Works only with SDK-style .NET projects and requires the .NET 10 SDK. Maintained by an individual developer, not an official .NET/Roslyn product.

Strongest · Documentation 18/20 Weakest · Reliability 9/20

Reliability

9/20

Security and permissions

14/20

Maintenance

14/20

Documentation

18/20

Setup experience

13/20

Why each score

Reliability 9/20: The README describes a consistent response envelope, structured error codes (SymbolNotFound, SolutionNotTrusted, etc.), and detailed behavior for all 67 tools, implying a mature error-handling design; it also references a CI badge and `dotnet test` commands. However, this is a purely static review — no actual CI workflow file or test suite content was available to verify these claims, nor can tool descriptions be confirmed against real behavior. Per the static calibration rule, reliability cannot exceed 12 without verifiable execution evidence (real CI plus committed tests covering key paths), so a low-to-mid score is given given the evidence is text-only.
Security and permissions 14/20: The server implements a VS/Rider-style trust model: analyzer execution requires explicit `trust_solution` (or auto-trust only for solutions passed on the CLI at startup), analyzer DLLs are restricted to the NuGet global packages folder, SDK install dir, or the solution's own bin/obj, and `list_trusted_paths`/`revoke_trust` allow inspection and revocation. Write-capable operations (`apply_code_action`, `rename_symbol`, `change_signature`) all offer preview/diff modes before touching disk, showing confirmation discipline for risky actions. However, analyzers still run in-process without sandboxing, and outbound network behavior is not further documented. This does not reach red-line severity but also falls short of full least-privilege coverage across all scenarios.
Maintenance 14/20: The repo is not archived, is MIT licensed, has 42 stars and only 1 open issue, and the version number (2.18.0) suggests substantial iterative releases. The README includes a build-status badge, a dedicated docs site, and a GitHub Sponsors link, indicating some governance and ownership. However, no concrete commit history, release dates, or issue-response times were available in the reviewed material, so update cadence and dependency maintenance cannot be confirmed, precluding a top score.
Documentation 18/20: The README is very comprehensive: it lists all tools with one-line descriptions, documents the uniform response envelope, structured error codes, the three-tier external-assembly model, runtime configuration, a security trust model (with a link to SECURITY.md), quick-start configs for multiple clients (npx/dnx/Docker/dotnet tool/Claude Code plugin), a performance benchmark table, hot-reload behavior, and explicit project-compatibility limitations (how legacy projects are skipped). Docs are layered, well-exemplified, and disclose limitations clearly. A small deduction applies because the 67 tools only get one-line descriptions without per-tool parameter-level JSON schema documentation.
Setup experience 13/20: Four-plus install/connection paths are provided (npx, dnx, `dotnet tool install`, Docker, Claude Code plugin) with concrete client configuration JSON for each, which is documented very clearly. Per the static calibration rule, setup cannot exceed 15 without verifiable execution evidence; actual installation also depends on having the .NET 10 SDK on PATH, the npm package is merely a launcher that installs a .NET global tool, and the `dnx` mechanism is relatively new and potentially fragile — steps are clear but the path remains somewhat manual/fragile.

Static review · not runListed 2026-09-07

Read the FMRS scoring method →

Fit and risk

What it can accessReads local filesWrites / deletes local filesRuns commands or code

Best for

.NET/C# developers using MCP clients like Claude Code or VS Code
AI-assisted development workflows needing deep static analysis and safe refactoring
Teams maintaining large SDK-style .NET solutions

Not for

Non-.NET/C# codebases
Legacy non-SDK-style csproj projects (skipped with a warning)
Environments without the .NET 10 SDK (unless using Docker)

Required permissions

Reads and analyzes .NET solutions and source files on disk
Tools like apply_code_action can write/modify source files (preview mode available)
Running Roslyn analyzer DLLs requires explicit trust via trust_solution

Risks and side effects

Analyzer DLLs execute in-process; the trust model restricts them to NuGet global packages, the dotnet SDK dir, or solution bin/obj, but grant trust carefully
apply_code_action, rename_symbol, and change_signature modify source files directly; prefer preview mode first
The stdio server has read/write access to the loaded codebase
Solutions must be NuGet-restored before loading or reference resolution may fail

Setup

Before you start

Runtime：.NET 10+ · Node.js · Docker

Other optional settings (1)

ROSLYN_CODELENS_OPEN_PROJECT_TIMEOUT_SECONDS optional Per-project MSBuild load timeout in seconds (default 300); projects exceeding it are skipped and reported in SkippedProjects. Set before starting the server.

npm: add "mcpServers": {"roslyn-codelens": {"type": "stdio", "command": "npx", "args": ["-y", "roslyn-codelens-mcp"]}} to your MCP client config; the .NET 10 SDK must be on PATH. NuGet: run dotnet tool install -g RoslynCodeLens.Mcp then configure command roslyn-codelens-mcp, or use dnx RoslynCodeLens.Mcp --yes. Claude Code: claude install gh:MarcelRoozekrans/roslyn-codelens-mcp. Docker is also supported, bind-mounting the solution at /workspace (solution must be restored).

.mcp.json

{"mcpServers":{"roslyn-codelens":{"type":"stdio","command":"npx","args":["-y","roslyn-codelens-mcp"]}}}

Shown for Claude Code. Other clients may use a different file or key (VS Code uses "servers") — the configurator below converts it.

.vscode/mcp.json

{
  "servers": {
    "roslyn-codelens": {
      "type": "stdio",
      "command": "npx",
      "args": [
        "-y",
        "roslyn-codelens-mcp"
      ]
    }
  }
}

Goes in your project's .vscode/mcp.json (VS Code uses a "servers" key).

Terminal

claude mcp add roslyn-codelens -- npx -y roslyn-codelens-mcp

Run it in a terminal; replace any <…> placeholders with your own values first.

Generate and validate client config

Check that it works

Connect the client and confirm the 67 tools (e.g. go_to_definition, get_diagnostics, trust_solution) appear in the tool list; then load a .sln and call list_solutions — seeing the loaded solution proves the connection works.

Troubleshooting

Verify the .NET 10 SDK is installed and on PATH (required by the npx launcher)
Solutions must be compilable SDK-style projects; legacy projects are skipped and listed in SkippedProjects
If diagnostics with analyzers return SolutionNotTrusted, authorize the solution via trust_solution
Project load timeouts (default 300s) are recorded as SkippedProjects; adjust ROSLYN_CODELENS_OPEN_PROJECT_TIMEOUT_SECONDS
Docker output reports container paths, and the mounted solution must be restored
If a symbol is not found, confirm the solution is loaded and active via list_solutions

Things to try

Once connected, you can ask your AI assistant things like:

Find all classes implementing IFoo and show the file each one lives in.
Analyze the exception flow of OrderService.Submit: which exceptions can escape this method?
Rename this method to CalculateTotal and show me a preview diff before writing anything.
Which public methods in my solution have no test coverage? Sort them by cyclomatic complexity.

Tools 67

find_implementations read-only

Find all classes/structs implementing an interface or extending a class

find_callers read-only

Find every call site for a method, property, or constructor

find_event_subscribers read-only

Find every += / -= site for an event symbol with resolved handler

find_tests_for_symbol read-only

List xUnit/NUnit/MSTest methods exercising a production symbol, with optional transitive walk

get_test_summary read-only

Per-project inventory of test methods with framework, attributes, data rows, and locations

find_uncovered_symbols read-only

Find public methods/properties no test reaches, sorted by cyclomatic complexity

generate_test_skeleton read-only

Emit a compilable test-class skeleton (as text) with auto-detected test framework

get_type_hierarchy read-only

Walk base classes, interfaces, and derived types

Show 59 more tools

get_di_registrations read-only

Scan for DI service registrations (generic, typeof-pair, and factory-lambda forms)

get_instantiation_options read-only

Answer 'how do I construct this type' with constructors, factories, DI registrations, and required members

get_project_dependencies read-only

Get the project reference graph

get_symbol_context read-only

One-shot context dump for any type

get_public_api_surface read-only

Enumerate every public/protected type and member in production projects

find_breaking_changes read-only

Diff the current API against a baseline JSON or DLL and report breaking changes

find_reflection_usage read-only

Detect dynamic/reflection-based usage

get_exception_flow read-only

Report exceptions that escape a method with their propagation paths

find_throw_sites read-only

Find every site an exception type is thrown, optionally including derived types

find_catch_blocks read-only

Find every catch for a type, flagging filtered, rethrowing, and swallowing handlers

find_references read-only

Find all references to a symbol, tagged with kinds like read/write/invocation

go_to_definition read-only

Find the source file and line where a symbol is defined

get_method_source read-only

Fetch full declaration source for one or many members by name in a single call

resolve_stack_trace read-only

Map a pasted .NET stack trace to file/line/symbol, undoing compiler name mangling

get_diagnostics read-only

List compiler errors, warnings, and Roslyn analyzer diagnostics

get_code_fixes read-only

Get available code fixes with structured text edits for any diagnostic

search_symbols read-only

Fuzzy workspace symbol search by name

get_nuget_dependencies read-only

List NuGet package references per project

find_attribute_usages read-only

Find types and members decorated with a specific attribute

find_obsolete_usage read-only

List every [Obsolete] call site grouped by deprecation message and severity

find_circular_dependencies read-only

Detect cycles in project or namespace dependency graphs

check_architecture read-only

Enforce layering rules against the real semantic type graph

get_complexity_metrics read-only

Report cyclomatic complexity, cognitive complexity, and max nesting depth per member

find_naming_violations read-only

Check .NET naming convention compliance

find_async_violations read-only

Detect sync-over-async, async void misuse, missing awaits, and fire-and-forget tasks

find_disposable_misuse read-only

Detect IDisposable/IAsyncDisposable instances not wrapped in using/await using

find_large_classes read-only

Find oversized types by member or line count

find_god_objects read-only

Find types combining high size with high cross-namespace coupling

find_unused_symbols read-only

Dead code detection via reference analysis with automatic filtering of tests and generated code

get_project_health read-only

Composite audit aggregating 7 quality dimensions per project

get_source_generators read-only

List source generators and their output per project

get_generated_code read-only

Inspect generated source code from source generators

inspect_external_assembly read-only

Browse types, members, and XML docs from closed-source assemblies

peek_il read-only

Decompile any method to ilasm-style IL bytecode

get_code_actions read-only

Discover available refactorings and fixes at any position

apply_code_action writes

Execute any Roslyn refactoring by title, with preview mode

rename_symbol writes

Solution-wide safe rename via Roslyn's Renamer with preview and conflict reporting

change_signature writes

Add, remove, and reorder method parameters and rewrite every call site

list_solutions read-only

List all loaded solutions and which one is currently active

set_active_solution writes

Switch the active solution by partial name

load_solution writes

Load an additional .sln/.slnx at runtime and make it active

unload_solution writes

Unload a loaded solution to free memory

rebuild_solution writes

Force a full reload of the analyzed solution

start_background_task writes

Queue a long-running tool to run in the background

get_task_status read-only

Get status, result, or error of a background task by taskId

list_running_tasks read-only

List background tasks running or completed within the last 5 minutes

trust_solution writes

Authorize a solution to run Roslyn analyzers

list_trusted_paths read-only

Inspect the persistent trust store and session-trusted solutions

revoke_trust writes

Revoke previously-granted trust for a solution path

analyze_data_flow read-only

Variable read/write/capture analysis within a statement range

analyze_control_flow read-only

Branch/loop reachability analysis within a statement range

analyze_change_impact read-only

Show all files, projects, and call sites affected by changing a symbol

get_type_overview read-only

Compound tool: type context + hierarchy + file diagnostics in one call

analyze_method read-only

Compound tool: method signature + callers + outgoing calls in one call

get_overloads read-only

Every overload of a method/constructor with full parameter detail

get_extension_methods read-only

Every extension member applicable to a type, including C# 14 extension blocks

get_operators read-only

Every user-defined and conversion operator on a type

get_call_graph read-only

Transitive caller/callee graph with depth bound and cycle detection

get_file_overview read-only

Compound tool: types in a file + file-scoped diagnostics in one call

Use cases

Give AI agents semantic navigation and call-graph analysis of .NET codebases

Perform safe solution-wide renames and signature changes

Audit code quality: complexity, naming, async misuse, dead code

Find untested public symbols via test intelligence and generate test skeletons

Inspect types and IL of closed-source NuGet dependencies

Supported clients

Claude Code

VS Code

Visual Studio

Listed from the project's documentation, not tested by this site.

Overview

Roslyn CodeLens is a Roslyn-based MCP server that gives AI agents deep semantic understanding of .NET/C# codebases over stdio. It exposes 67 tools for navigation, call graphs, diagnostics and code fixes, safe refactoring, code-quality auditing, test intelligence, DI graphs, and IL/external-assembly inspection. Installable via npm (npx launcher), NuGet (.NET global tool), Claude Code plugin, or Docker; requires the .NET 10 SDK.

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Source revision 45f6b21fdd17 Data synced 2026-10-19 Read the FMRS scoring method