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roslyn-codelens-mcp

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Roslyn-based MCP server giving AI agents deep semantic understanding of .NET/C# codebases — 67 tools for navigation, call graphs, diagnostics & code fixes, safe refactoring, code-quality auditing, test intelligence, DI graphs, and IL/external-assembly inspection.

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# Roslyn CodeLens MCP Server

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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](https://fronteir.ai/mcp/marcelroozekrans-roslyn-codelens-mcp).

## 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
- **find_event_subscribers** — Every += / -= site for an event symbol, with resolved handler and subscribe/unsubscribe tag
- **find_tests_for_symbol** — List xUnit/NUnit/MSTest methods that exercise a production symbol; opt-in transitive walk through helpers
- **get_test_summary** — Per-project inventory of test methods with framework, attribute kind, data-row count, location, and production symbols referenced
- **find_uncovered_symbols** — Public methods and properties no test transitively reaches; sorted by cyclomatic complexity for prioritization
- **generate_test_skeleton** — Emit a compilable test-class skeleton (as text) for a method or type. Auto-detects xUnit/NUnit/MSTest; surfaces constructor dependencies as TodoNotes; returns a suggested file path. Closes the loop with `find_uncovered_symbols`
- **get_type_hierarchy** — Walk base classes, interfaces, and derived types
- **get_di_registrations** — Scan for DI service registrations. Reads generic (`AddSingleton<IFoo, Foo>()`), single-generic, `typeof` pair and factory-lambda forms
- **get_instantiation_options** — "How do I construct this type?" in one call: constructors with full parameter detail, static factory methods declared anywhere in the solution (including on a separate factory type), DI registrations, and `required` members. Pass `fromProject` to learn whether *that* project can actually reach each option — it honours `InternalsVisibleTo`, so it answers "can my test project call this internal constructor?"
- **get_project_dependencies** — Get the project reference graph
- **get_symbol_context** — One-shot context dump for any type
- **get_public_api_surface** — Enumerate every public/protected type and member in production projects; flat, deterministically-sorted list suitable for API review or breaking-change baselines.
- **find_breaking_changes** — Diff the current API against a baseline JSON or DLL; report removed members, kind changes, and accessibility changes with Breaking/NonBreaking severity.
- **find_reflection_usage** — Detect dynamic/reflection-based usage
- **get_exception_flow** — What exceptions can escape a method: walks callees depth-bounded, propagates each throw up through every enclosing try/catch, and reports what still escapes with its propagation path; metadata callees contribute their documented exceptions
- **find_throw_sites** — Every place an exception type is thrown, optionally including derived types; rethrows flagged
- **find_catch_blocks** — Every catch for a type, optionally via base clauses; flags filtered, rethrowing, and empty (swallowing) handlers
- **find_references** — Find all references to any symbol (types, methods, properties, fields, events), each tagged with a kind (`read`, `write`, `readwrite`, `invocation`, `method_group`, `object_creation`, `cast`, `type_check`, `typeof`, `base_type`, `type_constraint`, `type_argument`, `declaration`, `attribute`, `nameof`, `xml_doc`) and reported per occurrence with a column; filter server-side with `kinds` (e.g. `["write","readwrite"]` for mutation sites)
- **go_to_definition** — Find the source file and line where a symbol is defined
- **get_method_source** — Full declaration source (XML docs, attributes, signature, body — original formatting) for one or many members by name in a single call: methods (all overloads), constructors, properties, indexers, fields, events; per-item statuses (`ok`/`notFound`/`ambiguous`/`metadata`/`unsupportedKind`) so a batch never fails wholesale
- **resolve_stack_trace** — Map a pasted .NET stack trace to file/line/symbol, undoing compiler name mangling (async/iterator state machines, lambdas, local functions, generic arity); handles inner-exception chains, log-prefixed lines, and Demystifier traces
- **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_obsolete_usage** — Every `[Obsolete]` call site grouped by deprecation message and severity, errors first; for planning migrations
- **find_circular_dependencies** — Detect cycles in project or namespace dependency graphs
- **check_architecture** — Enforce layering rules you supply (`forbid` and `allowOnly`) against the real semantic type graph rather than `using` directives; violations are grouped per boundary with a reference count and example sites
- **get_complexity_metrics** — Cyclomatic complexity, cognitive complexity and max nesting depth per member (methods, constructors, properties, indexers, operators). Cyclomatic counts paths and starts at 1; cognitive measures how hard the code is to follow and starts at 0. `metric` picks which one the threshold and sort use — cognitive is the better refactoring-priority signal, cyclomatic the better test-budget one
- **find_naming_violations** — Check .NET naming convention compliance
- **find_async_violations** — Sync-over-async, `async void` misuse, missing awaits, fire-and-forget tasks; per-violation report with severity
- **find_disposable_misuse** — `IDisposable`/`IAsyncDisposable` instances not wrapped in `using`/`await using`/returned/assigned to field; severity error/warning per violation.
- **find_large_classes** — Find oversized types by member or line count
- **find_god_objects** — Types combining high size with high cross-namespace coupling; sharper signal than raw size for SRP violations
- **find_unused_symbols** — Dead code detection via reference analysis. Auto-filters test methods (xUnit/NUnit/MSTest), MCP tool entry points, source-generator output, MEF-composed services, and interop-laid-out fields; filter counts surface in `summary.filteredOut`
- **get_project_health** — Composite audit aggregating 7 quality dimensions per project (complexity, large classes, naming, unused symbols, reflection, async violations, disposable misuse) with counts and top-N hotspots inline
- **get_source_generators** — List source generators and their output per project
- **get_generated_code** — Inspect generated source code from source generators
- **inspect_external_assembly** — Browse types, members, and XML docs from closed-source NuGet packages and referenced assemblies
- **peek_il** — Decompile any method to ilasm-style IL bytecode from closed-source or generated assemblies
- **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)
- **rename_symbol** — Solution-wide safe rename of a type or member via Roslyn's Renamer, with preview mode, conflict reporting, and a freshness check against on-disk edits
- **change_signature** — Add, remove, and reorder a method's parameters and rewrite every call site; handles named/optional arguments, `params` and extension methods, and reports the overrides and interface implementations it cascaded to
- **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)
- **load_solution** — Load an additional .sln/.slnx at runtime and make it the active solution
- **unload_solution** — Unload a loaded solution to free memory
- **rebuild_solution** — Force a full reload of the analyzed solution
- **start_background_task** — Queue a long-running tool (currently `rebuild_solution`) to run in the background; returns a `taskId` to poll
- **get_task_status** — Get the current status, result, or error of a background task by its `taskId`
- **list_running_tasks** — List background tasks running or completed within the last 5 minutes
- **trust_solution** — Authorize a solution to run Roslyn analyzers (required before `get_diagnostics` with `includeAnalyzers: true`)
- **list_trusted_paths** — Inspect the persistent trust store + session-trusted solutions
- **revoke_trust** — Revoke a previously-granted trust for a solution path
- **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_overloads** — Every overload of a method/constructor (source + metadata) with full parameter and modifier detail in one call
- **get_extension_methods** — Every extension member applicable to a type — including arrays, nullables and tuples — from the solution and referenced assemblies (so LINQ shows up), using Roslyn's own applicability rules; covers C# 14 `extension` blocks including properties and static members
- **get_operators** — Every user-defined operator and conversion operator on a type (source + metadata) with kind, signature, parameters, and source location. Includes synthesized record equality and .NET 7+ checked variants
- **get_call_graph** — Transitive caller/callee graph for a method, depth-bounded with cycle detection
- **get_file_overview** — Compound tool: types defined in a file + file-scoped diagnostics in one call

## Response shape

All list-returning tools wrap their results in a uniform envelope:

```json
{
  "items": [ ... ],
  "totalCount": 142,
  "truncated": false,
  "limit": 500,
  "summary": { ... }
}
```

When `truncated` is `true`, the items are the **top N by the tool's natural sort order** (severity-first, worst-first, by-project, etc.) — usually that's exactly what you want. Raise `limit` only if the missing tail items matter for the task.

Tools that include a `summary` aggregate today:

- `get_diagnostics` — `{ error, warning, info, hidden }` counts
- `find_references` — `{ byProject: { name: count }, byKind: { kind: count } }`
- `find_callers`, `find_attribute_usages` — `{ byProject: { name: count } }`
- `search_symbols`, `find_reflection_usage` — `{ byKind: {...} }`
- `find_unused_symbols` — `{ byKind, filteredOut: { testMethod, testContainer, mcpTool, generated, composition, interop } }`
- `find_naming_violations` — `{ byRule: {...} }`
- `get_complexity_metrics` — `{ max, avg, overThreshold, maxCognitive }` (the first three describe the selected `metric`)

Single-object tools (`get_type_overview`, `get_symbol_context`, `apply_code_action`, etc.) return their bespoke shape directly — the envelope only wraps list-returning tools.

## Error responses

When a tool can't proceed (symbol not resolved, solution not trusted, file not found, ambiguous match, etc.), the response is an `isError: true` content block carrying a structured JSON body:

```json
{
  "code": "SolutionNotTrusted",
  "message": "Solution 'Foo.sln' is not trusted for analyzer execution. ...",
  "details": { "solutionPath": "C:\\Foo.sln" }
}
```

Error codes (switch on `code` to handle each):

- `SymbolNotFound` — type / method / property could not be resolved.
- `SolutionNotTrusted` — `get_diagnostics` or `get_code_fixes` requested analyzers but the solution hasn't been authorized via `trust_solution`.
- `AmbiguousMatch` — `set_active_solution` / `unload_solution` matched multiple solutions; `details.matches` lists them.
- `FileNotFound` — file path or baseline doesn't exist (or isn't in any loaded project).
- `ProjectNotFound` — solution name didn't match any loaded solution.
- `InvalidArgument` — caller-supplied input was malformed, unsupported, or out of range.
- `Internal` — unexpected error not modeled above; `message` carries the underlying exception text.

**Cancellation:** the MCP framework's native cancellation is honored. Cancelling a `tools/call` request mid-flight terminates the operation; long-running tools (`get_diagnostics` with analyzers, `get_code_actions`, `apply_code_action`, `get_code_fixes`) check the token at hot-loop boundaries.

## External Assemblies

Metadata-origin symbols (from NuGet packages and referenced assemblies) are first-class citizens:

- **Tier 1 — Navigation** (`find_references`, `find_callers`, `find_implementations`): Accepts closed-source type and member names. Resolves them from assembly metadata and reports all source-level usage sites.
- **Tier 2 — Inspection** (`inspect_external_assembly`): Browse namespaces, types, members, and XML doc comments from any referenced assembly without decompiling.
- **Tier 3 — IL** (`peek_il`): Decompile a specific method to annotated ilasm-style IL using ICSharpCode.Decompiler — useful for understanding the internals of NuGet libraries.

Location-returning results include an `Origin` field (`source` or `metadata`) and an `IsGenerated` flag to distinguish hand-written code from closed-source or generated output.

## Runtime configuration

- `ROSLYN_CODELENS_OPEN_PROJECT_TIMEOUT_SECONDS` — per-project MSBuild load timeout (default `300`). When a project exceeds this duration during workspace open, it's recorded as a `SkippedProjects` entry with `kind: "Timeout"` and the rest of the solution still loads. Useful when a legacy or malformed project wedges the `BuildHost-net472` subprocess.

## Security: Trust Model

`get_diagnostics` and `get_code_fixes` can load Roslyn analyzers — DLLs that execute in-process. To prevent untrusted analyzers from running automatically, this server uses a VS/Rider-style trust model:

- **Solutions passed on the CLI at startup** are auto-trusted for the current session.
- **Other solutions** must be explicitly trusted via the `trust_solution` MCP tool.
- **Analyzer DLLs** must come from the user's NuGet global packages folder, the dotnet SDK install dir, or the solution's own `bin`/`obj`. Other paths are skipped.

Use the `list_trusted_paths` and `revoke_trust` tools to inspect and manage trust state. Persistent trust is stored at `%APPDATA%\roslyn-codelens\trust.json`.

See [SECURITY.md](SECURITY.md) for the full threat model.

## Quick Start

### npx (any MCP client)

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

The npm package ships no server code — it is a launcher that installs the `RoslynCodeLens.Mcp`
.NET global tool at a matching version and execs it, so the **.NET 10 SDK must be on `PATH`**.
Subsequent starts skip the install entirely and work offline.

### VS Code / Visual Studio (via dnx)

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

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

### Claude Code Plugin

```bash
claude install gh:MarcelRoozekrans/roslyn-codelens-mcp
```

### .NET Global Tool

```bash
dotnet tool install -g RoslynCodeLens.Mcp
```

Then add to your MCP client config:

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

### Docker

Runs without a .NET SDK on the host; the solution is bind-mounted at `/workspace`.

```bash
docker build -t roslyn-codelens-mcp .
docker run -i --rm -v "$PWD:/workspace" roslyn-codelens-mcp
```

The mounted solution must be restored for `MSBuildWorkspace` to resolve its
references, and tool output reports container paths rather than host paths — see
[docs/site/docs/getting-started/docker.md](docs/site/docs/getting-started/docker.md).

## Usage

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

```bash
# 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 10.0.7:

| Tool | Latency | Memory |
|------|--------:|-------:|
| `go_to_definition` | 2.1 µs | 576 B |
| `find_implementations` | 2.5 µs | 720 B |
| `get_project_dependencies` | 2.8 µs | 1.5 KB |
| `get_type_hierarchy` | 3.5 µs | 1.3 KB |
| `find_circular_dependencies` | 3.7 µs | 2.7 KB |
| `get_symbol_context` | 4.1 µs | 1.0 KB |
| `get_source_generators` | 16 µs | 23 KB |
| `analyze_data_flow` | 19 µs | 1.6 KB |
| `find_attribute_usages` | 72 µs | 904 B |
| `get_generated_code` | 78 µs | 24 KB |
| `analyze_control_flow` | 115 µs | 14 KB |
| `inspect_external_assembly` (summary) | 159 µs | 35 KB |
| `find_large_classes` | 265 µs | 3.5 KB |
| `get_di_registrations` | 478 µs | 16 KB |
| `inspect_external_assembly` (namespace) | 564 µs | 259 KB |
| `find_reflection_usage` | 705 µs | 19 KB |
| `get_complexity_metrics` | 781 µs | 25 KB |
| `get_code_actions` | 792 µs | 54 KB |
| `get_file_overview` | 797 µs | 101 KB |
| `get_diagnostics` | 822 µs | 99 KB |
| `get_nuget_dependencies` | 849 µs | 48 KB |
| `get_public_api_surface` | 885 µs | 247 KB |
| `get_type_overview` | 1.1 ms | 104 KB |
| `peek_il` | 1.1 ms | 34 KB |
| `find_disposable_misuse` | 3.5 ms | 286 KB |
| `find_uncovered_symbols` | 3.8 ms | 224 KB |
| `search_symbols` | 3.9 ms | 557 KB |
| `analyze_method` | 5.8 ms | 333 KB |
| `find_async_violations` | 7.0 ms | 335 KB |
| `find_tests_for_symbol` (direct) | 8.4 ms | 396 KB |
| `find_callers` | 10 ms | 337 KB |
| `find_tests_for_symbol` (transitive) | 12 ms | 399 KB |
| `find_naming_violations` | 15 ms | 788 KB |
| `find_unused_symbols` | 23 ms | 1.0 MB |
| `find_references` | 28 ms | 1013 KB |
| `analyze_change_impact` | 33 ms | 1.3 MB |
| Solution loading (one-time) | ~4.1 s | 16 MB |

## 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

## Project compatibility

The server analyses every project that MSBuildWorkspace can load under the .NET SDK runtime.

**Supported:** SDK-style projects (`<Project Sdk="...">`), any target framework — `net48`, `net6.0`, `net8.0`, `net10.0`, etc. .NET Framework targets work fine *as long as the csproj uses the SDK-style format*.

**Skipped (with a warning, not a crash):** legacy non-SDK-style projects (`<Project ToolsVersion="..." xmlns="http://schemas.microsoft.com/developer/msbuild/2003">`, typically `.NET Framework` projects authored in older versions of Visual Studio). These rely on `Microsoft.Common.props` imports from the .NET Framework MSBuild that ships with Visual Studio, which is not available in the .NET SDK MSBuild runtime.

When a solution contains legacy projects, the server:

1. Loads every SDK-style project normally — all tools work for those.
2. Skips each legacy project and records it in `LoadedSolution.SkippedProjects`.
3. Surfaces the skipped list via `list_solutions` (the `SkippedProjects` array on each `SolutionInfo`) and in the return message of `load_solution`. Each entry includes the project name, kind (`Legacy`), and reason.

**To analyse a legacy project,** convert it to SDK-style format (see [Microsoft's migration guide](https://learn.microsoft.com/en-us/dotnet/core/porting/project-structure)) or open the solution from a Visual Studio Developer Command Prompt so the full Visual Studio MSBuild is on `PATH`.

## Development

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

## Third-party licenses

- [ICSharpCode.Decompiler](https://github.com/icsharpcode/ILSpy) — MIT license (v8+). Used for IL disassembly in the `peek_il` tool.

## License

MIT

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