Roslyn-Backed MCP Server

Local-first MCP (Model Context Protocol) server for semantic C# analysis, navigation, validation, and refactoring on real .sln / .slnx / .csproj workspaces. It uses Roslyn and MSBuildWorkspace, runs over stdio, and does not require Visual Studio.

Correct package ID: Darylmcd.RoslynMcp  ·  CLI: roslynmcp  ·  Plugin: roslyn-mcp@roslyn-mcp-marketplace

What It Does

• Loads real C# solutions and projects with session-scoped workspaceIds.
• Exposes semantic navigation, diagnostics, build/test helpers, and preview/apply refactoring workflows over MCP.
• Ships as a .NET global tool, a Claude Code plugin, and a source-buildable stdio host.
• Publishes the authoritative live surface through server_info and roslyn://server/catalog.

Why Roslyn-Backed MCP

• No Visual Studio dependency — runs anywhere the .NET SDK runs (Windows, macOS, Linux, containers, CI).
• Production ops discipline — repeatable CI mirror (just ci), release verification scripts, and documented update paths for the global tool and release-pinned Claude Code plugin.
• Safe install defaults — no ${user_config.*} placeholder substitution that breaks prompt-skipping install flows; the server starts with compiled-in defaults and accepts literal overrides via project-scope .mcp.json.
• Authoritative live surface — every release publishes server_info + roslyn://server/catalog so clients can discover the exact tool/resource/prompt set and support tier (stable vs experimental) without guessing.
• Preview → apply discipline — refactoring tools issue preview tokens with TTLs and a verify step before mutating the workspace, so agents can dry-run multi-file edits.
• Three install paths — pick one: global tool (dotnet tool install), zero-install via dnx (.NET 10), or the Claude Code plugin.

Quick Start

Prerequisites

• .NET 10 SDK — source builds use the 10.0.400 floor in global.json (rollForward: latestFeature); published install paths retain their own package/runtime requirements

Option A — Install As A Global Tool

dotnet tool install -g Darylmcd.RoslynMcp

• Package ID: Darylmcd.RoslynMcp
• CLI command: roslynmcp
• Updates: dotnet tool update -g Darylmcd.RoslynMcp

Configure a filesystem boundary before first use. The global tool ships a binary, not a config — you write your own .mcp.json, and an unset boundary is fail-closed: every path-taking tool (workspace_load, edits, symbol lookups) rejects its input, and solution discovery returns nothing. Set ROSLYNMCP_SANCTIONED_ROOTS in your client config:

{
  "mcpServers": {
    "roslyn": {
      "type": "stdio",
      "command": "roslynmcp",
      "env": { "ROSLYNMCP_SANCTIONED_ROOTS": "." }
    }
  }
}

. resolves against the server process's working directory, which your MCP client chooses — use an absolute path if you want the boundary pinned regardless of how the server is launched. See Configuration. The Claude Code plugin and Desktop extension ship this default already; only hand-written configs need it.

Option B — Zero-Install Via dnx (.NET 10)

dnx is the .NET SDK's npx-equivalent: it resolves a tool package from NuGet on demand, without installing a global shim. Requires .NET 10 SDK 10.0.100 or later (dnx ships with the SDK).

One-shot smoke test:

dnx Darylmcd.RoslynMcp

The process should start and then appear to hang — that's expected; it's an MCP server waiting for protocol messages on stdin. dnx is noninteractive unless --interactive is explicitly requested, so MCP hosts need no consent flag.

.mcp.json snippet:

{
  "mcpServers": {
    "roslyn": {
      "type": "stdio",
      "command": "dnx",
      "args": [
        "Darylmcd.RoslynMcp",
        "--source",
        "https://api.nuget.org/v3/index.json"
      ]
    }
  }
}

Trade-offs vs. the global tool:

• ✅ No PATH pollution; no manual install step.
• ✅ Each cold start resolves to the latest version unless pinned (no dotnet tool update step).
• ⚠️ Cold-start cost on first invocation while the package downloads.
• ⚠️ For reproducible setups, pin the version in the package token: Darylmcd.RoslynMcp@<version>.

A copy-paste config also lives at docs/mcp-json-examples/dnx.mcp.json.

Option C — Claude Code Plugin

/plugin marketplace add darylmcd/Roslyn-Backed-MCP
/plugin install roslyn-mcp@roslyn-mcp-marketplace

The plugin bundles 32 skills and safety hooks, then launches the exact release-matched Darylmcd.RoslynMcp package through dnx; it does not require a global roslynmcp shim. The first launch requires NuGet access unless the package is already cached. For packaging, reinstall, and local plugin-dev details, see docs/setup.md and docs/reinstall.md.

Build And Run From Source

dotnet build RoslynMcp.slnx --nologo
dotnet test RoslynMcp.slnx --nologo
dotnet run --project src/RoslynMcp.Host.Stdio

Per-Client Config

The JSON shape is the same across MCP clients — only the file path differs. Drop one of the docs/mcp-json-examples/ snippets into the right location for your client:

Minimal config (works with Option A — global tool):

{
  "mcpServers": {
    "roslyn": {
      "type": "stdio",
      "command": "roslynmcp"
    }
  }
}

For the Option B (dnx) form, use the snippet from the previous section or copy docs/mcp-json-examples/dnx.mcp.json.

For NDJSON framing, handshake order, and minimal Python/C# client examples, see docs/stdio-client-integration.md.

Health Check

After installing and wiring up .mcp.json, paste this single prompt into your MCP client to verify the server is reachable and report its surface:

Call server_info and read the roslyn://server/catalog resource. Report back:

1. The server name and version.
2. The total tool / resource / prompt counts and their stable-vs-experimental split.
3. Whether any workspaces are currently loaded (and their IDs if so).
4. Any warnings or degraded-state flags.

If both calls succeed and the version matches what you installed, the install is healthy.

MCP Registry

The server is published to the official MCP Registry under the name io.github.darylmcd/roslyn-mcp. MCP-Registry-aware clients — and the downstream catalogs that mirror the registry (the GitHub MCP Registry, the VS Code and Visual Studio MCP galleries, and aggregators) — can discover and install the server by name.

Manifest: .claude-plugin/server.json — name io.github.darylmcd/roslyn-mcp, NuGet package Darylmcd.RoslynMcp, runtime dnx. Every release tag republishes it automatically via the publish-nuget workflow using GitHub OIDC.

You can also install directly without a registry-aware client via the Global Tool or Claude Code Plugin paths above.

Configuration

The server starts with built-in operational defaults. File-path access is the exception: configure ROSLYNMCP_SANCTIONED_ROOTS explicitly (usually . in a project-scope .mcp.json). Multiple roots use the platform path separator (; on Windows, : on macOS/Linux). An empty root list fails closed; see Setup. Other ROSLYNMCP_* values remain optional literal env overrides.

Copy-ready examples live in docs/mcp-json-examples/README.md. The full runtime/config surface is documented in ai_docs/runtime.md.

For log destinations, verbosity controls, correlation identifiers, and health probes, see the stdio observability contract.

Security

Loading a solution or project executes MSBuild evaluation. Treat workspaces as trusted code unless you run the server inside a sandbox, container, or VM.

• Only load repos you trust.
• Use isolation for untrusted workspaces.
• Path validation is defense in depth, not a substitute for trusting the loaded project graph.

The filesystem boundary is server-owned. ROSLYNMCP_SANCTIONED_ROOTS is configured by you, the operator — not by the connecting client. A client's MCP Roots can only narrow that boundary; they can never widen it or act as the sole authority. This is deliberate: the control exists to constrain the agent, so a model that is confused or prompt-injected into reading outside your project cannot do so, even if it asks. Sibling-worktree widening needs two independent opt-ins — the server operator setting ROSLYNMCP_ALLOW_ROOT_EXPANSION=true and the request setting expandSanctionedRoots=true — so request input alone never widens access.

Paths are canonicalized component-by-component, resolving every symlink and junction in the ancestor chain before comparison, so a file under a linked ancestor cannot present an in-boundary logical path while pointing outside it.

See SECURITY.md for disclosure policy.

Upgrading From 2.x

Path validation is now bounded by a server-owned root list instead of the client's (deprecated) roots/list capability. Two things to do before upgrading:

1. Set ROSLYNMCP_SANCTIONED_ROOTS. An unset boundary is fail-closed — every path-taking tool rejects its input. . is the normal project-scoped value; see Configuration for the delimiter and Option A for a copy-ready snippet. If you need to defer, ROSLYNMCP_PATH_VALIDATION_FAIL_OPEN=true restores the old unbounded behavior as a temporary measure. From this release the server warns at startup and reports server_info.pathBoundary when the boundary is missing, so the state is visible before your first call rather than after it.
2. Stop relying on roots/list for discovery. Query-anchored solution discovery no longer calls it and scans only configured roots. Pass a file-path argument, configure a root containing exactly one solution, call workspace_load explicitly, or pass a workspaceId.

If you install via the Claude Code plugin or the Desktop extension, both ship the default — but update both layers together. A binary-only update leaves a stale config with no boundary set, which is the fail-closed case above. Rationale and full detail: ADR 0002.

Live Surface

The current release exposes 174 tools (113 stable / 61 experimental), 14 resources (9 stable / 5 experimental), and 20 prompts (all experimental).

Use the running server for the authoritative live catalog and support tiers:

• server_info for a human-readable summary
• roslyn://server/catalog for the machine-readable contract
• roslyn://server/resource-templates for resource URI templates

Stable families include workspace/session management, semantic navigation, diagnostics, build/test helpers, and preview/apply refactoring flows. Experimental families include broader project mutation, scaffolding, orchestration, direct text-edit helpers, and prompts.

Repository Layout

• src/RoslynMcp.Host.Stdio/ — stdio host, tool/resource/prompt wiring, logging
• src/RoslynMcp.Core/ — DTOs, contracts, abstractions, preview-store types
• src/RoslynMcp.Roslyn/ — Roslyn workspace, analysis, diagnostics, refactoring, execution services
• tests/RoslynMcp.Tests/ — integration and regression coverage
• skills/ — bundled Claude Code skill definitions
• hooks/ — Claude Code safety hooks

Docs

• docs/setup.md — packaging, Docker, tool install, plugin install, CI artifacts
• docs/compatibility.md — MCP client compatibility matrix (Claude Code, Cursor, VS Code, Claude Desktop)
• docs/stdio-client-integration.md — custom MCP client integration
• docs/product-contract.md — stable vs experimental surface contract
• docs/release-policy.md — release gates and compatibility rules
• AGENTS.md — bootstrap entry point for AI agents working in this repo
• ai_docs/README.md — canonical AI-doc routing index

Filing Surface-Test Findings

If you find a bug or behaviour gap while running /mcp-server-surface-test against your own C# repo, share it back via the Surface-test finding issue template. The shipped skill renders findings into a copy-paste body block by default; pass --auto-file and the skill calls gh issue create for you.

P0 / area: security findings are refused for public filing — see SECURITY.md for the private-disclosure path.

Support

• Bugs and feature requests: GitHub Issues
• Contribution guidelines: CONTRIBUTING.md
• Security disclosures: SECURITY.md