integrating-language-servers

Language Server Protocol (LSP) Extensions in Visual Studio

The Language Server Protocol (LSP) enables extensions to provide language features — IntelliSense, diagnostics, go-to-definition, find references, formatting, code actions, and more — by connecting Visual Studio to an external language server process that speaks LSP (JSON-RPC v2.0).

LSP is intended for adding support for new languages not already built into Visual Studio. It is not designed to extend existing built-in languages like C# or C++.

LSP is the standard way to add full language support (completion, hover, diagnostics, navigation, formatting) for a language that VS doesn't natively support. The key benefit is architecture: your language server runs in a separate process and communicates over a well-defined protocol, meaning you can reuse the same server across VS Code, Sublime Text, and other editors. VS provides two integration models: in-process via MEF (ILanguageClient) and out-of-process via LanguageServerProvider (VisualStudio.Extensibility).

When to use LSP vs. alternatives:

• Full language support for a new language (completion, diagnostics, navigation, formatting) → LSP (this skill)
• Simple syntax coloring without full language support → TextMate grammar (see vs-textmate-grammar)
• Extending C#/VB with analyzers and code fixes → Roslyn DiagnosticAnalyzer / CodeFixProvider
• Adding completion to an existing language without a full server → vs-editor-completion
• Inline metadata above code elements → CodeLens (see vs-codelens)

File organization

MyExtension/
├── LanguageServer/
│   ├── MyLanguageServerProvider.cs    ← (VS.Extensibility) or MyLanguageClient.cs (VSSDK)
│   └── MyContentDefinition.cs         ← (VSSDK only) content type + file extension mapping
├── Grammars/                           ← optional TextMate grammar for syntax highlighting
│   ├── mylang.tmLanguage.json
│   └── mylang.tmTheme
├── Server/
│   └── my-language-server.exe          ← the LSP server binary (bundled or installed separately)
├── MyExtension.csproj
└── source.extension.vsixmanifest       ← (VSSDK only)

Implementation checklist

• Create or obtain the language server executable
• Define the content type and file extension mapping
• Create the language server provider/client class
• Register in .vsixmanifest with MEF asset type (Toolkit/VSSDK only)
• Bundle the server binary in the VSIX

---

1. VisualStudio.Extensibility (out-of-process, recommended)

The VisualStudio.Extensibility SDK provides a LanguageServerProvider base class that runs out-of-process and communicates with the language server via an IDuplexPipe. No .vsct file, no MEF exports, no VSIX manifest assets — everything is declared in code.

Note: This API is currently in preview. The #pragma warning disable VSEXTPREVIEW_LSP directive is required.

NuGet package: Microsoft.VisualStudio.Extensibility Key namespaces: Microsoft.VisualStudio.Extensibility.LanguageServer, Microsoft.VisualStudio.Extensibility.Editor, Microsoft.VisualStudio.RpcContracts.LanguageServerProvider Additional dependency: Nerdbank.Streams (for FullDuplexStream / DuplexPipe)

Step 1: Define custom document types

If your language's file extensions are not natively recognized by Visual Studio, define custom document types inside your provider class:

[VisualStudioContribution]
public static DocumentTypeConfiguration MyLangDocumentType => new("mylang")
{
    FileExtensions = [".mylang", ".ml"],
    BaseDocumentType = LanguageServerBaseDocumentType,
};

LanguageServerBaseDocumentType is a built-in base type available to all language server providers.

Step 2: Create the language server provider

using System.Diagnostics;
using System.IO;
using System.IO.Pipelines;
using System.Reflection;
using System.Threading;
using System.Threading.Tasks;
using Microsoft.VisualStudio.Extensibility;
using Microsoft.VisualStudio.Extensibility.Editor;
using Microsoft.VisualStudio.Extensibility.LanguageServer;
using Microsoft.VisualStudio.RpcContracts.LanguageServerProvider;
using Nerdbank.Streams;

namespace MyExtension;

#pragma warning disable VSEXTPREVIEW_LSP // Type is for evaluation purposes only and is subject to change or removal in future updates.

[VisualStudioContribution]
internal class MyLanguageServerProvider : LanguageServerProvider
{
    [VisualStudioContribution]
    public static DocumentTypeConfiguration MyLangDocumentType => new("mylang")
    {
        FileExtensions = [".mylang", ".ml"],
        BaseDocumentType = LanguageServerBaseDocumentType,
    };

    public override LanguageServerProviderConfiguration LanguageServerProviderConfiguration =>
        new("My Language Server",
            [DocumentFilter.FromDocumentType(MyLangDocumentType)]);

    public override Task<IDuplexPipe?> CreateServerConnectionAsync(CancellationToken cancellationToken)
    {
        ProcessStartInfo info = new()
        {
            FileName = Path.Combine(
                Path.GetDirectoryName(Assembly.GetExecutingAssembly().Location)!,
                "Server",
                "my-language-server.exe"),
            RedirectStandardInput = true,
            RedirectStandardOutput = true,
            UseShellExecute = false,
            CreateNoWindow = true,
        };

        Process process = new() { StartInfo = info };

        if (process.Start())
        {
            return Task.FromResult<IDuplexPipe?>(new DuplexPipe(
                PipeReader.Create(process.StandardOutput.BaseStream),
                PipeWriter.Create(process.StandardInput.BaseStream)));
        }

        return Task.FromResult<IDuplexPipe?>(null);
    }

    public override Task OnServerInitializationResultAsync(
        ServerInitializationResult startState,
        LanguageServerInitializationFailureInfo? initializationFailureInfo,
        CancellationToken cancellationToken)
    {
        if (startState == ServerInitializationResult.Failed)
        {
            // Disable the server so it won't try to activate again.
            this.Enabled = false;
        }

        return base.OnServerInitializationResultAsync(startState, initializationFailureInfo, cancellationToken);
    }
}

#pragma warning restore VSEXTPREVIEW_LSP

Sending initialization options

Pass custom data to the server's initialize message by setting LanguageServerOptions.InitializationOptions in the constructor:

public MyLanguageServerProvider(ExtensionCore container, VisualStudioExtensibility extensibilityObject, TraceSource traceSource)
    : base(container, extensibilityObject)
{
    this.LanguageServerOptions.InitializationOptions = JToken.Parse(@"{""myOption"": true}");
}

Enabling / disabling at runtime

The Enabled property (default true) controls whether Visual Studio activates your server. Setting it to false sends a stop message to any running server and prevents future activations until set back to true.

Localized display name

Use a string-resources.json file for localization:

{
  "MyExtension.MyLanguageServerProvider.DisplayName": "My Language Server"
}

Then reference the token in your configuration:

public override LanguageServerProviderConfiguration LanguageServerProviderConfiguration =>
    new("%MyExtension.MyLanguageServerProvider.DisplayName%",
        [DocumentFilter.FromDocumentType(MyLangDocumentType)]);

Official sample

See the Rust Language Server Provider sample for a complete working example.

---

2. VSIX Community Toolkit (in-process)

The Community Toolkit does not add a separate LSP API. Language server integration uses the same VSSDK ILanguageClient MEF-based approach described below. The toolkit can simplify package setup, but the LSP components are standard VSSDK APIs.

---

3. VSSDK (in-process)

⚠️ Use ILanguageClient — do NOT use the legacy LanguageService base class

Visual Studio has two approaches for language integration:

• ILanguageClient (LSP-based, current) — Uses the Language Server Protocol. You implement a thin MEF client that launches and connects to an external language server. Use this approach.
• Microsoft.VisualStudio.Package.LanguageService (legacy, pre-LSP) — Requires building a custom scanner/parser with IScanner, ParseSource, COM interop, and registry entries. Do not use this approach for new extensions. It predates LSP and is far more complex with no LSP interoperability.

NuGet package and namespaces

NuGet package: Microsoft.VisualStudio.LanguageServer.Client Key namespace: Microsoft.VisualStudio.LanguageServer.Client

The package also brings in Newtonsoft.Json and StreamJsonRpc. Do not update these transitive packages beyond the versions shipped with your target Visual Studio version — the assemblies are loaded from the VS install directory, not from your VSIX.

Step 1: Define the content type

Map your file extension to a content type. The base definition must derive from CodeRemoteContentTypeName:

using System.ComponentModel.Composition;
using Microsoft.VisualStudio.LanguageServer.Client;
using Microsoft.VisualStudio.Utilities;

namespace MyExtension;

public static class MyLangContentDefinition
{
    [Export]
    [Name("mylang")]
    [BaseDefinition(CodeRemoteContentDefinition.CodeRemoteContentTypeName)]
    internal static ContentTypeDefinition MyLangContentTypeDefinition = null!;

    [Export]
    [FileExtension(".mylang")]
    [ContentType("mylang")]
    internal static FileExtensionToContentTypeDefinition MyLangFileExtensionDefinition = null!;
}

Step 2: Implement ILanguageClient

using System;
using System.Collections.Generic;
using System.ComponentModel.Composition;
using System.Diagnostics;
using System.IO;
using System.Reflection;
using System.Threading;
using System.Threading.Tasks;
using Microsoft.VisualStudio.LanguageServer.Client;
using Microsoft.VisualStudio.Threading;
using Microsoft.VisualStudio.Utilities;

namespace MyExtension;

[ContentType("mylang")]
[Export(typeof(ILanguageClient))]
public class MyLanguageClient : ILanguageClient
{
    public string Name => "My Language Extension";

    public IEnumerable<string>? ConfigurationSections => null;

    public object? InitializationOptions => null;

    public IEnumerable<string>? FilesToWatch => null;

    public event AsyncEventHandler<EventArgs>? StartAsync;
    public event AsyncEventHandler<EventArgs>? StopAsync;

    public async Task OnLoadedAsync()
    {
        if (StartAsync != null)
        {
            await StartAsync.InvokeAsync(this, EventArgs.Empty);
        }
    }

    public Task<Connection?> ActivateAsync(CancellationToken token)
    {
        var info = new ProcessStartInfo
        {
            FileName = Path.Combine(
                Path.GetDirectoryName(Assembly.GetExecutingAssembly().Location)!,
                "Server",
                "my-language-server.exe"),
            RedirectStandardInput = true,
            RedirectStandardOutput = true,
            UseShellExecute = false,
            CreateNoWindow = true,
        };

        var process = new Process { StartInfo = info };

        if (process.Start())
        {
            return Task.FromResult<Connection?>(
                new Connection(
                    process.StandardOutput.BaseStream,
                    process.StandardInput.BaseStream));
        }

        return Task.FromResult<Connection?>(null);
    }

    public Task OnServerInitializedAsync()
    {
        return Task.CompletedTask;
    }

    public Task OnServerInitializeFailedAsync(Exception e)
    {
        return Task.CompletedTask;
    }
}

Step 3: Register the MEF asset

In source.extension.vsixmanifest, add this inside the <Assets> element:

<Asset Type="Microsoft.VisualStudio.MefComponent"
       d:Source="Project"
       d:ProjectName="%CurrentProject%"
       Path="|%CurrentProject%|" />

Without this, Visual Studio will not discover your MEF-exported ILanguageClient.

Activation lifecycle

1. Visual Studio calls OnLoadedAsync().
2. Your code invokes the StartAsync event to signal "ready to start."
3. Visual Studio calls ActivateAsync() — you launch the server process and return a Connection wrapping stdin/stdout streams.
4. Visual Studio sends initialize / initialized LSP messages over the connection.
5. Visual Studio calls OnServerInitializedAsync() (or OnServerInitializeFailedAsync on failure).

Transport options

The Connection class accepts any pair of streams. Common transports:

Transport	How to connect
stdio	Redirect StandardInput / StandardOutput of the server process
Named pipe	Use NamedPipeClientStream for both read and write
TCP socket	Use TcpClient.GetStream()

Providing settings to the language server

1. Implement ConfigurationSections to return setting prefixes (e.g., "mylang").
2. Bundle a default settings JSON file (e.g., MyLangSettings.json) as VSIX content.
3. Add a .pkgdef file registering the settings file:

   [$RootKey$\OpenFolder\Settings\VSWorkspaceSettings\MyLangExtension]
   @="$PackageFolder$\MyLangSettings.json"
   

4. Register the .pkgdef as a Microsoft.VisualStudio.VsPackage asset in the VSIX manifest.

Users override settings by creating a .vs/VSWorkspaceSettings.json file in their workspace.

Custom messages (beyond standard LSP)

Implement ILanguageClientCustomMessage2 on your language client class for custom JSON-RPC messages:

[ContentType("mylang")]
[Export(typeof(ILanguageClient))]
public class MyLanguageClient : ILanguageClient, ILanguageClientCustomMessage2
{
    private JsonRpc? customMessageRpc;

    public object? CustomMessageTarget { get; } = new MyCustomMessageTarget();

    public Task AttachForCustomMessageAsync(JsonRpc rpc)
    {
        this.customMessageRpc = rpc;
        return Task.CompletedTask;
    }

    // ... rest of ILanguageClient implementation
}

Middle layer (intercepting LSP messages)

Use ILanguageClientMiddleLayer2<T> to intercept and modify LSP messages between VS and the server. Do not use the obsolete ILanguageClientMiddleLayer interface — it will be removed in a future version.

public class MyMiddleLayer : ILanguageClientMiddleLayer2<JToken>
{
    public bool CanHandle(string methodName) =>
        methodName == "textDocument/publishDiagnostics";

    public Task HandleNotificationAsync(string methodName, JToken methodParam, Func<JToken, Task> sendNotification)
    {
        // Filter or modify diagnostics before they reach VS
        return sendNotification(methodParam);
    }

    public Task<JToken?> HandleRequestAsync(string methodName, JToken methodParam, Func<JToken, Task<JToken?>> sendRequest)
    {
        return sendRequest(methodParam);
    }
}

TextMate grammar for syntax highlighting

LSP does not define syntax colorization. To provide highlighting, bundle a TextMate grammar:

1. Add a Grammars/ folder with .tmLanguage.json (or .tmLanguage, .plist) and optional .tmTheme files.
2. Add a .pkgdef to register the grammar repository:

   [$RootKey$\TextMate\Repositories]
   "MyLang"="$PackageFolder$\Grammars"
   

3. Set all grammar files and the .pkgdef to Build Action = Content and Include in VSIX = true.

Supported LSP features in Visual Studio

LSP feature	Supported
initialize / initialized / shutdown / exit	Yes
textDocument/completion + completionItem/resolve	Yes
textDocument/hover	Yes
textDocument/signatureHelp	Yes
textDocument/definition	Yes
textDocument/references	Yes
textDocument/documentHighlight	Yes
textDocument/documentSymbol	Yes
textDocument/codeAction	Yes
textDocument/formatting / rangeFormatting	Yes
textDocument/rename	Yes
textDocument/publishDiagnostics	Yes
workspace/symbol	Yes
workspace/executeCommand	Yes
workspace/applyEdit	Yes
window/showMessage / showMessageRequest / logMessage	Yes
textDocument/codeLens / codeLens/resolve	No
textDocument/documentLink / documentLink/resolve	No
textDocument/onTypeFormatting	No

Official samples

• VSSDK sample: VSSDK-Extensibility-Samples/LanguageServerProtocol

---

Troubleshooting

• Language server doesn't activate: For VSSDK, verify the MefComponent asset type is in .vsixmanifest. For Extensibility, confirm your LanguageServerProvider has the correct [VisualStudioContribution] attribute and DocumentFilter matches your content type.
• MissingMethodException or TypeLoadException at runtime: You've updated Newtonsoft.Json or StreamJsonRpc beyond the version shipped with your target VS. Pin to the VS-bundled version and use binding redirects if your server needs a different version.
• Diagnostics not appearing in Error List: Ensure your server publishes textDocument/publishDiagnostics notifications. Verify the ContentType on your ILanguageClient matches the file type, and that the document URI format matches what VS expects (file:///C:/...).
• Completion items show but selecting one inserts wrong text: Check completionItem/resolve — VS calls resolve lazily. If your server doesn't implement it, VS uses the label field as the insert text.
• Server starts but no features work: Check the LSP initialize response capabilities. VS only activates features your server declares in its capabilities object.
• Syntax highlighting missing even though LSP is working: LSP doesn't define syntax colorization. Bundle a TextMate grammar (see vs-textmate-grammar).

See also

• vs-textmate-grammar
• vs-editor-completion
• vs-codelens
• vs-error-list
• vs-editor-quickinfo

Key links

Do NOT use the legacy LanguageService base class (from Microsoft.VisualStudio.Package) — pre-LSP, far more complex, and unmaintained. Use ILanguageClient (VSSDK) or LanguageServerProvider (Extensibility). Old docs may reference it; do not follow.

Do NOT use ILanguageClientMiddleLayer (non-generic) — obsolete. Use ILanguageClientMiddleLayer2<T> instead.

Do NOT update Newtonsoft.Json or StreamJsonRpc beyond VS-shipped versions — version mismatches cause MissingMethodException or silent protocol failures.

Do NOT forget the MefComponent asset type in .vsixmanifest for VSSDK ILanguageClient — without it, the language server never activates.

Do NOT use ISuggestedAction or custom taggers for diagnostics in LSP languages — use textDocument/publishDiagnostics and textDocument/codeAction instead.

Do NOT confuse LSP with extending built-in VS languages (C#, C++, F#) — LSP is for new languages. Use Roslyn analyzers for C#/VB.

• Add an LSP extension (VSSDK)
• Language Server Provider (VisualStudio.Extensibility)
• Language Configuration (for local editor features)
• Language Server Protocol specification