getcellm · kaspermarstal · Aug 10, 2026
diff --git a/Cellm.sln b/Cellm.sln
@@ -14,6 +14,8 @@ Project("{B7DD6F7E-DEF8-4E67-B5B7-07EF123DB6F0}") = "Cellm.Installer", "src\Cell
 EndProject
 Project("{FAE04EC0-301F-11D3-BF4B-00C04F79EFBC}") = "Cellm.Installer.CustomActions", "src\Cellm.Installers\CustomActions\Cellm.Installer.CustomActions.csproj", "{02AB9A3F-CF5E-8F75-666D-6D8AE65C85E1}"
 EndProject
+Project("{9A19103F-16F7-4668-BE54-9A1E7A4F7556}") = "Cellm.Mcp", "src\Cellm.Mcp\Cellm.Mcp.csproj", "{7F995EA7-C647-4323-9689-9FCBF3A2AE7A}"
+EndProject
 Global
 	GlobalSection(SolutionConfigurationPlatforms) = preSolution
 		Debug|Any CPU = Debug|Any CPU
@@ -90,6 +92,22 @@ Global
 		{02AB9A3F-CF5E-8F75-666D-6D8AE65C85E1}.Release|x64.Build.0 = Release|Any CPU
 		{02AB9A3F-CF5E-8F75-666D-6D8AE65C85E1}.Release|x86.ActiveCfg = Release|Any CPU
 		{02AB9A3F-CF5E-8F75-666D-6D8AE65C85E1}.Release|x86.Build.0 = Release|Any CPU
+		{7F995EA7-C647-4323-9689-9FCBF3A2AE7A}.Debug|Any CPU.ActiveCfg = Debug|Any CPU
+		{7F995EA7-C647-4323-9689-9FCBF3A2AE7A}.Debug|Any CPU.Build.0 = Debug|Any CPU
+		{7F995EA7-C647-4323-9689-9FCBF3A2AE7A}.Debug|ARM64.ActiveCfg = Debug|Any CPU
+		{7F995EA7-C647-4323-9689-9FCBF3A2AE7A}.Debug|ARM64.Build.0 = Debug|Any CPU
+		{7F995EA7-C647-4323-9689-9FCBF3A2AE7A}.Debug|x64.ActiveCfg = Debug|Any CPU
+		{7F995EA7-C647-4323-9689-9FCBF3A2AE7A}.Debug|x64.Build.0 = Debug|Any CPU
+		{7F995EA7-C647-4323-9689-9FCBF3A2AE7A}.Debug|x86.ActiveCfg = Debug|Any CPU
+		{7F995EA7-C647-4323-9689-9FCBF3A2AE7A}.Debug|x86.Build.0 = Debug|Any CPU
+		{7F995EA7-C647-4323-9689-9FCBF3A2AE7A}.Release|Any CPU.ActiveCfg = Release|Any CPU
+		{7F995EA7-C647-4323-9689-9FCBF3A2AE7A}.Release|Any CPU.Build.0 = Release|Any CPU
+		{7F995EA7-C647-4323-9689-9FCBF3A2AE7A}.Release|ARM64.ActiveCfg = Release|Any CPU
+		{7F995EA7-C647-4323-9689-9FCBF3A2AE7A}.Release|ARM64.Build.0 = Release|Any CPU
+		{7F995EA7-C647-4323-9689-9FCBF3A2AE7A}.Release|x64.ActiveCfg = Release|Any CPU
+		{7F995EA7-C647-4323-9689-9FCBF3A2AE7A}.Release|x64.Build.0 = Release|Any CPU
+		{7F995EA7-C647-4323-9689-9FCBF3A2AE7A}.Release|x86.ActiveCfg = Release|Any CPU
+		{7F995EA7-C647-4323-9689-9FCBF3A2AE7A}.Release|x86.Build.0 = Release|Any CPU
 	EndGlobalSection
 	GlobalSection(SolutionProperties) = preSolution
 		HideSolutionNode = FALSE

diff --git a/src/Cellm.Mcp/Cellm.Mcp.csproj b/src/Cellm.Mcp/Cellm.Mcp.csproj
@@ -0,0 +1,16 @@
+<Project Sdk="Microsoft.NET.Sdk">
+
+  <PropertyGroup>
+    <OutputType>Exe</OutputType>
+    <TargetFramework>net9.0-windows</TargetFramework>
+    <ImplicitUsings>enable</ImplicitUsings>
+    <Nullable>enable</Nullable>
+    <RestorePackagesWithLockFile>true</RestorePackagesWithLockFile>
+  </PropertyGroup>
+
+  <ItemGroup>
+    <PackageReference Include="Microsoft.Extensions.Hosting" Version="10.0.10" />
+    <PackageReference Include="ModelContextProtocol" Version="2.1.0" />
+  </ItemGroup>
+
+</Project>
diff --git a/src/Cellm.Mcp/CellmClient.cs b/src/Cellm.Mcp/CellmClient.cs
@@ -0,0 +1,63 @@
+using System.IO.Pipes;
+using System.Security.Principal;
+using System.Text;
+using System.Text.Json;
+
+namespace Cellm.Mcp;
+
+internal class CellmClient
+{
+    private static readonly TimeSpan _connectTimeout = TimeSpan.FromSeconds(5);
+    private static readonly JsonSerializerOptions _jsonOptions = new(JsonSerializerDefaults.Web);
+    private readonly string _pipeName;
+
+    public CellmClient()
+        : this(GetPipeName())
+    {
+    }
+
+    internal CellmClient(string pipeName)
+    {
+        _pipeName = pipeName;
+    }
+
+    public async Task<PromptResponse> PromptAsync(PromptRequest request, CancellationToken cancellationToken)
+    {
+        await using var pipe = new NamedPipeClientStream(
+            serverName: ".",
+            pipeName: _pipeName,
+            direction: PipeDirection.InOut,
+            options: PipeOptions.Asynchronous);
+
+        try
+        {
+            await pipe.ConnectAsync(_connectTimeout, cancellationToken).ConfigureAwait(false);
+        }
+        catch (TimeoutException ex)
+        {
+            throw new InvalidOperationException("Cellm is not running in Excel.", ex);
+        }
+
+        using var reader = new StreamReader(pipe, Encoding.UTF8, detectEncodingFromByteOrderMarks: false, leaveOpen: true);
+        using var writer = new StreamWriter(pipe, new UTF8Encoding(encoderShouldEmitUTF8Identifier: false), leaveOpen: true)
+        {
+            AutoFlush = true
+        };
+
+        await writer.WriteLineAsync(JsonSerializer.Serialize(request, _jsonOptions)).ConfigureAwait(false);
+
+        var responseJson = await reader.ReadLineAsync(cancellationToken).ConfigureAwait(false)
+            ?? throw new InvalidOperationException("Cellm closed the connection without returning a response.");
+
+        return JsonSerializer.Deserialize<PromptResponse>(responseJson, _jsonOptions)
+            ?? throw new InvalidOperationException("Cellm returned an invalid response.");
+    }
+
+    private static string GetPipeName()
+    {
+        var user = WindowsIdentity.GetCurrent().User?.Value
+            ?? throw new InvalidOperationException("Unable to identify the current Windows user.");
+
+        return $"Cellm-{user}";
+    }
+}
diff --git a/src/Cellm.Mcp/CellmTools.cs b/src/Cellm.Mcp/CellmTools.cs
@@ -0,0 +1,40 @@
+using System.ComponentModel;
+using ModelContextProtocol.Server;
+
+namespace Cellm.Mcp;
+
+[McpServerToolType]
+internal class CellmTools(CellmClient client)
+{
+    [McpServerTool(
+        Name = "cellm_prompt",
+        Title = "Run a Cellm prompt in Excel",
+        Destructive = true,
+        Idempotent = false,
+        OpenWorld = true,
+        UseStructuredContent = true)]
+    [Description("Writes a PROMPT or PROMPTMODEL formula to an Excel cell and waits for Excel to return the result.")]
+    public Task<PromptResponse> PromptAsync(
+        [Description("The name of the open Excel workbook.")] string workbook,
+        [Description("The name of the worksheet in that workbook.")] string worksheet,
+        [Description("The target cell address, for example B2.")] string cell,
+        [Description("The prompt text sent to the configured model.")] string prompt,
+        [Description("Optional cell or range addresses on the same worksheet to include as context.")] string[]? ranges = null,
+        [Description("Optional provider and model in provider/model form. Omit this to use Cellm's configured default.")] string? providerAndModel = null,
+        [Description("Whether Cellm may replace an existing value or formula in the target cell.")] bool overwrite = false,
+        CancellationToken cancellationToken = default)
+    {
+        return client.PromptAsync(
+            new PromptRequest(
+                Version: 1,
+                Method: "prompt",
+                Workbook: workbook,
+                Worksheet: worksheet,
+                Cell: cell,
+                Prompt: prompt,
+                Ranges: ranges ?? [],
+                ProviderAndModel: providerAndModel,
+                Overwrite: overwrite),
+            cancellationToken);
+    }
+}
diff --git a/src/Cellm.Mcp/Program.cs b/src/Cellm.Mcp/Program.cs
@@ -0,0 +1,19 @@
+using Cellm.Mcp;
+using Microsoft.Extensions.DependencyInjection;
+using Microsoft.Extensions.Hosting;
+using Microsoft.Extensions.Logging;
+
+var builder = Host.CreateApplicationBuilder(args);
+
+builder.Logging.AddConsole(options =>
+{
+    options.LogToStandardErrorThreshold = LogLevel.Trace;
+});
+
+builder.Services
+    .AddSingleton<CellmClient>()
+    .AddMcpServer()
+    .WithStdioServerTransport()
+    .WithTools<CellmTools>();
+
+await builder.Build().RunAsync();
diff --git a/src/Cellm.Mcp/PromptRequest.cs b/src/Cellm.Mcp/PromptRequest.cs
@@ -0,0 +1,12 @@
+namespace Cellm.Mcp;
+
+internal record PromptRequest(
+    int Version,
+    string Method,
+    string Workbook,
+    string Worksheet,
+    string Cell,
+    string Prompt,
+    IReadOnlyList<string> Ranges,
+    string? ProviderAndModel,
+    bool Overwrite);
diff --git a/src/Cellm.Mcp/PromptResponse.cs b/src/Cellm.Mcp/PromptResponse.cs
@@ -0,0 +1,11 @@
+namespace Cellm.Mcp;
+
+public record PromptResponse(
+    int Version,
+    string Status,
+    string Workbook,
+    string Worksheet,
+    string Cell,
+    string? Formula = null,
+    object? Value = null,
+    string? Error = null);
diff --git a/src/Cellm.Mcp/Properties/AssemblyInfo.cs b/src/Cellm.Mcp/Properties/AssemblyInfo.cs
@@ -0,0 +1,3 @@
+using System.Runtime.CompilerServices;
+
+[assembly: InternalsVisibleTo("Cellm.Tests")]
diff --git a/src/Cellm.Mcp/README.md b/src/Cellm.Mcp/README.md
@@ -0,0 +1,14 @@
+# Cellm MCP server
+
+This project exposes Cellm to MCP clients over standard input/output. Excel must be running with the Cellm add-in loaded.
+
+For a development build, configure the client to run:
+
+```json
+{
+  "command": "dotnet",
+  "args": ["C:\\path\\to\\Cellm.Mcp.dll"]
+}
+```
+
+The first tool, `cellm_prompt`, writes a normal `PROMPT` or `PROMPTMODEL` formula to an explicit workbook, worksheet, and cell. It then waits for Excel's result. Cancelling the MCP call stops waiting but does not replace Cellm's existing Excel cancellation behavior.
Original file line number	Diff line number	Diff line change
		@@ -0,0 +1,3 @@
		using System.Runtime.CompilerServices;

		[assembly: InternalsVisibleTo("Cellm.Tests")]