diff --git a/src/CodingWithCalvin.MCPServer.Server/Tools/BuildTools.cs b/src/CodingWithCalvin.MCPServer.Server/Tools/BuildTools.cs
index 0efd610..6778ec4 100644
--- a/src/CodingWithCalvin.MCPServer.Server/Tools/BuildTools.cs
+++ b/src/CodingWithCalvin.MCPServer.Server/Tools/BuildTools.cs
@@ -8,6 +8,9 @@ namespace CodingWithCalvin.MCPServer.Server.Tools;
 [McpServerToolType]
 public class BuildTools
 {
+    private const string DebugSessionActiveMessage =
+        "A debug session is currently active. Stop debugging first using debugger_stop before building.";
+
     private readonly RpcClient _rpcClient;
     private readonly JsonSerializerOptions _jsonOptions;
 
@@ -17,27 +20,48 @@ public BuildTools(RpcClient rpcClient)
         _jsonOptions = new JsonSerializerOptions { WriteIndented = true };
     }
 
+    private async Task<bool> IsDebuggingActiveAsync()
+    {
+        var status = await _rpcClient.GetDebuggerStatusAsync();
+        return status.Mode != "Design";
+    }
+
     [McpServerTool(Name = "build_solution", Destructive = false)]
-    [Description("Build the entire solution. The build runs asynchronously; use build_status to check progress. Returns immediately after starting the build.")]
+    [Description("Build the entire solution. The build runs asynchronously; use build_status to check progress. Returns immediately after starting the build. If a debug session is active, the build cannot proceed — use debugger_stop first.")]
     public async Task<string> BuildSolutionAsync()
     {
+        if (await IsDebuggingActiveAsync())
+        {
+            return DebugSessionActiveMessage;
+        }
+
         var success = await _rpcClient.BuildSolutionAsync();
         return success ? "Build started" : "Failed to start build (is a solution open?)";
     }
 
     [McpServerTool(Name = "build_project", Destructive = false)]
-    [Description("Build a specific project. The build runs asynchronously; use build_status to check progress. IMPORTANT: Requires the full path to the .csproj file, not just the project name. Use project_list first to get the correct path.")]
+    [Description("Build a specific project. The build runs asynchronously; use build_status to check progress. IMPORTANT: Requires the full path to the .csproj file, not just the project name. Use project_list first to get the correct path. If a debug session is active, the build cannot proceed — use debugger_stop first.")]
     public async Task<string> BuildProjectAsync(
         [Description("The full absolute path to the project file (.csproj). Get this from project_list. Supports forward slashes (/) or backslashes (\\).")] string projectName)
     {
+        if (await IsDebuggingActiveAsync())
+        {
+            return DebugSessionActiveMessage;
+        }
+
         var success = await _rpcClient.BuildProjectAsync(projectName);
         return success ? $"Build started for project: {projectName}" : $"Failed to build project: {projectName}";
     }
 
     [McpServerTool(Name = "clean_solution", Destructive = true, Idempotent = true)]
-    [Description("Clean the entire solution by removing all build outputs (bin/obj folders). The clean runs asynchronously; use build_status to check progress.")]
+    [Description("Clean the entire solution by removing all build outputs (bin/obj folders). The clean runs asynchronously; use build_status to check progress. If a debug session is active, the clean cannot proceed — use debugger_stop first.")]
     public async Task<string> CleanSolutionAsync()
     {
+        if (await IsDebuggingActiveAsync())
+        {
+            return DebugSessionActiveMessage;
+        }
+
         var success = await _rpcClient.CleanSolutionAsync();
         return success ? "Clean started" : "Failed to start clean (is a solution open?)";
     }