Set FUNCTIONS_WORKER_RUNTIME to custom if EnableMcpCustomHandlerPreview is set (#4703)

Author: soninaren

src/Cli/func/Actions/HostActions/StartHostAction.cs

@@ -18,6 +18,7 @@
 using Microsoft.AspNetCore.Hosting;
 using Microsoft.Azure.WebJobs.Extensions.Http;
 using Microsoft.Azure.WebJobs.Script;
+using Microsoft.Azure.WebJobs.Script.Config;
 using Microsoft.Azure.WebJobs.Script.Configuration;
 using Microsoft.Azure.WebJobs.Script.Description;
 using Microsoft.Azure.WebJobs.Script.WebHost;
@@ -367,6 +368,11 @@ private void UpdateEnvironmentVariables(IDictionary<string, string> secrets)
                     Environment.SetEnvironmentVariable($"AzureFunctionsJobHost__functions__{i}", EnabledFunctions[i]);
                 }
             }
+
+            if (FeatureFlags.IsEnabled(Constants.EnableMcpCustomHandlerFeatureFlag, SystemEnvironment.Instance))
+            {
+                Environment.SetEnvironmentVariable(Constants.FunctionsWorkerRuntime, Constants.Languages.Custom);
+            }
         }
 
         /// <summary>

src/Cli/func/Common/Constants.cs

@@ -95,6 +95,7 @@ internal static partial class Constants
         public const string Dotnet = "dotnet";
         public const string InProcDotNet8EnabledSetting = "FUNCTIONS_INPROC_NET8_ENABLED";
         public const string AzureDevSessionsRemoteHostName = "AzureDevSessionsRemoteHostName";
+        public const string EnableMcpCustomHandlerFeatureFlag = "EnableMcpCustomHandlerPreview";
 
         // forwardedHttpUrl sample format: https://n12abc3t-<port>.asse.devtunnels.ms/
         public const string AzureDevSessionsPortSuffixPlaceholder = "<port>";

src/Cli/func/ConfigurationProfiles/McpCustomHandlerConfigurationProfile.cs

@@ -12,8 +12,6 @@ internal class McpCustomHandlerConfigurationProfile : IConfigurationProfile
     {
         // This feature flag enables MCP (Model Context Protocol) support for custom handlers
         // This flag is not required locally, but is required when deploying to Azure environments.
-        private const string McpFeatureFlag = "EnableMcpCustomHandlerPreview";
-
         public string Name { get; } = "mcp-custom-handler";
 
         public async Task ApplyAsync(WorkerRuntime workerRuntime, bool force = false)
@@ -102,19 +100,19 @@ internal async Task ApplyLocalSettingsAsync(WorkerRuntime workerRuntime, bool fo
                                     .Where(f => !string.IsNullOrWhiteSpace(f))
                                     .ToList();
 
-            if (!flags.Contains(McpFeatureFlag, StringComparer.OrdinalIgnoreCase))
+            if (!flags.Contains(Constants.EnableMcpCustomHandlerFeatureFlag, StringComparer.OrdinalIgnoreCase))
             {
-                flags.Add(McpFeatureFlag);
+                flags.Add(Constants.EnableMcpCustomHandlerFeatureFlag);
                 values[Constants.AzureWebJobsFeatureFlags] = string.Join(",", flags);
                 updatedFeatureFlag = true;
 
                 if (!hasFlagsKey)
                 {
-                    SetupProgressLogger.Ok(Constants.LocalSettingsJsonFileName, $"Added feature flag '{McpFeatureFlag}'");
+                    SetupProgressLogger.Ok(Constants.LocalSettingsJsonFileName, $"Added feature flag '{Constants.EnableMcpCustomHandlerFeatureFlag}'");
                 }
                 else
                 {
-                    SetupProgressLogger.Ok(Constants.LocalSettingsJsonFileName, $"Appended feature flag '{McpFeatureFlag}'");
+                    SetupProgressLogger.Ok(Constants.LocalSettingsJsonFileName, $"Appended feature flag '{Constants.EnableMcpCustomHandlerFeatureFlag}'");
                 }
             }