Fix stack overflow by using McpServerDefaultOptions instead of resolving IOptions during tool creation

Co-authored-by: eiriktsarpalis <2813363+eiriktsarpalis@users.noreply.github.com>

Author: Copilot

src/ModelContextProtocol/McpServerBuilderExtensions.cs

@@ -48,8 +48,36 @@ public static partial class McpServerBuilderExtensions
             if (toolMethod.GetCustomAttribute<McpServerToolAttribute>() is not null)
             {
                 builder.Services.AddSingleton((Func<IServiceProvider, McpServerTool>)(toolMethod.IsStatic ?
-                    services => McpServerTool.Create(toolMethod, options: new() { Services = services, SerializerOptions = serializerOptions ?? services.GetService<IOptions<McpServerOptions>>()?.Value.JsonSerializerOptions, SchemaCreateOptions = schemaCreateOptions ?? services.GetService<IOptions<McpServerOptions>>()?.Value.SchemaCreateOptions }) :
-                    services => McpServerTool.Create(toolMethod, static r => CreateTarget(r.Services, typeof(TToolType)), new() { Services = services, SerializerOptions = serializerOptions ?? services.GetService<IOptions<McpServerOptions>>()?.Value.JsonSerializerOptions, SchemaCreateOptions = schemaCreateOptions ?? services.GetService<IOptions<McpServerOptions>>()?.Value.SchemaCreateOptions })));
+                    services =>
+                    {
+                        var effectiveSerializerOptions = serializerOptions;
+                        var effectiveSchemaCreateOptions = schemaCreateOptions;
+                        
+                        // Try to get server-wide defaults if not explicitly provided
+                        if (effectiveSerializerOptions is null || effectiveSchemaCreateOptions is null)
+                        {
+                            var defaultOptions = services.GetService<McpServerDefaultOptions>();
+                            effectiveSerializerOptions ??= defaultOptions?.JsonSerializerOptions;
+                            effectiveSchemaCreateOptions ??= defaultOptions?.SchemaCreateOptions;
+                        }
+                        
+                        return McpServerTool.Create(toolMethod, options: new() { Services = services, SerializerOptions = effectiveSerializerOptions, SchemaCreateOptions = effectiveSchemaCreateOptions });
+                    } :
+                    services =>
+                    {
+                        var effectiveSerializerOptions = serializerOptions;
+                        var effectiveSchemaCreateOptions = schemaCreateOptions;
+                        
+                        // Try to get server-wide defaults if not explicitly provided
+                        if (effectiveSerializerOptions is null || effectiveSchemaCreateOptions is null)
+                        {
+                            var defaultOptions = services.GetService<McpServerDefaultOptions>();
+                            effectiveSerializerOptions ??= defaultOptions?.JsonSerializerOptions;
+                            effectiveSchemaCreateOptions ??= defaultOptions?.SchemaCreateOptions;
+                        }
+                        
+                        return McpServerTool.Create(toolMethod, static r => CreateTarget(r.Services, typeof(TToolType)), new() { Services = services, SerializerOptions = effectiveSerializerOptions, SchemaCreateOptions = effectiveSchemaCreateOptions });
+                    }));
             }
         }
 

src/ModelContextProtocol/McpServerDefaultOptions.cs

@@ -0,0 +1,21 @@
+using Microsoft.Extensions.AI;
+using System.Text.Json;
+
+namespace ModelContextProtocol.Server;
+
+/// <summary>
+/// Holds default options for MCP server primitives (tools, prompts, resources).
+/// This is separate from McpServerOptions to avoid circular dependencies during service resolution.
+/// </summary>
+internal sealed class McpServerDefaultOptions
+{
+    /// <summary>
+    /// Gets or sets the default JSON serializer options.
+    /// </summary>
+    public JsonSerializerOptions? JsonSerializerOptions { get; set; }
+
+    /// <summary>
+    /// Gets or sets the default JSON schema creation options.
+    /// </summary>
+    public AIJsonSchemaCreateOptions? SchemaCreateOptions { get; set; }
+}

src/ModelContextProtocol/McpServerServiceCollectionExtensions.cs

@@ -21,10 +21,23 @@ public static IMcpServerBuilder AddMcpServer(this IServiceCollection services, A
     {
         services.AddOptions();
         services.TryAddEnumerable(ServiceDescriptor.Transient<IConfigureOptions<McpServerOptions>, McpServerOptionsSetup>());
+        
+        // Capture default options from the configuration callback to avoid circular dependencies
+        // when resolving IOptions<McpServerOptions> from within tool/prompt/resource factories
+        var defaultOptions = new McpServerDefaultOptions();
         if (configureOptions is not null)
         {
+            var tempOptions = new McpServerOptions();
+            configureOptions(tempOptions);
+            defaultOptions.JsonSerializerOptions = tempOptions.JsonSerializerOptions;
+            defaultOptions.SchemaCreateOptions = tempOptions.SchemaCreateOptions;
+            
             services.Configure(configureOptions);
         }
+        
+        // Register the default options as a singleton that can be safely resolved
+        // without circular dependencies
+        services.TryAddSingleton(defaultOptions);
 
         return new DefaultMcpServerBuilder(services);
     }

tests/ModelContextProtocol.Tests/Configuration/McpServerJsonSerializerOptionsTests.cs

@@ -48,13 +48,11 @@ public void WithTools_UsesServerWideOptions_WhenNoExplicitOptionsProvided()
             PropertyNamingPolicy = JsonNamingPolicy.SnakeCaseLower
         };
 
-        services.Configure<McpServerOptions>(options =>
+        var builder = services.AddMcpServer(options =>
         {
             options.JsonSerializerOptions = customOptions;
         });
 
-        var builder = services.AddMcpServer();
-
         // Act - WithTools should pick up the server-wide options with snake_case naming policy
         builder.WithTools<TestTools>();
         var serviceProvider = services.BuildServiceProvider();
@@ -85,13 +83,11 @@ public void WithPrompts_UsesServerWideOptions_WhenNoExplicitOptionsProvided()
             PropertyNamingPolicy = JsonNamingPolicy.SnakeCaseLower
         };
 
-        services.Configure<McpServerOptions>(options =>
+        var builder = services.AddMcpServer(options =>
         {
             options.JsonSerializerOptions = customOptions;
         });
 
-        var builder = services.AddMcpServer();
-
         // Act - WithPrompts should pick up the server-wide options with snake_case naming policy
         builder.WithPrompts<TestPrompts>();
         var serviceProvider = services.BuildServiceProvider();
@@ -120,13 +116,11 @@ public void WithResources_UsesServerWideOptions_WhenNoExplicitOptionsProvided()
             PropertyNamingPolicy = JsonNamingPolicy.SnakeCaseLower
         };
 
-        services.Configure<McpServerOptions>(options =>
+        var builder = services.AddMcpServer(options =>
         {
             options.JsonSerializerOptions = customOptions;
         });
 
-        var builder = services.AddMcpServer();
-
         // Act - WithResources should pick up the server-wide options with snake_case naming policy
         builder.WithResources<TestResources>();
         var serviceProvider = services.BuildServiceProvider();