Allowed to set a custom JsonSchemaGeneratorOptions for MCP tool schema #543

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1. fvdcx

   added

   enhancementNew feature or requestNew feature or request

   on Jun 25, 2025
2. stephentoub commented on Jun 25, 2025

   

   stephentoub

   on Jun 25, 2025

   Last edited by stephentoub

   Contributor

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   For DateTime, DateTimeOffset, and TimeSpan specifically, that should automatically be handled by dotnet/extensions#6540.

   Even without that change, however, you can already control the schema being generated via the TransformSchemaNode on the SchemaCreateOptions passed to McpServerTool.Create. For example, this code:

   using ModelContextProtocol.Server;
   using System.Text.Json;
   using System.Text.Json.Serialization;
   
   var tool = McpServerTool.Create((TemporalData input) => "received data");
   
   Console.WriteLine(JsonSerializer.Serialize(tool.ProtocolTool.InputSchema, new JsonSerializerOptions { WriteIndented = true }));
   
   public class TemporalData
   {
       [JsonPropertyName("dateTimeValue")]
       public DateTime DateTimeValue { get; set; }
   
       [JsonPropertyName("dateTimeOffsetValue")]
       public DateTimeOffset DateTimeOffsetValue { get; set; }
   
       [JsonPropertyName("timeSpanValue")]
       public TimeSpan TimeSpanValue { get; set; }
   }

   outputs schema like you cite in your issue:

   {
     "type": "object",
     "properties": {
       "input": {
         "type": "object",
         "properties": {
           "dateTimeValue": {
             "type": "string"
           },
           "dateTimeOffsetValue": {
             "type": "string"
           },
           "timeSpanValue": {
             "$comment": "Represents a System.TimeSpan value.",
             "type": "string"
           }
         }
       }
     },
     "required": [
       "input"
     ]
   }

   But, if I change the Create call to be this:

   var tool = McpServerTool.Create((TemporalData input) => "received data", new()
   {
       SchemaCreateOptions = new()
       {
           TransformSchemaNode = (ctx, node) =>
           {
               Type? t = ctx.PropertyInfo?.PropertyType;
               if (t == typeof(DateTime) || t == typeof(DateTimeOffset))
               {
                   node["format"] = "date-time";
               }
               else if (t == typeof(TimeSpan))
               {
                   node["format"] = "duration";
               }
   
               return node;
           },
       },
   });

   then I get output like this:

   {
     "type": "object",
     "properties": {
       "input": {
         "type": "object",
         "properties": {
           "dateTimeValue": {
             "type": "string",
             "format": "date-time"
           },
           "dateTimeOffsetValue": {
             "type": "string",
             "format": "date-time"
           },
           "timeSpanValue": {
             "$comment": "Represents a System.TimeSpan value.",
             "type": "string",
             "format": "duration"
           }
         }
       }
     },
     "required": [
       "input"
     ]
   }
3. fvdcx commented on Jun 26, 2025

   

   fvdcx

   on Jun 26, 2025

   Author

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   Yeah, that should work. I see your example is using the dynamic way to create a tool

   var tool = McpServerTool.Create((TemporalData input) => "received data", new()
   {
       SchemaCreateOptions = new()
       {
           TransformSchemaNode = (ctx, node) =>
           {
               Type? t = ctx.PropertyInfo?.PropertyType;
               if (t == typeof(DateTime) || t == typeof(DateTimeOffset))
               {
                   node["format"] = "date-time";
               }
               else if (t == typeof(TimeSpan))
               {
                   node["format"] = "duration";
               }
   
               return node;
           },
       },
   });
   

   Is it possible to add SchemaCreateOptions on a specific static mcp tool (existing method with adding mcpservertool attribute)

           [McpServerTool(Name = "test_tool"), Description("")]
           public async Task<CallToolResponse> TestToolMethod(
               [Description("")] Foo foo)
           {
           }
   
4. eiriktsarpalis commented on Jun 26, 2025

   

   eiriktsarpalis

   on Jun 26, 2025

   Member

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   I think we can close this given that dotnet/extensions#6540 got merged. Note that this won't be addressed in MCP until a new version of MEAI.Abstractions has been published.
5. eiriktsarpalis

   closed this as completedon Jun 26, 2025
6. stephentoub commented on Jun 26, 2025

   

   stephentoub

   on Jun 26, 2025

   Contributor

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   Is it possible to add SchemaCreateOptions on a specific static mcp tool (existing method with adding mcpservertool attribute)

   Not today, no. In theory we could extend the attribute with the ability to supply a Type name and a member name and use reflection to access the options instance, but that starts to get complicated, and the attribute-based scheme is intended to represent the simple/majority cases. The McpServerTool type is provided when you need much more control, with the Create methods, with a DelegatingMcpServerTool type to make it easy to wrap another McpServerTool instance and override whatever members you need to, to write your own derived type, etc. The attribute-based support is itself just a small wrapper around using that class; you can see here that it's just getting all methods with the relevant attribute and calling McpServerTool.Create with each; you can do the same with your own helper if you want more customization.