Write null fields so tool results match their output schemas

.agents/skills/json-serialization/SKILL.md

@@ -9,7 +9,7 @@ metadata:
 
 ## The rule
 
-Tool arguments, tool results, resources and prompt attachments all use one set of options: `McpSerializer.Options` in `src/Presentation/Serialization/McpSerializer.cs`. It starts from the SDK's `McpJsonUtilities.DefaultOptions`, then sets camelCase names and leaves nulls out, and puts `AppJsonSerializerContext` first in the resolver chain so the listed types use generated code. The options are read-only once built.
+Tool arguments, tool results, resources and prompt attachments all use one set of options: `McpSerializer.Options` in `src/Presentation/Serialization/McpSerializer.cs`. It starts from the SDK's `McpJsonUtilities.DefaultOptions`, then sets camelCase names and writes nulls, because tool output schemas list nullable fields such as `nextOffset` as required and strict clients reject a result that leaves them out, and puts `AppJsonSerializerContext` first in the resolver chain so the listed types use generated code. The options are read-only once built.
 
 | Used by | How |
 | --- | --- |
@@ -35,7 +35,7 @@ Results are trees. A review inside a movie carries the movie's id and title, not
 var options = new JsonSerializerOptions(McpJsonUtilities.DefaultOptions)
 {
     PropertyNamingPolicy = JsonNamingPolicy.CamelCase,
-    DefaultIgnoreCondition = JsonIgnoreCondition.WhenWritingNull
+    DefaultIgnoreCondition = JsonIgnoreCondition.Never
 };
 options.TypeInfoResolverChain.Insert(0, AppJsonSerializerContext.Default);
 options.MakeReadOnly();

src/Presentation/Serialization/AppJsonSerializerContext.cs

@@ -9,7 +9,6 @@ namespace CleanModelContextProtocol.Presentation.Serialization;
 /// </summary>
 [JsonSourceGenerationOptions(
     PropertyNamingPolicy = JsonKnownNamingPolicy.CamelCase,
-    DefaultIgnoreCondition = JsonIgnoreCondition.WhenWritingNull,
     PropertyNameCaseInsensitive = true,
     WriteIndented = false,
     GenerationMode = JsonSourceGenerationMode.Default)]

src/Presentation/Serialization/McpSerializer.cs

@@ -21,7 +21,7 @@ private static JsonSerializerOptions CreateOptions()
         var options = new JsonSerializerOptions(McpJsonUtilities.DefaultOptions)
         {
             PropertyNamingPolicy = JsonNamingPolicy.CamelCase,
-            DefaultIgnoreCondition = JsonIgnoreCondition.WhenWritingNull
+            DefaultIgnoreCondition = JsonIgnoreCondition.Never
         };
         options.TypeInfoResolverChain.Insert(0, AppJsonSerializerContext.Default);
         options.MakeReadOnly();

tests/Presentation.Tests.Integration/Tools/MovieToolTests.cs

@@ -1,6 +1,8 @@
 namespace CleanModelContextProtocol.Presentation.Tests.Integration.Tools;
 
+using System.Text.Json;
 using ModelContextProtocol;
+using ModelContextProtocol.Protocol;
 using Presentation.Models;
 using Shouldly;
 using Xunit;
@@ -55,6 +57,20 @@ public async Task SearchMovies_ShouldPage_ThroughEveryMovie()
         titles.Count.ShouldBe(50);
     }
 
+    [Fact]
+    public async Task SearchMovies_ShouldWrite_NullNextOffset_OnTheLastPage()
+    {
+        // Arrange
+        var arguments = new Dictionary<string, object?> { ["query"] = "no movie has this title" };
+
+        // Act
+        var result = await this.Client.CallToolAsync("movies_search", arguments);
+
+        // Assert
+        result.Content.OfType<TextContentBlock>().First().Text.ShouldContain("\"nextOffset\":null");
+        result.StructuredContent.ShouldNotBeNull().GetProperty("nextOffset").ValueKind.ShouldBe(JsonValueKind.Null);
+    }
+
     [Fact]
     public async Task SearchMovies_ShouldFind_MovieByTitle()
     {