Produce deprecated property in JsonSchema for obsolete types

src/libraries/System.Text.Json/src/System/Text/Json/Schema/JsonSchema.cs

@@ -25,6 +25,7 @@ internal sealed class JsonSchema
         internal const string DefaultPropertyName = "default";
         internal const string MinLengthPropertyName = "minLength";
         internal const string MaxLengthPropertyName = "maxLength";
+        internal const string DeprecatedPropertyName = "deprecated";
 
         public static JsonSchema CreateFalseSchema() => new(false);
         public static JsonSchema CreateTrueSchema() => new(true);
@@ -95,6 +96,9 @@ public JsonSchema() { }
         public int? MaxLength { get => _maxLength; set { VerifyMutable(); _maxLength = value; } }
         private int? _maxLength;
 
+        public bool? Deprecated { get => _deprecated; set { VerifyMutable(); _deprecated = value; } }
+        private bool? _deprecated;
+
         public JsonSchemaExporterContext? ExporterContext { get; set; }
 
         public int KeywordCount
@@ -124,6 +128,7 @@ public int KeywordCount
                 Count(HasDefaultValue);
                 Count(MinLength != null);
                 Count(MaxLength != null);
+                Count(Deprecated != null);
 
                 return count;
 
@@ -255,6 +260,11 @@ public JsonNode ToJsonNode(JsonSchemaExporterOptions options)
                 objSchema.Add(MaxLengthPropertyName, (JsonNode)maxLength);
             }
 
+            if (Deprecated is { } deprecated)
+            {
+                objSchema.Add(DeprecatedPropertyName, (JsonNode)deprecated);
+            }
+
             return CompleteSchema(objSchema);
 
             JsonNode CompleteSchema(JsonNode schema)

src/libraries/System.Text.Json/src/System/Text/Json/Schema/JsonSchemaExporter.cs

@@ -5,6 +5,7 @@
 using System.Diagnostics;
 using System.Diagnostics.CodeAnalysis;
 using System.Globalization;
+using System.Reflection;
 using System.Runtime.InteropServices;
 using System.Text.Json.Nodes;
 using System.Text.Json.Serialization;
@@ -423,6 +424,13 @@ private static JsonSchema MapJsonSchemaCore(
 
             JsonSchema CompleteSchema(ref GenerationState state, JsonSchema schema)
             {
+                if (HasObsoleteAttribute(typeInfo.Type) ||
+                    HasObsoleteAttribute(propertyInfo?.AttributeProvider))
+                {
+                    JsonSchema.EnsureMutable(ref schema);
+                    schema.Deprecated = true;
+                }
+
                 if (schema.Ref is null)
                 {
                     if (IsNullableSchema(state.ExporterOptions))
@@ -471,6 +479,28 @@ private static void ValidateOptions(JsonSerializerOptions options)
             options.MakeReadOnly();
         }
 
+        private static bool HasObsoleteAttribute(ICustomAttributeProvider? attributeProvider)
+        {
+            if (attributeProvider is null)
+            {
+                return false;
+            }
+
+            // Identify ObsoleteAttribute using its full type name rather than typeof(ObsoleteAttribute).
+            // On downlevel targets System.Text.Json compiles in an internal ObsoleteAttribute polyfill
+            // that would otherwise shadow the framework type, causing the typeof comparison to never match
+            // the ObsoleteAttribute applied by user code.
+            foreach (object attribute in attributeProvider.GetCustomAttributes(inherit: true))
+            {
+                if (attribute.GetType().FullName == "System.ObsoleteAttribute")
+                {
+                    return true;
+                }
+            }
+
+            return false;
+        }
+
         private static bool IsPolymorphicTypeThatSpecifiesItselfAsDerivedType(JsonTypeInfo typeInfo)
         {
             Debug.Assert(typeInfo.PolymorphismOptions is not null);

src/libraries/System.Text.Json/tests/Common/JsonSchemaExporterTests.TestTypes.cs

@@ -1193,6 +1193,22 @@ of the type which points to the first occurrence. */
                     }
                     """);
 
+#pragma warning disable CS0612 // Type or member is obsolete
+            yield return new TestData<MyObsoleteType>(
+                Value: new() { MyString = "str", MyObsoleteString = "str", MyObsoleteInnerType = new() },
+                ExpectedJsonSchema: """
+                    {
+                        "type": ["object","null"],
+                        "properties": {
+                          "MyString": { "type": ["string","null"] },
+                          "MyObsoleteString": { "type": ["string","null"], "deprecated": true },
+                          "MyObsoleteInnerType": { "type": ["object","null"], "deprecated": true }
+                        },
+                        "deprecated": true
+                    }
+                    """);
+#pragma warning restore CS0612 // Type or member is obsolete
+
             // Collection types
             yield return new TestData<int[]>([1, 2, 3], ExpectedJsonSchema: """{"type":["array","null"],"items":{"type":"integer"}}""");
             yield return new TestData<List<bool>>([false, true, false], ExpectedJsonSchema: """{"type":["array","null"],"items":{"type":"boolean"}}""");
@@ -1638,6 +1654,22 @@ public readonly struct StructDictionary<TKey, TValue>(IEnumerable<KeyValuePair<T
             IEnumerator IEnumerable.GetEnumerator() => ((IEnumerable)_dictionary).GetEnumerator();
         }
 
+        [Obsolete]
+        public sealed class MyObsoleteType
+        {
+            public string? MyString { get; set; }
+
+            [Obsolete]
+            public string? MyObsoleteString { get; set; }
+
+            public MyInnerObsoleteType? MyObsoleteInnerType { get; set; }
+
+            [Obsolete]
+            public sealed class MyInnerObsoleteType
+            {
+            }
+        }
+
         public record TestData<T>(
             T? Value,
             string ExpectedJsonSchema,

src/libraries/System.Text.Json/tests/System.Text.Json.SourceGeneration.Tests/Serialization/JsonSchemaExporterTests.cs

@@ -117,6 +117,9 @@ public sealed partial class JsonSchemaExporterTests_SourceGen()
         [JsonSerializable(typeof(ClassWithJsonPointerEscapablePropertyNames))]
         [JsonSerializable(typeof(ClassWithOptionalObjectParameter))]
         [JsonSerializable(typeof(ClassWithPropertiesUsingCustomConverters))]
+#pragma warning disable CS0612 // Type or member is obsolete
+        [JsonSerializable(typeof(MyObsoleteType))]
+#pragma warning restore CS0612 // Type or member is obsolete
         // Collection types
         [JsonSerializable(typeof(int[]))]
         [JsonSerializable(typeof(List<bool>))]