Fix for1959

README.md

@@ -513,6 +513,8 @@ var gitHubApi = RestService.For<IGitHubApi>("https://api.github.com",
     });
 ```
 
+When using `System.Text.Json` polymorphism features such as `[JsonDerivedType]` / `[JsonPolymorphic]`, Refit serializes request bodies using the **declared Refit method parameter type** rather than the boxed runtime `object`. This ensures type discriminators configured on the base contract are preserved in outgoing request payloads.
+
 #### XML Content
 
 XML requests and responses are serialized/deserialized using _System.Xml.Serialization.XmlSerializer_.

Refit.Tests/SerializedContentTests.cs

@@ -560,14 +560,16 @@ public async Task RestService_CanUseSourceGeneratedSystemTextJsonMetadata()
         )
         {
             HttpMessageHandlerFactory = () => new StubHttpMessageHandler(
-                _ => new HttpResponseMessage(HttpStatusCode.OK)
-                {
-                    Content = new StringContent(
-                        "{\"name\":\"Road Runner\",\"company\":\"ACME\",\"createdAt\":\"1949-09-17\"}",
-                        Encoding.UTF8,
-                        "application/json"
-                    )
-                }
+                _ => Task.FromResult(
+                    new HttpResponseMessage(HttpStatusCode.OK)
+                    {
+                        Content = new StringContent(
+                            "{\"name\":\"Road Runner\",\"company\":\"ACME\",\"createdAt\":\"1949-09-17\"}",
+                            Encoding.UTF8,
+                            "application/json"
+                        )
+                    }
+                )
             )
         };
 
@@ -581,9 +583,59 @@ public async Task RestService_CanUseSourceGeneratedSystemTextJsonMetadata()
         Assert.Contains(typeof(User), resolver.RequestedTypes);
     }
 
+    [Fact]
+    public async Task RestService_SerializesBodyUsingDeclaredPolymorphicBaseType()
+    {
+        string? serializedBody = null;
+        var settings = new RefitSettings(
+            new SystemTextJsonContentSerializer(
+                new JsonSerializerOptions(JsonSerializerDefaults.Web)
+                {
+                    TypeInfoResolver = PolymorphicRequestJsonSerializerContext.Default
+                }
+            )
+        )
+        {
+            HttpMessageHandlerFactory = () => new StubHttpMessageHandler(async request =>
+            {
+                serializedBody = await request.Content!.ReadAsStringAsync();
+                return new HttpResponseMessage(HttpStatusCode.OK)
+                {
+                    Content = new StringContent("{}", Encoding.UTF8, "application/json")
+                };
+            })
+        };
+
+        var api = RestService.For<IPolymorphicRequestApi>(BaseAddress, settings);
+        await api.CreateWeapon(new LaserWeaponRequest { Name = "Photon" });
+
+        Assert.NotNull(serializedBody);
+        Assert.Contains("\"$type\":\"laser\"", serializedBody, StringComparison.Ordinal);
+        Assert.Contains("\"name\":\"Photon\"", serializedBody, StringComparison.Ordinal);
+    }
+
+    [JsonPolymorphic(TypeDiscriminatorPropertyName = "$type")]
+    [JsonDerivedType(typeof(LaserWeaponRequest), "laser")]
+    public abstract class CreateWeaponRequest
+    {
+        public string? Name { get; set; }
+    }
+
+    public sealed class LaserWeaponRequest : CreateWeaponRequest { }
+
+    public interface IPolymorphicRequestApi
+    {
+        [Post("/weapons")]
+        Task CreateWeapon(CreateWeaponRequest request);
+    }
+
     [JsonSerializable(typeof(User))]
     internal sealed partial class SerializedContentJsonSerializerContext : JsonSerializerContext { }
 
+    [JsonSerializable(typeof(CreateWeaponRequest))]
+    [JsonSerializable(typeof(LaserWeaponRequest))]
+    internal sealed partial class PolymorphicRequestJsonSerializerContext : JsonSerializerContext { }
+
     [JsonSerializable(typeof(ObjectValueContainer))]
     [JsonSerializable(typeof(string))]
     internal sealed partial class ObjectValueContainerJsonSerializerContext : JsonSerializerContext { }
@@ -599,13 +651,13 @@ sealed class TrackingTypeInfoResolver(IJsonTypeInfoResolver innerResolver) : IJs
         }
     }
 
-    sealed class StubHttpMessageHandler(Func<HttpRequestMessage, HttpResponseMessage> responder)
+    sealed class StubHttpMessageHandler(Func<HttpRequestMessage, Task<HttpResponseMessage>> responder)
         : HttpMessageHandler
     {
         protected override Task<HttpResponseMessage> SendAsync(
             HttpRequestMessage request,
             CancellationToken cancellationToken
-        ) => Task.FromResult(responder(request));
+        ) => responder(request);
     }
 
     sealed class NewtonsoftFieldNameModel

Refit/RequestBuilderImplementation.cs

@@ -1003,7 +1003,10 @@ void AddBodyToRequest(RestMethodInfoInternal restMethod, object param, HttpReque
                     case BodySerializationMethod.Json:
 #pragma warning restore CS0618 // Type or member is obsolete
                     case BodySerializationMethod.Serialized:
-                        var content = serializer.ToHttpContent(param);
+                        var declaredBodyType = restMethod.ParameterInfoArray[
+                            restMethod.BodyParameterInfo.Item3
+                        ].ParameterType;
+                        var content = SerializeBody(serializer, param, declaredBodyType);
                         switch (restMethod.BodyParameterInfo.Item2)
                         {
                             case false:
@@ -1200,6 +1203,25 @@ void AddVersionToRequest(HttpRequestMessage ret)
         }
 #endif
 
+        static readonly MethodInfo SerializeBodyMethod =
+            typeof(RequestBuilderImplementation).GetMethod(
+                nameof(SerializeBodyGeneric),
+                BindingFlags.Static | BindingFlags.NonPublic
+            )!;
+
+        static HttpContent SerializeBody(
+            IHttpContentSerializer serializer,
+            object? body,
+            Type declaredBodyType
+        )
+        {
+            var serializeMethod = SerializeBodyMethod.MakeGenericMethod(declaredBodyType);
+            return (HttpContent)serializeMethod.Invoke(null, [serializer, body])!;
+        }
+
+        static HttpContent SerializeBodyGeneric<T>(IHttpContentSerializer serializer, object? body) =>
+            serializer.ToHttpContent((T)body!);
+
         IEnumerable<KeyValuePair<string, string?>> ParseQueryParameter(
             object? param,
             ParameterInfo parameterInfo,