Add __typename to projection interface fields

graphql-dgs-codegen-core/src/main/kotlin/com/netflix/graphql/dgs/codegen/generators/java/ClientApiGenerator.kt

@@ -807,6 +807,16 @@ class ClientApiGenerator(
         val javaType = subProjection.first
         val codeGenResult = subProjection.second
 
+        // Automatically include __typename when the field type is interface to maintain parity with fragment projections
+        if (type is InterfaceTypeDefinition) {
+            javaType.addInitializerBlock(
+                CodeBlock
+                    .builder()
+                    .addStatement("getFields().put(\$S, null)", TypeNameMetaFieldDef.name)
+                    .build(),
+            )
+        }
+
         val javaFile = JavaFile.builder(getPackageName(), javaType.build()).build()
         return CodeGenResult(clientProjections = listOf(javaFile)).merge(codeGenResult)
     }

graphql-dgs-codegen-core/src/test/kotlin/com/netflix/graphql/dgs/codegen/clientapi/ClientApiGenProjectionTest.kt

@@ -583,6 +583,46 @@ class ClientApiGenProjectionTest {
         )
     }
 
+    @Test
+    fun testInterfaceProjectionIncludesTypenameAutomatically() {
+        val schema =
+            """
+            type Query {
+                account: Account
+            }
+
+            type Account {
+                subscriber: Subscriber
+            }
+
+            interface Subscriber {
+                id: ID
+            }
+
+            type PremiumSubscriber implements Subscriber {
+                id: ID
+                tier: String
+            }
+
+            type BasicSubscriber implements Subscriber {
+                id: ID
+            }
+            """.trimIndent()
+
+        val codeGenResult =
+            CodeGen(
+                CodeGenConfig(
+                    schemas = setOf(schema),
+                    packageName = BASE_PACKAGE_NAME,
+                    generateClientApi = true,
+                ),
+            ).generate()
+
+        val subscriberProjection = codeGenResult.clientProjections.first { it.typeSpec().name() == "SubscriberProjection" }
+        assertThat(subscriberProjection.typeSpec().initializerBlock().toString()).contains("__typename")
+        assertCompilesJava(codeGenResult)
+    }
+
     @Test
     fun testScalarsDontGenerateProjections() {
         val schema =