diff --git a/codegen/smithy-kotlin-codegen/src/main/kotlin/software/amazon/smithy/kotlin/codegen/model/ShapeExt.kt b/codegen/smithy-kotlin-codegen/src/main/kotlin/software/amazon/smithy/kotlin/codegen/model/ShapeExt.kt
index b6b097277..2401238a2 100644
--- a/codegen/smithy-kotlin-codegen/src/main/kotlin/software/amazon/smithy/kotlin/codegen/model/ShapeExt.kt
+++ b/codegen/smithy-kotlin-codegen/src/main/kotlin/software/amazon/smithy/kotlin/codegen/model/ShapeExt.kt
@@ -21,7 +21,6 @@ import software.amazon.smithy.rulesengine.language.EndpointRuleSet
 import software.amazon.smithy.rulesengine.traits.EndpointRuleSetTrait
 import software.amazon.smithy.rulesengine.traits.EndpointTestCase
 import software.amazon.smithy.rulesengine.traits.EndpointTestsTrait
-import kotlin.streams.toList
 
 /**
  * Get all shapes of a particular type from the model.
@@ -193,6 +192,15 @@ val Shape.isSparse: Boolean
 val Shape.isStreaming: Boolean
     get() = hasTrait<StreamingTrait>()
 
+/**
+ * Returns boolean indicating if operations explicitly set HTTP payload is a union
+ */
+fun OperationShape.payloadIsUnionShape(model: Model): Boolean {
+    val requestShape = model.expectShape<StructureShape>(input.get())
+    val payload = requestShape.findMemberWithTrait<HttpPayloadTrait>(model)?.targetOrSelf(model)
+    return payload is UnionShape
+}
+
 /**
  * Test if a member targets an event stream
  */
diff --git a/codegen/smithy-kotlin-codegen/src/main/kotlin/software/amazon/smithy/kotlin/codegen/rendering/protocol/HttpBindingProtocolGenerator.kt b/codegen/smithy-kotlin-codegen/src/main/kotlin/software/amazon/smithy/kotlin/codegen/rendering/protocol/HttpBindingProtocolGenerator.kt
index 4a671bfb1..09cb6b5a2 100644
--- a/codegen/smithy-kotlin-codegen/src/main/kotlin/software/amazon/smithy/kotlin/codegen/rendering/protocol/HttpBindingProtocolGenerator.kt
+++ b/codegen/smithy-kotlin-codegen/src/main/kotlin/software/amazon/smithy/kotlin/codegen/rendering/protocol/HttpBindingProtocolGenerator.kt
@@ -12,7 +12,10 @@ import software.amazon.smithy.kotlin.codegen.core.*
 import software.amazon.smithy.kotlin.codegen.lang.KotlinTypes
 import software.amazon.smithy.kotlin.codegen.lang.toEscapedLiteral
 import software.amazon.smithy.kotlin.codegen.model.*
-import software.amazon.smithy.kotlin.codegen.rendering.serde.*
+import software.amazon.smithy.kotlin.codegen.rendering.serde.deserializerName
+import software.amazon.smithy.kotlin.codegen.rendering.serde.formatInstant
+import software.amazon.smithy.kotlin.codegen.rendering.serde.parseInstant
+import software.amazon.smithy.kotlin.codegen.rendering.serde.serializerName
 import software.amazon.smithy.kotlin.codegen.utils.getOrNull
 import software.amazon.smithy.model.Model
 import software.amazon.smithy.model.knowledge.HttpBinding
@@ -267,7 +270,15 @@ abstract class HttpBindingProtocolGenerator : ProtocolGenerator {
             writer.write("val payload = #T(context, input)", opBodySerializerFn)
             writer.write("builder.body = #T.fromBytes(payload)", RuntimeTypes.Http.HttpBody)
         }
+        renderContentTypeHeader(ctx, op, writer, resolver)
+    }
 
+    protected open fun renderContentTypeHeader(
+        ctx: ProtocolGenerator.GenerationContext,
+        op: OperationShape,
+        writer: KotlinWriter,
+        resolver: HttpBindingResolver = getProtocolHttpBindingResolver(ctx.model, ctx.service),
+    ) {
         resolver.determineRequestContentType(op)?.let { contentType ->
             writer.withBlock("if (builder.body !is HttpBody.Empty) {", "}") {
                 write("builder.headers.setMissing(\"Content-Type\", #S)", contentType)