From 68328d957380de11b79ab4063ea76205e4a95390 Mon Sep 17 00:00:00 2001
From: Fahad Zubair <fahadzub@amazon.com>
Date: Thu, 12 Feb 2026 23:14:17 +0000
Subject: [PATCH 1/2] Handle implicit event stream payload members

---
 .../HttpBoundProtocolPayloadGenerator.kt      |  7 ++++++-
 .../parse/EventStreamUnmarshallerGenerator.kt | 19 +++++++++++++++++++
 2 files changed, 25 insertions(+), 1 deletion(-)

diff --git a/codegen-core/src/main/kotlin/software/amazon/smithy/rust/codegen/core/smithy/protocols/HttpBoundProtocolPayloadGenerator.kt b/codegen-core/src/main/kotlin/software/amazon/smithy/rust/codegen/core/smithy/protocols/HttpBoundProtocolPayloadGenerator.kt
index 6ee4284452e..819da5dccb5 100644
--- a/codegen-core/src/main/kotlin/software/amazon/smithy/rust/codegen/core/smithy/protocols/HttpBoundProtocolPayloadGenerator.kt
+++ b/codegen-core/src/main/kotlin/software/amazon/smithy/rust/codegen/core/smithy/protocols/HttpBoundProtocolPayloadGenerator.kt
@@ -95,7 +95,12 @@ class HttpBoundProtocolPayloadGenerator(
         // need to take ownership.
         return if (payloadMemberName == null) {
             ProtocolPayloadGenerator.PayloadMetadata(takesOwnership = false)
-        } else if (operationShape.isInputEventStream(model)) {
+        } else if (
+            when (httpMessageType) {
+                HttpMessageType.REQUEST -> operationShape.isInputEventStream(model)
+                HttpMessageType.RESPONSE -> operationShape.isOutputEventStream(model)
+            }
+        ) {
             ProtocolPayloadGenerator.PayloadMetadata(takesOwnership = true)
         } else {
             val member = shape.expectMember(payloadMemberName)
diff --git a/codegen-core/src/main/kotlin/software/amazon/smithy/rust/codegen/core/smithy/protocols/parse/EventStreamUnmarshallerGenerator.kt b/codegen-core/src/main/kotlin/software/amazon/smithy/rust/codegen/core/smithy/protocols/parse/EventStreamUnmarshallerGenerator.kt
index 39fcb5dfc61..4509ebf52f1 100644
--- a/codegen-core/src/main/kotlin/software/amazon/smithy/rust/codegen/core/smithy/protocols/parse/EventStreamUnmarshallerGenerator.kt
+++ b/codegen-core/src/main/kotlin/software/amazon/smithy/rust/codegen/core/smithy/protocols/parse/EventStreamUnmarshallerGenerator.kt
@@ -243,6 +243,25 @@ class EventStreamUnmarshallerGenerator(
                         }
                     }
                 }
+                // Handle implicit payload members: members with neither @eventPayload nor @eventHeader.
+                // Per the Smithy spec, these are collectively serialized as a protocol-specific document
+                // in the message body.
+                val implicitMembers = unionStruct.members().filter {
+                    !it.hasTrait<EventPayloadTrait>() && !it.hasTrait<EventHeaderTrait>()
+                }
+                if (implicitMembers.isNotEmpty() && payloadMember == null) {
+                    val parser = protocol.structuredDataParser().errorParser(unionStruct)
+                    if (parser != null) {
+                        rustTemplate(
+                            """
+                            builder = #{parser}(&message.payload()[..], builder)
+                                .map_err(|err| #{Error}::unmarshalling(format!("failed to unmarshall: {err}")))?;
+                            """,
+                            "parser" to parser,
+                            *codegenScope,
+                        )
+                    }
+                }
                 rustTemplate(
                     "Ok(#{UnmarshalledMessage}::Event(#{Output}::$unionMemberName(builder.build())))",
                     "Output" to unionSymbol,

From 2613e17baabf9f07b5b1870ece26ed14bd4a641b Mon Sep 17 00:00:00 2001
From: Fahad Zubair <fahadzub@amazon.com>
Date: Fri, 13 Feb 2026 14:35:23 +0000
Subject: [PATCH 2/2] Lint format

---
 .../protocols/parse/EventStreamUnmarshallerGenerator.kt    | 7 ++++---
 1 file changed, 4 insertions(+), 3 deletions(-)

diff --git a/codegen-core/src/main/kotlin/software/amazon/smithy/rust/codegen/core/smithy/protocols/parse/EventStreamUnmarshallerGenerator.kt b/codegen-core/src/main/kotlin/software/amazon/smithy/rust/codegen/core/smithy/protocols/parse/EventStreamUnmarshallerGenerator.kt
index 4509ebf52f1..4b00d1e1633 100644
--- a/codegen-core/src/main/kotlin/software/amazon/smithy/rust/codegen/core/smithy/protocols/parse/EventStreamUnmarshallerGenerator.kt
+++ b/codegen-core/src/main/kotlin/software/amazon/smithy/rust/codegen/core/smithy/protocols/parse/EventStreamUnmarshallerGenerator.kt
@@ -246,9 +246,10 @@ class EventStreamUnmarshallerGenerator(
                 // Handle implicit payload members: members with neither @eventPayload nor @eventHeader.
                 // Per the Smithy spec, these are collectively serialized as a protocol-specific document
                 // in the message body.
-                val implicitMembers = unionStruct.members().filter {
-                    !it.hasTrait<EventPayloadTrait>() && !it.hasTrait<EventHeaderTrait>()
-                }
+                val implicitMembers =
+                    unionStruct.members().filter {
+                        !it.hasTrait<EventPayloadTrait>() && !it.hasTrait<EventHeaderTrait>()
+                    }
                 if (implicitMembers.isNotEmpty() && payloadMember == null) {
                     val parser = protocol.structuredDataParser().errorParser(unionStruct)
                     if (parser != null) {