From 79f74e79a3d8a0089c2bb98bf6e02bfe48995c56 Mon Sep 17 00:00:00 2001
From: PratikDhanave <i.pratikdhanave@gmail.com>
Date: Mon, 21 Sep 2026 11:45:29 +0530
Subject: [PATCH] Scope Response.Update AdditionalProperties by MessageID

Response.Update copied an update's AdditionalProperties into BOTH the target
message and the response, so message-scoped provider metadata leaked to the
response level and response-scoped metadata leaked onto the current message.

Scope them to the message when the update carries a MessageID and to the
response otherwise, matching .NET's ChatResponseExtensions.ProcessUpdate
(which Response.Update ports).
---
 agent/response.go      |  8 ++++++--
 agent/response_test.go | 32 ++++++++++++++++++++++++++++++++
 2 files changed, 38 insertions(+), 2 deletions(-)

diff --git a/agent/response.go b/agent/response.go
index 0a74bedd..827828ba 100644
--- a/agent/response.go
+++ b/agent/response.go
@@ -191,7 +191,11 @@ func (resp *Response) Update(update *ResponseUpdate) {
 		msg.CreatedAt = update.CreatedAt
 	}
 	msg.Contents = append(msg.Contents, update.Contents...)
-	if update.AdditionalProperties != nil {
+	// AdditionalProperties are scoped to the message when the update carries a
+	// MessageID, otherwise to the response (below), matching .NET's
+	// ChatResponseExtensions.ProcessUpdate. Copying to both would duplicate and
+	// mis-attribute provider metadata.
+	if update.AdditionalProperties != nil && update.MessageID != "" {
 		if msg.AdditionalProperties == nil {
 			msg.AdditionalProperties = make(map[string]any)
 		}
@@ -213,7 +217,7 @@ func (resp *Response) Update(update *ResponseUpdate) {
 	if !isValidCreatedAt(resp.CreatedAt) && isValidCreatedAt(update.CreatedAt) {
 		resp.CreatedAt = update.CreatedAt
 	}
-	if update.AdditionalProperties != nil {
+	if update.AdditionalProperties != nil && update.MessageID == "" {
 		if resp.AdditionalProperties == nil {
 			resp.AdditionalProperties = make(map[string]any)
 		}
diff --git a/agent/response_test.go b/agent/response_test.go
index 7d45feee..8cb7461a 100644
--- a/agent/response_test.go
+++ b/agent/response_test.go
@@ -455,6 +455,38 @@ func TestResponse_CreatedAt(t *testing.T) {
 	}
 }
 
+// AdditionalProperties are scoped to the message when the update carries a
+// MessageID and to the response otherwise, matching .NET's ProcessUpdate. They
+// must not land on both.
+func TestResponse_Update_AdditionalPropertiesScopedByMessageID(t *testing.T) {
+	// With a MessageID -> message only.
+	withID := &agent.Response{}
+	withID.Update(&agent.ResponseUpdate{
+		MessageID:            "msg1",
+		AdditionalProperties: map[string]any{"k": "v"},
+		Contents:             message.Contents{&message.TextContent{Text: "hi"}},
+	})
+	if withID.Messages[0].AdditionalProperties["k"] != "v" {
+		t.Errorf("message AdditionalProperties = %v, want k=v", withID.Messages[0].AdditionalProperties)
+	}
+	if len(withID.AdditionalProperties) != 0 {
+		t.Errorf("response AdditionalProperties = %v, want empty (scoped to message)", withID.AdditionalProperties)
+	}
+
+	// Without a MessageID -> response only.
+	noID := &agent.Response{}
+	noID.Update(&agent.ResponseUpdate{
+		AdditionalProperties: map[string]any{"k": "v"},
+		Contents:             message.Contents{&message.TextContent{Text: "hi"}},
+	})
+	if noID.AdditionalProperties["k"] != "v" {
+		t.Errorf("response AdditionalProperties = %v, want k=v", noID.AdditionalProperties)
+	}
+	if len(noID.Messages[0].AdditionalProperties) != 0 {
+		t.Errorf("message AdditionalProperties = %v, want empty (scoped to response)", noID.Messages[0].AdditionalProperties)
+	}
+}
+
 func TestResponse_Update_AdditionalProperties(t *testing.T) {
 	resp := &agent.Response{}