[fix]: preserve bare-string tool call actions in streaming deep copy

framework/streaming/responses.go

@@ -358,6 +358,10 @@ func deepCopyResponsesMessage(original schemas.ResponsesMessage) schemas.Respons
 				copyAction := *original.ResponsesToolMessage.Action.ResponsesMCPApprovalRequestAction
 				copy.ResponsesToolMessage.Action.ResponsesMCPApprovalRequestAction = &copyAction
 			}
+
+			// Bare-string actions (e.g. image_generation_call's "generate") live in a
+			// field the Framework's pinned Core release may predate; copy it by name.
+			copyOptionalStringFieldByName(copy.ResponsesToolMessage.Action, original.ResponsesToolMessage.Action, "ResponsesToolCallActionStr")
 		}
 
 		if original.ResponsesToolMessage.Caller != nil {
@@ -502,7 +506,7 @@ func copyRawMessageFieldByName(dst *schemas.ResponsesMessage, src schemas.Respon
 	}
 }
 
-func copyOptionalStringFieldByName(dst *schemas.ResponsesToolMessage, src *schemas.ResponsesToolMessage, fieldName string) {
+func copyOptionalStringFieldByName(dst, src any, fieldName string) {
 	srcField := reflect.ValueOf(src).Elem().FieldByName(fieldName)
 	if !srcField.IsValid() || srcField.IsNil() {
 		return

framework/streaming/responses_test.go

@@ -2,6 +2,7 @@ package streaming
 
 import (
 	"fmt"
+	"reflect"
 	"strings"
 	"testing"
 	"time"
@@ -154,6 +155,34 @@ func TestDeepCopyResponsesStreamResponseCopiesToolCaller(t *testing.T) {
 	}
 }
 
+// TestDeepCopyResponsesMessagePreservesBareStringAction verifies that a
+// bare-string action (e.g. image_generation_call's "generate") survives the
+// accumulator's deep copy instead of being dropped as an empty action struct.
+func TestDeepCopyResponsesMessagePreservesBareStringAction(t *testing.T) {
+	action := &schemas.ResponsesToolMessageActionStruct{}
+	actionField := reflect.ValueOf(action).Elem().FieldByName("ResponsesToolCallActionStr")
+	if !actionField.IsValid() {
+		t.Skip("released core dependency predates bare-string tool call actions")
+	}
+	actionField.Set(reflect.ValueOf(schemas.Ptr("generate")))
+
+	copied := deepCopyResponsesMessage(schemas.ResponsesMessage{
+		ResponsesToolMessage: &schemas.ResponsesToolMessage{
+			Action: action,
+		},
+	})
+	if copied.ResponsesToolMessage == nil || copied.ResponsesToolMessage.Action == nil {
+		t.Fatalf("unexpected copied message: %#v", copied)
+	}
+	copiedField := reflect.ValueOf(copied.ResponsesToolMessage.Action).Elem().FieldByName("ResponsesToolCallActionStr")
+	if copiedField.IsNil() || copiedField.Elem().String() != "generate" {
+		t.Fatal("bare-string action was not preserved")
+	}
+	if copiedField.Pointer() == actionField.Pointer() {
+		t.Fatal("bare-string action pointer was aliased")
+	}
+}
+
 // TestBuildResponsesMessageAccumulatesReasoningSummary verifies reasoning
 // summary deltas (no content index) concatenate into a single summary entry.
 func TestBuildResponsesMessageAccumulatesReasoningSummary(t *testing.T) {