diff --git a/backend/internal/service/openai_codex_transform.go b/backend/internal/service/openai_codex_transform.go
index a3b69dee..5b5f2bc3 100644
--- a/backend/internal/service/openai_codex_transform.go
+++ b/backend/internal/service/openai_codex_transform.go
@@ -1030,7 +1030,7 @@ func filterCodexInputWithOptions(input []any, opts codexInputFilterOptions) []an
 				return id
 			}
 			if strings.HasPrefix(id, "call_") {
-				return "fc" + strings.TrimPrefix(id, "call_")
+				return "fc_" + strings.TrimPrefix(id, "call_")
 			}
 			return "fc_" + id
 		}