diff --git a/spec/tracespec/span_key.go b/spec/tracespec/span_key.go
index 46f681d..b4d4c8b 100644
--- a/spec/tracespec/span_key.go
+++ b/spec/tracespec/span_key.go
@@ -11,6 +11,11 @@ const (
 	ReasoningDuration = "reasoning_duration" // The duration during the reasoning process. The unit is microseconds.
 )
 
+// Tags for tool-type span.
+const (
+	ToolCallID = "tool_call_id"
+)
+
 // Tags for retriever-type span
 const (
 	RetrieverProvider = "retriever_provider" // Data retrieval providers, such as Elasticsearch (ES), VikingDB, etc.