diff --git a/src/interface.jl b/src/interface.jl
index 2d90dd1..d29e392 100644
--- a/src/interface.jl
+++ b/src/interface.jl
@@ -10,6 +10,7 @@ using JSON3, DataStructures
 
 
 @kwdef mutable struct agent
+    availableRole=["system", "user", "assistant"]
     sessionId::Int= 1
     maxUserMsg::Int= 10
 
@@ -34,6 +35,10 @@ julia> addNewMessage(agent1, "user", "Where should I go to buy snacks")
 ````
 """
 function addNewMessage(a::agent, role::String, content::String)
+  if role ∉ a.availableRole   # guard against typo
+    error("role is not in agent.availableRole")
+  end
+
   # check whether user messages exceed limit
   userMsg = 0
   for i in a.messages