diff --git a/langchain/agents/chat/base.py b/langchain/agents/chat/base.py
index 0935362a..971d8332 100644
--- a/langchain/agents/chat/base.py
+++ b/langchain/agents/chat/base.py
@@ -73,9 +73,9 @@ class ChatAgent(Agent):
             SystemMessagePromptTemplate.from_template(template),
             HumanMessagePromptTemplate.from_template("{input}\n\n{agent_scratchpad}"),
         ]
-        return ChatPromptTemplate(
-            input_variables=["input", "agent_scratchpad"], messages=messages
-        )
+        if input_variables is None:
+            input_variables = ["input", "agent_scratchpad"]
+        return ChatPromptTemplate(input_variables=input_variables, messages=messages)
 
     @classmethod
     def from_llm_and_tools(