passing input_variables to Prompt template in langchain

Question

I have code below in ReAct prompt template. I have input_variables like tools, tool_names, etc. But when we are calling, not all input_variables are passed. Then, to my understanding, it should throw an error. But no error is thrown. why?

# Create the ReAct template
react_template = """Answer the following questions as best you can. You have access to the following tools:

{tools}

Use the following format:

Question: the input question you must answer
Thought: you should always think about what to do
Action: the action to take, should be one of [{tool_names}]
Action Input: the input to the action
Observation: the result of the action
... (this Thought/Action/Action Input/Observation can repeat N times)
Thought: I now know the final answer
Final Answer: the final answer to the original input question

Begin!

Question: {input}
Thought:{agent_scratchpad}"""

prompt = PromptTemplate(
    template=react_template,
    input_variables=["tools", "tool_names", "input", "agent_scratchpad"]
)
from langchain.agents import AgentExecutor, create_react_agent

# Construct the ReAct agent
agent = create_react_agent(llm, tools, prompt)
agent_executor = AgentExecutor(
    agent=agent, tools=tools, verbose=True, handle_parsing_errors=True
)

"tools", "tool_names", and "agent_scratchpad" are filled out by create_react_agent, so you just need to pass in the "input". — cottontail
– cottontail, Commented Sep 25 at 20:41

colonel · Accepted Answer · 2025-10-22 19:31:28Z

You are not seeing any error because you’re not calling prompt.format() directly. It’s being used indirectly inside create_react_agent.

How PromptTemplate normally works:

prompt = PromptTemplate(
    template=react_template,
    input_variables=["tools", "tool_names", "input", "agent_scratchpad"]
)

You’re telling LangChain that this template has these four placeholders, and you are expect all of them to be provided when formatting.

So ideally, when you call:

prompt.format(input="Hi")

should raise a KeyError because tools, tool_names, and agent_scratchpad are missing.

But, you are doing :

agent = create_react_agent(llm, tools, prompt)
agent_executor = AgentExecutor(agent=agent, tools=tools, verbose=True, handle_parsing_errors=True)

This wraps your prompt inside a custom chain which injects (or overrides) your variables automatically (tools, tool_names, agent_scratchpad).

In order to raise an error, put this inside your script
prompt.format(input="Hi")

It will Raises KeyError: 'tools'

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