step is used to store historical data of the conversation. Since the Wasm file does not provide a way to store historical data, we need to use step if a task requires information from multiple messages in a conversation.
Every time Host calls functions in Wasm, all the functions except
expect_image will return a step value. Host will store step and include it as a parameter in the following function calls. In this way, Wasm can get all the necessary information in previous messages.