How to make sure that LLM always returns a response in a specific format (e.g., JSON)?

To ensure that an LLM (Large Language Model) always returns a response in a specific format, such as JSON, you can use the following approaches:

• Prompt engineering — explicitly specify in the prompt that the response should be in JSON format, with an example structure. For example:

Please respond in JSON format with fields "name", "age", and "city".

• Use templates and instructions — define a response template for the model to follow.

• Post-processing of the response — if the model occasionally outputs invalid JSON, apply parsers with correction attempts or re-prompt the model with clarification.

• Use specialized models or APIs that support structured responses — some services support formatted output.

• Implement validation and checking — after receiving the response, verify if it conforms to the JSON schema, and request a repeat if there are errors.

Thus, the key is clear formatting instructions in the prompt and processing the result on the client side.