When using Claude, you have the unique ability to guide its responses by prefilling the Assistant message. This powerful technique allows you to direct Claude’s actions, skip preambles, enforce specific formats like JSON or XML, and even help Claude maintain character consistency in role-play scenarios.

In some cases where Claude is not performing as expected, a few prefilled sentences can vastly improve Claude’s performance. A little prefilling goes a long way!

How to prefill Claude’s response

To prefill, include the desired initial text in the Assistant message (Claude’s response will continue from where the Assistant message leaves off):

import anthropic

client = anthropic.Anthropic()
response = client.messages.create(
        {"role": "user", "content": "What is your favorite color?"},
        {"role": "assistant", "content": "As an AI assistant, I don't have a favorite color, But if I had to pick, it would be green because"}  # Prefill here


Example 1: Controlling output formatting and skipping the preamble

Power user tip: Prefilling { forces Claude to skip the preamble and directly output the JSON object. This is cleaner, more concise, and easier for programs to parse without additional processing.

Example 2: Maintaining character in roleplay scenarios

Role-play tip: Prefilling a bracketed [ROLE_NAME] can remind Claude stay in character, even for longer and more complex conversations. This is especially powerful when combined with role prompting in the system parameter.