In the API, you can start Claude’s response by including a partial assistant message:
response = client.messages.create(
model="claude-sonnet-4-6",
max_tokens=1024,
messages=[
{"role": "user", "content": "List the top 5 Python web frameworks"},
{"role": "assistant", "content": "```json\n["}
]
)Claude will continue from where you left off, ensuring JSON output. This works for:
- Forcing output format: Start with
{or[for JSON - Setting tone: Prefill with “Certainly! Here’s a concise…”
- Language control: Start the response in the desired language
- Structured output: Begin with a template Claude should fill in