Force claude to output JSON by pre-filling the response with ```json

claude_json_prefill.py

import anthropic

client = anthropic.Anthropic()

message = client.messages.create(
    model="claude-3-5-sonnet-20241022",
    max_tokens=1000,
    temperature=0,
    system="Always respond in JSON.",
    messages=[
        {"role": "user", "content": [{"type": "text", "text": "Please write a poem."}]},
        {"role": "assistant", "content": [{"type": "text", "text": "```json"}]},
    ],
    stop_sequences=["```"],
)

assert message.content[0].type == "text"

print(message.content[0].text)