Support Claude's prefill feature

[simonw]

https://docs.anthropic.com/en/docs/build-with-claude/prompt-engineering/prefill-claudes-response

[simonw]

Documented in the new model options section (partly maintained by Cog): https://github.com/simonw/llm-anthropic/blob/ffd23912afaa0ba7674c6f0e16229bb1eeced06a/README.md#model-options

[simonw]

Example usage:

llm -m claude-3.5-sonnet -o prefill '{' 'Fun data about pelicans'

{
    "Pelican Fun Facts": [
        {
            "Size": "One of the largest flying birds, with wingspans up to 10 feet (3 meters)",
            "Pouch": "Can hold up to 3 gallons (11 liters) of water - more than 3 times what their stomach can hold",
            "Diet": [
                "Can eat up to 4 pounds (1.8 kg) of fish per day",
                "Don't store fish in their pouch - it's just used as a fishing net"
            ],
            "Cool Features": [
                "Can dive from 60+ feet high to catch fish",
                "Have internal air sacs that help them float and protect them during dives",
                "Can live up to 25-30 years in the wild",
                "All young pelicans are born blind"
            ],
            "Social Behavior": "Often fish cooperatively in groups, herding fish into shallow water",
            "Interesting Tidbit": "Baby pelicans will eat up to 150 pounds of fish before they can fly"
        }
    ]
}

[simonw]

More complex example (from the README):

llm -m claude-3.5-haiku 'Short python function describing a pelican' \
  -o prefill '```python' \
  -o hide_prefill true \
  -o stop_sequences '```'

def describe_pelican():
    """
    Returns a brief description of a pelican's characteristics.
    
    Returns:
        str: A descriptive string about pelicans
    """
    return "A large seabird with a distinctive long, pouched bill, known for diving into water to catch fish and often found along coastal areas."