Luis Claude 3.7 offers "extended thinking" as a toggle.

Maybe these are two separate models. It looks as if the "thinking" flag being passed to the API changes this behavior.
curl https://api.anthropic.com/v1/messages \
--header "x-api-key: $ANTHROPIC_API_KEY" \
--header "anthropic-version: 2023-06-01" \
--header "content-type: application/json" \
--data \
'{
"model": "claude-3-7-sonnet-20250219",
"max_tokens": 20000,
"thinking": {
"type": "enabled",
"budget_tokens": 16000
},
"messages": [
{
"role": "user",
"content": "Are there an infinite number of prime numbers such that n mod 4 == 3?"
}
]
}'