Reasoning

Most chat models here reason: they write their thinking first, then the answer. You choose how hard they think, can cap how long, and get the thinking back beside the answer.

How hard to think#

Effort runs from minimal through low, medium, high, xhigh and max to ultra. Leave it out and the model decides. Each model lists the levels it takes on its page, and a level it does not take is a 400.

curl https://api.vultrinference.com/v1/chat/completions \
  -H "Authorization: Bearer $VULTR_INFERENCE_API_KEY" \
  -H "Content-Type: application/json" \
  -d '{
    "model": "deepseek-v4.1-flash",
    "reasoning": {"effort": "low"},
    "messages": [{"role": "user", "content": "Is 221 a prime number?"}]
  }'

import os

from openai import OpenAI

client = OpenAI(base_url="https://api.vultrinference.com/v1", api_key=os.environ["VULTR_INFERENCE_API_KEY"])

reply = client.chat.completions.create(
    model="deepseek-v4.1-flash",
    messages=[{"role": "user", "content": "Is 221 a prime number?"}],
    # Not one of OpenAI's own fields, so the SDK sends it as extra body
    extra_body={"reasoning": {"effort": "low"}},
)
message = reply.choices[0].message
print("thought:", getattr(message, "reasoning", None))
print("answer:", message.content)
print("reasoning tokens:", reply.usage.completion_tokens_details.reasoning_tokens)

To turn thinking off, send effort none or "reasoning": {"enabled": false}. A few models think whatever you send.

A cap on thinking#

A budget caps how many tokens the model may spend thinking. It must be below the cap on the whole answer.

Not every model can hold to a budget; those that can say supports_max_tokens in their catalog entry. The others ignore it on chat completions and responses, and refuse it with a 400 on messages. On messages, thinking: {"type": "adaptive"} leaves the amount to the model.

Where the thinking comes back#

On chat completions it is always reasoning, whichever engine serves the model, and delta.reasoning in a stream. SDKs and compatibility shows a whole answer.

On messages, each thinking block comes with a signature. Send the block back on the next turn and the model sees its earlier thinking. With display: "omitted" the block arrives with empty text, but still with its signature.

Billing#

Thinking tokens are output tokens: completion_tokens already includes them, and they are billed at the model's output price.