Reasoning

Gemma 4 can think step by step before it answers, which helps with math, code and multi-step problems. Thinking is off by default because it adds output tokens and time.

Turning it on

Set reasoning_effort to low, medium or high. none turns thinking off.

import os
from openai import OpenAI

client = OpenAI(base_url="https://api.fastrun.si/v1", api_key=os.environ["FASTRUN_API_KEY"])

res = client.chat.completions.create(
    model="google/gemma-4-31b-it",
    messages=[{"role": "user", "content": "How many prime numbers are there between 100 and 150?"}],
    reasoning_effort="medium",
)
msg = res.choices[0].message
print("thinking:", getattr(msg, "reasoning", None))
print("answer:", msg.content)

curl https://api.fastrun.si/v1/chat/completions \
  -H "Authorization: Bearer $FASTRUN_API_KEY" \
  -H "Content-Type: application/json" \
  -d '{
    "model": "google/gemma-4-31b-it",
    "reasoning_effort": "medium",
    "messages": [{"role": "user", "content": "How many prime numbers are there between 100 and 150?"}]
  }'

You can also turn it on through the chat template with "chat_template_kwargs": {"enable_thinking": true}. In the OpenAI Python SDK, pass it as extra_body={"chat_template_kwargs": {"enable_thinking": True}}.

Where the thinking goes

The thinking comes in a separate reasoning field, and content holds only the answer:

{
  "role": "assistant",
  "reasoning": "Check each odd number from 101 to 149 ...",
  "content": "There are 10 primes between 100 and 150."
}

When streaming, it arrives in delta.reasoning before the answer starts in delta.content.

Some clients read a field named reasoning_content. fastrun uses reasoning.

Cost and limits

Thinking tokens are output tokens: they count toward completion_tokens, max_tokens and the price. Leave enough room in max_tokens for both the thinking and the answer, or the answer may be cut off. Reasoning works together with tool calling and structured outputs.

Get started

Models

Guides

Turning it on

Where the thinking goes

Cost and limits

​Turning it on

​Where the thinking goes

​Cost and limits

Turning it on

Where the thinking goes

Cost and limits