Grok 4.6 vs GPT-5.6 Terra: STEM depth or long context

xAI's Grok 4.6 and OpenAI's GPT-5.6 Terra charge almost the same for input and are both pitched as capable all-rounders. Two numbers separate them: Grok's output costs half as much, and Terra's context window is twice as large. Past that, the choice comes down to what each is built for.

The short answer: Grok 4.6 for math, science and research-heavy reasoning; GPT-5.6 Terra when you need a long context and one model that covers a wide range of tasks.

Specs and price

Input costs nearly the same. Output is where they split: Grok 4.6's output costs about half of Terra's. Both models charge more on long prompts: Grok above 200K tokens ($4.40 input, $13.20 output per million), Terra above 272K ($4.22 input, $18.99 output). Prices are what a USD account pays today; the current figures are on the pricing page.

A lower rate doesn't guarantee a lower bill, since models differ in how many tokens they write for the same task. Measure output tokens on your own prompts before you decide on price.

Where each one is stronger

Grok 4.6 is pitched by xAI as frontier-level in three areas: coding, knowledge work and STEM. It aims to reach top-tier results on math and science reasoning and on research tasks, at a mid-tier price.

GPT-5.6 Terra is the balanced model of OpenAI's GPT-5.6 family and covers everyday coding, reasoning and agent work. Its context window is about twice Grok 4.6's, which matters as soon as you want to hand it a whole repository or a long set of documents in one request.

Things to watch

Grok 4.6: 500K tokens is a modest window at this price. For whole-codebase work, Terra has the edge.

GPT-5.6 Terra: it trades peak performance in any one field for overall balance. On hard STEM problems, Grok 4.6 may land where Terra doesn't. Terra also doesn't take PDF input on FastMetal at the moment, so extract the text first or send pages as images.

Which one to use

• Grok 4.6: math and science reasoning, research support, coding where you want to try a model built for depth in a few areas.
• GPT-5.6 Terra: long-context jobs, reuse of OpenAI-tuned prompts, or one model that has to handle many kinds of task.

xAI has since released Grok 4.7, at the same price as Grok 4.6 on FastMetal; it's worth including if you're testing Grok today.

Try both

Same key, same balance; only model changes:

curl https://api.fastmetal.ai/v1/chat/completions \
  -H "Authorization: Bearer $FASTMETAL_API_KEY" \
  -H "Content-Type: application/json" \
  -d '{"model": "grok-4.6", "messages": [{"role": "user", "content": "Hello"}]}'

curl https://api.fastmetal.ai/v1/chat/completions \
  -H "Authorization: Bearer $FASTMETAL_API_KEY" \
  -H "Content-Type: application/json" \
  -d '{"model": "gpt-5.6-terra", "messages": [{"role": "user", "content": "Hello"}]}'

Real answers from both models to the same prompts are on the comparison page, and details are on the Grok 4.6 and GPT-5.6 Terra pages.