GPT-5.6 Terra vs Claude Sonnet 5: which mid-tier model to use

Flagships are expensive and small models fall short, so most production work ends up on a mid-tier model. GPT-5.6 Terra and Claude Sonnet 5 are OpenAI's and Anthropic's entries at that tier.

The short answer: pick Sonnet 5 for multi-step agents and coding, where staying consistent over a long run matters. Pick Terra when your stack and prompts are already built around OpenAI models.

Specs and price

Input costs the same. Output on Terra costs 20% more, and Terra's rates step up to $4.22 input and $18.99 output per million on prompts over 272K tokens; Sonnet 5 has one rate across its window. Prices are what a USD account pays today; the current figures are always on the pricing page.

The per-token rate is only half of the bill. How many tokens each model writes for the same task can matter more than a 20% rate difference, so measure output on your own prompts before deciding on price.

Where each one is stronger

GPT-5.6 Terra is the balanced model of the GPT-5.6 family, built to cover everyday coding, reasoning and agent work at a lower rate than the flagship Sol. If your system was written against OpenAI models, Terra behaves the way your prompts expect.

Claude Sonnet 5 is Anthropic's mid-tier model, positioned as the balance point between speed and intelligence. Its reputation is in coding and agent work: it follows instructions consistently and holds its output together over long workflows.

Things to watch

Terra: it covers a lot of ground without standing out in one area. Test it on the hardest prompts in your workload before committing.

Sonnet 5: it behaves differently from OpenAI models in the details, tool-call style among them. Prompts tuned for OpenAI models may need some rework.

Neither route takes PDF input on FastMetal at the moment. Send text or images, or extract the document's text first.

Which one to use

• GPT-5.6 Terra: you already have OpenAI-tuned prompts, you want to stay within one GPT-5.6 family across tiers, or the work is everyday tasks with images.
• Claude Sonnet 5: multi-step agents, coding assistance, and long-context work where the model has to keep following the same instructions.

Anthropic has since released Claude Sonnet 5.5 at the same price as Sonnet 5. If you're choosing a Sonnet today, compare it too; we measured it against GPT-6.1 Sol in GPT-6.1 Sol vs Claude Sonnet 5.5.

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": "gpt-5.6-terra", "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": "anthropic-claude-sonnet-5", "messages": [{"role": "user", "content": "Hello"}]}'

Real answers from both models to the same prompts are on the comparison page, and details are on the GPT-5.6 Terra and Claude Sonnet 5 pages.