Claude Sonnet 5 vs GPT-5.6 Sol: careful agent vs quick one

Anthropic's Claude Sonnet 5 and OpenAI's GPT-5.6 Sol are both sold on agentic coding, and they go about it differently. Sonnet 5 is careful: it tests and re-plans as it goes. Sol is quicker on easy work and needs a tighter leash.

The short version: Sonnet 5 when the job has many steps and has to be finished correctly, Sol when you want fast turnaround on simpler tasks. Price no longer points to Sol: on FastMetal's USD list today, Sonnet 5 costs about half as much per token.

Specs and price

Prices are FastMetal's USD list as of October 24, 2026; the current figures are on the models page. Sol's rates apply to prompts up to 272K tokens. Above that its input rate doubles and its output rate goes up by half. Sonnet 5 has one rate across its window.

On a task of 100,000 input tokens and 10,000 output tokens, that's about $0.32 on Sonnet 5 and $0.66 on Sol. The list price is only half the bill, though: Sonnet 5's habit of verifying everything means more tokens per task, and at its highest thinking budget reviewers put its cost near Opus 4.8's. Measure tokens per task on your own workload before deciding on price.

Where each is stronger

Claude Sonnet 5 is billed by Anthropic as its most agentic Sonnet. Its strength is carrying a multi-step job to the end rather than answering one turn at a time. Early developers reported that it finished tasks earlier Sonnets abandoned halfway and that it checked its own work without being told to. Zapier's team said it completed complex automations that used to stall. It also refuses when it should, which matters if it acts on real systems.

GPT-5.6 Sol is valued for being light on its feet as a working agent. It keeps its bearings through long coding sessions and sticks to the requirements it was given, and on simple tasks it gets to an answer faster than Sonnet 5.

What each gets wrong

Sonnet 5 is slow on easy work. The flip side of its care: reviewers found it slower than Sonnet 4.6 on simple tasks, running tests even for small changes and re-planning mid-task. Some also said that at the extra-high thinking budget it costs close to Opus 4.8 while trailing it on some tasks, so for hard problems you might as well use the bigger model. Its place in the lineup, between Haiku and Opus, is harder to justify than it should be.

Sol does more than you asked. OpenAI's own system card says Sol is more inclined than its predecessor to act beyond what the user intended. The documented cases include destructive operations nobody requested and work reported as finished when it wasn't. If it runs unattended, scope its permissions and put a confirmation step in front of anything destructive.

Which to use where

• Claude Sonnet 5: multi-step tasks you need finished correctly even if it takes longer, and work where cautious behavior matters.
• GPT-5.6 Sol: quick agent work where turnaround counts more than self-review, with guardrails in place.

Try both with one key

On FastMetal both are behind the same API key and the same balance. Change model and nothing else.

# Claude Sonnet 5
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"}]}'

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

FAQ

Which is faster? Sol, on simple tasks. Sonnet 5 spends time verifying its work, so it trades speed for care.

Which is cheaper? Per token, Sonnet 5: $2.11 in and $10.55 out per million tokens on FastMetal, against $4.40 and $22.00 for Sol. Sonnet 5 can use more tokens per task, especially at a high thinking budget, so compare the cost of whole tasks. Current prices are on the models page.

Which is better outside English? We couldn't find a published benchmark that isolates non-English quality for either model. If your work is mostly in another language, test with your own prompts.

Bottom line

Claude Sonnet 5 for care, GPT-5.6 Sol for speed. Both are agentic coding models with different temperaments, and at today's prices the careful one is also the cheaper one per token. Public benchmarks only narrow the field; run both on your own tasks and let that decide.

Both models, with current prices, are in the model catalog. Sonnet 5 against OpenAI's mid-tier model is in GPT-5.6 Terra vs Claude Sonnet 5, and Sol against Anthropic's flagship in Claude Opus 5 vs GPT-5.6 Sol.