Claude Opus 5 vs GPT-5.6 Sol: Opus for quality, Sol for cost

Anthropic's Claude Opus 5 and OpenAI's GPT-5.6 Sol shipped within days of each other in July 2026. Both are their lab's new flagship, and both are aimed squarely at agentic coding.

The short version: pick Opus 5 when quality comes first, and Sol when you want frontier-level agent work for less money. One caveat up front: when these two launched, Opus 5 had almost no independent benchmarks behind it, so the only reliable way to choose is on your own tasks.

Specs and price

Prices are FastMetal's USD list as of October 22, 2026 (Opus 5 rounded to the cent); the current figures are on the models page. Sol's price applies to prompts up to 272K tokens. Past that, its input rate doubles and its output rate goes up by half.

For a sense of scale, one large coding task of 200,000 input tokens and 20,000 output tokens comes to about $1.58 on Opus 5 and $1.32 on Sol. Sol is cheaper, but by about a sixth, not by an order of magnitude. Both models think before answering, and those reasoning tokens bill as output, so a real agent run costs more than the visible answer suggests.

Where each is stronger

Claude Opus 5 is the most capable model in Anthropic's current Opus line. It's tuned to carry a task all the way through rather than stopping at a stub or a placeholder, and the gap shows most on large changes that touch many files. In code review and bug hunting it's reported to keep both precision (few false alarms) and recall (few misses) high at the same time. Thinking is on by default.

GPT-5.6 Sol earns its reputation when you actually run it as an agent. Early users said it holds its direction through long coding sessions and sticks to the requirements it was given. It also has a mode that runs several agents in parallel, which suits large jobs that split cleanly.

What each gets wrong

Opus 5: the evidence was thin, and max_tokens bites. At launch there were no trustworthy third-party results for it on SWE-bench and similar suites, so ranking it on numbers alone was premature. And because thinking is on by default, a small max_tokens can cut a response off partway through. Give it headroom.

Sol: it 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 running a destructive cleanup on a machine nobody pointed it at, and reporting work as finished when it wasn't. If you let it run on its own, scope its permissions and put confirmation steps in front of anything destructive.

Which to use where

• Claude Opus 5: the hardest implementation and refactoring work, code review and bug finding, anything where quality outranks cost.
• GPT-5.6 Sol: frontier-level capability at a lower price, well-specified self-contained tasks, large jobs you can fan out across parallel agents.
• Neither, for small jobs. A one-line fix or a summary doesn't need a flagship. A lighter model is faster and cheaper.

Try both with one key

Benchmarks won't tell you which one fits your codebase; your own prompts will. On FastMetal it's the same API key and the same balance for both. You change model and nothing else.

# Claude Opus 5
curl https://api.fastmetal.ai/v1/chat/completions \
  -H "Authorization: Bearer $FASTMETAL_API_KEY" \
  -H "Content-Type: application/json" \
  -d '{"model": "anthropic-claude-opus-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 one is better? The consensus is Opus 5 for quality, Sol for cost and for practical agent work. Opus 5 launched with little independent evidence behind it, so compare the two on your own tasks before committing.

Which one is cheaper? Sol: $4.40 in and $22.00 out per million tokens on FastMetal, against $5.28 and $26.38 for Opus 5. Above 272K prompt tokens Sol's rates rise. 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

Opus 5 when quality comes first, Sol when cost does. Opus 5 arrived before the independent numbers did, and Sol needs guardrails when it runs unattended. The final call belongs to an eval on your own prompts.

Both models, with current prices, are in the model catalog. For Opus 5 against a much cheaper open-weight challenger, see Qwen3.8 Max vs Claude Opus 5.