Claude Opus 4.8 vs GPT-5.6 Sol: The Value Flagships

Claude Opus 4.8 shipped May 28, 2026 as Anthropic's flagship tier and recommended starting point for complex agentic coding. GPT-5.6 Sol went GA July 9, 2026 as the only GPT-5.6 tier with max reasoning effort and ultra mode. They land within 10% of each other on price, which makes this the rare comparison where cost mostly cancels out:

On cost, the only daylight is output: both charge $5 per million input tokens, while Sol's $30 output rate runs 20% above Opus's $25. A story consuming 500k input and 100k output tokens costs about $5.00 on Opus versus $5.50 on Sol, real, but rarely decision-grade. Deeper single-model breakdowns: Claude Opus 4.8 for coding and GPT-5.6 Sol for coding.

Here is what makes this pairing unusual. In the Fable 5 vs Sol comparison, the vendors publish entirely different benchmarks and nothing lines up. Opus and Sol are different: both have SWE-bench Pro numbers, so for once there is a shared yardstick. Claude Opus 4.8 scores 69.2% on the Scale leaderboard; third-party runs place GPT-5.6 Sol at 64.6%. That is a 4.6-point lead for Opus on repo-level issue resolution, at essentially the same price.

The caveat matters, so here it is plainly: Sol's 64.6% comes from third-party runs, not OpenAI-published numbers. OpenAI has published no SWE-bench results for Sol at all, and third-party harnesses can differ from vendor runs in scaffold, retries, and budget. The direction of the gap is consistent with everything else we know, but the exact size of it should be held loosely until more replications land.

It is also one yardstick, not the whole story. Opus posts 88.6% on SWE-bench Verified (swebench.com leaderboard), where Sol has no number; Sol posts 88.8% on Terminal-Bench 2.1, where Opus has none. The shared benchmark tells you who leads on resolving real GitHub issues; it says nothing about the terminal-agent work where Sol's published record is strongest.

Beyond benchmarks, the two flagships spend their compute differently. Opus 4.8's effort defaults to high, it is thorough on multi-file changes without being told to be, and its 1M-token context holds a large repo in one session. Sol's headline is the opposite shape: an exclusive max reasoning effort level, plus ultra mode, which runs parallel subagents natively and lifts Terminal-Bench 2.1 from 88.8% to 91.9% in OpenAI's launch materials.

Note the pricing asymmetry hiding in the capability story: Opus's speed lever, fast mode, is priced as a separate tier (Fast mode $10/$50), while Sol's ultra mode is a capability of the same $5/$30 model, it spends more tokens rather than charging a higher rate.

Because these two are so close on price, the cheapest experiment in AI-assisted engineering is running them head-to-head on your own backlog. That is the setup AIDEN is built for: a macOS desktop app that orchestrates your existing Claude Code and Codex CLIs on one kanban board, BYOK, local-first, every story gated behind an approved spec. Assign the same story once to Opus and once to Sol, each on its own git branch, and review the two PRs side by side, or split the backlog by task shape, repo-heavy stories to Opus, terminal-heavy ones to Sol. The full lineup lives at AI models for coding, and the tooling comparison at Claude Code vs Codex.