Flagship
Sol · the Sun
The strongest, priciest tier.
Best at coding, science, cybersecurity. Unlocks two exclusives: "max reasoning" (lets the model think longer) and "ultra mode" (splits work across many sub-agents).
AI Writings · Model Watch
GPT-5.6 Sol vs Claude Fable 5 & Opus 4.8.
OpenAI just launched GPT-5.6 in three tiers: Sol (the strongest), Terra (balanced) and Luna (cheap, fast). This piece compares the strongest one — Sol — with Anthropic's two top models in plain language: where each is strong, where each is weak, what they cost, and which to pick.
OpenAI announcing the three GPT-5.6 models: Sol, Terra and Luna — via OpenAI.
OpenAI changed how it names models. From now on the number is the "generation" (5.6), while Sol / Terra / Luna are three fixed capability tiers — think Sun, Earth and Moon. Future generations keep these same names, just stronger.
The twist: the strongest one — Sol — isn't open to everyone yet. It shipped as a controlled preview, available first to around 20 government-approved companies, because Sol is also good at sensitive areas like cybersecurity. The wide public release comes later. This mirrors Anthropic: Claude also splits a safe version (Fable) from a guardrails-lifted one for approved partners (Mythos).
01 · The GPT-5.6 trio
Sol, Terra, Luna.
Three names for three tiers: the higher you go, the smarter but pricier. Pick by the job — the strongest isn't always the right fit.
Balanced
Terra · the Earth
Balanced for everyday work.
Roughly as capable as the previous GPT-5.5 but about half the price. The sensible default for most office work, writing and customer support.
Budget
Luna · the Moon
Cheapest, fastest.
Prioritizes low cost and speed for high-volume, simple work: classifying, summarizing, quick replies. Not for hard problems that need deep reasoning.
Part A · Technical comparison
02 · Head to head, technically
Sol vs Fable 5 vs Opus 4.8.
Comparing each side's strongest. Fable 5 is Anthropic's most powerful public model; Opus 4.8 is the familiar, cheaper workhorse. Swipe sideways to see the whole table.
| Criteria | GPT-5.6 SolOpenAI | Claude Fable 5Anthropic | Claude Opus 4.8Anthropic |
|---|---|---|---|
| Positioning | OpenAI's next-gen, strongest model | Strongest public model ("Mythos" class) | Workhorse, balanced price vs performance |
| Command-line coding Terminal-Bench 2.1 |
88.8% (ultra: 91.9%) |
83.4 – 84.3% | Below Fable 5 |
| Real GitHub bug-fixing SWE-Bench Pro |
Not published | 80.3% | 69.2% |
| Price (per million tokens) | ~$5 in / $30 out | $10 in / $50 out | $5 in / $25 out |
| Standout trait | "Ultra" mode with many sub-agents; strong at cybersecurity | Leads on long & complex tasks, spreadsheets (ViBench) | ~4× less likely than the old version to miss flaws in its own code |
| Availability | Controlled preview → wide release later | Globally available (some sensitive queries answered by Opus 4.8 instead) | Widely available |
Figures from OpenAI & Anthropic announcements and public benchmark tables as of July 2026. Each vendor picks the metrics that flatter it, so treat these as rough references, not absolute truth.
03 · Strengths & weaknesses
Who's good at what?
There's no "best at everything" model. The race is splitting: each side wins on a different kind of work.
OpenAI
GPT-5.6 Sol
-
Strong: command line & speed
Leads on running terminal commands, replies fast, and is about half the price of Fable 5 per output token.
-
Strong: "ultra" mode
Splits one big job across many sub-agents — great for complex problems needing several directions at once.
-
Weak: hard to actually get
It's a controlled preview, not open to all. If you need it now and stable, your turn may not have come.
-
Weak: missing proof
No published score on real bug-fixing (SWE-Bench Pro) — where Claude leads — so it's hard to claim it wins on hard, long tasks.
Anthropic
Fable 5 & Opus 4.8
-
Strong: long & complex work
Fable 5 clearly leads on SWE-Bench Pro (80.3%) — fixing real GitHub bugs. The longer and more multi-step the task, the wider the gap.
-
Strong: trustworthy & ready
Already globally available, rarely misses flaws in its own code, strong on spreadsheets and real office tasks.
-
Strong: Opus 4.8 is cheap & solid
Just $5 in / $25 out — cheapest here — and still beats GPT-5.5 on real bug-fixing. The safe pick for most projects.
-
Weak: pricey & sometimes gated
Fable 5 is twice Sol's price on output; some sensitive questions get routed to Opus 4.8 to answer, so it isn't always "the strongest model" you asked for.
Part B · Who it's for
04 · Who it's for
Who are you — which model fits?
The technical numbers above only matter against your real work. Here are six common user types — each gets one primary recommendation, plus a fallback.
Group 01
Developers & engineering teams
Best pickGPT-5.6 Sol
Terminal coding, multi-step automation, needing fast replies and low output cost. Sol leads on the command line and is about half the price of Fable 5.
No preview yet? Use Claude Fable 5 or Opus 4.8 for a stable option today.
Group 02
Teams on complex, long-running projects
Best pickClaude Fable 5
Multi-step systems, big refactors, high reliability needed. Fable 5 clearly leads on real bug-fixing (80.3%) and rarely misses flaws in its own code.
Want to save: run Opus 4.8 for most work, escalate to Fable 5 only on the hardest parts.
Group 03
SMEs & business operators
Best pickClaude Opus 4.8
You need something cheap, stable and ready now for operations, accounting, customer support. Opus 4.8 is the cheapest top-tier ($5/$25) yet solid.
A safe starting point before considering pricier tiers when you truly need them.
Group 04
Marketing, content & office work
Best pickGPT-5.6 Terra
Writing, emails, summaries, spreadsheets. You don't need the strongest tier — Terra is as capable as the prior generation but about half the price, plenty for daily work.
Equivalent alt: Claude Opus 4.8 if you're already on Claude.
Group 05
Startups building AI products, cost-sensitive
Best pickSol / Terra
When tokens are your marginal cost, low output price drives your margin. Sol is half the price of Fable 5; Terra is cheaper still for lighter work.
Caveat: if you need stability in production, weigh Opus 4.8 to avoid depending on a preview.
Group 06
Individuals & tight budgets
Best pickLuna / Opus 4.8
Q&A, studying, small daily tasks. Prioritize cheap and fast: Luna is GPT-5.6's budget tier; Opus 4.8 is the balanced choice if you want more reliability.
You don't need Sol or Fable 5 — no point paying for power you won't use.
05 · Takeaway
The race is splitting — no more "one model wins everything".
GPT-5.6 Sol is fast, cheap and strong at the command line — but still hard to get and short on proof for the hardest tasks. Claude Fable 5 leads on long, complex work but costs twice as much. Opus 4.8 is the balanced pick: cheap and always available.
For business, the lesson is unchanged: measure on your own real work, choose the cheapest model that still hits the quality you need, and don't switch vendors just because there's a new name. Durable advantage lives in the process around AI, not in using this week's hottest model.
"There is no best model — only the one that best fits the job you need done and the budget you have."
Note: figures compiled from OpenAI & Anthropic announcements and public benchmark tables as of 07/2026. New-model information changes fast — re-verify pricing and availability before deciding.