Three frontier-adjacent API models landed within 24 hours of each other in late September 2026: Claude Opus 5.5, GPT-6 Sol with its cheaper sibling Luna, and Grok 4.7. Their list prices look easy to rank, but the real bill depends on context thresholds, caching rules and how many tokens each model burns per task. This guide puts all four SKUs on the same workloads and shows where the headline numbers mislead.
This comparison is based on official documentation, published specifications, and vendor pricing verified as of October 26, 2026. Anthropic's model page was opened directly. OpenAI's two launch posts on X were retrieved directly, but OpenAI's pricing and model pages and xAI's docs could not be opened in this session. Figures from those pages are marked with a dagger (†) and were cross-checked across several independent trackers that cite them. Cost figures marked "derived estimate" are my own arithmetic on list prices, not vendor numbers.
Which models are actually being compared
The lineup is not like-for-like. OpenAI's flagship is GPT-6 Astra at $10/$50 per million tokens; OpenAI's launch post positions Sol and Luna as cheaper tiers beneath it. A week later, GPT-6.1 Sol arrived at the same list price and is now described as the replacement for GPT-6 Sol†. So "Sol" below means both, and the differences are called out where they exist.
| Spec | Claude Opus 5.5 | GPT-6 Sol / 6.1 Sol† | GPT-6 Luna† | Grok 4.7† |
|---|---|---|---|---|
| API ID | claude-opus-5-5 | gpt-6-sol / gpt-6.1-sol | gpt-6-luna | grok-4.7 |
| Released | Sep 22, 2026 | Sep 22 / Sep 29, 2026 | Sep 22, 2026 | Sep 21, 2026 |
| Context window | 1M | 1.05M | 1.05M | 500K |
| Max output | 128K | 128K | 128K | Not clearly published |
| Input / output per 1M | $4 / $20 | $2 / $10 | $0.10 / $0.50 | $2 / $6 |
| Cached input per 1M | $0.20 | $0.20 (6) / $0.10 (6.1) | $0.01 | $0.50 |
| Knowledge cutoff | Jun 2026 | Apr 20 (6) / Apr 30 (6.1), 2026 | May 18, 2026 | May 2026 |
Opus 5.5's row comes from Anthropic's model overview (the Chinese-language edition of the page, which carries the same spec table). The same page lists Sonnet 5.5 at $2/$10 and Fable 5.1 at $10/$50, which matters later: Sonnet 5.5 has the same list price as Sol.
Same workloads, one price scale (derived estimate)
Per-million-token prices hide the structure of real requests. The table below applies list prices to four fixed workloads. Assumptions: no batch discount, and Claude's 1M window billed at the flat rate (reported by trackers; I could not open Anthropic's pricing page itself).
| Workload (derived estimate) | Opus 5.5 | GPT-6.1 Sol | GPT-6 Luna | Grok 4.7 |
|---|---|---|---|---|
| A. One request, 100K in / 20K out | $0.80 | $0.40 | $0.02 | $0.32 |
| B. 1M in + 1M out (blended) | $24.00 | $12.00 | $0.60 | $8.00 |
| C. One request, 300K in / 20K out | $1.60 | $1.50 | $0.075 | $1.44 |
| D. Agent step: 150K cached prefix + 5K new in / 5K out | $0.15 | $0.075 | $0.0045 | $0.115 |
| E. Same as D, but 200K cached prefix | $0.16 | $0.08 | $0.005 | $0.28 |
Three readings follow. In A and B, Sol costs half of Opus 5.5 and Grok sits between them, so the order matches list price. In C, the gap nearly vanishes: Sol, Grok and Opus land within 10% of each other because OpenAI and xAI both raise rates on long prompts. In E, Grok goes from second-cheapest to most expensive of the three flagship-class models, purely because its prompt crossed 200K tokens. Workload E uses a long-tier cached rate of $1.00 per million, which I took from a tracker, so treat that row as the least certain.
Where the list price stops telling the truth
Threshold cliffs
OpenAI defines short context as up to 272K input tokens. Above that, the whole request is billed at 2x input and cache rates and 1.5x output†, so Sol becomes $4/$15. Grok 4.7 doubles at 200K†: $4 input, $1 cached, $12 output, and trackers report the higher rate applies to the entire request, not only the excess. Claude Opus 5.5 is the only one of the three with no reported long-context premium. If your prompts routinely sit between 200K and 272K, Sol is the safer choice; above 272K, the price advantage over Claude mostly disappears.
Token volume, not token price
Per Artificial Analysis (relayed through secondary coverage†), Grok 4.7 at xhigh effort used roughly 81K output tokens per Intelligence Index task and cost $3.74 per task, against $0.72 for GPT-6.1 Sol. That is about five times more per task on a model whose list output price is 40% lower. Grok's own price per token did not change from 4.6; its cost per task roughly doubled. I could not retrieve a comparable per-task figure for Opus 5.5, so it is absent here rather than guessed.
Thinking you cannot turn off
On Opus 5.5, adaptive thinking is always on and cannot be disabled; you steer depth with the effort parameter, whose default is medium (Anthropic model overview). GPT-6.1 Sol dropped the "none" effort setting, so its minimum is low†. GPT-6 Sol and Luna still allow none†, and that matters because Chat Completions function calling works only at effort none; at any other effort you need the Responses API†. Cheap, fast, tool-calling workloads therefore belong on Luna or the older Sol, not 6.1.
Where vendor claims and independent numbers diverge
- "50% cheaper." OpenAI Developers describes the cut against GPT-5.6 promotional pricing, not against the earlier regular price. Luna's output rate fell 58%, its input 50%†.
- Sol versus Claude. OpenAI's launch charts claim Sol beats or matches Anthropic models on several agentic benchmarks at lower cost. On Artificial Analysis's broad Intelligence Index v4.3.2, as relayed†, Opus 5.5 scores about 58, Sonnet 5.5 about 56, GPT-6.1 Sol about 52 and Grok 4.7 46. Index versions are rebased, so only compare scores carrying the same version.
- Grok 4.7's output limit. One tracker lists 450K output tokens; another says xAI publishes no separate maximum. Until xAI's docs say otherwise, do not design around 450K.
- Grok 4.7's size. Coverage repeated a 2.1-trillion-parameter figure, but it traces to pre-launch posts by Elon Musk, not xAI's model documentation†.
Which model for which job
- High-volume extraction, routing, summarization: Luna. At $0.02 for workload A it is 15 to 40 times cheaper than the other three. OpenAI's own positioning for Luna is clerical, high-volume work†.
- Cost-sensitive coding agents with a stable, cached prefix: GPT-6.1 Sol. Workload D shows it at half of Opus's cost, helped by the $0.10 cached rate.
- Best independent reasoning score and flat long-context pricing: Opus 5.5, accepting roughly 2x Sol's list price. Verify on your own tasks first; the index gap is a single benchmark.
- Cursor or Grok Build users: Grok 4.7 is the default model there†. Outside those tools, the verbosity penalty and 200K cliff weaken its price case.
- Same price tier as Sol: Claude Sonnet 5.5 lists at $2/$10 on Anthropic's page, so a Sol-versus-Sonnet test is a fairer cost comparison than Sol-versus-Opus.
What each one cannot do
- Opus 5.5: thinking cannot be disabled; forced tool use returns an error; the older computer_20251124 tool is no longer accepted on the Claude API; thinking blocks are tied to the model that produced them (Anthropic model overview and its linked changes). Fast mode is separately priced.
- GPT-6 Sol and Luna: text and image input, text-only output, no audio or video†. At launch they were not in ChatGPT's regular Chat mode, only in ChatGPT Work and Codex (Luna also in the desktop app for Free and Go users)†. They are not offered on the API Free Tier†. GPT-6.1 Sol has no "none" effort.
- Grok 4.7: 500K context is half of Claude's and under half of OpenAI's. Batch API is not supported†. The Fast variant, billed at 2x, exists only inside Cursor and Grok Build†. Input is text and image only†.
FAQ
Is GPT-6 Sol the same as GPT-6.1 Sol?
No. They are separate API IDs (gpt-6-sol and gpt-6.1-sol) with the same $2/$10 list price. GPT-6.1 Sol cuts cached input to $0.10 and removes the "none" effort setting†.
Which of the three is cheapest?
GPT-6 Luna by a wide margin. Among the larger models, Grok 4.7 has the lowest output price, but its per-task cost can exceed Sol's because it spends more tokens.
Does Claude Opus 5.5 charge more for long prompts?
Trackers report a flat rate across the 1M window. Anthropic's pricing page should be your final check before budgeting a long-context workload.
Last verified: October 26, 2026. Models in this segment have shipped roughly monthly, so re-check list prices before committing a budget.
Sources
Pages marked "referenced" could not be opened directly in this session; their figures were cross-checked through independent trackers.
- Anthropic Claude Platform Docs — Claude Opus 5.5 overview (opened directly)
- OpenAI on X — GPT-6 Sol and Luna rollout announcement (opened directly)
- OpenAI Developers on X — Sol and Luna API pricing announcement (opened directly)
- OpenAI API Docs — Pricing (referenced)
- OpenAI API Docs — GPT-6 Sol model page (referenced)
- OpenAI API Docs — GPT-6 Luna model page (referenced)
- OpenAI API Docs — GPT-6.1 Sol model page (referenced)
- xAI Docs — Models and pricing (referenced)
- xAI — Introducing Grok 4.7 (referenced)
- Artificial Analysis — Benchmarking Grok 4.7 (referenced)