GPT-5.6 vs Grok 4.6: Level on the Index, Split on Context and Price

On the Artificial Analysis Intelligence Index scale that both were measured against through early September 2026, OpenAI's GPT-5.6 — whose flagship tier, GPT-5.6 Sol, is what the gpt-5.6 API name routes to — and SpaceXAI's Grok 4.6 scored the same number: 61. An independent index placing two rival flagships level is the rarest kind of result in a frontier comparison, and it is where this matchup gets interesting rather than where it ends. The models that tied are sold very differently. GPT-5.6 Sol, the flagship OpenAI released on July 9 and repositioned as its value tier when GPT-6 Astra launched on September 3, lists at $4.00 per million input tokens and $20.00 per million output after an August 24 price cut, and reads up to roughly 1.05 million tokens of context. Grok 4.6, which xAI released on August 12, lists at $2.00 / $6.00 and tops out at 500,000 tokens. Same score on the independent board, and then the two models diverge on how far you can push a single request — and on what pushing it costs.

This page exists now for a concrete reason: the two rate cards and the two ceilings moved within weeks of each other. Grok 4.6 shipped August 12 and on August 28 became available in normal Grok chats on web and mobile after an initial two weeks confined to Grok Build and the API. GPT-5.6 Sol's promotional cut landed August 24, and ten days later GPT-6 Astra's launch quietly demoted Sol from flagship to value flagship. Both of the models below are now what you reach for when you want frontier-adjacent reasoning without paying Astra-class prices — which is exactly why the 61-to-61 tie has become the live decision point rather than a trivia question.

What "tied at 61" means, and what it does not

The score needs a date and a ruler. Artificial Analysis measured GPT-5.6 Sol at ~61 and Grok 4.6 at 61 on the index scale in use through early September — the scale this blog's other GPT-5.6 matchups cite — with Grok ranked 11th of the 202 models AA tracks and Sol within a couple of ranks of it at the same score. AA then recalibrated the index on September 7 (v4.3), a version that compresses scores: GPT-5.6 Sol measures 47 there, GPT-6 Astra and Claude Fable 5.1 measure 53, and no overall Grok 4.6 score has been published on the new scale yet. The tie below is therefore a statement about the pre-recalibration September scale, and it is best read as relative position: on the most recent index that scored both, these two were level, and both sat just behind the Claude Opus 5 class.

One more caveat on the tie, and it matters for how you use it. The two scores were produced at different effort settings — GPT-5.6 Sol at max reasoning, OpenAI's highest dial setting (which runs four parallel subagents on hard requests), and Grok 4.6 at high, xAI's default on its low-to-xhigh dial. Both are "give it everything" configurations, but they are not the same configuration, and the tie is between those two specific settings rather than between every setting the models offer. What the equal score does establish is that neither model holds a general-intelligence advantage the other camp can point to on an independent composite — so a buyer choosing between them has to look past the index, at context, price, and the evidence each side can actually show.

Two rate cards, two cliffs

Both vendors bill a long prompt as a premium event, and both bill the entire request at the higher tier once a threshold is crossed — that is the shared mechanic that makes this price comparison legible. OpenAI's rate for GPT-5.6 Sol is $4.00 / $20.00 per million with cached reads at $0.40, and once a request passes roughly 272K input tokens the whole request is repriced to $8.00 / $30.00 — the long-context structure Epoch AI documented on September 8. xAI's rate for Grok 4.6 is $2.00 / $6.00 with cached reads at $0.50, and once a prompt passes 200K tokens the entire request moves to $4.00 / $12.00.

• Released — GPT-5.6: July 9, 2026 (public API; the gpt-5.6 alias reaches the Sol tier). Grok 4.6: August 12, 2026.

• List price per 1M (in / out) — GPT-5.6 Sol: $4.00 / $20.00, cached reads $0.40 (promo through at least Nov 21, 2026). Grok 4.6: $2.00 / $6.00, cached reads $0.50.

• Long-prompt tier — GPT-5.6 Sol: whole request to $8.00 / $30.00 past ~272K input. Grok 4.6: whole request to $4.00 / $12.00 at 200K input.

• Context / output ceiling — GPT-5.6 Sol: ~1.05M in, 128K out. Grok 4.6: 500K in, no published output cap.

• Index (independent) — GPT-5.6 Sol (max) ~61 vs Grok 4.6 (high) 61, pre-recalibration September scale.

• Modality — both accept text and image input and produce text.

Run the worked numbers and the pattern is unambiguous: at every prompt length Grok 4.6 can serve, it is the cheaper call, because its repriced ceiling still lands at or below GPT-5.6 Sol's standard rate.

• 100K in / 8K out — GPT-5.6 Sol: 0.10 × $4 + 0.008 × $20 = $0.56. Grok 4.6: 0.10 × $2 + 0.008 × $6 = $0.25. Grok is roughly 55% cheaper on the request.

• 400K in / 30K out (both repriced) — GPT-5.6 Sol: 0.40 × $8 + 0.03 × $30 = $4.10. Grok 4.6: 0.40 × $4 + 0.03 × $12 = $1.96. Grok is still roughly half the price even after its own cliff.

• 600K in / 30K out — GPT-5.6 Sol: 0.60 × $8 + 0.03 × $30 = $5.70. Grok 4.6: cannot — 600K exceeds its 500K context window. This is the band where Sol is the only option, and where its price stops looking premium.

The cliff geometry differs in one way that favors GPT-5.6 Sol: its threshold (272K) sits higher than Grok's (200K), so there is a band — roughly 200K to 272K of input — where Grok has already repriced and Sol has not. Even there Grok tends to win on dollars, because its repriced output rate of $12 is still 40% below Sol's standard $20. The economics only flip in Sol's favor past 500K tokens, which is exactly the region Grok's context window cannot enter. If your prompt lengths stay under 200K — true of most chat, RAG, and code-assist traffic — Grok 4.6 is cheaper at scale by close to a factor of two on blended cost, and the whole-request cliff means you should treat 200K as a planning boundary rather than a soft suggestion.

What Sol's extra half-million tokens are actually for

Grok 4.6's 500K window fits more real workloads than the spec sheet suggests — whole-codebase reads, long agent transcripts, big retrieval contexts — and its lack of a published output cap helps on the generation side: for a single call that must emit a very long artifact, Grok has no documented ceiling while GPT-5.6 Sol stops at 128K output tokens. GPT-5.6 Sol's advantage sits in the 500K-to-1.05M band, and the workloads that genuinely need it are narrower than the marketing implies: agents that hold an entire repository plus a long task history in context, very long meeting or research transcripts analyzed in one pass, and retrieval jobs over corpora too big to chunk into Grok-sized windows. If none of your pipelines touch that band, Sol's extra context is headroom you are paying the $4.00 input rate to not use.

The two models also steer reasoning differently. GPT-5.6 Sol exposes a low / medium / high / max effort dial, where max runs four parallel subagents that coordinate on hard tasks — effectively a small agent swarm per difficult request, powerful but expensive in output tokens, and the setting behind the ~61 index score. Grok 4.6 runs a single model with a low-to-xhigh dial (default high) and server-side tools for search and code execution, which makes its behavior more predictable to budget: one request, one model, a cost you can estimate from the rate card. For a developer who wants deterministic spend, Grok's single-model design is operationally simpler; for the hardest long-horizon tasks, Sol's max-effort swarm is the more capable configuration — and the reason its best score and its worst bills come from the same setting.

The evidence each side can actually show

Neither model has an independent coding scoreboard advantage, so the citable evidence splits by vendor. OpenAI reports GPT-5.6 Sol at roughly 96% on SWE-bench Verified — the strongest coding citation either model can show, but a reported evaluation with no independent audit published yet — and Sol's real-world edge includes living inside the Codex and ChatGPT tooling. xAI reports Grok 4.6 at 94.9% on GPQA Diamond, which it bills as the highest score tested on that benchmark, and cites agentic evaluations averaging roughly $0.84 of API cost per comprehensive task; both are vendor numbers. On speed the two are closer than the price gap suggests: AA measured Grok 4.6 at 64.9 output tokens per second and GPT-5.6 Sol in the same mid-60s range on standard serving, with OpenAI's separate Cerebras-powered "Ultrafast" preview (up to roughly 750 tokens per second) still limited and unpriced. Read the whole evidence table as: index tie, coding receipts on both sides with no independent audit on either, and no speed gap that changes the decision.

Which default is rational in September 2026

Choose Grok 4.6 when your traffic fits under 200K tokens of input — the price is close to half at every length it serves, the independent index says the models are level, and a single-model dial with server-side tools keeps the bill predictable. Choose GPT-5.6 Sol when you genuinely need the 500K-to-1.05M context band, when your stack is already Codex- or ChatGPT-native and the reported ~96% SWE-bench figure gives procurement a coding receipt, or when you want the option of the four-subagent max-effort configuration for the hardest long-horizon tasks. And watch two dates: GPT-5.6 Sol's $4/$20 promotion is only guaranteed through at least November 21, 2026, after which this comparison moves, and xAI has already teased a Grok 4.7 — either event could reprice the matchup you are about to standardize on.

Running both without re-architecting

Because the tie on the index means this is a ceiling-and-price decision rather than a quality one, the practical pattern for most teams is to route rather than pick. GPT-5.6 Sol and Grok 4.6 are both on OrcaRouter at their providers' list prices, passed through at zero markup — OpenAI's $4/$20 and xAI's $2/$6 are the numbers on the listing, and when either vendor changes a rate the new price is live on the same key the same day. With one OpenAI-compatible endpoint you can send sub-200K traffic to Grok 4.6, escalate prompts that need Sol's longer context or its max-effort configuration, and use automatic failover so a rate limit on one provider path is a routing event rather than an outage.

The price half of this comparison is the part that moves — OpenAI's Sol promotion is only guaranteed through November 21 and xAI has a 4.7 on the roadmap — so the reference this blog keeps updated is its GPT-5.6 Sol pricing page, date-stamped and re-verified whenever the rate card changes.

The two-line answer

If your prompts fit under 500K tokens — and most do — Grok 4.6 delivers the same independent index score as GPT-5.6 Sol at close to half the price at every length it can serve, with no published output cap and a predictable single-model dial. GPT-5.6 Sol earns its premium in the band Grok cannot reach: past 500K tokens of context, and inside the OpenAI tooling where its reported ~96% SWE-bench Verified and the Terra and Luna tiers beneath it give you a family rather than a single model. The tie on the index means this decision is about ceilings and dollars, not capability — so measure your longest real prompts before you commit, because that single number picks the winner.