Claude Opus 5.5 vs Grok 4.6: One Model Reads, the Other Acts

Claude Opus 5.5 and Grok 4.6 are the two cheapest ways to buy a frontier-class model that will hold half a million tokens of context — and they are not the same product. On one measurement Grok 4.6 is three points off the absolute ceiling: 94.9 on GPQA Diamond, a graduate-level science question set where Anthropic's flagship sits in the same band. On the next one it is thirty-eight points back. On Terminal-Bench 4.0, the agentic terminal benchmark that asks a model to finish real work in a shell, Artificial Analysis scored Grok 4.6 at 21.21% and Claude Opus 5.5 at 59.60%. That is not a misprint in either direction, and the whole shape of this pairing is in explaining why both numbers are true at once.

Two releases, one price bracket, four months apart

SpaceXAI shipped Grok 4.6 on August 12, 2026 as the current flagship of the Grok line, at $2.00 per million input tokens and $6.00 output, with a 500,000-token context window and a text, image and file input surface. Anthropic shipped Claude Opus 5.5 on September 22, 2026, roughly six weeks later, at $4.00/$20.00 with a 1,000,000-token context window and 128,000 output tokens. Both support configurable reasoning effort, tool calling and structured outputs; both speak an OpenAI-compatible chat surface as well as their vendor's own format.

So the naive reading is a clean trade: Grok 4.6 is half the price on input and roughly a third on output, and Claude Opus 5.5 answers with twice the context window. That trade is real, and on long-document work it may be the only thing that matters. It is also not where the interesting divergence is, because the two models have been measured on the same independent harness and did not land in the same place on the axes that matter for agentic work.

What the catalogue says on the axes both were measured on

OrcaRouter's catalogue carries benchmark provenance per row — each figure names the evaluator it came from — so the pairs below are all from one source across both models, Artificial Analysis, rather than a vendor number on one side and a third party on the other:

• GPQA Diamond — Claude Opus 5.5 is not listed on this axis in our catalogue; Grok 4.6 scores 94.9%

• Humanity's Last Exam — 61.4% for Claude Opus 5.5 against 42.9% for Grok 4.6

• SciCode — 66.9% against 56.5%

• Long-Context Recall — 84.7% against 80.3%

• Terminal-Bench 4.0 — 59.60% against 21.21%

• AA Intelligence Index — 57.6 against 44.3

The GPQA row is deliberately left blank on the Anthropic side rather than filled from a vendor deck. Grok 4.6's 94.9 there is the strongest number on its card and it is genuine — an independent measurement, not a SpaceXAI claim — and it says something real about the model: when the task is a well-formed question with one defensible answer, Grok 4.6 performs at the frontier. Read it next to Terminal-Bench 4.0's 21.21% and the shape becomes legible. Producing a correct answer about science and executing a five-step task in a shell against a real filesystem are different competencies, and Grok 4.6 is much further along the first than the second.

One caveat on that pairing before it gets used as a verdict: the two models' Artificial Analysis figures were recorded at different evaluation dates, and Grok 4.6's are the older set. The comparison is between a model evaluated in August and one evaluated in September on a harness that has itself been revised across that window. The direction of the gap is consistent across five separate axes, which is why we treat it as signal, but the exact point values should be read as an August-versus-September comparison, not a same-day one.

An index built by someone who is not selling either

There is a second independent measure, and it agrees on the direction while being much less willing to make fine distinctions. The Epoch Capabilities Index, built by Epoch AI as a composite of more than fifty benchmarks and rescaled so that Claude 3.5 Sonnet sits at 130 and GPT-5 at 150, places Claude Opus 5.5 at 167.35 in the file we read on October 2, 2026. Grok 4.6 is at 156.61, from an August 12 evaluation. That is a 10.74-point gap on a scale where roughly five points was observed to correspond to a doubling of METR's time-horizon measure at the index's launch — wide, and comfortably outside the two models' published intervals of 164.0 to 172.0 and 154.66 to 158.93, which do not overlap at all.

Worth noting what this index does not settle. It puts Claude Sonnet 5.5 at 165.2 and Claude Fable 5.1 at 164.82, both above Grok 4.6, and both of those are cheaper per million output tokens than Grok 4.6's second-tier rate. Anyone choosing on capability-per-dollar alone has a third and fourth option in the same vendor family, and the pairing in this article is about something else: what the two models are each good at.

The rate cards, and the row that only appears on one of them

Grok 4.6's rate card has a step in it that Claude Opus 5.5's does not: requests above 200,000 prompt tokens bill at double the base rate, so input goes from $2.00 to $4.00 and output from $6.00 to $12.00, with the cache read moving from $0.50 to $1.00. Claude Opus 5.5 charges $4.00/$20.00 with $0.20 cache reads flat across the whole 1,000,000-token window. That inversion is the practical consequence of buying long context from either model, and it flips the sticker comparison once a workload actually grows:

• Price at 30,000 input tokens — Claude Opus 5.5 is the expensive one, at roughly 28 cents for 30,000 in and 8,000 out, against about 11 cents for Grok 4.6

• Price at 250,000 input tokens — the order reverses, because Grok 4.6 has stepped to its doubled tier while Claude Opus 5.5 has not

• Cache reads — $0.20 per million for Claude Opus 5.5 against $0.50, rising to $1.00 above the step, for Grok 4.6

• Maximum output — 128,000 tokens for Claude Opus 5.5; Grok 4.6's output ceiling is not published in the catalogue record

Grok 4.6 also carries one gap worth stating plainly rather than leaving to be discovered later. Its record lists no maximum output figure at all — the field is unmeasured, not zero — so a workload that depends on very long single responses has no published number to plan against on that side of the comparison.

The performance column that should not be read

Our catalogue also carries a serving-performance panel per model, and on Grok 4.6 it is the most misleading thing on the page. The card reads a p50 latency of 10,000 milliseconds and an error rate of 97.58% over a seven-day window, with 26,184 tokens served in that period. Those fields do not describe a slow model. They describe a model that was barely called: at that traffic level the sample is too thin for a latency distribution to mean anything, and the error rate reflects a handful of attempts rather than a service quality. Treating that block as evidence that Grok 4.6 is slow would be reading a measurement artifact as a measurement.

Claude Opus 5.5's panel, by contrast, has a population behind it — a p50 in the 4.4-second range across a seven-day window with daily samples, and 156 million tokens served in the same period. Even that should be read as what it is: our own playground traffic, which is a sample of our users rather than a property of the model.

Which one to route, and how to find out on your own work

The split the numbers describe is stable enough to act on. Claude Opus 5.5 is the choice for agentic and long-horizon work — the Terminal-Bench 4.0 gap of 59.60% against 21.21% is the largest separation on any axis both models were measured on, and it is the axis that predicts whether a model can be handed a task rather than a question. It is also the choice when the prompt is genuinely long, because the rate card does not step and the window is twice the size. Grok 4.6 is the choice for question-shaped work at volume: a 94.9% GPQA Diamond score at $2.00/$6.00 inside the first 200,000 tokens is a very good deal for retrieval-and-answer workloads that never grow into the doubled tier.

Both are on OrcaRouter at the provider's list price with 0% markup — Claude Opus 5.5 at $4.00/$20.00 with $0.20 cache reads, Grok 4.6 at $2.00/$6.00 with the above-200K tier applied exactly as SpaceXAI sets it — behind one key, so the split above can be tested on your own prompt set instead of argued about. Where both are routed, automatic failover sits underneath, which is worth having when the cheaper model is the one holding the agentic path.

The verdict this pairing earns: Grok 4.6 is the better reader and Claude Opus 5.5 is the better worker. The 94.9 on GPQA Diamond and the 21.21 on Terminal-Bench are the same model measured twice, and knowing which of those two numbers your workload looks like is the entire decision.