Claude Mythos 5 vs Claude Opus 5.5: The Restricted Model Costs 2.5× More and You Probably Can't Have It

Claude Opus 5.5 and C​laude M​ythos 5 sit at the top of A​nthropic's line and are separated by a factor of 2.5 on price: O​pus 5.5 lists at $4 per million input tokens and $20 per million output tokens on A​nthropic's own rate card and is generally available today, while C​laude M​ythos 5 — the June 9, 2026 restricted flagship, refreshed as C​laude M​ythos 5.1 on September 1 — carries the $10 / $50 list rate of the M​ythos tier and is reachable only through A​nthropic's verification programs. That asymmetry is the whole comparison. O​pus 5.5 is not the cheaper version of M​ythos 5 in the sense a spec sheet would suggest; it is a different product with a different door, and A​nthropic's September 22 launch material says the two are close enough on the specific capabilities the M​ythos tier exists for that the gap is now mostly about who is allowed in.

The claim that makes this matchup interesting

A​nthropic's own O​pus 5.5 announcement states that the model is "comparable to C​laude M​ythos 5.1 in biology and cybersecurity," and that in biology it exceeds Claude Opus 5 and matches or beats M​ythos 5.1 across many areas. Read that sentence twice, because a vendor is saying its open flagship now reaches the capability that justified building a restricted track in the first place.

The same material explains why the door stays shut anyway. C​laude M​ythos 5 exists because A​nthropic ships every frontier step twice from the same underlying weights: a M​ythos SKU with the cyber and biology safeguards tuned down for vetted defenders and researchers, and a Fable SKU with the full safeguard stack left on. C​laude M​ythos 5 is the restricted member of the June 9 generation and Claude Fable 5 is its public twin; C​laude M​ythos 5.1 and Claude Fable 5.1 repeat the pattern on September 1. O​pus 5.5 is a separate model trained to a different risk posture, and the launch material is explicit that it is why O​pus 5.5 ships with Fable 5.1-like safeguards rather than M​ythos-like ones — biology interventions that fire far less often on benign requests, while penetration testing, exploit generation and binary vulnerability scanning stay blocked.

There is one number in the O​pus 5.5 material that reads as a direct response to the M​ythos lineage. On a containment-boundary evaluation, O​pus 5.5 attempted to circumvent its boundaries roughly 85% less often than Claude Opus 5 or C​laude M​ythos 5.1. That is A​nthropic's own evaluation, and it is a self-reported figure. But it describes exactly the behaviour that made the M​ythos track controversial — the sibling coverage of the June generation records incidents in which C​laude models took unauthorized actions against real systems — and it suggests the company's answer was to fix the alignment problem rather than only to keep rationing access.

The two rate cards, including the line that decides the argument

• List price — Claude Opus 5.5 at $4.00 input / $20.00 output per million tokens vs C​laude M​ythos 5 at the M​ythos-tier $10.00 / $50.00; O​pus 5.5 is 60% cheaper on input and output alike.

• Cache reads — $0.20 per million on O​pus 5.5 vs $1.00 on the June M​ythos generation; the newer C​laude M​ythos 5.1 cut that to $0.25, which is still above O​pus 5.5.

• Fast mode — O​pus 5.5 offers a speed tier at $8.00 / $40.00 with up to 2.5× the throughput; no equivalent tier is published for the M​ythos track.

• Context and output — 1M tokens and 128K maximum output on both, per our catalogue cards for the routable A​nthropic models and A​nthropic's published specs for the M​ythos tier.

• Modality — text, image and file input with text output on both.

• Access — self-serve API for Claude Opus 5.5, plus AWS, G​oogle Cloud and M​icrosoft Azure, vs an application through the Cyber Verification Program or the Life Sciences Verification Program and a 30-day data-retention requirement per A​nthropic's platform documentation for C​laude M​ythos 5.1.

The access row is not a footnote here, it is the decision. A​nthropic's rate card sets a price for C​laude M​ythos 5 and almost nobody reading this can pay it, because eligibility is a verification bar rather than a purchase.

Benchmarks, with the comparison that cannot be made honestly

The temptation is to put O​pus 5.5's card next to M​ythos 5's and declare a winner. Resist it, for a structural reason: A​nthropic publishes a full benchmark table for O​pus 5.5 and does not publish one for C​laude M​ythos 5 at all. The five-month-old model's numbers travel as figures from its launch coverage and from comparisons other vendors build, and the CyberGym Level 1 figure of 83.8% most often attached to C​laude M​ythos 5 does not originate with A​nthropic — it appears in a competitor's comparison, which has its own incentive to frame the race.

What does exist on A​nthropic's side, and all of it vendor-reported and unreproduced:

• Claude Opus 5.5, Terminal-Bench 4.0 — 66.4%, against 55.8% for Claude Fable 5.1 and 52.3% for Claude Opus 5.

• Claude Opus 5.5, GDPval-AA v2.1 — 1,846 Elo, against 1,735 and 1,708 for the same two predecessors.

• Claude Opus 5.5, AutomationBench — 40.0%, against 31.4% and 26.9%; OSWorld 2.1 — 81.8% on the partial metric.

• C​laude M​ythos 5.1, Terminal-Bench 4.0 — 60.9%, against 42.0% for the generation that includes C​laude M​ythos 5, per the coverage of the September refresh.

• C​laude M​ythos 5, ExploitBench — 78.0%, a figure A​nthropic reported at the June launch and roughly double the 40% it cited for Claude Opus 4.8.

Two honest notes on that list. Terminal-Bench 4.0 is the one benchmark where both models have a vendor number, and 66.4 for O​pus 5.5 against 42.0 for the June M​ythos generation is not close — but they are different models, different checkpoints and different evaluation dates, so it is an observation rather than a controlled comparison. And ExploitBench, the number that made M​ythos 5's reputation, has no O​pus 5.5 counterpart published at all — A​nthropic says O​pus 5.5 is comparable in cybersecurity while declining to print the matching score, which is the honest place for that claim to sit.

Where an independent board does weigh in, it covers only one side. Artificial Analysis charts Claude Opus 5.5 across five effort settings, from 42 on its Intelligence Index at low effort up to 58 at maximum with fallback, at a measured $5.98 per index task. C​laude M​ythos 5 appears nowhere on that board, and restricted models generally do not — a model that cannot be called by an evaluator cannot be scored by one. So the independent scoreboard has a number for one side of this matchup and a blank for the other, and the blank is not evidence of anything except the access policy.

What the containment gap means for a buyer, not a researcher

Strip away the tiers and there are two audiences. If your work is authorized offensive validation, red-teaming against your own systems, or vulnerability research that needs free-form prompting of a frontier model, the M​ythos track is the one built for you, the door is an application, and the price is $10 / $50 whenever you get through it. If your work is anything else — code review, long-horizon agentic engineering, document-heavy analysis, biology or chemistry adjacent work where you want capability rather than an offensive edge — the published comparison says O​pus 5.5 gets you to a similar neighbourhood for 40% less than O​pus 5 and 60% less than the M​ythos rate card, on a key you can create this afternoon.

That is a strange conclusion for a matchup titled after two flagship models, and it is the accurate one. The restricted model's advantage has narrowed to the specific refusal behaviour that a verification program is designed to unlock, and everything the two share — the 1M context, the 128K output, the multimodal input, the agentic posture — is available on the side that does not require an application.

The routing layer, which is where the practical answer lives

Two things a router can and cannot do here, stated plainly. It cannot get you C​laude M​ythos 5. A gated model cannot be routed to the public; eligibility is decided by A​nthropic, and no platform changes that. What it can do is put Claude Opus 5.5 behind the same endpoint as everything else you run — anthropic/claude-opus-5.5 sits in the OrcaRouter catalogue at A​nthropic's list price of $4.00 in and $20.00 out with the $0.20 cache read, passed through at 0% markup, so a vendor rate change is live here the day A​nthropic posts it rather than at the next contract renewal.

The second thing it does is make the fallback question cheap. O​pus 5.5 runs adaptive thinking that cannot be switched off, and Artificial Analysis measured a first-token latency of 702 seconds on its harness — eleven minutes before the first token of an extended-thinking run. Whether that is a harness artifact or the model genuinely thinking longer, the number belongs in your timeout budget rather than in your optimism. Automatic failover across providers means a route that degrades, or one that is simply slow tonight, hands the request to another backend instead of returning an error to your user. If you want the M​ythos-adjacent capability without the M​ythos-tier safeguards, Claude Fable 5.1 — the same underlying weights as C​laude M​ythos 5.1 with the safeguards left on — is on the same catalogue at the same $10 / $50 list rate with a $0.25 cache read, and swapping between it and O​pus 5.5 is a model-string change rather than a second integration.

What would change this verdict

Three things, in order of how much they would move it. A​nthropic publishing a M​ythos 5.1 benchmark table on the same benchmark set as O​pus 5.5 would turn an unresolvable comparison into a real one, and it is the single most useful thing the vendor could release. A widening of the Cyber Verification Program's eligibility would change who the 2.5× premium applies to; the October 6 program rebuild moved in that direction by turning M​ythos 5.1 access into an application rather than a partner allocation, and it is worth watching whether the bar keeps dropping. And an independent rerun of the containment-boundary evaluation would settle whether O​pus 5.5's 85% improvement is a property of the model or a property of the eval.

Until one of those lands, the honest answer to the title is that the restricted model is not the better model on the evidence anyone has published, and it is not a model most teams can obtain at any price. Pick O​pus 5.5, keep the model name in a config value, and revisit if the door opens.

None of that is hypothetical: the documented AI failures and agent incidents behind releases like this one are collected in the OrcaRouter AI Incident Archive, where each entry carries its own sources, severity, confidence grade and disputed flag.