Model comparison

Claude Mythos 5.1 vs GPT-5.5 Pro

Anthropic against OpenAI, compared on context, price, and verified benchmark results.

Catalog record checked September 3, 2026Individual provider fields may changeEvidence confidence: Medium — see receipts below

Anthropic

Claude Mythos 5.1

Frontier

OpenAI

GPT-5.5 Pro

Frontier

AI model capability comparison
Specification	Claude Mythos 5.1	GPT-5.5 Pro
Provider	Anthropic	OpenAI
Tier	Frontier	Frontier
Context window	Not verifiedUnverified	1.05M
Max output	Not verifiedUnverified	128K
Input / 1M tokens	Winner: $10	$30
Output / 1M tokens	Winner: $50	$180
Weights	Closed	Closed
Parameters	cyber/bio research variant of Fable 5.1	Not disclosedUnverified
Reasoning levels	adaptive	none, low, medium, high, xhigh, max
Modalities	text, image	text, image
API model id	claude-mythos-5	gpt-5.5-pro
Released	September 1, 2026	April 23, 2026
Artificial Analysis Intelligence Index [xhigh] (2026-08-14)	Not verifiedUnverified	60

Prices are USD per million tokens at standard rates, excluding batch and caching discounts. Bold indicates the better figure where one is objectively better. Values we could not confirm from the provider are shown as “Not verified” rather than estimated.

Benchmark receipts

2026-08-14: Artificial AnalysisComposite index blending reasoning, knowledge, and coding evals into one 0–100 score. Comparability: directly comparable — AA occasionally rebaselines the index scale between snapshots — a score captured on one date is only comparable to same-snapshot scores (check measuredAt).

Pricing tiers: Claude Mythos 5.1: Starts at $10/$50 per MTok — priced with Fable 5.1, which shares the same underlying model. API id claude-mythos-5 replaced the deprecated claude-mythos-preview. · GPT-5.5 Pro: Standard $30/$180 per MTok; no cached-input discount. Batch $15/$90.

FrontierRecord checked September 3, 2026

Claude Mythos 5.1

Claude Mythos 5.1 is Anthropic's most capable model for cybersecurity and biology research — the Fable 5.1 model with fewer cyber/bio safeguards, restricted to vetted US organisations.

Best for

Cyber-defence research
Biology research
Claude Security workloads

Watch out

Invite-only (vetted US organisations, verification programmes); 30-day data retention applies by default. Same underlying model as Fable 5.1 — most teams should just use Fable.

FrontierRecord checked September 3, 2026

GPT-5.5 Pro

GPT-5.5 Pro is OpenAI's highest-effort tier for maximal accuracy on the hardest tasks.

Best for

Maximum-accuracy reasoning
Research
Hard enterprise workloads

Watch out

At $30/$180 per MTok it is the most expensive OpenAI tier; reserve for work that needs it.

When the cheaper one wins

Claude Mythos 5.1 is cheaper on output at $50 per million tokens against $180 for GPT-5.5 Pro — about 3.6×. Use the cheaper tier for classification, extraction, summarisation, and any task where the expensive model’s extra score does not change the accepted output. The expensive one only pays if your hardest task actually fails on the cheap tier. These are standard-tier API rates, excluding batch and cache discounts.

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Evidence confidence: Medium

How strong and complete the evidence behind this comparison is — not a prediction of which model is better.

Pricing verified on both sides — Input and output rates are verified for both models.
2/5 core specs verified on both sides — Not published for at least one side: context window, max output, parameter count.
No shared named benchmark — No benchmark has been measured on both models.
Verified within the last 90 days — Newest catalog check was 8 days ago.
Both models carry source citations — Each side has at least two catalog sources on record.

Source receipts

Each catalog figure was checked against the provider or an independent second source on the date shown.

Claude Mythos 5.1: Anthropic — Claude Mythos (accessed 2026-09-03)
Claude Mythos 5.1: Anthropic — model deprecations (claude-mythos-5) (accessed 2026-09-03)
GPT-5.5 Pro: OpenAI — Introducing GPT-5.5 (accessed 2026-08-29)
GPT-5.5 Pro: OpenAI API pricing (gpt-5.5-pro $30/$180) (accessed 2026-08-29)

Related comparisons

Diving deeper on one model? Claude Mythos 5.1 · GPT-5.5 Pro

Common questions

Claude Mythos 5.1 vs GPT-5.5 Pro

Answered from the verified figures on this page rather than general guidance.

Is Claude Mythos 5.1 or GPT-5.5 Pro cheaper for input?

Claude Mythos 5.1 is cheaper at $10 per million input tokens, against $30 for GPT-5.5 Pro — roughly 3.0× the price. Output tokens usually dominate a real bill, so weigh the output rate more heavily than the input rate. These are base rates; Claude Mythos 5.1 has tiered pricing: Starts at $10/$50 per MTok — priced with Fable 5.1, which shares the same underlying model. API id claude-mythos-5 replaced the deprecated claude-mythos-preview. GPT-5.5 Pro has tiered pricing: Standard $30/$180 per MTok; no cached-input discount. Batch $15/$90.

Is Claude Mythos 5.1 or GPT-5.5 Pro cheaper for output?

Claude Mythos 5.1 is cheaper at $50 per million output tokens, against $180 for GPT-5.5 Pro — roughly 3.6× the price. Output tokens usually dominate a real bill, so weigh the output rate more heavily than the input rate. These are base rates; Claude Mythos 5.1 has tiered pricing: Starts at $10/$50 per MTok — priced with Fable 5.1, which shares the same underlying model. API id claude-mythos-5 replaced the deprecated claude-mythos-preview. GPT-5.5 Pro has tiered pricing: Standard $30/$180 per MTok; no cached-input discount. Batch $15/$90.

Do Claude Mythos 5.1 and GPT-5.5 Pro support the same reasoning levels?

Claude Mythos 5.1 exposes adaptive, while GPT-5.5 Pro exposes none, low, medium, high, xhigh, max.

Should I use Claude Mythos 5.1 or GPT-5.5 Pro?

Both sit in the frontier tier, so the choice usually comes down to price and context rather than capability. Claude Mythos 5.1 suits cyber-defence research; GPT-5.5 Pro suits maximum-accuracy reasoning.

Next step

Choosing between them

The verified figures that separate this pair, computed from the catalog rather than restated boilerplate.

Input price: Claude Mythos 5.1 costs $10 per 1M tokens versus $30 for GPT-5.5 Pro — a 3x difference at the headline tier.

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