Model comparison
Claude Mythos 5.1 vs GPT-5.5
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
Frontier
| Specification | Claude Mythos 5.1 | GPT-5.5 |
|---|---|---|
| Provider | Anthropic | OpenAI |
| Tier | Frontier | Frontier |
| Context window | Not verifiedUnverified | 1.05M |
| Max output | Not verifiedUnverified | 128K |
| Input / 1M tokens | $10 | Winner: $5 |
| Output / 1M tokens | $50 | Winner: $30 |
| 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 |
| Released | September 1, 2026 | April 23, 2026 |
| Artificial Analysis Intelligence Index [xhigh] (2026-08-14) | Not verifiedUnverified | 58 |
| SWE-bench Verified (2026-09-01) | Not verifiedUnverified | 82.6 |
| Terminal-Bench 2.1 (2026-07-27) | Not verifiedUnverified | 83.4 |
| GPQA Diamond (2026-07-27) | Not verifiedUnverified | 93.5 |
| Humanity's Last Exam (2026-04-23) | Not verifiedUnverified | 52.2 |
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
- 2026-09-01: vals.ai SWE-bench Verified leaderboard (independent, mini-swe-agent bash-only); official SWE-bench not publishedReal GitHub issue resolution: does the model's patch pass the hidden tests. Comparability: comparable with caveat — Post-audit vendor claims and pre-audit scores sit on different task trust levels; scaffolding (agent harness, compute budget) also dominates results. Never aggregate across scaffolds.
- 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).
- 2026-07-27: Kimi K3 model card (vendor-reported, xhigh); announcement charts show 93.6 — minor conflict preservedAgentic terminal work: multi-step tasks executed in a sandboxed shell environment. Comparability: comparable with caveat — Scores across Terminal-Bench major versions (v1 vs v2) are NOT comparable; harness configuration (container, time limits) also shifts results.
- 2026-04-23: OpenAI — Introducing GPT-5.5 (vendor, with tools; 41.4 no tools)Frontier-knowledge ceiling: extremely hard multi-domain questions written to be near-impossible without deep expertise. Comparability: comparable with caveat — Subset/tool configurations (text-only vs with-tools) differ between vendors and materially change scores; must match configuration to compare.
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: Standard $5/$30 per MTok; >272K input billed 2x in / 1.5x out. Cached input $0.50/MTok; batch $2.50/$15.
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.
GPT-5.5
GPT-5.5 is OpenAI's previous flagship — strong on Terminal-Bench 2.0 (82.7%) and 1M-token context.
Best for
- Reasoning
- Agentic coding
- Long-context work
Watch out
Superseded by GPT-5.6 Sol on quality; still a capable, widely integrated model.
When the cheaper one wins
GPT-5.5 is cheaper on output at $30 per million tokens against $50 for Claude Mythos 5.1 — about 1.7×. 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.
Run the model pickerEvidence 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: OpenAI — Introducing GPT-5.5 (accessed 2026-08-29)
- GPT-5.5: OpenAI API pricing (gpt-5.5 $5/$30) (accessed 2026-08-29)
Related comparisons
- Claude Fable 5.1 vs Claude Mythos 5.1
- Claude Fable 5.1 vs GPT-5.5
- Claude Fable 5 vs Claude Mythos 5.1
- Claude Fable 5 vs GPT-5.5
- Claude Haiku 4.5 vs Claude Mythos 5.1
- Claude Mythos 5.1 vs Claude Opus 5
Diving deeper on one model? Claude Mythos 5.1 · GPT-5.5
Common questions
Claude Mythos 5.1 vs GPT-5.5
Answered from the verified figures on this page rather than general guidance.
Is Claude Mythos 5.1 or GPT-5.5 cheaper for input?
GPT-5.5 is cheaper at $5 per million input tokens, against $10 for Claude Mythos 5.1 — roughly 2.0× 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 has tiered pricing: Standard $5/$30 per MTok; >272K input billed 2x in / 1.5x out. Cached input $0.50/MTok; batch $2.50/$15.
Is Claude Mythos 5.1 or GPT-5.5 cheaper for output?
GPT-5.5 is cheaper at $30 per million output tokens, against $50 for Claude Mythos 5.1 — roughly 1.7× 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 has tiered pricing: Standard $5/$30 per MTok; >272K input billed 2x in / 1.5x out. Cached input $0.50/MTok; batch $2.50/$15.
Do Claude Mythos 5.1 and GPT-5.5 support the same reasoning levels?
Claude Mythos 5.1 exposes adaptive, while GPT-5.5 exposes none, low, medium, high, xhigh, max.
Should I use Claude Mythos 5.1 or GPT-5.5?
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 suits reasoning.
Next step
Choosing between them
The verified figures that separate this pair, computed from the catalog rather than restated boilerplate.
- Input price: GPT-5.5 costs $5 per 1M tokens versus $10 for Claude Mythos 5.1 — a 2x difference at the headline tier.
Choosing a harness rather than a model — Cursor, Copilot, Claude Code, Windsurf, or Lovable? Compare agentic harnesses · Latest releases.