Model comparison
Claude Mythos 5.1 vs Upstage Solar Pro 4
Anthropic against Upstage, 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
Upstage
Upstage Solar Pro 4
Frontier
| Specification | Claude Mythos 5.1 | Upstage Solar Pro 4 |
|---|---|---|
| Provider | Anthropic | Upstage |
| Tier | Frontier | Frontier |
| Context window | Not verifiedUnverified | 524K |
| Max output | Not verifiedUnverified | Not verifiedUnverified |
| Input / 1M tokens | $10 | Winner: $0.30 |
| Output / 1M tokens | $50 | Winner: $1.20 |
| Weights | Closed | Closed |
| Parameters | cyber/bio research variant of Fable 5.1 | Not disclosedUnverified |
| Reasoning levels | adaptive | low, medium, high |
| Modalities | text, image | text |
| API model id | claude-mythos-5 | solar-pro-4 |
| Released | September 1, 2026 | August 10, 2026 |
| Artificial Analysis Intelligence Index (2026-08-14) | Not verifiedUnverified | 50 |
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. · Upstage Solar Pro 4: List $0.30/$1.20 per MTok; live promo rate is $0.03/$0.12 (90% off through 2026-09-10). Closed API. 524K context.
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.
Upstage Solar Pro 4
Solar Pro 4 is Upstage's closed flagship with a 524K context at a low launch price.
Best for
- Long-context
- Cost-sensitive API
- Korean + English
Watch out
Closed API; promo pricing may end.
When the cheaper one wins
Upstage Solar Pro 4 is cheaper on output at $1.20 per million tokens against $50 for Claude Mythos 5.1 — about 42×. 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)
- Upstage Solar Pro 4: Upstage — Solar Pro 4 (accessed 2026-08-29)
- Upstage Solar Pro 4: Upstage (accessed 2026-08-29)
Related comparisons
- Claude Fable 5.1 vs Claude Mythos 5.1
- Claude Fable 5.1 vs Upstage Solar Pro 4
- Claude Fable 5 vs Claude Mythos 5.1
- Claude Fable 5 vs Upstage Solar Pro 4
- 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 · Upstage Solar Pro 4
Common questions
Claude Mythos 5.1 vs Upstage Solar Pro 4
Answered from the verified figures on this page rather than general guidance.
Is Claude Mythos 5.1 or Upstage Solar Pro 4 cheaper for input?
Upstage Solar Pro 4 is cheaper at $0.30 per million input tokens, against $10 for Claude Mythos 5.1 — roughly 33× 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. Upstage Solar Pro 4 has tiered pricing: List $0.30/$1.20 per MTok; live promo rate is $0.03/$0.12 (90% off through 2026-09-10). Closed API. 524K context.
Is Claude Mythos 5.1 or Upstage Solar Pro 4 cheaper for output?
Upstage Solar Pro 4 is cheaper at $1.20 per million output tokens, against $50 for Claude Mythos 5.1 — roughly 42× 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. Upstage Solar Pro 4 has tiered pricing: List $0.30/$1.20 per MTok; live promo rate is $0.03/$0.12 (90% off through 2026-09-10). Closed API. 524K context.
Do Claude Mythos 5.1 and Upstage Solar Pro 4 support the same reasoning levels?
Claude Mythos 5.1 exposes adaptive, while Upstage Solar Pro 4 exposes low, medium, high.
Should I use Claude Mythos 5.1 or Upstage Solar Pro 4?
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; Upstage Solar Pro 4 suits long-context.
Next step
Choosing between them
The verified figures that separate this pair, computed from the catalog rather than restated boilerplate.
- Input price: Upstage Solar Pro 4 costs $0.30 per 1M tokens versus $10 for Claude Mythos 5.1 — a 33.3x difference at the headline tier.
Choosing a harness rather than a model — Cursor, Copilot, Claude Code, Windsurf, or Lovable? Compare agentic harnesses · Latest releases.