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Model comparison

Claude Mythos 5.1 vs GPT-5.6 Sol

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

Catalog record checked September 26, 2026Individual provider fields may changeEvidence confidence: Medium — see receipts below

Anthropic

Claude Mythos 5.1

Frontier

vs

OpenAI

GPT-5.6 Sol

Frontier

AI model capability comparison
SpecificationClaude Mythos 5.1GPT-5.6 Sol
ProviderAnthropicOpenAI
TierFrontierFrontier
Context window1MWinner: 1.05M
Max output128K128K
Input / 1M tokens$10Winner: $4
Output / 1M tokens$50Winner: $20
WeightsClosedClosed
Parameterscyber/bio research variant of Fable 5.1previous-generation reasoning + coding model
Reasoning levelsadaptivenone, low, medium, high, xhigh, max
Modalitiestext, imagetext, image
API model idclaude-mythos-5-1gpt-5.6-sol
ReleasedSeptember 1, 2026July 9, 2026
Artificial Analysis Intelligence Index [max] (2026-09-26)Not verifiedUnverified47
Terminal-Bench 2.1 (2026-07-09)Not verifiedUnverified88.8
GPQA Diamond (2026-07-09)Not verifiedUnverified94.6

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

Where each score comes from, and how far it can be compared across models.

  • 2026-09-26
    Artificial Analysis

    Composite index blending reasoning, knowledge, and coding evals into one 0–100 score.

    Comparable with caveatOnly same-version scores are comparable. v4.3.2 was rebaselined (the top score fell from 66 on v4.1.1 to ~58), so v4.1.1 figures must not be compared with v4.3.2 figures — check measuredAt.

  • 2026-07-09
    OpenAI — GPT-5.6 evals table (vendor)

    Agentic terminal work: multi-step tasks executed in a sandboxed shell environment.

    Comparable with caveatNot comparable with Terminal-Bench 3.0 or 4.0 (different task sets) or v1; harness configuration (container, time limits) also shifts results.

Pricing tiers

Claude Mythos 5.1: $10/$50 per MTok — priced with Fable 5.1, which shares the same underlying model. Cache reads $0.25/MTok; batch $5/$25. Invite-only (vetted US organisations). API id claude-mythos-5-1; the separate, older claude-mythos-5 (released 2026-06-09) remains Active.

GPT-5.6 Sol: Launch list was $5/$30; cut to $4/$20 on 2026-08-21, promo held at least through 2026-11-21. Above 272K input the whole request bills 2x input / 1.5x output ($8/$30). Cached input $0.40/MTok; cache writes 1.25x input; batch $2/$10. Knowledge cutoff Feb 16 2026.

FrontierRecord checked September 26, 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 26, 2026

GPT-5.6 Sol

GPT-5.6 Sol was OpenAI's GPT-5.6 flagship for complex reasoning and coding with a 1.05M-token context — superseded on 2026-09-22 by GPT-6 Sol at half the price.

Best for

  • Complex reasoning
  • Agentic coding
  • Hard knowledge work

Watch out

Superseded: GPT-6 Sol lists $2/$10, half this row's $4/$20 promo rate, and scores higher on the Artificial Analysis index. Still served; 'max' effort and 'ultra' multi-agent mode multiply token use.

When the cheaper one wins

GPT-5.6 Sol is cheaper on output at $20 per million tokens against $50 for Claude Mythos 5.1 — about 2.5×. 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 picker

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.
  • 5/5 core specs verified on both sides — All core specifications verified for both models.
  • No shared named benchmark — No benchmark has been measured on both models.
  • Verified within the last 90 days — Newest catalog check was 2 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.

Related comparisons

Diving deeper on one model? Claude Mythos 5.1 · GPT-5.6 Sol

Common questions

Claude Mythos 5.1 vs GPT-5.6 Sol

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

Is Claude Mythos 5.1 or GPT-5.6 Sol cheaper for input?
GPT-5.6 Sol is cheaper at $4 per million input tokens, against $10 for Claude Mythos 5.1 — roughly 2.5× 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: $10/$50 per MTok — priced with Fable 5.1, which shares the same underlying model. Cache reads $0.25/MTok; batch $5/$25. Invite-only (vetted US organisations). API id claude-mythos-5-1; the separate, older claude-mythos-5 (released 2026-06-09) remains Active. GPT-5.6 Sol has tiered pricing: Launch list was $5/$30; cut to $4/$20 on 2026-08-21, promo held at least through 2026-11-21. Above 272K input the whole request bills 2x input / 1.5x output ($8/$30). Cached input $0.40/MTok; cache writes 1.25x input; batch $2/$10. Knowledge cutoff Feb 16 2026.
Is Claude Mythos 5.1 or GPT-5.6 Sol cheaper for output?
GPT-5.6 Sol is cheaper at $20 per million output tokens, against $50 for Claude Mythos 5.1 — roughly 2.5× 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: $10/$50 per MTok — priced with Fable 5.1, which shares the same underlying model. Cache reads $0.25/MTok; batch $5/$25. Invite-only (vetted US organisations). API id claude-mythos-5-1; the separate, older claude-mythos-5 (released 2026-06-09) remains Active. GPT-5.6 Sol has tiered pricing: Launch list was $5/$30; cut to $4/$20 on 2026-08-21, promo held at least through 2026-11-21. Above 272K input the whole request bills 2x input / 1.5x output ($8/$30). Cached input $0.40/MTok; cache writes 1.25x input; batch $2/$10. Knowledge cutoff Feb 16 2026.
Which has the larger context window, Claude Mythos 5.1 or GPT-5.6 Sol?
GPT-5.6 Sol accepts 1.05M tokens against 1M for Claude Mythos 5.1. This only matters if you routinely send very long documents or large codebases.
Do Claude Mythos 5.1 and GPT-5.6 Sol support the same reasoning levels?
Claude Mythos 5.1 exposes adaptive, while GPT-5.6 Sol exposes none, low, medium, high, xhigh, max.
Should I use Claude Mythos 5.1 or GPT-5.6 Sol?
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-5.6 Sol suits complex reasoning.

Next step

Choosing between them

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

  • Input price: GPT-5.6 Sol costs $4 per 1M tokens versus $10 for Claude Mythos 5.1 — a 2.5x difference at the headline tier.
  • Context: GPT-5.6 Sol takes 1.05M against 1M for Claude Mythos 5.1 — only decisive if your prompts approach the smaller window.
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