Claude Mythos Preview vs Claude Opus 5.5: Specs & Benchmark Comparison
Claude Mythos Preview and Claude Opus 5.5 both come from Anthropic, Claude Mythos Preview was released in April 2026, and Claude Opus 5.5 followed 5 months later in September 2026.
The two models share 3 published benchmarks. Claude Opus 5.5 leads on 3 of them. The widest gaps are on SWE-Bench Pro, where Claude Opus 5.5 scores 89.9% against 77.8%; SWE-bench Multilingual, where Claude Opus 5.5 scores 93.9% against 87.3%. Averaged across everything we track, Claude Mythos Preview sits at 85.1% and Claude Opus 5.5 at 71.1%.
Claude Opus 5.5 is the cheaper API at $4 per million input tokens and $20 per million output tokens, roughly 6 times cheaper than Claude Mythos Preview at $25 and $125. Both accept text and images as input. On tooling, only Claude Opus 5.5 supports function calling and only Claude Opus 5.5 offers structured output.
| Characteristic | Claude Mythos Preview | Claude Opus 5.5 |
|---|---|---|
| Company | Anthropic | Anthropic |
| Release Date | April 7, 2026 | September 22, 2026 |
| Parameters | — | — |
| Multimodal | Yes | Yes |
| Context (input) | — | 1.0M |
| Context (output) | — | 128K |
| Input Price / 1M | $25.00 | $4.00 |
| Output Price / 1M | $125.00 | $20.00 |
| Average Score | 85.1% | 71.1% |
| Benchmarks | ||
| SWE-Bench Pro | 77.8% | 89.9% |
| SWE-bench Multilingual | 87.3% | 93.9% |
| SWE-Bench Multimodal | 59.0% | 61.4% |
Visual Benchmark Comparison
Verdict
Claude Opus 5.5 leads in 2 out of 3 comparison categories.
Both models show comparable average scores: Claude Mythos Preview — 0.9, Claude Opus 5.5 — 0.7.
Claude Opus 5.5 is 6.3x cheaper: input $4.00/1M vs $25.00/1M tokens.
Claude Opus 5.5 is newer: released 9/22/2026 vs 4/7/2026.
More About These Models
Related Comparisons
Frequently Asked Questions
Which is better for coding — Claude Mythos Preview or Claude Opus 5.5?
Which model is cheaper — Claude Mythos Preview or Claude Opus 5.5?
Which has a larger context window — Claude Mythos Preview or Claude Opus 5.5?
The Claude Mythos Preview and Claude Opus 5.5 comparison is updated for 2026. Data includes benchmark results, API pricing, context window size and other specifications. For more detailed information, visit the Claude Mythos Preview or Claude Opus 5.5 page. See also the complete list of AI model comparisons.