Claude Mythos Preview vs GPT-5.3-Codex
Claude Mythos Preview (2026) and GPT-5.3-Codex (2026) are agentic coding models from Anthropic and OpenAI. Claude Mythos Preview ships a 1M-token context window, while GPT-5.3-Codex ships a 400K-token context window. On SWE-bench Verified, Claude Mythos Preview leads by 8.9 pts. On pricing, GPT-5.3-Codex costs $1.75/1M input tokens versus $25/1M for the alternative. This comparison covers specs, pricing, capabilities, benchmarks, provider availability, and production fit.
GPT-5.3-Codex is ~1329% cheaper at $1.75/1M; pay for Claude Mythos Preview only for coding workflow support.
Decision scorecard
Local evidence first| Signal | Claude Mythos Preview | GPT-5.3-Codex |
|---|---|---|
| Decision fit | Coding, RAG, and Agents | Coding, RAG, and Agents |
| Context window | 1M | 400K |
| Cheapest output | $125/1M tokens | $14/1M tokens |
| Provider routes | 2 tracked | 2 tracked |
| Shared benchmarks | SWE-bench Verified leader | 2 rows |
Decision tradeoffs
- Claude Mythos Preview leads the largest shared benchmark signal on SWE-bench Verified by 8.9 points.
- Claude Mythos Preview has the larger context window for long prompts, retrieval packs, or transcript analysis.
- Claude Mythos Preview uniquely exposes Multimodal in local model data.
- Local decision data tags Claude Mythos Preview for Coding, RAG, and Agents.
- GPT-5.3-Codex has the lower cheapest tracked output price at $14/1M tokens.
- Local decision data tags GPT-5.3-Codex for Coding, RAG, and Agents.
Monthly cost at traffic
Estimate token spend from the cheapest tracked input and output prices on this page.
Claude Mythos Preview
$51,250
Cheapest tracked route: Anthropic
GPT-5.3-Codex
$4,900
Cheapest tracked route: OpenRouter
Estimated monthly gap: $46,350. Batch, cache, and negotiated pricing are excluded from this local estimate.
Switch friction
- No overlapping tracked provider route is sourced for Claude Mythos Preview and GPT-5.3-Codex; plan for SDK, billing, or endpoint changes.
- GPT-5.3-Codex is $111/1M tokens lower on cheapest tracked output pricing before cache, batch, or negotiated discounts.
- Check replacement coverage for Multimodal before moving production traffic.
- No overlapping tracked provider route is sourced for GPT-5.3-Codex and Claude Mythos Preview; plan for SDK, billing, or endpoint changes.
- Claude Mythos Preview is $111/1M tokens higher on cheapest tracked output pricing, so quality gains need to justify the spend.
- Claude Mythos Preview adds Multimodal in local capability data.
Specs
| Specification | ||
|---|---|---|
| Released | 2026-04-07 | 2026-02-05 |
| Context window | 1M | 400K |
| Parameters | — | — |
| Architecture | decoder only | decoder only |
| License | Proprietary | Proprietary |
| Knowledge cutoff | 2025-12 | 2025-08 |
Pricing and availability
| Pricing attribute | Claude Mythos Preview | GPT-5.3-Codex |
|---|---|---|
| Input price | $25/1M tokens | $1.75/1M tokens |
| Output price | $125/1M tokens | $14/1M tokens |
| Providers |
Capabilities
| Capability | Claude Mythos Preview | GPT-5.3-Codex |
|---|---|---|
| Vision | Yes | Yes |
| Multimodal | Yes | No |
| Reasoning | Yes | Yes |
| Function calling | Yes | Yes |
| Tool use | Yes | Yes |
| Structured outputs | Yes | Yes |
| Code execution | Yes | Yes |
Benchmarks
| Benchmark | Claude Mythos Preview | GPT-5.3-Codex |
|---|---|---|
| SWE-bench Verified | 93.9 | 85.0 |
| τ-bench | 89.2 | 77.8 |
Deep dive
On shared benchmark coverage, SWE-bench Verified has Claude Mythos Preview at 93.9 and GPT-5.3-Codex at 85, with Claude Mythos Preview ahead by 8.9 points; τ-bench has Claude Mythos Preview at 89.2 and GPT-5.3-Codex at 77.8, with Claude Mythos Preview ahead by 11.4 points. The largest visible gap is 11.4 points on τ-bench, which matters most when that benchmark mirrors your workload. Treat isolated benchmark wins as directional, because provider routing, prompt style, and tool access can move real application results.
The capability footprint differs most on multimodal input: Claude Mythos Preview. Both models share vision, reasoning mode, function calling, and tool use, so the practical split is not just feature count. Use those differences to decide whether the page is about raw model quality, agentic coding support, multimodal ingestion, or predictable structured API behavior.
For cost, Claude Mythos Preview lists $25/1M input and $125/1M output tokens, while GPT-5.3-Codex lists $1.75/1M input and $14/1M output tokens on the cheapest tracked provider. A 70/30 input-output blend puts GPT-5.3-Codex lower by about $49.58 per million blended tokens. Availability is 2 providers versus 2, so concentration risk also matters.
Choose Claude Mythos Preview when coding workflow support and larger context windows are central to the workload. Choose GPT-5.3-Codex when coding workflow support and lower input-token cost are more important. For production, rerun your own prompts through the exact provider, region, and tool stack you plan to ship.
FAQ
Which has a larger context window, Claude Mythos Preview or GPT-5.3-Codex?
Claude Mythos Preview supports 1M tokens, while GPT-5.3-Codex supports 400K tokens. That gap matters most for long documents, large codebases, retrieval-heavy agents, and conversations where earlier context must remain visible.
Which is cheaper, Claude Mythos Preview or GPT-5.3-Codex?
GPT-5.3-Codex is cheaper on tracked token pricing. Claude Mythos Preview costs $25/1M input and $125/1M output tokens. GPT-5.3-Codex costs $1.75/1M input and $14/1M output tokens. Provider discounts or batch pricing can still change the final bill.
Is Claude Mythos Preview or GPT-5.3-Codex open source?
Claude Mythos Preview is listed under Proprietary. GPT-5.3-Codex is listed under Proprietary. License labels affect whether you can self-host, redistribute weights, or rely only on hosted APIs, so confirm the upstream license before deployment.
Which is better for vision, Claude Mythos Preview or GPT-5.3-Codex?
Both Claude Mythos Preview and GPT-5.3-Codex expose vision. The better choice depends on benchmark fit, context budget, pricing, and whether your provider route exposes the same capability surface. Use this as a quick comparison signal, then confirm the provider-specific limits before committing to production.
Which is better for multimodal input, Claude Mythos Preview or GPT-5.3-Codex?
Claude Mythos Preview has the clearer documented multimodal input signal in this comparison. If multimodal input is mission-critical, validate it against the provider endpoint because model-level support and API-level exposure can differ.
Where can I run Claude Mythos Preview and GPT-5.3-Codex?
Claude Mythos Preview is available on Anthropic and Microsoft Foundry. GPT-5.3-Codex is available on OpenRouter and OpenAI API. Provider coverage can affect latency, region availability, compliance posture, and fallback options.
Continue comparing
Last reviewed: 2026-05-16. Data sourced from public model cards and provider documentation.