LLM Reference

Claude Mythos Preview vs GPT-5.3-Codex

Claude Mythos Preview (2026) and GPT-5.3-Codex (2026) compare a standalone API model against a coding-specialized model. 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 page treats the result as workflow and deployment fit, not a universal model winner.

Treat this as a product-type comparison: Claude Mythos Preview is standalone API model, while GPT-5.3-Codex is coding-specialized model. Choose based on workflow fit before reading any benchmark or price row as decisive.

Decision scorecard

Local evidence first
SignalClaude Mythos PreviewGPT-5.3-Codex
Product typeStandalone API modelCoding-specialized model
Best forreasoning-heavy apps, multimodal apps, and tool-calling agentscustom coding agents, code generation, and tool loops
Decision fitCoding, RAG, and AgentsCoding, RAG, and Agents
Context window1m400k
Cheapest output$125/1M tokens$14/1M tokens
Provider routes2 tracked3 tracked
Shared benchmarksSWE-bench Verified leader2 shared

Decision tradeoffs

Choose Claude Mythos Preview when...
  • Claude Mythos Preview holds a shared-benchmark lead on SWE-bench Verified, ahead 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.
Choose GPT-5.3-Codex when...
  • GPT-5.3-Codex has the lower cheapest tracked output price at $14/1M tokens.
  • GPT-5.3-Codex has broader tracked provider coverage for fallback and route flexibility.
  • GPT-5.3-Codex uniquely exposes Computer use in local model data.
  • 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 route or tier on this page.

Lower estimate GPT-5.3-Codex

Claude Mythos Preview

$51,250

Cheapest tracked route/tier: Anthropic

GPT-5.3-Codex

$4,900

Cheapest tracked route/tier: OpenRouter

Estimated monthly gap: $46,350. Batch, cache, alternate speed tiers, and negotiated pricing are excluded from this local estimate.

Switch friction

Claude Mythos Preview -> GPT-5.3-Codex
  • 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.
  • GPT-5.3-Codex adds Computer use in local capability data.
GPT-5.3-Codex -> Claude Mythos Preview
  • 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.
  • Check replacement coverage for Computer use before moving production traffic.
  • Claude Mythos Preview adds Multimodal in local capability data.

Specs

Specification
Released2026-05-012026-02-05
Context window1m400k
Parameters——
ArchitectureDecoder OnlyDecoder Only
LicenseProprietaryProprietary
OpennessProprietaryProprietary
WeightsNot releasedNot released
CodeUnknownUnknown
Commercial useCommercial use: conditionalCommercial use: conditional
Knowledge cutoff2025-122025-08

Pricing and availability

Pricing attributeClaude Mythos PreviewGPT-5.3-Codex
Input price$25/1M tokens$1.75/1M tokens
Output price$125/1M tokens$14/1M tokens
Providers
AnthropicMicrosoft Foundry
OpenRouterOpenAI APIVercel AI Gateway

Capabilities

CapabilityClaude Mythos PreviewGPT-5.3-Codex
VisionYesYes
MultimodalYesNo
ReasoningYesYes
JSON / Tool useYesYes
Structured outputsYesYes
Code executionYesYes
IDE integrationNoNo
Computer useNoYes
Parallel agentsNoNo

Benchmarks

BenchmarkClaude Mythos PreviewGPT-5.3-Codex
SWE-bench Verified93.985.0
τ-bench89.277.8

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Last reviewed: 2026-07-26. Data sourced from public model cards and provider documentation.