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Model comparison

Claude Opus 5.5 vs GPT-5.3-Codex

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

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

Anthropic

Claude Opus 5.5

Frontier

vs

OpenAI

GPT-5.3-Codex

Frontier

AI model capability comparison
SpecificationClaude Opus 5.5GPT-5.3-Codex
ProviderAnthropicOpenAI
TierFrontierFrontier
Context windowWinner: 1M400K
Max output128K128K
Input / 1M tokens$4Winner: $1.75
Output / 1M tokens$20Winner: $14
WeightsClosedClosed
ParametersNot disclosedUnverifiedNot disclosedUnverified
Reasoning levelsadaptivelow, medium, high, xhigh
Modalitiestext, imagetext, image
API model idclaude-opus-5-5gpt-5.3-codex
ReleasedSeptember 22, 2026February 5, 2026
Artificial Analysis Intelligence Index [max] (2026-09-26)57.6Not verifiedUnverified
Artificial Analysis Intelligence Index [xhigh] (2026-09-26)Not verifiedUnverified32.5
Humanity's Last Exam [max] (2026-09-26)61.4Not verifiedUnverified
Terminal-Bench 4.0 [max] (2026-09-26)59.6Not verifiedUnverified
Terminal-Bench 4.0 [xhigh] (2026-09-22)66.4Not verifiedUnverified

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.

Pricing tiers

Claude Opus 5.5: $4/$20 per MTok, no long-context surcharge. Cache reads $0.20; 5-minute cache writes $5, 1-hour writes $8. Batch $2/$10; Fast mode $8/$40. Batches API allows 300K output with the output-300k-2026-03-24 beta header. Knowledge cutoff Jun 2026; retirement not before 2027-09-22.

GPT-5.3-Codex: Standard $1.75/$14 per MTok; cached input $0.175/MTok; Fast mode $3.50/$28. Still listed in the API as a coding-specialised model.

FrontierRecord checked September 26, 2026

Claude Opus 5.5

Claude Opus 5.5 is Anthropic's default model for agentic coding and knowledge work — Anthropic says it performs near Fable 5.1 on most work at about 40% lower running cost than Opus 5, and it leads the Artificial Analysis Intelligence Index v4.3.2.

Best for

  • Complex agentic coding
  • Enterprise knowledge work
  • Default frontier model

Watch out

Breaking changes from Opus 5: adaptive thinking cannot be disabled (effort runs low to max, default medium) and forced tool use returns an error. Verbose at max effort, so cap output on cost-sensitive routes.

FrontierRecord checked September 26, 2026

GPT-5.3-Codex

GPT-5.3-Codex is OpenAI's coding-specialised API model at $1.75/$14 — the Codex agent itself now runs GPT-6 Sol, Luna and Astra.

Best for

  • Autonomous coding
  • Repo-scale refactors
  • Test generation

Watch out

Tuned for coding, not general chat; for new coding work compare GPT-6 Sol ($2/$10) first.

When the cheaper one wins

GPT-5.3-Codex is cheaper on output at $14 per million tokens against $20 for Claude Opus 5.5 — about 1.4×. 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.

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Evidence confidence: High

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.
  • 4/5 core specs verified on both sides — Not published for at least one side: parameter count.
  • 1 shared named benchmark with differing scores — Measured on: Artificial Analysis Intelligence Index.
  • 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 Opus 5.5 · GPT-5.3-Codex

Common questions

Claude Opus 5.5 vs GPT-5.3-Codex

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

Is Claude Opus 5.5 or GPT-5.3-Codex cheaper for input?
GPT-5.3-Codex is cheaper at $1.75 per million input tokens, against $4 for Claude Opus 5.5 — roughly 2.3× 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 Opus 5.5 has tiered pricing: $4/$20 per MTok, no long-context surcharge. Cache reads $0.20; 5-minute cache writes $5, 1-hour writes $8. Batch $2/$10; Fast mode $8/$40. Batches API allows 300K output with the output-300k-2026-03-24 beta header. Knowledge cutoff Jun 2026; retirement not before 2027-09-22. GPT-5.3-Codex has tiered pricing: Standard $1.75/$14 per MTok; cached input $0.175/MTok; Fast mode $3.50/$28. Still listed in the API as a coding-specialised model.
Is Claude Opus 5.5 or GPT-5.3-Codex cheaper for output?
GPT-5.3-Codex is cheaper at $14 per million output tokens, against $20 for Claude Opus 5.5 — roughly 1.4× 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 Opus 5.5 has tiered pricing: $4/$20 per MTok, no long-context surcharge. Cache reads $0.20; 5-minute cache writes $5, 1-hour writes $8. Batch $2/$10; Fast mode $8/$40. Batches API allows 300K output with the output-300k-2026-03-24 beta header. Knowledge cutoff Jun 2026; retirement not before 2027-09-22. GPT-5.3-Codex has tiered pricing: Standard $1.75/$14 per MTok; cached input $0.175/MTok; Fast mode $3.50/$28. Still listed in the API as a coding-specialised model.
Which has the larger context window, Claude Opus 5.5 or GPT-5.3-Codex?
Claude Opus 5.5 accepts 1M tokens against 400K for GPT-5.3-Codex. This only matters if you routinely send very long documents or large codebases.
Do Claude Opus 5.5 and GPT-5.3-Codex support the same reasoning levels?
Claude Opus 5.5 exposes adaptive, while GPT-5.3-Codex exposes low, medium, high, xhigh.
Should I use Claude Opus 5.5 or GPT-5.3-Codex?
Both sit in the frontier tier, so the choice usually comes down to price and context rather than capability. Claude Opus 5.5 suits complex agentic coding; GPT-5.3-Codex suits autonomous coding.

Next step

Choosing between them

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

  • Input price: GPT-5.3-Codex costs $1.75 per 1M tokens versus $4 for Claude Opus 5.5 — a 2.3x difference at the headline tier.
  • Context: Claude Opus 5.5 takes 1M against 400K for GPT-5.3-Codex — only decisive if your prompts approach the smaller window.
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