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Model comparison

Claude Mythos 5 vs GPT-5.3 Codex

Data verified

Head-to-head evidence from 4 shared benchmark results across 2 categories. Overall scores shown here use the public BenchAlign v5 ranking lane.

83.93/100
Margin
17.2pts
← winning
66.69/100
2 category wins0 category wins

Public leaderboard positions: Claude Mythos 5 #1 (Supported); GPT-5.3 Codex #26 (Supported). Intervals and evidence labels describe ranking uncertainty, not a guarantee for a specific workload.

Evidence parity. Claude Mythos 5 and GPT-5.3 Codex share 4 comparable benchmark results. 2 of 8 categories are comparable. 11 results are unique to Claude Mythos 5; 17 to GPT-5.3 Codex.

Updated July 23, 2026
Shared results
4
Claude Mythos 5 only
11
GPT-5.3 Codex only
17
Comparable categories
2 / 8

Pick Claude Mythos 5 if you want the stronger benchmark profile. GPT-5.3 Codex only becomes the better choice if you want the cheaper token bill.

Confidence note. This is a partial-evidence comparison with 4 shared benchmark results across 2 evidence categories; 2 of 8 categories currently have scoreable aggregates for both models. Treat the verdict as directional until coverage is more balanced.

Why this result

Claude Mythos 5 is clearly ahead on the BenchAlign aggregate, 83.93 to 66.69. The gap is large enough that you do not need to squint at the spreadsheet to see the difference.

Claude Mythos 5's sharpest advantage is in coding, where it averages 89.7 against 67.2. The single biggest benchmark swing on the page is SWE-bench Pro, 80.3% to 56.8%.

Claude Mythos 5 is also the more expensive model on tokens at $10.00 input / $50.00 output per 1M tokens, versus $1.75 input / $14.00 output per 1M tokens for GPT-5.3 Codex. That is roughly 3.6x on output cost alone. Claude Mythos 5 gives you the larger context window at 1M+, compared with 400K for GPT-5.3 Codex.

Category breakdown

Exact category averages are shown below. Not measured means BenchLM does not have enough sourced public coverage for that model and category.

Category scores and score margins for Claude Mythos 5 and GPT-5.3 Codex
CategoryClaude Mythos 5ΔGPT-5.3 Codex
CodingClaude Mythos 589.7Margin← 22.5GPT-5.3 Codex67.2
AgenticClaude Mythos 587.0Margin← 15.6GPT-5.3 Codex71.4
KnowledgeClaude Mythos 568.5MarginNo overlapGPT-5.3 CodexNot measured
MathClaude Mythos 597.6MarginNo overlapGPT-5.3 CodexNot measured
MultimodalClaude Mythos 593.5MarginNo overlapGPT-5.3 CodexNot measured

Decisive benchmark drivers

The largest measured benchmark gaps in this matchup, with exact reported values.

More
A · Claude Mythos 5B · GPT-5.3 Codex

  1. SWE-bench Pro

    Coding
    Source ↗
    A 80.3%B 56.8%
    Winner: Claude Mythos 5Δ 23.5
    SWE-bench Pro: Claude Mythos 5 scored 80.3%; GPT-5.3 Codex scored 56.8%. Claude Mythos 5 wins this benchmark.

  2. OSWorld-Verified

    Agentic
    Source ↗
    A 85%B 64.7%
    Winner: Claude Mythos 5Δ 20.3
    OSWorld-Verified: Claude Mythos 5 scored 85%; GPT-5.3 Codex scored 64.7%. Claude Mythos 5 wins this benchmark.

  3. Terminal-Bench 2.0

    Agentic
    Source ↗
    A 88%B 77.3%
    Winner: Claude Mythos 5Δ 10.7
    Terminal-Bench 2.0: Claude Mythos 5 scored 88%; GPT-5.3 Codex scored 77.3%. Claude Mythos 5 wins this benchmark.

  4. SWE-bench Verified

    Coding
    Source ↗
    A 95.5%B 85%
    Winner: Claude Mythos 5Δ 10.5
    SWE-bench Verified: Claude Mythos 5 scored 95.5%; GPT-5.3 Codex scored 85%. Claude Mythos 5 wins this benchmark.

Operational comparison

Runtime and commercial metrics are compared only when both models have a complete sourced value.

MetricClaude Mythos 5GPT-5.3 CodexComparison
Input / output priceUSD per 1M tokensClaude Mythos 5$10 input / $50 outputGPT-5.3 Codex$1.75 input / $14 outputGPT-5.3 Codex has the lower combined listed price.
Generation speedtokens per secondClaude Mythos 5Not availableGPT-5.3 Codex79 tok/sA complete speed comparison is not available.
First-answer latencyseconds to first tokenClaude Mythos 5Not availableGPT-5.3 Codex88.26 sA complete latency comparison is not available.
Context windowmaximum listed tokensClaude Mythos 51M+GPT-5.3 Codex400KClaude Mythos 5 lists the larger context window.

Benchmark Deep Dive

AgenticClaude Mythos 5 wins
BenchmarkClaude Mythos 5GPT-5.3 CodexResult
Terminal-Bench 2.0Source 88%77.3%Claude Mythos 5 leads
OSWorld-VerifiedSource 85%64.7%Claude Mythos 5 leads
BrowseCompSource 88%—Not comparable
ExploitGymSource 17.5%—Not comparable
τ²-bench resultsSource —86%Not comparable
Gert LabsSource —57.47%Not comparable
JobBenchSource —33.7%Not comparable
CodingClaude Mythos 5 wins
BenchmarkClaude Mythos 5GPT-5.3 CodexResult
SWE-bench VerifiedSource 95.5%85%Claude Mythos 5 leads
SWE-bench ProSource 80.3%56.8%Claude Mythos 5 leads
Terminal-Bench 2.0Source 88.0%—Not comparable
SWE-RebenchSource —58.2%Not comparable
Vibe Code BenchSource —61.77%Not comparable
AA-SciCodeSource —53.2%Not comparable
Reasoning
BenchmarkClaude Mythos 5GPT-5.3 CodexResult
AA-LCRSource —74.0%Not comparable
CritPtSource —16.9%Not comparable
Knowledge
BenchmarkClaude Mythos 5GPT-5.3 CodexResult
GPQASource 94.1%—Not comparable
HLESource 64.5%—Not comparable
HLE w/o toolsSource 59%—Not comparable
Artificial Analysis Intelligence IndexSource —44.3%Not comparable
AA-GPQA DiamondSource —91.5%Not comparable
AA-HLESource —39.9%Not comparable
AA-Omniscience IndexSource —9.9%Not comparable
AA-Omniscience AccuracySource —51.8%Not comparable
AA-Omniscience Hallucination RateSource —86.9%Not comparable
Math
BenchmarkClaude Mythos 5GPT-5.3 CodexResult
USAMO 2026Source 97.6%—Not comparable
Multilingual
BenchmarkClaude Mythos 5GPT-5.3 CodexResult
SWE MultilingualSource 92.2%—Not comparable
Multimodal
BenchmarkClaude Mythos 5GPT-5.3 CodexResult
SWE-bench MultimodalSource 54.9%—Not comparable
CharXivSource 93.5%—Not comparable
CharXiv w/o toolsSource 88.9%—Not comparable
AA-MMMU-ProSource —78.5%Not comparable
Design Arena WebsiteSource —1193Not comparable
Inst. Following
BenchmarkClaude Mythos 5GPT-5.3 CodexResult
AA-IFBenchSource —75.4%Not comparable
Frequently Asked Questions (3)

Which is better, Claude Mythos 5 or GPT-5.3 Codex?

Claude Mythos 5 is ahead on BenchLM's BenchAlign leaderboard, 83.93 to 66.69. The biggest single separator in this matchup is SWE-bench Pro, where the scores are 80.3% and 56.8%.

Which is better for coding, Claude Mythos 5 or GPT-5.3 Codex?

Claude Mythos 5 has the edge for coding in this comparison, averaging 89.7 versus 67.2. Inside this category, SWE-bench Pro is the benchmark that creates the most daylight between them.

Which is better for agentic tasks, Claude Mythos 5 or GPT-5.3 Codex?

Claude Mythos 5 has the edge for agentic tasks in this comparison, averaging 87 versus 71.4. Inside this category, OSWorld-Verified is the benchmark that creates the most daylight between them.

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