Head-to-head comparison
GPT-5.3-Codex (OpenAI) and Step 3.7 Flash (StepFun) compared on the ModelCap Index, API price, context window, provider availability, weight access and every public benchmark board they share. Figures are the same ones shown on the live rankings; nothing here is a hidden score.
Snapshot as of 18 August 2026
As of 18 August 2026, GPT-5.3-Codex holds the stronger ModelCap Index position (#20 vs #65); Step 3.7 Flash is 12.2× cheaper per output token ($1.15/1M vs $14.00/1M); GPT-5.3-Codex offers the longer context window (400K vs 262K tokens); and Step 3.7 Flash ships open weights.
Evidence: 1 public benchmark observation across 1 board · 1 public benchmark observation across 1 board. Prices are the lowest listed API offer per million tokens observed on the OpenRouter catalogue.
Scores are the sources' own published figures for each model's best evaluated configuration; ModelCap never re-runs a benchmark.
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GPT-5.3-Codex ranks higher on the ModelCap Index as of 18 August 2026: #20 against #65. That is a capability ranking built from public benchmark evidence with published uncertainty; whether it is "better" for you also depends on price, context and where you can run it.
Step 3.7 Flash is cheaper on output tokens: $1.15/1M against $14.00/1M. Input tokens are $1.75/1M for GPT-5.3-Codex and $0.20/1M for Step 3.7 Flash. Prices are the lowest listed API offer ModelCap observed, in USD per million tokens.
GPT-5.3-Codex has the larger context window: 400K tokens against 262K.
The two models do not share a coding benchmark board on ModelCap yet, so no head-to-head coding score is published; the ModelCap Index position is the closest overall signal.
GPT-5.3-Codex: API only. Step 3.7 Flash: Open weights. Open weights mean the checkpoint can be downloaded and self-hosted under its licence; API-only models are available solely through hosted endpoints.
ModelCap currently lists 2 API providers for GPT-5.3-Codex and 3 for Step 3.7 Flash, from the OpenRouter catalogue snapshot the site serves; each model page lists the providers and their prices.
Every figure comes from the sealed ModelCap dataset published 18 August 2026; the page re-renders within a minute of each data refresh, and the ModelCap Index positions are the same ones shown on the live rankings.