GPT-5.3-Codex vs Step 3.5 Flash
GPT-5.3-Codex (2026) and Step 3.5 Flash (2026) are agentic coding models from OpenAI and StepFun. GPT-5.3-Codex ships a not-yet-sourced context window, while Step 3.5 Flash ships a 256K-token context window. On pricing, Step 3.5 Flash costs $0.1/1M input tokens versus $1.75/1M for the alternative. This comparison covers specs, pricing, capabilities, benchmarks, provider availability, and production fit. It focuses on practical selection signals rather than broad model-family marketing.
Step 3.5 Flash is ~1650% cheaper at $0.1/1M; pay for GPT-5.3-Codex only for coding workflow support.
Specs
| Released | 2026-02-05 | 2026-01-29 |
| Context window | — | 256K |
| Parameters | — | 196B (11B active) |
| Architecture | decoder only | mixture of experts |
| License | Proprietary | Open Source |
| Knowledge cutoff | - | - |
Pricing and availability
| GPT-5.3-Codex | Step 3.5 Flash | |
|---|---|---|
| Input price | $1.75/1M tokens | $0.1/1M tokens |
| Output price | $14/1M tokens | $0.3/1M tokens |
| Providers |
Capabilities
| GPT-5.3-Codex | Step 3.5 Flash | |
|---|---|---|
| Vision | ||
| Multimodal | ||
| Reasoning | ||
| Function calling | ||
| Tool use | ||
| Structured outputs | ||
| Code execution |
Benchmarks
No shared benchmark rows are currently sourced for this pair.
Deep dive
The capability footprint differs most on multimodal input: GPT-5.3-Codex, reasoning mode: Step 3.5 Flash, function calling: GPT-5.3-Codex, tool use: GPT-5.3-Codex, structured outputs: GPT-5.3-Codex, and code execution: GPT-5.3-Codex. Both models share the core language-model surface, 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, GPT-5.3-Codex lists $1.75/1M input and $14/1M output tokens, while Step 3.5 Flash lists $0.1/1M input and $0.3/1M output tokens on the cheapest tracked provider. A 70/30 input-output blend puts Step 3.5 Flash lower by about $5.26 per million blended tokens. Availability is 1 providers versus 1, so concentration risk also matters.
Choose GPT-5.3-Codex when coding workflow support are central to the workload. Choose Step 3.5 Flash when reasoning depth 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. This keeps the decision grounded in measurable tradeoffs instead of brand-level assumptions. It also helps separate model capability from provider packaging, which can change cost and latency.
FAQ
Which is cheaper, GPT-5.3-Codex or Step 3.5 Flash?
Step 3.5 Flash is cheaper on tracked token pricing. GPT-5.3-Codex costs $1.75/1M input and $14/1M output tokens. Step 3.5 Flash costs $0.1/1M input and $0.3/1M output tokens. Provider discounts or batch pricing can still change the final bill.
Is GPT-5.3-Codex or Step 3.5 Flash open source?
GPT-5.3-Codex is listed under Proprietary. Step 3.5 Flash is listed under Open Source. 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 multimodal input, GPT-5.3-Codex or Step 3.5 Flash?
GPT-5.3-Codex 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.
Which is better for reasoning mode, GPT-5.3-Codex or Step 3.5 Flash?
Step 3.5 Flash has the clearer documented reasoning mode signal in this comparison. If reasoning mode is mission-critical, validate it against the provider endpoint because model-level support and API-level exposure can differ.
Which is better for function calling, GPT-5.3-Codex or Step 3.5 Flash?
GPT-5.3-Codex has the clearer documented function calling signal in this comparison. If function calling is mission-critical, validate it against the provider endpoint because model-level support and API-level exposure can differ.
Where can I run GPT-5.3-Codex and Step 3.5 Flash?
GPT-5.3-Codex is available on OpenRouter. Step 3.5 Flash is available on OpenRouter. Provider coverage can affect latency, region availability, compliance posture, and fallback options. Use this as a quick comparison signal, then confirm the provider-specific limits before committing to production.
Continue comparing
Last reviewed: 2026-04-24. Data sourced from public model cards and provider documentation.