GPT-5.3-Codex vs Step 3.5 Flash
GPT-5.3-Codex (2026) and Step 3.5 Flash (2026) compare a coding-specialized model against a standalone API model. GPT-5.3-Codex ships a 400k-token context window, while Step 3.5 Flash ships a 256k-token context window. On SWE-bench Verified, GPT-5.3-Codex leads by 10.6 pts. On pricing, Step 3.5 Flash costs $0.10/1M input tokens versus $1.75/1M 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: GPT-5.3-Codex is coding-specialized model, while Step 3.5 Flash is standalone API model. Choose based on workflow fit before reading any benchmark or price row as decisive.
Decision scorecard
Local evidence first| Signal | GPT-5.3-Codex | Step 3.5 Flash |
|---|---|---|
| Product type | Coding-specialized model | Standalone API model |
| Best for | custom coding agents, code generation, and tool loops | reasoning-heavy apps |
| Decision fit | Coding, RAG, and Agents | Coding, Agents, and Long context |
| Context window | 400k | 256k |
| Cheapest output | $14/1M tokens | $0.30/1M tokens |
| Provider routes | 3 tracked | 1 tracked |
| Shared benchmarks | SWE-bench Verified leader | 1 rows |
Decision tradeoffs
- GPT-5.3-Codex holds a shared-benchmark lead on SWE-bench Verified, ahead by 10.6 points.
- GPT-5.3-Codex has the larger context window for long prompts, retrieval packs, or transcript analysis.
- GPT-5.3-Codex has broader tracked provider coverage for fallback and procurement flexibility.
- GPT-5.3-Codex uniquely exposes Vision, Function calling, and Tool use in local model data.
- Local decision data tags GPT-5.3-Codex for Coding, RAG, and Agents.
- Step 3.5 Flash has the lower cheapest tracked output price at $0.30/1M tokens.
- Local decision data tags Step 3.5 Flash for Coding, Agents, and Long context.
Monthly cost at traffic
Estimate token spend from the cheapest tracked input and output route or tier on this page.
GPT-5.3-Codex
$4,900
Cheapest tracked route/tier: OpenRouter
Step 3.5 Flash
$155
Cheapest tracked route/tier: OpenRouter
Estimated monthly gap: $4,745. Batch, cache, alternate speed tiers, and negotiated pricing are excluded from this local estimate.
Switch friction
- Provider overlap exists on OpenRouter; start route-level A/B tests there.
- Step 3.5 Flash is $13.70/1M tokens lower on cheapest tracked output pricing before cache, batch, or negotiated discounts.
- Check replacement coverage for Vision, Function calling, and Tool use before moving production traffic.
- Provider overlap exists on OpenRouter; start route-level A/B tests there.
- GPT-5.3-Codex is $13.70/1M tokens higher on cheapest tracked output pricing, so quality gains need to justify the spend.
- GPT-5.3-Codex adds Vision, Function calling, and Tool use in local capability data.
Specs
| Specification | ||
|---|---|---|
| Released | 2026-02-05 | 2026-01-29 |
| Context window | 400k | 256k |
| Parameters | — | 196B (11B active) |
| Architecture | decoder only | mixture of experts |
| License | Proprietary | Proprietary |
| Openness | Proprietary | Proprietary |
| Commercial use | Commercial use with conditions | Commercial use with conditions |
| Knowledge cutoff | 2025-08 | - |
Pricing and availability
| Pricing attribute | GPT-5.3-Codex | Step 3.5 Flash |
|---|---|---|
| Input price | $1.75/1M tokens | $0.10/1M tokens |
| Output price | $14/1M tokens | $0.30/1M tokens |
| Providers |
Capabilities
| Capability | GPT-5.3-Codex | Step 3.5 Flash |
|---|---|---|
| Vision | Yes | No |
| Multimodal | No | No |
| Reasoning | Yes | Yes |
| Function calling | Yes | No |
| Tool use | Yes | No |
| Structured outputs | Yes | No |
| Code execution | Yes | No |
| IDE integration | No | No |
| Computer use | Yes | No |
| Parallel agents | No | No |
Benchmarks
| Benchmark | GPT-5.3-Codex | Step 3.5 Flash |
|---|---|---|
| SWE-bench Verified | 85.0 | 74.4 |
Deep dive
On shared benchmark coverage, SWE-bench Verified has GPT-5.3-Codex at 85 and Step 3.5 Flash at 74.4, with GPT-5.3-Codex ahead by 10.6 points. The largest visible gap is 10.6 points on SWE-bench Verified, which matters most when that benchmark mirrors your workload. Treat isolated benchmark wins as directional, because provider routing, prompt style, and tool access can move real application results.
The capability footprint differs most on vision: GPT-5.3-Codex, 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 reasoning mode, 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 on the cheapest tracked provider, while Step 3.5 Flash lists $0.10/1M input and $0.30/1M 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 3 providers versus 1, so concentration risk also matters.
Choose GPT-5.3-Codex when coding workflow support, larger context windows, and broader provider choice are central to the workload. Choose Step 3.5 Flash when provider fit 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.
FAQ
Which has a larger context window, GPT-5.3-Codex or Step 3.5 Flash?
GPT-5.3-Codex supports 400k tokens, while Step 3.5 Flash supports 256k tokens. That gap matters most for long documents, large codebases, retrieval-heavy agents, and conversations where earlier context must remain visible.
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 $0.10/1M input and $0.30/1M 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 Proprietary. 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 vision, GPT-5.3-Codex or Step 3.5 Flash?
GPT-5.3-Codex has the clearer documented vision signal in this comparison. If vision is mission-critical, validate it against the provider endpoint because model-level support and API-level exposure can differ. Use this as a quick comparison signal, then confirm the provider-specific limits before committing to production.
Which is better for reasoning mode, GPT-5.3-Codex or Step 3.5 Flash?
Both GPT-5.3-Codex and Step 3.5 Flash expose reasoning mode. The better choice depends on benchmark fit, context budget, pricing, and whether your provider route exposes the same capability surface. Use this as a quick comparison signal, then confirm the provider-specific limits before committing to production.
Where can I run GPT-5.3-Codex and Step 3.5 Flash?
GPT-5.3-Codex is available on OpenRouter, OpenAI API, and Vercel AI Gateway. Step 3.5 Flash is available on OpenRouter. Provider coverage can affect latency, region availability, compliance posture, and fallback options.
Continue comparing
Last reviewed: 2026-06-10. Data sourced from public model cards and provider documentation.