Model Comparison

GPT-5.1 Codex vs Step-3.5-FlashWhich is better in 2026?

Both models are evenly matched across the benchmarks. Step-3.5-Flash is 19.6x cheaper per token.

Verdict: GPT-5.1 Codex vs Step-3.5-Flash — which is better?

GPT-5.1 Codex (by OpenAI) and Step-3.5-Flash (by StepFun) are two of the AI models people compare most. Here is how they stack up on benchmarks, price and capabilities, and which one to pick in 2026.

GPT-5.1 Codex outperforms in 1 benchmarks (Terminal-Bench 2.0), while Step-3.5-Flash is better at 1 benchmark (SWE-Bench Verified). Both models are evenly matched across the benchmarks.

On price, Step-3.5-Flash is roughly 19.6x cheaper per token on a blended 3:1 input/output basis, which adds up quickly at production volume.

GPT-5.1 Codex also accepts a larger context window (400,000 input tokens), making it the stronger choice for long documents and large codebases.

Choose GPT-5.1 Codex if…

  • you process long inputs — it offers a 400,000 token context window

Choose Step-3.5-Flash if…

  • cost matters — it's about 19.6x cheaper per token
  • you want the most recent training data — it shipped Feb 2026
  • you need open weights you can self-host or fine-tune

Performance Benchmarks

Comparative analysis across standard metrics

2 benchmarks

GPT-5.1 Codex outperforms in 1 benchmarks (Terminal-Bench 2.0), while Step-3.5-Flash is better at 1 benchmark (SWE-Bench Verified).

Both models are evenly matched across the benchmarks.

Sat Jul 25 2026 • llm-stats.com

Arena Performance

Human preference votes

Pricing Analysis

Price comparison per million tokens

Step-3.5-Flash costs less

For input processing, GPT-5.1 Codex ($1.25/1M tokens) is 12.5x more expensive than Step-3.5-Flash ($0.10/1M tokens).

For output processing, GPT-5.1 Codex ($10.00/1M tokens) is 25.0x more expensive than Step-3.5-Flash ($0.40/1M tokens).

In conclusion, GPT-5.1 Codex is more expensive than Step-3.5-Flash.*

* Using a 3:1 ratio of input to output tokens

Lowest available price from all providers
Sat Jul 25 2026 • llm-stats.com
OpenAI
GPT-5.1 Codex
Input tokens$1.25
Output tokens$10.00
Best providerOpenAI
StepFun
Step-3.5-Flash
Input tokens$0.10
Output tokens$0.40
Best providerStepFun
Notice missing or incorrect data?Start an Issue→

Context Window

Maximum input and output token capacity

GPT-5.1 Codex accepts 400,000 input tokens compared to Step-3.5-Flash's 65,536 tokens. GPT-5.1 Codex can generate longer responses up to 128,000 tokens, while Step-3.5-Flash is limited to 8,192 tokens.

OpenAI
GPT-5.1 Codex
Input400,000 tokens
Output128,000 tokens
StepFun
Step-3.5-Flash
Input65,536 tokens
Output8,192 tokens
Sat Jul 25 2026 • llm-stats.com

Input Capabilities

Supported data types and modalities

GPT-5.1 Codex supports multimodal inputs, whereas Step-3.5-Flash does not.

GPT-5.1 Codex can handle both text and other forms of data like images, making it suitable for multimodal applications.

GPT-5.1 Codex

Text
Images
Audio
Video

Step-3.5-Flash

Text
Images
Audio
Video

License

Usage and distribution terms

GPT-5.1 Codex is licensed under a proprietary license, while Step-3.5-Flash uses Apache 2.0.

License differences may affect how you can use these models in commercial or open-source projects.

GPT-5.1 Codex

Proprietary

Closed source

Step-3.5-Flash

Apache 2.0

Open weights

Release Timeline

When each model was launched

GPT-5.1 Codex was released on 2025-11-19, while Step-3.5-Flash was released on 2026-02-02.

Step-3.5-Flash is 3 months newer than GPT-5.1 Codex.

GPT-5.1 Codex

Nov 19, 2025

8 months ago

Step-3.5-Flash

Feb 2, 2026

5 months ago

2mo newer

Knowledge Cutoff

When training data ends

GPT-5.1 Codex has a documented knowledge cutoff of 2024-09-30, while Step-3.5-Flash's cutoff date is not specified.

We can confirm GPT-5.1 Codex's training data extends to 2024-09-30, but cannot make a direct comparison without Step-3.5-Flash's cutoff date.

GPT-5.1 Codex

Sep 2024

Step-3.5-Flash

—

Provider Availability

GPT-5.1 Codex is available from OpenAI. Step-3.5-Flash is available from StepFun.

GPT-5.1 Codex

openai logo
OpenAI
Input Price:Input: $1.25/1MOutput Price:Output: $10.00/1M

Step-3.5-Flash

stepfun logo
StepFun
Input Price:Input: $0.10/1MOutput Price:Output: $0.40/1M
* Prices shown are per million tokens

Outputs Comparison

Notice missing or incorrect data?Start an Issue discussion→

Key Takeaways

OpenAI
Larger context window (400,000 tokens)
Supports multimodal inputs
Higher Terminal-Bench 2.0 score (52.8% vs 51.0%)
StepFun
Less expensive input tokens
Less expensive output tokens
Has open weights
Higher SWE-Bench Verified score (74.4% vs 73.7%)

Detailed Comparison

Interactive Arena

Judge for yourself.

Run your own prompts against GPT-5.1 Codex and Step-3.5-Flash side-by-side, then vote on the output you prefer.

GPT-5.1 Codex
✓ Preferred
Step-3.5-Flash
Open in Playground
AI Model Comparison Table
Feature
OpenAI
GPT-5.1 Codex
StepFun
Step-3.5-Flash

FAQ

Common questions about GPT-5.1 Codex vs Step-3.5-Flash.

Which is better, GPT-5.1 Codex or Step-3.5-Flash?

Both models are evenly matched across the benchmarks. GPT-5.1 Codex is made by OpenAI and Step-3.5-Flash is made by StepFun. The best choice depends on your use case — compare their benchmark scores, pricing, and capabilities above.

How does GPT-5.1 Codex compare to Step-3.5-Flash in benchmarks?

GPT-5.1 Codex scores SWE-Bench Verified: 73.7%, SWE-Lancer: 66.3%, Terminal-Bench 2.0: 52.8%. Step-3.5-Flash scores AIME 2025: 97.3%, Tau-bench: 88.2%, LiveCodeBench v6: 86.4%, IMO-AnswerBench: 85.4%, SWE-Bench Verified: 74.4%.

Is GPT-5.1 Codex cheaper than Step-3.5-Flash?

Step-3.5-Flash is 12.5x cheaper for input tokens. GPT-5.1 Codex costs $1.25/M input and $10.00/M output via openai. Step-3.5-Flash costs $0.10/M input and $0.40/M output via stepfun.

What are the context window sizes for GPT-5.1 Codex and Step-3.5-Flash?

GPT-5.1 Codex supports 400K tokens and Step-3.5-Flash supports 66K tokens. A larger context window lets you process longer documents, conversations, or codebases in a single request.

What are the main differences between GPT-5.1 Codex and Step-3.5-Flash?

Key differences include context window (400K vs 66K), input pricing ($1.25 vs $0.10/M), multimodal support (yes vs no), licensing (Proprietary vs Apache 2.0). See the full comparison above for benchmark-by-benchmark results.

Who makes GPT-5.1 Codex and Step-3.5-Flash?

GPT-5.1 Codex is developed by OpenAI and Step-3.5-Flash is developed by StepFun.

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