# Gemini 3.5 Flash vs GPT-5.3 Codex

> Side-by-side benchmark comparison for Gemini 3.5 Flash and GPT-5.3 Codex across agentic, coding, multimodal, knowledge, reasoning, multilingual, and math tasks.

- Canonical page: https://benchlm.ai/compare/gemini-3-5-flash-vs-gpt-5-3-codex
- Last updated: October 2, 2026

- Shared sourced benchmarks: 6
- HTML indexing: indexable
- Ranking lane: BenchAlign v5.8

## Quick Verdict

Pick Gemini 3.5 Flash if you want the stronger benchmark profile. GPT-5.3 Codex only makes more sense when its price, context window, or workload-specific category wins matter more than the overall score.

## Summary

- Gemini 3.5 Flash leads overall 63.95 to 62.49.
- The clearest category separation is in reasoning, where the averages are 62.8 for Gemini 3.5 Flash and 79.6 for GPT-5.3 Codex.
- The biggest single benchmark swing is OSWorld-Verified in Agentic, with scores of 78.4% and 64.7%.
- Gemini 3.5 Flash is the cheaper option on output tokens, which matters if you expect large responses or heavy interactive use.
- Gemini 3.5 Flash also has the larger context window at 1M.

## Model Snapshot

| Property | Gemini 3.5 Flash | GPT-5.3 Codex |
|----------|----------|----------|
| Creator | Google | OpenAI |
| Type | Proprietary | Proprietary |
| Reasoning | Reasoning | Reasoning |
| Context | 1M | 400K |
| Overall Score | 63.95 | 62.49 |
| Benchmarks Covered | 27 | 10 |
| Pricing (input/output) | $1.50 / $9.00 | $1.75 / $14.00 |

## Category Breakdown

### Agentic

- Winner: Directional only
- Gemini 3.5 Flash public-lane score: 50.7 (Supported · #42/119)
- GPT-5.3 Codex public-lane score: 55.4 (Estimated · #33/119)

| Benchmark | Gemini 3.5 Flash | GPT-5.3 Codex | Winner |
|-----------|-----------|-----------|--------|
| Terminal-Bench 2.1 | 76.2% | Coming soon | Coming soon |
| MCP Atlas | 83.6% | Coming soon | Coming soon |
| Toolathlon | 56.5% | Coming soon | Coming soon |
| OSWorld-Verified | 78.4% | 64.7% | Gemini 3.5 Flash |
| Finance Agent v2 | 57.9% | Coming soon | Coming soon |
| Gert Labs | 61.85% | 57.47% | Gemini 3.5 Flash |
| ResearchClawBench | 18.0% | Coming soon | Coming soon |
| Terminal-Bench 2.1 (Vals) | 74.2% | Coming soon | Coming soon |
| Terminal-Bench 2.0 | Coming soon | 77.3% | Coming soon |
| JobBench | Coming soon | 33.7% | Coming soon |

### Coding

- Winner: GPT-5.3 Codex
- Gemini 3.5 Flash public-lane score: 52.3 (Supported · #39/144)
- GPT-5.3 Codex public-lane score: 56 (Supported · #29/144)

| Benchmark | Gemini 3.5 Flash | GPT-5.3 Codex | Winner |
|-----------|-----------|-----------|--------|
| Terminal-Bench 2.1 | 76.2% | Coming soon | Coming soon |
| SWE-bench Pro | 55.1% | 56.8% | GPT-5.3 Codex |
| Vibe Code Bench | 48.68% | 61.77% | GPT-5.3 Codex |
| cursorBench31 | 49.8% | Coming soon | Coming soon |
| CursorBench 3.2 | 48.8% | Coming soon | Coming soon |
| LiveCodeBench (Vals) | 87.6% | 87.3% | Gemini 3.5 Flash |
| SWE-bench (Vals) | 78.8% | 78.0% | Gemini 3.5 Flash |
| SWE-bench Verified | Coming soon | 85% | Coming soon |
| SWE-Rebench | Coming soon | 58.2% | Coming soon |

### Reasoning

- Winner: Not comparable
- Gemini 3.5 Flash public-lane score: 62.8 (#20/27)
- GPT-5.3 Codex public-lane score: 79.6 (Unranked · 2 rankable rows)

| Benchmark | Gemini 3.5 Flash | GPT-5.3 Codex | Winner |
|-----------|-----------|-----------|--------|
| MRCRv2 | 77.3% | Coming soon | Coming soon |
| MRCR 1M | 26.6% | Coming soon | Coming soon |
| ARC-AGI-2 | 72.1% | Coming soon | Coming soon |

### Multimodal & Grounded

- Winner: Not comparable
- Gemini 3.5 Flash public-lane score: 87.8 (#6/49)
- GPT-5.3 Codex public-lane score: 76.7 (Unranked · 1 rankable row)

| Benchmark | Gemini 3.5 Flash | GPT-5.3 Codex | Winner |
|-----------|-----------|-----------|--------|
| CharXiv | 84.2% | Coming soon | Coming soon |
| MMMU-Pro | 83.6% | Coming soon | Coming soon |
| Blueprint-Bench 2 | 33.6% | Coming soon | Coming soon |

### Knowledge

- Winner: Directional only
- Gemini 3.5 Flash public-lane score: 64 (Supported · #30/171)
- GPT-5.3 Codex public-lane score: 64.9 (Estimated · #26/171)

| Benchmark | Gemini 3.5 Flash | GPT-5.3 Codex | Winner |
|-----------|-----------|-----------|--------|
| GPQA-D | 92.7% | Coming soon | Coming soon |
| HLE | 40.2% | Coming soon | Coming soon |
| GPQA Diamond (Vals) | 92.7% | Coming soon | Coming soon |
| MMLU-Pro (Vals) | 89.5% | Coming soon | Coming soon |

### Instruction Following

- Winner: Directional only
- Gemini 3.5 Flash public-lane score: 84.1 (#43/125)
- GPT-5.3 Codex public-lane score: 91.2 (#14/125)

### Mathematics

- Winner: Not comparable
- Gemini 3.5 Flash public-lane score: 55.1 (Unranked · 2 rankable rows)
- GPT-5.3 Codex public-lane score: Coming soon

| Benchmark | Gemini 3.5 Flash | GPT-5.3 Codex | Winner |
|-----------|-----------|-----------|--------|
| FrontierMath v2 (Tiers 1-3) | 38.966% | Coming soon | Coming soon |
| FrontierMath v2 (Tier 4) | 14.583% | Coming soon | Coming soon |

## FAQ

### Which is better overall, Gemini 3.5 Flash or GPT-5.3 Codex?

Gemini 3.5 Flash is ahead overall on BenchLM right now.

### Where is the biggest gap between Gemini 3.5 Flash and GPT-5.3 Codex?

The widest category gap is in reasoning, where the averages are 62.8 for Gemini 3.5 Flash and 79.6 for GPT-5.3 Codex.

### How many benchmarks does Gemini 3.5 Flash cover on BenchLM?

Gemini 3.5 Flash currently has 27 sourced benchmark scores on BenchLM.

### How many benchmarks does GPT-5.3 Codex cover on BenchLM?

GPT-5.3 Codex currently has 10 sourced benchmark scores on BenchLM.

## Related Comparisons

- [Gemini 3.5 Flash vs Gemini 3.5 Flash Cyber](/compare/gemini-3-5-flash-vs-gemini-3-5-flash-cyber)
- [GPT-5.3 Codex vs Gemini 3.5 Flash Cyber](/compare/gemini-3-5-flash-cyber-vs-gpt-5-3-codex)
- [Gemini 3.5 Flash vs GPT-5.3-Codex-Spark](/compare/gemini-3-5-flash-vs-gpt-5-3-codex-spark)
- [GPT-5.3 Codex vs GPT-5.3-Codex-Spark](/compare/gpt-5-3-codex-vs-gpt-5-3-codex-spark)

## Explore More

- [Gemini 3.5 Flash profile](/models/gemini-3-5-flash)
- [GPT-5.3 Codex profile](/models/gpt-5-3-codex)
- [Compare Pricing](/llm-pricing)
- [Alternative Finder](/tools/alternative-finder)
- [LLM Selector](/tools/llm-selector)
- [Overall Rankings](/best/overall)