Gemini 3.5 Flash vs Grok 4.20 (Reasoning)
Too close to call on our weighted score (Gemini 3.5 Flash 69, Grok 4.20 (Reasoning) 68). The right pick depends on what you value most.
GoogleGemini 3.5 Flash
69/100- ECI154.5
- Price$1.50 / $9.00
- Context1.05M
xAIGrok 4.20 (Reasoning)
68/100- ECI152.0
- Price$1.25 / $2.50
- Context1M
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Make it a three-way comparison.
Too close to call
It is close. Our weighted score puts them within a point (Gemini 3.5 Flash 69/100, Grok 4.20 (Reasoning) 68/100), so choose by what matters most for your work: Gemini 3.5 Flash for raw capability and Grok 4.20 (Reasoning) on price. The score weighs capability 50%, price 25%, inputs & features 15%, context window 10%.
- CapabilityGemini 3.5 FlashCapabilities Index (ECI): Gemini 3.5 Flash 154.5 · Grok 4.20 (Reasoning) 152.0
- Lowest priceGrok 4.20 (Reasoning)Grok 4.20 (Reasoning) $1.56 · Gemini 3.5 Flash $3.38 per 1M tokens (3:1 blend)
- Longest contextGemini 3.5 FlashGemini 3.5 Flash 1,048,576 · Grok 4.20 (Reasoning) 1,000,000 tokens
- Widest inputsGemini 3.5 FlashGemini 3.5 Flash: Text, Images, PDFs, Audio, Video · Grok 4.20 (Reasoning): Text, Images, PDFs
- Self-hostingNo open weightsBoth are available only through APIs
| Measure | Weight | Gemini 3.5 Flash | Grok 4.20 (Reasoning) |
|---|---|---|---|
| CapabilityCapabilities Index (ECI) | 50% | 84 | 81 |
| Price | 25% | 25 | 41 |
| Inputs & features | 15% | 100 | 80 |
| Context window | 10% | 61 | 60 |
| Overall | 100% | 69/100 | 68/100 |
Every spec in one table
Highlighted cells lead their row. Dashes mean the data is not published.
| Specification | Gemini 3.5 FlashGoogle | Grok 4.20 (Reasoning)xAI |
|---|---|---|
| Capability | ||
| Capabilities Index (ECI) | 154.5 (best) | 152.0 |
| ECI rank | #33 of 148 (best) | #41 of 148 |
| GPQA DiamondGraduate-level science questions | 92.8% (best) | 89.3% |
| FrontierMath Tiers 1–3Research-level mathematics | 62.8% (best) | 44.9% |
| OTIS Mock AIME 2024–2025Competition mathematics | 95.6% (best) | 92.2% |
| SWE-bench VerifiedFixing real GitHub issues | 79.3% | — |
| SimpleQA VerifiedShort factual questions | 66.2% (best) | 30.2% |
| Price per million tokens | ||
| Input | $1.50 | $1.25 (best) |
| Output | $9.00 | $2.50 (best) |
| Cached input | $0.15 (best) | $0.20 |
| Blended (3:1) | $3.38 | $1.56 (best) |
| Long-context rate | Same rate | Over 200K: $2.50 / $5.00 |
| Price source | Official Google API | Official xAI API |
| Limits | ||
| Context window | 1,048,576 tokens (best) | 1,000,000 tokens |
| Max output | 65,536 tokens (best) | 30,000 tokens |
| Inputs and features | ||
| Text | Yes | Yes |
| Images | Yes | Yes |
| PDFs | Yes | Yes |
| Audio | Yes | No |
| Video | Yes | No |
| Reasoning | Yesminimal · low · medium · high | Yes |
| Tool calling | Yes | Yes |
| Structured output | Yes | Yes |
| Availability | ||
| Weights | Proprietary | Proprietary |
| API model ID | gemini-3.5-flash | grok-4.20-0309-reasoning |
| API providers | 32 (best) | 11 |
| Released | May 19, 2026 | Mar 9, 2026 |
| Knowledge cutoff | Jan 2025 | — |
What would a month cost?
Enter your expected volume in millions of tokens. List prices only; caching and batch discounts would lower these.
- Gemini 3.5 Flash$33.00
- Grok 4.20 (Reasoning)$17.50
Which should you choose?
Which is better: Gemini 3.5 Flash or Grok 4.20 (Reasoning)?
It is close. Our weighted score puts them within a point (Gemini 3.5 Flash 69/100, Grok 4.20 (Reasoning) 68/100), so choose by what matters most for your work: Gemini 3.5 Flash for raw capability and Grok 4.20 (Reasoning) on price. The score weighs capability 50%, price 25%, inputs & features 15%, context window 10%.
Which is cheaper, Gemini 3.5 Flash or Grok 4.20 (Reasoning)?
Grok 4.20 (Reasoning) is cheaper at $1.25 input / $2.50 output per million tokens (official xAI API price). Gemini 3.5 Flash costs $1.50 input / $9.00 output per million tokens (official Google API price). At a typical mix of three input tokens to one output token, that is $1.56 per million tokens for Grok 4.20 (Reasoning) versus $3.38 for Gemini 3.5 Flash (2.2× as much).
Which scores higher on benchmarks?
Gemini 3.5 Flash scores higher on the Capabilities Index (ECI): Gemini 3.5 Flash 154.5 (#33 of 148) and Grok 4.20 (Reasoning) 152.0 (#41 of 148). The confidence ranges of the top two overlap (152.5–156.6 vs 149.3–154.4), so treat the gap as small. On individual benchmarks: GPQA Diamond — Gemini 3.5 Flash 92.8%, Grok 4.20 (Reasoning) 89.3%; FrontierMath Tiers 1–3 — Gemini 3.5 Flash 62.8%, Grok 4.20 (Reasoning) 44.9%; OTIS Mock AIME 2024–2025 — Gemini 3.5 Flash 95.6%, Grok 4.20 (Reasoning) 92.2%; SimpleQA Verified — Gemini 3.5 Flash 66.2%, Grok 4.20 (Reasoning) 30.2%.
Which is better for coding?
There are no published SWE-bench Verified results for Grok 4.20 (Reasoning) yet, so there is no like-for-like coding score. On overall capability, Gemini 3.5 Flash leads, which tends to carry over to coding, but test on your own codebase. Both support tool calling for agent workflows.
Which has the bigger context window?
Gemini 3.5 Flash has the largest context window at 1,048,576 tokens, against 1,000,000 for Grok 4.20 (Reasoning). Maximum output per response: Gemini 3.5 Flash up to 65,536, Grok 4.20 (Reasoning) up to 30,000 tokens.
Which can read images, PDFs, audio or video?
Gemini 3.5 Flash accepts text, images, PDFs, audio and video; Grok 4.20 (Reasoning) accepts text, images and PDFs. Gemini 3.5 Flash handles the widest range of inputs.
Are any of these open source?
No. Gemini 3.5 Flash and Grok 4.20 (Reasoning) are proprietary and only available through APIs and apps.
Which is newer?
Gemini 3.5 Flash is the newest, released May 19, 2026. Grok 4.20 (Reasoning) came out Mar 9, 2026. Knowledge cutoff: Gemini 3.5 Flash Jan 2025.
How do you decide the winner?
Each model gets a 0–100 score on capability (50%, independent benchmark results); price (25%, blended price per million tokens (3 input : 1 output), log scale); inputs & features (15%, image, PDF, audio and video input, tool calling, structured output and reasoning); context window (10%, maximum tokens per request, log scale). Dimensions missing for any model are dropped and the remaining weights rescaled, so every model is judged on the same evidence. Specs and prices come from public model listings and the labs’ own API pages; capability scores come from independent benchmark runs. Data updated Oct 4, 2026.
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