Gemini 3.1 Pro preview Customtools vs Grok 4.20 Reasoning
Gemini 3.1 Pro preview Customtools (Google Vertex AI, 1,048,576-token context) versus Grok 4.20 Reasoning (Google Vertex AI, 2,000,000-token context). Grok 4.20 Reasoning is cheaper by 73% on a blended token mix. Gemini 3.1 Pro preview Customtools uniquely supports audio input and pdf input. Use the live calculator below to plug your real usage shape into both, then route the winner via Agent Command Center for shadow A/B without code changes.
Bottom line — Gemini 3.1 Pro preview Customtools vs Grok 4.20 Reasoning
Gemini 3.1 Pro preview Customtools and Grok 4.20 Reasoning target overlapping workloads but differ sharply on economics. Grok 4.20 Reasoning runs roughly 73% cheaper on a blended input-plus-output token mix, which translates to approximately $7,950 per month at mid-market volume (100K requests/day). The gap compounds at enterprise scale, making the cost axis the first filter most teams apply when deciding between these two models.
Grok 4.20 Reasoning ships a 2,000,000-token context window, 1.9x larger than Gemini 3.1 Pro preview Customtools's 1,048,576 tokens. That headroom matters for long-document RAG pipelines, multi-turn agent sessions that accumulate tool-call history, and codebases where the entire repository needs to fit in a single prompt. If your average prompt stays under 1,048,576 tokens, the extra context on Grok 4.20 Reasoning is insurance you may never use — and Gemini 3.1 Pro preview Customtools may win on other axes.
On capability surface area, the models diverge: Gemini 3.1 Pro preview Customtools supports audio input where the other does not; Gemini 3.1 Pro preview Customtools supports pdf input where the other does not. These differences are binary — either your workload needs the capability or it does not. Check whether any critical path in your agent pipeline depends on a capability only one model provides before committing to a migration.
For teams evaluating both models, the recommended path is a shadow A/B test: route production traffic through an OpenAI-compatible gateway, mirror a percentage to the candidate model, score both responses with an automated evaluator (faithfulness, tool-call correctness, latency), and compare cohort-level metrics over two weeks. Future AGI Agent Command Center supports this pattern with a single `base_url` change and built-in evaluators from the ai-evaluation SDK.
Live workload comparison
Same workload run through both models. The cheaper one is highlighted.
strategy: cost-optimized
primary:
model: xai-grok-4-20-reasoning
provider: vertex-ai
fallback:
model: gemini-3-1-pro-preview-customtools
provider: vertex-ai
shadow: { sample_rate: 0.05 } # mirror 5% of traffic to compare quality live| Gemini 3.1 Pro preview Customtools | Grok 4.20 Reasoning | |
|---|---|---|
| Input price | $2.00/M | $1.25/M |
| Output price | $12.00/M | $2.50/M |
| Context window | 1,048,576 | 2,000,000 |
| Max output | 65,536 | 2,000,000 |
| Function calling | ✓ | ✓ |
| Vision | ✓ | ✓ |
| Audio input | ✓ | — |
| Reasoning | ✓ | ✓ |
| Prompt caching | ✓ | ✓ |
| Structured output | ✓ | ✓ |
| Pricing verified | Oct 22, 2026 | Oct 22, 2026 |
Cost at scale: monthly spend at three usage volumes
Estimated monthly cost assuming 1,000 input + 200 output tokens per request — a realistic chat-agent shape. Adjust your own usage in the calculator at the top of this page for an exact number.
| Scale | Gemini 3.1 Pro preview Customtools | Grok 4.20 Reasoning | Delta |
|---|---|---|---|
| Startup 10K requests/day | $1,320 /mo | $525 /mo | $795/mo |
| Mid-market 100K requests/day | $13,200 /mo | $5,250 /mo | $7,950/mo |
| Enterprise 1M requests/day | $132,000 /mo | $52,500 /mo | $79,500/mo |
At enterprise scale (1M requests/day), a difference of even ~10% in unit price compounds into thousands of dollars per month. Cached input pricing and batch tiers can shift this further — both are surfaced on each model's own page.
When to choose which
Picked from the data above — not vendor marketing. Match the rules to your workload, not the other way around.
You're cost-sensitive at scale — Grok 4.20 Reasoning runs ~73% cheaper on a blended in+out token mix, compounding into thousands of dollars per month at production volume.
Your agent listens to calls or voice notes — Gemini 3.1 Pro preview Customtools accepts audio input directly, the other requires an ASR preprocessing hop.
Capability diff — what you gain and lose on the swap
A specific list of what each model has that the other doesn't. If your workload depends on a row in Only Gemini 3.1 Pro preview Customtools, switching to Grok 4.20 Reasoning means re-architecting that path (and vice versa).
- • Audio input
- • PDF input
Capabilities both share (6)
- ✓ Function calling
- ✓ Vision input
- ✓ Streaming
- ✓ Structured output (JSON schema)
- ✓ Prompt caching
- ✓ Native reasoning mode
Migration considerations
Concrete differences to wire through your stack before you flip traffic from one to the other.
- Context window changes up 91% when moving from Gemini 3.1 Pro preview Customtools (1,048,576) to Grok 4.20 Reasoning (2,000,000). Re-check any prompt that relies on cramming long history or documents.
- Max output tokens differ: 65,536 on Gemini 3.1 Pro preview Customtools vs 2,000,000 on Grok 4.20 Reasoning. Long-form generation tasks may truncate differently — adjust streaming UI and chunking accordingly.
- Gemini 3.1 Pro preview Customtools has capabilities Grok 4.20 Reasoning lacks: Audio input, PDF input. Switching to Grok 4.20 Reasoning means re-architecting any flow that depends on these.
How to A/B test Gemini 3.1 Pro preview Customtools vs Grok 4.20 Reasoning in production
If you're stuck between the two, run them side-by-side on real traffic. Four steps the Future AGI team uses internally:
- 1. Point your existing OpenAI SDK at
https://gateway.futureagi.com/v1. No code change beyondbase_urland a virtual key. - 2. Mark Gemini 3.1 Pro preview Customtools primary, mirror 20% of traffic to Grok 4.20 Reasoning in shadow mode. Both responses are logged; only the primary is served to users.
- 3. Score every shadow response with an evaluator — faithfulness, tool-call correctness, response latency, cost. Built-in evaluators in ai-evaluation cover the common axes.
- 4. Compare cohort-level metrics after two weeks. Switch primary when the candidate wins on what matters to your workload — and stays within your latency budget.
Full walkthrough on the Agent Command Center page.
FAQ — Gemini 3.1 Pro preview Customtools vs Grok 4.20 Reasoning
Which is cheaper, Gemini 3.1 Pro preview Customtools or Grok 4.20 Reasoning? ▾
Grok 4.20 Reasoning is cheaper by roughly 73% on a blended input + output token mix. Input prices are $2.00/M for Gemini 3.1 Pro preview Customtools versus $1.25/M for Grok 4.20 Reasoning; output prices are $12.00/M versus $2.50/M. The exact savings depend on your input:output ratio — use the live calculator above to plug in your own request shape.
What is the context window of Gemini 3.1 Pro preview Customtools versus Grok 4.20 Reasoning? ▾
Gemini 3.1 Pro preview Customtools supports up to 1,048,576 tokens of context. Grok 4.20 Reasoning supports up to 2,000,000 tokens. Grok 4.20 Reasoning has the larger window by a factor of 1.9x, which matters for long-document RAG, multi-turn agent sessions, and tasks that need to keep an entire codebase in working memory.
Do Gemini 3.1 Pro preview Customtools and Grok 4.20 Reasoning both support tool calling? ▾
Yes — both Gemini 3.1 Pro preview Customtools and Grok 4.20 Reasoning support native function calling. Both also support structured output via JSON schema, so an agent can be ported between them with the same tool definitions.
Which model supports prompt caching for cost reduction? ▾
Both Gemini 3.1 Pro preview Customtools and Grok 4.20 Reasoning support prompt caching. Cached input tokens are typically discounted 50–90% versus uncached input, depending on the provider. For agents with a stable system prompt + retrieval context, the cached pricing tier is the real unit economics number to track.
When should I choose Gemini 3.1 Pro preview Customtools over Grok 4.20 Reasoning? ▾
Your agent listens to calls or voice notes — Gemini 3.1 Pro preview Customtools accepts audio input directly, the other requires an ASR preprocessing hop.
When should I choose Grok 4.20 Reasoning over Gemini 3.1 Pro preview Customtools? ▾
You're cost-sensitive at scale — Grok 4.20 Reasoning runs ~73% cheaper on a blended in+out token mix, compounding into thousands of dollars per month at production volume.
How do I A/B test Gemini 3.1 Pro preview Customtools against Grok 4.20 Reasoning in production? ▾
Route both through an OpenAI-compatible gateway like Future AGI Agent Command Center with shadow mode enabled. Send 100% of traffic to your primary model, mirror 10–20% to the candidate, score every response with an evaluator (faithfulness, tool-call correctness, response time), and compare cohort-level metrics for two weeks. Switch when the candidate wins on the metrics that matter to your workload and stays within your latency budget.