Claude Opus 4.8 Default vs Grok 4.20 Reasoning
Claude Opus 4.8 Default (Google Vertex AI, 1,000,000-token context) versus Grok 4.20 Reasoning (Google Vertex AI, 2,000,000-token context). Grok 4.20 Reasoning is cheaper by 88% on a blended token mix. Claude Opus 4.8 Default uniquely supports 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 — Claude Opus 4.8 Default vs Grok 4.20 Reasoning
Claude Opus 4.8 Default and Grok 4.20 Reasoning target overlapping workloads but differ sharply on economics. Grok 4.20 Reasoning runs roughly 88% cheaper on a blended input-plus-output token mix, which translates to approximately $24,750 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, 2.0x larger than Claude Opus 4.8 Default's 1,000,000 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 1,000,000 tokens, the extra context on Grok 4.20 Reasoning is insurance you may never use — and Claude Opus 4.8 Default may win on other axes.
On capability surface area, the models diverge: Claude Opus 4.8 Default 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: claude-opus-4-8-default
provider: vertex-ai
shadow: { sample_rate: 0.05 } # mirror 5% of traffic to compare quality live| Claude Opus 4.8 Default | Grok 4.20 Reasoning | |
|---|---|---|
| Input price | $5.00/M | $1.25/M |
| Output price | $25.00/M | $2.50/M |
| Context window | 1,000,000 | 2,000,000 |
| Max output | 128,000 | 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 | Claude Opus 4.8 Default | Grok 4.20 Reasoning | Delta |
|---|---|---|---|
| Startup 10K requests/day | $3,000 /mo | $525 /mo | $2,475/mo |
| Mid-market 100K requests/day | $30,000 /mo | $5,250 /mo | $24,750/mo |
| Enterprise 1M requests/day | $300,000 /mo | $52,500 /mo | $247,500/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 ~88% cheaper on a blended in+out token mix, compounding into thousands of dollars per month at production volume.
Your workload needs long context — Grok 4.20 Reasoning fits 2,000,000 tokens versus the other model's 1,000,000, enough headroom for full books, large codebases, or 100+ page documents in one shot.
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 Claude Opus 4.8 Default, switching to Grok 4.20 Reasoning means re-architecting that path (and vice versa).
- • 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 100% when moving from Claude Opus 4.8 Default (1,000,000) to Grok 4.20 Reasoning (2,000,000). Re-check any prompt that relies on cramming long history or documents.
- Max output tokens differ: 128,000 on Claude Opus 4.8 Default vs 2,000,000 on Grok 4.20 Reasoning. Long-form generation tasks may truncate differently — adjust streaming UI and chunking accordingly.
- Claude Opus 4.8 Default has capabilities Grok 4.20 Reasoning lacks: PDF input. Switching to Grok 4.20 Reasoning means re-architecting any flow that depends on these.
How to A/B test Claude Opus 4.8 Default 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 Claude Opus 4.8 Default 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 — Claude Opus 4.8 Default vs Grok 4.20 Reasoning
Which is cheaper, Claude Opus 4.8 Default or Grok 4.20 Reasoning? ▾
Grok 4.20 Reasoning is cheaper by roughly 88% on a blended input + output token mix. Input prices are $5.00/M for Claude Opus 4.8 Default versus $1.25/M for Grok 4.20 Reasoning; output prices are $25.00/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 Claude Opus 4.8 Default versus Grok 4.20 Reasoning? ▾
Claude Opus 4.8 Default supports up to 1,000,000 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 2.0x, which matters for long-document RAG, multi-turn agent sessions, and tasks that need to keep an entire codebase in working memory.
Do Claude Opus 4.8 Default and Grok 4.20 Reasoning both support tool calling? ▾
Yes — both Claude Opus 4.8 Default 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 Claude Opus 4.8 Default 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 Claude Opus 4.8 Default over Grok 4.20 Reasoning? ▾
On the data this page surfaces, Claude Opus 4.8 Default is the right pick when Grok 4.20 Reasoning's lower price or different capability profile aren't a fit for your workload. Run the live calculator above against your actual usage shape to confirm.
When should I choose Grok 4.20 Reasoning over Claude Opus 4.8 Default? ▾
You're cost-sensitive at scale — Grok 4.20 Reasoning runs ~88% cheaper on a blended in+out token mix, compounding into thousands of dollars per month at production volume. Your workload needs long context — Grok 4.20 Reasoning fits 2,000,000 tokens versus the other model's 1,000,000, enough headroom for full books, large codebases, or 100+ page documents in one shot.
How do I A/B test Claude Opus 4.8 Default 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.