Claude Fable 5 vs Grok 4
Claude Fable 5 (Azure AI Foundry, 1,000,000-token context) versus Grok 4 (Azure AI Foundry, 131,072-token context). Grok 4 is cheaper by 70% on a blended token mix. Claude Fable 5 uniquely supports vision input and pdf input. Across 1 public benchmark we tracked, Claude Fable 5 wins 1 and Grok 4 wins 0. 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 Fable 5 vs Grok 4
Claude Fable 5 and Grok 4 target overlapping workloads but differ sharply on economics. Grok 4 runs roughly 70% cheaper on a blended input-plus-output token mix, which translates to approximately $42,000 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.
Claude Fable 5 ships a 1,000,000-token context window, 7.6x larger than Grok 4's 131,072 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 131,072 tokens, the extra context on Claude Fable 5 is insurance you may never use — and Grok 4 may win on other axes.
On capability surface area, the models diverge: Claude Fable 5 supports vision input where the other does not; Claude Fable 5 supports pdf input where the other does not; Claude Fable 5 supports prompt caching 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: grok-4
provider: azure-ai-foundry
fallback:
model: claude-fable-5
provider: azure-ai-foundry
shadow: { sample_rate: 0.05 } # mirror 5% of traffic to compare quality live| Claude Fable 5 | Grok 4 | |
|---|---|---|
| Input price | $10.00/M | $3.00/M |
| Output price | $50.00/M | $15.00/M |
| Context window | 1,000,000 | 131,072 |
| Max output | 128,000 | 131,072 |
| Function calling | ✓ | ✓ |
| Vision | ✓ | — |
| Audio input | — | — |
| Reasoning | ✓ | — |
| Prompt caching | ✓ | — |
| Structured output | ✓ | ✓ |
| Pricing verified | Aug 6, 2026 | Aug 6, 2026 |
Benchmark comparison
Side-by-side public benchmark scores. Greener bar = winner.
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 Fable 5 | Grok 4 | Delta |
|---|---|---|---|
| Startup 10K requests/day | $6,000 /mo | $1,800 /mo | $4,200/mo |
| Mid-market 100K requests/day | $60,000 /mo | $18,000 /mo | $42,000/mo |
| Enterprise 1M requests/day | $600,000 /mo | $180,000 /mo | $420,000/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 runs ~70% cheaper on a blended in+out token mix, compounding into thousands of dollars per month at production volume.
Your workload needs long context — Claude Fable 5 fits 1,000,000 tokens versus the other model's 131,072, enough headroom for full books, large codebases, or 100+ page documents in one shot.
Your inputs include screenshots, diagrams, or product photos — Claude Fable 5 accepts image input natively, the other doesn't.
Your tasks involve multi-step planning or math-heavy reasoning — Claude Fable 5 ships a native reasoning mode that explicitly thinks before responding, the other doesn't.
You re-send the same large system prompt across requests — Claude Fable 5 supports prompt caching, cutting input cost on repeat hits.
On arena-elo, Claude Fable 5 scores 50.0 points higher — if your workload pattern matches that benchmark's task shape, the gap is meaningful.
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 Fable 5, switching to Grok 4 means re-architecting that path (and vice versa).
- • Vision input
- • PDF input
- • Prompt caching
- • Native reasoning mode
Capabilities both share (3)
- ✓ Function calling
- ✓ Streaming
- ✓ Structured output (JSON schema)
Benchmark winners — by the numbers
For each public benchmark that has scores for both models, the higher score and the size of the gap. Benchmarks are noisy — treat anything under a 2-point delta as effectively tied.
| Benchmark | Claude Fable 5 | Grok 4 | Winner | Δ |
|---|---|---|---|---|
| arena-elo | 1509.0 | 1459.0 | Claude Fable 5 | +50.0 |
Migration considerations
Concrete differences to wire through your stack before you flip traffic from one to the other.
- Context window changes down 87% when moving from Claude Fable 5 (1,000,000) to Grok 4 (131,072). Re-check any prompt that relies on cramming long history or documents.
- Max output tokens differ: 128,000 on Claude Fable 5 vs 131,072 on Grok 4. Long-form generation tasks may truncate differently — adjust streaming UI and chunking accordingly.
- Claude Fable 5 has capabilities Grok 4 lacks: Vision input, PDF input, Prompt caching, Native reasoning mode. Switching to Grok 4 means re-architecting any flow that depends on these.
How to A/B test Claude Fable 5 vs Grok 4 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 Fable 5 primary, mirror 20% of traffic to Grok 4 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 Fable 5 vs Grok 4
Which is cheaper, Claude Fable 5 or Grok 4? ▾
Grok 4 is cheaper by roughly 70% on a blended input + output token mix. Input prices are $10.00/M for Claude Fable 5 versus $3.00/M for Grok 4; output prices are $50.00/M versus $15.00/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 Fable 5 versus Grok 4? ▾
Claude Fable 5 supports up to 1,000,000 tokens of context. Grok 4 supports up to 131,072 tokens. Claude Fable 5 has the larger window by a factor of 7.6x, which matters for long-document RAG, multi-turn agent sessions, and tasks that need to keep an entire codebase in working memory.
Do Claude Fable 5 and Grok 4 both support tool calling? ▾
Yes — both Claude Fable 5 and Grok 4 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.
Can Claude Fable 5 and Grok 4 process images? ▾
Claude Fable 5 accepts native image input. Grok 4 does not — you would need to route image-heavy workloads through Claude Fable 5 or add a separate vision model in front of Grok 4.
Which model supports prompt caching for cost reduction? ▾
Claude Fable 5 supports prompt caching; the other does not. If your agent has a stable system prompt + retrieval context block that repeats across requests, Claude Fable 5 gives you a 50–90% discount on those repeated input tokens at the provider level.
When should I choose Claude Fable 5 over Grok 4? ▾
Your workload needs long context — Claude Fable 5 fits 1,000,000 tokens versus the other model's 131,072, enough headroom for full books, large codebases, or 100+ page documents in one shot. Your inputs include screenshots, diagrams, or product photos — Claude Fable 5 accepts image input natively, the other doesn't. Your tasks involve multi-step planning or math-heavy reasoning — Claude Fable 5 ships a native reasoning mode that explicitly thinks before responding, the other doesn't. You re-send the same large system prompt across requests — Claude Fable 5 supports prompt caching, cutting input cost on repeat hits. On arena-elo, Claude Fable 5 scores 50.0 points higher — if your workload pattern matches that benchmark's task shape, the gap is meaningful.
When should I choose Grok 4 over Claude Fable 5? ▾
You're cost-sensitive at scale — Grok 4 runs ~70% cheaper on a blended in+out token mix, compounding into thousands of dollars per month at production volume.
How do I A/B test Claude Fable 5 against Grok 4 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.