Neither wins outright: Anthropic: Claude Fable 5.1 is the pick when you need file / PDF input, xAI: Grok 4 Fast when cost matters at scale.
| Spec | Anthropic: Claude Fable 5.1 | xAI: Grok 4 Fast |
|---|---|---|
| Provider | Anthropic | X Ai |
| Input price (per 1M tokens) | $10.00 | $0.20 |
| Output price (per 1M tokens) | $50.00 | $0.50 |
| Blended price (3:1 input/output) | $20.00 | $0.28 |
| Cheaper than … of all models we track | 3% | 64% |
| Context window | 1M tokens (~1,500 pages) | 2M tokens (~3,000 pages) |
| Larger context than … of all models | 69% | 99% |
| Max output per response | 128K tokens (~96,000 words) | 30K tokens (~22,500 words) |
| Input types | text, image, file | text, image |
| Output types | text | text |
| Free variant | No | No |
| Listed since | 2026-09-01 | 2025-09-19 |
Monthly cost at list price for five common workloads. Token counts per unit are stated so you can scale them to your own traffic. A workload that does not fit a model's context window is marked rather than priced.
| Workload (per month) | Anthropic: Claude Fable 5.1 | xAI: Grok 4 Fast | Difference |
|---|---|---|---|
| Customer-support chatbot 50,000 conversations — each a 6-turn conversation (1,500 in / 400 out tokens) | $1,750 | $25.00 | xAI: Grok 4 Fast saves $1,725 |
| RAG search over documents 20,000 questions — each a question answered from ~8 retrieved passages (6,000 in / 500 out tokens) | $1,700 | $29.00 | xAI: Grok 4 Fast saves $1,671 |
| Coding / tool-using agent 2,000 tasks — each a multi-step task with tool calls and file context (60,000 in / 6,000 out tokens) | $1,800 | $30.00 | xAI: Grok 4 Fast saves $1,770 |
| Batch summarisation 10,000 documents — each a 6,000-word report summarised to a page (8,000 in / 600 out tokens) | $1,100 | $19.00 | xAI: Grok 4 Fast saves $1,081 |
| Long-document analysis 500 documents — each a 150-page contract or codebase read in one pass (100,000 in / 2,000 out tokens) | $550 | $10.50 | xAI: Grok 4 Fast saves $540 |
| Capability | Anthropic: Claude Fable 5.1 | xAI: Grok 4 Fast | What it lets you do |
|---|---|---|---|
| Tool / function calling | ✓ Yes | ✓ Yes | call your APIs and run agent loops |
| Structured output (JSON schema) | ✓ Yes | ✓ Yes | return JSON that validates against your schema |
| Reasoning / thinking mode | ✓ Yes | ✓ Yes | spend extra tokens thinking before answering hard problems |
| Image input | ✓ Yes | ✓ Yes | read screenshots, charts and scanned pages |
| Audio input | — | — | take speech or audio directly |
| File / PDF input | ✓ Yes | — | accept documents without your own parsing step |
| Image output | — | — | generate images, not just text |
| Built-in web search | ✓ Yes | ✓ Yes | answer from live web results |
| Seed (reproducible sampling) | — | ✓ Yes | repeat a generation for tests and evals |
| Log-probabilities | — | ✓ Yes | read token confidence for classification and scoring |
From each model's published parameters and input/output types. “—” means not advertised, not proven absent.
Neither wins outright: Anthropic: Claude Fable 5.1 is the pick when you need file / PDF input, xAI: Grok 4 Fast when cost matters at scale. In short: Anthropic: Claude Fable 5.1 when you need file / PDF input, you generate long outputs in one go, you want the newer model; xAI: Grok 4 Fast when cost matters at scale, you need to fit long inputs in one request, you need seed (reproducible sampling), you need log-probabilities.
xAI: Grok 4 Fast is cheaper: $0.28 vs $20.00 per 1M tokens at a typical 3:1 input/output mix, 99% less. For 50,000 support conversations a month that is $25.00 against $1,750.
Anthropic: Claude Fable 5.1 costs $10.00 per 1M input tokens and $50.00 per 1M output tokens; xAI: Grok 4 Fast costs $0.20 input and $0.50 output. Output tokens are billed separately, so long answers shift the gap.
xAI: Grok 4 Fast: 2M tokens, about 3,000 pages, against 1M (about 1,500 pages) for Anthropic: Claude Fable 5.1.
Tool calling — Anthropic: Claude Fable 5.1: yes, xAI: Grok 4 Fast: yes. JSON-schema structured output — Anthropic: Claude Fable 5.1: yes, xAI: Grok 4 Fast: yes.
Yes, both accept image input.
No free variant is listed for either; both are pay-per-token.
Anthropic: Claude Fable 5.1 can write up to 128K tokens (about 96,000 words) per response; xAI: Grok 4 Fast up to 30K (about 22,500 words).
Anthropic: Claude Fable 5.1 was listed on 2026-09-01; xAI: Grok 4 Fast on 2025-09-19.
Claude Fable 5.1 improves on Claude Fable 5 across the board, with the biggest gains in agentic coding, long-running agentic workflows, and knowledge work: long code refactors, front-end and visual...
Full Anthropic: Claude Fable 5.1 specs →Grok 4 Fast is xAI's latest multimodal model with SOTA cost-efficiency and a 2M token context window. It comes in two flavors: non-reasoning and reasoning. Read more about the model on xAI's [news post](http://x.ai/news/grok-4-fast). Reasoning can be enabled/disabled using the `reasoning` `enabled` parameter in the API. [Learn more in our docs](https://openrouter.ai/docs/use-cases/reasoning-tokens#controlling-reasoning-tokens)
Full xAI: Grok 4 Fast specs →