As of: 2026-11-13 13:49 CET.
Researched and written by AI agents (Claude Opus 5.5 in Claude Code), with a human editor. How we work: https://postcutoff.com/about/
Canonical page: https://postcutoff.com/m/claude-fable-5-1/

# Claude Fable 5.1

Anthropic, Claude 5 family. Status: current. Type: reasoning model. Released: 2026-09-01. Training cutoff: 2026-06.

## How to call it

| Provider | Model id | Endpoint | Docs |
|---|---|---|---|
| Anthropic API (Claude API) | `claude-fable-5-1` | https://api.anthropic.com/v1/messages | https://platform.claude.com/docs/en/models/fable-5-1/overview |
| AWS Bedrock (Messages API / Mantle) | `anthropic.claude-fable-5-1` |  | https://platform.claude.com/docs/en/build-with-claude/claude-in-amazon-bedrock |
| Google Cloud Vertex AI | `claude-fable-5-1` |  | https://platform.claude.com/docs/en/build-with-claude/claude-on-vertex-ai |
| Microsoft Foundry (Azure) | `claude-fable-5-1` |  | https://platform.claude.com/docs/en/build-with-claude/claude-in-microsoft-foundry |
| Claude Platform on AWS | `claude-fable-5-1` |  | https://platform.claude.com/docs/en/build-with-claude/claude-platform-on-aws |
| OpenRouter | `anthropic/claude-fable-5.1` |  | https://openrouter.ai/anthropic/claude-fable-5.1 |
| Web app |  |  | https://claude.ai |

## Pricing

- Input: $10 per 1M tokens
- Output: $50 per 1M tokens
- Cache read: $0.25 per 1M tokens
- Unit as published: per 1M tokens (USD); Batch API 50% off
- Source: https://platform.claude.com/docs/en/about-claude/pricing
- Last checked: 2026-11-09

## Specs

- Input: text, image, pdf
- Output: text
- Context window: 1,000,000 tokens
- Max output: 128,000 tokens
- Open weights: no
- Licence: proprietary

## What it has not seen

- Published training cutoff: 2026-06 (June 2026).
- AI events after it on Post-Cutoff: 1,795 (409 major, 81 historic), as of 2026-11-13.
- Already missing at launch: 191.
- Briefings of the news since June 2026: https://postcutoff.com/briefings/cutoff-2026-06-s.md (short), https://postcutoff.com/briefings/cutoff-2026-06-m.md (medium), https://postcutoff.com/briefings/cutoff-2026-06.md (full).
- Its card, with five things it has never heard of: https://postcutoff.com/you/claude-fable-5-1/
- In our own test, asked without tools, Claude Fable 5.1 placed its knowledge far before its published June 2026 cutoff (case 021). https://postcutoff.com/why/021-claude-5x-effective-cutoff-far-before-published/

## What stands out

- Scientific discovery (protein design): In Anthropic's launch examples, its protein designs reached about 10x higher binding affinity than competition winners, with a hit rate near 50%. Source: https://www.anthropic.com/claude-fable-and-mythos-5-1
- Rare-bug hunting: Anthropic reports it found the cause of a one-in-a-million crash that engineers had not explained for years. Source: https://www.anthropic.com/claude-fable-and-mythos-5-1
- Top CursorBench score: Scored 73.4% on CursorBench 3.2.0 at max effort, which Cursor called the most capable model it had run. Source: https://www.anthropic.com/claude-fable-and-mythos-5-1
- Preserved thinking and content provenance (first): Thinking blocks are bound to the model and the conversation, and editing earlier turns invalidates them. Also adds per-message effort, turn-scoped system messages and content provenance. Source: https://platform.claude.com/docs/en/models/fable-5-1/overview
- Cheaper cache reads: Cache reads cost $0.25/MTok (0.025x input). Anthropic cites up to 45% savings on agentic work compared with Fable 5. Source: https://platform.claude.com/docs/en/about-claude/pricing

## Notes

Superseded on 2026-11-09 by Claude Fable 5.2 ($8/$40, same API family; see claude-fable-5-2). Anthropic's most capable widely released model; thinking always on (adaptive, effort low..max, default high); forced tool_choice any/tool returns 400; no prefill; 30-day data retention required (no ZDR unless authorized); no Priority Tier. Batch $5/$25.

Best for the hardest reasoning and long-horizon agentic work (coding, research, documents/spreadsheets/slides, computer use). Successor to Claude Fable 5 at the same price with cheaper cache reads. Same model is offered as Claude Mythos 5.1 to Project Glasswing participants. Handle `stop_reason: "refusal"` (safety classifiers) and consider the server-side `fallbacks` parameter.

```bash
curl https://api.anthropic.com/v1/messages \
 -H "x-api-key: $ANTHROPIC_API_KEY" -H "anthropic-version: 2023-06-01" -H "content-type: application/json" \
 -d '{"model":"claude-fable-5-1","max_tokens":16000,"messages":[{"role":"user","content":"Hello"}]}'
```

Sources:
- Model page: https://platform.claude.com/docs/en/models/fable-5-1/overview
- Announcement: https://www.anthropic.com/claude-fable-and-mythos-5-1
- Pricing: https://platform.claude.com/docs/en/about-claude/pricing
- Models overview: https://platform.claude.com/docs/en/about-claude/models/overview
- Deprecations: https://platform.claude.com/docs/en/about-claude/model-deprecations

Model page: https://postcutoff.com/m/claude-fable-5-1/