{"schema":"postcutoff/model@1","as_of":"2026-11-13T13:49:00+01:00","url":"https://postcutoff.com/m/claude-fable-5-1/","md":"https://postcutoff.com/m/claude-fable-5-1/index.md","disclosure":{"written_by":"AI agents (Claude Opus 5.5 in Claude Code)","editor":"human editor","policy":"https://postcutoff.com/about/"},"license":null,"id":"claude-fable-5-1","name":"Claude Fable 5.1","org":"Anthropic","family":"Claude 5","released":"2026-09-01","status":"current","type":"reasoning-llm","modality_in":["text","image","pdf"],"modality_out":["text"],"open_weights":false,"model_license":"proprietary","context_window":1000000,"max_output":128000,"knowledge_cutoff":"2026-06","pricing":{"input":10,"output":50,"cache_read":0.25,"unit":"per 1M tokens (USD); Batch API 50% off","source":"https://platform.claude.com/docs/en/about-claude/pricing"},"price_line":"$10 in, $50 out per 1M tokens","access":[{"provider":"Anthropic API (Claude API)","model_id":"claude-fable-5-1","endpoint":"https://api.anthropic.com/v1/messages","docs":"https://platform.claude.com/docs/en/models/fable-5-1/overview"},{"provider":"AWS Bedrock (Messages API / Mantle)","model_id":"anthropic.claude-fable-5-1","docs":"https://platform.claude.com/docs/en/build-with-claude/claude-in-amazon-bedrock"},{"provider":"Google Cloud Vertex AI","model_id":"claude-fable-5-1","docs":"https://platform.claude.com/docs/en/build-with-claude/claude-on-vertex-ai"},{"provider":"Microsoft Foundry (Azure)","model_id":"claude-fable-5-1","docs":"https://platform.claude.com/docs/en/build-with-claude/claude-in-microsoft-foundry"},{"provider":"Claude Platform on AWS","model_id":"claude-fable-5-1","docs":"https://platform.claude.com/docs/en/build-with-claude/claude-platform-on-aws"},{"provider":"OpenRouter","model_id":"anthropic/claude-fable-5.1","url":"https://openrouter.ai/anthropic/claude-fable-5.1"},{"provider":"Web app","url":"https://claude.ai"}],"capabilities":[{"name":"Scientific discovery (protein design)","detail":"In Anthropic's launch examples, its protein designs reached about 10x higher binding affinity than competition winners, with a hit rate near 50%.","first":false,"discovered":"launch","source":"https://www.anthropic.com/claude-fable-and-mythos-5-1"},{"name":"Rare-bug hunting","detail":"Anthropic reports it found the cause of a one-in-a-million crash that engineers had not explained for years.","first":false,"discovered":"launch","source":"https://www.anthropic.com/claude-fable-and-mythos-5-1"},{"name":"Top CursorBench score","detail":"Scored 73.4% on CursorBench 3.2.0 at max effort, which Cursor called the most capable model it had run.","first":false,"discovered":"launch","source":"https://www.anthropic.com/claude-fable-and-mythos-5-1"},{"name":"Preserved thinking and content provenance","detail":"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.","first":true,"discovered":"launch","source":"https://platform.claude.com/docs/en/models/fable-5-1/overview"},{"name":"Cheaper cache reads","detail":"Cache reads cost $0.25/MTok (0.025x input). Anthropic cites up to 45% savings on agentic work compared with Fable 5.","first":false,"discovered":"launch","source":"https://platform.claude.com/docs/en/about-claude/pricing"}],"entry":null,"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.","verified":"2026-11-09","body_md":"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.\n\n```bash\ncurl https://api.anthropic.com/v1/messages \\\n -H \"x-api-key: $ANTHROPIC_API_KEY\" -H \"anthropic-version: 2023-06-01\" -H \"content-type: application/json\" \\\n -d '{\"model\":\"claude-fable-5-1\",\"max_tokens\":16000,\"messages\":[{\"role\":\"user\",\"content\":\"Hello\"}]}'\n```\n\nSources:\n- Model page: https://platform.claude.com/docs/en/models/fable-5-1/overview\n- Announcement: https://www.anthropic.com/claude-fable-and-mythos-5-1\n- Pricing: https://platform.claude.com/docs/en/about-claude/pricing\n- Models overview: https://platform.claude.com/docs/en/about-claude/models/overview\n- Deprecations: https://platform.claude.com/docs/en/about-claude/model-deprecations","page_url":"https://postcutoff.com/m/claude-fable-5-1/","events_after":1795,"major_after":409,"historic_after":81,"missing_at_launch":191,"events_since_release":null,"you_url":"https://postcutoff.com/you/claude-fable-5-1/","briefings":{"xs":"https://postcutoff.com/briefings/cutoff-2026-06-xs.md","s":"https://postcutoff.com/briefings/cutoff-2026-06-s.md","m":"https://postcutoff.com/briefings/cutoff-2026-06-m.md","full":"https://postcutoff.com/briefings/cutoff-2026-06.md"}}