What Is This?

This repository contains the complete system prompt (~1,580 lines, ~120KB) for Claude Fable 5, Anthropic's most capable generally-available AI model as of June 2026. The prompt was extracted from a live claude.ai session and reveals the inner architecture, behavioral guidelines, tool definitions, and safety frameworks that govern how Claude Fable 5 thinks, responds, and interacts.

This is the most detailed look yet into how Anthropic engineers its flagship model's behavior.

Key Revelations

Mythos-Class: A New Model Tier Above Opus

Claude Fable 5 belongs to an entirely new "Mythos-class" model tier that sits above Claude Opus in capability. The prompt confirms:

"This iteration of Claude is Claude Fable 5, the first model in Anthropic's new Claude 5 family and part of a new Mythos-class model tier that sits above Claude Opus in capability."

• Claude Fable 5 and Claude Mythos 5 share the same underlying model
• Claude Mythos 5 is available without safety measures to approved organizations only
• Model strings: claude-fable-5, claude-opus-4-8, claude-sonnet-4-6, claude-haiku-4-5-20251001

The Full Claude Product Ecosystem

The prompt reveals a surprisingly expansive product lineup:

Claude Cowork can use all of the above as tools — making it a meta-agent that orchestrates other Claude agents.

"Claudeception" — API Inside Artifacts

Claude can make Anthropic API calls from within Artifacts, creating AI-powered interactive applications. The prompt uses Sonnet 4 as the internal model:

model: "claude-sonnet-4-20250514" // Always use Sonnet 4

This means Claude can build applications that call Claude — a recursive AI architecture.

22+ Tools in the Arsenal

The prompt defines an extensive toolset including: web_search, web_fetch, bash_tool, create_file, str_replace, image_search, places_search, places_map_display_v0, recipe_display_v0, ask_user_input_v0, message_compose_v1, suggest_connectors, search_mcp_registry, recommend_claude_apps, fetch_sports_data, weather_fetch, present_files, and more.

Aggressive Copyright Compliance

The prompt contains an extremely detailed copyright enforcement system with hard limits:

• 15+ words from any single source = SEVERE VIOLATION
• ONE quote per source MAXIMUM — after one quote, that source is "CLOSED"
• Default to paraphrasing — quotes should be "rare exceptions"
• Never reproduce song lyrics, poems, or haikus in any form

Self-Harm & Mental Health: Unprecedented Depth

The prompt includes remarkably nuanced mental health guidelines:

• No substitution techniques that mimic self-harm (ice cubes, rubber bands, sour candy)
• No causal narratives for disordered eating the user hasn't made themselves
• Acknowledge bad experiences with crisis services without totalizing the system
• Never foster over-reliance on Claude or encourage continued engagement

Knowledge Cutoff: End of January 2026

Claude's reliable knowledge cutoff is confirmed as end of January 2026, with the current date set to June 9, 2026.

Repository Structure

claude-fable-5-system-prompt/
├── README.md                              ← You are here
├── LICENSE                                ← MIT License
├── system-prompt/
│   ├── full-system-prompt.md              ← Complete formatted system prompt
│   ├── tool-definitions.md                ← All 22+ tool schemas extracted
│   └── analysis/
│       ├── key-findings.md                ← Top discoveries and insights
│       ├── model-architecture.md          ← What we learn about the model stack
│       ├── behavioral-guidelines.md       ← Deep dive into behavior engineering
│       └── fable5-vs-opus-comparison.md   ← Mythos-class vs Opus-class comparison
├── highlights/
│   └── surprising-revelations.md          ← Most shocking/viral findings
├── scripts/
│   └── verify_signatures.py               ← Curation integrity validator
└── assets/
    └── fable5-hero.png                    ← Banner image asset

Table of Contents — System Prompt

How to Use This Repo

Read the full prompt: Start with system-prompt/full-system-prompt.md for the complete, formatted system prompt with section anchors.

Quick highlights: Jump to highlights/surprising-revelations.md for the most interesting findings.

Deep analysis: The system-prompt/analysis/ directory contains breakdowns of the model architecture, behavioral engineering, and a detailed Fable 5 vs Opus 4.8 Comparison.

Verify Integrity: Run the signature checker script to verify that your clone's content and curation attributions are original:

python scripts/verify_signatures.py

Why This Matters

System prompts are the invisible architecture behind AI behavior. This prompt reveals:

1. How Anthropic engineers personality — from warm tone requirements to anti-bullet-point philosophy
2. The Mythos tier system — a previously unknown model hierarchy above Opus
3. Recursive AI architecture — Claude calling Claude inside Artifacts
4. Unprecedented safety nuance — mental health guidelines more detailed than most clinical protocols
5. The agentic future — Cowork as a meta-agent orchestrating Claude Code, Chrome, Excel, and PowerPoint

This is a primary source document for understanding how frontier AI models are shaped.

Fun Quirks & Valuable Learnings

Beyond the architecture, the prompt reveals some hilarious and fascinating rules that govern Claude's personality:

• "Claudeception": Fable 5 is explicitly allowed to use the Anthropic API (via Sonnet 4) from within the interactive artifacts it generates.
• The Anti-Bullet-Point Crusade: The prompt aggressively bans Claude from using bullet points or bold text in standard prose, noting it wants to avoid looking like stereotypical "AI slop." It even notes: "Claude never uses bullet points when declining a task; the additional care helps soften the blow."
• No Sycophantic Gratitude: Claude is explicitly told to never thank the user just for reaching out, and to never beg the user to keep talking.
• Military-Grade Copyright: More than 15 words quoted from a single source is defined as a "SEVERE VIOLATION". The prompt includes a 5-step checklist Claude must run before quoting text.
• Clinical Nuance: It forbids suggesting "rubber bands or ice cubes" as self-harm substitutes, noting (like a graduate psychology textbook) that sensory substitutions reinforce neural pathways rather than breaking them.

For the full list of deep dives, see highlights/surprising-revelations.md.

Community & Contributing

We want to make this repository the premier research hub for Anthropic's Mythos-class models. Found a novel jailbreak, a behavioral quirk, or want to add an interactive tool?

Please check our Contributing Guidelines and open an Issue using our templates, or submit a PR with your analysis. We actively welcome community discussions!

Contributors

Disclaimer

This repository is for educational and research purposes. The system prompt was extracted from a publicly accessible claude.ai session. All rights to the original content belong to Anthropic. This repo does not claim ownership of Anthropic's intellectual property.

License

MIT — See LICENSE for details.