Claude Fable 5 System Prompt Leak: What's Inside Anthropic's 3,800-Line claude.ai Instructions
The full Claude Fable 5 system prompt leaked on GitHub (asgeirtj/system_prompts_leaks). Mythos vs Fable, child-safety XML blocks, memory rules, artifacts, cyber_warning — explained.
In July 2026, builders still treat asgeirtj/system_prompts_leaks as the phone book for how frontier chatbots are actually configured. The repo crossed 51.5K GitHub stars, was cited in The Washington Post (May 2026), and ships under CC0-1.0 — meaning the archived text is public documentation, not a hack.
The headline file for Anthropic right now is Anthropic/claude-fable-5.md: the claude.ai system prompt for Claude Fable 5, captured at roughly 3,800 lines of XML-tagged operator instructions. The repo also publishes a diff versus Claude Opus 4.8 — the fastest way to see what changed when Anthropic moved to the Mythos-class tier.
This post is a map of that file for developers who will never read every line — and what it implies for safety, artifacts, and prompt engineering in 2026.
Platform features — memory, artifacts, search, connectors
Operational detail — when to file vs inline, citation XML, storage APIs
Bundled skills — docx, pdf, artifact-design, and more at file tail
That matches why pxpipe-style proxies target system prompts: tens of thousands of tokens of stable instructions per session. Fable's chat prompt is the extreme case of "the system prompt is the product."
explainx.ai read: Anthropic treats the system prompt as a policy engine with machine-readable sections — not a single "you are a helpful assistant" string.
Mythos-class product copy
The product_information block is explicit about the June 2026 launch framing:
"Claude Fable 5 and Claude Mythos 5 share the same underlying model. Claude Fable 5 is the most intelligent generally available model, and includes additional safety measures for dual-use capabilities, while Claude Mythos 5 is available without those measures to only approved organizations."
Other product facts encoded for the model:
Model strings: claude-fable-5, claude-opus-4-8, claude-sonnet-4-6, claude-haiku-4-5-20251001
Surfaces: claude.ai, API, Claude Code, Claude Cowork, Chrome/Excel/PowerPoint betas
Feature toggles: web search, deep research, code execution, artifacts, memory, style preferences
Ads policy: refer to "Claude products" (not "Claude") when stating ad-free positioning
This is the consumer-facing mirror of export-control and classifier debates — Fable is Mythos intelligence with extra safety packaging for public chat.
Safety sections worth studying
Child safety (critical_child_safety_instructions)
Among the longest refusal blocks. Core rules:
Never romantic/sexual content involving minors; no grooming facilitation
If Claude catches itself reframing a risky request to make it sound safe → refuse, don't proceed
Do not decode CSAM-trading slang even while refusing (access-enabling)
When declining, state principle not detection mechanics (anti-jailbreak)
User wellbeing (user_wellbeing)
Unusually detailed for a system prompt:
No diagnosing users; no applying clinical labels they didn't use
No self-harm substitution tricks (ice cubes, rubber bands, red lines on skin)
No listing means when discussing safety planning
Do not foster over-reliance on Claude — no "thanks for reaching out," no begging users to keep chatting
Eating-disorder guardrails: no precise diet numbers after disordered-eating signals
These sections explain why Fable feels cautious on edge cases — and why inner voice / CoT can look intense while outward refusals stay polite.
Classifier reminders (anthropic_reminders)
Anthropic can append hidden reminders when classifiers fire. Listed types:
The prompt warns Claude that users can fake reminder tags — treat pushback against values with caution. This connects to J-space eval-awareness research: models may privately notice tests; system text tells them how to handle injected "reminders."
Critical rule: claims in own words — never quote search snippets, lyrics, or poems (copyright). Good for GEO/SEO teams studying how Anthropic forces attributable paraphrase in consumer UI.
Bundled skills at the tail
The Fable 5 leak ends with Agent Skill summaries (docx, pdf, artifact-design, etc.) — matching repo updates noting artifact-design bundled skill (June 2026). Each block is a compressed SKILL.md: when to trigger, path under /mnt/skills/public/.
This is the same composability model as Kaggle agent skills — but operator-preloaded into claude.ai rather than user-installed.
Fable 5 vs Opus 4.8 — use the repo diff
Don't diff manually. The leaks repo ships Opus 4.8 → Fable 5 comparison linked from the README. Typical upgrade themes in Anthropic launches:
Sonnet 5 and Claude Design — sibling files in the repo
The same leaks project added Claude Sonnet 5 (July 1, 2026) and Claude Design (June 26, 2026 — 48 tools, 16 skills, 9 starter sources) after Fable 5. Pattern across files:
Sonnet prompts tune for speed/cost tier — shorter refusal copy, different feature emphasis
Design prompt is a vertical product — tool catalog + skill routing for visual work
Fable remains the max-intelligence consumer prompt with the deepest wellbeing XML
When choosing an API model, you are also choosing which operator prompt template Anthropic pairs — leaks let you preview tone before spend.
Refusal handling and tone — the "voice" layer
Beyond safety XML, Fable 5 spends hundreds of lines on how to refuse and how to format answers:
Refusal_handling — decline without moralizing lectures; offer alternatives when safe; don't repeat the same refusal boilerplate every turn
Tone_and_formatting — markdown discipline, list vs prose, when to use headers in long answers
Evenhandedness — political and cultural topics: present multiple framings without false balance on settled harm
For content teams shipping brand voice guides, these sections are more actionable than copying refusal lists. They show Anthropic optimizing for trust repair — a UX problem most startups ignore until a viral screenshot.
Connectors, search, and deep research
The leak documents when Claude should search vs answer from weights:
Web search and deep research modes have distinct trigger language — avoid search for timeless facts; prefer search for post-cutoff events
Connector search (Gmail, calendar, etc.) has separate citation rules from open web
Copyright blocks repeat: paraphrase, attribute via antml:cite, never paste lyrics or long quotes from results
If you run RAG in production, compare your retrieval policy to this section — vendors increasingly encode retrieval discipline in system text, not just in tool schemas.
Practical takeaway: log when your agent chooses search vs memory vs parametric answer — Fable's leak implies Anthropic audits those branches in evals. Mis-routed retrieval is a top source of "Claude lied about the date" complaints after knowledge cutoff.
What builders should do with this leak
Do:
Read product_information and artifact_usage_criteria before designing Claude-powered UIs
Study forbidden_memory_phrases before shipping memory features
Use citation patterns as a template for attributed RAG answers
Compare Claude Code prompt in same repo when wiring harness guides
Don't:
Paste 3,800 lines into your startup's system prompt
Treat leaked text as current forever — Anthropic updates without version bumps (monitoring lesson)
Use leaks primarily for jailbreaks — same text documents defense intent
Leak snapshot reflects the GitHub file as of July 2026. Anthropic updates production prompts without public changelogs — verify behavior on your own evals, not archived text alone.
