Claude Code Fable 5.2 system prompt leaked: 950K characters, ~240K tokens of overhead
zainhas · x · 2026-11-10
Security researcher elderplinius (Pliny the Liberator) has published the full system instructions and tool definitions for Fable 5.2 as used in Claude Code — 950,000 characters across 7,832 lines (935KB), now hosted in his GitHub repo CL4R1T4S, with the Claude web app prompt promised next. The excerpt opens with "You are Claude Code, Anthropic's official CLI..." and includes behavior and product-information sections.
Reblogger zainhas did the math: the full instructions plus tools come to roughly 240,000 tokens of context in Claude Code — a sizable hidden per-session cost, sparking discussion about prompt size and pricing.
More from coding & agent
- YC founder pivots to agent sandboxes, sees revenue run rate grow ~15X — ycombinator · 2026-11-10
- AI Engineer Code Summit returns to SF Nov 10-12 with Databricks PR-repair workshop — matei_zaharia · 2026-11-10
- Theo shows t3code running 22 parallel AI coding threads with an empty inbox — _AustinCalvert_ · 2026-11-10
- Stanford launches CS329Z on building AI agents from scratch, first lecture video out — stanfordnlp · 2026-11-10
- herdr ships mobile apps for iPhone, iPad and Android to manage waiting agents — johnlindquist · 2026-11-10
- Dev builds disposable numbers for AI agents to talk via A2A protocol — RightTechSoftware · 2026-11-10