Fableit teaches your AI coding assistant to work like a careful senior engineer. It is a small ruleset you install once. After that, the model checks facts before stating them, reads the code before changing it, fixes root causes instead of symptoms, finishes the whole task instead of narrating the next step, verifies its own work before reporting, and tells you honestly what it did and did not do.

It packages the engineering process of Claude Fable 5.1 (released September 1, 2026) so that any model in any agent tool can run it: Claude Code, OpenCode, Cursor, Codex CLI, Gemini CLI, aider, and anything else that reads an instructions file. Fableit is for every model, with or without Fable access.

v1.1.0 (2026-09-02): distilled from Fable 5.1. The ruleset now carries the behaviors Anthropic documented for 5.1 (see below), costs the same or fewer tokens per rule, and the [FABLEIT] statusline chip works on Windows and sits next to any statusline you already have. Run npx @seedexr/fableit@latest to upgrade an existing install. (Older notice: v1.0.0 blocked prompts on Windows; v1.0.1 fixed it, details in PR #1.)

Anthropic's Fable 5.1 launch material (the announcement, the system card, and the "Prompting Claude Fable 5.1" guide) says the same thing the Fable 5 analysis said: the lead over older models is not raw intelligence on short tasks. It is learnable behavior, and it shows up on long, complex work:

• Goal persistence: 5.1 "keeps its own records, reprioritizes as things change, and picks up where it left off" (Shopify). Re-anchor on the goal after every error or detour instead of drifting.
• Killing incorrect beliefs: treat your theory as a hypothesis and drop it the moment evidence contradicts it. That is how 5.1 "correctly identified the root cause of every broken build" Red Hat tested.
• Self-verification as a reflex: 5.1 "can write its own tests to check its work", and the system card found it falsely claims a task is done less often than previous models. Test your own work before presenting it.
• Keep looking before asking: exhaust what you can find yourself before interrupting the human.
• Finish the whole task: a step you have decided on is something to run, not to announce. Never end a turn on "Next, I'll…".
• Effort and tokens as a resource: 5.1 at medium effort roughly matches Fable 5 at lower cost. Think hard on the hard parts, move fast through the mechanical parts, batch tool calls, edit surgically, draft once.
• Abstain when you do not know: the one measured regression in 5.1 is that it abstains less, so it gets more answers right and more answers wrong. Fableit's grounding protocol is the counterweight: familiarity is not knowledge, and "I don't know yet, checking" is the rule.
• Memory discipline: with persistent memory, Fable improved 3x more than Opus.

Fableit injects those behaviors as an always-on ruleset: ground → comprehend → reason → design → execute → verify → report. In plain terms: check the facts, read the code, think, plan, do the work, test it, then report honestly.

Two files matter, and they say the same thing at different sizes:

• SKILL.md is the full blueprint (about 4,000 words, roughly 5,400 tokens), written for the model to read on demand when it needs the complete process, with the published sources cited at the end.
• hooks/fableit-instructions.js holds the condensed ruleset that gets injected into every session. One source of truth for every host.

Once installed, the ruleset is added to the model's instructions automatically at the start of every session. You never have to remember to turn it on. A [FABLEIT] chip in the statusline shows it is active.

Fableit is published on npm as @seedexr/fableit, owned by the SeedeXR organization. (It was previously also published under the short name fableit; that name is now deprecated and frozen. Use @seedexr/fableit. Existing fableit installs keep working.)

Recommended: the installer wires everything (hooks, skill, statusline chip).

npx @seedexr/fableit

# or straight from GitHub:
npx github:SeedeXR/fableit

What the installer does, step by step:

• Adds three hooks to settings.json in your Claude config dir:
  • SessionStart injects the ruleset when a session begins,
  • SubagentStart makes sub-agents follow the same rules,
  • UserPromptSubmit tracks level switches like /fableit ultra.
• Installs the fableit skill and copies the package to ~/.claude/fableit.
• Adds the [FABLEIT] / [FABLEIT:LITE] / [FABLEIT:ULTRA] statusline chip. It runs on Node, so it works on Windows, macOS, and Linux. If you already have a statusline (your own script, another plugin's chip such as [PONYTAIL]), fableit keeps it: the chip script runs your existing command first and appends [FABLEIT] on the same line. Uninstalling hands your original statusline back.

It refuses to touch a settings.json it cannot parse. Restart Claude Code or /clear to activate.

Alternative: install through the plugin marketplace.

/plugin marketplace add SeedeXR/fableit
/plugin install fableit@fableit

Skill only (no hooks, load on demand with /fableit):

git clone https://github.com/SeedeXR/fableit ~/.claude/skills/fableit

To remove everything the installer added (hooks, skill, flag, chip, OpenCode entry, installed copy), while leaving your other settings untouched:

npx @seedexr/fableit uninstall

npx @seedexr/fableit opencode

This wires the plugin into ~/.config/opencode/opencode.json, pointing at the stable ~/.claude/fableit copy. Or: npm i -g @seedexr/fableit and add "plugin": ["@seedexr/fableit"] to your opencode.json. The plugin appends the ruleset to every turn's system prompt and registers the /fableit command.

Any tool that reads an instructions file can run fableit: append the ruleset to whatever file your tool loads.

npx @seedexr/fableit print >> AGENTS.md      # Codex, aider, many others
npx @seedexr/fableit print >> .cursorrules   # Cursor
npx @seedexr/fableit print >> GEMINI.md      # Gemini CLI
npx @seedexr/fableit print lite              # smaller variant, to stdout

Fableit ships three intensity levels. Each level includes everything from the one before it and adds more.

Token counts are estimates (words × 1.35, a fair proxy for current Claude tokenizers) and are guarded by a test so the ruleset cannot quietly grow. The 1.0.x ruleset was 721 / 1,017 / 1,127 words with roughly a third fewer rules, so the per-rule cost went down. On downgrade (/fableit lite from full), the hook sends a one-line directive instead of re-injecting the ruleset.

Which one to pick: lite is the core discipline and the cheapest in tokens. full, the default, adds the behaviors that matter on long tasks. ultra adds a strict "no evidence, no claim" gate, built for models that tend to guess confidently.

Switch anytime with /fableit lite|full|ultra|off (both Claude Code and OpenCode). Turn it off with stop fableit or normal mode. All states, including off, persist until changed. Set the default level with the FABLEIT_DEFAULT_MODE env var or ~/.config/fableit/config.json ({"defaultMode": "ultra"}).

The behaviors above are process, not parameters. A model that re-anchors on the goal after every error, refuses to state unobserved facts, finishes the task instead of announcing it, spends tokens only where they change the result, and runs a verification gate before reporting closes most of the gap this ruleset was distilled from. The ultra gate exists precisely for models that hallucinate confidently: no evidence, no claim.

• Introducing Claude Fable 5.1 and Claude Mythos 5.1 (Anthropic, Sept 1, 2026)
• Claude Fable 5.1 & Claude Mythos 5.1 System Card (Anthropic, Sept 1, 2026)
• Prompting Claude Fable 5.1 and What's new in Claude Fable 5.1 (Anthropic docs)
• Claude Fable 5.1 tops the Artificial Analysis Intelligence Index
• Claude Fable 5 and Claude Mythos 5 (the analysis the first edition distilled)

npm test        # node --test (unit, token budget, installer + chip, hook bash-validity)

PRs welcome. Keep the spirit: it is a decision process, not a style guide. Every addition should be a behavior a model can actually execute, grounded in something observed or published, not vibes. The token-budget test will fail if the injected ruleset grows; raise the ceiling only with a reason.