fableplan

Fableplan — plan with Fable 5, execute with Sonnet

Claude Code has a built-in opusplan model setting ("Opus in plan mode, Sonnet otherwise") but no Fable equivalent. This skill recreates it by combining opusplan with the ANTHROPIC_DEFAULT_OPUS_MODEL environment variable override, which redirects the "opus" alias to Fable 5.

The target file is the user's global settings: ~/.claude/settings.json.

Arguments

• (none) — enable fableplan with claude-fable-5
• 1m — enable fableplan with claude-fable-5[1m] (1M context window)
• off — revert to the previous configuration

Enabling (no argument, or 1m)

1. Read ~/.claude/settings.json. If it does not exist, create it with an empty object first.
2. Merge the following keys, preserving every other existing setting:
   • If a model key already exists and is different from "opusplan", save its current value under the key fableplanPreviousModel (top-level, custom key — extra keys are allowed in settings.json) so it can be restored later.
   • Set "model": "opusplan".
   • Under "env" (create the object if missing, preserve its other entries), set "ANTHROPIC_DEFAULT_OPUS_MODEL" to "claude-fable-5" (or "claude-fable-5[1m]" if the argument is 1m).
3. Validate that the resulting file is valid JSON.
4. Tell the user:
   • Fableplan is enabled: plan mode runs on Fable 5, execution runs on Sonnet.
   • They must restart their Claude Code session for the model change to take effect.
   • The UI will display "Opus Plan Mode" — that label is cosmetic; plan mode actually runs Fable 5.

Disabling (off)

1. Read ~/.claude/settings.json.
2. Remove "ANTHROPIC_DEFAULT_OPUS_MODEL" from "env". If "env" becomes empty, remove the "env" key entirely.
3. Restore the model:
   • If fableplanPreviousModel exists, set "model" to that value and remove the fableplanPreviousModel key.
   • Otherwise remove the "model" key (falls back to the account default) and tell the user they can pick a model with /model.
4. Validate JSON and tell the user to restart the session.

Important caveats (mention them when enabling for the first time)

• ANTHROPIC_DEFAULT_OPUS_MODEL is an undocumented override — a future CLI update could change or remove it.
• Fable 5 is not available on every plan/account. If requests fail after enabling, run /fableplan off to revert.
• Anything else that resolves the "opus" alias (e.g. fast mode) will also point to Fable while enabled.