What is opus-fable-playbook?

fable-mode is a Claude Code plugin that tries to make Claude Opus 4.8 behave more like Claude Fable 5 when used inside Claude Code. The project's own explanation is direct about its limits: Opus's reasoning depth comes from its underlying model weights, not from configuration, so this playbook cannot close that capability gap. What it can do is transplant Fable's behavior patterns and catch Opus when it drifts away from them, using rules enforced at the harness level. The doctrine that defines Fable's behavior was transcribed by Fable 5 itself, then turned into an output style plus seven hooks that check Opus's responses against that doctrine while it works. On top of this, the project includes an eval loop that measures how closely Opus's output matches golden transcripts recorded from real Fable 5 sessions, so changes to the doctrine can be tested against real evidence rather than guesswork. There are a few ways to use it. You can run a single session immediately by pointing Claude Code at the plugin directory and a settings file, without installing anything. For repeated use, you install it from a plugin marketplace and activate the Fable output style with a command, or merge the provided settings file into your own project or user settings so it persists automatically. A status command reports the current posture at any time. For SDK or headless use without hooks, you can append just the doctrine text to the system prompt, and if you cannot use a plugin at all, the doctrine can be pasted into a CLAUDE.md file instead. The README reports results from 24 test probes, each judged against golden Fable transcripts and averaged over multiple runs, with a companion study measuring how much the scores vary from run to run. It states that fable-mode produces real, measurable improvements in a few specific areas, such as staying focused on outcomes and finishing turns properly, alongside a couple of measured regressions, while several other dimensions show no meaningful difference from using Opus without the plugin. Full documentation covering installation, hooks, environment variables, and troubleshooting lives in the project's docs folder.