I had Fable 5 write down its own behavioural rules (how it structures answers, when it ends a turn, when it just does the work instead of asking permission) and packaged them into a Claude Code plugin that runs Opus 4.8 under that doctrine. Yes, Fable helped me build its own impersonator. The doctrine itself is output-styles/fable.md in the repo. There’s three layers: that output style in the system prompt; seven harness hooks that catch drift ( hooks/stop-gate.sh blocks the classic “I’ll now go do X” turn ending and makes the model do X… there’s also one that denies cat file in favour of the Read tool, and one that makes it report failing tests verbatim instead of “mostly working”); and an eval loop of 12 probes ( evals/probes/ ), an 8-dimension rubric ( evals/rubric.md ), judged pairwise against golden transcripts generated by actual Fable 5 ( evals/golden/ ). Numbers, because vibes don’t count: after two tuning iterations, Opus-with-plugin leads or ties vanilla Opus on 6 of 8 dimensions. Both eval reports are committed, you can check docs/2026-07-02-baseline-report.md (iteration 1, where it lost on two dimensions) and docs/2026-07-03-iteration-2-report.md (after evidence-driven fixes). It still loses a couple; delegation is weights, not prompting. The first iteration made “don’t bury findings” worse because Opus learned to append a contentless “everything verified, nothing pending” closer to satisfy the stop hook (0.1.2 entry). And one “golden Fable transcript” turned out to be Opus: the serving pool silently rerouted one prompt; caught by asserting per-model cost attribution, regenerated on the pinned pool (0.1.1 entry). Also: three of Fable’s own goldens tripped my “don’t end with ‘want me to…?’” pattern. Fable legitimately ends some turns with decision questions, so the gate only blocks continuation offers now (the calibration tests are in tests/run.sh ). It can’t make Opus reason deeper . It makes Opus behave better and measures the gap instead . Try it out: /plugin marketplace add rennf93/opus-fable-playbook then /plugin install fable-mode@opus-fable-playbook Or one session, no install: claude --plugin-dir <clone> --settings <clone>/profiles/opus-fable.settings.json . Full guide: docs/guide.md . Repo here. submitted by /u/PA100T0
Originally posted by u/PA100T0 on r/ClaudeCode