Migrating CLAUDE.md files to Claude Fable 5's Goal-Reason-Boundary structure

Christopher Hoeben's guide on Dev.to describes a two-phase migration from Claude Opus 4.8 CLAUDE.md files and system prompts to Claude Fable 5. The first phase is automated: developers invoke `/claude-api migrate this project to claude-fable-5` inside Claude Code, which pauses to confirm scope (full working directory, a subdirectory, or a specific file list), then performs the model ID swap, applies breaking parameter changes, replaces prefills, applies effort calibration for the target model, and produces a manual-review checklist. The skill also detects client platforms — Amazon Bedrock, Vertex AI, Claude Platform on AWS, and Microsoft Foundry — and automatically adjusts model ID formats and feature changes for each.

The guide argues that prescriptive instructions tuned for Opus 4.8 degrade Fable 5 output, so developers should delete sequential scaffolding ("First do X, then Y"), forced step labels, and rigid output templates.

The second phase is manual rewriting. The guide argues that prescriptive instructions tuned for Opus 4.8 degrade Fable 5 output, so developers should delete sequential scaffolding ("First do X, then Y"), forced step labels, and rigid output templates. Every surviving constraint is mapped into one of four explicit blocks: Goal, Reason, Boundary, and Verification. A concrete before-and-after example shows a legacy CLAUDE.md fragment with seven imperative rules rewritten into a structured block where process-order prescriptions and cosmetic guardrails are removed entirely, and a single Verification step condenses former guardrails into one self-check the model runs before emitting output.

The guide also addresses how to convert negative constraints — common in Opus 4.8 files — into measurable Boundary values. Subjective prohibitions like "don't be verbose" become hard limits such as "Keep responses under 200 words"; latency hints become millisecond budgets; dependency rules become maximum counts; and stylistic negations like "don't use jargon" become objectively measurable standards such as a Flesch Reading Ease score threshold. The guide states that every subjective guardrail should resolve to a numeric threshold, a bounded enumeration, or an objectively measurable standard, enabling Fable 5 to self-verify against the Boundary. The source text is truncated before the guide's conclusion.

Key facts

1. 01The migration is initiated with the Claude Code skill command `/claude-api migrate this project to claude-fable-5`.
2. 02The skill detects client platforms — Amazon Bedrock, Vertex AI, Claude Platform on AWS, and Microsoft Foundry — and adjusts model ID formats automatically.
3. 03Prescriptive instructions tuned for Opus 4.8 are described as degrading Claude Fable 5 output.
4. 04The new prompt architecture uses four explicit blocks: Goal, Reason, Boundary, and Verification.
5. 05Sequential scaffolding such as 'First do X, then Y' is removed because Fable 5 determines its own execution order.
6. 06Negative constraints (e.g., 'don't be verbose') are replaced with measurable Boundary values like word counts, millisecond budgets, or coverage percentages.
7. 07Constraints that cannot be expressed as a scalar should be replaced with an explicit allowed set rather than a negated forbidden set.

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