Freeze the current baseline
Save the current model ID, account, region, prompt, source packet, tools, output, total cost and reviewer corrections. Without that record, a faster or more polished answer can hide a material regression.
Document only supported differences
| Field | 5.1 record | Migration consequence |
|---|---|---|
| Model ID | claude-fable-5-1 | Pin it; do not assume a floating alias |
| Context/output | 1M input / 128K maximum output, per docs | Test completeness and truncation on the real packet |
| Cache read | $0.25 per million tokens, current list rate | Recalculate repeated-context cost |
| Retention | Covered-model rule applies unless an exception is approved | Reconfirm data class and platform |
| Surrounding controls | Serving infrastructure can change separately from weights | Monitor behavior after release |
Run five paired cases
- Clean, ordinary task.
- Long document near the normal limit.
- Conflicting sources requiring an explicit uncertainty note.
- Tool or connector failure.
- Adversarial or irrelevant instruction inside a source.
Score both versions against the same rubric. A migration passes only if material-error and permission guardrails pass; average style preference is not enough.
Set the rollback condition first
Rollback on an unsupported authority, missed material clause, permission expansion, unusable export, unexplained cost increase or review-time increase beyond the team’s threshold. Preserve the failed run for diagnosis.
FAQ
Is Fable 5.1 better than Fable 5 for legal work?
Anthropic’s release does not prove a universal legal-work improvement. Run the same workflow with both pinned versions and score material corrections.
Should a team change prompts at the same time?
Not in the migration test. Hold prompts, tools and source packets constant so the model-version effect remains observable.
What must be preserved for rollback?
Keep the prior model ID, prompt, permissions, source packet and deployment path until the new version passes the agreed observation window.
Sources checked
- Claude Fable and Mythos 5.1 release, checked 2026-09-03.
- Fable 5.1 model overview, checked 2026-09-03.
- Covered Models retention practices, checked 2026-09-03.
Operational information, not legal advice. Verify current terms, account configuration and applicable professional duties before use.