Executed local preflight · revision 1.0
Fable 5.1 request-shape preflight
Detect documented Fable 5.1 request-shape hazards before a live migration test.
Local result
6/6 cases matched the expected classification
The validator caught the intentionally incompatible request bodies and left safe shapes ready. This is a local software-test result, not evidence of live API acceptance.
2 ready1 review3 blocked
Test environment
- Models represented
- claude-fable-5-1 and claude-opus-5
- Provider / endpoint
- None; no API call performed
- SDK
- None
- Runtime
- v24.19.0 local validator
- Run timestamp
- Fixture count
- 6
- Fixture SHA-256
827b3e897e18616f29e943aa511c81ce09a5e7937589f7f0b40039ff5154279d
Protocol
- Load the six frozen, synthetic request fixtures.
- Classify documented forced-tool, thinking-mode, prefix-stability, and downgrade-route hazards.
- Compare every classification with the fixture expectation.
- Fail the command if any case diverges.
npm run test:migrationDecision relevance
A pass can justify moving a request body into a controlled live evaluation. It cannot justify production cutover, because provider validation, output quality, latency, token use, and tool behavior are outside this test.
Fixture results
| Case | Classification | Local finding |
|---|---|---|
| Auto tool choice with a strict schema | ready | No encoded hazard detected. |
| Named tool forced on Fable 5.1 | blocked | Fable 5.1 rejects forced tool choice. Use auto or none. |
| Thinking disabled on Fable 5.1 | blocked | Fable 5.1 does not accept explicitly disabled thinking. |
| Append-only continued conversation | ready | No encoded hazard detected. |
| Earlier system and message content rewritten | blocked | The system prompt changed after a Fable 5.1 thinking block. The prior conversation is not an unchanged prefix of the request. |
| Fable 5.1 conversation routed back to Opus 5 | review | A route from Fable 5.1 to an older model is one-way for thinking blocks; expect the target model to re-plan. |
Open the secret-free result artifact. Its fixture digest is shown above so reruns can identify the exact input set.
Limitations
- A passing fixture means only that the local request shape avoids the encoded hazards.
- It does not prove Anthropic API acceptance, output quality, latency, cost, or tool-call reliability.
- Operators must rerun live workload evaluations in their own provider, region, tier, SDK, and account context.
Rerun conditions
- Anthropic changes the Fable 5.1 migration or API error documentation
- The request builder, system prompt, tool set, compaction, or routing policy changes
- A new target model or provider surface is introduced
- The 30-day verification window expires
Rules encoded from primary sources
- Migrating to Claude Fable 5.1 and Claude Mythos 5.1Anthropic primary source · checked Sep 11, 2026
- Claude API errorsAnthropic primary source · checked Sep 11, 2026
Revision and corrections
- Initial pilot edition; no material correction recorded.