[BUG] `best` alias never falls back to Opus when Fable 5 is not usable — hard error at top level, silent Sonnet 5 as a subagent model

Version: 2.1.240 · Plan: Pro, usage credits turned off · Provider: Anthropic API

best is documented as "Uses Fable 5 where your organization has access to it, otherwise the latest Opus model". On an account where Fable 5 can't actually run — Pro with usage credits switched off — best still resolves to Fable 5 and never reaches the Opus fallback. It fails differently depending on where it's used.

1. Top level — hard error instead of Opus

$ claude --model best -p "Respond with only the word: ok" --output-format json
result: Fable 5 requires usage credits. Switch to another model, or manage usage
        credits at claude.ai/settings/usage, to continue.
modelUsage: {}

Expected: the request runs on the latest Opus. Actual: nothing runs.

2. Subagent frontmatter — silent downgrade to Sonnet 5

An agent with model: best, spawned through the Agent tool with no per-invocation model, ran all 108 of its assistant turns on claude-sonnet-5. The parent session was on claude-opus-5 ("model": "opus" in settings), CLAUDE_CODE_SUBAGENT_MODEL was unset in the environment and in every settings file, and no warning appeared in either transcript. Control, same session and same agent with only the frontmatter changed to model: opus: the transcript shows claude-opus-5.

So neither documented outcome happens: not the latest Opus that best promises, and not the inherited model that a blocked value is documented to fall back to, with an interactive warning naming both models.

Before the credits were switched off, the same model: best agent failed with You're out of usage credits. Run /usage-credits to keep using Fable 5 — confirming best resolves to Fable 5 whenever the account is anywhere near it, and only ever misses the Opus fallback.

Related: #82960 covers the silent Sonnet 5 fallback for subagent dispatch. This report is about the best alias itself skipping its documented Opus fallback; the silent Sonnet is how that surfaces through the path in #82960.