[Bug] Fable 5 rejects trivial arithmetic with contradictory error response (is_error: true, subtype: success)
Bug Description
Title: Fable 5 hard-rejects trivial arithmetic ("2+2") in a fresh headless session; response is self-contradictory (is_error: true, subtype: success)
Environment: macOS, Claude Code CLI, non-interactive (headless) invocation, fresh session, no prior context.
Steps to reproduce:
echo "2+2" | claude --model claude-fable-5 -p --output-format json </dev/null
Expected: Fable 5 answers "4". If a safeguard genuinely applies, the response should at minimum be internally consistent.
Actual: top-level is_error is true while subtype is "success" — contradictory. modelUsage carries the key claude-fable-5, confirming Fable 5 actually served the request rather than silently routing elsewhere. The result field reads:
"API Error: Fable 5's safeguards flagged this message (https://www.anthropic.com/legal/aup). This sometimes happens with safe, normal conversations. Claude Code can't respond to this message with Fable 5. Try rephrasing the request in a new session or change your model. Learn more: https://support.claude.com/en/articles/15363606"
Request ID: req_011CdPmhLTnPR38GAaAWwdCK
Control (same machine, same minute, identical command shape, model swapped):
echo "2+2" | claude --model claude-opus-5 --effort xhigh -p --output-format json </dev/null
Result: is_error: false, modelUsage key claude-opus-5, correct answer "4".
Why this may be worth more than the existing reports: issue #73784 and its siblings all involve session context that plausibly contributed to the false positive (fraud / abuse-mitigation vocabulary), and they end in a fallback to another model. This case has no session context at all — the entire input is the literal string 2+2 in a fresh headless session — and it ends in a hard refusal, not a fallback. It is the minimal reproducer with the context variable removed.
Additional context: this account's usage dashboard shows 0% historical Fable 5 usage — every call has failed this way. This blocks a multi-model orchestration pipeline that assigns Fable 5 to a first-position judgment role.
Request: please investigate the trigger on this literal input, and confirm whether Request ID req_011CdPmhLTnPR38GAaAWwdCK is sufficient to pull server-side logs.
Environment Info
- Platform: darwin
- Terminal: ghostty
- Version: 2.1.220
- Feedback ID: a31abbad-7dd5-48b5-ac23-8e708ca63dba
Errors
[]