The one-shot reasoning-off retry changes a request parameter that is part
of the provider cache key on config-sensitive providers (Anthropic renders
thinking/effort into the prompt; OpenAI lists reasoning.effort as
prefix-affecting), so that request is a deliberate single cache miss.
Pin the bound: the request AFTER it must carry the configured reasoning
again and the system prompt must be byte-identical across the whole retry
sequence. Sabotage-verified (sticky flag -> test fails on request 3).
Docstring on _consume_ephemeral_reasoning_off states the cost honestly.
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