deferred_tool_results may arrive with a non-empty prompt, so the absence of a
prompt cannot be the only test for a resumption. Reproduced: resuming a live
question that way replaced its search result with the earlier-question notice
while the deferred result arrived alongside it.
_is_resumption accepts either signal — no prompt, or a history ending with a
request the model has not answered or a response whose tool calls have no
returns. A settled history ends with the previous answer, so a genuinely new
question is unaffected.
A new prompt on top of an unanswered tail is ambiguous and now counts as a
continuation: compacting costs the answer if it is one, while not compacting
only costs a larger request.
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