A run carrying no prompt is continuing a question rather than asking one:
pydantic-ai resumes that way for deferred tool results, interruptions and
suspended responses. len(ctx.messages) then counts the live question's own
messages, so its search result was replaced by the earlier-question notice and
the model was asked to answer with the evidence removed. Reproduced: resuming
with an in-flight history left a notice where the only evidence was.
Switch compaction off for the whole run when ctx.prompt is None. The absence of
a prompt is the signal rather than the message layout — the resume shapes differ
from each other, and deriving the boundary from layout is what broke this to
begin with.
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