Codex found the session-id anchor too blunt. It assumes a new id means an
unrelated transcript, which holds for `/new` and for resuming something
else — but `fork()` mints a fresh id while cloning the transcript and
keeping the same recovery state running. Attribution and routing both
expired there, so immediately after `/fork` an unproven fallback
bootstrapped as the current model with `isFallback: false`: the run was
re-credited to a model that never produced any of it, and mislabelled as
the configured primary. Exactly the bug the anchor exists to prevent,
reopened for the one switch that is a continuation.
`AgentSession.fork()` now re-tags both onto the new id after the fork
succeeds, moving only state that belonged to the pre-fork id so an id left
behind by an earlier switch stays expired.
enieuwy
a5be3ea0df