TransientError and SourceDeadError read like caller-facing contracts in
the taxonomy table, but the retry loop catches both and repackages them as
AllSourcesExhausted, so they never arrive. That is only discoverable by
reading middleware/retry.py, and a downstream project wrote a whole design
section on the false premise before checking.
The new table states the split outright, including that
GovernanceBackendError now sits on the caller-facing side and
SourceNotConfiguredError deliberately does not join the retryable family.
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