The internal model_switched event — emitted when a turn falls back from the primary model — now tracks two additional trigger reasons: "server_error" (retryable 5xx pivot) and "last_resort" (non-retryable error on the primary). Previously only model_not_found, permission_denied, and overloaded were tracked.
Evidence
Fallback trigger enum (search for "last_resort" near "model_not_found" and "overloaded" in the fallback model schema)
Strings lifted out of the shipped bundle, so the claim above can be checked against them.