GPT-5.6 Responses-Lite models receive tool_choice "auto" (the forced
hosted choice is invalid under the lite shape, #5771/#5772), so the model
may answer without invoking the hosted web_search tool. The codex search
parser accepted any non-empty answer, returning a stale completion with
zero sources as a successful search.
callCodexSearch now tracks response.web_search_call.* events (and
web_search_call output items) and throws CodexNoWebSearchError when none
occurred. The candidate chain treats that error as retryable, advancing
default lite models to a non-lite model that forces web_search, and
surfaces a clear failure when the model was explicitly configured.
Fixes#6988
(cherry picked from commit a276cd0b3df1d0d041faf0a63fabcbb884e36a91)
roboomp
e5ea31b22c