The Codex Responses handler kept arguments that arrived only on
response.output_item.done (and custom tool input that arrived on
response.output_item.added) in the builder for history replay, but never
emitted them as tool-call deltas, so the runtime executed the call with an
empty argument string ("path is required").
Route every builder write through one forwardInput helper that also emits
the delta, so the builder holds exactly what the runtime has received. The
added handler forwards whole custom input, the delta handler forwards each
fragment, and the done handler forwards the final input only when nothing
reached the runtime yet, so streamed fragments are never duplicated.
Handler-level coverage spans function and custom calls, with and without
the added item, streamed or final-only, plus custom input on the added
item; a second test drives the same shapes through StreamCompletion and
zeroruntime.CollectStream. Against the unfixed handler the final-only and
input-on-added cases report an empty argument string at tool-call-end.
Closes#1194
Co-Authored-By: Claude Fable 5.1 <[email protected]>
