PR #35718 added a per-slot "cumulative-resend" latch to the universal
streaming tool-call accumulator to fix DeepSeek / Baidu Qianfan (#35592).
The latch fires when a delta is a strict superset of the accumulated
buffer (len(_new) > len(_prev) and _new.startswith(_prev)) and then
REPLACES the buffer instead of appending.
That superset test is not an unambiguous cumulative signature. A normal
incremental stream can emit a single fragment that restates an already-
accumulated prefix — trivially common in large code-patch arguments with
repeated lines / indentation — which trips the latch and clobbers the
accumulated buffer, corrupting the tool call. Observed in the wild on
Anthropic Opus (the primary model) building a large patch: corrupted /
short arguments → finish_reason='length' dead-end → session killed.
A guessing heuristic that can silently clobber a tool-call buffer has no
place on the path every provider and model shares. Reverting restores the
known-good plain `+=` accumulator. The #35592 narrow provider bug should
be re-addressed provider-gated so it is structurally impossible to touch
Anthropic / OpenAI incremental streams, rather than via a heuristic on the
shared path.
Reverts ca03486b6.
2b5268f716