ToolArgsRevealController.setTarget initialized new entries with revealed=0, so the first message_update returned { __partialJson: "" } even when the provider had already parsed a complete chunk. For renderers without exposeRawPartialJson (e.g. write), the throttled re-parse + cached displayArgs short-circuited every subsequent setTarget, leaving the preview body blank until tool_execution_end.
Seed revealed with the full incoming partialJson length on entry creation (clamped to a surrogate-safe boundary). The first frame now carries the parsed path/content immediately; subsequent message_updates extend target and the reveal ticks pace only the newly arrived bytes — no field is ever truncated because the seeded prefix is the longest the entry has seen so far.
Fixes#3881
roboomp
88be72e4d4