ACP Tool-Result Text Projection

Problem

ACP tool-result display fields are assembled after model-facing response finalization. A complete display value can therefore appear in both structured content and rawOutput, producing a much larger ACP frame than the bounded model response. Live delivery and history replay also leave through different paths, so applying a limit at only one of them would leave a replay bypass.

Contract

Each eligible field has a fixed 65,536-byte budget measured as the UTF-8 byte length of that field's JSON serialization. Eligible content is an array made only of canonical ACP text-content blocks with no extra fields. Eligible rawOutput is a primitive string. The fields are evaluated independently and remain present after projection.

A2UI tool updates are exempt as a whole because the daemon extracts command JSON after the child ACP wire boundary. Structured, diff, terminal, media, mixed, and otherwise non-canonical content remains unchanged. The projection does not change the canonical transcript, model response, artifact metadata, or offline export.

Projection

String size is computed with a linear scanner that matches native JSON escaping for controls, quotes, backslashes, Unicode, valid surrogate pairs, and lone surrogates. Oversized strings retain an approximately 20 percent head and 80 percent tail around a fixed transport-truncation marker. Selected slices are copied so a bounded preview does not retain an oversized backing string.

For multi-block content, the array and object wrappers count toward the same field budget. A deterministic max-min allocation preserves block order and lets small blocks remain complete. Reduced blocks reserve marker space before sharing the remaining payload budget. Fit is decided with a cumulative early-stopping scan, then each block is scanned only through its largest possible allocation. If the empty structure or minimum marker set cannot fit, the field collapses to one canonical omission marker.

The projector does not stringify an original oversized field, join blocks, or perform repeated binary searches. Native serialization is used only to verify the already bounded result. Applying the projector twice is a no-op after the first projection.

Boundaries

Live updates are projected in Session.sendUpdate(). Replay updates are projected when the replay collector accepts them, covering bulk load, qwen/session/loadUpdates, paged transcript routes, and virtual subagent replay. The canonical replay machine and transcript update constructors remain unchanged, and offline export continues to replay through its dedicated export context without this transport projection.

Compatibility and Non-Goals

ACP schemas, capabilities, and field types do not change. Keeping both content and rawOutput avoids a wire-deduplication compatibility decision. When no artifact exists, the marker makes no recovery claim.

This design is not a universal ACP frame limit. Structured payload bounds, generic NDJSON caps, backpressure, replay aggregate limits, Headless display projection, diagnostics, and artifact lifecycle remain separate work tracked by #8091, #8447, #8448, and the later phases of #7306.