[Bug]: ACP (acpRegistry) Claude turn stays on "Thinking"; final reply hidden until user presses Stop #17203

Before submitting

• I searched existing issues and did not find a duplicate.
• I included enough detail to reproduce or investigate the problem.

Area

apps/server

Steps to reproduce

This happens often: several times per thread, across many threads.

1. Start a thread with the acpRegistry harness, model Claude Opus 5.5, effort "low", in Agent mode with Full access.
2. Send any prompt. Short replies like "ok" or "is working now." trigger it as easily as long tool-using turns.
3. The agent finishes its turn and writes its final reply.

Expected behavior

The final reply renders as soon as the agent writes it. The run then completes and the thread goes idle.

Actual behavior

• The UI shows a reasoning summary (e.g. "A short, simple response seems best here."), then a "Thinking" row with the stop button active. The "Working for Xs" timer keeps counting.
• The reply the agent already wrote is not shown.
• Sometimes only its first line shows (e.g. a header sentence) and the rest is hidden behind "Thinking".
• The reply only appears after I press Stop, followed by "Run interrupted · Run interrupted by user".
• From the agent's side the turn had already ended normally.

This looks like the ACP-path version of #15197. That issue covers the Claude adapter, where the final message renders but the run stays "running". Here the final message content itself is held back until the run is cancelled, so I lose replies unless I interrupt every turn.

Impact

Major degradation or frequent failure

Version or commit

T3 Code desktop, macOS (Apple Silicon), October 2026 build. Provider: acpRegistry → Claude Opus 5.5.

Environment

macOS. Long-running thread with many turns, an async background shell, and a summarized/compacted conversation history.

Logs or stack traces

Screenshots available on request:

• (1) a stuck "Thinking" row with no reply shown;
• (2) the same turn after pressing Stop, where the reply appears above "Run interrupted by user".