Summary

ACP streaming sometimes drops the final tail of assistant output.
The run still finishes with stopReason: "end_turn", but the last chunk text is incomplete.

Steps to reproduce

1. Start ACP bridge:
   openclaw acp --verbose
2. Send:
   {"jsonrpc":"2.0","id":1,"method":"initialize","params":{"protocolVersion":1}}
3. Create session:
   {"jsonrpc":"2.0","id":2,"method":"session/new","params":{"cwd":"/home/jaesbit/Work/openclaw-tests","mcpServers":[]}}
4. Send long prompt with explicit end marker:
   {"jsonrpc":"2.0","id":3,"method":"session/prompt","params":{"sessionId":"<SESSION_ID>","prompt":[{"type":"text","text":"Please write a long test message in English, around 25 lines, with a title, a numbered list, and a final closing marker: END-OF-LONG-TEST-OK."}]}}
5. Inspect session/update notifications and final id:3 response.

Expected behavior

Final streamed assistant text should be complete (including the full tail/end marker context) before end_turn.

Actual behavior

• Multiple session/update events arrive with sessionUpdate: "agent_message_chunk".
• The final text chunk is truncated (tail missing).
• Then ACP returns:
  {"jsonrpc":"2.0","id":3,"result":{"stopReason":"end_turn"}}

Environment

• Clawdbot version: OpenClaw CLI 2026.2.9 observed locally (openclaw --version)
• OS: Linux (host environment)
• Install method (pnpm/npx/docker/etc): global npm install (~/.npm-global/...)

Logs or screenshots

Raw ACP sample (redacted):

• initialize succeeds with protocolVersion:1
• session/new succeeds and returns sessionId
• session/update emits several agent_message_chunk events
• Last chunk ends early (example tail: ...missing/c)
• Final response is still stopReason:"end_turn"

Additional scope notes:

• Reproduced on node-connected path (paired node setup).
• In OpenClaw Web UI, full message is visible.
• Direct same-instance ACP→gateway path has not been tested yet.