ACP stdio transport crashes on oversized tool_call_update payloads (truncate large text fields)

[zztdandan]

Description

Describe the bug you encountered

When OpenCode is used with the Agent Client Protocol (ACP) Python SDK over stdio, oversized JSON-RPC update payloads can trigger a fatal stdio channel/port error on the SDK side.
This breaks ACP communication for the current client session.

What happened?

tool_call_update messages may contain very large text fields (for example from read output), and a single emitted JSON message can exceed the practical stdio boundary (~64KB observed in this integration path).
After that, ACP updates may fail with a fatal stdio-side error and the session becomes unstable/unusable.

Plugins

ACP / Agent Client Protocol Python SDK over stdio

OpenCode version

1.2.10

Steps to reproduce

1. Run OpenCode with ACP Python SDK connected via stdio.
2. Trigger a tool call that emits large text content in update payloads (e.g. read on a moderately large file, or large rawOutput.output).
3. Observe ACP update delivery failure / fatal stdio-side error once payload size becomes too large.

Screenshot and/or share link

frame_20260311T012529745579Z_32e0a22071284adfa27899e8ced26c40.txt
frame_20260311T034910517070Z_46893e1cb388476ca5a9cfeb5ee96777.txt
these are two large acp stdio return i catched

Operating System

win11 WSL unbunt 22.04

Terminal

vscode