ACP: messages are duplicated

[ask4fusora]

Description

I used opencode acp on Windows, and when I send a message, either the response gets duplicated, or the request gets duplicated.
I deduced this is an OpenCode problem because this wasn't reproducible with other ACPs such as Gemini CLI, Copilot CLI.
The duplication is really weird, on VSCode (VSCode ACP extension), the response gets duplicated. On Zed Editor, the request gets duplicated. On JetBrains Idea, the request gets duplicated but prepend to the response.

Plugins

None

OpenCode version

1.4.3

Steps to reproduce

1. Connect to a code editor with opencode acp.
2. Try sending a message.
3. See the response.

Screenshot and/or share link

Operating System

Windows 11

Terminal

No response

[zooloo303]

Same issue here

[OpeOginni]

@ask4fusora Im only able to reproduce the one with Zed,

Which Jetbrains IDE are you making use of?

Which extension on VS code exactly are you using, cause I can see two

[ask4fusora]

For the JetBrains, I was using IntelliJ Idea 2025.3.3. With the built-in AI Chat, with these configuration:

IntelliJ IDEA 2025.3.3
Build #IU-253.31033.145, built on February 20, 2026
Source revision: 212d61deaf3a8
Runtime version: 21.0.10+1-b1163.108 amd64 (JCEF 137.0.17)
VM: OpenJDK 64-Bit Server VM by JetBrains s.r.o.
Toolkit: sun.awt.windows.WToolkit
Windows 11.0
GC: G1 Young Generation, G1 Concurrent GC, G1 Old Generation
Memory: 2048M
Cores: 16
Registry:
  ide.experimental.ui=true
  llm.show.ai.promotion.window.on.start=false
  maven.packagesearch.enabled=false
Non-Bundled Plugins:
  IdeaVIM (2.30.0)
  com.intellij.plugins.vscodekeymap (253.28294.218)
  com.intellij.ml.llm (253.31033.188)
  com.demonwav.minecraft-dev (2025.3-1.8.14)
Kotlin: 253.31033.145-IJ

{
  "agent_servers": {
    "OpenCode": {
      "command": "opencode",
      "args": ["acp"],
    }
}

For the VSCode, I was actually using Antigravity, which is a folk of it, using omercnet.vscode-acp.

[OpeOginni]

For the Intelij I saw they have an agents app plugin that is made by the team and I didn't have to setup any configs, try that, it works well for me. I'll update with a screenshot when I get to my computer

[ask4fusora]

Hmmm, for me, even if I use the OpenCode from the registry, the result is still the same duplication of the request prepended to the response.

[willmcgugan]

Opencode is sending a "user_message_chunk" update when it receives the user's prompt. Other agents only do this when resuming a session.

Here's a partial log from Toad. I think the first line is erroneous.

[agent] {"jsonrpc":"2.0","method":"session/update","params":{"sessionId":"ses_27e14d00fffejSTVnYZCaPVJH9","update":{"sessionUpdate":"user_message_chunk","messageId":"msg_d81eb33ca001QiYYstpbohw8Dp","content":{"type":"text","text":"hello"}}}}
[agent] {"jsonrpc":"2.0","method":"session/update","params":{"sessionId":"ses_27e14d00fffejSTVnYZCaPVJH9","update":{"sessionUpdate":"agent_thought_chunk","messageId":"msg_d81eb33d9001ncCMg0o5tSBEmw","content":{"type":"text","text":"The user"}}}}

Issue reported here: batrachianai/toad#278

[OpeOginni]

This might seem to do it too #21708