System prompt block Qwen3.5 natural thinking process

[JeroBnd]

Description

Qwen 3.5 don't think naturally on opencode.

Qwen 3.5 naturally have a long thinking chain with a specific structure that isn't present on opnecode.

I track the origin of this behavior to the next phrase of the default system prompt (packages/opencode/src/session/prompt/default.txt [line 14]): "Output text to communicate with the user; all text you output outside of tool use is displayed to the user."

Plugins

none

OpenCode version

1.2.24

Steps to reproduce

Extract this phrase from the system prompt and use in a simple chat, you will see the think chain shorter.

Screenshot and/or share link

Response to "hola" (hello in spanish) on Opencode

Response to "hola" (hello in spanish) on lm-sttudio with the system prompt

Natural Qwen 3.5 thinking (without this system prompt)

Operating System

Ubuntu 24.10

Terminal

No response

[github-actions]

This issue might be a duplicate of existing issues. Please check:

• Qwen/Qwen3.5-397B-A17B-FP8: opencode never show the thinking process block. #16963: Same symptom — Qwen3.5 thinking process not shown in opencode (same OpenCode version 1.2.24, same root behavior)
• default system prompt materiallly freaks out / degrades reasoning of every high reasoning model #10927: Related — default system prompt degrading reasoning in high-reasoning models

If your issue is distinct (e.g., you've specifically traced the cause to a particular line in the system prompt), please add a comment explaining how it differs from #16963.

[JeroBnd]

This issue its different of #16963. In #16963 the thinking block it isn' showing at all. In this case these case the thinking block is showing but the prompt is breaking the chain of thought of Qwen 3.5 model. This is reproducible outside opencode.