Plan mode can write and edit files via bash

Description

I wanted to create a new skill and plan out what should be done.
The model (Claude Sonnet 4.6) forgot that it was in plan mode and just started the implementation. It could not use the write-tool (which is expected), so it used bash commands

$ cat > SKILL.md << 'SKILLEOF'
...
SKILLEOF
echo "SKILL.md done"

to create the files.

This never happened to me before.
As far as I can see in the docs, the model can ask for bash and edit permissions in plan mode. I gave it access to my .config/opencode/skills folder but nothing else. I was not prompted to allow bash commands of any kind.

I tried to reproduce the problem, but obviously Sonnet doesn't want to do that now when instructed explicitly.

Plugins

none

OpenCode version

1.18.9

Steps to reproduce

No response

Screenshot and/or share link

Operating System

Ubuntu 24.04

Terminal

Alacritty