Internal server error (and Type validation failed)

Description

During the last week opencode (with Go) repeatedly interrupts with and Internal server error and does not do any work. It usually starts after some work (e.g. when it gets to about 25% of context).
It happens both with glm-5.3 flash and qwen3.8 flash.
Sometimes it gives also the following error:
Type validation failed: Value: {}.
Error message: [
{
"code": "invalid_union",
"errors": [],
"note": "No matching discriminator",
"discriminator": "type",
"path": [
"type"
],
"message": "Invalid input"
}
]
Previously it was working fine. Now it is unusable.

Plugins

none

OpenCode version

1.18.30

Steps to reproduce

Open opencode.
Start working. Wait until it starts to give internal server error

Screenshot and/or share link

Operating System

MacOS 26.6.2

Terminal

Bash, zed (zsh)