opencode run --format json fails with UnknownError (1.18.23 → 1.18.29)

Description

Summary

opencode run --format json consistently fails with UnknownError, making the JSON output mode unusable for programmatic consumption.

Version

$ opencode --version
1.18.29

Originally observed on v1.18.23; still reproduces identically on v1.18.29. Platform: macOS (arm64).

Repro

$ opencode run --format json "reply with the single word ok"
{"type":"error","timestamp":1788875942731,"sessionID":"ses_f7eaf57b4ffeRoCprFrTfUsZZJ","error":{"name":"UnknownError","data":{"message":"Unexpected server error. Check server logs for details.","ref":"err_4014dc86"}}}
$ echo $?
1

Notes

• Reproduced identically on v1.18.23 in both standalone mode and opencode run --attach --format json.
• Verified again on v1.18.29 (standalone mode) on 2026-09-08 with OPENCODE/OPENCODE_AUTO env vars unset (i.e. not nested inside an existing opencode session).
• Exit code 1, no JSON result payload is ever emitted — only the error line.
• This blocks any tooling that spawns opencode run --format json and parses stdout for events/messages (e.g. driving opencode sessions programmatically).

Plugins

No response

OpenCode version

1.18.29

Steps to reproduce

No response

Screenshot and/or share link

No response

Operating System

macOS

Terminal

Ghostyy