bug: OpenCode error runs show empty message body

[nathanschram]

Description

When OpenCode encounters an error (e.g. reading a nonexistent file), the completion message shows error · opencode · 34s · step 1 with an empty body — no error message or explanation is visible to the user.

Reproduction

1. Send read the file /nonexistent/impossible/path.txt to the OpenCode dev chat
2. OpenCode runs, encounters the error
3. Completion message shows: error · opencode · 34s · step 1 followed directly by the footer (dir, resume line, cost)
4. No error description between the status line and footer

Expected behaviour

The error message or at least a brief description of what went wrong should be visible to the user, similar to how Claude ("That file doesn't exist"), Codex, Pi, and Gemini all show explanatory text.

Comparison

Engine	Error handling
Claude	"That file doesn't exist" ✅
Codex	"path does not exist" ✅
OpenCode	(blank) ❌
Pi	Detailed explanation ✅
Gemini	"outside the workspace boundaries" ✅

Possible cause

OpenCode may exit with a non-zero code without producing an answer in its final events. The CompletedEvent gets error status but no answer text. The runner may need to capture stderr or the last text output as the error message.

Environment

• v0.35.0rc6, @untether_dev_bot, OpenCode engine

[nathanschram]

Integration test result: NOT FIXED in rc6

Tested via @untether_dev_bot on 2026-03-18.

Reproduction:

• Sent read the file /nonexistent/impossible/path.txt to OpenCode dev chat
• Response: error · opencode · 30s · step 1 followed directly by footer — empty body

Comparison (same prompt, same session):

Engine	Error body
Claude	"That file doesn't exist" ✅
Codex	"I couldn't read... because it does not exist" ✅
OpenCode	(blank) ❌
Pi	"I couldn't read... file not found (ENOENT)" ✅
Gemini	"outside the workspace boundaries" ✅

Log evidence:

runner.completed action_count=1 answer_len=0 engine=opencode error=None ok=True

Note answer_len=0 and ok=True — OpenCode completed "successfully" with no answer text despite the error. The CompletedEvent.answer fallback to error text is not triggering because the event has ok=True (not an error status from OpenCode's perspective).

The unit tests pass because they test the fallback when ok=False, but OpenCode may report the error within normal completion flow (ok=True, answer empty) rather than as an error event.

Suggested fix: Also fall back to state.last_text or the last Text event content when answer is empty, regardless of ok status.

[nathanschram]

Created #150 with the broader fix needed — the reason="stop" path at line 331 needs a fallback when state.last_text is empty, not just the Error event path at line 368.