BUG: task-tool subagents exit completed with empty results on empty provider completions (finish: unknown, 0 tokens) #236

Summary

Subagent tasks dispatched via the task tool exit as completed with an empty task_result when the provider returns an empty completion (0 input/output tokens, cost 0, finish reason unknown). The parent cannot distinguish "subagent finished with nothing" from "every model call failed."

Over the last 3 days on the canonical server: ~130 such empty completions across every agent class (parent sessions, subagents, compaction), 12 task-tool parts in error state, 24 task parts stuck at running (zombies from interrupted sessions).

Context

• Engine: fork build merge-upstream-2026-08-16 (server binary; extension pin v1.18.10-amicode.4)
• OS: Darwin 24.6.0 arm64
• Provider path: opencode-go (glm-5.3) via the shared gateway
• Discovered while diagnosing recurring "subagents dying" reports in autoresearch campaigns — the campaign ledger's "instant completion, zero artifacts" experimenter failures are this bug.

Expected vs actual

Expected: an empty provider completion (0 tokens, finish unknown) is a transport/endpoint error — the loop should retry with backoff or surface an error; a subagent must never exit completed on one.

Actual: assistant messages record tokens {in:0, out:0}, cost 0, finish "unknown" (up to ~84s elapsed per call); the subagent retries a few turns, then exits completed with an empty result.

Sibling error states observed in the same window: Task cancelled / Tool execution aborted (parent interruption), and Unknown agent type: … (agent-set drift — a separate class).

intake: not-ready — mature per the maturity contract before picking up
suggested_path: B
diagnostics: inline
upstream: anomalyco/opencode#41469 (open)