subagent: finish:"error" / MALFORMED_FUNCTION_CALL with empty content is reported to the parent as a successful completion #52378

Description

Summary

OpenCode reports a failed subagent run to the parent as a successful one.

A Task subagent's last assistant message ended with finish:"error", rawFinish:"MALFORMED_FUNCTION_CALL", content:[], and 0 output tokens. OpenCode then set the child session to idle with outcome:"succeeded", and the parent got state="completed" with "Subagent completed without a text response." The parent cannot tell a provider failure from a real empty answer. In a review panel, an orchestrator can read that as a clean review. This happened 3 times in one session.

Expected Behavior

• The child ends in an error state (for example idle outcome:"failed", or the existing error outcome), with the provider error attached.
• The parent receives state="error", and the tool result carries the finish reason (MALFORMED_FUNCTION_CALL) and the provider error text.
• Optional: retry the failed step once automatically. This failure may be transient (see Frequency below).

Actual Behavior

Evidence from ~/.local/share/opencode/opencode.db (sqlite3 -readonly). I removed prompts and file contents. All other fields are as stored, except providerState, which is trimmed.

Child ses_f0ca68c11ffescNxFG4bNp3AjF, last assistant message (seq 33, 2026-09-30 19:25:35):

{
  "agent": "prosecutor-tests",
  "model": { "id": "vmc/adlc-prosecutor-tests", "providerID": "vercel", "variant": "default" },
  "content": [],
  "finish": "error",
  "rawFinish": "MALFORMED_FUNCTION_CALL",
  "providerState": {
    "routing": {
      "originalModelId": "google/gemini-3.8-flash",
      "finalProvider": "google",
      "modelAttempts": [
        {
          "canonicalSlug": "google/gemini-3.8-flash",
          "success": false,
          "providerAttempts": [
            {
              "provider": "google",
              "success": false,
              "error": "stream finished with error: MALFORMED_FUNCTION_CALL"
            }
          ]
        }
      ]
    }
  },
  "tokens": { "input": 3771, "output": 0, "reasoning": 37, "cache": { "read": 20366, "write": 0 } }
}

Next event (seq 36, 19:25:37):

{ "time": { "created": 1790789137877 }, "outcome": "succeeded" }

Parent (root session), synthetic message seq 2311:

<subagent sessionID="ses_f0ca68c11ffescNxFG4bNp3AjF" state="completed" description="P5 panel 2 tests">
Subagent completed without a text response.
</subagent>

The same pattern appears in two other child sessions:

session_v2.idle_outcome is succeeded for all three. Each child made 4–5 valid tool calls before the failing step.

Query used:

sqlite3 -readonly ~/.local/share/opencode/opencode.db "select seq, data from session_message where session_id='<child>' and type in ('assistant','idle') order by seq desc limit 2"

Additional Context

• Frequency: 3 out of 5 invocations of this subagent in one session. The same model worked earlier that day.
• Impact: the parent retried by hand, then switched to a different model. The cost was small (≈ $0.12), but an empty "completed" result looks like "no findings" to a reviewing orchestrator.
• Workaround: tell the orchestrator to treat an empty subagent reply as a failed run, retry once, then switch to another model.
• rawFinish is mapped in packages/opencode/src/session/llm/ai-sdk.ts. The error seems to get lost where finish:"error" becomes the child's idle outcome or the parent's subagent state.
• Provider: vercel (AI Gateway). Virtual model vmc/adlc-prosecutor-tests → google/gemini-3.8-flash, routed to Google.
• Install/channel: latest (Homebrew anomalyco/tap/opencode-v2, plus OpenChamber's bundled opencode-cli, driven via opencode serve). Shell: /bin/zsh.

Possibly related. These issues also report a failed stream that reaches the parent as an empty success, but none of them covers v2 with finish:"error" / MALFORMED_FUNCTION_CALL:

• Subagent stream error can surface as empty task result #38866: Subagent stream error can surface as empty task result (v1.18.3).
• Sub-agent Task silently returns empty result when the final response stream fails (e.g. 503) — no error, no retry, completed work lost #40527: Sub-agent Task silently returns empty result when the final response stream fails (v1.18.13, 503).
• Aborted provider stream recorded as clean stop (finish=unknown, zero usage, no text) — subagent returns empty with no error #37852: Aborted provider stream recorded as clean stop (finish=unknown), subagent returns empty with no error.
• Session idles silently when a turn ends with stop and no model output #50250: Session idles silently when a turn ends with stop and no model output.

Plugins

@adlc/opencode, opencode.browser, openchamber-agent-tool

OpenCode version

Sessions recorded by v2.0.18 (session_v2.version, OpenChamber's bundled opencode-cli). The same machine also has Homebrew opencode-v2 v2.0.20. I have not checked whether 2.0.19 or 2.0.20 changed this code path.

Steps to reproduce

Intermittent. It depends on the model sending a malformed tool call.

1. Configure a subagent pinned to a Gemini Flash model (here, through the Vercel AI Gateway).
2. From a parent session, invoke it with the Task tool, with a tool-heavy brief (read or grep over a large diff).
3. After several valid tool steps, the provider stream sometimes ends with stream finished with error: MALFORMED_FUNCTION_CALL.
4. Look at the parent: the task result is state="completed" with "Subagent completed without a text response."

Screenshot and/or share link

No response

Operating System

macOS (Darwin 25.6.0 arm64)

Terminal

xterm-256color (sessions driven through OpenChamber via opencode serve)