[Bug] OpenCode structured output broken — format must be nested in info.format, not body root

[shreyasp]

Summary

no-mistakes sends structured output (json_schema) to OpenCode at the message body root, but OpenCode >= 1.17.x expects it nested inside info.format. This breaks the review, document, and agent-driven test steps — all three fail with "opencode returned no text output".

This is a more specific root cause for #277 — the "schema rejection" errors in the daemon logs show exactly where the format is misplaced.

Environment

no-mistakes version v1.30.1 (59dfa25) 2026-06-21T22:28:43Z
OpenCode 1.17.9
macOS arm64

Reproduction

1. Configure no-mistakes to use OpenCode

# ~/.no-mistakes/config.yaml
agent: opencode

2. Push a branch through the gate

git push no-mistakes feat/test-opencode-review
no-mistakes axi run --intent "test opencode review" --yes

3. Observe the failure

step review failed: agent review: opencode returned no text output

4. Check daemon logs

~/.no-mistakes/logs/daemon.log

Shows repeated schema rejections:

WARN schema rejection kind=Body reason="Expected OutputFormatJsonSchema, got
{\"type\":\"json_schema\",\"schema\":{...},\"retryCount\":1}
  at [0][\"info\"][\"format\"]"

The at [0]["info"]["format"] path tells us exactly where OpenCode expects the format.

Root Cause Location

internal/agent/opencode_http.go:sendMessage() — around line 84:

// CURRENT (broken) — format at message body root:
if len(schema) > 0 {
    body["format"] = map[string]any{
        "type":       "json_schema",
        "schema":     json.RawMessage(schema),
        "retryCount": 1,
    }
}

OpenCode 1.17.x expects format nested under info:

{
  "role": "user",
  "parts": [{"type": "text", "text": "..."}],
  "info": {
    "format": {
      "type": "json_schema",
      "schema": {...}
    }
  }
}

Verification (API-level proof)

Testing against opencode serve directly:

• Root-level format → rejected: "Expected OutputFormatJsonSchema"
• Nested in info.format → accepted: 200 OK, streaming response with structured output

Proposed Fix

// FIX — nest format inside info:
if len(schema) > 0 {
    info := map[string]any{
        "format": map[string]any{
            "type":   "json_schema",
            "schema": json.RawMessage(schema),
        },
    }
    body["info"] = info
}

Bonus: The info object also accepts "model" for per-message model selection:

info["model"] = "github-copilot/claude-sonnet-4.6"

This would allow the review/document/test agents to use a specific model without changing the global OpenCode config.

Use Case Context

We deploy no-mistakes in a Go 1.26 microservice platform (~200K LOC). Our org disabled Claude Code subscriptions, so we can only use opencode as the agent. All agent-driven steps (review, document, test evidence) fail because of this format nesting issue.

Impact: Cannot use the review step at all with OpenCode. Forced to --skip=review,document on every run — every PR goes through without AI code review.

Related

• OpenCode + DeepSeek V4 structured review fails as "opencode returned no text output" #277 — "OpenCode + DeepSeek V4 structured review fails" — same symptom, this issue identifies the root cause
• post-receive hook: set -- --gate "$(pwd)" can resolve to ".", daemon rejects with "invalid gate path: ." #269 — post-receive hook gate path resolves to . — worktree compatibility
• axi run cannot recover when gate branch ref exists but no pipeline run was recorded #294 — "axi run cannot recover when gate branch ref exists" — worktree workaround