cmux-second-opinion — Second Opinion from Another AI

Get an independent code or spec review from a different AI agent in a separate cmux workspace.

Language Rules

1. Auto-detect input language → output in the same language
2. Japanese input → Japanese output, use references/second-opinion-guide.ja.md
3. English input → English output, use references/second-opinion-guide.md
4. Explicit override takes priority

Prerequisites

BEFORE requesting a second opinion, verify cmux environment:

echo $CMUX_SOCKET_PATH

• If empty or unset → stop with error:
  • EN: "Error: Not running inside a cmux session. Please start Claude Code from within cmux."
  • JA: "エラー: cmux セッション内で実行されていません。cmux 内で Claude Code を起動してください。"
• If set → proceed

Review Type Detection

Determine the review type from user input:

User Input Pattern	Type
"review this diff", "code review", "この変更をレビュー", "diff を見て"	code
"review specs", "spec review", "仕様書をレビュー", ".specs/ を見て"	spec
(ambiguous)	Ask user to clarify

Review Criteria Mode

Determine how to use review_rules.md:

Mode	review_rules.md	Additional Instruction	Trigger
Default	Pass to reviewer	"Also share any issues you notice beyond these rules"	(no mode specified)
No criteria	Do not pass	"Review freely from your own perspective"	"freely", "自由にレビュー", "ルールなしで"
Criteria only	Pass to reviewer	(none — strictly follow rules)	"strictly", "ルールに従って", "基準のみ"

Detect review_rules.md:

find . -name "review_rules.md" -maxdepth 3 2>/dev/null

If not found → automatically fall back to "No criteria" mode.

Agent Selection

Select a reviewer that differs from the parent agent to ensure a different perspective.

Parent Agent	Default Reviewer	Auto-Approve Command
Claude Code	Codex	codex --dangerously-bypass-approvals-and-sandbox
Codex	Claude Code	claude --dangerously-skip-permissions
(unknown)	Ask user	—

User can override: "have Claude review this", "Gemini にレビューさせて", etc.

Verify reviewer is available:

which codex    # or whichever agent was selected

Code Review Flow

Step 1: Get Diff

# Staged + unstaged changes vs HEAD
git diff HEAD

# Or vs base branch (if on a feature branch)
git diff main...HEAD

If diff is empty → error: "No changes to review." / "レビュー対象の変更がありません。"

Step 2: Build Review Prompt

Construct the prompt in the same language as the user's input (auto-detected via Language Rules). The reviewer should receive instructions and produce output in that language.

Default mode (English):

Review the following code changes.
## Review Criteria
{content of review_rules.md}
In addition to the above criteria, share any issues you notice from your own perspective.
## Diff
{diff content}
## Output Format
### Critical (must fix)
### Improvements
### Good points

Default mode (日本語):

以下のコード変更をレビューしてください。
## レビュー基準
{review_rules.md の内容}
上記の基準に加えて、独自の視点で気づいた点があれば指摘してください。
## 変更差分
{diff の内容}
## 出力形式
### 重大（修正必須）
### 改善提案
### 良い点

No criteria mode (EN):

Review the following code changes from your own perspective.
Focus on code quality, security, performance, and best practices.
## Diff
{diff content}
## Output Format
### Critical (must fix) / ### Improvements / ### Good points

No criteria mode (JA):

以下のコード変更を自由な視点でレビューしてください。
コード品質、セキュリティ、パフォーマンス、ベストプラクティスの観点でお願いします。
## 変更差分
{diff の内容}
## 出力形式
### 重大（修正必須） / ### 改善提案 / ### 良い点

Criteria only mode: Same as default but without the "also share" / "独自の視点" instruction.

Step 3: Launch Reviewer, Send Prompt, Collect Results

1. Create surface: cmux new-split right → extract surface handle from output (OK surface:{N} workspace:{N})
   • Do NOT use cmux new-workspace — it may create a non-terminal surface that cannot receive commands. Use cmux new-split right (or down) instead.
2. Launch reviewer: cmux send --surface surface:{N} "{reviewer_command}\n"
3. Detect prompt: poll with cmux read-screen (3s intervals, 15s timeout)
4. Send review prompt: cmux send + cmux send-key return
5. Monitor completion: graduated polling (5s → 10s → 30s)
6. Collect results: cmux read-screen --surface surface:{N} --scrollback
7. Do NOT auto-close the reviewer pane. The user may want to review the output in the pane, scroll through history, or ask follow-up questions to the reviewer directly.

Spec Review Flow

Step 1: Locate Spec Files

ls .specs/{feature}/requirement.md .specs/{feature}/design.md .specs/{feature}/tasks.md 2>/dev/null

If no spec files found → error: "Spec files not found." / "仕様書が見つかりません。"

If {feature} is not specified, scan .specs/ and ask user to select.

Step 2: Build Review Prompt

Construct in the same language as user input (per Language Rules).

English:

Review the following specification documents.
{if default or criteria-only mode}
## Review Criteria
{content of review_rules.md} (apply relevant review perspectives only)
{end}
## Specifications
### requirement.md / ### design.md / ### tasks.md
{contents}
## Review Perspectives
- Missing or contradictory requirements?
- Design consistent with requirements?
- Task breakdown appropriate?
- Technically infeasible aspects?
- Security concerns?
## Output Format
### Critical Issues / ### Improvements / ### Good Points

日本語:

以下の仕様書をレビューしてください。
{if デフォルト or 基準のみモード}
## レビュー基準
{review_rules.md の内容}（該当する観点のみ適用）
{end}
## 仕様書
### requirement.md / ### design.md / ### tasks.md
{内容}
## レビュー観点
- 要件の漏れ・矛盾はないか
- 設計と要件の整合性はあるか
- タスク分解は適切か
- 技術的に実現困難な点はないか
- セキュリティ上の懸念はないか
## 出力形式
### 重大な問題 / ### 改善提案 / ### 良い点

Important: Do NOT pass inspection-report.md to the reviewer. The value of a second opinion is an independent perspective.

Step 3: Launch, Send, Collect

Same as Code Review Flow Step 3 (use cmux new-split right, not new-workspace).

Result Report

After collecting the reviewer's output, structure it in the same language as user input:

English:

## Second Opinion Result
**Reviewer**: {agent_name} | **Type**: {code / spec} | **Criteria Mode**: {mode}
### Summary
- Critical: {n} / Improvements: {n} / Good points: {n}
### Critical (Must Fix) / ### Improvements / ### Good Points

日本語:

## セカンドオピニオン結果
**レビュアー**: {agent_name} | **種別**: {code / spec} | **基準モード**: {mode}
### サマリー
- 重大: {n} 件 / 改善提案: {n} 件 / 良い点: {n} 件
### 重大（修正必須） / ### 改善提案 / ### 良い点

Report to the user in the parent session.

Error Handling

Situation	Response
CMUX_SOCKET_PATH not set	Error: not in cmux session
Reviewer agent not installed	Error + suggest alternative agent
Diff is empty	Error: no changes to review
Spec files not found	Error: spec directory not found
Reviewer fails to start	Report error; suggest checking the pane
Reviewer crashes during review	Collect partial output; report what was captured
review_rules.md not found	Fall back to "no criteria" mode (not an error)

Notes

• This skill provides the mechanism for second opinions. Review quality depends on the reviewer agent's capabilities.
• For general task delegation (not review-specific), use cmux-delegate.
• For conversation forking, use cmux-fork.
• For detailed usage examples and troubleshooting, see the reference guide.