Second Opinion

Second Opinion

Shell out to external LLM CLIs for an independent code review powered by a separate model. Supports OpenAI Codex CLI and Google Gemini CLI.

When to Use

• Getting a second opinion on code changes from a different model
• Reviewing branch diffs before opening a PR
• Checking uncommitted work for issues before committing
• Running a focused review (security, performance, error handling)
• Comparing review output from multiple models

When NOT to Use

• Neither Codex CLI nor Gemini CLI is installed
• No API key or subscription configured for either tool
• Reviewing non-code files (documentation, config)
• You want Claude's own review (just ask Claude directly)

Safety Note

Gemini CLI is invoked with --yolo, which auto-approves all tool calls without confirmation. This is required for headless (non-interactive) operation but means Gemini will execute any tool actions its extensions request without prompting.

Quick Reference

# Codex
codex review --uncommitted
codex review --base <branch>
codex review --commit <sha>

# Gemini (code review extension)
gemini -p "/code-review" --yolo -e code-review
# Gemini (headless with diff — see references/ for full pattern)
git diff HEAD > /tmp/review-diff.txt
{ printf '%s\n\n' 'Review this diff for issues.'; cat /tmp/review-diff.txt; } \
  | gemini -p - --yolo -m gemini-3-pro-preview

Invocation

1. Gather context interactively

Use AskUserQuestion to collect review parameters in one shot. Adapt the questions based on what the user already provided in their invocation (skip questions they already answered).

Combine all applicable questions into a single AskUserQuestion call (max 4 questions).

Question 1 — Tool (skip if user already specified):

header: "Review tool"
question: "Which tool should run the review?"
options:
  - "Both Codex and Gemini (Recommended)" → run both in parallel
  - "Codex only"                          → codex review
  - "Gemini only"                         → gemini CLI

Question 2 — Scope (skip if user already specified):

header: "Review scope"
question: "What should be reviewed?"
options:
  - "Uncommitted changes" → --uncommitted / git diff HEAD
  - "Branch diff vs main" → --base (auto-detect default branch)
  - "Specific commit"     → --commit (follow up for SHA)

Question 3 — Project context (skip if neither CLAUDE.md nor AGENTS.md exists):

Check for CLAUDE.md first, then AGENTS.md in the repo root. Only show this question if at least one exists.

header: "Project context"
question: "Include project conventions file so the review
  checks against your standards?"
options:
  - "Yes, include it"
  - "No, standard review"

Note: Project context only applies to Gemini and to Codex with --uncommitted. For Codex with --base/--commit, the positional prompt is not supported — inform the user that Codex will review without custom instructions in this mode (it still reads AGENTS.md if one exists in the repo).

Question 4 — Review focus (always ask):

header: "Review focus"
question: "Any specific focus areas for the review?"
options:
  - "General review"    → no custom prompt
  - "Security & auth"   → security-focused prompt
  - "Performance"       → performance-focused prompt
  - "Error handling"    → error handling-focused prompt

2. Run the tool directly

Do not pre-check tool availability. Run the selected tool immediately. If the command fails with "command not found" or an extension is missing, report the install command from the Error Handling table below and skip that tool (if "Both" was selected, run only the available one).

Diff Preview

After collecting answers, show the diff stats:

# For uncommitted:
git diff --stat HEAD

# For branch diff:
git diff --stat <branch>...HEAD

# For specific commit:
git diff --stat <sha>~1..<sha>

If the diff is empty, stop and tell the user.

If the diff is very large (>2000 lines changed), warn the user that high-effort reasoning on a large diff will be slow and ask whether to proceed or narrow the scope.

Skipping Inapplicable Checks

After determining the diff scope, skip checks that don't apply to the files actually changed.

Dependency Scanning

Only run /security:scan-deps when the diff touches dependency manifest files. Check with:

git diff --name-only <scope> \
  | grep -qiE '(package\.json|package-lock|yarn\.lock|pnpm-lock|Gemfile|\.gemspec|requirements\.txt|setup\.py|setup\.cfg|pyproject\.toml|poetry\.lock|uv\.lock|Cargo\.toml|Cargo\.lock|go\.mod|go\.sum|composer\.json|composer\.lock|Pipfile)'

If no dependency files are in the diff, skip the scan even when security focus is selected. The scan analyzes the entire project's dependency tree regardless of diff scope, so it adds significant time for zero value when dependencies weren't touched.

Auto-detect Default Branch

For branch diff scope, detect the default branch name:

git symbolic-ref refs/remotes/origin/HEAD 2>/dev/null \
  | sed 's@^refs/remotes/origin/@@' || echo main

Codex Invocation

See references/codex-invocation.md for full details on command syntax, prompt passing, and model fallback.

Summary:

• Model: gpt-5.3-codex, reasoning: xhigh
• --uncommitted takes a positional prompt
• --base and --commit do NOT accept custom prompts (Codex reads AGENTS.md if present, but the skill will not create one; note this limitation to the user)
• Falls back to gpt-5.2-codex on auth errors
• Output is verbose — summarize findings, don't dump raw (see references/codex-invocation.md § Parsing Output)
• Set timeout: 600000 on the Bash call

Gemini Invocation

See references/gemini-invocation.md for full details on flags, scope mapping, and extension usage.

Summary:

• Model: gemini-3-pro-preview, flags: --yolo, -e, -m
• For uncommitted general review: gemini -p "/code-review" --yolo -e code-review
• For branch/commit diffs: pipe git diff into gemini -p
• Security extension name is gemini-cli-security (not security)
• /security:analyze is interactive-only — use -p with a security prompt instead
• Run /security:scan-deps only when security focus is selected AND the diff touches dependency manifest files (see Diff-Aware Optimizations)
• Set timeout: 600000 on the Bash call

Scope mapping for git diff (Gemini has no built-in scope flags):

Running Both

When the user picks "Both" (the default):

1. Run Codex and Gemini in parallel — issue both Bash tool calls in a single response. Both commands are read-only (they review diffs via external APIs) so there is no shared state or git lock contention.
2. Collect both results, then present with clear headers:

## Codex Review (gpt-5.3-codex)
<codex output>

## Gemini Review (gemini-3-pro-preview)
<gemini output>

Summarize where the two reviews agree and differ.

Error Handling

Examples

Both tools (default):

User: /second-opinion
Claude: [asks 4 questions: tool, scope, context, focus]
User: picks "Both", "Branch diff", "Yes include CLAUDE.md", "Security"
Claude: [detects default branch = main]
Claude: [shows diff --stat: 6 files, +103 -15]
Claude: [runs Codex review with security prompt]
Claude: [runs Gemini review with security prompt + dep scan]
Claude: [presents both reviews, highlights agreements/differences]

Codex only with inline args:

User: /second-opinion check uncommitted changes for bugs
Claude: [scope known: uncommitted, focus known: custom]
Claude: [asks 2 questions: tool, project context]
User: picks "Codex only", "No context"
Claude: [shows diff --stat: 3 files, +45 -10]
Claude: [runs codex review --uncommitted with prompt]
Claude: [presents review]

Gemini only:

User: /second-opinion
Claude: [asks 4 questions]
User: picks "Gemini only", "Uncommitted", "No", "General"
Claude: [shows diff --stat: 2 files, +20 -5]
Claude: [runs gemini -p "/code-review" --yolo -e code-review]
Claude: [presents review]

Large diff warning:

User: /second-opinion
Claude: [asks questions] → user picks "Both", "Uncommitted", "General"
Claude: [shows diff --stat: 45 files, +3200 -890]
Claude: "Large diff (3200+ lines). High-effort reasoning will be
  slow. Proceed, or narrow the scope?"
User: "proceed"
Claude: [runs both reviews]