Gemini

Gemini CLI Skill

Overview

This skill provides guidance for invoking Google's Gemini AI models via the command-line interface in headless (non-interactive) mode. Use Gemini for:

• Comparative analysis or second opinions
• Scenarios where the user explicitly requests Gemini
• Multi-modal analysis (images, audio, video)

Consider gemini-3-pro-preview to be a peer of Opus, and gemini-3-flash-preview to be a peer of Sonnet.

Instructions

Basic Headless Invocation

The Gemini CLI supports headless mode through positional arguments and standard input:

# Prompt via positional argument (preferred)
gemini "Your prompt here"

# With specific model selection
gemini -m gemini-3-pro-preview "Your prompt here"
gemini -m gemini-3-flash-preview "Your prompt here"

# Prompt via stdin
echo "Your prompt here" | gemini

Output Control

# JSON output for parsing
gemini --output-format json "Your prompt here"

# Stream responses (for long outputs)
gemini --output-format stream-json "Your prompt here"

# Save output to file
gemini "Your prompt here" > output.txt

Configuration Options

For complete details on all available configuration options, settings files, and environment variables, see the Gemini CLI Configuration Guide.

Examples

Example 1: Batch Code Analysis

for file in src/*.py; do
    echo "Analyzing $file..."
    result=$(cat "$file" | gemini --output-format json "Find potential bugs and suggest improvements")
    echo "$result" | jq -r '.response' > "reports/$(basename "$file").analysis"
    echo "Completed analysis for $(basename "$file")" >> reports/progress.log
done

Example 2: QA

gemini "Review the newly implemented higher-kinded types (HKTs) for ..."

Example 3: User Simulation and Feedback

# Simulate user perspective on new feature
gemini -m gemini-3-flash-preview \
  --include-directories boundary docs \
  --output-format json \
  "Act as a new user encountering this interface for the first time. What's confusing? What delights you?" \
  > user/feedback/gemini-ux-review-$(date +%Y%m%d).json

# Quick usability check on CLI help text
cat boundary/commands.ss | gemini "Is this help text clear to a beginner? Suggest improvements."

Example 4: Second Opinion/Adversarial Review

# Challenge architectural decisions
gemini -m gemini-3-pro-preview \
  --include-directories core/types \
  "Review this type system design. What edge cases might break it? What performance issues do you foresee?" \
  > docs/peer-review/type-system-critique-$(date +%Y%m%d).md

# Adversarial review of security-sensitive code
gemini --include-directories boundary \
  --output-format json \
  "You are a security auditor. Find vulnerabilities in validate.ss. Consider injection attacks, bypasses, and edge cases." \
  | jq -r '.findings[]' > reports/security-review.txt

Example 5: Tech Debt Report

# Generate comprehensive tech debt inventory
gemini -m gemini-3-pro-preview \
  --include-directories core boundary \
  --output-format json \
  "Analyze this codebase for technical debt. Identify: duplicated code, missing tests, complex functions, outdated patterns, performance bottlenecks, and documentation gaps. Prioritize by impact." \
  > reports/tech-debt-$(date +%Y%m%d).json

# Focus on specific subsystem
gemini --include-directories core/types \
  "Identify refactoring opportunities in this module. What would make it more maintainable?" \
  | tee reports/types-refactor-suggestions.md

Best Practices

1. Use Positional Args: Use gemini "prompt" not the deprecated -p flag
2. Escape Properly: Quote prompts containing special characters or multiple lines
3. Use Pipes: Leverage stdin for complex prompts from files or command output
4. Model Selection by Use Case:
   • QA/code review: Use gemini-3-flash-preview (faster, cheaper for bulk tasks)
   • Deep analysis/architecture: Use gemini-3-pro-preview (more thorough reasoning)
   • General/uncertain: Trust "auto" mode
5. Error Handling: Check exit codes (non-zero = failure) and stderr for API errors
6. Rate Limiting: Be mindful of API quotas in automated scripts
7. JSON Output: When using --output-format json, the response is in the .response field

Piping and Composition

Combine with shell tools for powerful workflows:

# Process file through Gemini, save result
cat input.txt | gemini "Summarize this" > summary.txt

# Chain with other commands
gemini --output-format json "Generate test data in JSON" | jq '.items[]' | while read item; do ...; done

# Use heredoc for multi-line prompts
gemini "$(cat << 'EOF'
Your multi-line
prompt here
EOF
)"

QA Reviews

For code review tasks, use gemini-3-flash-preview (faster, cheaper for bulk reviews). Reserve gemini-3-pro-preview for architectural design reviews or complex analysis.

# Code review - use flash model
gemini -m gemini-3-flash-preview \
  --include-directories <target-dir> \
  "Review this code for bugs, logic errors, edge cases. Report findings with file:line format."

Integration with The Fold

When using Gemini within The Fold context:

1. Complement, don't replace: Use Gemini Pro for second opinions on designs, or as a subagent to handle parallel workstreams
2. Document results: Save Gemini review outputs to docs/peer-review/ for reference
3. Forum posts: Share interesting Gemini insights via (msg 'channel ...)
4. Validation: Cross-check critical Gemini outputs with core validation logic

IMPORTANT: Model Version Reminder

ALWAYS use Gemini 3 models, NOT Gemini 2.5:

Gemini 3 models catch significantly more issues in QA reviews. If you find yourself typing "2.5", stop and use "3" instead.

# CORRECT
gemini -m gemini-3-flash-preview "QA review..."

# WRONG - DO NOT USE
gemini -m gemini-2.5-flash "QA review..."  # NO!

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