Tool for AI-powered code review, wrapping around git diff

aireview.sh

#!/usr/bin/env bash
# An original project of Bill Mill (https://billmill.org/):
# See: https://notes.billmill.org/blog/2025/07/An_AI_tool_I_find_useful.html
#      https://github.com/llimllib/personal_code/blob/master/homedir/.local/bin/review
# LICENSE: unlicense. This is free and unencumbered software released into the public domain.
# see unlicense.org for full license

set -euo pipefail

# Configuration
MAX_TOKENS=${AIREVIEW_MAX_TOKENS:-50000}
DEFAULT_CONTEXT=${AIREVIEW_DEFAULT_CONTEXT:-10}

function usage {
  cat <<EOF
aireview [--verbose] [--context TEXT|-] [--help] [git-diff-arguments...]

Ask an LLM to review code changes. This tool passes arguments directly to 'git diff',
allowing you to use any git diff syntax or options.

Options:
  -c, --context TEXT  Add additional context for the review, appended to the system prompt. Will be concatenated if provided multiple times
      --context -     Read additional context from stdin
  -h, --help          Show this help message
  -v, --verbose       Enable verbose output

Review Examples:
  # Review unstaged changes
  aireview

  # Review with additional context
  aireview --context "Focus your review on possible authentication bypasses"

  # Review with context from stdin
  cat PR_DESCRIPTION.md | aireview --context -

  # Review with context from a command
  git log -1 --pretty=%B | aireview --context -

  # Review staged changes
  aireview --cached

  # Review changes between HEAD and main
  aireview main

  # Review changes between two branches
  aireview main feature-branch
  # OR
  aireview main..feature-branch

  # Review only changes since branch diverged from main
  aireview main...feature-branch

  # Review a remote branch
  aireview origin/main..origin/feature-branch

  # Limit review to specific files
  aireview main -- src/components/

  # Adjust context lines
  aireview -U5 main

Dot Notation:
  - Two dots (A..B): Direct comparison between A and B
  - Three dots (A...B): Compare common ancestor of A and B with B

Depends on:
- llm: https://github.com/simonw/llm
- bat: https://github.com/sharkdp/bat (optional)
EOF
  exit "${1:-0}"
}

git_args=()
has_unified_context=false
context_value=$DEFAULT_CONTEXT
additional_context=""

readonly RED='\033[0;31m'
readonly BLUE='\033[0;34m'
readonly RESET='\033[0m'

info() {
  printf "${BLUE}• %s${RESET}\n" "$1" >&2
}

error() {
  printf "${RED}❌ %s${RESET}\n" "$1" >&2
  exit 1
}

usage_error() {
  printf "${RED}❌ %s${RESET}\n" "$1" >&2
  usage 1
}

estimate_tokens() {
  local content="$1"
  local chars=${#content}
  
  # Code tends to have more tokens per character than prose
  local chars_per_token=3
  echo $((chars / chars_per_token))
}

find_optimal_context() {
  local min_context=1
  local max_context="$context_value"
  local optimal_context=$min_context
  
  while [[ $min_context -le $max_context ]]; do
    local test_context=$(( (min_context + max_context) / 2 ))
    
    # Replace unified context in git args for testing
    local test_git_args=()
    for arg in "${git_args[@]}"; do
      if [[ "$arg" =~ ^-U[0-9]+$ ]]; then
        test_git_args+=("-U$test_context")
      elif [[ "$arg" =~ ^--unified=[0-9]+$ ]]; then
        test_git_args+=("--unified=$test_context")
      else
        test_git_args+=("$arg")
      fi
    done
    
    # Test this context level
    local test_diff_output
    test_diff_output=$(git diff "${test_git_args[@]}" 2>/dev/null || error "Git diff command failed during context optimization")
    local test_tokens
    test_tokens=$(estimate_tokens "$test_diff_output")
    
    if [[ $test_tokens -le $MAX_TOKENS ]]; then
      optimal_context=$test_context
      min_context=$((test_context + 1))
    else
      max_context=$((test_context - 1))
    fi
  done
  
  echo $optimal_context
}

# Process only our custom arguments, pass everything else to git
while [[ $# -gt 0 ]]; do
  case "$1" in
    -v|--verbose)
      set -x
      shift
      ;;
    -c|--context)
      shift
      if [[ $# -gt 0 ]]; then
        # Read from stdin for context
        if [[ "$1" == "-" ]]; then
          if [[ -t 0 ]]; then
            usage_error "No stdin input available for --context -"
          fi
          new_context=$(cat)
          if [[ -z "$new_context" ]]; then
            usage_error "Empty input from stdin for --context -"
          fi
        else
          new_context=$1
        fi
        if [[ -n "$additional_context" ]]; then
          additional_context="${additional_context}

$new_context"
        else
          additional_context="$new_context"
        fi
        shift
      else
        usage_error "Missing value for --context option"
      fi
      ;;
    -U[0-9]*)
      # Form: -U10
      has_unified_context=true
      context_value="${1#-U}"
      git_args+=("$1")
      shift
      ;;
    -U)
      # Form: -U 10
      has_unified_context=true
      shift
      if [[ $# -gt 0 && "$1" =~ ^[0-9]+$ ]]; then
        context_value="$1"
        # normalize to `-U10` to ease our checking later on
        git_args+=("-U$1")
        shift
      else
        usage_error "Missing value for -U option"
      fi
      ;;
    --unified=*)
      # Form: --unified=10
      has_unified_context=true
      context_value="${1#--unified=}"
      git_args+=("$1")
      shift
      ;;
    -h|--help)
      usage
      ;;
    *)
      # Store all other arguments to pass to git diff
      git_args+=("$1")
      shift
      ;;
  esac
done

if ! command -v llm >/dev/null 2>&1; then
  error "Missing required command llm. On mac: brew install llm"
fi

# Default unified context if none specified. The idea here is to increase the
# context (git defaults to 3 lines) so that the LLM has more context for its
# review. Later on we'll check if this generates too much output and shorten it
# if so
if [[ "$has_unified_context" == false ]]; then
  git_args=("-U$context_value" "${git_args[@]}")
fi

# Run git diff
if [[ ${#git_args[@]} -gt 0 ]]; then
  diff_output=$(git diff "${git_args[@]}" 2>/dev/null || error "Git diff command failed. Check your arguments.")
else
  diff_output=$(git diff 2>/dev/null || error "Git diff command failed. Check your arguments.")
fi

if [[ -z "$diff_output" ]]; then
  error "No changes found to review."
fi

# Estimate token count and reduce context if needed
estimated_tokens=$(estimate_tokens "$diff_output")

if [[ $estimated_tokens -gt $MAX_TOKENS ]]; then
  info "Optimizing context to fit token limits"
  
  # Find optimal context using binary search
  optimal_context=$(find_optimal_context)
  
  info "Using context of $optimal_context lines"

  # Replace unified context in git args
  new_git_args=()
  for arg in "${git_args[@]}"; do
    if [[ "$arg" =~ ^-U[0-9]+$ ]]; then
      new_git_args+=("-U$optimal_context")
    elif [[ "$arg" =~ ^--unified=[0-9]+$ ]]; then
      new_git_args+=("--unified=$optimal_context")
    else
      new_git_args+=("$arg")
    fi
  done

  # Re-run git diff with optimal context
  new_diff_output=$(git diff "${new_git_args[@]}" 2>/dev/null || error "Git diff command failed with reduced context.")
  new_estimated_tokens=$(estimate_tokens "$new_diff_output")

  if [[ $new_estimated_tokens -gt $MAX_TOKENS ]]; then
    error "Changes are too large even with minimal context. Try reviewing fewer files."
  fi

  diff_output="$new_diff_output"
fi

prompt="Please review this PR as if you were a senior engineer.

## Focus Areas
- Architecture and design decisions
- Potential bugs and edge cases
- Performance considerations
- Security implications
- Code maintainability and best practices
- Test coverage

## Review Format
- Start with a brief summary of the PR purpose and changes
- List strengths of the implementation
- Identify issues and improvement opportunities (ordered by priority)
- Provide specific code examples for suggested changes where applicable

Please be specific, constructive, and actionable in your feedback. Output the review in markdown format."

# Add the additional context if provided
if [[ -n "$additional_context" ]]; then
  prompt="$prompt

## Additional Context
$additional_context"
fi

if command -v bat >/dev/null 2>&1; then
  echo "$diff_output" | llm -s "$prompt" | bat --paging=never --style=plain --language=markdown
else
  echo "$diff_output" | llm -s "$prompt"
fi
exit_status=${PIPESTATUS[1]}

if [[ "$exit_status" -eq 130 ]]; then
  # User pressed Ctrl+C, exit silently
  exit 130
elif [[ "$exit_status" -ne 0 ]]; then
  echo "Error: LLM command failed." >&2
  exit 1
fi