Generate a PROJECT_INDEX.md at the project root that maps the entire codebase for fast session orientation. This eliminates the need for Claude to rediscover project structure each session.

When to Run

•At project start (after initial setup)
•After significant structural changes (new modules, renamed directories, added dependencies)
•If $ARGUMENTS is "update": refresh the existing index rather than regenerating from scratch

What to Capture

Scan the project and build a structured index covering:

1. Project Overview

•Project name and description (from README.md or CLAUDE.md)
•Language/framework (detect from config files: pyproject.toml, package.json, go.mod, Cargo.toml)
•Last indexed date

2. Directory Structure

•ASCII tree of the top-level layout (2 levels deep max)
•Purpose annotation for each major directory

3. Entry Points

•Main executable / entry point files
•CLI commands (if any)
•API routes or server startup (if applicable)

4. Core Modules

For each significant module/package:

•Path: where it lives
•Purpose: one-line description
•Key exports: main functions, classes, or types
•Dependencies: what it imports from other modules

5. Configuration Files

•List all config files with their purpose (linter, formatter, build, CI, etc.)

6. Test Commands

•How to run tests (from Makefile or package scripts)
•Test file locations
•Coverage status (if known from tasks/tests.md)

7. Dependencies

•Core dependencies (from lock files or config)
•Dev dependencies

8. Key Decisions

•Link to tasks/todo.md for architectural decisions
•Link to tasks/lessons.md for accumulated knowledge

Output Format

Write to PROJECT_INDEX.md at the project root:

markdown

# Project Index — {project name}

> Auto-generated by /index. Last updated: {date}
> Read this file at session start for fast project orientation.

## Overview
{language} project — {description}

## Structure
{ascii tree}

## Entry Points
{entry points}

## Modules
{module table}

## Configuration
{config files table}

## How to Run
| Command | What It Does |
|---------|-------------|
| ... | ... |

## Dependencies
{dependency list}

## Key References
- Plan: tasks/todo.md
- Lessons: tasks/lessons.md
- Tests: tasks/tests.md

Rules

•Keep the index concise — this is a map, not documentation. One line per module, not paragraphs.
•Don't include file contents — just paths, purposes, and relationships.
•Skip generated files, build artifacts, and node_modules/venv/target directories.
•If the project is small (<20 files), keep the index to under 100 lines.
•If the project is large (>100 files), focus on the top-level architecture and key modules — don't index every file.
•Update tasks/tests.md coverage summary if test structure has changed.