diff --git a/CLAUDE.md b/CLAUDE.md
index 5d4c1c4..adef195 100644
--- a/CLAUDE.md
+++ b/CLAUDE.md
@@ -1,6 +1,6 @@
 # CLAUDE.md
 
-This file provides guidance to Claude Code (claude.ai/code) when working with code in this repository.
+This file provides guidance to AI coding agents (Claude Code, Codex, Gemini CLI, and similar) when working with code in this repository. The content is intentionally agent-agnostic; only the filename follows the `CLAUDE.md` convention.
 
 ## Project Overview
 
@@ -8,6 +8,31 @@ sgraph is a Python library for working with hierarchic graph structures, typical
 
 The library is maintained by Softagram and is used for building information models about analyzed software. See also [sgraph-mcp-server](https://github.com/softagram/sgraph-mcp-server) for AI agent integration.
 
+## Repository Model (Fork-Based Contribution)
+
+The canonical repository is `softagram/sgraph`. Contributors are expected to work from **personal forks** using the standard fork-and-PR model.
+
+A typical working clone has two remotes:
+
+| Remote | Purpose | Example URL |
+|--------|---------|-------------|
+| `origin` | Your personal fork (push target for feature branches) | `git@github.com:<you>/sgraph.git` |
+| `upstream` | The canonical Softagram repository (source of truth for `main`) | `git@github.com:softagram/sgraph.git` |
+
+**Important for coding agents:** when checking whether a branch's commits are already merged, always compare against `upstream/main`, not just `origin/main`. A contributor's fork may lag behind upstream, so commits that appear "unmerged" relative to `origin/main` can in fact already be live in the canonical repo.
+
+```bash
+# Always fetch both before reasoning about merge state
+git fetch origin main
+git fetch upstream main
+
+# Are this branch's commits in the canonical main?
+git rev-list --left-right --count upstream/main...HEAD
+# → "N 0" means all HEAD commits are already in upstream/main (safe to switch/delete branch)
+```
+
+New contributors: fork `softagram/sgraph` on GitHub, clone your fork as `origin`, and add `softagram/sgraph` as `upstream`. Open PRs from feature branches on your fork against `softagram/sgraph:main`.
+
 ## Development Commands
 
 ### Environment Setup