diff --git a/hooks/commit_msg.sh b/hooks/commit_msg.sh
new file mode 100755
index 0000000..4aa2345
--- /dev/null
+++ b/hooks/commit_msg.sh
@@ -0,0 +1,173 @@
+#!/usr/bin/env bash
+# commit_msg.sh — génère un message de commit depuis les changements en cours
+# Fonctionne avec git (fichiers stagés) et jj (working copy).
+#
+# Usage : bash .claude/hooks/commit_msg.sh
+
+set -euo pipefail
+
+log()  { printf '\033[1;34m==>\033[0m %s\n' "$*" >&2; }
+info() { printf '    \033[0;37m%s\033[0m\n' "$*" >&2; }
+ok()   { printf '    \033[0;32m✓\033[0m %s\n' "$*" >&2; }
+
+# --- Détection du VCS ---
+
+_detect_vcs() {
+    if jj root &>/dev/null; then
+        echo "jj"
+    elif git rev-parse --git-dir &>/dev/null; then
+        echo "git"
+    else
+        echo "none"
+    fi
+}
+
+# --- Fichiers modifiés ---
+
+_changed_files() {
+    case "$VCS" in
+        jj)  jj diff --name-only ;;
+        git) git diff --staged --name-only ;;
+    esac
+}
+
+# --- Diff lisible par fichier ---
+
+_readable_diff() {
+    local file="$1"
+    local raw_diff
+
+    case "$VCS" in
+        jj)  raw_diff=$(jj diff -- "$file") ;;
+        git) raw_diff=$(git diff --staged -- "$file") ;;
+    esac
+
+    [[ -z "$raw_diff" ]] && return 0
+
+    local max_len
+    max_len=$(grep '^[+-]' <<< "$raw_diff" 2>/dev/null \
+        | awk '{ if (length > m) m = length } END { print m+0 }')
+
+    if (( max_len <= 500 )); then
+        printf '%s' "$raw_diff"
+        return
+    fi
+
+    # Pretty-print pour les formats pathologiques (JSON, JS minifié…)
+    local ext="${file##*.}"
+    local pretty_old pretty_new
+
+    case "$ext" in
+        json)
+            case "$VCS" in
+                jj)
+                    pretty_old=$(jj file show "${file}@-" 2>/dev/null | python3 -m json.tool 2>/dev/null || true)
+                    pretty_new=$(jj file show "$file"    2>/dev/null | python3 -m json.tool 2>/dev/null || true)
+                    ;;
+                git)
+                    pretty_old=$(git show "HEAD:${file}" 2>/dev/null | python3 -m json.tool 2>/dev/null || true)
+                    pretty_new=$(git show ":${file}"     2>/dev/null | python3 -m json.tool 2>/dev/null || true)
+                    ;;
+            esac
+            ;;
+        js|mjs|cjs|css|ts)
+            local node_fmt='
+                const chunks = [];
+                process.stdin.on("data", d => chunks.push(d));
+                process.stdin.on("end", () => {
+                    const src = chunks.join("");
+                    const out = src
+                        .replace(/([{(])/g,  "$1\n  ")
+                        .replace(/([;}])/g,  "\n$1\n")
+                        .replace(/,\s*/g,    ",\n  ");
+                    process.stdout.write(out);
+                });'
+            case "$VCS" in
+                jj)
+                    pretty_old=$(jj file show "${file}@-" 2>/dev/null | node -e "$node_fmt" 2>/dev/null || true)
+                    pretty_new=$(jj file show "$file"     2>/dev/null | node -e "$node_fmt" 2>/dev/null || true)
+                    ;;
+                git)
+                    pretty_old=$(git show "HEAD:${file}" 2>/dev/null | node -e "$node_fmt" 2>/dev/null || true)
+                    pretty_new=$(git show ":${file}"     2>/dev/null | node -e "$node_fmt" 2>/dev/null || true)
+                    ;;
+            esac
+            ;;
+        *)
+            case "$VCS" in
+                jj)
+                    pretty_old=$(jj file show "${file}@-" 2>/dev/null | fold -s -w 120 || true)
+                    pretty_new=$(jj file show "$file"     2>/dev/null | fold -s -w 120 || true)
+                    ;;
+                git)
+                    pretty_old=$(git show "HEAD:${file}" 2>/dev/null | fold -s -w 120 || true)
+                    pretty_new=$(git show ":${file}"     2>/dev/null | fold -s -w 120 || true)
+                    ;;
+            esac
+            ;;
+    esac
+
+    if [[ -n "$pretty_old" && -n "$pretty_new" ]]; then
+        diff <(printf '%s\n' "$pretty_old") <(printf '%s\n' "$pretty_new") \
+            --label "a/${file}" --label "b/${file}" -u || true
+    else
+        printf '%s' "$raw_diff"
+    fi
+}
+
+# --- Main ---
+
+VCS=$(_detect_vcs)
+
+if [[ "$VCS" == "none" ]]; then
+    echo "ERREUR : ni git ni jj détecté dans ce répertoire." >&2
+    exit 1
+fi
+
+log "VCS détecté : $VCS"
+
+changed_files=$(_changed_files)
+
+if [[ -z "$changed_files" ]]; then
+    echo "Aucun fichier modifié." >&2
+    exit 1
+fi
+
+file_count=$(wc -l <<< "$changed_files" | tr -d ' ')
+log "Fichiers modifiés : $file_count"
+
+summaries=""
+n=0
+
+while IFS= read -r file; do
+    diff=$(_readable_diff "$file")
+    [[ -z "$diff" ]] && continue
+
+    n=$((n + 1))
+    log "[$n/$file_count] Résumé de $file …"
+
+    summary=$(printf '%s' "$diff" \
+        | aichat "In 2-3 lines, summarise what this diff changes in the file '$file'. Be concise and technical.")
+
+    while IFS= read -r line; do info "$line"; done <<< "$summary"
+
+    summaries+="### $file
+$summary
+
+"
+done <<< "$changed_files"
+
+if [[ -z "$summaries" ]]; then
+    echo "Aucun diff non-vide trouvé." >&2
+    exit 1
+fi
+
+log "Génération du message de commit ($n fichier(s)) …"
+
+result=$(printf '%s' "$summaries" \
+    | aichat "From these per-file summaries of a diff, write a single conventional git commit message in English. First line: short imperative summary (max 72 chars). Then a blank line. Then a short paragraph with more detail if needed. Output only the commit message, nothing else.")
+
+ok "Terminé"
+printf '\n' >&2
+
+printf '%s\n' "$result"
diff --git a/skills/commit-message.md b/skills/commit-message.md
new file mode 100644
index 0000000..cbc9e68
--- /dev/null
+++ b/skills/commit-message.md
@@ -0,0 +1,23 @@
+---
+name: commit-message
+description: Génère un message de commit depuis les changements en cours (git ou jj) via aichat
+---
+
+# Génération de message de commit
+
+Quand l'utilisateur demande de générer un message de commit ou de committer :
+
+1. Lance le script :
+   ```sh
+   bash .claude/hooks/commit_msg.sh
+   ```
+   Le script détecte automatiquement git ou jj, résume chaque fichier modifié via aichat,
+   puis génère un message de commit conventionnel.
+
+2. Présente le message généré à l'utilisateur pour validation.
+
+3. Si l'utilisateur valide, effectue le commit :
+   - **git** : `git commit -m "<message>"`
+   - **jj** : `jj describe -m "<message>"`
+
+Le script écrit sa progression sur stderr et le message final sur stdout.