diff --git a/bin/kei b/bin/kei
index 1169f30..2d6c6bc 100755
--- a/bin/kei
+++ b/bin/kei
@@ -8,6 +8,7 @@
 #   kei                     # splash → claude (interactive REPL)
 #   kei --no-splash         # skip splash → exec claude
 #   kei --status            # status only, don't launch claude
+#   kei message ...         # inter-session mailbox (send/inbox/list) — see kei-message.sh
 #   kei [args...]           # splash → claude args... (forwarded verbatim)
 #
 # The splash shows: substrate version, agent count, last sleep run,
@@ -17,6 +18,15 @@
 
 set -e
 
+# --- subcommand dispatch (before splash) ---------------------------------
+# `kei message ...` → the mailbox CLI; everything else falls through to launch.
+case "${1:-}" in
+  message|msg|m)
+    shift
+    exec "$HOME/.claude/scripts/kei-message.sh" "$@"
+    ;;
+esac
+
 # --- args ----------------------------------------------------------------
 SPLASH=1
 STATUS_ONLY=0
diff --git a/hooks/mailbox-inject.sh b/hooks/mailbox-inject.sh
new file mode 100755
index 0000000..b5ca36d
--- /dev/null
+++ b/hooks/mailbox-inject.sh
@@ -0,0 +1,57 @@
+#!/bin/sh
+# mailbox-inject — pull-inbox for kei-message. On every UserPromptSubmit, inject
+# any messages addressed to THIS session (by cwd-basename or the broadcast
+# channel "all") that arrived since last turn, into the session context, so
+# Claude sees what other sessions sent. Per-session read cursor dedups; first
+# turn starts fresh (no history dump). Never blocks (always exit 0).
+# Event: UserPromptSubmit. Bypass: KEI_MAILBOX_BYPASS=1.
+
+[ "${KEI_MAILBOX_BYPASS:-}" = "1" ] && exit 0
+command -v jq >/dev/null 2>&1 || exit 0
+
+INPUT=$(cat)
+SID=$(printf '%s' "$INPUT" | jq -r '.session_id // empty' 2>/dev/null)
+CWD=$(printf '%s' "$INPUT" | jq -r '.cwd // empty' 2>/dev/null)
+[ -n "$CWD" ] || CWD="$PWD"
+me="$(basename "$CWD")"
+
+MBOX="$HOME/.claude/mailbox"
+LOG="$MBOX/messages.jsonl"
+mkdir -p "$MBOX"
+CUR="$MBOX/.cursor-${SID:-$me}"
+
+# Highest id currently in the bus (0 if the log doesn't exist yet / is empty).
+if [ -f "$LOG" ]; then
+  maxid=$(jq -s 'map(.id) | max // 0' "$LOG" 2>/dev/null || echo 0)
+else
+  maxid=0
+fi
+[ -n "$maxid" ] || maxid=0
+
+# First fire for this session: record baseline cursor, show nothing. Done even
+# when the bus is still empty — so messages that arrive AFTER this point (but
+# before the session's next turn) are not missed.
+if [ ! -f "$CUR" ]; then
+  echo "$maxid" > "$CUR"
+  exit 0
+fi
+
+# Nothing to read yet.
+[ -f "$LOG" ] || { echo "$maxid" > "$CUR"; exit 0; }
+
+last=$(cat "$CUR" 2>/dev/null || echo 0)
+case "$last" in ''|*[!0-9]*) last=0 ;; esac
+
+new=$(jq -r --argjson last "$last" --arg me "$me" '
+  select(.id > $last)
+  | select(.to == $me or .to == "all")
+  | select(.from != $me)
+  | "  • \(.from) -> \(.to): \(.body)"' "$LOG" 2>/dev/null)
+
+# Advance cursor past everything seen this turn.
+echo "$maxid" > "$CUR"
+
+if [ -n "$new" ]; then
+  printf '[kei mailbox] new message(s) for this session (%s):\n%s\n  (reply: kei message send --to <name> "...")\n' "$me" "$new"
+fi
+exit 0
diff --git a/install/lib-scaffold.sh b/install/lib-scaffold.sh
index 0903c86..272d97d 100644
--- a/install/lib-scaffold.sh
+++ b/install/lib-scaffold.sh
@@ -66,20 +66,19 @@ copy_sleep_scripts() {
   fi
 }
 
-# KeiSei tamagotchi statusline — copy the renderer + state updater into
-# ~/.claude/scripts/. Zero binary deps (pure bash, state under ~/.claude/pet/),
-# always available regardless of profile. The statusLine + pet-update hooks
-# are wired into settings.json by the settings-snippet merge (lib-hooks.sh).
+# Pure-bash scripts → ~/.claude/scripts/ (tamagotchi renderer + state updater,
+# kei-message mailbox CLI, any future scripts). Zero binary deps, always
+# available regardless of profile. statusLine + pet-update + mailbox-inject
+# hooks are wired into settings.json by the settings-snippet merge (lib-hooks.sh).
 copy_pet_scripts() {
-  local pet_sh src dst="$HOME_DIR/.claude/scripts"
+  local src dst="$HOME_DIR/.claude/scripts" name
   [ -d "$KIT_DIR/scripts" ] || return 0
   mkdir -p "$dst"
-  for pet_sh in keisei-pet.sh keisei-pet-update.sh; do
-    src="$KIT_DIR/scripts/$pet_sh"
-    if [ -f "$src" ]; then
-      cp -f "$src" "$dst/$pet_sh"
-      chmod +x "$dst/$pet_sh"
-    fi
+  for src in "$KIT_DIR/scripts/"*.sh; do
+    [ -f "$src" ] || continue
+    name="$(basename "$src")"
+    cp -f "$src" "$dst/$name"
+    chmod +x "$dst/$name"
   done
 }
 
diff --git a/scripts/kei-message.sh b/scripts/kei-message.sh
new file mode 100755
index 0000000..4e56c5a
--- /dev/null
+++ b/scripts/kei-message.sh
@@ -0,0 +1,62 @@
+#!/bin/sh
+# kei-message — minimal persistent mailbox so ANY Claude Code session can message
+# ANY other (not just Agent-Teams teammates). Append-only jsonl bus; the
+# mailbox-inject.sh UserPromptSubmit hook pulls unread into each session's
+# context per turn. Identity = basename of the session's cwd (or --from/--to a
+# name), plus the broadcast channel "all".
+#
+#   kei message send [--to <name|all>] [--from <name>] <text...>
+#   kei message inbox            # messages addressed to me (cwd) or all
+#   kei message list             # whole bus (recent)
+#   kei message channels         # known recipient names
+#
+# Store: ~/.claude/mailbox/messages.jsonl  (one JSON object per line)
+
+set -eu
+command -v jq >/dev/null 2>&1 || { echo "kei message: jq required" >&2; exit 1; }
+
+MBOX="$HOME/.claude/mailbox"
+LOG="$MBOX/messages.jsonl"
+mkdir -p "$MBOX"
+[ -f "$LOG" ] || : > "$LOG"
+
+me="$(basename "$PWD")"
+cmd="${1:-inbox}"
+[ $# -gt 0 ] && shift || true
+
+case "$cmd" in
+  send)
+    to="all"; body=""
+    while [ $# -gt 0 ]; do
+      case "$1" in
+        --to)   to="$2";   shift; shift ;;
+        --from) me="$2";   shift; shift ;;
+        --)     shift; body="$body $*"; break ;;
+        *)      body="$body $1"; shift ;;
+      esac
+    done
+    body="${body# }"
+    [ -n "$body" ] || { echo "usage: kei message send [--to <name|all>] <text>" >&2; exit 1; }
+    id="$(date +%s)$(date +%N 2>/dev/null | cut -c1-6 || printf '000000')"
+    jq -cn --argjson id "$id" --arg ts "$(date -u +%FT%TZ)" \
+           --arg from "$me" --arg to "$to" --arg body "$body" \
+       '{id:$id, ts:$ts, from:$from, to:$to, body:$body}' >> "$LOG"
+    echo "-> sent to '$to' (from '$me')"
+    ;;
+  inbox|read)
+    while [ $# -gt 0 ]; do case "$1" in --me) me="$2"; shift; shift ;; *) shift ;; esac; done
+    jq -r --arg me "$me" '
+      select(.to==$me or .to=="all")
+      | "[\(.ts|sub("T";" ")|sub("Z";""))] \(.from) -> \(.to): \(.body)"' "$LOG" | tail -20
+    ;;
+  list|all)
+    jq -r '"[\(.ts|sub("T";" ")|sub("Z";""))] \(.from) -> \(.to): \(.body)"' "$LOG" | tail -40
+    ;;
+  channels|names|who)
+    jq -r '.to, .from' "$LOG" 2>/dev/null | sort -u | grep -v '^$' || true
+    ;;
+  *)
+    echo "kei message: send [--to <name|all>] <text> | inbox | list | channels" >&2
+    exit 1
+    ;;
+esac
diff --git a/settings-snippet.json b/settings-snippet.json
index 16d55ab..457632e 100644
--- a/settings-snippet.json
+++ b/settings-snippet.json
@@ -241,6 +241,11 @@
           {
             "type": "command",
             "command": "~/.claude/scripts/keisei-pet-update.sh prompt"
+          },
+          {
+            "type": "command",
+            "command": "~/.claude/hooks/mailbox-inject.sh",
+            "statusMessage": "kei mailbox pull-inbox..."
           }
         ]
       }