diff --git a/hooks-handlers/post-tool-use.py b/hooks-handlers/post-tool-use.py
index ad48c04..4fa77b9 100755
--- a/hooks-handlers/post-tool-use.py
+++ b/hooks-handlers/post-tool-use.py
@@ -24,6 +24,30 @@ PLUGIN_ROOT = os.environ.get("CLAUDE_PLUGIN_ROOT", str(Path(__file__).parent.par
 BUDDYMON_DIR = Path.home() / ".claude" / "buddymon"
 CATALOG_FILE = Path(PLUGIN_ROOT) / "lib" / "catalog.json"
 
+# Each CC session gets its own state file keyed by process group ID.
+# All hooks within one session share the same PGRP, giving stable per-session state.
+SESSION_KEY = str(os.getpgrp())
+SESSION_FILE = BUDDYMON_DIR / "sessions" / f"{SESSION_KEY}.json"
+
+
+def get_session_state() -> dict:
+    """Read the current session's state file, falling back to global active.json."""
+    session = load_json(SESSION_FILE)
+    if not session:
+        # No session file yet — inherit from global default
+        global_active = load_json(BUDDYMON_DIR / "active.json")
+        session = {
+            "buddymon_id": global_active.get("buddymon_id"),
+            "challenge": global_active.get("challenge"),
+            "session_xp": 0,
+        }
+    return session
+
+
+def save_session_state(state: dict) -> None:
+    SESSION_FILE.parent.mkdir(parents=True, exist_ok=True)
+    save_json(SESSION_FILE, state)
+
 KNOWN_EXTENSIONS = {
     ".py": "Python", ".js": "JavaScript", ".ts": "TypeScript",
     ".jsx": "JavaScript/React", ".tsx": "TypeScript/React",
@@ -62,15 +86,13 @@ def get_state():
 
 
 def add_session_xp(amount: int):
-    active_file = BUDDYMON_DIR / "active.json"
-    roster_file = BUDDYMON_DIR / "roster.json"
-
-    active = load_json(active_file)
-    active["session_xp"] = active.get("session_xp", 0) + amount
-    buddy_id = active.get("buddymon_id")
-    save_json(active_file, active)
+    session = get_session_state()
+    session["session_xp"] = session.get("session_xp", 0) + amount
+    buddy_id = session.get("buddymon_id")
+    save_session_state(session)
 
     if buddy_id:
+        roster_file = BUDDYMON_DIR / "roster.json"
         roster = load_json(roster_file)
         if buddy_id in roster.get("owned", {}):
             roster["owned"][buddy_id]["xp"] = roster["owned"][buddy_id].get("xp", 0) + amount
@@ -153,8 +175,7 @@ def is_starter_chosen():
 
 
 def get_active_buddy_id():
-    active = load_json(BUDDYMON_DIR / "active.json")
-    return active.get("buddymon_id")
+    return get_session_state().get("buddymon_id")
 
 
 def get_active_encounter():
diff --git a/hooks-handlers/session-start.sh b/hooks-handlers/session-start.sh
index 5ecf3fc..fa02855 100755
--- a/hooks-handlers/session-start.sh
+++ b/hooks-handlers/session-start.sh
@@ -7,8 +7,31 @@ source "${PLUGIN_ROOT}/lib/state.sh"
 
 buddymon_init
 
-ACTIVE_ID=$(buddymon_get_active)
-SESSION_XP=$(buddymon_get_session_xp)
+# Per-session state — keyed by process group ID so parallel sessions are isolated.
+SESSION_KEY=$(python3 -c "import os; print(os.getpgrp())")
+SESSION_FILE="${BUDDYMON_DIR}/sessions/${SESSION_KEY}.json"
+mkdir -p "${BUDDYMON_DIR}/sessions"
+
+# Create session file if missing, inheriting buddy from global active.json
+if [[ ! -f "${SESSION_FILE}" ]]; then
+    python3 << PYEOF
+import json, os
+active = {}
+try:
+    active = json.load(open('${BUDDYMON_DIR}/active.json'))
+except Exception:
+    pass
+session_state = {
+    "buddymon_id": active.get("buddymon_id"),
+    "challenge": active.get("challenge"),
+    "session_xp": 0,
+}
+json.dump(session_state, open('${SESSION_FILE}', 'w'), indent=2)
+PYEOF
+fi
+
+ACTIVE_ID=$(python3 -c "import json; d=json.load(open('${SESSION_FILE}')); print(d.get('buddymon_id',''))" 2>/dev/null)
+SESSION_XP=$(python3 -c "import json; d=json.load(open('${SESSION_FILE}')); print(d.get('session_xp',0))" 2>/dev/null)
 
 # Load catalog for buddy display info
 CATALOG="${PLUGIN_ROOT}/lib/catalog.json"
diff --git a/hooks-handlers/session-stop.sh b/hooks-handlers/session-stop.sh
index 47a3cae..0aecef8 100755
--- a/hooks-handlers/session-stop.sh
+++ b/hooks-handlers/session-stop.sh
@@ -6,10 +6,29 @@ source "${PLUGIN_ROOT}/lib/state.sh"
 
 buddymon_init
 
-ACTIVE_ID=$(buddymon_get_active)
-SESSION_XP=$(buddymon_get_session_xp)
+SESSION_KEY=$(python3 -c "import os; print(os.getpgrp())")
+SESSION_FILE="${BUDDYMON_DIR}/sessions/${SESSION_KEY}.json"
+
+ACTIVE_ID=$(python3 -c "
+import json, sys
+try:
+    d = json.load(open('${SESSION_FILE}'))
+    print(d.get('buddymon_id', ''))
+except Exception:
+    print('')
+" 2>/dev/null)
+
+SESSION_XP=$(python3 -c "
+import json, sys
+try:
+    d = json.load(open('${SESSION_FILE}'))
+    print(d.get('session_xp', 0))
+except Exception:
+    print(0)
+" 2>/dev/null)
 
 if [[ -z "${ACTIVE_ID}" ]] || [[ "${SESSION_XP}" -eq 0 ]]; then
+    [[ -f "${SESSION_FILE}" ]] && rm -f "${SESSION_FILE}"
     exit 0
 fi
 
@@ -20,17 +39,20 @@ SUMMARY=$(python3 << PYEOF
 import json, os
 
 catalog_file = '${CATALOG}'
-active_file = '${BUDDYMON_DIR}/active.json'
+session_state_file = '${SESSION_FILE}'
 roster_file = '${BUDDYMON_DIR}/roster.json'
 session_file = '${BUDDYMON_DIR}/session.json'
-encounters_file = '${BUDDYMON_DIR}/encounters.json'
 
 catalog = json.load(open(catalog_file))
-active = json.load(open(active_file))
+session_state = json.load(open(session_state_file))
 roster = json.load(open(roster_file))
-session = json.load(open(session_file))
+session = {}
+try:
+    session = json.load(open(session_file))
+except Exception:
+    pass
 
-buddy_id = active.get('buddymon_id')
+buddy_id = session_state.get('buddymon_id')
 if not buddy_id:
     print('')
     exit()
@@ -39,7 +61,7 @@ b = (catalog.get('buddymon', {}).get(buddy_id)
      or catalog.get('evolutions', {}).get(buddy_id) or {})
 display = b.get('display', buddy_id)
 
-xp_earned = active.get('session_xp', 0)
+xp_earned = session_state.get('session_xp', 0)
 level = roster.get('owned', {}).get(buddy_id, {}).get('level', 1)
 total_xp = roster.get('owned', {}).get(buddy_id, {}).get('xp', 0)
 xp_needed = level * 100
@@ -61,7 +83,7 @@ commits = session.get('commits_this_session', 0)
 tools = session.get('tools_used', 0)
 langs = session.get('languages_seen', [])
 challenge_completed = session.get('challenge_completed', False)
-challenge = active.get('challenge')
+challenge = session_state.get('challenge')
 
 lines = [f"\n## 🐾 Session complete — {display}"]
 lines.append(f"**+{xp_earned} XP earned** this session")
@@ -104,17 +126,10 @@ print('\n'.join(lines))
 PYEOF
 )
 
-# Reset session XP + clear challenge so next session assigns a fresh one
-python3 << PYEOF
-import json
-active_file = '${BUDDYMON_DIR}/active.json'
-active = json.load(open(active_file))
-active['session_xp'] = 0
-active['challenge'] = None
-json.dump(active, open(active_file, 'w'), indent=2)
-PYEOF
+# Clean up this session's state file — each session is ephemeral
+rm -f "${SESSION_FILE}"
 
-# Reset session file
+# Reset shared session.json for legacy compatibility
 buddymon_session_reset
 
 SUMMARY_JSON=$(python3 -c "import json,sys; print(json.dumps(sys.argv[1]))" "${SUMMARY}" 2>/dev/null)
diff --git a/hooks-handlers/user-prompt-submit.py b/hooks-handlers/user-prompt-submit.py
index 3620128..843c4e1 100644
--- a/hooks-handlers/user-prompt-submit.py
+++ b/hooks-handlers/user-prompt-submit.py
@@ -16,6 +16,27 @@ PLUGIN_ROOT = os.environ.get("CLAUDE_PLUGIN_ROOT", str(Path(__file__).parent.par
 BUDDYMON_DIR = Path.home() / ".claude" / "buddymon"
 CATALOG_FILE = Path(PLUGIN_ROOT) / "lib" / "catalog.json"
 
+SESSION_KEY = str(os.getpgrp())
+SESSION_FILE = BUDDYMON_DIR / "sessions" / f"{SESSION_KEY}.json"
+
+
+def get_session_state() -> dict:
+    try:
+        with open(SESSION_FILE) as f:
+            return json.load(f)
+    except Exception:
+        global_active = {}
+        try:
+            with open(BUDDYMON_DIR / "active.json") as f:
+                global_active = json.load(f)
+        except Exception:
+            pass
+        return {
+            "buddymon_id": global_active.get("buddymon_id"),
+            "challenge": global_active.get("challenge"),
+            "session_xp": 0,
+        }
+
 
 def load_json(path):
     try:
@@ -55,9 +76,8 @@ def main():
     enc_data["active_encounter"] = enc
     save_json(enc_file, enc_data)
 
-    # Resolve buddy display name
-    active = load_json(BUDDYMON_DIR / "active.json")
-    buddy_id = active.get("buddymon_id")
+    # Resolve buddy display name from session-specific state
+    buddy_id = get_session_state().get("buddymon_id")
     buddy_display = "your buddy"
     if buddy_id:
         catalog = load_json(CATALOG_FILE)
diff --git a/skills/buddymon/SKILL.md b/skills/buddymon/SKILL.md
index 2b56da9..4fdc6b1 100644
--- a/skills/buddymon/SKILL.md
+++ b/skills/buddymon/SKILL.md
@@ -118,12 +118,57 @@ Greet them and explain the encounter system.
 
 ## `assign <name>` — Assign Buddy
 
+Assignment is **per-session** — each Claude Code window can have its own buddy.
+It writes to the session state file only, not the global default.
+
 Fuzzy-match `<name>` against owned Buddymon (case-insensitive, partial).
 If ambiguous, list matches and ask which.
 If no name given, list roster and ask.
 
-On match, update `active.json` (buddy_id, reset session_xp, set challenge).
-Show challenge proposal with Accept / Decline / Reroll.
+On match, show challenge proposal with Accept / Decline / Reroll, then write:
+
+```python
+import json, os, random
+from pathlib import Path
+
+BUDDYMON_DIR = Path.home() / ".claude" / "buddymon"
+PLUGIN_ROOT = os.environ.get("CLAUDE_PLUGIN_ROOT", "")
+catalog = json.load(open(f"{PLUGIN_ROOT}/lib/catalog.json"))
+
+SESSION_KEY = str(os.getpgrp())
+SESSION_FILE = BUDDYMON_DIR / "sessions" / f"{SESSION_KEY}.json"
+SESSION_FILE.parent.mkdir(parents=True, exist_ok=True)
+
+roster = json.load(open(BUDDYMON_DIR / "roster.json"))
+
+buddy_id = "Debuglin"  # replace with matched buddy id
+buddy_catalog = (catalog.get("buddymon", {}).get(buddy_id)
+                 or catalog.get("evolutions", {}).get(buddy_id) or {})
+challenges = buddy_catalog.get("challenges", [])
+
+# Load or init session state
+try:
+    session_state = json.load(open(SESSION_FILE))
+except Exception:
+    session_state = {}
+
+session_state["buddymon_id"] = buddy_id
+session_state["session_xp"] = 0
+session_state["challenge"] = random.choice(challenges) if challenges else None
+
+json.dump(session_state, open(SESSION_FILE, "w"), indent=2)
+
+# Also update global default so new sessions inherit this assignment
+active = {}
+try:
+    active = json.load(open(BUDDYMON_DIR / "active.json"))
+except Exception:
+    pass
+active["buddymon_id"] = buddy_id
+json.dump(active, open(BUDDYMON_DIR / "active.json", "w"), indent=2)
+```
+
+Show challenge proposal with Accept / Decline / Reroll (updating `session_state["challenge"]` accordingly).
 
 ---