diff --git a/server.go b/server.go
index 44545fe..b3d0732 100644
--- a/server.go
+++ b/server.go
@@ -279,6 +279,8 @@ func New(sink *olog.Sink, opts ...ServerOption) *Server {
 			return storeWrite(s.transcriptStore, name, data)
 		},
 		OnRename: func(oldID, newID string) {
+			s.RemoveGroup("agent", oldID)
+			s.AddGroup("agent", newID)
 			oldPrefix := "/s/" + oldID
 			newPrefix := "/s/" + newID
 			for _, c := range s.conns {
@@ -326,15 +328,14 @@ func (s *Server) InGroup(group, user string) bool {
 }
 
 // fileOwnerGroup returns the uid and gid for a given path.
-// Session files are owned by the session ID with group "agent".
+// Session files are owned by the session's principal with group "agent".
 // Everything else is owned by "ollie" with group "ollie".
 func (s *Server) fileOwnerGroup(path string) (uid, gid string) {
 	if strings.HasPrefix(path, "/s/") {
 		parts := strings.SplitN(strings.TrimPrefix(path, "/s/"), "/", 2)
 		if len(parts) >= 1 && parts[0] != "new" {
-			// Check if parts[0] is a session (not a script like sh, bfg, etc.)
 			if sess := s.sessionStore.Session(parts[0]); sess != nil {
-				return parts[0], "agent"
+				return sess.Uname(), "agent"
 			}
 		}
 	}
@@ -771,9 +772,9 @@ func (s *Server) attach(cs *connState, fc *plan9.Fcall) *plan9.Fcall {
 	qid := plan9.Qid{Type: QTDir, Path: 0}
 	cs.fids[fc.Fid] = &fid{path: "/", qid: qid}
 	s.log.Debug("Tattach uname=%q", fc.Uname)
-	// Assign group membership based on whether uname is a session ID.
+	// Assign group membership based on whether uname is a session principal.
 	if fc.Uname != "" {
-		if sess := s.sessionStore.Session(fc.Uname); sess != nil {
+		if s.sessionStore.SessionByUname(fc.Uname) != nil {
 			s.AddGroup("agent", fc.Uname)
 		} else {
 			s.AddGroup("user", fc.Uname)
diff --git a/store/session.go b/store/session.go
index c7ef325..87a0a61 100644
--- a/store/session.go
+++ b/store/session.go
@@ -7,6 +7,7 @@ import (
 	"sort"
 	"strings"
 	"sync"
+	"sync/atomic"
 
 	"ollie/pkg/agent"
 	"ollie/pkg/backend"
@@ -21,6 +22,7 @@ import (
 type Session struct {
 	mu         sync.RWMutex
 	id         string
+	uname      string // immutable user principal (numeric UID), set at creation
 	Core       agent.Core
 	Ctx        context.Context
 	cancel     context.CancelFunc
@@ -37,6 +39,7 @@ func NewSession(id string, core agent.Core, ctx context.Context, cancel context.
 }
 
 func (sess *Session) RunnableID() string { return sess.id }
+func (sess *Session) Uname() string     { return sess.uname }
 
 func (sess *Session) Cancel() {
 	sess.cancel()
@@ -120,9 +123,15 @@ type SessionStoreConfig struct {
 // SessionStore implements Store for session management.
 type SessionStore struct {
 	*storeConfig
-	cfg      SessionStoreConfig
-	mu       sync.RWMutex
-	sessions map[string]*Session
+	cfg       SessionStoreConfig
+	mu        sync.RWMutex
+	sessions  map[string]*Session
+	nextUID   atomic.Uint32 // incrementing principal counter
+}
+
+// nextUname generates the next uname atomically.
+func (s *SessionStore) nextUname() string {
+	return fmt.Sprintf("%d", s.nextUID.Add(1))
 }
 
 func NewSessionStore(cfg SessionStoreConfig) *SessionStore {
@@ -136,6 +145,7 @@ func NewSessionStore(cfg SessionStoreConfig) *SessionStore {
 		cfg:      cfg,
 		sessions: make(map[string]*Session),
 	}
+	ss.nextUID.Store(9999)
 	ss.storeConfig = &storeConfig{
 		StatFn:   ss.stat,
 		ListFn:   ss.list,
@@ -236,6 +246,18 @@ func (s *SessionStore) Session(id string) *Session {
 	return s.sessions[id]
 }
 
+// SessionByUname returns the session with the given uname (principal), or nil.
+func (s *SessionStore) SessionByUname(uname string) *Session {
+	s.mu.RLock()
+	defer s.mu.RUnlock()
+	for _, sess := range s.sessions {
+		if sess.uname == uname {
+			return sess
+		}
+	}
+	return nil
+}
+
 // OpenStore returns a RunnableStore for the given session ID.
 func (s *SessionStore) OpenStore(id string) (RunnableStore, error) {
 	if sess := s.Session(id); sess != nil {
@@ -358,6 +380,7 @@ func (s *SessionStore) createSession(args []string) error {
 
 	var core agent.Core
 	var allowTools []string
+	uname := s.nextUname()
 	if s.cfg.NewCore != nil {
 		var err error
 		core, err = s.cfg.NewCore(sessID, agentName, cwd)
@@ -393,6 +416,7 @@ func (s *SessionStore) createSession(args []string) error {
 		}
 
 		var execOpts []execute.Option
+		execOpts = append(execOpts, execute.WithMount())
 		if s.cfg.Strict {
 			execOpts = append(execOpts, execute.WithStrict())
 		}
@@ -409,7 +433,7 @@ func (s *SessionStore) createSession(args []string) error {
 			"execute": execute.Decl(cwd, execOpts...),
 		})
 
-		env := agent.BuildAgentEnv(cfg, newDisp(), cwd, []string{"OLLIE_SESSION_ID=" + sessID})
+		env := agent.BuildAgentEnv(cfg, newDisp(), cwd, []string{"OLLIE_SESSION_ID=" + sessID, "OLLIE_UNAME=" + uname})
 
 		core = agent.NewAgentCore(agent.AgentCoreConfig{
 			Backend:       be,
@@ -417,6 +441,7 @@ func (s *SessionStore) createSession(args []string) error {
 			AgentsDir:     s.cfg.AgentsDir,
 			SessionsDir:   s.cfg.SessionsDir,
 			SessionID:     sessID,
+			Uname:         uname,
 			CWD:           cwd,
 			Env:           env,
 			NewDispatcher: newDisp,
@@ -429,6 +454,7 @@ func (s *SessionStore) createSession(args []string) error {
 	sess.AllowTools = allowTools
 
 	s.mu.Lock()
+	sess.uname = uname
 	s.sessions[sessID] = sess
 	s.mu.Unlock()