diff --git a/pkg/memory/jsonl.go b/pkg/memory/jsonl.go
index e12e2c5ab3..afe374166c 100644
--- a/pkg/memory/jsonl.go
+++ b/pkg/memory/jsonl.go
@@ -86,14 +86,14 @@ func (s *JSONLStore) metaPath(key string) string {
 
 // sanitizeKey converts a session key to a safe filename component.
 // Mirrors pkg/session.sanitizeFilename so that migration paths match.
-//
-// Note: this is a lossy mapping — "telegram:123" and "telegram_123"
-// both produce the same filename. This is an intentional tradeoff:
-// keys with colons (e.g. from channels) are by far the common case,
-// and a bidirectional encoding (like URL-encoding) would complicate
-// file listings and debugging.
+// Replaces ':' with '_' (session key separator) and '/' and '\' with '_'
+// so composite IDs (e.g. Telegram forum "chatID/threadID", Slack "channel/thread_ts")
+// do not create subdirectories or break on Windows.
 func sanitizeKey(key string) string {
-	return strings.ReplaceAll(key, ":", "_")
+	s := strings.ReplaceAll(key, ":", "_")
+	s = strings.ReplaceAll(s, "/", "_")
+	s = strings.ReplaceAll(s, "\\", "_")
+	return s
 }
 
 // readMeta loads the metadata file for a session.
diff --git a/pkg/session/manager.go b/pkg/session/manager.go
index 07f981df17..a31dbd55c9 100644
--- a/pkg/session/manager.go
+++ b/pkg/session/manager.go
@@ -146,12 +146,15 @@ func (sm *SessionManager) TruncateHistory(key string, keepLast int) {
 }
 
 // sanitizeFilename converts a session key into a cross-platform safe filename.
-// Session keys use "channel:chatID" (e.g. "telegram:123456") but ':' is the
-// volume separator on Windows, so filepath.Base would misinterpret the key.
-// We replace it with '_'. The original key is preserved inside the JSON file,
-// so loadSessions still maps back to the right in-memory key.
+// Replaces ':' with '_' (session key separator) and '/' and '\' with '_' so
+// composite IDs (e.g. Telegram forum "chatID/threadID") do not create
+// subdirectories or break on Windows. The original key is preserved inside
+// the JSON file, so loadSessions still maps back to the right in-memory key.
 func sanitizeFilename(key string) string {
-	return strings.ReplaceAll(key, ":", "_")
+	s := strings.ReplaceAll(key, ":", "_")
+	s = strings.ReplaceAll(s, "/", "_")
+	s = strings.ReplaceAll(s, "\\", "_")
+	return s
 }
 
 func (sm *SessionManager) Save(key string) error {
@@ -162,10 +165,9 @@ func (sm *SessionManager) Save(key string) error {
 	filename := sanitizeFilename(key)
 
 	// filepath.IsLocal rejects empty names, "..", absolute paths, and
-	// OS-reserved device names (NUL, COM1 … on Windows).
-	// The extra checks reject "." and any directory separators so that
-	// the session file is always written directly inside sm.storage.
-	if filename == "." || !filepath.IsLocal(filename) || strings.ContainsAny(filename, `/\`) {
+	// OS-reserved device names (NUL, COM1 … on Windows). sanitizeFilename
+	// already replaced '/' and '\' with '_', so no subdirs are created.
+	if filename == "." || !filepath.IsLocal(filename) {
 		return os.ErrInvalid
 	}
 
diff --git a/pkg/session/manager_test.go b/pkg/session/manager_test.go
index 5ef5f4349c..bc56159668 100644
--- a/pkg/session/manager_test.go
+++ b/pkg/session/manager_test.go
@@ -17,6 +17,7 @@ func TestSanitizeFilename(t *testing.T) {
 		{"slack:C01234", "slack_C01234"},
 		{"no-colons-here", "no-colons-here"},
 		{"multiple:colons:here", "multiple_colons_here"},
+		{"agent:main:telegram:group:-1003822706455/12", "agent_main_telegram_group_-1003822706455_12"},
 	}
 
 	for _, tt := range tests {
@@ -64,11 +65,21 @@ func TestSave_RejectsPathTraversal(t *testing.T) {
 	tmpDir := t.TempDir()
 	sm := NewSessionManager(tmpDir)
 
-	badKeys := []string{"", ".", "..", "foo/bar", "foo\\bar"}
+	// Invalid names that must still be rejected.
+	badKeys := []string{"", ".", ".."}
 	for _, key := range badKeys {
 		sm.GetOrCreate(key)
 		if err := sm.Save(key); err == nil {
 			t.Errorf("Save(%q) should have failed but didn't", key)
 		}
 	}
+
+	// Keys containing path separators are sanitized (no subdirs created).
+	sm.GetOrCreate("foo/bar")
+	if err := sm.Save("foo/bar"); err != nil {
+		t.Fatalf("Save(\"foo/bar\") after sanitize should succeed: %v", err)
+	}
+	if _, err := os.Stat(filepath.Join(tmpDir, "foo_bar.json")); os.IsNotExist(err) {
+		t.Errorf("expected foo_bar.json in storage (sanitized from foo/bar)")
+	}
 }