diff --git a/internal/session/session_dir_test.go b/internal/session/session_dir_test.go
new file mode 100644
index 00000000..764bfceb
--- /dev/null
+++ b/internal/session/session_dir_test.go
@@ -0,0 +1,70 @@
+package session
+
+import (
+	"strings"
+	"testing"
+)
+
+// TestEncodeCwdForDir verifies the working-directory → session-directory
+// name encoding strips characters that are illegal on Windows (notably the
+// drive-letter colon, see issue #18) while preserving the previous output
+// for the typical Unix paths.
+func TestEncodeCwdForDir(t *testing.T) {
+	tests := []struct {
+		name string
+		cwd  string
+		want string
+	}{
+		{
+			name: "unix absolute path",
+			cwd:  "/home/user/proj",
+			want: "home--user--proj",
+		},
+		{
+			name: "unix relative path",
+			cwd:  "proj/sub",
+			want: "proj--sub",
+		},
+		{
+			name: "windows drive root",
+			cwd:  `C:\test`,
+			want: "C--test",
+		},
+		{
+			name: "windows nested path",
+			cwd:  `C:\Users\User\code`,
+			want: "C--Users--User--code",
+		},
+		{
+			name: "windows secondary drive",
+			cwd:  `S:\work\repo`,
+			want: "S--work--repo",
+		},
+		{
+			name: "windows mixed separators",
+			cwd:  `C:\Users/User\code`,
+			want: "C--Users--User--code",
+		},
+		{
+			name: "windows other illegal chars stripped",
+			cwd:  `C:\a<b>c|d?e*f"g`,
+			want: "C--abcdefg",
+		},
+	}
+
+	for _, tc := range tests {
+		t.Run(tc.name, func(t *testing.T) {
+			got := encodeCwdForDir(tc.cwd)
+			if got != tc.want {
+				t.Errorf("encodeCwdForDir(%q) = %q, want %q", tc.cwd, got, tc.want)
+			}
+			// Encoded directory must never contain characters that are
+			// illegal in Windows directory names.
+			for _, bad := range []string{":", "<", ">", "\"", "|", "?", "*", "\\", "/"} {
+				if strings.Contains(got, bad) {
+					t.Errorf("encodeCwdForDir(%q) = %q contains illegal char %q", tc.cwd, got, bad)
+				}
+			}
+		})
+	}
+}
diff --git a/internal/session/tree_manager.go b/internal/session/tree_manager.go
index a8914e02..b3959846 100644
--- a/internal/session/tree_manager.go
+++ b/internal/session/tree_manager.go
@@ -1350,15 +1350,44 @@ func (tm *TreeManager) buildTreeNodeDepth(id string, depth int, visited map[stri
 // --- Path conventions ---
 
 // DefaultSessionDir returns the default session storage directory for a cwd.
-// Convention: ~/.kit/sessions/--<cwd-path>--/
+// Convention: ~/.kit/sessions/<encoded-cwd>, where path separators are
+// encoded as "--" with no leading or trailing dashes — e.g.
+// /home/user/proj becomes home--user--proj. See encodeCwdForDir for the
+// full encoding rules (including Windows path handling).
 func DefaultSessionDir(cwd string) string {
 	home, err := os.UserHomeDir()
 	if err != nil {
 		home = "."
 	}
-	// Convert path separators to double dashes.
-	safeCwd := strings.ReplaceAll(cwd, string(filepath.Separator), "--")
+	return filepath.Join(home, ".kit", "sessions", encodeCwdForDir(cwd))
+}
+
+// encodeCwdForDir converts a working-directory path into a single, filesystem-
+// safe directory name. Path separators are replaced with double dashes and
+// characters that are illegal in Windows directory names — most importantly
+// the colon that follows the drive letter (e.g. `C:\foo` → `C--foo`) — are
+// stripped. The result is identical to the previous Unix-only encoding for
+// paths that do not contain such characters, so existing session directories
+// are preserved.
+func encodeCwdForDir(cwd string) string {
+	// Convert both `/` and `\` to double dashes so encoding is stable across
+	// platforms and remains correct on Windows where `filepath.Separator`
+	// would otherwise miss forward-slash style paths.
+	safeCwd := strings.ReplaceAll(cwd, "\\", "--")
+	safeCwd = strings.ReplaceAll(safeCwd, "/", "--")
 	// Remove leading separator replacement.
 	safeCwd = strings.TrimPrefix(safeCwd, "--")
-	return filepath.Join(home, ".kit", "sessions", safeCwd)
+	// Strip characters that are illegal in directory names on Windows
+	// (`< > : " | ? *`). On Unix these characters are legal but rare in
+	// practice; stripping them keeps the encoding portable.
+	replacer := strings.NewReplacer(
+		":", "",
+		"<", "",
+		">", "",
+		"\"", "",
+		"|", "",
+		"?", "",
+		"*", "",
+	)
+	return replacer.Replace(safeCwd)
 }