Tcl/Tk 9.1 represents the current development work toward a stable release planned for September 2026, building on the foundation of Tcl/Tk 9.0 with new language features and interface improvements. The update is positioned as a step toward the final 9.1.0 release, which was made available as source code on September 29, 2026.
In Tcl 9.1, a new unicode command provides Unicode normalization, complemented by C routines Tcl_UtfToNormalized*. A timer command offers monotonic clock access with microsecond resolution, and lfilter enables selecting items from a list. The interp set command allows variable access in child interpreters, while the subst command gains optional backslashes, commands, and variables switches. The switch command now accepts an -integer option, and many C99 math routines become available as expr functions. Applications must call an initialization routine such as Tcl_FindExecutable or TclZipfs_AppHook. Additional C‑level additions include Tcl_IsEmpty, Tcl_GetEncodingNameForUser, Tcl_AttemptCreateHashEntry, and list‑manipulation routines Tcl_ListObjRange, Tcl_ListObjRepeat, and Tcl_ListObjReverse. The C time API switches to long long values instead of the older Tcl_Time structure. Platform‑specific changes consist of case‑insensitive filesystem paths on macOS, revised auto_execok and exec search behavior on Windows, updated searches for script libraries and encodings, memory‑efficient internals for large lists, and extended support for 64‑bit sizes.
In Tk 9.1, accessibility support for screen readers is added, along with initial support for bidirectional text and right‑to‑left languages. A new ttk::toggleswitch widget is introduced, together with a tk attribtable command. The send command has been revised and improved on Aqua, and Tk now handles negative screen distances correctly. The available states for ttk::treeview and ttk::notebook widgets are extended, and the Tk_CanvasTextInfo interface is improved. Label widgets can display rotated text, message boxes and dialogs are limited to the physical screen width, listbox selection colors have been improved, and obsolete support for Windows XP appearances has been removed.