# Font Size (OSC 7770)

> Set mintty's font size, or change it by steps, without resizing the window.

- **Sequence:** `OSC 7770 ; Pt ST`
- **ECMA-48:** [§8.3.89 OSC – Operating System Command, p. 51](https://archive.org/details/ecma-48-5th-edition-june-1991/page/n64/mode/1up)
- **Specification:** [mintty's control sequences: font size](https://github.com/mintty/mintty/wiki/CtrlSeqs#font-size)

`OSC 7770 ; Pt ST` is mintty's: it changes the font size and leaves the
window the size it was, so the number of rows and columns changes instead.
`Pt` is a size, a `+` or `-` step to zoom by, or empty to go back to the
configured size. `OSC 7770 ; ? ST` asks for the size, and the answer is the
sequence that would set it, ended the way the request was. A `Pt` with
anything in it but digits and a sign is ignored
([control sequences](https://github.com/mintty/mintty/wiki/CtrlSeqs#font-size),
[`termout.c`](https://github.com/mintty/mintty/blob/3.8.3/src/termout.c#L4921)).
[OSC 7777](https://control-codes.page/osc/font-window-size/index.html.md) resizes the window with the font.

A default xterm has no `OSC 7770` and ignores it
([the OSC numbers it knows](https://github.com/ThomasDickey/xterm-snapshots/blob/xterm-411a/misc.c#L3476-L3504)).
The cases follow xterm.

libghostty-vt does not recognize `OSC 7770` either: its OSC parser in
`osc.zig` has no state for the number, so the string is dropped, and the
cases pass.

## Support

| Terminal | Version | Support | Notes |
| --- | --- | --- | --- |
| VT220 | [manual](https://vt100.net/docs/vt220-rm/), 2nd ed., 1984 | No | OSC is not among the controls it recognizes ([§4.2](https://vt100.net/docs/vt220-rm/chapter4.html#S4.2)) |
| VT510 | [manual](https://vt100.net/docs/vt510-rm/), 1st ed., 1993 | No | ignores an OSC string ([chapter 4](https://vt100.net/docs/vt510-rm/chapter4.html)) |
| xterm | patch 412, [xterm-411a](https://github.com/ThomasDickey/xterm-snapshots/tree/xterm-411a) | No | ignored |
| libghostty-vt | [83edd49](https://github.com/ghostty-org/ghostty/tree/83edd491e3024ae5e50393d62877b8897da1cccd) | No | ignored |
| Konsole | 26.11.70, [a24c3d71](https://invent.kde.org/utilities/konsole/-/tree/a24c3d71be3684f24d030aa2dbe9bd723d985233) | No | not among the OSC numbers it handles ([`Vt102Emulation.h`](https://invent.kde.org/utilities/konsole/-/blob/a24c3d71be3684f24d030aa2dbe9bd723d985233/src/Vt102Emulation.h#L189-L205)) |
| ConEmu | build 230724, its [documentation](https://conemu.github.io/en/AnsiEscapeCodes.html) | No | not in its [ANSI escape codes](https://conemu.github.io/en/AnsiEscapeCodes.html) |
| mintty | 3.8.3, its [documentation](https://github.com/mintty/mintty/wiki/CtrlSeqs) | Yes | since 0.5.3 ([control sequences](https://github.com/mintty/mintty/wiki/CtrlSeqs#font-size)) |

## Validation

### OSC7770-1: Setting it changes nothing on screen

Input, one step per line:

```text
A
\e]7770;+2\a
B
```

Expected screen:

```text
|AB____|
cursor 1,3
reply none
```

### OSC7770-2: A query is not answered

Input, one step per line:

```text
\e]7770;?\a
```

Expected screen:

```text
|______|
reply none
```

---

This is the Markdown version of <https://control-codes.page/osc/font-size/>. On that page every validation case runs live in libghostty-vt, the terminal emulation core of Ghostty, compiled to WebAssembly.

The validation cases are written in the notation described in <https://control-codes.page/notation/index.html.md>.