.mkit Files

A .mkit file is Molkit’s native document format. It stores your entire document as JSON: every structure, annotation, animation, and presentation slide, plus document settings and metadata. Use it whenever you want to come back to a drawing later and keep it fully editable. For sharing a finished image instead, use the SVG or PNG exporters.

Saving

Press Ctrl+S or choose File > Save to save the active document.

The first time you save an untitled document, Molkit prompts you for a filename and then writes a .mkit file. After that, Ctrl+S writes back to the same file.

Save vs Save As

Save (Ctrl+S) writes to the current file. Save As (Ctrl+Shift+S, or File > Save As...) always prompts for a new name and starts a fresh file.

How the prompt looks depends on your browser:

Chrome and Edge use the File System Access API. Molkit keeps a handle to the file you saved, so later Ctrl+S presses write straight back to the same file on disk with no dialog. Save As opens the native file picker.
Firefox and Zen do not expose that API. Molkit asks for a filename in an in-app dialog, then downloads the .mkit file through the browser. Each save produces a new download, so your browser’s download settings control where the file lands.

The Firefox and Zen path also writes a private in-browser copy used only for session recovery (see autosave, below). The Save dialog there offers two checkboxes: pretty-print the JSON (larger but human-readable), and embed any custom fonts the document uses so the file renders correctly on another machine.

Opening

Press Ctrl+O or choose File > Open... to open a .mkit file.

Chrome and Edge show the native file picker; other browsers use a standard file chooser. Opened files load into a new tab, and the view fits to the content. Files with the older .json extension from earlier versions also open.

What a .mkit file contains

The file is a JSON object with a format field set to molkit, a formatVersion field, and a canvas object holding the document. The canvas includes:

Structures: atoms and bonds, with their positions and per-object styling.
Annotations: text, arrows, shapes, brackets, and projections.
Animation states and presentation slides, when present.
Document settings: page setup, color scheme, and structure settings.
Metadata for the document and per-structure metadata.

To keep files small, Molkit drops properties that match their defaults and rounds coordinates to two decimal places on save. Those defaults are restored when the file loads, so nothing is lost.

Version compatibility

The current format version is 2. Molkit reads version 2 files and migrates older files on open (for example, converting legacy textbox annotations and older animation data). If you open a file created by a newer version than your build supports, Molkit warns you that it may not load correctly but still attempts to read it.

Recovery

Saving to a .mkit file is separate from Molkit’s automatic in-browser recovery. If the app or browser closes unexpectedly, Molkit can restore unsaved work from its autosave store. See the autosave page for how that works and how it interacts with open files.