applescript-decompiler

Decompiler for compiled AppleScript (.scpt) files. Parses the Fasd UAS binary format and reconstructs readable AppleScript source from the bytecode — including handlers, control flow (if/repeat/try/tell blocks), object specifiers, and literals.

Built on (and a significant extension of) Jinmo/applescript-disassembler.

Installation

pip install applescript-decompiler

Or from a checkout:

pip install .

Requires Python 3.11+.

Usage

Command line

asdec compiled.scpt

========================================
Function: areaOfCircle
Arguments: none
========================================
if (not ({real, integer} contains class of var_0)) then
    error "Radius must be number."
else
end if
return ((var_0 * var_0) * |«class pi  »|)

As a library

from applescript_decompiler import decompile_file

source = decompile_file("compiled.scpt")
print(source)

How it works

• applescript_decompiler.fas parses the Fasd UAS serialization format (a port of the FasLoad routine from the original AppleScript runtime): reference tables, value blocks, records, literals, and embedded bytecode.
• applescript_decompiler.opcodes knows the 256-entry instruction set and disassembles handler bytecode.
• applescript_decompiler.decompiler interprets each handler's instructions against a simulated value stack, emitting AppleScript statements as it goes.

Limitations

Decompilation reconstructs source from bytecode, and some information is discarded at compile time or only meaningful with an application's terminology dictionary. Expect:

• Local variable names are not stored in compiled scripts, so they appear as var_0, var_1, … Handler argument names (including typed and destructuring patterns), globals, and properties are recovered.
• Labeled parameters of application/scripting-addition commands are not reconstructed. A command's name is recovered (e.g. make, display dialog), but its arguments are rendered positionally rather than with their with properties / given labels.
• use statements, property initializers, and script object structure (inheritance, nesting) are not reconstructed; only the handlers they contain are emitted.
• Output is per-handler, framed with =/Function:/Arguments: headers, not a single recompilable file. The handler bodies are valid AppleScript for most inputs; pathological scripts can still leave a stray marker.

A trailing return of the last expression is emitted for every handler (the compiler stores it as the result), which is harmless but not always present in the original.

Development

uv sync          # install dev dependencies
uv run pytest    # run the test suite

The test suite compiles a corpus of AppleScript sources with osacompile (macOS only) and verifies that decompilation round-trips: output must contain no unknown markers, balance its blocks, recompile cleanly, and leave no opcode unhandled.

License

MIT — see LICENSE. Original disassembler © 2017 Jinmo; decompiler extension © 2026 n0kovo.