Originally published at: macOS 26.4’s Script Editor Won’t Open Some Older AppleScripts - TidBITS
Shortly after the release of macOS 26.4 Tahoe (see “OS 26.4 Adds AI-Generated Playlist Playground, Separates Family Sharing Purchases,” 25 March 2026), several TidBITS Talk users began reporting problems with running certain older AppleScripts, while other scripts continued to work normally. The affected scripts did not appear to be corrupted, but Script Editor refused to open them, making editing impossible. Fortunately, community members have identified a workaround.
The Problem: Obsolete Script Formats
Allen Gainsford first reported the issue, noting that many of his BBEdit AppleScripts no longer worked after upgrading to macOS 26.4. Attempting to run them produced macOS error code -1758 (errOSADataFormatObsolete), indicating that the script’s compiled data format could not be read. More human-readable expansions of error code -1758 appear in a 1999 Apple Event Errors document (“Data couldn’t be read because its format is obsolete”) and a 2016 Open Scripting Architecture Errors document (“The data format is obsolete”). Script Editor also wouldn’t open them, displaying the error message “The document ‘filename’ could not be opened.”
Further investigation by Dafuki revealed a nuanced situation. In his testing, many AppleScripts—even some dating back to 2006—continued to run without issues when triggered from apps like BBEdit and Keyboard Maestro. However, others wouldn’t open, and after failing to open one script, Script Editor would then refuse to open any subsequent scripts until it was quit and relaunched.
He found that the issue appears to involve older compiled scripts that rely on legacy storage formats—hence the “data format is obsolete” errors. Allen Gainsford even found that one affected script had a 0-byte data fork and a 26 KB resource fork, indicating that the actual script was stored in the resource fork—a legacy structure macOS supports only for backward compatibility.
Although Script Editor’s 2.11 version number didn’t change from macOS 26.3.1 to macOS 26.4, Apple did increment its build number from 233 to 234, indicating that it received some changes.
The Fix: Recompile with Script Debugger
Dafuki’s solution involves using Script Debugger, a professional AppleScript development environment from Late Night Software. Although developer Mark Alldritt retired Script Debugger in January 2025, he generously made the final version available as a free download.
To recover an affected script:
- Download and install Script Debugger if you don’t already have it.
- Open the problematic script in Script Debugger.
- Save the script as a new file, which effectively recompiles it into a modern format.
The new file may be smaller—Dafuki reported one script dropping from 40 KB to 10 KB—and Script Editor should be able to open and run it.
How Widespread Is This?
Even though I have now opened hundreds of random AppleScripts dating back to the early 2000s (everything I could find on my Mac via a search), none of them suffer from this problem. Nor have I found discussions of AppleScript issues in macOS 26.4 anywhere other than TidBITS Talk, suggesting the problem isn’t widespread.
However, the fact that two regular members of TidBITS Talk—a relatively small group with only 6500 participants—both encountered this problem immediately upon updating to macOS 26.4 suggests a reproducible issue under certain conditions rather than simple file corruption.
The conditions necessary to trigger the problem—likely involving older compiled AppleScripts that use legacy binary formats or resource forks—may be highly unusual. Most Mac users don’t use AppleScript, and most of those who do aren’t using scripts they wrote decades ago. But clearly some TidBITS readers do!
If you discover scripts that won’t open, don’t panic: they almost certainly still work. You just need Script Debugger to modernize the file format so Script Editor can read them again. Also, be sure to leave feedback with Apple or file a bug using Feedback Assistant (in the Utilities folder in your Applications folder).