Which results in:

NPP_SAVE: C:\Temporary\BitToggle.html
"C:\Program Files (x86)\tidy-5.4.0\bin\tidy.exe"  -modify -indent --indent-spaces 2 --wrap 120 "C:\Temporary\BitToggle.html"
Process started (PID=33372) >>>
Error: Can't open "C:\Temporary\BitToggle.html"

I don’t know why you got that. If you open cmd.exe (not in Notepad++) and send "C:\Program Files (x86)\tidy-5.4.0\bin\tidy.exe" -modify -indent --indent-spaces 2 --wrap 120 "C:\Temporary\BitToggle.html" , does it give the same error? Because if it gives an error there, then there is something wrong in the tidy syntax; but if that command doesn’t given an error in cmd.exe, then we need to try to figure out what’s actually different between the two.

Assuming the command works when BitToggle.html is not open in Notepad++ and you run that command from the cmd.exe command line, my initial guess is that tidy.exe wants an exclusive lock on the file, and if it’s open in Notepad++, then it can’t get that lock. But that’s just a guess. So the first thing you need to do is confirm whether or not the command works when it doesn’t involve Notepad++ or NppExec.

NPP_OPEN: "C:\Temporary\BitToggle.html
NPP_RELOAD
; about to start a child process: "NPP_RELOAD"
CreateProcess() failed with error code 2:
The system cannot find the file specified.

That one’s easier: there’s a missing endquote. In your script, you showed:

// 1. Temporarily save the current document state
NPP_SAVE
// 2. Run Tidy silently, format the active file, and overwrite it
"C:\Program Files (x86)\tidy-5.4.0\bin\tidy.exe"  -modify -indent --indent-spaces 2 --wrap 120 "$(FULL_CURRENT_PATH)"
// 3. Force Notepad++ to reload the updated text cleanly
NPP_OPEN "$(FULL_CURRENT_PATH)
// 4. to see the formatted changes
NPP_RELOAD

Do you notice on the NPP_OPEN line? There is no closing quote after the $(FULL_CURRENT_PATH), and command interpreters do strange things with unclosed quotes. So add that endquote.

NPP_RELOAD

Did you get this from AI? Because NPP_RELOAD is not found anywhere in the NppExec repository. I think your AI may have hallucinated that.

If I use Plugins > NppExec > Help/Manual and go to section 4. Advanced Usage > 4.6. Using external tools > 4.6.15. Triggering certain actions while saving a file (*), about halfway down, I find npp_sendmsg NPPM_RELOADFILE 0 "$(FULL_CURRENT_PATH)" as the syntax for reloading a file. (*: no, I didn’t find it that way; I searched the manual for reload, and that’s just the section that had it.)

So, assuming the tidy.exe syntax is correct, then the script should be:

// 1. Temporarily save the current document state
NPP_SAVE
// 2. Run Tidy silently, format the active file, and overwrite it
"C:\Program Files (x86)\tidy-5.4.0\bin\tidy.exe"  -modify -indent --indent-spaces 2 --wrap 120 "$(FULL_CURRENT_PATH)"
// 3. Force Notepad++ to reload the updated text cleanly
NPP_OPEN "$(FULL_CURRENT_PATH)"
// 4. to see the formatted changes
NPP_SENDMSG NPPM_RELOADFILE 0 "$(FULL_CURRENT_PATH)"

I just went to the https://binaries.html-tidy.org/ and unzipped tidy 5.4 (I put it in c:\usr\local\apps\tidy-5.4.0\), and created a C:\Temporary\BitToggle.html with some HTML in it. When I ran the script I showed (but with my path to tidy.exe rather than yours), it worked for me:

NPP_SAVE: c:\Temporary\BitToggle.html
"C:\usr\local\apps\tidy-5.4.0\tidy.exe"  -modify -indent --indent-spaces 2 --wrap 120 "c:\Temporary\BitToggle.html"
Process started (PID=20256) >>>
Info: Document content looks like HTML5
No warnings or errors were found.


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<<< Process finished (PID=20256). (Exit code 0)
NPP_OPEN: c:\Temporary\BitToggle.html
NPP_SENDMSG: NPPM_RELOADFILE 0 "$(FULL_CURRENT_PATH)"
================ READY ================

So that works for me. Given that it worked from NppExec with it open in Notepad++ for me, I don’t know why you got the first error when you ran the script…