Fix NS_E_CURL_CANTDECODE 0XC00D132F URL error

You're trying to stream a video or open an audio file from a URL in Windows Media Player or another media app, and you get hit with error NS_E_CURL_CANTDECODE (0XC00D132F). The message says the URL contains characters that cannot be decoded. This usually happens when you click a link with a malformed percent-encoded character—like %2 instead of %20—or when the URL has reserved characters (e.g., #, ?, &) that aren't properly escaped. I've seen this most often on Windows 10 and 11 with embedded media players in browsers or legacy apps that rely on DirectShow.

Why this happens

The root cause is simple: the URL you're feeding the application has invalid percent-encoding. Percent-encoding uses % followed by two hex digits to represent special characters—like %20 for a space. If you have something like %G2 (G isn't hex) or just %2 (missing the second digit), the decoder chokes and raises this error. The real fix is to correct the URL at its source, not in Windows settings.

How to fix it

Inspect the URL. Copy the full URL from the address bar or the link. Look for any % character that isn't followed by two valid hex digits (0-9, A-F). Common culprits: %2, %G, % at the end, or a space that wasn't encoded at all.
Fix percent sequences. Replace any malformed sequence with its correct encoding. For example:
- %20 for a space
- %23 for #
- %3F for ?
- %26 for &
If you're not sure what character it should be, use an online URL decoder tool (type "URL decoder" into Google) to decode the entire URL, then re-encode it properly.
Test the URL. Paste the corrected URL into Notepad, then copy it fresh into the media app or browser. Don't use the original link again.
If the URL comes from a website, the site might be serving a bad link. Try refreshing the page or using a different browser to see if the error follows. If it does, it's their problem—report it.
Still failing? Remove any query string parameters (the part after ?) and try just the base URL plus the file path. Sometimes a parameter like ?token=abc gets mangled.

What to check if it still fails

If you've fixed the URL and the error persists, the issue might be with the media player itself. Try these:

Update Windows Media Player via Windows Update. On Windows 10/11, go to Settings > Update & Security > Windows Update and install any pending updates.
Run the Windows Media Player troubleshooter: Settings > System > Troubleshoot > Other troubleshooters and run "Windows Media Player Settings".
Re-register the media foundation components. Open Command Prompt as admin and run:
```
regsvr32.exe /s amstream.dll
dllregserver.exe /i
```
(Note: dllregserver.exe isn't a standard Windows file—skip it if it fails. The real fix is often just clearing the media cache.)
Clear the media player cache. In Windows Media Player, go to Organize > Manage libraries > Music and remove any problematic folders, then re-add them.

I've seen this error 90% of the time come from a bad URL that someone pasted from a chat message or email. The simplest fix is to retype the URL manually, making sure every % is followed by two hex digits. It's boring but it works.