Update 2015-10-12: My new advice for getting Sprint data to work on a Nexus 5 phone running CyanogenMod 12 is "Don't bother." I never did get it working right, and had to reboot at least once a day to get it working. I've since reverted back to KitKat. Original post follows, but if you want my advice it's "Stick with CM11."

First, let's get one thing out of the way: if you're using a custom Android ROM on your phone (or any device that can receive text messages), you're going to want to make sure it's up-to-date. There's a vulnerability in an Android component called Stagefright that is potentially devastating; it allows an attacker to gain control by doing nothing more than send a text message, and there are now attacks in the wild.

If you've got the stock firmware on your phone, and your phone is relatively recent, you should get the patch to fix this vulnerability automatically. (If, for example, your phone is running Lollipop, either because it came with it or automatically updated to it, you're probably good.)

But if you're running a custom ROM and don't have automatic updates enabled, you're going to want to check on whether you're running a current version that includes the Stagefright fix.

I'm a CyanogenMod user. If you're using the latest version of CyanogenMod 11.0, 12.0, or 12.1, then you've got the Stagefright fix.

I recently took the opportunity to upgrade my phone to the latest 11.x series to get the fix. And I figured while I was at it, why not upgrade to 12.1 and see if it's any good?

So I installed CyanogenMod 12.1, and everything looked like it was working fine at first -- when I was using it in my own house, on my wifi network. It wasn't until a day or two later that I realized my Sprint data connection wasn't working.

It took rather more searching than it should have, but it turns out there's an easy solution (albeit an annoying one if you've already got your phone set up the way you want it, because it involves wiping it to factory again).

mjs2011 at XDA Developers links to a sprint.zip file assembled by somebody named Motcher41, and gives these instructions for use:

The fix should be flashed during initial installation, so:

  1. Flash ROM
  2. Gapps
  3. SU (if necessary)
  4. Sprint APN Fix zip

I can confirm that you don't need to worry about setting up root before sprint.zip; it's fine if you do it afterward (my recovery, for example, sets up su right before reboot). However, I can confirm that you need to install sprint.zip after Gapps and before your first boot; if you install it before Gapps or after your first boot then it won't work.

Update 2015-09-30: After a few days my data connection quit working again. I rebooted to recovery, reinstalled sprint.zip, and it started working again. So never mind about not working if you install it after you've already booted the ROM; it will still work just as well. Unfortunately, "just as well" appears to mean "just for a few days"; I'm not sure what happened that changed my settings to make it stop working, but if I figure it out I'll update this post again.

You may notice that the linked thread is old (it's from November 2013) and was written in reference to pre-11.0 versions of CyanogenMod. However, I can confirm that it applies to the 12.x series too. This issue appears to be a regression; CM fixed it in version 11 but then broke it again in version 12.

So if you're a Sprint customer and you just installed CyanogenMod 12 on your phone and then discovered Sprint data was no longer working, this is what you're gonna wanna do to fix it.