There was a cartridge with an assembler & text editor plus a little CPU RAM. (4k? My memory is gone but it certainly wasn't much even for the 80s.) You'd still need to use cassette tape to save your work. Ah it was this: https://www.arcadeshopper.com/wp/ti-99-4a-faq-mini-memory/

For actually usable development you could buy a TI expansion box, 32K RAM expansion, and 5 1/4" floppy drive. This cost the equivalent of like two thousand bucks today. Less than an Apple ][, somewhat more than a base C-64, but a lot more than the TI-99 itself.

My parents were very indulgent in this. Once I had this setup, I bought a third-party Forth and coded my own Forth assembler vocabulary, and finally had a reasonably capable dev env, for maybe a year before leaving for college. But still had basically no way to share my work (wasn't online).