Idea how tech espionage could work in stellaris:
1. You send your spies to get the tech from the XY empire
2. You steal papers on said tech, or maybe a random tech you don't have
everything went well or enemy empire knows you stole it and is angry and wars you
3. To understand the tech, you would have to research it, but since you stole it, you get the card
4. Various outcomes are possible: You learn the tech and can use it
You still don't understand it, but can try again after you researched similar lesser techs
Prototype malfunctions and pops on that planet are unhappy
Planet explodes
Sun explodes
A tear in reality opens and the unbidden eat you
Something like that.
Just to ensure you only steal techs you really really want and must have.