On Towards the Stars: Spies
[This post is part of a larger strategy guide for Civilization 6]
Civ is a game that is all about snowballing. Past investments lead to future gains along a lot of different axes, and as a result once you're behind it is difficult to catch up.
Spies are a mechanism to counteract the snowball. You want to primarily think of them as a way to slow down your opponents without having to explicitly spin up a massive war machine. They are less powerful than an army, but also less costly; and if you're meaningfully behind in military power, spies may be the only way to achieve concrete outcomes. As a result, by the information era you ideally want to have 3+ master spies.
Getting an upgraded spy is generally pretty straightforward.
Send your spy to an enemy city with a commercial hub;
Run 'Gather Sources;
Run 'Siphon Funds' until your spy is leveled up.
You basically always want to follow this pattern — it maximizes the chance that your spy survives the first mission. I generally will run ‘Siphon Funds’ all the way until the spy is level 3, unless (due to upgrades) some other spy mission becomes higher likelihood to succeed.
I think there's a lot of things you can do with spies, and the best option is highly situational. Still, here's a rough stack rank of how you might think about deploying them.
Stop an imminent win
If you know someone is really close to winning, use your spies to stop them. This is easiest against opponents going for science victories using 'Disrupt Rocketry' (the rest of the victory types all have passive growth options that make it hard to disrupt them once they are really going, and there's literally no way to stop a religious victory with spies).
Increase diplomatic visibility during or in the lead up to war
Every level of diplomatic visibility that you have over your opponent is equivalent to +3 combat strength. When you run the "Listening Post" mission with a spy, you get +1 visibility (+2 if using an upgraded spy) for the duration of the mission. Like I've said everywhere else in this guide, military bonuses are critical to winning multiplayer games. A +6 combat bonus can be devastating in wartime.
Remove an enemy suzerain
Fabricate Scandal is basically the only way to remove deeply entrenched city state suzerains in the late game. If your opponent has a city state bonus that they absolutely depend on (see: Akkad, Mohenjo-Daro), Fabricate Scandal can be devastating to their momentum. Also, obviously, you can use this particular spy ability along with a bunch of saved up envoys to capture a key city state yourself.
Damage enemy production
Production is the key to ramping, so removing production is a great way to slow your opponents. Look to blow up dams in critical cities that have many nearby commercial and industrial zones, or critical industrial zones that provide factory and energy benefits to many nearby cities. Removing governors from big cities can also be quite helpful.
Counter-spy
So far we've discussed how to use spies when you're behind. But when you're way in the lead, most of the other spy options aren't going to be that useful. In these cases, counterspies are your bread and butter. Keep a spy in key industrial zones, space ports, and cities to ensure you keep your lead1.
What if my spy has to evade capture?
Always return by foot! If your spy gets captured, it’s near impossible to get it back — people are rarely incentivized to trade spies. As a result, you want to either get killed or try as hard as possible to avoid getting caught at all. Thus: return by foot. It takes longer for your spy to be active again, but the trade is worth it.
This is especially valuable if your spy has upgrades that benefit staying in home turf.