Basic Spy strategy
The Spy specializes in deception and excels at punishing mistakes made by the enemy team. Although he is fragile and possesses some of the weakest weapons in the game, he can backstab enemies and sap Engineer buildings. Additionally, the ability to see enemy names and health levels allows him to pick off weakened foes with his Revolver. The Spy will almost always operate behind enemy lines, using Cloaking and disguises to move about undetected. Because of the threat he poses, enemies will frequently Spy-check their teammates. Although the Spy can hide or blend in with the enemy team, he can easily be exposed through various methods, including the Pyro's flames, Jarate, or even a simple collision with a foe. Through cunning and effective use of his various tools, a good Spy can avoid drawing enemy attention while effectively picking off important targets.
Revolver + reskins
Although weaker than the primary weapons used by other classes, the Revolver is extremely accurate, so you can use it to pick off weakened enemies from a distance or should you be discovered. There are several situations where you'll want to choose it for combat over the Knife, such as when your backstab fails, you cannot safely approach a target, or when a Sniper has a Razorback equipped.
The Ambassador comes with small penalties in damage and firing rate, but has the ability to headshot enemies. If you are confident about your aim, use your ability to see enemy health as an indicator of which enemies can easily be taken out from a distance. There is a set period after firing where the Ambassador cannot headshot, so it's better to space your shots out instead of firing rapidly.
The L'Etranger primarily increases your Cloak duration by 40%, making it easier to maneuver behind enemy lines. In combat, it deals reduced damage but slightly refills your Cloak when you shoot an enemy, converting firepower into potential fleeing power. It can be used to safely refill your Cloak from a distance and pairs well with cloaking devices that have specific recharge methods, like the Cloak and Dagger or Dead Ringer.
The Enforcer deals extra damage when attacking while undisguised, making it useful for direct confrontations but not for ambushing. The increased damage is accompanied by a reduced firing speed, so accuracy is rewarded. Because the buff only applies when completely undisguised, the Enforcer should be used with caution, generally as a last resort to force an escape by taking down a pursuing enemy.
The Diamondback's decrease in base damage makes it a poor weapon for direct encounters. However, the guaranteed critical hits gained upon the destruction of a sapped building allow you to more easily escape from enemy territory after a successful attack. Like when using the Ambassador, use your ability to see enemy health to determine who can be picked off. Work with your allies to ensure that buildings are destroyed; for example, if an Engineer is actively guarding his nest of buildings, simply disable his Sentry Gun and run interference while your teammates destroy it. You will still receive critical hits, even if the final hit wasn't done by your Sapper.
Knife + reskins
The Knife is typically used to backstab targets and deals pitiful damage otherwise. To get into position for a backstab, use your Cloak and disguises to maneuver behind your enemies without drawing attention to yourself. Switch to your Revolver in any situation where you can't land a backstab.
Your Eternal Reward + reskins
Your Eternal Reward removes your ability to initially disguise, making you rely more on your Cloak ability to find good hiding places. Upon a successful backstab, your victim will silently disappear while you instantly take their profile as your disguise; no death notice will appear to the enemy team. Once you obtain a disguise, you are free to approach your enemies from behind as usual. Although backstabs by Your Eternal Reward don't appear in the kill feed, don't rely on the "silent killer" aspect of the weapon; dead enemies will still leave behind weapons and be able to communicate using text or voice chat. If you're stuck with a slow disguise, you can Cloak at any time to move at normal walking speed.
The Conniver's Kunai allows you to steal the health of your victim and eliminate any negative status effects upon a successful backstab. While stolen health can overheal your health up to 180, the weapon reduces your initial health to 60, meaning that you can be killed by a single stray projectile. When using the Conniver's Kunai, you can either play aggressively, landing continuous backstabs to maintain your health, or pick off single targets and use the stolen health to safely escape.
When you kill an enemy with the Big Earner, your Cloak will be refilled by 30%, regardless of any restrictions for the Cloaking device you are using. However, it comes with a moderate health reduction which allows for stronger weapons like the Soldier's Rocket Launcher to kill you in one shot. Overall, you'll have a smaller margin of error when behind enemy lines but can escape more easily with your Cloak once you kill an enemy.
Like Your Eternal Reward, the Spy-cicle does not announce a death notice to the enemy team upon a successful backstab. Additionally, you retain the ability to manually disguise. Unlike Your Eternal Reward, backstabs visibly turn victims into ice, alerting enemies that you are nearby. If you are hit by fire while the Spy-cicle is your active weapon, it will make a distinct sound and melt, temporarily disabling its use but protecting you from being ignited again for two seconds; this allows you to extinguish yourself while invisible by changing weapons. Pairing the Spy-cicle with the Dead Ringer can give you some immunity to the routine Spy-checks made by enemy Pyros.
The Disguise Kit is one of your major infiltration tools. If near allies, it's best to disguise as another friendly class or an enemy Spy so if seen by an enemy you won't raise suspicion. Upon donning a disguise, you will be briefly immersed in a cloud of team-colored smoke. This can easily alert the enemy to your team affiliation and whereabouts,so when changing your disguise make sure you are out of enemy vision or cloaked. A disguise takes one second to become effective, so avoid the common mistake of approaching an enemy before the change is complete. Once you have successfully navigated behind enemy lines, try to not behave like a Spy; for example, reloading your Revolver while disguised will mimic your disguise's reload animation, allowing you to better blend into an enemy crowd. Even if you're disguised, don't linger in front of enemies for too long, as you are still vulnerable to Spy-checking. Attacking with the Revolver or Knife will cause your disguise to disappear, so find a safe place to redisguise afterwards.
Invisibility Watch + reskins
Out of all your watches, the Invisibility Watch has the quietest decloaking sound and allows you to cover the furthest distance while invisible. However, its limited Cloak meter and delayed activation time give you the smallest margin of error to work with when in enemy territory. To keep your Cloak filled and ready for emergencies, deactivate it when in safe areas and frequently grab metal pickups and weapons from dead players. Although you are invisible, you are not invincible; upon seeing an ammo box disappear, competent enemies may Spy-check the area. If in enemy territory, always stay on the move to decrease the chances of you being detected or cornered.
As long as you remain still, the Cloak and Dagger can be used to remain invisible indefinitely, allowing you to camp an area to pick off important targets, such as enemy Medics. Its meter drains based on your average movement speed; traveling a certain distance over a set period of time will drain a certain amount of Cloak, regardless of whether you move slowly and steadily or in quick bursts. The Cloak and Dagger cannot be recharged by collecting metal pickups or dropped weapons, making it more difficult for you to escape from enemies once discovered. The Cloak and Dagger's decloaking sound, while still fairly quiet, is distinct and noticeable, so refrain from decloaking within the range of any enemies. Continue to be mobile, even when remaining in one area; enemies will be encouraged to Spy-check if you stay in the same place too long. Likewise, don't just use the Cloak and Dagger to stay out of harm's way; being alive does not necessarily equate to being useful.
While you cannot manually Cloak with the Dead Ringer active, it will instantly Cloak should you take any type of damage. This also produces a fake corpse, displays a fake kill notification to the enemy team, and shields you from 90% of further incoming damage while cloaked. Simply "dying" from fall damage is a sure sign that you're a Spy. To make your feign death more convincing, consider disguising as an ally or letting the enemy damage you for a bit before taking out the Dead Ringer. If disguised as an enemy, the fake corpse will look like you, alerting them to your presence. If disguised as an ally, it will look like the ally you are disguised as, allowing you to give your enemies a false sense of security as you change disguises. Once cloaked, the Dead Ringer's damage reduction allows you to safely navigate across battlefields and behind enemy lines. Prioritize navigating away from fire, as further damage taken while cloaked will decrease your Cloak time. While enemies will not see you upon getting hit or being collided with, they can still reveal you using effects such as fire, bleeding, or Jarate. Although collecting metal can prolong your Cloak, taking damage will decrease it. Thus, the increased Cloak drain rate and loud decloak noise means that finding a safe area is a priority.
Sapper + reskins
The Sapper will instantly disable an Engineer's building, destroying it over time. It can only be removed by the Engineer's melee weapon, the Pyro's Homewrecker, or the Neon Annihilator. You have an unlimited supply and can place them extremely rapidly on any building within melee range. Typically, you'll first backstab an Engineer, then quickly sap any nearby buildings; you'll want to begin with Sentry Guns to prevent them from attacking you. While your weapons deal reduced damage to any building that has your Sapper on it, it may be worth further attacking a building to speed up its destruction.
The Red-Tape Recorder is capable of reversing the construction of Engineer buildings. When compared to the standard Sapper, it takes moderately longer to destroy any Engineer building. However, it takes a very short amount of time to downgrade buildings to their level one version. If you're having trouble dismantling a guarded Engineer nest, use the Red-Tape Recorder to weaken instead of destroy it and allow teammates to clean up.