Kill assist

A Kill Assist

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A Kill assist is awarded to a player when an enemy that they have wounded is killed by a team-mate. If multiple team-mates have wounded an enemy, the player who dealt the most damage (without killing the enemy) is awarded the assist. Each kill assist is worth half a point. Only one assist is possible for every kill, unlike captures, which award points to multiple players.

If a Medic is healing a team-mate with the Medigun, and that team-mate scores a kill, the Medic is always awarded a kill assist, regardless of any other damage dealt by other players. In addition, a player that is killed while on fire will give an assist to the Pyro that most recently burned him. These take priority over normal kill assists, though not over a Medic assist.

Note that a Spy will not earn a kill assist if his team-mate destroys a sapped Engineer building. On the other hand, the Spy still receives a point for having sapped a building.

If a disguised Spy is being healed by an enemy Medic, the Medic will receive an assist if the Spy backstabs a team-mate or destroys a building.

Snipers can gain a kill assist if an enemy is killed by a team-mate after he has soaked them with Jarate. This takes precedence over Medic assists.

Soldier can gain a kill assist if an enemy is killed by a team-mate while he is boosting them with the Buff Banner. This does not take precedence over Medic assists.

Example

Consider the example demonstrated in the image

A BLU Pyro called Assister is using a Flamethrower on a Red Scout called Target.
Assister stops attacking. Target has suffered direct damage from the Flamethrower and is now on fire.
A BLU Demoman called Killer places Sticky bombs beneath Target and then detonates them.
The Sticky bombs kill Target. They also happen to kill Killer because he's too close when they detonate.

Result

Killer is awarded a Kill because the Sticky bombs dealt the last damage to Target.
Assister is awarded a Kill assist for damaging Target prior to Killer.
Assister is not awarded a Kill because his attack was not the one that killed Target.
The console notification for a Kill with a Kill assist: Killer + Assister (Killer's Weapon) Target. A zoomed-in view of the notification is featured at the bottom of the image.
Killer's death from the Sticky bomb splash damage does not affect either his Kill award or Assister's Kill assist award.

Additional notes

A Kill assist is worth half a point. Half points are not displayed, however, so a player's score will only appear to rise with every two assists.
If the person who receives a kill assist is being dominated by the victim, they will get a revenge kill. It is possible for both the killer and the assister to get a revenge kill if the victim was dominating both. Similarly, it is possible to dominate a player by getting four kill assists on them.
Unlike Pyros receiving assists when someone who was lit by them is killed, a Medic will not receive an assist if a player he has healed or overhealed recently scores a kill and he isn't healing him at that moment. A Medic will only receive an assist kill if his patient kills someone while he's being healed.
It is actually possible to assist yourself by throwing Jarate and then killing your target or by by hitting them with the Sandman's baseball and killing them. If the enemy kills himself (fall damage, the Demoman's explosives, the Soldier's rockets or his suicide grenade) you'll still be awarded for the kill.
Even though no damage is done, if a Pyro compression blasts an enemy who is killed shortly afterward, the Pyro receives a kill assist.