Killstreak Kits are tool items that can be applied to a specific weapon, upgrading it into a Killstreak weapon.
When a Killstreak weapon is equipped, a kill counter appears in the player's HUD, tracking the number of kills made with any Killstreak weapon equipped and resetting upon death. The number of kills in a player's streak is displayed both on the scoreboard next to that player's name as well as in the killfeed in the top right corner following the player's name, which increases with each consecutive kill. Every 5 kills (20 in Mann vs. Machine), a message will be displayed to the server displaying the player's name and their current streak. Other kills by the player with a different weapon do not count toward their current Killstreak unless that weapon is also a Killstreak weapon. If a player is carrying multiple Killstreak weapons, the killfeed will display only the number of kills each weapon has obtained, but the messages to the server and the player's HUD "Streak" indicator will add the player's kills from all their Killstreak weapons.
A Medic using a Killstreak Medi Gun will not see any indication in the killfeed, although kill assists with the Medi Gun still count towards the player's Killstreak and are displayed in the HUD. An Engineer wielding a Killstreak Wrench will see an indication in the killfeed for both a Wrench kill and a Sentry Gun kill. Additionally, while playing Mann vs. Machine, kills made with the Projectile Shield upgrade will also go towards Medic's Medi Gun Killstreak.
Higher tiers of Killstreak weapons also have cosmetic effects; while standard Killstreak weapons only have the kill counter, Specialized Killstreak weapons also have a colored sheen, and Professional Killstreak weapons add a colored particle effect to the user's eyes. These eye particle effects start at 5 kills and are visually minimal, while at 10 kills the effect becomes greatly noticeable.
Killstreak Kits can be applied to weapons of any quality. If a Killstreak Kit is applied to a stock item, it becomes a Unique item and the player will subsequently find it as an item drop. The newly found item will exist separate to the original stock item. The player can then choose which item to equip through the character loadout menu. If the weapon is restored to remove the Killstreak, the weapon will remain in the player's inventory.
Standard Killstreak Kit
Standard Killstreak Kits are the most common variety and are rewarded after every completed tour of Operation Two Cities which may be applied to its assigned weapon. Upon application, it will add a HUD kill counter in addition with the ability to display your killstreak in the killfeed for everyone to see, indicated by a number and a small arrow next to the kill icon. At the end of each round, the player with the highest killstreak is displayed on the final scoreboard.
Specialized Killstreak Kit
The next tier of Killstreak Kits. In addition to a kill counter, a Specialized Kit also applies a colored sheen to the weapon. As the player's streak increases, the sheen becomes more intense. Specialized Kits are obtained by completing Specialized Killstreak Kit Fabricators, which can be found as a random reward from completing Operation Two Cities.
The following Sheens are available:
Professional Killstreak Kit
The final tier of Killstreak Kits, Professional Kits are the rarest variation. In addition to a kill counter and a colored sheen, Professional Killstreak Kits also cause the weapon to add a particle effect to the user's eyes. Professional Kits are obtained by completing Professional Killstreak Kit Fabricators, which can be found as a rare random reward from completing Operation Two Cities.
While the weapon is active and the player has a streak of at least five, the eye effect will become visible. This effect, similar to that of the Eyelander, will match the color of the weapon's sheen. The effect will become more intense when the streak passes 10 kills.
The following effects are available:
- Cerebral Discharge
- Fire Horns
- Kills made while dead contribute to a weapon's killstreak, though as it is properly reset on both death and respawn, this has little effect.
- If a killstreak weapon is used to kill an enemy and is then changed in the loadout and changed back, the counter will start at 1 again in the killfeed, but the streak overlay will remain the same number.
- Killstreaks that are ended due to environmental deaths, provided another player isn't credited for finishing them off, are credited to ERRORNAME.
- Kills made with the Wrangler count towards the killstreak of an Engineer's wrench, even if only the wrench has killstreaks enabled.
- Killstreak notifications override the "silent killer" attribute of Your Eternal Reward and its reskins, causing them to show in the enemy team's killfeed.
- Certain passively-visible weapons and taunts can inherit the sheen of a different killstreak weapon:
- The Gunslinger, Demoman shields, and Spy watches appear with the same sheen as the weapon in use.
- If a Ham Shank Heavy or a Gunslinger Engineer use the High Five! taunt, their hand will take on the sheen of their primary weapon.
- If a Demoman uses the Spent Well Spirits taunt, the bottle will take on the sheen of their Grenade Launcher or Stickybomb Launcher.
- The Hypno-Beam effect does not display through wearable glasses.