Stealth

Stealth is a mechanic which was first introduced in Update 7.

Stealth Attack
As long as an enemy is unaware of your presence, a stealth attack may be performed on them. To perform a stealth attack, approach an enemy from behind, and activate the stealth attack prompt. Successfully performing a stealth attack will deal a massive amount of damage during a special animation. This damage is not usually affected by armor or resistances (see stealth damage multipliers). Some enemies will not die from a single stealth attack, if they have a large amount of health or if the melee weapon is too weak.

Enemy Eligibility
Mechanical enemies do not grant a stealth attack prompt. Mechanical enemies immune to stealth attacks are listed below.

Grineer Corpus
 * Roller
 * MOAs
 * Ospreys
 * Cameras
 * Turrets

Some Bosses, such as Captain Vor and Sgt. Nef Anyo, can be stealth attacked. The damage done will usually not be sufficient enough to kill them in one hit. Non-humanoid Bosses such as Phorid and Jackal are immune to stealth attacks.

Special Warden enemies grant bonus damage with stealth attacks, allowing a larger chance of instantly killing them, which is crucial in order to complete their host mission with the most points.

Damage Multipliers
Melee attacks are affected by a stealth damage multiplier when attacking unalerted enemies, or when cloaked by an ability (Smoke Screen and Invisibilty). These damage multipliers change the color of regular damage pop-ups from white to yellow (similiar to critical hits). Critical hits while under these effects will still appear in the color yellow but will multiply the 400% melee damage multiplier by the amount of critical damage on the weapon. E.g. 400% x 3x critical = 1200% damage.

As of Update 13, all stealth attacks have a 400% damage multiplier that is affected by resistances, although the number may not be exactly 4 times the damage shown in the arsenal, as the arsenal does not show decimals.

Prior to Update 13.8.1 Stealth melee attacks count as critical hits for the Berserker mod.

The thrown stealth attacks from the Kestrel, Glaive, and Glaive Prime are affected by enemy armor resistances. It is unknown if this is a bug or not.

Animation
Stealth attacks use the same animations as rear finishers. Like normal finishers, stealth attack animations vary according to what class of melee weapon the player is carrying. Examples of stealth attack animations along with other finisher moves can be found in the melee gallery page.

Detection

 * Noise caused by breaking objects or firing loud weapons will alert nearby enemies. Non-silent Warframe abilities will also alert enemies. Certain weapons, however, are silent, such as the Paris, Kunai, and Despair. These weapons may be used to perform long-range stealth attacks. Certain weapons, when used at very long range, like the Latron, Snipetron and Vulkar, are less likely to attract attention. Be sure to kill your target before it recuperates and alerts attention, so preferably in a single hit.
 * Loud weapons create "3D" noise. The bigger the noise, the further it travels, and the more likely it will alert an enemy. Banshee eliminates this noise with her Silence.
 * The Corpus Turret is the only enemy weapon that is "silent" and won't generally attract attention if it attacks players.
 * Bumping into enemies will cause an alert whether cloaked or not.
 * Enemy range-of-view before alert is <180 degrees, so crouching still in a corner is an effective way to expose the back of a Grineer that just walked through a door. Enemies are bad at detecting objects at a distance, or anything somewhat above them. Hiding behind terrain will conceal your presence.
 * When enemies are on alert, they will become highly sensitive to your presence. Their range of vision will widen and will be able to spot you or your sentinel even from far away unless you become cloaked.
 * Performing an unsuccessful stealth kill will alert the enemy (this happens often when in a high level area, or when stealth attacking heavy Grineer).
 * This includes failing to kill an enemy with a silent or silenced weapon. Once they clearly notice you or fire their own weapon, enemies nearby will become alerted.
 * In order to put the whole level in alert, an enemy needs to activate a control panel. Killing everyone before a control panel is touched (they take about 1.5s to complete the alert) can still allow you to remain undetected. However, gunfire will put enemies in other map tiles in alert temporarily. These enemies keep watch behind cover, or patrol with their guns raised and are more sensitive to your presence. They will "cool down" and can be stealth killed after a short period of time. Cloaking and avoiding enemy line of sight will also lower their alertness.
 * Even when the control panel has been accessed and an alarm has been given, enemies will lose track of you when the last group of enemies you killed is not close enough to them to set them on alert. They will wander as if you were not detected and it is possible to finish a Stealth side mission this way.
 * Cameras make loud alarms when they see you, but again, if they are destroyed and nothing is in range to take advantage of their alert, you're safe. Rooms with two cameras are difficult to get past undetected without destroying them.
 * By sprinting through the map and leaving the current room quickly, you can sometimes avoid the alarm being set off, or the alarm will go off but since you are in another section of the level enemies won't notice. Using a frame with a sprint mod can increase this chance of escaping.
 * If an alarm was set off, you can reset the alarm by going to a terminal and hacking it after clearing a room without further spread of notice. Do this quickly before reinforcements come and you can resume your stealth run.

Silenced Weapons

 * Bows (Paris, Dread, Paris Prime, Cernos) are completely silent, able to multi-kill, and can pin corpses out of sight.
 * Lanka.
 * Bolto and Akbolto
 * Explosive weapons (Ogris, Torid, Bows with Thunderbolt) are, oddly enough, silent.
 * All melee weapons, including the Glaive and Kestrel.
 * Helios's Deconstructor is also silent.
 * Throwing weapons: Despair, Kunai, Hikou.
 * Cloaking based warframe abilities (Smoke Screen, Invisibility).
 * Cloaking based warframe abilities can also render any weapon fire silent.
 * Offensive based warframe abilities (Shuriken, Slash Dash etc.): silent only if cast under cloak.
 * Mag's abilities Pull and Shield Polarize.
 * Ash's Teleport: silent only if cast under cloak, otherwise teleporting to an enemy will cause you to bump into it and cause an alert.
 * All of Vauban's abilities
 * Stinger, the default weapon included with Djinn Sentinel, is silenced.
 * Hush and Suppress, are rifles and pistols mods respectively that render any weapons completely silent at maximum rank.

Tips

 * Be patient.
 * Check your surroundings to sneak up on an unsuspecting enemy without interruption.
 * Crouch for better stealth runs. You'll have a smaller model so you can hide your silhouette better. The speed at which you move is also more convenient for timing your melee than running, and you will be less likely to bump into the enemy before the prompt appears. Also, it seems that enemies are less likely to notice you while you are crouching.
 * If enemies see a corpse that has not disappeared yet, they might be alerted for a short period of time. Be sure not to be in their line of sight.
 * Staggered enemies don't make sounds, but they become alert and will alert others if given a chance to recover. If you did not kill an enemy with a single hit, keep it staggered by stealthy means and you'll be fine. Cold procs will lengthen the amount of time an enemy stays staggered.
 * If you have a Shade with Ghost ability, consider sprinting toward an enemy to activate it quickly.
 * Rolling is a great way to move quickly while crouching.
 * Beware of breakable reinforced glass, unless it is part of the player's tactic to cause a controlled lock down to eliminate all enemies inside a chamber. This action will not cause a high alert.
 * Having a Sentinel equipped may be detrimental to your stealth ability, as it does not hide behind low cover.
 * If you do bring a Sentinel make sure to remove their targeting precept mod or the weapon itself.
 * If you do not see a prompt, consider dealing a channeled attack for more damage.
 * Kunai, Hikou and Despair are good weapons to quickly eliminate a small pack of enemies without alerting anyone.
 * Stand to the side of a door to open it and peer slightly by moving the camera.
 * Do not eliminate an enemy if it is being watched by another enemy.
 * If using a bow or a slow fire rate weapon, be sure you are killing your targets in one shot to avoid alerting your target.
 * You can see around the left edge of obstacles by changing the camera to hang over your left shoulder (default is H).
 * It is advised to use a Codex Scanner while you are doing a stealth mission, due to the fact that the Codex Scanner can show you enemies within 50 ingame meters even through walls and behind cover.

Grievances
Since the concept of stealth is still a new one, there are some flaws that have not been addressed.
 * The practicality of attempting assassination on heavy and shielded units.
 * If you aren't at the same altitude as the target, the prompt will not appear easily.
 * Bumping into an enemy will put them in an alert status and void your attempt at stealth killing them.
 * Prompt will appear and disappear unexpectedly when the enemy are in alert.
 * Not all enemies can be stealth killed. It is hard to know whether an enemy type can be stealth killed until the player approaches behind the target.
 * After each stealth attack, you finish standing up, which might reveal your position. You will need to release and re-press the crouch button to return to crouch position. If you are using a toggle crouch key, simply press it during the animation to return to crouching when the attack completes.
 * Dropping off a ledge will also cause you to stand up upon landing.
 * There appears to be no inherent benefits of stealth gameplay aside from the prompt for an assassination and lower enemy spawn rate, which is counterproductive given that weapons level via kill experience. Though stealth can be a great end-game challenge for more experienced players.
 * A glitch can occur when stealth attacking Infested Ancients, Heavy Gunners or Corpus Techs. Upon entering the stealth attack animation, the enemy will shrink to roughly the player's size. (This shrinking glitch can also occur when using Ash's Blade Storm. Blade Storm has a possibility to also shrink Grineer soldiers, as well as a few bosses, including Lieutenant Lech Kril.