Sneak

Sneak is a Fallout, Fallout 2, Fallout 3, Fallout: New Vegas, Fallout Tactics, Van Buren and J.E. Sawyer's Fallout Role-Playing Game skill.

Fallout, Fallout 2 and Fallout Tactics

 * Fallout:
 * $$\text{Initial level}\%=25+\text{Agility}$$


 * Fallout 2 and Fallout Tactics:
 * $$\text{Initial level}\%=5+\text{Agility}\times3$$

Sneak only covers sneaking as such, while pickpocketing is covered by the Steal skill. In Fallout and Fallout 2, one cannot run and sneak at the same time without the Silent Running perk.

How close one can sneak to a target before detected depends on a combination of several factors:
 * Target's Perception statistic.
 * Sneaking character's Sneak skill.
 * Sneaking character's stance plays a much smaller role than the above two factors (Fallout Tactics).
 * The light level from the day/night cycle plays a small factor too, but again a much smaller one than the Perception statistic and the Sneak skill.

Where the target is facing does not impact one's ability to successfully sneak. The target character can detect equally well in all directions even directly behind them.

Fallout 3

 * $$\text{Initial level}=2+(2\times\text{Agility})+\left\lceil\frac{\text{Luck}}{2}\right\rceil$$

Example: A starting Agility of 5 and Luck of 5.
 * $$2+(2\times5)+\left\lceil\frac{5}{2}\right\rceil=15$$

Sneaking serves to be advantageous in Fallout 3, allowing the player character to move closer undetected and avoiding taking damage for the longest time possible, and maximizing the damage potential through the use of "sneak-attacks." Sneaking increases the chance to loot and steal, and allows the ability to avoid some undesired confrontations, thus working as an offensive and defensive skill.

Unlike its predecessors, in Fallout 3, the player character can both sneak and run at the same time (i.e. move faster while crouching) regardless of perks. However, without the Silent Running perk, they will suffer a detection penalty while running.

Sneak states
The sneaking state is displayed while crouching and alerts the player character to detection and possible detection.
 * [HIDDEN] means being undetected. In this state, attacking a target guarantees the sneak attack critical bonus, pickpocketing is available, and it is safe to steal, lockpick, or otherwise access "owned" property.
 * [DETECTED] means being in sight by one or more targets. Pickpocketing is enabled, but almost always fails. Attempting to steal, lockpick, or otherwise access "owned" property will fail, and may result in friendlies turning hostile or confiscating stolen goods. Sometimes an unfriendly target will see it and this will be shown instead of [CAUTION] because they haven't identified one as hostile or a target.
 * [CAUTION] means one or more unfriendly targets are suspicious of presence and are searching for the character. After a time the enemy will dismiss the disruption and the state will return to [HIDDEN]. Pickpocketing is disabled for the enemy searching for the player character. Attacking in this state will not cause a sneak-critical hit.
 * [DANGER] means hostile NPCs are aware of one's presence, but they do not know the current location. They will actively search at or around their last known location. If [DANGER] is flashing, it means that the enemy has the player character in sight and will engage them. The state can be returned to [HIDDEN] by moving away and staying out of sight for an extended period of time. As soon as the conditions are satisfied for any state, it replaces the one above it on the HUD even though one may still be in the [HIDDEN] state to several targets around them. Essentially, it reports the worst detection state towards the player character of all potential targets. Once all targets that satisfy the most-elevated state are dead (as in killing everyone actively pursuing the target in [DANGER]), the state will revert to the highest remaining state amongst the remaining targets.

Factors in successful sneaking

 * Sneak skill: The sneak skill applies whenever the player character crouches. The higher the Sneak skill, the easier it is to remain undetected while attacking, stealing, or picking someone's pocket.
 * Cloaking fields: The Stealth Boy and Chinese stealth armor will wrap the user in a cloaking field that makes them more difficult to detect while sneaking, rendering the character virtually undetectable by visual sighting. The stealth field effect is extremely powerful, and its value matters very little, so the Chinese stealth armor's stealth field of 5 works almost (but not quite) as well as the Stealth Boy's 75. It should be noted that this field only increases one's sneaking ability, and the field will be negated if leaving sneak mode in any conventional manner, such as firing a loud gun, since the cloaking field does not mask sound-based detection. The stealth suit only has a +15 Sneak versus the Stealth Boy's +100, but its effects are not limited by time. Cloaking can also be stacked, so the stealth armor and Stealth Boy can give the player character a combined field of 80.
 * Enemy's Perception: Enemies with low Perception are easier to sneak around than enemies with high Perception. A target's Perception will be impaired (a -4 reduction) if they have a crippled head.
 * Line of sight: While out of sight an enemy can only detect the target through sound.
 * Light level: The target is more difficult to see in dark areas or at night if outdoors. If the Pip-Boy light is active, it will give away the position.
 * Distance: The farther away the target is from the seeker(s) the less likely they would be detected.
 * Movement: The player character is hardest to detect when motionless and easiest to detect when running. One can avoid the sound aspect of this by the Silent Running perk. Moving faster while within line of sight will still make it easier to be seen, but one will no longer get any penalty for movement speed to sound made. Note that looking around counts as movement, but not as much as movement over ground. If standing in shadows with a direct line of sight to a target not aware of the location, looking around may very well make the target investigate, or immediately end with being found.
 * Contact: Touching or running into targets while crouched will immediately alert them of position. Regardless of how hidden one is, bumping into a target, friendly or otherwise, will immediately result in detection. This also applies to robots that have been deactivated using the Robotics Expert perk.
 * Equipped weight: The total weight of all equipped items (weapons count only if they are in one's hands) makes footstep volume louder for non-player characters. Silent Running removes this penalty entirely, allowing full sneaking ability in any equipment. The footsteps are still audible while sneaking with the perk, but non-player characters will not hear it.
 * Sound: Most guns make noise. Louder guns make one easier to detect. The silenced 10mm pistol, BB gun, dart gun, Infiltrator, Perforator , all melee weapons and all unarmed weapons are considered entirely silent, and thus will not break sneak mode. Grenades will also not break sneak mode, as they only make noise at where they explode. Hitting but not killing an enemy with silent weapons still causes the [CAUTION] alert. All other weapons will most likely be heard, depending on one's distance from the target. Missile launchers and Fat Man units make noise at the location and then at the impact point as well. The Pip-Boy's radio has no effect on sneaking.
 * Dead bodies: When an enemy spots a dead body, it will often enter the [CAUTION] state and move toward the dead body. This can often create a more or less perpetual [CAUTION] state while one remains in the area. If a dead enemy can be grabbed and dragged out of the way before it is found, the player character can retain their hidden state.

Sneak attacking
A Sneak Attack Critical occurs in Fallout 3, Fallout: New Vegas, Fallout 4, and Fallout 76 when attacking while [HIDDEN]. Successfully hitting the target will result in a guaranteed critical strike (unless the weapon does not allow critical hits), while at the same time doubling (ranged) or quintupling (melee/unarmed) total damage.

Pickpocketing
While sneaking and [HIDDEN] using the "activate" function while facing a non-player character (when in touching range) will allow pickpocketing them; in other words, steal items that they are personally "holding." This will open the inventories of both the player character and the target. Items can be transferred either way, and thus items can also be planted on them (also known as reverse-pickpocketing, or the "Shady Sands Shuffle").

For each item the player character steals or plants in a non-player character's inventory, there is a chance of being detected, with the formula being:
 * $$40+(0.6\times\text{Player Sneak})-\left\lceil\frac{\text{Item Value}}{2}\right\rceil-(0.6\times\text{Target NPC Sneak})=\text{Success Chance}$$

Since item value affects the overall chance of pickpocketing, this can lead to unusual situations where a full set of power armor in nearly-broken condition may have a higher chance of being taken than a well-maintained 9mm pistol. Additionally, the maximum and minimum chance of a successful pickpocket attempt is capped at 85% and 5% respectively. This means any item with a value of 190 caps or more will only have a 5% chance of success, even with 100 Sneak skill or with using a Stealth Boy.

If detected, the non-player character will automatically reclaim the item(s) taken and the player character will not be able to pickpocket that non-player character again (at least not straight away - leaving the area and returning 24 hours later should allow the player character to re-attempt this).

The player character cannot steal items a non-player character currently has equipped. To steal an item the non-player character is wearing or using (armor or weapon), the player character can reverse-pickpocket a better item (higher DR for armor, higher damage value for weapons) into their inventory and then leave and re-enter the area. The non-player character will equip the item with the highest stats and then the player character may try to steal the item they were using before. For melee weapons, reverse-pickpocket a better ranged weapon onto the non-player character along with the appropriate ammunition. When the non-player character is reloaded, it will instead equip that new weapon. Steal the melee weapon and the ammunition they were given, and reload the non-player character. The non-player character will be unarmed and the player character can steal the ranged weapon back. Keep in mind that just because the player character can do more damage with a specific ranged weapon doesn't mean that the non-player character will (usually but not always). Experiment to get the desired effect.

Successfully stealing the ammunition from an non-player character with a gun equipped will immediately render that non-player character unable to fire their gun. The amount of ammunition an non-player character is carrying will not match the amount they could fire if not pickpocketed (which is actually unlimited as long as they have 1 round for that weapon); this discrepancy can be attributed to gameplay purposes. This tactic can be used to influence the outcome of both non-player character/PC and non-player character/Non-player character conflicts. Concerning non-player character/Non-player character conflicts, this allows remaining non-hostile to non-player characters on both sides of a conflict (by not needing to intervene with gunfire) while still ensuring victory for one side or another. Mind that after some time, possibly the general respawn period of 72h game time, ammunition will be restored on NPCs.

Non-player characters will notice the pickpocketing of other non-player characters, hence the [DETECTED] sneak state, but in large open areas one can maneuver into gaps or around corners to avoid line-of-sight and return to [HIDDEN]. Non-player characters lying in beds may appear to have their back in one's direction, but still have one in sight, this is because as far as the game is concerned they are not facing the way they are lying down, that is merely an animation. To get "behind" an non-player character lying in bed, go to the foot or head of the bed.

The player character can also use pickpocketing offensively to plant a live explosive. This will detonate after a short time, automatically killing the target regardless of the PC's Explosives skill or the target's health or resistances.

The Mothership Zeta add-on added another way to pickpocket a friendly non-player character. Reverse-pickpocket a cryo mine or cryo grenade onto the non-player character. After exiting the pickpocket menu, the cryo charge will go off but only stun or knockdown the non-player character. The non-player character can then be pickpocketed like any disabled target, with the exception that the non-player character won't turn hostile when they recover.

Stealing items
Picking up non-player character-owned items (i.e. items whose text name is red) in the game world while sneaking and [HIDDEN] will allow stealing items undetected, though Karma penalties may still apply.

Ways to increase Sneak

 * Permanent
 * Leveling up (up to 10 + INT + 3 with Educated perk))
 * Bobblehead - Sneak (+10)
 * Chinese Army: Special Ops Training Manual (+1 or +2 with Comprehension)
 * Thief perk (+5 with each rank)
 * Tag! perk (+15)
 * Silent Running perk (+10, removes movement speed penalties to sneak)
 * Survival Expert perk (+0 - +6, depending on quest results)
 * Temporary
 * Fire ant nectar (+2-8 depending on current Agility, a perfect Agility of 10 will see no benefit.)
 * Recon armor (+5)
 * Shady hat (+5)
 * Poplar's hood (+10, additional +2 if less than 10 Agility)
 * Stealth Boy (+100, +75 cloaking field)
 * Chinese stealth armor (+15, also adds +5 cloaking field while crouched)
 * Lag-Bolt's shades (+3)

Fallout: New Vegas
Most aspects of the Sneak skill are the same as for Fallout 3 above. This includes the initial statistic, the sneak meter, stealing and pickpocketing, and Sneak Attack Criticals. Exceptions and new aspects are described below.

Sneak in quests and exploration

 * 25 Sneak can be used to convince Trudy to fight the Powder Gangers as an alternative to 25 Speech.
 * 50 Sneak is required to smuggle improved holdout weapons into New Vegas Casinos, as well as the interrogation room at Camp McCarran.
 * 50 Sneak is required to convince Mister Holdout to sell improved holdout weapons.
 * 75 Sneak is needed to convince Father Elijah that hiding in the Sierra Madre is easy, and effectively get him to come down into the Sierra Madre vault.

Ways to improve Sneak

 * Permanent
 * Leveling up (up to 10 + INT/2 + 2 with Educated perk)
 * Chinese Army: Special Ops Training Manual (+3 or +4 with Comprehension perk)
 * Agility Implant (+1 depending on current Agility, a perfect Agility of 10 will see no benefit)
 * Tag! (+15)
 * Skilled (+5)
 * Temporary
 * Fire ant nectar (+2-8 depending on current Agility, a perfect Agility of 10 will see no benefit.)
 * Recon armor (+5)
 * Stealth Boy (+100, +75 cloaking field)
 * Chinese stealth armor (+5)
 * ¡La Fantoma! (+10 or +20 with Comprehension perk)
 * Assassin suit (+10)
 * Salt-Upon-Wounds' helmet (+5)
 * Stealth suit Mk II (+15 or +25 with the Firmware 1.1 Upgrade)
 * Dead Horses stalker armor (+5)
 * Blood-Nap (+10)

Bugs
The "Sneak" skill has no difference in effect from 50 - 100. Only factors are the amount of light, running (pre-perk), stepping on some obstacles, and line of sight.