Survival (Fallout 4)

Survival mode (sometimes called Survival difficulty) is a unique gameplay mode in Fallout 4 with significant differences to other modes. Survival mode was added to the game for all platforms on March 29, 2016. It replaced the existing Survival difficulty, which was renamed to Very Hard.

General features
Survival mode can be accessed in the options menu. Unlike Hardcore mode used in Fallout: New Vegas, Survival mode can be set before starting a new game. This mode also cannot be toggled freely. Once Survival mode is activated, turning it off will permanently disable it from being reactivated for that save file.

In survival mode, autosave and manually saving from the menu has been disabled. The only means of physically saving the game is to sleep in a bed, on a mattress or in a sleeping bag. The exit save function is still available, but is a temporary save that is deleted automatically upon loading. Additionally, the use of console commands is disabled.

Ammunition now has a set weight, per round, based on the ammunition type. For example the 10mm round has a weight of 0.025 per round, where as the mini nuke has a weight of 12. If one becomes overencumbered, their Agility and Endurance will drop by 2, they will suffer from Fatigue and they will take constant damage to their legs. Rather than permitting fast travel while overencumbered, the perk Strong Back eliminates these negative effects. Further to this, carry weight for the Sole Survivor and their companions has also been reduced.

Locations take 35 in-game days for non-cleared locations and 80 in-game days for cleared locations to repopulate enemies and loot, as opposed to the standard 7 and 20 days respectively. Additionally, the workshop's inventory has been restricted, which may affect the availability of production items.

While fixed enemy locations have increased repopulation times, the occurrence of random enemy encounters is two days or less.  * It is possible to start to sleep for an hour, then cancel out. Time will not progress, but the effects of one hour of sleep will be applied, and the game will be saved.

Travel and navigation
Fast travel is disabled, with the exception of entering the institute, which can only be entered via fast travel while they are an available faction. Being a member of the Institute however, will not allow one to bypass fast travel's elimination as the only outward teleportation point is the C.I.T. ruins. However, traveling via Vertibird using a Vertibird signal grenade is unaffected.

Alongside the changes to fast travel, the compass's range of detection has been significantly restricted to a matter of yards for locations of interest and completely removed for NPC detection, hostile or otherwise. Enemies will still show on the compass if they are quest targets, e.g. in the Taking Point or Randolph Safehouse quests.

Combat
In combat, there have been several key changes. Firstly enemies have had their attack strength (damage output) and overall health/damage resistances increased. The Sole Survivor's damage output is reduced relative to enemies. This generally results in more challenging battles. Also the encounter frequency of higher level enemies has been increased, adding an additional layer of difficulty.

Experience Points however, have been doubled. Additionally a new perk, Adrenaline, is automatically applied to the Sole Survivor for every five enemies they kill. This perk gradually increases up to a 50% increase in damage, but the bonus is reduced upon sleeping, relative to the time slept.

Health and well-being
Prior to Survival mode being added, only Fallout: New Vegas '  hardcore mode made food and drink a necessity. In Survival mode, the Sole Survivor must remain hydrated, eat periodically and have a regular sleep cycle in order to survive. Failure to manage one's well being will result in negative effects such as Fatigue, Hunger and Thirst, reducing maximum AP, and increasing the risk of contracting an illness. Illness risks come from various sources, such as food, drink, chem use, being hit by many creatures' unarmed attacks, immersion in water, or rain. Immune deficiency can be caused by certain chems, further increasing the risk of disease. Poor sleep quality (more specifically: sleeping too frequently, for shorter periods) may increase the risk of disease, meaning that a pattern of using low-quality sleeping places (sleeping bags, mattresses) may indirectly increase the risk of disease (see Disease risks section below for more details).

Healing
Additionally, sleeping in a bed for less than 7 hours will not heal all Hit Points (HP). There is a sliding scale of health recovery for sleep durations of less than 7 hours. Furthermore, the healing rates of all health-restorative items are significantly slowed, rather than being instant, and the use of medicinal items such as stimpaks, RadAway and chems can result in thirst, hunger and immune deficiency, which will then increase the risk for the Sole Survivor becoming ill. However, visiting a doctor for purposes of restoring health, removing rads and curing addictions still carry no side effects. Paying a doctor to "Cure Health" will remove all diseases, in addition to restoring the player character to full hit points, at no extra charge.

Survival mode adds antibiotics, a rare loot item, craftable, also obtainable from some doctors and vendors, which will immediately cure all diseases, with no side effects. Antibiotics can be self-administered "in the field" and do not require travelling to a doctor. Survival mode also adds various herbal remedies of different types. These can be crafted, found in loot, or bought from vendors, and are more common than antibiotics. Each type of herbal remedy will reduce the chance of contracting certain specific disease types only (not all types). The herbal remedies are preventive rather curative: they must be taken prior to exposure, or at least, prior to sleeping (see below section on Disease risks).

Crippling
If one is crippled in any form, this no longer recovers with time and must be rectified with a stimpak, by sleeping in a bed, mattress or sleeping bag for two hours, or by visiting a doctor for treatment. The third rank of Adamantium Skeleton however, largely negates this effect, as it makes the Sole Survivor's limbs immune from the effects of crippling.

Companions
As noted above, carry weight of companions has been reduced.

When downed in combat, companions will no longer heal themselves automatically and thus require a stimpak to recover from their injuries. If left unattended, they will leave the Sole Survivor's traveling party and return to their most recent settlement to recover. Codsworth will require a Robot repair kit for healing if Automatron add-on is installed.

Generic robotic companions constructed at the robot workbench as part of the Automatron add-on function differently from other companions. As they no longer hold essential status, they will explode on death, becoming unrecoverable.

Wellness
There are five stages in each category of Wellness that span in-game hours. Decreases in Wellness condition are called Fatigue (not to be confused with the illness of the same name). Decreases in Wellness are communicated to the player with a "Fatigue" pop up warning. The increasing thresholds for each level of Wellness condition are on a curve. For example, it takes about 6 hours without eating to go from Well-fed to Peckish to Hungry, then a further 12 hours to go from Hungry to Famished.

The tables in the sections below show the approximate times that it takes for a character to go from completely satisfied to the listed condition in each area. In-game time is expressed in "hours" and real-time is expressed in "minutes." These progressions can be sped up when using certain items - Stimpaks and many chems inducing dehydration, RadAway causing hunger, etc.

Hunger and Thirst
Generally speaking, the value that an item contributes or hinders one's wellness is equal to the monetary value of the item used (as shown in the Pip-Boy inventory, not as shown during trading with a vendor). Benefit effects tend to use the full value, while negative effects tend to use a fraction of the negative value. High monetary values (positive or negative) are capped at a maximum effect value that varies by Wellness level.

There is also a minimum effect value, generally equal to half the size of current Wellness band (for most bands, but not all). The effect of this is that most hunger and thirst conditions can be cleared by eating or drinking two items of any type, regardless of how low their monetary value is. An implication of this is to eat low value food and drink to deal with hunger and thirst, while saving higher value items for their buff effects.

Disease risk
Unlike Hunger and Thirst, Disease uses a minimum (and maximum) base percent chance ("risk pool"), which is increased by a flat additive percent value based on the category of item or exposure event involved. This additive value ranges from +1% (normal food) to +12% for High-risk foods, with a typical value being around +5% (eg creature bites/claws) to +7% per exposure (eg chems and Standard-risk foods).

Only the high-risk events (which include bites, most chems, swimming and cannibalism as well as the riskier foods) also trigger an immediate, intra-day check for contracting a disease. Other exposure events (rad treatments, caffeine drinks, normal food, rain) simply accumulate risk in the disease risk pool. The disease risk pool is checked upon going to sleep, or on any high risk event (subject to some cool-off timers). Whenever the disease risk pool is checked, it is either cleared down fully to the minimum (if a new disease is contracted) or drained slightly (if no new disease is contracted).

Disease risk is directly increased by overall Fatigue (the inverse of Wellness). Fatigue is a weighted average of Hunger, Thirst and Sleep factors. Hence it is important to minimise all these aggravating risk factors, particularly prior to sleeping.

Taking herbal remedies will reduce the disease chance by -20%, for the diseases that the specific remedy protects against. However the remedy must be taken no more than one game hour prior to the risk check (sleeping or high-risk event).

If the player character is in an immune suppressed state (a side effect of RadAway and Lorenzo's Serum) then the total disease risk is increased x 120%. (It may be advisable not to sleep until this effect state has expired.)

Any amount of sleeping will clear down the disease risk pool to its minimum value, adjusted for Wellness. Low quality sleep (on mattresses etc) may increase the overall chance of getting a disease, if the effects of Fatigue outweigh the effects of reducing the risk pool to the minimum level, particularly considering that more frequent sleep will lead to more frequent disease checks.

Generally speaking, the best time to take disease prevention measures is just before sleep or (if it can be anticipated) just before high-risk exposure. It is a very good idea to be fully fed and fully hydrated at the point of going to sleep, particularly if you are also tired and/or have had high exposure to disease risk factors.

Disease risk values
Chems trigger +7% to the risk pool, immediate check. (From a script comment it looks like all addictive Chems will trigger this.)
 * Special cases:
 * +0% to pool for Purified Water, no check
 * +2% to pool for Nuka-Cola, no check

Thirst
Thirst will negatively impact the Intelligence, Luck and Perception stats, until corrected. Drinking dirty water, either bottled or direcly from open water sources such as puddles and rivers, increases the possibility of contracting an illness. As a means of ensuring one has a steady supply of water, the Sole Survivor can now fill empty bottles with dirty or purified water. Dirty water is obtained from sources such as rivers, puddles or ponds. Purified water is obtained from water pumps and water fountains, or direct (without needing to supply an empty bottle) from the workshop inventory of a settlement with a water supply surplus, as in a vanilla game.

Nuka-Cola (and variants) give only 40% of the expected thirst reduction, based on their monetary value (capped), and cause increased hunger equal to 20% of their monetary value (capped).

While sleeping, the increase of thirst is 25% of normal, ie 75% less than standard.

Hunger
While the Sole Survivor is hungry (including Peckish - anything other than Well Fed), the healing and bonus properties of normal (non-wild) food consumption is suppressed until enough is consumed to make the survivor "well-fed."

Eating normal food that grants a bonus (e.g. the +1 Agility of deathclaw steak) while not Well Fed will remove the bonus if it is already active. For example, eating a deathclaw steak while not hungry to gain the bonus, then becoming Peckish, then eating a second deathclaw steak to become well-fed will negate the +1 Agility bonus and would require eating a third deathclaw steak to regain it. This is probably a bug.

The amount of food that must be eaten to reduce hunger is relative to its quality (which is equal to its nominal price), i.e, a ribeye steak satisfies hunger more than a grilled radroach. Certain foods, such as soups and certain crops, will satisfy both hunger and thirst, to the same value as their nominal price, so in effect have double worth or double effect.

Wild plants in general do not reduce hunger, but they do apply their other buffs regardless of whether the player is Well Fed or not. Razorgrain also behaves this way, which is almost certainly a bug. Certain wild plants (wild corn, wild mutfruit, wild tarberry, silt bean) behave like normal plants, not wild plants, in this regard.

The Cannibal perk has a special interaction called Dark Craving when eating a corpse is used to satisfy hunger.

While sleeping, the increase of hunger is 25% of standard, so 75% less.

Sleep
Sleeping for at least one game hour on any bed, mattress or sleeping bag will save the game, and remove any Fatigue the player character has developed from being tired (but not Fatigue from hunger or thirst). The Sole Survivor cannot sleep more than 3 game hours in a sleeping bag or a tent, and more than 5 hours on a mattress. To sleep for longer amounts of time, it is required to use a real bed. Attempting to sleep for longer than these times will result in one waking up at the time threshold.

More frequent sleeping indirectly increases the risk of contracting a disease, particularly if there is exposure to disease risk events between sleeps. This is because while sleeping clears down the accumulated increased disease risk, a higher minimum risk is imposed by Fatigue (including hunger and thirst), and a disease check is forced each time the player goes to sleep, checked prior to any reduction in the disease risk pool.

Sleeping for seven game hours or more allows one to recover full health and if sleeping in a bed as opposed to a mattress or sleeping bag, will ensure one is well rested and minimizes risk of illness. Being asleep for any period of time, in any bed, of any type, will force a check for illness. It will then clear the disease risk pool down to the Fatigue-adjusted minimum (see the Disease risk section) regardless of how brief the period of sleep is.

The sleep counter runs for 14 game hours after waking up fully rested. In order to minimise Fatigue from lack of sleep, with a regular sleep time each day, the optimum sleep duration would be 10 game hours (if there is no combat, see below); otherwise the player character will get the Tired warning at a different time on each successive day.

Entering combat (and potentially any AP-using activity such as sprinting) increments the sleep counter by 15 game minutes, i.e. brings the next sleep Fatigue condition 15 minutes closer.

Consuming caffeinated drinks such as Nuka-Cola will delay the need to sleep for some time. However, caffeinated drinks have hunger increasing effects as well as reduced thirst quenching effects, and alcoholic drinks will typically cause increased Thirst rather than reducing it.

Note: The in-game / real-time data in the tables above are approximations only.

Disease types
There is no correlation between player character actions and any particular disease type. Disease type is almost totally random, the only exception being that the game will not assign the same disease type consecutively. (But might well assign first one disease, then a different one, then the first one again).

It is not possible to contract more than one disease per game day, and it is not possible to contract a disease on the first game day. Subject to this, it is possible to have multiple diseases at the same time, and it is theoretically possible to contract all disease types simultaneously.

Disease durations

 * 144 hrs Fatigue (game time value)
 * 72 hrs Infection
 * 60 hrs Insomnia
 * 96 hrs Lethargy
 * 168 hrs Parasites (NeedMoreFood)
 * 48 hrs Weakness

It is possible that (like hunger and thirst) these counters run down more slowly during sleep, or don't run down at all. (Unconfirmed)

Bugs

 * If the squirrel stew is used to satisfy both the Parched and Peckish effects simultaneously while an other food bonus is already in effect, this food bonus will become permanent. The bonus description will disappear from the Status menu the moment the squirrel stew is eaten but the bonus itself will remain and can be checked in the SPECIAL page or in the affected stat section if such bonus involves Damage Resistance, Energy Resistance or others. Some bonus cannot be checked without console commands, so if one wishes to avoid accidentally exploiting this glitch, it's best not to use the squirrel stew while an other food bonus is in effect.
 * When first joining the Railroad after completing Tradecraft, the message "You can now fast travel to and from Railroad HQ" still appears, despite the fact that fast travel is disabled in Survival mode.
 * Starting in Survival mode and later changing the game mode will not change the water production from water purifiers.
 * Cleansing the Commonwealth for Knight Rhys seems to have a bug when getting the Clear Breakheart Banks quest. No flag appears on the super mutants, and no objective is given for finishing quest. Since use of the console isn't allowed, no fix is available.
 * As noted above in the Hunger section, Razorgrain behaves like a wild plant for nourishment purposes, and three wild plants behave like their domesticated equivalents.