When a spellcaster takes damage while maintaining concentration on a spell, they need to make a Constitution saving throw to keep the spell active. The Difficulty Class (DC) of the save is the higher of 10 or half the damage taken (rounded down).

There are two feats that can help a spellcaster with this check:

War Caster gives you advantage on Constitution saves to maintain concentration when you take damage.

gives you advantage on Constitution saves to maintain concentration when you take damage. Resilient grants you proficiency in the saving throw of your choice.

Which one is the better feat for maintaining concentration on a spell?

Assumptions

Spellcasters first invest in their spellcasting ability score. Spellcasters will have +2 to Constitution at Level 1. Spellcasters prioritize Constitution over Dexterity. No metamaggin’.

Assumption 1. A caster has the maximum investment in their spellcasting ability score. Assumption 1 is probably a safe bet based upon our polling, since over 70% of casters max their spellcasting ability score first. It might also be worth looking into a “straddle” where a player first takes an ASI, then a feat, and finally bumps their spellcasting score to 20.

Assumption 2. Spellcasters have a +2 to Constitution at Level 1. This assumption is based on the point buy/standard array system. We can safely extrapolate from our polling data that players who spend their ASIs first on maxing their ability scores also value maxing that ability score at the character creation stage. After that primary ability score, casters most often benefit from Dexterity (which prevents them from getting hit and helps them move first in combat) and Constitution (which helps them stay alive when they take hits and maintain concentration on spellcasting). A 15 is your ability score cap in a standard point buy scenario. Most often, casters are not going to max a secondary stat over their spellcasting while also choosing a beneficial race to boost that stat above 15. In most cases, that puts the cap at +2 for a Level 1 character. However, since we know the importance of Constitution for casters, +2 is also a safe floor.

Assumption 3. Spellcasters want to invest in Constitution first. While there’s an argument to be had for Dexterity, we’re talking about a character that will be spending a feat on maintaining concentration. That signals a player that it invested in improving their Constitution saving throws. Our polling supports spellcasters’ inclination to invest Constitution over Dexterity (75%):

Assumption 4. We assume you’re not playing a Sorcerer, since they already gain proficiency in Constitution saving throws. If that’s the case, Resilient is a worse investment than an ASI for you.

Analysis

Max ASI

If we work on the assumption that a spellcaster will first max out their spellcasting stat before investing into maintaining concentration, Level 12 will be the key level for analysis, since that’s when the character will be taking the feat.

When they get access to the feat at level 12, most spellcasters will have 16 Constitution. That’s because if you max out your spellcasting score, you’ll start at 17 in your spellcasting ability score. Then, two ASIs will leave you with one leftover point to spend.

If you’re planning on taking War Caster, you would start with 15 Constitution and spend the extra point to increase your Constitution to 16 for the +3 bonus, before going into the feat.

If you’re planning on taking Resilient (CON), you can start with 15 Constitution and use the feat to get to +3. Or, you can start with 14 Constitution and use the floating ASI to get the bump to 16. There’s little point to starting at 15, using the ASI, and taking Resilient, since it will leave you with an odd ability score.

It’s helpful to gain access to this extra ASI because it can help you boost your Dexterity score. Getting hit less often means making fewer concentration checks. This can make up some of the Armor Class (AC) ground that the Resilient character may be giving up by sacrificing a shield.

16 CON @ Level 12

War Caster starts out with a small advantage against lower DCs.

When you can take the feat at level 12, War Caster gives you better odds until the DC hits 19 (37 damage). At 38 damage and up, Resilient is the better feat by a significant margin.

This appears to make War Caster the better option, since most hits that trigger concentration checks aren’t going to pack such a punch. The average mage taking 38 damage a hit will be dead after three hits. Maintaining concentration becomes secondary to staying alive.

16 CON @ Level 13-16

After just one level, War Caster’s advantage evaporates.

At level 13, the proficiency bonus increase pushes Resilient firmly into the lead. It does equal or better than War Caster against every DC.

The green dotted line here shows your odds of maintaining concentration if you take both feats.

These numbers stay the same until you’re ready to select your next feat at level 16.

16 CON @ Level 17+

At higher levels, Resilient’s advantage only grows larger.

Whether you increase in level or delay the feat to raise your Constitution ability score, Resilient is always the better option at higher levels. If you’re taking the feat at level 12, I recommend taking Resilient (CON), unless your campaign is ending at that level.

Straddle

Since many players use their first ASI to boost their spellcasting ability score, but then take a feat before maxing it out, we can also perform the analysis for taking a feat at Level 8.

This analysis is a little different, since it’s possible we won’t get the floating ASI from maxing out our spellcasting score to round out our Constitution score. However, the War Caster can start with a 17 in its spellcasting score and split the level 4 ASI to get up to 16 Constitution. So, let’s look at 16 and 15 Constitution scores.

16 CON @ Level 8

If you’re taking a feat at level 8, War Caster comes out strongly ahead.

Warcaster sees a significant advantage all the way up to DC 23. There’s not a lot of CR 8 enemies dealing out 46 damage per hit. This makes War Caster a better option in the mid-level range.

15 CON @ Level 8

Notice how this difference is smaller with an odd Constitution score.

An odd Constitution score will benefit the Resilient (CON) user, who will also get the ability score increase.

The Lesson

Need a feat faster?

Go get War Caster.

Maxing CHA-WIS-INT?

Wait for Resilient.



If you want to check the actual percentages or run your own numbers, you can find the concentration save calculator on the ThinkDM Patreon.