Luck Stat

Luck tilts the level-up reward rarity roll toward higher tiers. It does not affect XP gain, pickup magnetism, weapon damage, or any other roll outside the level-up card selection — only the rarity weights on _rollRarity in engine/world/leveling.ts.

Higher luck shifts probability mass off common and onto uncommon / rare / epic / legendary, with the strongest pull toward legendary.

Stat shape on the ship

Two fields on ship cooperate (engine/core/state.ts:300-301):

Field	Starts at	Role
ship.luck	0	Flat base luck. Set by artifact triggers (Caretaker, Bloodlust, Hunter, Lodestone, Absorption Barrier, etc.) granting a numeric bonus like +30, +90, +160. Also accumulates from the Luck modifier’s passive flat additions via the artifact flat-bonus system.
ship.luckMult	0	Multiplier on top of luck. The Luck modifier (data/modifiers.ts) pumps this stat: +0.10 per rank, capped at 20 ranks → +2.00 additive at L20 (+200%).

The roll uses the combined value:

effectiveLuck = ship.luck * (1 + ship.luckMult)

So luckMult only does anything once ship.luck > 0. A run with the Luck modifier maxed but no luck-granting artifacts active gets effectiveLuck = 0 * (1 + 2) = 0 — same baseline as no luck at all. The modifier amplifies a base; it doesn’t create one.

How luck shifts the rarity roll

_rollRarity() in engine/world/leveling.ts:171-191 multiplies each rarity’s base weight by (1 + effectiveLuck * _LUCK_BIAS[i]):

Rarity	Base weight	Mult	_LUCK_BIAS
common	50	1.0×	0
uncommon	30	1.25×	0.01
rare	15	1.5×	0.02
epic	4	1.75×	0.03
legendary	1	2.0×	0.04

common is always weight-50 — luck never starves it. The other four scale faster per point of effective luck, with legendary scaling fastest (4× the rate of uncommon). This is intentional so that stacking luck has a visible legendary payoff rather than just inflating uncommons.

Example: effectiveLuck = 50

Rarity	Weight = base × (1 + 50 × bias)
common	50 × 1.00 = 50.0
uncommon	30 × 1.50 = 45.0
rare	15 × 2.00 = 30.0
epic	4 × 2.50 = 10.0
legendary	1 × 3.00 = 3.0

Total weight 138.0 (vs 100 baseline). Legendary share goes from 1% → ~2.2%; common share drops from 50% → ~36%.

The Luck modifier

data/modifiers.ts:268-282:

• ID: luck
• Icon: 🎲
• Category: utility, rarity common
• Max level: 20
• Effect: { stat: 'luckMult', mode: 'flat', base: 0.10, perLevel: 0, k: -1 } — flat +0.10 per rank to ship.luckMult. At L20 that’s +2.00 (+200% additive).
• Description: “Better odds of rare level-up cards”

luckMult is in _FLAT_AS_PERCENT_STATS (engine/world/leveling.ts:211) so reward cards display rank gains as percentages (e.g. +10%).

Artifacts that grant luck

The flat ship.luck base comes from artifacts. Every artifact has a passive flat-bonus stat (ARTIFACT_FLAT_BONUSES in data/artifacts/index.ts) and most luck-themed artifacts also fire a modify_stat action on their trigger to spike ship.luck for a duration.

Passive flat-bonus → luck

These artifacts contribute flat luck just by being equipped:

Artifact	Trigger archetype
crate_buster	Crate broken
killstreak_rain	Kill-streak milestone
bloodlust	Kill-streak milestone
refresh	Various
event_reward	Event resolved
caretaker	player_healed
lodestone	Crate broken
hunter	weapon_fire counter

Burst +luck on trigger

These artifacts apply a temporary +luck buff via modify_stat when their signal fires:

Artifact	Trigger	Tier 1 → Tier 4 luckBonus	Duration
caretaker	player_healed	30 / 45 / 65 / 90	8–15s
bloodlust	Kill-streak milestone	60 / 90 / 130 / 180	15–20s
lodestone	Crate breaks	40 / 60 / 90 / 130	10–18s
hunter	Every N shots fired (50→30)	50 / 75 / 110 / 160	6–10s
absorption_barrier	Survive timer seconds without damage	35 / 50 / 70 / 100	until hit

The modify_stat calls all target stat: 'luck' in flat mode, so they stack additively with each other and with the passive flat-bonus on the same artifact. stacking: 'refresh' + maxStacks: 1 on most of them means re-triggering refreshes the duration rather than stacking new instances of the same source.

What luck does not affect

• Pickup magnet range (uses ship.magnetRange)
• XP gain per warp crystal (uses ship.xpGainMult)
• Weapon damage, fire rate, crit, anything in the fire pipeline
• Affix roll pools / affix drop tables
• Boss / event spawn odds
• Crate contents

Luck is strictly a level-up card rarity tilt. Every other “rarity” or “odds” system in the game uses its own roll function.

Related

• gameplay/concepts/rarity-tints.md — how rarity colors map to mults
• gameplay/concepts/pity-system.md — separate system for gacha pulls (not affected by luck)
• engine/world/leveling.ts — _rollRarity() and _LUCK_BIAS
• data/modifiers.ts — Luck modifier definition
• data/artifacts/ — Caretaker, Bloodlust, Lodestone, Hunter, Absorption Barrier