Progression — XP, Levels, the Power Curve

What it is. The curve that converts time-and-skill spent into durable power. XP fills a bar, the bar pops a level, the level buys a gain. The design work is not the bar — it is the shape of the curve and what each level is worth.

Player fantasy / why it’s fun. “I am becoming stronger, and I can feel it.” Every session ends with the character measurably ahead of where it started. The pull is the next gain always sitting just inside reach.

When to use / when NOT

Use it when	Skip it when
The fun is a rising arc — you build toward power	Mastery is the whole point; power is flat (Tetris, chess)
Sessions need a between-run ledger of “I earned something”	A run should reset clean every time with no carryover
You want a difficulty dial that the player turns by playing	Handing power out would erase the challenge that IS the game

If the game is pure execution (genre-precision-platformer, genre-bullet-hell), don’t bolt on XP — the level is the reward. A platformer’s “progression” is new verbs (genre-metroidvania), not bigger numbers.

Variants

Variant	Power comes from	Feels like	Example
Vertical	Bigger numbers on the same actions	Overwhelming what used to threaten you	Diablo levels; Vampire Survivors’ in-run levels
Horizontal	New options, not more damage	A wider toolbox, same lethality	Zelda items; Hades’ boon variety
In-run (soft)	Growth reset each run	A build arc per session	VS level-ups; roguelite per-run picks
Meta (hard)	Growth persists between runs	A campaign under the runs	See `system-meta-progression`
Milestone	Discrete unlocks at thresholds	Earned checkpoints, not a smooth ramp	”Beat act 2 → new class”

Most games layer two: an in-run vertical ramp under a meta horizontal one (Hades, Dead Cells). Name which axis carries the fantasy before you tune.

Tuning levers

Lever	What it controls	Healthy range / rule
XP-to-next curve	How fast levels come	Gentle geometric (×1.1–1.4/level). Flat = trivial; steep = a wall
Power-per-level	The size of a gain	Each level must be felt — no “+0.3% crit” filler levels
Time-to-first-level	The onboarding hook	Fast (< 60–90s of play). The first pop teaches the loop
Power-vs-challenge gap	Whether growth trivialises	Enemies/costs must ramp with the player (law 3 inequality)
Level cap / soft cap	Where the curve flattens	Cap before gains stop being felt, or switch axes (vertical→horizontal)
Catch-up	Whether a behind player recovers	Optional; XP scaling on lower levels. Pairs with `system-difficulty-and-dda`

The one number that breaks games: power outrunning challenge. State it as a law-3 inequality — enemy HP(t) grows ≥ player DPS(t) — and assert it, exactly as horde-survival asserts superlinear spawn pressure against multiplicative build growth ([[FUN.md §6]]).

How it wires to Hayao

Power is state. XP, level, and stat totals live in world.state; the level-up is a sim step, and the pick that spends it is an input action (never a direct mutation) so replays and goldens stay honest — the same rule incremental/idle lives by ([[FUN.md §14]]).
The pop is choreography. The sim resolves the level instantly and returns a “leveled-up” beat; the view (flash, chime, particles on LAYER_HUD) replays it and is cosmetic = true, deletable without changing world.hash() ([[FUN.md §6]]).
The curve is content, not code. Pace levels the way src/content/ generates and ramps levels: express the difficulty series and feed it to assertRamp/rampIssues (grep docs/API.md) so “the curve has no deserts and no walls” is a test, not a vibe.
Meta power persists through SaveManager — see system-meta-progression.

Fails when…

Filler levels. A gain you can’t feel is a chore bar. Fewer, bigger levels beat many tiny ones.
The curve deserts. A long stretch with no reachable next gain — the incremental sin ([[FUN.md §14]]). Assert first-gain times per tier.
Power laps challenge. The build trivialises the content and the game turns to autopilot; or challenge laps power and it’s a wall. Both break the CI window ([[FUN.md §6]]).
Vertical-only forever. Numbers with no new decisions get boring; introduce a horizontal axis (options, system-skill-trees) once damage stops thrilling.
Grind to progress. If the only path to the next level is repeating a solved encounter, you’ve built a timer, not a game.

Verify

Pacing/no-deserts and the payback discipline: [[FUN.md §14]] (incremental).
Power-vs-challenge as an asserted inequality: [[FUN.md §6]] (horde) and law 3 (derive constraints).
Ramp shape as a test: assertRamp/rampIssues over the level/curve series — the same instrument src/content/ ships.
Skill-delta: intended play out-earns null play by a margin (universal law 2).

Composes with

system-skill-trees — where horizontal power branches into builds.
system-economy — XP is one faucet; levels are one sink among many.
system-meta-progression — the persistent layer under run-scoped growth.
system-mastery-curve — power should raise the ceiling, not replace skill.
system-reward-schedules — the drip that fills the XP bar between levels.
pattern-pacing-and-tension — the level curve IS a pacing curve.