Difficulty & Dynamic Difficulty

What it is. How hard the game is, over time and per player. Three tools: a difficulty curve (a designed sequence of spikes and breathers), a director (a system that spawns pressure in response to state), and assist modes (honest knobs that let anyone find their edge). The goal is the flow channel — challenge tracking skill, never far above or below (pattern-mastery-and-flow).

Player fantasy / why it’s fun. Being tested at exactly your level. A curve that breathes lets tension land — a boss hits harder after a quiet corridor. A director makes a run feel authored to you. Assist modes let the humane version of the fantasy exist (Celeste): the game meets you, it doesn’t gate-keep you.

When to use / when NOT

Tool	Reach for it when	Don’t when
Authored curve	linear/campaign content; you control order	fully procedural — shape the generator’s band instead
Director (DDA)	survival, colony sim, replayable runs; you want reactive pressure	tight puzzles — perfect information forbids hidden dials (§12)
Assist modes	any single-player skill game	competitive PvP where fairness is symmetry

Two ditches. Static difficulty leaves half your players bored and the other half walled out. A hidden rubber-band that punishes doing well (“do better, get punished”) betrays trust the moment it’s noticed. DDA must nudge, never lie.

Variants

Variant	How it decides	Feels like	Anchor
Authored curve	fixed schedule of encounters	a designed rollercoaster	most campaigns
Threat-budget director	spends a rising budget on spawns	escalating siege	`anchor-rimworld`‘s storyteller
Performance DDA	reads hp/deaths/time, adjusts spawns	”the game read me”	Left-4-Dead-style
Player-chosen tiers	difficulty menu	honest self-selection	Doom, most action games
Assist toggles	per-axis mercy (speed, i-frames, skip)	“I set my own edge”	`anchor-celeste` assist mode

Tuning levers

Lever	Effect	Watch for
Spike/breather rhythm	tension arc	monotone-rising = exhausting; flat = dull
Director budget slope	how fast pressure grows	superlinear needed to outpace build growth (§6)
DDA sensitivity	how sharply it reacts	too twitchy = visibly rubber-bandy
DDA bounds	floor/ceiling on adjustment	unbounded = trivializes or walls
Assist granularity	per-axis vs. one slider	one slider forces all-or-nothing

How it wires to Hayao

The director already exists. initDirector(waves) + pollDirector(waves, state, world.time, world.rng) (grep docs/API.md, src/content/dsl.ts) is a deterministic pressure authorer: waves fire on sim time, weighted spawns roll through world.rng, and catch-up after a restore never drops spawns. Keep DirectorState in world.state so pressure is hashed and replayable.
Performance DDA reads the probe. The same numbers world.probe() exposes (hp floor, time alive, deaths) are the DDA inputs — bounded, ordered, and part of world.hash() so difficulty can’t silently escape determinism.
The curve is checkable. assertRamp(difficulty, opts) / rampIssues (grep docs/API.md) assert the shape — a designed ramp with breathers, not monotonicity. Author the sequence, then prove it breathes.
Assist modes are tuning. Model them as declared tuning: knobs (docs/WORKSHOP.md); their resolved values live in world.hash() via world.tune(key), so an assisted run is still a first-class, replayable artifact — not a second-class cheat path.

Fails when…

Monotone difficulty. Always-harder with no breather has no arc — tension can’t spike without a valley (FUN.md §8, pattern-pacing-and-tension).
Visible rubber-band. Players who notice they’re punished for playing well stop trying — the trust cost outweighs the smoothing.
Unbounded DDA. No floor/ceiling and the director either trivializes the game or spirals it out of reach.
Assist as shame. Locking achievements or nagging on assist turns a humane tool into a punishment. Assist is a setting, not a confession (Celeste).
Sublinear survival pressure. In hordes, linear spawn loses to multiplicative build growth — the ramp must be superlinear (§6).

Verify

Curve breathes: assertRamp/rampIssues on the difficulty series — each wave ≥ ~55% of prior, finale peaks (FUN.md §8).
Win-rate window, not a point: a competent bot should land inside a band (e.g. 11–19 of 20) — both edges break CI (FUN.md §11).
Null loses at every tier: the do-nothing/undefended run fails on easy too (FUN.md law 4).
Determinism under DDA: golden-hash a scripted run at each setting; tuning values are in world.hash() (WORKSHOP knob-change semantics).

Composes with

system-encounter-design — the units a curve or director sequences.
system-onboarding — the first ten minutes are the gentlest slope; the curve starts here.
system-accessibility — assist modes are the accessible face of difficulty.
pattern-pacing-and-tension — spikes/breathers are pacing made mechanical.
pattern-mastery-and-flow — the flow channel is the target the curve tracks.