Skill Trees — Branching Unlocks & Builds

What it is. A graph of unlocks the player buys with a progression currency (points, cells, essence). The graph’s branches — and the fact that you can’t afford all of them — turn “get stronger” into “become a kind of strong.”

Player fantasy / why it’s fun. “This build is mine.” The tree is a canvas; the player paints a strategy and then gets to test it. The pull is the road not taken — every point spent is a small identity claim.

When to use / when NOT

Use it when	Skip it when
You want players to express strategy through pre-commitment	There’s no meaningful choice — every node is strictly good
Replay value comes from “try a different build”	One optimal path exists; the tree is a formality
Power is horizontal (`system-progression`) and needs structure	A flat list of upgrades reads clearer (small games)

A tree with no exclusivity is just a shopping list. If the player will eventually own every node, the branches are decoration — either add scarcity (can’t afford all) or opposition (this node locks that one).

Variants

Variant	Structure	The choice is	Example
Prerequisite web	Nodes gate deeper nodes	Order & reach	Path of Exile’s lattice
Point-buy, no gates	Flat pool, scarce points	What to skip	Classic RPG stat spend
Mutually-exclusive forks	Pick A xor B at each fork	Identity	”Fire OR frost, never both”
Class/archetype trees	Separate trees per role	Which fantasy	Diablo class trees
Draft-as-tree	The run’s picks form an implicit build	Emergent shape	Hades boons; StS deck

The fork variant carries the most meaning per node and the most respec pressure — see levers.

Tuning levers

Lever	What it controls	Healthy range / rule
Point scarcity	How much you must skip	You should afford ~40–70% of a tree per playthrough — enough to specialise, not complete
Branch exclusivity	Whether a choice costs another	At least the capstones should be exclusive, or builds converge
Node granularity	Fewer big nodes vs many small	Big nodes = legible builds; tiny nodes = fiddly filler
Capstones	The payoff that defines a build	Each branch needs a “this is why you went here” node
Respec cost	Reversibility	Free in experimental games; costed where commitment is the point
Gating depth	Prereqs before the good stuff	Shallow — burying power behind 8 filler nodes reads as grind

Respec is grace ([[FUN.md law 5]], the anti-frustration mindset). A tree the player is afraid to touch is a tree they don’t engage with. Default to generous respec unless permanent commitment is an explicit pillar.

How it wires to Hayao

The tree is data. A node graph (id, cost, prereqs, effect) in world.state; unlocking is an input action that spends a currency and flips a flag. The effect is applied by pure logic reading owned-node flags — so a build is a reproducible input sequence, and clone-and-score bots can pilot builds to measure them ([[FUN.md law 7]]).
Exclusivity is a solver constraint. “Can’t own A and B” is a rule the pure logic enforces and a test asserts. Prove every branch is viable by piloting each and asserting it beats a null build — the skill-delta proof ([[FUN.md law 2]]), applied per branch, is your balance instrument for system-build-diversity.
The tree UI is chrome. Draw it with showScreen() (DOM menu) — it’s not sim, so it doesn’t belong in world.hash(). Grep docs/API.md; per CLAUDE.md invariant 4, menus are DOM.
Persist unlocks via SaveManager when the tree is meta (system-meta-progression).

Fails when…

A trap node. A branch that’s strictly worse wastes the player’s one irreversible choice — punishing, not deep. Prove each branch viable.
A God node. One branch dominates; everyone funnels to it and the tree collapses to a line. The win-rate window per build catches this.
No exclusivity. Own-everything trees have no decisions. Add scarcity or forks.
Filler gating. Prereq chains of dead nodes to reach the good one — grind wearing a tree costume.
Respec you fear. No respec + a trap node = a bricked character. Give an out.

Verify

Per-branch skill delta — each build beats a null build: [[FUN.md law 2]].
Build viability as a win-rate window, both edges break CI: [[FUN.md §11]] (deckbuilder’s balance instrument).
Build as a reproducible pilot (clone-and-score): [[FUN.md law 7]].

Composes with

system-progression — the tree spends what progression earns.
system-build-diversity — the tree is the machinery that makes many builds real.
system-tech-tree — the same graph shape at the strategy scale (a whole faction).
system-meta-progression — persistent trees between runs.
system-economy — skill points are a currency; the tree is a sink.