Tech Tree — Research Gating & the Ramp of Options

What it is. A tech tree is a system-skill-trees at the strategy scale: a graph of researchable capabilities — units, buildings, upgrades, ages — where each unlock widens what the player can do, and prerequisites impose an order. Where a skill tree shapes a character, a tech tree shapes a civilization’s whole arc.

Player fantasy / why it’s fun. “My tech path is my strategy.” The order you research is a plan; a well-timed unlock is a power spike (anchor-age-of-empires age-ups). The pull is the branch just out of reach — the tech that would answer what’s beating you.

When to use / when NOT

Use it when	Skip it when
Strategy is expressed through what you unlock, when (RTS, 4X)	The game has a fixed toolset from the start
Build orders and tech timing are a skill	Research would just be a passive timer
You want power spikes to punctuate a long session	Sessions are too short to ramp options

The tech tree is the RTS/4X engine of pattern-pacing-and-tension: each age or era is a plateau, each unlock a spike. If unlocks don’t create timing decisions (rush this or bank for that?), you have a checklist, not a tree.

Variants

Variant	Structure	The decision is	Example
Age/era gates	Discrete tiers; advance to unlock a bracket	When to age up (spike vs eco)	AoE ages; Civ eras
Prerequisite web	Deep dependency graph	Route through the tree	Civ tech web; Factorio science
Exclusive branches	Pick a path, lock the other	Identity/commitment	Civ policy trees; StarCraft tech choices
Cost-gated flat	No prereqs, priced by power	Priority	Simple upgrade shops
Faction trees	Different tree per faction	Asymmetric strategy	See `system-faction-asymmetry`

Tuning levers

Lever	What it controls	Healthy range / rule
Tier/age spacing	Time between power spikes	Long enough to feel the plateau, short enough to keep momentum
Branch exclusivity	Whether a path costs another	Some branches exclusive, or every game converges to the same tech
Research cost curve	The ramp of options	Rising cost per tier; but each tier’s value must rise with it
Gate depth	Prereqs before a payoff tech	Shallow — burying the fun behind filler tech is grind
Counter-availability	Can you tech into an answer?	Yes — a tech tree is where RTS counters live; every threat needs a reachable response
Snowball vs comeback	Does a tech lead compound?	Add catch-up / comeback techs, or leaders run away (`pattern-feedback-loops`)

How it wires to Hayao

The tree is a data graph + pure logic. Nodes (id, cost, prereqs, unlocks) in world.state; researching is an input action that spends resources and flips availability flags; unit/ability availability is pure logic reading those flags. So a build order is a reproducible input script a bot can pilot ([[FUN.md law 7]]).
Strategy is the balance test. The RTS truth ([[FUN.md §9]]): the intended line (e.g. turtle → tech → counterpush) must beat a naive line (attack-move / one-tech rush). Pilot both, assert the delta — that’s your proof the tech choices matter (law 2).
Counters must be reachable. Every enemy threat needs a tech-reachable answer; assert it the way tower-defense proves counter duels from both sides ([[FUN.md §8]]) and RTS proves every counter edge wins its NvN ([[FUN.md §9]]).
The tech panel is chrome — showScreen() DOM; the research state is sim (CLAUDE.md invariant 4). Generate/ramp the option-unlock curve as content (src/content/, assertRamp).

Fails when…

A dominant path. One tech order is always best; the tree collapses to a line. Skill-delta between paths should be small; each viable path should beat null.
Unreachable counters. A threat with no tech answer is unfair — the RTS/TD counter proof fails from one side.
Filler gating. Dead prereq techs padding the route to the good one — grind.
Runaway snowball. A tech lead that compounds into an unbeatable lead with no comeback tech (pattern-feedback-loops).
Spikes with no plateau. Constant unlocks = no pacing; the age-up thrill needs the plateau before it.

Verify

Intended tech line beats naive line (strategy is the test): [[FUN.md §9]], law 2.
Every counter edge wins its duel, both sides: [[FUN.md §9]] (RTS) / [[FUN.md §8]] (TD).
Build order as a reproducible pilot: [[FUN.md law 7]].
Option ramp shape as a test: assertRamp/rampIssues over the unlock curve.

Composes with

system-skill-trees — the same graph shape at the character scale.
system-resource-loops — research consumes the production chain’s output.
system-economy — research points/cost are a currency and a sink.
system-faction-asymmetry — different trees are a core asymmetry lever.
system-counter-systems — the tree is where reachable counters live.
pattern-pacing-and-tension — ages/eras are the plateau-and-spike engine.