# GUTS — Game Design Document (v1, round 1)

Owner: Lane C (with F sign-off). This v1 settles the open questions from the original
concept discussion; C refines numbers/encounters in-lane, but the **decisions** below are
integrator-settled for v1.

> **⚠ Lane C round-1 refinements** (2026-07-16) — changed in place, per charter:
> - **§Structure** — "6–10 min/level" replaced with **per-level** targets. L2 is deliberately
>   the short one: it is the speed level, and a 6-minute esophagus is a corridor.
> - **§Enemies** — esophagus roster filled in (eosinophil swarm, candida bloom). All three
>   L2 enemies are real esophageal pathology; see LANE_C_NOTES.md §L2 roster.
> - **§Structure** — L2's signature mechanic now names the three real anatomical
>   constrictions, which are the level's three setpieces.
>
> Not changed, and not up for relitigation in-lane: the flow-locked hybrid, the 5+1 level
> structure, the coat/hull split, the boss roster. Boss *specs* (phases/telegraphs/windows)
> are in LANE_C_NOTES.md §Boss specs and fold into the GDD once F signs them off.

## Fantasy & tone

You are ENDO-1, a one-pilot micro-probe injected to clear a patient's digestive tract of a
hostile infestation, mouth to exit. Tone: playful-gross medical sci-fi — *Fantastic Voyage*
wonder, *Star Fox* arcade flow, a wink of *Osmos*-y biology-nerd humor. The body is not the
enemy; it's the weather. Enemies are the infestation + an immune system that can't tell
you're friendly.

## DECISION — movement model: "flow-locked" hybrid

Neither pure rail nor pure 6DOF — **the anatomy picks the mode**:

- **Tube mode** (esophagus, intestines): peristalsis is a forward current you cannot fully
  fight. Base flow speed comes from the biome (`world.biomeAt(s).flow`); throttle gives
  ±40%, boost gives short +120% bursts. Full analog movement in the cross-section disc
  (mouse/stick aims, ship banks). You steer *within* the tube; the tube carries you. This is
  the Star Fox layer: readable, forward, fast.
- **Arena mode** (oral cavity, stomach, boss lairs): the same controller unlocked to free
  6DOF, Descent-style, no gravity. Slower, spatial, exploratory. This is where fights and
  secrets breathe.
- Transitions are diegetic: sphincters are literal gates between modes.

Rationale: pure rail wastes the stomach; pure 6DOF makes the esophagus a boring corridor.
The hybrid also keeps collision cheap in the 90% case (tube = radius test).

## DECISION — structure: 5 levels + gates + 1 secret

Each level ends at a real anatomical valve, staged as a gate ritual or boss:

| # | level | mode | signature mechanic | exit gate |
|---|---|---|---|---|
| 1 | **Oral Cavity** | arena (tutorial) | dodging molar crush-cycles, saliva tides | **Epiglottis** — timing gate: dive on the swallow |
| 2 | **Esophagus** | tube (speed run) | surf peristalsis crests for boost; the three real constrictions (**aortic squeeze** → **diaphragmatic ring gate**) as setpieces; **reflux surge** chase finale | **Cardiac sphincter** — outrun the acid wall through it |
| 3 | **Stomach** | arena (the acid sea) | pH as environmental damage; mucus shallows are safe lanes; churn cyclones | **BOSS: Pyloric Guardian** — the sphincter itself |
| 4 | **Small Intestine** | tube (the long maze) | villi forest occludes sightlines; bile slicks; immune hunters | **MINIBOSS: Tapeworm chase** → ileocecal valve |
| 5 | **Large Intestine** | tube→arena (gas swamps) | flammable gas pockets: shooting inside one detonates it (hurts everyone — risk/reward); darkness + your scanner light | **BOSS: The Blockage** — then triumphant ejection finale |
| S | **Appendix** (secret) | arena | hidden lair off the cecum, bonus loot + lore | — |

**Per-level targets** (C, round 1 — replaces a flat "6–10 min/level", which flattened the
levels into each other). Length is an identity, not a budget: the campaign should *breathe*.

| # | level | par | first playthrough | why |
|---|---|---|---|---|
| 1 | Oral Cavity | 5:00 | ~6 min | tutorial: slow, safe, over-explained on purpose |
| 2 | **Esophagus** | **3:30** | ~4–5 min | **the sprint.** Shortest level in the game. You cannot stop in a flow-locked tube, so its length is bounded by physics: even a timid run is 4:22 and a full-throttle run is 2:47 (measured — `levels/index.js --sim`) |
| 3 | Stomach | 9:00 | ~10 min | the big room + first real boss; arenas need time to breathe |
| 4 | Small Intestine | 10:00 | ~11 min | the long maze — the campaign's endurance beat |
| 5 | Large Intestine | 9:00 | ~10 min | darkness, gas risk/reward, the final boss |

~45 min campaign including deaths and retries. Built to replay for medals.

## Player systems

- **Hull** (HP, pickups only) + **Mucus coat** (regenerating shield; drained continuously by
  ambient pH/enzymes — biome-dependent — and by hits). The coat is the pH mechanic: in the
  stomach open acid, coat drain is brutal; hugging mucus shallows pauses it.
- **Weapons:** primary **lysozyme cannon** (fast pellet stream, infinite, heat-limited);
  secondary **antacid torpedo** (limited ammo, AoE, *also neutralizes an acid zone for ~10 s*
  — the dual-use is the interesting bit).
- **Boost** with a short i-frame window at spend; recharges from nutrient orbs.
- **Pickups:** nutrient orbs (score+boost), mucin globs (coat refill), B12 cell (hull),
  antacid ammo, **biopsy samples** (3 hidden per level — the collectible).

## Enemies (v1 roster — B implements behaviors, D makes them pretty later)

All emissive-coded (ART_BIBLE): hostile = hot amber/red, projectiles = white-hot,
neutral flora = cyan (turns hostile if shot).

| enemy | biome | behavior archetype |
|---|---|---|
| amylase droplet | mouth | slow floater, corrosive contact aura |
| bolus chunk | esophagus | tumbling debris in the flow (crush). Drifts *slower* than the current, so it reads as traffic you overtake, never as a chaser |
| eosinophil swarm | esophagus | small fast seekers (eosinophilic esophagitis: immune cells that mistake ENDO-1 for the parasite they were built for). Faster than the flow — you cannot outrun them, only shoot or bank |
| candida bloom | esophagus | wall-mounted plaque colony firing spore clusters (candida esophagitis). Owns an arc of the cross-section: turns "which side do I fly" into a decision |
| pepsin swarm | stomach | small fast seekers, dissolve on kill |
| churn cyclone | stomach | environmental vortex, flings you |
| macrophage | small int. | big slow hunter; grab → mash to escape; eats other corpses to grow |
| antibody turret (Peyer's patch) | small int. | wall-mounted, fires homing Y-shaped darts |
| gut flora colony | large int. | neutral biofilm; shot ⇒ angry E. coli swarm |
| gas vent | large int. | hazard: thrust-shove + flammable cloud |

Bosses: **Pyloric Guardian** (armored sphincter; destroy mucosal nodes during open-cycles
while dodging acid spouts, arena slowly churns), **Tapeworm** (tube chase; shoot segment
weak-points as it coils around the canal ahead/behind), **The Blockage** (compacted-mass
fortress with embedded turrets and gas pockets; peel it layer by layer, finale = the canal
itself expels you both).

## Scoring & replay

Star-Fox-style medals per level: score (kills × style multiplier, chained kills), par time,
biopsy samples, damageless-gate bonus. Combo meter feeds the music layer (E). Endless
"colonoscopy run" mode is V2 (see V2_IDEAS.md).

## Difficulty & assists

Three difficulties scale coat-drain, enemy density, boss timing windows. Assist toggles
(flow auto-center, aim magnetism) — arcade games live or die on feel accessibility.

## Open for C to design in-lane (not settled)

Exact level lengths/pacing curves, encounter tables per segment, par times, pickup economy,
boss phase timings, secret placement, tutorialization beats of level 1.