Repo: . Canvas 2D, fixed timestep physics, tile maps, WebAudio synth, the Gemini client, and the level validator all ship in one file so GitHub Pages can host it with zero build step.

SOLSTICE-jump/

├── index.html      # The entire game (HTML + CSS + JS)

├── jump.wmv        # Gameplay demo video

├── README.md

└── working-doc.md  # Dev notes

How I Built It

I started with one question: what if changing time changed geometry?

From there I worked iteratively with Gemini: describing mechanics, getting code back, playtesting, and sending fixes until the game felt right. Levels are ASCII grids. # is stone. S and M are phase locked blocks. W is water or ice depending on the sky. o is a sun mote. E is the Shrine. The parser pads uneven rows so a sloppy AI row never crashes the game.

Physics and feel

Movement uses coyote time and jump buffering because platformers live or die on forgiveness. Shifting phases runs through a trap check first. If the new phase would crush the player inside solid tile, the shift is denied with a little shake and a sour note. You are never soft locked by your own power.

Proving levels are fair

Early playtests caught a nasty bug: motes floating four tiles in the air when the jump apex only reaches about two. That felt like a secret rule instead of a skill check. With Gemini I rebuilt placement around three reachable patterns: a short hop, a climbable staircase, or a moon stair you only see after shifting to night.

Then we added a phase aware reachability search (BFS) that understands walk, step up, jump arcs, falls, and day/night toggles. Every campaign level, every sample realm, and every runtime Gemini output gets filtered through it before you play. If the Shrine is unreachable, the level is rejected. If a mote is unreachable, it gets stripped so 100% collection stays honest.

A greedy hold jump bot was also run against the campaign until it cleared all five levels including the finale. That caught missing player spawn markers and floating steps that looked cool but broke progression.

Campaign levels

1. 
   I · First Light · learn SHIFT and the moon bridge


2. 
   II · Meltwater · deadly water by day, frozen ice by night


3. 
   III · Twilight Rhythm · alternating sun stone and moon stone


4. 
   IV · Frostfall · every mechanic combined


5. 
   V · The Longest Day · the finale; light the great Shrine

Beating the campaign unlocks Endless Solstice.

How Gemini was used to build the game

Gemini was not a one off feature bolted on at the end. It was the main development partner.

For the codebase, I described systems in plain language (physics, tile parsing, phase shift, WebAudio, UI) and iterated with Gemini until everything landed in one self contained index.html. Bug fixes, the reachability validator, the verification bot, and the Endless Mode pipeline all went through the same loop: prompt, generate, test, refine.

For campaign and sample levels, Gemini helped draft and revise ASCII maps against the jump limits and tile rules. I still validated every map with the in game BFS before shipping, which is how unreachable motes and missing spawn points got caught.

For this post, Gemini helped structure and polish the writeup so judges could actually follow the project.

How Gemini shows up inside the game

This is the second layer, and the one judges can play with directly.

Endless Mode uses a pluggable LevelManager with a clear fallback chain:

1. Ask Google Gemini to invent a brand new map at runtime (if the player enabled AI and supplied their own API key via the ✦ AI panel)


2. Pull from a shuffled pool of eight verified sample realms from the build


3. Echo a random known realm so loading never dead ends

So Gemini shows up twice: once as the dev tool that built SOLSTICE, and once as a live level designer players can turn on in Endless Mode.

The runtime path sends a detailed level design brief: tile legend, jump limits (about two tiles up, three across), staircase rules, mote counts, and a full JSON example of a real solvable level from the game. The model must answer with structured JSON via responseSchema, matching fields like name, sub, tip, startDay, and rows.

Implementation details judges might care about:

• The client tries the official @google/genai SDK with streaming first, then falls back to raw REST if the CDN is blocked


• 
  AbortController plus a 30 second timeout so a slow network never freezes the run


• API keys live in localStorage on the player's machine only. No key? Endless Mode still works on the built in pool. Runtime AI is a bonus layer, not a paywall


• Every generated map passes the same reachability validator as campaign content before it hits the canvas


• The HUD shows where the current realm came from (Gemini AI, Sample realm, or Echo)

Gemini is not generating flavor text off to the side. At build time it helped write the game. At runtime it can dream up the next lawful puzzle inside rules strict enough to reject bad output automatically.

Polish

Visuals are all procedural: gold sun motes, indigo night sky, star fields, phase crossfades, particle bursts on shift and collect. Audio is tiny synthesized bleeps so the file stays portable.

Prize Category

Best Google AI Usage

This category fits SOLSTICE on two levels.

Build time: Gemini helped write the full game: physics, rendering, audio, level data, the reachability validator, and the Endless Mode pipeline. I steered the design and kept iterating until every campaign level was provably solvable.

Runtime: Gemini is embedded in the playable product. Endless Mode can call the same model live to invent new maps, with structured JSON output, streaming, graceful fallback, and the same validator that rejects unfair levels.

That is end to end Google AI usage: from jam concept to shippable code to a feature players can switch on with their own API key.

Players opt in through the in game ✦ AI settings panel. The game never blocks if they skip runtime generation. Built in sample realms and echo fallback keep Endless Mode running either way.

I am not submitting for Best Ode to Alan Turing this round. The theme here is solstice light and the turning of time rather than computing history.

Thanks for reading. If you try Endless Mode with Gemini enabled, tell me the weirdest lawful realm it conjured. I am curious which shift puzzle it thinks is clever.

Links

• Play (recommended):
  


• Repo:
  


• Jam: https://dev.to/challenges/june-game-jam-2026-06-03