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+# Experiment_ReinforcementLearning — cursor-following lunar lander
+
+A cursor-following lunar lander demo: user moves the mouse, a PPO-controlled
+ship chases it against gravity. Deliberate strict separation between game
+logic, backend, frontend, and training.
+
+Live repo: `https://gitea.tes.gd/admin/Experiment_ReinforcementLearning`
+
+## Architecture
+
+Four processes with narrow, stable interfaces:
+
+```
+[React UI] ──WebSocket──▶ [ASP.NET backend] ──stdin/stdout──▶ [Game CLI (.NET)]
+                                                                     ▲
+                                                                     │ same protocol
+                                                                     │
+                                              [Python PPO trainer] ──┘
+```
+
+- **`GameCli/`** — C# .NET 8 console app. Stateless step-driven executable.
+  Reads one action per line on stdin, writes one observation JSON per line on
+  stdout. Owns physics, reward, observation. No networking, no UI knowledge.
+- **`Backend/`** — ASP.NET Core Web API. WebSocket at `/ws/game`. Per
+  connection: spawns one `GameCli` subprocess, runs a 50 Hz tick loop that
+  calls the singleton `PolicyRunner` (ONNX) for each action.
+- **`Frontend/`** — Vite + React + TypeScript. Opens a WebSocket via a
+  same-origin proxy (`/ws` → `localhost:5100`), sends `mousemove` as world
+  coords, renders the ship on a `<canvas>`.
+- **`Training/`** — Python. `LanderCliEnv` wraps the **same** `GameCli` binary
+  as a `gym.Env`, so training physics and inference physics are identical by
+  construction. Produces `models/ppo_lander.onnx`.
+
+Design spec: `docs/superpowers/specs/2026-07-17-cursor-following-lander-design.md`
+Implementation plans: `docs/superpowers/plans/2026-07-17-{01-04}-*.md`
+
+## Build & test
+
+Everything runs from the repo root.
+
+```bash
+# Game CLI (25 xUnit tests)
+dotnet test --filter FullyQualifiedName~GameCli.Tests
+
+# Backend (8 xUnit tests — includes end-to-end WebSocket smoke)
+dotnet publish GameCli/GameCli.csproj -c Release -o publish/GameCli
+dotnet test --filter FullyQualifiedName~Backend.Tests
+
+# Training (11 pytest tests)
+source .venv/bin/activate
+cd Training && pytest && cd ..
+
+# Frontend
+cd Frontend && npx tsc --noEmit && npm run build && cd ..
+```
+
+## Run the demo locally
+
+```bash
+# 1. Publish CLI (idempotent)
+dotnet publish GameCli/GameCli.csproj -c Release -o publish/GameCli
+
+# 2. Start backend (loads models/ppo_lander.onnx at startup)
+ASPNETCORE_URLS=http://localhost:5100 \
+  dotnet run --project Backend --no-launch-profile
+
+# 3. In another shell, start Vite dev server
+cd Frontend && npm run dev  # → http://localhost:5173/
+```
+
+Open `http://localhost:5173/`. Status bar top-left shows connection state and
+step counter; blue crosshair follows your mouse; white triangle is the ship.
+
+## Wire protocol reference
+
+**CLI stdio** (line-delimited JSON):
+
+- CLI startup: `{"init":{"world":[1,1],"dt":0.02,"obs_dim":7,"n_actions":4}}`
+- Step in: `{"action":<0-3>,"target":[x,y]}`
+- Reset in: `{"cmd":"reset","seed":<int>?}`
+- Step out: `{"obs":[7 floats],"state":{x,y,angle,engine},"reward":<f>,"done":<b>,"step":<i>}`
+- Actions: `0`=noop, `1`=left thruster, `2`=main engine, `3`=right thruster.
+- Observation: `[dx, dy, vx, vy, sin(angle), cos(angle), angular_velocity]`.
+
+**WebSocket** (`ws://localhost:5100/ws/game` or same-origin via Vite proxy):
+
+- Client → server: `{"type":"cursor","x":<0-1>,"y":<0-1>}`
+- Server → client (on connect): `{"type":"init","world":[1,1]}`
+- Server → client (each 20 ms tick):
+  `{"type":"state","x":..,"y":..,"angle":..,"engine":..,"target":[..,..],"step":..}`
+
+Both are mirrored in code (`GameCli/Protocol.cs`, `Backend/GameProcess.cs`,
+`Backend/WireMessages.cs`, `Frontend/src/protocol.ts`).
+
+## Coordinate & physics conventions
+
+- World is `[0, 1] × [0, 1]`. Y increases downward (screen-native).
+- `angle = 0` = ship pointing up. Positive angle rotates clockwise.
+- No walls, no ground. Ship can drift off-screen; only the reward pulls it back.
+- Fixed `dt = 0.02` (50 Hz). Explicit Euler integration.
+- Physics constants live in `GameCli/Physics.cs`; reward constants in
+  `GameCli/Reward.cs`. All `public const`, easy to tune.
+
+## PPO training
+
+```bash
+source .venv/bin/activate
+
+# Smoke run (~30 s):
+python Training/train.py --steps 10000 --n-envs 2 --use-dummy --max-episode-steps 200
+
+# Real training (~hours on CPU):
+python Training/train.py --steps 2000000 --n-envs 8
+
+# Export to ONNX (backend reads this file):
+python Training/export_onnx.py \
+  --checkpoint checkpoints/ppo_lander_final.zip \
+  --out models/ppo_lander.onnx
+```
+
+Backend loads `models/ppo_lander.onnx` at startup and fails fast if it's
+missing. Restart the backend after re-exporting to pick up the new weights.
+
+### Current state of the model
+
+`models/ppo_lander.onnx` was trained for only 10k PPO steps as a
+pipeline smoke — essentially random with slight bias. The ship falls off the
+world in a few seconds under this policy. To see actual cursor-following
+behavior, run the 2M-step training pass above (see PPO training section).
+
+## Config paths & env vars
+
+Backend resolves paths at startup by walking up from `AppContext.BaseDirectory`
+to find `GameCli.sln` (the repo-root marker), then joining a relative config
+path.
+
+- `Lander:CliPath` (default `publish/GameCli/GameCli`)
+- `Lander:ModelPath` (default `models/ppo_lander.onnx`)
+- Env-var overrides: `LANDER__CliPath`, `LANDER__ModelPath` (ASP.NET
+  double-underscore convention).
+
+## Repo conventions
+
+- Every commit lands on `main` (no PR flow — greenfield project).
+- Author: `meelstorm <meelstorm@gmail.com>`. Use
+  `-c user.email=meelstorm@gmail.com -c user.name=meelstorm` when committing.
+- Conventional commit prefixes: `feat(gamecli):`, `feat(backend):`,
+  `feat(training):`, `feat(frontend):`, `fix(...)`, `test(...)`, `chore:`.
+- `.gitignore` covers `bin/`, `obj/`, `node_modules/`, `dist/`, `publish/`,
+  `.venv/`, `checkpoints/`, `models/`, `tensorboard/`, `*.onnx`.
+- The trained model is NOT in git — it's a build artifact.
+
+## Known limitations & future work
+
+- **Model is undertrained** — see "Current state of the model" above.
+- **No walls** — a badly-trained policy loses to gravity within a second and
+  the ship drifts off-screen forever. Design choice: reward should be enough
+  to keep a well-trained policy in-world.
+- **Angle wrap in reward is applied** but not in `Physics.Step` — the ship's
+  `angle` field grows unbounded across many revolutions. Observation uses
+  sin/cos so it's immune; reward wraps to `[-π, π]` internally.
+- **StrictMode WebSocket double-mount** in dev — React's StrictMode mounts
+  the `useGameSocket` effect twice, so you'll see one benign
+  "closed before established" WebSocket warning per page load.
+- **Single-player only** — one CLI subprocess per connection; multi-user
+  demo is fine at demo scale but there's no shared world.
+
+## Debugging tips
+
+- **Backend can't find CLI or ONNX at startup** — the repo-root walker looks
+  for `GameCli.sln`. Both `.sln` and `.slnx` formats work.
+- **`dotnet test` in `Backend.Tests` uses the CLI** — publish first
+  (`dotnet publish GameCli -c Release -o publish/GameCli`) so
+  `EndToEndSmokeTests` can find the binary.
+- **`torch.onnx.export` on torch 2.13+** — pass `dynamo=False` to force the
+  legacy TorchScript exporter unless `onnxscript` is installed.
+- **Vite WS proxy failing** — verify with a Node script bypassing the proxy
+  (`ws://localhost:5100/ws/game` direct). If direct works but proxy doesn't,
+  check `Frontend/vite.config.ts` proxy block.