Debug Tools

The @yagejs/debug package provides a toggleable debug overlay with physics shape visualization, HUD text, performance stats, and a contributor system for custom debug views.

DebugPlugin Setup

import { DebugPlugin } from "@yagejs/debug";

engine.use(new DebugPlugin({
  startEnabled: true,     // show debug overlay on launch
  toggleKey: "Backquote", // key to toggle (default: backtick `)
}));

Press the toggle key (` by default) to show/hide the debug overlay at runtime.

DebugPlugin Config

Property	Type	Default	Description
startEnabled	boolean	false	Show overlay on launch
toggleKey	string	"Backquote"	Key code to toggle overlay
stepKey	string	"Period"	Key to advance one frozen frame
maxGraphics	number	256	Graphics object pool size
maxHudLines	number	32	Max HUD text lines
flags	Record<string, boolean>	—	Initial flag overrides

Built-In Debug Views

When the debug overlay is active, the plugin automatically shows:

• Physics colliders — colored outlines around all collider shapes:
  • Green: dynamic bodies
  • Gray: static bodies
  • Blue: kinematic bodies
  • Yellow: sensor colliders
• FPS counter — frames per second in the HUD
• Entity count — total entities in the current scene
• System timing — per-system execution time breakdown

Custom Debug Contributors

Implement DebugContributor to add your own debug visualizations:

import type { DebugContributor, WorldDebugApi, HudDebugApi } from "@yagejs/debug/api";

class WallDebugContributor implements DebugContributor {
  readonly name = "walls";
  readonly flags = ["show-walls"] as const;

  private readonly shapes: Array<{ x: number; y: number; w: number; h: number }>;

  constructor(shapes: Array<{ x: number; y: number; w: number; h: number }>) {
    this.shapes = shapes;
  }

  drawWorld(api: WorldDebugApi): void {
    if (!api.isFlagEnabled("show-walls")) return;

    for (const s of this.shapes) {
      const g = api.acquireGraphics();
      if (!g) break;
      g.rect(s.x, s.y, s.w, s.h)
        .stroke({ color: 0xff0000, width: 2 / api.cameraZoom });
    }
  }

  drawHud(api: HudDebugApi): void {
    api.addLine(`Walls: ${this.shapes.length}`);
  }
}

Register it in your scene:

import { DebugRegistryKey } from "@yagejs/debug/api";

onEnter(): void {
  const registry = this.service(DebugRegistryKey);
  registry.register(new WallDebugContributor(this.wallShapes));
}

WorldDebugApi

Method	Description
acquireGraphics()	Get a pooled Graphics object (returns undefined if pool exhausted)
cameraZoom	Current camera zoom level (scale line widths by 1/cameraZoom)
isFlagEnabled(flag)	Check if a debug flag is active

HudDebugApi

Method	Description
addLine(text)	Add a line to the HUD text overlay
isFlagEnabled(flag)	Check if a debug flag is active
screenWidth / screenHeight	Screen dimensions for custom positioning

Debug Flags

Contributors can declare flags for toggling specific views:

class MyContributor implements DebugContributor {
  readonly name = "my-debug";
  readonly flags = ["show-paths", "show-hitboxes"] as const;

  drawWorld(api: WorldDebugApi): void {
    if (api.isFlagEnabled("show-paths")) {
      // draw paths...
    }
    if (api.isFlagEnabled("show-hitboxes")) {
      // draw hitboxes...
    }
  }
}

Set flags programmatically:

registry.setFlag("my-debug", "show-paths", true);

Inspector Extensions

Visual debug overlays and programmatic debug helpers are separate concerns. Use DebugRegistry contributors for drawing, and register inspector extensions when a plugin wants to expose imperative helpers to tests or the browser console.

import type { EngineContext } from "@yagejs/core";
import { InspectorKey } from "@yagejs/core";

install(context: EngineContext): void {
  const inspector = context.resolve(InspectorKey);

  inspector.addExtension("inventory", {
    listItems: () => this.inventory.snapshot(),
    grantItem: (id: string) => this.inventory.grant(id),
  });
}

Consume that extension from test code or the browser console:

const inventory = window.__yage__.inspector.getExtension<{
  listItems(): string[];
  grantItem(id: string): void;
}>("inventory");

inventory?.grantItem("rocket-boots");
inventory?.listItems();

DebugPlugin uses the same pattern for renderer-aware helpers:

const debug = window.__yage__.inspector.getExtension("debug");
debug?.getCameraStack?.();
debug?.getLayerTransform?.("game", "world");

StatsStore

StatsStore provides rolling-window statistics for performance monitoring:

import { StatsStore } from "@yagejs/debug";

const stats = new StatsStore();

// Push samples each frame
stats.push("updateTime", performance.now() - start);

// Read stats
stats.average("updateTime");  // rolling average
stats.latest("updateTime");   // most recent sample

Frame-step mode and agent-driven debugging

DebugPlugin always installs the frozen-clock implementation. To step exact frames, freeze through the Inspector and then advance from code or with the step key:

window.__yage__.inspector.time.freeze();
window.__yage__.inspector.time.step(1);
window.__yage__.inspector.time.thaw();

The configured stepKey only advances frames while the inspector clock is frozen. This is useful for debugging physics, animations, or frame-specific logic.

Throwaway Inspector specs

The Inspector + frozen clock + scripted input together make a tight short-loop for LLM-assisted debugging and gameplay validation. The intended workflow is a throwaway Playwright spec — write it, run it, delete it. Not a CI fixture, not a permanent regression suite. A scratch session.

Suppose you just changed the jump arc. Drop a spec that boots the platformer, holds Right for 30 frames, fires the jump action, steps 45 frames, and asserts the player landed on a specific ledge entity. Run it. If it passes, delete it. If it fails, iterate.

import { test, expect } from "@playwright/test";

test("can the player jump onto the ledge?", async ({ page }) => {
  await page.goto("/platformer.html");
  await page.waitForFunction(() => window.__yage__?.inspector);

  const result = await page.evaluate(async () => {
    const i = window.__yage__.inspector;
    i.setSeed(42);            // pin RNG for reproducible runs
    i.time.freeze();           // stop auto-advance
    await i.input.hold("ArrowRight", 30);
    await i.input.fireAction("jump", 1);
    i.time.step(45);           // advance 45 fixed-timestep frames
    return i.snapshotJSON();
  });

  // Optional: capture a frame for visual inspection.
  // await page.screenshot({ path: "/tmp/probe.png" });

  expect(result).toContain('"name":"player"');
});

The two spec styles serve very different purposes:

	Permanent Inspector tests in CI	Throwaway agent-vision specs
Frequency	Rare	Common
Lifetime	Lives with the repo	Deleted after one session
What’s asserted	Reproducibility (“snapshots match across reruns”)	Gameplay outcome (“player landed at x>200”)
Brittleness to balance changes	Stable by design	Brittle on purpose — that’s why they’re throwaway
Example	e2e/specs/inspector-determinism.spec.ts	”did my jump-arc change still let the player reach the ledge?”

Always advance via inspector.time.step(N) (loops one fixed-timestep frame at a time) over clock.step(bigDt) (collapses the interval into one fat frame). Component.update, tweens, and AI logic only see one update at the full bigDt under the latter, which diverges from real gameplay even though physics still substeps correctly.

Honest limits

These are truths, not bugs to fix — keep them in mind when interpreting probe results:

• Visuals: snapshotJSON() covers structural state — positions, components, scene stack — not pixel output. page.screenshot() helps but agent-grade interpretation of pixels is imperfect. Combine both for confidence.
• Audio: no introspection surface, and WebAudio doesn’t pause in step mode. Probes can’t tell you whether a sound effect played at the right frame.
• Wall-clock leaks: setTimeout, Date.now(), and raw performance.now() reads bypass the frame clock. No callers in core YAGE today, but custom plugins might.
• step(bigDt) ≠ stepFrames(N) for variable-update logic. Always prefer the latter in probes.

Logging

The Logger class is a category-tagged, ring-buffered logger built into the engine. Unlike the debug overlays above, it lives in @yagejs/core, not @yagejs/debug — you don’t need the DebugPlugin installed to use it. Every Engine has a Logger attached at construction time, and the game loop auto-updates the logger’s frame counter so every log entry records the frame it was emitted on.

Access it as engine.logger, or through DI via LoggerKey:

import { Logger, LogLevel, LoggerKey } from "@yagejs/core";

// Direct access on the engine
engine.logger.info("physics", "Shape spawned", { x: 100, y: 200 });

// From inside a System or Component
class SpawnSystem extends System {
  private logger!: Logger;

  init(context: EngineContext) {
    this.logger = context.resolve(LoggerKey);
  }

  update(dt: number) {
    if (wave.complete) {
      this.logger.warn("gameplay", "Wave ended with no kills");
    }
  }
}

Log Methods

All four methods take the same shape: (category, message, data?). The category is a free-form string — use whatever taxonomy suits your game ("physics", "ai", "input", "gameplay"). data is an arbitrary object that gets attached to the entry for later inspection.

Method	Level	Typical use
logger.debug(cat, msg, data?)	Debug	Chatter that only matters when actively diagnosing
logger.info(cat, msg, data?)	Info	Normal lifecycle events (scene change, entity spawn)
logger.warn(cat, msg, data?)	Warn	Recoverable problems (missing texture, late frame)
logger.error(cat, msg, data?)	Error	Broken invariants — something a human should look at

LoggerConfig

Pass a logger option when constructing the engine to configure level, category filter, ring buffer size, or a custom output sink:

import { Engine, LogLevel } from "@yagejs/core";

const engine = new Engine({
  logger: {
    level: LogLevel.Info,              // drop anything below Info
    categories: ["physics", "ai"],     // only accept these categories (empty = all)
    bufferSize: 1000,                  // keep the last 1000 entries
    output: (entry) => {               // optional: override console output
      console.log(`[${entry.category}] ${entry.message}`, entry.data);
    },
  },
});

LogLevel is an enum: Debug (0), Info (1), Warn (2), Error (3), None (4). Anything below the configured level is silently dropped at the log site — entries never enter the ring buffer, so there’s no cost to leaving debug calls in shipping code with level: LogLevel.Info.

Inspecting the Ring Buffer

The ring buffer is what makes the logger useful beyond console.log. Each entry is a LogEntry carrying level, category, message, optional data, a timestamp, and the frame number the game loop was on when the entry was emitted. You can pull recent entries at any time:

// Grab the last N entries (default: everything in the buffer)
const recent: LogEntry[] = engine.logger.getRecent(20);

// Or pre-formatted as a string, handy for crash dumps
const dump: string = engine.logger.formatRecentLogs(50);
console.log(dump);

A common pattern is to catch an error, dump the recent logs, and ship the result to a crash reporter:

window.addEventListener("error", (ev) => {
  reportCrash({
    error: ev.error,
    recentLogs: engine.logger.formatRecentLogs(100),
  });
});

Because every entry carries a frame number, you can correlate log output with specific frames seen in the debug overlay or inspector — very useful for reproducing frame-specific bugs.

Custom Output Sinks

The output option replaces the default console.* handler. Use it to ship logs somewhere else — an in-game debug panel, a remote telemetry service, or a file (in Electron/Tauri builds):

const engine = new Engine({
  logger: {
    output: (entry) => {
      // Forward warnings and errors to a remote logging backend
      if (entry.level >= LogLevel.Warn) {
        telemetry.send("game-log", {
          level: LogLevel[entry.level],
          category: entry.category,
          message: entry.message,
          data: entry.data,
          frame: entry.frame,
          timestamp: entry.timestamp,
        });
      }

      // Also mirror to the console in dev
      if (import.meta.env.DEV) {
        console.log(`[${entry.category}]`, entry.message, entry.data);
      }
    },
  },
});

Custom sinks don’t replace the ring buffer — entries are still stored in-memory and accessible via getRecent() regardless of what the sink does.