diff --git a/src/lib/app-state.svelte.ts b/src/lib/app-state.svelte.ts
new file mode 100644
index 0000000..5f7bbd1
--- /dev/null
+++ b/src/lib/app-state.svelte.ts
@@ -0,0 +1,116 @@
+import { detect, otsuThreshold } from './detect';
+import { prepareForExport } from './geometry';
+import { loadPhoto } from './load-image';
+import { shapeFromPolygons } from './margin';
+import type { CV } from './opencv-loader';
+import { scaleFromLine } from './scale';
+import type { Point, RasterImage, Shape } from './types';
+
+export type Tool = 'none' | 'scale' | 'seed' | 'draw';
+
+const DEFAULT_THRESHOLD = 128;
+
+export class AppState {
+  // Raw: the cv module and pixel buffers must never be wrapped in reactive proxies.
+  cv = $state.raw<CV | null>(null);
+  cvError = $state<string | null>(null);
+  image = $state.raw<RasterImage | null>(null);
+  loadError = $state<string | null>(null);
+
+  tool = $state<Tool>('none');
+  scalePoints = $state<Point[]>([]);
+  lengthMm = $state<number | null>(null);
+  threshold = $state(DEFAULT_THRESHOLD);
+  invert = $state(false);
+  seed = $state<Point | null>(null);
+  marginMm = $state(-2);
+  mode = $state<'auto' | 'manual'>('auto');
+  manualOuter = $state<Point[]>([]);
+  manualHoles = $state<Point[][]>([]);
+  drawing = $state<Point[]>([]);
+
+  pxPerMm = $derived(
+    this.scalePoints.length === 2 && this.lengthMm !== null
+      ? scaleFromLine(this.scalePoints[0], this.scalePoints[1], this.lengthMm)
+      : null,
+  );
+  scaleInvalid = $derived(this.scalePoints.length === 2 && this.lengthMm !== null && this.pxPerMm === null);
+  marginPx = $derived(this.pxPerMm ? this.marginMm * this.pxPerMm : 0);
+
+  shapePx = $derived.by((): Shape | null => {
+    const { cv, image } = this;
+    if (!cv || !image) return null;
+    if (this.mode === 'manual') {
+      return shapeFromPolygons(cv, image.width, image.height, { outer: this.manualOuter, holes: this.manualHoles }, this.marginPx);
+    }
+    return detect(cv, image, { threshold: this.threshold, invert: this.invert, seed: this.seed, marginPx: this.marginPx });
+  });
+
+  exportShape = $derived(this.shapePx && this.pxPerMm ? prepareForExport(this.shapePx, this.pxPerMm) : null);
+
+  setCv(cv: CV) {
+    this.cv = cv;
+    if (this.image) this.threshold = otsuThreshold(cv, this.image);
+  }
+
+  setImage(image: RasterImage) {
+    this.image = image;
+    this.tool = 'none';
+    this.scalePoints = [];
+    this.seed = null;
+    this.clearManual();
+    this.threshold = this.cv ? otsuThreshold(this.cv, image) : DEFAULT_THRESHOLD;
+  }
+
+  async loadFile(file: Blob) {
+    try {
+      this.loadError = null;
+      this.setImage(await loadPhoto(file));
+    } catch {
+      this.loadError = 'Could not read this image file.';
+    }
+  }
+
+  autoThreshold() {
+    if (this.cv && this.image) this.threshold = otsuThreshold(this.cv, this.image);
+  }
+
+  handleImageClick(p: Point) {
+    const image = this.image;
+    if (!image || p.x < 0 || p.y < 0 || p.x >= image.width || p.y >= image.height) return;
+    switch (this.tool) {
+      case 'scale':
+        if (this.scalePoints.length === 1) {
+          this.scalePoints.push(p);
+          this.tool = 'none';
+        } else {
+          this.scalePoints = [p];
+        }
+        break;
+      case 'seed':
+        this.seed = p;
+        this.tool = 'none';
+        break;
+      case 'draw':
+        this.drawing.push(p);
+        break;
+    }
+  }
+
+  finishPolygon() {
+    if (this.drawing.length >= 3) {
+      if (this.manualOuter.length === 0) this.manualOuter = this.drawing;
+      else this.manualHoles.push(this.drawing);
+      this.mode = 'manual';
+    }
+    this.drawing = [];
+  }
+
+  clearManual() {
+    this.manualOuter = [];
+    this.manualHoles = [];
+    this.drawing = [];
+    this.mode = 'auto';
+    if (this.tool === 'draw') this.tool = 'none';
+  }
+}
diff --git a/tests/app-state.test.ts b/tests/app-state.test.ts
new file mode 100644
index 0000000..ed65ec1
--- /dev/null
+++ b/tests/app-state.test.ts
@@ -0,0 +1,120 @@
+import { describe, expect, it } from 'vitest';
+import { AppState } from '../src/lib/app-state.svelte';
+
+const image = { width: 100, height: 80, data: new Uint8ClampedArray(100 * 80 * 4) };
+
+function withImage() {
+  const app = new AppState();
+  app.setImage(image);
+  return app;
+}
+
+describe('AppState', () => {
+  it('starts with sensible defaults', () => {
+    const app = new AppState();
+    expect(app.marginMm).toBe(-2);
+    expect(app.mode).toBe('auto');
+    expect(app.tool).toBe('none');
+    expect(app.shapePx).toBeNull();
+    expect(app.exportShape).toBeNull();
+  });
+
+  it('sets the scale from two clicks, then leaves the scale tool', () => {
+    const app = withImage();
+    app.tool = 'scale';
+    app.handleImageClick({ x: 10, y: 10 });
+    expect(app.tool).toBe('scale');
+    app.handleImageClick({ x: 70, y: 10 });
+    expect(app.tool).toBe('none');
+    app.lengthMm = 30;
+    expect(app.pxPerMm).toBeCloseTo(2);
+    expect(app.marginPx).toBeCloseTo(-4);
+  });
+
+  it('restarts the scale line on a third click', () => {
+    const app = withImage();
+    app.tool = 'scale';
+    app.handleImageClick({ x: 10, y: 10 });
+    app.handleImageClick({ x: 70, y: 10 });
+    app.tool = 'scale';
+    app.handleImageClick({ x: 5, y: 5 });
+    expect(app.scalePoints).toEqual([{ x: 5, y: 5 }]);
+    expect(app.pxPerMm).toBeNull();
+  });
+
+  it('flags an invalid scale and keeps the margin at 0 until the scale is valid', () => {
+    const app = withImage();
+    app.tool = 'scale';
+    app.handleImageClick({ x: 10, y: 10 });
+    app.handleImageClick({ x: 10, y: 10 });
+    app.lengthMm = 50;
+    expect(app.scaleInvalid).toBe(true);
+    expect(app.marginPx).toBe(0);
+  });
+
+  it('ignores clicks outside the image', () => {
+    const app = withImage();
+    app.tool = 'seed';
+    app.handleImageClick({ x: -1, y: 10 });
+    app.handleImageClick({ x: 50, y: 80 });
+    expect(app.seed).toBeNull();
+    expect(app.tool).toBe('seed');
+  });
+
+  it('sets the seed and leaves the seed tool', () => {
+    const app = withImage();
+    app.tool = 'seed';
+    app.handleImageClick({ x: 20, y: 30 });
+    expect(app.seed).toEqual({ x: 20, y: 30 });
+    expect(app.tool).toBe('none');
+  });
+
+  it('draws an outline then holes, switching to manual mode', () => {
+    const app = withImage();
+    app.tool = 'draw';
+    [{ x: 1, y: 1 }, { x: 90, y: 1 }, { x: 90, y: 70 }].forEach((p) => app.handleImageClick(p));
+    app.finishPolygon();
+    expect(app.manualOuter).toHaveLength(3);
+    expect(app.mode).toBe('manual');
+    [{ x: 20, y: 20 }, { x: 30, y: 20 }, { x: 30, y: 30 }].forEach((p) => app.handleImageClick(p));
+    app.finishPolygon();
+    expect(app.manualHoles).toHaveLength(1);
+    expect(app.drawing).toEqual([]);
+  });
+
+  it('discards a polygon with fewer than 3 points', () => {
+    const app = withImage();
+    app.tool = 'draw';
+    app.handleImageClick({ x: 1, y: 1 });
+    app.finishPolygon();
+    expect(app.manualOuter).toEqual([]);
+    expect(app.mode).toBe('auto');
+  });
+
+  it('clearManual returns to auto mode', () => {
+    const app = withImage();
+    app.tool = 'draw';
+    [{ x: 1, y: 1 }, { x: 90, y: 1 }, { x: 90, y: 70 }].forEach((p) => app.handleImageClick(p));
+    app.finishPolygon();
+    app.clearManual();
+    expect(app.mode).toBe('auto');
+    expect(app.manualOuter).toEqual([]);
+    expect(app.tool).toBe('none');
+  });
+
+  it('a new image resets scale, seed and manual work', () => {
+    const app = withImage();
+    app.tool = 'scale';
+    app.handleImageClick({ x: 10, y: 10 });
+    app.seed = { x: 5, y: 5 };
+    app.manualOuter = [{ x: 1, y: 1 }, { x: 2, y: 2 }, { x: 3, y: 1 }];
+    app.mode = 'manual';
+    app.setImage(image);
+    expect(app.scalePoints).toEqual([]);
+    expect(app.seed).toBeNull();
+    expect(app.manualOuter).toEqual([]);
+    expect(app.mode).toBe('auto');
+    expect(app.tool).toBe('none');
+    expect(app.threshold).toBe(128);
+  });
+});