diff --git a/src/lib/server/ai/image-preprocess.ts b/src/lib/server/ai/image-preprocess.ts
new file mode 100644
index 0000000..3b51d0f
--- /dev/null
+++ b/src/lib/server/ai/image-preprocess.ts
@@ -0,0 +1,38 @@
+import sharp from 'sharp';
+
+const MAX_EDGE = 1600;
+const JPEG_QUALITY = 85;
+
+export type PreprocessedImage = {
+  buffer: Buffer;
+  mimeType: 'image/jpeg';
+};
+
+// Resize auf max 1600px lange Kante, JPEG re-encode, Metadata strippen.
+// sharp liest HEIC/HEIF transparent, wenn libheif im libvips-Build enthalten ist
+// (in Alpine's vips-dev + in den offiziellen sharp-Prebuilds).
+export async function preprocessImage(input: Buffer): Promise<PreprocessedImage> {
+  const pipeline = sharp(input, { failOn: 'error' }).rotate(); // respect EXIF orientation
+  const meta = await pipeline.metadata();
+  if (!meta.width || !meta.height) {
+    throw new Error('Unable to read image dimensions');
+  }
+
+  const longEdge = Math.max(meta.width, meta.height);
+  const resized =
+    longEdge > MAX_EDGE
+      ? pipeline.resize({
+          width: meta.width >= meta.height ? MAX_EDGE : undefined,
+          height: meta.height > meta.width ? MAX_EDGE : undefined,
+          withoutEnlargement: true
+        })
+      : pipeline;
+
+  // Default-Verhalten seit sharp 0.33: alle Metadata (EXIF/IPTC/XMP) werden
+  // gestripped. Nur `.keepMetadata()`/`.keepExif()` würde sie erhalten.
+  const buffer = await resized
+    .jpeg({ quality: JPEG_QUALITY, mozjpeg: true })
+    .toBuffer();
+
+  return { buffer, mimeType: 'image/jpeg' };
+}
diff --git a/tests/unit/image-preprocess.test.ts b/tests/unit/image-preprocess.test.ts
new file mode 100644
index 0000000..9ce14c0
--- /dev/null
+++ b/tests/unit/image-preprocess.test.ts
@@ -0,0 +1,71 @@
+import { describe, it, expect } from 'vitest';
+import sharp from 'sharp';
+import { preprocessImage } from '../../src/lib/server/ai/image-preprocess';
+
+async function makeTestImage(
+  width: number,
+  height: number,
+  format: 'jpeg' | 'png' | 'webp' = 'jpeg'
+): Promise<Buffer> {
+  return sharp({
+    create: {
+      width,
+      height,
+      channels: 3,
+      background: { r: 128, g: 128, b: 128 }
+    }
+  })
+    .toFormat(format)
+    .toBuffer();
+}
+
+describe('preprocessImage', () => {
+  it('resizes a landscape image so long edge <= 1600px', async () => {
+    const input = await makeTestImage(4000, 2000);
+    const { buffer, mimeType } = await preprocessImage(input);
+    const meta = await sharp(buffer).metadata();
+    expect(Math.max(meta.width ?? 0, meta.height ?? 0)).toBeLessThanOrEqual(1600);
+    expect(Math.max(meta.width ?? 0, meta.height ?? 0)).toBeGreaterThan(1000);
+    expect(mimeType).toBe('image/jpeg');
+  });
+
+  it('resizes a portrait image so long edge <= 1600px', async () => {
+    const input = await makeTestImage(2000, 4000);
+    const { buffer } = await preprocessImage(input);
+    const meta = await sharp(buffer).metadata();
+    expect(Math.max(meta.width ?? 0, meta.height ?? 0)).toBeLessThanOrEqual(1600);
+  });
+
+  it('does not upscale smaller images', async () => {
+    const input = await makeTestImage(800, 600);
+    const { buffer } = await preprocessImage(input);
+    const meta = await sharp(buffer).metadata();
+    expect(meta.width).toBe(800);
+    expect(meta.height).toBe(600);
+  });
+
+  it('converts PNG input to JPEG output', async () => {
+    const input = await makeTestImage(1000, 1000, 'png');
+    const { buffer, mimeType } = await preprocessImage(input);
+    const meta = await sharp(buffer).metadata();
+    expect(meta.format).toBe('jpeg');
+    expect(mimeType).toBe('image/jpeg');
+  });
+
+  it('strips EXIF metadata', async () => {
+    const input = await sharp({
+      create: { width: 100, height: 100, channels: 3, background: '#888' }
+    })
+      .withMetadata({ exif: { IFD0: { Copyright: 'test' } } })
+      .jpeg()
+      .toBuffer();
+    const { buffer } = await preprocessImage(input);
+    const meta = await sharp(buffer).metadata();
+    expect(meta.exif).toBeUndefined();
+  });
+
+  it('rejects non-image buffers', async () => {
+    const notAnImage = Buffer.from('hello world');
+    await expect(preprocessImage(notAnImage)).rejects.toThrow();
+  });
+});