diff --git a/apps/api/src/lib/file-validation.ts b/apps/api/src/lib/file-validation.ts
index 65e95160..b18465f7 100644
--- a/apps/api/src/lib/file-validation.ts
+++ b/apps/api/src/lib/file-validation.ts
@@ -229,10 +229,12 @@ export async function validateImageBuffer(
     detectedFormat = "tga";
   }
 
-  // SVGZ: gzip-compressed SVG, detected by extension + gzip magic
+  // SVGZ: gzip-compressed SVG, detected by extension + gzip magic.
+  // Return early because Sharp cannot read compressed SVGZ directly;
+  // decompression happens later in the route pipeline.
   if (!detectedFormat && ext === "svgz") {
     if (buffer.length >= 2 && buffer[0] === 0x1f && buffer[1] === 0x8b) {
-      detectedFormat = "svg";
+      return { valid: true, format: "svg", width: 0, height: 0 };
     }
   }
 
diff --git a/apps/api/src/routes/tools/watermark-image.ts b/apps/api/src/routes/tools/watermark-image.ts
index 4a61ce42..f9babe2f 100644
--- a/apps/api/src/routes/tools/watermark-image.ts
+++ b/apps/api/src/routes/tools/watermark-image.ts
@@ -23,6 +23,7 @@ export function registerWatermarkImage(app: FastifyInstance) {
     let mainBuffer: Buffer | null = null;
     let watermarkBuffer: Buffer | null = null;
     let filename = "image";
+    let watermarkFilename = "watermark";
     let settingsRaw: string | null = null;
 
     try {
@@ -36,6 +37,7 @@ export function registerWatermarkImage(app: FastifyInstance) {
           const buf = Buffer.concat(chunks);
           if (part.fieldname === "watermark") {
             watermarkBuffer = buf;
+            watermarkFilename = sanitizeFilename(part.filename ?? "watermark");
           } else {
             mainBuffer = buf;
             filename = sanitizeFilename(part.filename ?? "image");
@@ -117,7 +119,7 @@ export function registerWatermarkImage(app: FastifyInstance) {
       }
       mainBuffer = await autoOrient(mainBuffer);
 
-      const valWm = await validateImageBuffer(watermarkBuffer, "watermark");
+      const valWm = await validateImageBuffer(watermarkBuffer, watermarkFilename);
       if (!valWm.valid) {
         return reply.status(400).send({ error: `Invalid watermark image: ${valWm.reason}` });
       }
diff --git a/tests/integration/color-palette.test.ts b/tests/integration/color-palette.test.ts
index 89c9a9d1..10a6600b 100644
--- a/tests/integration/color-palette.test.ts
+++ b/tests/integration/color-palette.test.ts
@@ -158,7 +158,7 @@ describe("Error handling", () => {
     expect(result.error).toBeDefined();
   });
 
-  it("returns 422 for corrupted image data", async () => {
+  it("returns 400 for corrupted image data", async () => {
     const badBuffer = Buffer.from("not an image at all");
     const { body: payload, contentType } = makeFilePayload(badBuffer, "bad.png", "image/png");
     const res = await app.inject({
@@ -170,7 +170,8 @@ describe("Error handling", () => {
         authorization: `Bearer ${adminToken}`,
       },
     });
-    expect(res.statusCode).toBe(422);
+    // validateImageBuffer catches corrupt data before processing
+    expect(res.statusCode).toBe(400);
   });
 });
 
diff --git a/tests/integration/compare.test.ts b/tests/integration/compare.test.ts
index 9a1d9bf2..8f39ca11 100644
--- a/tests/integration/compare.test.ts
+++ b/tests/integration/compare.test.ts
@@ -463,7 +463,7 @@ describe("Compare", () => {
 
   // ── Branch coverage: multipart parse error (lines 35-39) ────────────
 
-  it("returns 422 when corrupt image data fails processing", async () => {
+  it("returns 400 when corrupt image data fails validation", async () => {
     // Create a buffer that looks like an image but corrupts Sharp
     const corruptBuffer = Buffer.from("not a real image content at all");
     const { body, contentType } = createMultipartPayload([
@@ -481,10 +481,10 @@ describe("Compare", () => {
       body,
     });
 
-    // Should return 422 due to processing failure
-    expect(res.statusCode).toBe(422);
+    // validateImageBuffer catches corrupt data before processing
+    expect(res.statusCode).toBe(400);
     const result = JSON.parse(res.body);
-    expect(result.error).toMatch(/comparison failed/i);
+    expect(result.error).toBeDefined();
   });
 
   // ── Branch coverage: 1x1 tiny images (line 117-121 area) ───────────
@@ -649,7 +649,7 @@ describe("Compare", () => {
 
   // ── Branch coverage: both corrupt images fail processing ───────────
 
-  it("returns 422 when both images are corrupt", async () => {
+  it("returns 400 when both images are corrupt", async () => {
     const corrupt1 = Buffer.from("this is not an image");
     const corrupt2 = Buffer.from("neither is this one");
     const { body, contentType } = createMultipartPayload([
@@ -667,9 +667,10 @@ describe("Compare", () => {
       body,
     });
 
-    expect(res.statusCode).toBe(422);
+    // validateImageBuffer catches corrupt data before processing
+    expect(res.statusCode).toBe(400);
     const result = JSON.parse(res.body);
-    expect(result.error).toMatch(/comparison failed/i);
+    expect(result.error).toBeDefined();
   });
 
   // ── Branch coverage: HEIF content format input ─────────────────────