diff --git a/apps/api/src/routes/tools/watermark-image.ts b/apps/api/src/routes/tools/watermark-image.ts
index 4a61ce42..810482ad 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 wmFilename = "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;
+            wmFilename = 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, wmFilename);
       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..46f5f0e0 100644
--- a/tests/integration/color-palette.test.ts
+++ b/tests/integration/color-palette.test.ts
@@ -170,7 +170,7 @@ describe("Error handling", () => {
         authorization: `Bearer ${adminToken}`,
       },
     });
-    expect(res.statusCode).toBe(422);
+    expect(res.statusCode).toBe(400);
   });
 });
 
diff --git a/tests/integration/compare.test.ts b/tests/integration/compare.test.ts
index 9a1d9bf2..02e31a25 100644
--- a/tests/integration/compare.test.ts
+++ b/tests/integration/compare.test.ts
@@ -481,10 +481,9 @@ describe("Compare", () => {
       body,
     });
 
-    // Should return 422 due to processing failure
-    expect(res.statusCode).toBe(422);
+    expect(res.statusCode).toBe(400);
     const result = JSON.parse(res.body);
-    expect(result.error).toMatch(/comparison failed/i);
+    expect(result.error).toMatch(/invalid/i);
   });
 
   // ── Branch coverage: 1x1 tiny images (line 117-121 area) ───────────
@@ -667,9 +666,9 @@ describe("Compare", () => {
       body,
     });
 
-    expect(res.statusCode).toBe(422);
+    expect(res.statusCode).toBe(400);
     const result = JSON.parse(res.body);
-    expect(result.error).toMatch(/comparison failed/i);
+    expect(result.error).toMatch(/invalid/i);
   });
 
   // ── Branch coverage: HEIF content format input ─────────────────────
diff --git a/tests/integration/watermark-image.test.ts b/tests/integration/watermark-image.test.ts
index 9aca327d..369d12d2 100644
--- a/tests/integration/watermark-image.test.ts
+++ b/tests/integration/watermark-image.test.ts
@@ -226,9 +226,9 @@ describe("watermark-image", () => {
       body,
     });
 
-    expect(res.statusCode).toBe(422);
+    expect(res.statusCode).toBe(400);
     const json = JSON.parse(res.body);
-    expect(json.error).toContain("Processing failed");
+    expect(json.error).toMatch(/invalid/i);
   });
 
   // ── HEIC input handling ───────────────────────────────────────────
@@ -353,9 +353,9 @@ describe("watermark-image", () => {
       body,
     });
 
-    expect(res.statusCode).toBe(422);
+    expect(res.statusCode).toBe(400);
     const json = JSON.parse(res.body);
-    expect(json.error).toContain("Processing failed");
+    expect(json.error).toMatch(/invalid/i);
   });
 
   // ── Tiny 1x1 main image ──────────────────────────────────────────