diff --git a/src/lib/server/order-assistant-core.test.ts b/src/lib/server/order-assistant-core.test.ts
index 762f5f8..323a891 100644
--- a/src/lib/server/order-assistant-core.test.ts
+++ b/src/lib/server/order-assistant-core.test.ts
@@ -550,6 +550,19 @@ describe("buildOrderLineFillPatch", () => {
     expect(patch?.markup_percent).toBe(50);
   });
 
+  it("clamps a loss-making markup at 0, like every other order-line write", () => {
+    // The customer's price is below the quote's cost. The margin chip
+    // shows the real margin from cost + selling; the stored markup
+    // follows the app's convention of never going negative.
+    const patch = buildOrderLineFillPatch(
+      fillLine({ selling_price: 8 }),
+      fillQuoteLine({ cost_price: 10, selling_price: 12 }),
+      FILL_CTX,
+    );
+    expect(patch?.markup).toBe(0);
+    expect(patch?.markup_percent).toBe(0);
+  });
+
   it("adds the price doubt when the customer price differs by more than 5%", () => {
     const patch = buildOrderLineFillPatch(
       fillLine({ selling_price: 16.64 }),
diff --git a/src/lib/server/order-assistant-core.ts b/src/lib/server/order-assistant-core.ts
index 243855f..eefbcf5 100644
--- a/src/lib/server/order-assistant-core.ts
+++ b/src/lib/server/order-assistant-core.ts
@@ -260,7 +260,11 @@ export function buildOrderLineFillPatch(
   // The resulting selling price: the customer's own price when there is
   // one, otherwise the quoted price.
   const selling = customerPrice > 0 ? customerPrice : quotedPrice;
-  const markup = cost > 0 && selling > 0 ? round2(((selling - cost) / cost) * 100) : 0;
+  // Clamped at 0, like every other write to an order line's markup
+  // (`orders-service.ts` stores `Math.max(0, computedMarkup)`). A line
+  // that sells below cost still shows its true margin on screen, because
+  // the margin chip works from cost and selling, not from this field.
+  const markup = cost > 0 && selling > 0 ? Math.max(0, round2(((selling - cost) / cost) * 100)) : 0;
 
   const supplier = (line.supplier ?? "").trim() || (quoteLine.supplier ?? "");
   const sku = (line.sku ?? "").trim() || (quoteLine.sku ?? "");