diff --git a/src/lib/server/stock-quick-draft-service.test.ts b/src/lib/server/stock-quick-draft-service.test.ts
index eaaa8b4..2feed77 100644
--- a/src/lib/server/stock-quick-draft-service.test.ts
+++ b/src/lib/server/stock-quick-draft-service.test.ts
@@ -160,8 +160,12 @@ const { store, pbMock, clearStore } = vi.hoisted(() => {
   return { store, pbMock, clearStore };
 });
 
-vi.mock("pocketbase", () => ({ default: vi.fn().mockImplementation(() => pbMock) }));
+vi.mock("pocketbase", async (importOriginal) => {
+  const actual = await importOriginal<typeof import("pocketbase")>();
+  return { ...actual, default: vi.fn().mockImplementation(() => pbMock) };
+});
 
+import { ClientResponseError } from "pocketbase";
 import {
   addQuickDraftLine,
   getStockItemSupplierHistory,
@@ -169,6 +173,7 @@ import {
   removeQuickDraftLine,
   searchStockItemsForQuickDraft,
   listSuppliersForQuickDraft,
+  updateQuickDraftLine,
 } from "./stock-quick-draft-service";
 
 beforeEach(() => clearStore());
@@ -640,6 +645,132 @@ describe("removeQuickDraftLine", () => {
   });
 });
 
+describe("updateQuickDraftLine", () => {
+  async function createDraftLine() {
+    const supplierId = (
+      await pbMock.collection("suppliers").create({ suppliername: "S" })
+    ).id;
+    const purchase = await pbMock.collection("stock_purchases").create({
+      supplier: supplierId,
+      status: "draft",
+      purchase_number: "PO26070101STK",
+    });
+    return pbMock.collection("stock_purchase_items").create({
+      purchase: purchase.id,
+      stock_item: "item-1",
+      description: "115mm Cutting Disc",
+      quantity: 10,
+      cost_price: 18.5,
+    });
+  }
+
+  it("updates quantity on the line and returns ok", async () => {
+    const line = await createDraftLine();
+
+    const result = await updateQuickDraftLine(pbMock as never, {
+      lineId: line.id,
+      quantity: 25,
+    });
+
+    expect(result).toEqual({ ok: true });
+    const updated = await pbMock.collection("stock_purchase_items").getOne(line.id);
+    expect(updated.quantity).toBe(25);
+    expect(updated.cost_price).toBe(18.5);
+  });
+
+  it("updates cost_price on the line and returns ok", async () => {
+    const line = await createDraftLine();
+
+    const result = await updateQuickDraftLine(pbMock as never, {
+      lineId: line.id,
+      costPrice: 20,
+    });
+
+    expect(result).toEqual({ ok: true });
+    const updated = await pbMock.collection("stock_purchase_items").getOne(line.id);
+    expect(updated.quantity).toBe(10);
+    expect(updated.cost_price).toBe(20);
+  });
+
+  it("updates both fields in one call", async () => {
+    const line = await createDraftLine();
+
+    const result = await updateQuickDraftLine(pbMock as never, {
+      lineId: line.id,
+      quantity: 30,
+      costPrice: 21.5,
+    });
+
+    expect(result).toEqual({ ok: true });
+    const updated = await pbMock.collection("stock_purchase_items").getOne(line.id);
+    expect(updated.quantity).toBe(30);
+    expect(updated.cost_price).toBe(21.5);
+  });
+
+  it("rejects when neither quantity nor costPrice is supplied", async () => {
+    const line = await createDraftLine();
+
+    const result = await updateQuickDraftLine(pbMock as never, { lineId: line.id });
+
+    expect(result.ok).toBe(false);
+    if (result.ok) return;
+    expect(result.reason).toBe("validation");
+  });
+
+  it("rejects non-positive quantity", async () => {
+    const line = await createDraftLine();
+
+    const result = await updateQuickDraftLine(pbMock as never, {
+      lineId: line.id,
+      quantity: 0,
+    });
+
+    expect(result.ok).toBe(false);
+    if (result.ok) return;
+    expect(result.reason).toBe("validation");
+  });
+
+  it("rejects negative cost_price", async () => {
+    const line = await createDraftLine();
+
+    const result = await updateQuickDraftLine(pbMock as never, {
+      lineId: line.id,
+      costPrice: -1,
+    });
+
+    expect(result.ok).toBe(false);
+    if (result.ok) return;
+    expect(result.reason).toBe("validation");
+  });
+
+  it("returns line_not_found when the line does not exist", async () => {
+    const originalCollection = pbMock.collection;
+    const collectionSpy = vi.spyOn(pbMock, "collection").mockImplementation((name) => {
+      const collection = originalCollection(name);
+      if (name !== "stock_purchase_items") return collection;
+      return {
+        ...collection,
+        update: vi.fn().mockRejectedValue(
+          new ClientResponseError({
+            status: 404,
+            response: { message: "line not found" },
+          }),
+        ),
+      };
+    });
+
+    const result = await updateQuickDraftLine(pbMock as never, {
+      lineId: "does-not-exist",
+      quantity: 1,
+    });
+    collectionSpy.mockRestore();
+
+    expect(result.ok).toBe(false);
+    if (result.ok) return;
+    expect(result.reason).toBe("line_not_found");
+  });
+});
+
 describe("listOpenDrafts", () => {
   it("returns an empty array when there are no draft stock_purchases", async () => {
     const drafts = await listOpenDrafts(pbMock as never);
diff --git a/src/lib/server/stock-quick-draft-service.ts b/src/lib/server/stock-quick-draft-service.ts
index dd02015..bcac10a 100644
--- a/src/lib/server/stock-quick-draft-service.ts
+++ b/src/lib/server/stock-quick-draft-service.ts
@@ -505,6 +505,55 @@ export async function removeQuickDraftLine(
   }
 }
 
+/* ============================================================================
+ * updateQuickDraftLine
+ * ========================================================================== */
+
+export type UpdateQuickDraftLineResult =
+  | { ok: true }
+  | { ok: false; reason: "validation" | "line_not_found" | "update_failed"; message: string };
+
+/**
+ * Updates the quantity and/or cost price on a `stock_purchase_items` row.
+ * Used by the modal's inline line editor for correcting draft values.
+ *
+ * Returns `{ ok: true }` on success, or `{ ok: false, reason, message }`
+ * on validation or update failure. At least one field is required;
+ * quantity must be positive and cost price must be non-negative.
+ */
+export async function updateQuickDraftLine(
+  pb: StockQuickDraftPb,
+  input: { lineId: string; quantity?: number; costPrice?: number },
+): Promise<UpdateQuickDraftLineResult> {
+  if (typeof input.lineId !== "string" || input.lineId.length === 0) {
+    return { ok: false, reason: "validation", message: "lineId is required" };
+  }
+  if (input.quantity === undefined && input.costPrice === undefined) {
+    return { ok: false, reason: "validation", message: "at least one of quantity or costPrice is required" };
+  }
+  if (input.quantity !== undefined && (typeof input.quantity !== "number" || input.quantity <= 0)) {
+    return { ok: false, reason: "validation", message: "quantity must be > 0" };
+  }
+  if (input.costPrice !== undefined && (typeof input.costPrice !== "number" || input.costPrice < 0)) {
+    return { ok: false, reason: "validation", message: "costPrice must be >= 0" };
+  }
+
+  const patch: { quantity?: number; cost_price?: number } = {};
+  if (input.quantity !== undefined) patch.quantity = input.quantity;
+  if (input.costPrice !== undefined) patch.cost_price = input.costPrice;
+
+  try {
+    await pb.collection(STOCK_PURCHASE_ITEMS).update(input.lineId, patch);
+    return { ok: true };
+  } catch (err) {
+    const status = (err as { status?: number }).status;
+    if (status === 404) {
+      return { ok: false, reason: "line_not_found", message: err instanceof Error ? err.message : "line not found" };
+    }
+    return { ok: false, reason: "update_failed", message: err instanceof Error ? err.message : "line update failed" };
+  }
+}
+
 /* ============================================================================
  * listOpenDrafts
  * ========================================================================== */