diff --git a/src/app/(app)/stock-orders/[id]/page.test.tsx b/src/app/(app)/stock-orders/[id]/page.test.tsx
index 63aeb1b..ce8c10d 100644
--- a/src/app/(app)/stock-orders/[id]/page.test.tsx
+++ b/src/app/(app)/stock-orders/[id]/page.test.tsx
@@ -36,10 +36,14 @@ const state = vi.hoisted(() => ({
   }>,
   getOneCalls: [] as Array<{ collection: string; id: string }>,
   usersFetched: false,
+  // Toggleable per-test so the "pencil hidden when received" test
+  // can flip the seeded purchase status without re-mocking.
+  purchaseStatus: "sent" as "sent" | "received" | "draft" | "cancelled",
   reset() {
     this.updateCalls.length = 0;
     this.getOneCalls.length = 0;
     this.usersFetched = false;
+    this.purchaseStatus = "sent";
   },
 }));
 
@@ -62,7 +66,7 @@ vi.mock("@/lib/pocketbase", () => {
           return {
             id,
             purchase_number: "PO26080601STK",
-            status: "sent",
+            status: state.purchaseStatus,
             supplier: "sup-1",
             notes: "",
             email_payload: {},
@@ -372,3 +376,97 @@ describe("<StockOrderDetailPage /> — Cancel", () => {
     );
   });
 });
+
+describe("<StockOrderDetailPage /> — inline cost edit", () => {
+  it("shows a pencil next to each cost when status is sent", async () => {
+    await renderPage();
+    // Seeded data has cost_price=2.5 for si-1; the page renders the
+    // formatted value next to a Pencil button.
+    const pencils = await screen.findAllByTestId(/^stock-order-cost-edit-/);
+    expect(pencils.length).toBeGreaterThanOrEqual(2);
+  });
+
+  it("clicking the pencil swaps the cell to a number input pre-filled with the current cost", async () => {
+    await renderPage();
+    const pencil = await screen.findByTestId("stock-order-cost-edit-si-1");
+    fireEvent.click(pencil);
+    const input = await screen.findByTestId("stock-order-cost-input-si-1");
+    expect((input as HTMLInputElement).value).toBe("2.5");
+  });
+
+  it("Enter saves via stock_purchase_items.update with the new cost_price", async () => {
+    await renderPage();
+    fireEvent.click(await screen.findByTestId("stock-order-cost-edit-si-1"));
+    const input = await screen.findByTestId(
+      "stock-order-cost-input-si-1",
+    ) as HTMLInputElement;
+    fireEvent.change(input, { target: { value: "3.75" } });
+    fireEvent.keyDown(input, { key: "Enter" });
+    await waitFor(() =>
+      expect(
+        state.updateCalls.some(
+          (c) =>
+            c.collection === "stock_purchase_items" &&
+            c.id === "si-1" &&
+            c.patch.cost_price === 3.75,
+        ),
+      ).toBe(true),
+    );
+  });
+
+  it("Escape cancels without writing", async () => {
+    await renderPage();
+    fireEvent.click(await screen.findByTestId("stock-order-cost-edit-si-1"));
+    const input = await screen.findByTestId(
+      "stock-order-cost-input-si-1",
+    ) as HTMLInputElement;
+    fireEvent.change(input, { target: { value: "99" } });
+    fireEvent.keyDown(input, { key: "Escape" });
+    // Input disappears, no PB update was issued.
+    await waitFor(() =>
+      expect(
+        screen.queryByTestId("stock-order-cost-input-si-1"),
+      ).not.toBeInTheDocument(),
+    );
+    expect(
+      state.updateCalls.some((c) => c.id === "si-1"),
+    ).toBe(false);
+  });
+
+  it("rejects negative or non-numeric input with an inline error and no PB write", async () => {
+    await renderPage();
+    fireEvent.click(await screen.findByTestId("stock-order-cost-edit-si-1"));
+    const input = await screen.findByTestId(
+      "stock-order-cost-input-si-1",
+    ) as HTMLInputElement;
+    // Clear the pre-filled value first, then enter empty — the page
+    // treats "" as <0 and shows the inline error without writing.
+    fireEvent.change(input, { target: { value: "" } });
+    fireEvent.keyDown(input, { key: "Enter" });
+    await waitFor(() =>
+      expect(
+        screen.getByTestId("stock-order-cost-error-si-1"),
+      ).toBeInTheDocument(),
+    );
+    expect(
+      state.updateCalls.some(
+        (c) => c.collection === "stock_purchase_items" && c.id === "si-1",
+      ),
+    ).toBe(false);
+  });
+
+  it("does NOT show pencils once the purchase status moves to 'received'", async () => {
+    // Override the seeded purchase status for this one test. We
+    // don't reset state — the hoisted collection mock reads the
+    // current `state.purchaseStatus` so we can flip it in place.
+    state.purchaseStatus = "received";
+    try {
+      await renderPage();
+      expect(
+        screen.queryAllByTestId(/^stock-order-cost-edit-/),
+      ).toHaveLength(0);
+    } finally {
+      state.purchaseStatus = "sent";
+    }
+  });
+});
diff --git a/src/app/(app)/stock-orders/[id]/page.tsx b/src/app/(app)/stock-orders/[id]/page.tsx
index 58cbe51..54834a8 100644
--- a/src/app/(app)/stock-orders/[id]/page.tsx
+++ b/src/app/(app)/stock-orders/[id]/page.tsx
@@ -46,6 +46,7 @@ import {
   Truck,
   ChevronsUpDown,
   Ban,
+  Pencil,
 } from "lucide-react";
 import { toast } from "sonner";
 import getPocketBase from "@/lib/pocketbase";
@@ -173,6 +174,18 @@ export default function StockOrderDetailPage({
   const [sendMessageLoading, setSendMessageLoading] = useState(false);
   // Per-line override (editable while status="sent"). Keyed by item id.
   const [overrides, setOverrides] = useState<Record<string, string>>({});
+  // Per-line cost edit (also editable while status="sent"). When
+  // null, the line renders its existing cost_price + pencil; when
+  // set, the cell renders an input pre-filled with the current value
+  // and a Save callback writes to `stock_purchase_items.cost_price`.
+  // Distinct from `overrides` (which is for received-qty) so the two
+  // editors don't share state. Both share the same `sent`-only gate.
+  const [costEdits, setCostEdits] = useState<
+    Record<
+      string,
+      { value: string; saving: boolean; error: string | null }
+    >
+  >({});
   // Driver picker state. The picker is a small inline dropdown in the
   // header — it lists all PB users (so the operator can pick any
   // driver), and treats the user named "delivery" (case-insensitive)
@@ -583,6 +596,65 @@ export default function StockOrderDetailPage({
     }
   };
 
+  /**
+   * Inline cost edit. Mirrors the pattern in
+   * `src/components/collection-confirm/items-table.tsx` so the same
+   * mental model works for the operator on both pages.
+   *
+   * Only callable while the purchase is in `sent` — once received,
+   * historical pricing is locked (audit trail).
+   *
+   * Writes to `stock_purchase_items.cost_price` via the client PB
+   * (the REST routes that mirror this are AGENT_API_KEY-gated, so
+   * the page's own PB client is the canonical write path).
+   */
+  const saveCostEdit = async (lineId: string) => {
+    const draft = costEdits[lineId];
+    if (!draft) return;
+    const trimmed = draft.value.trim();
+    const parsed = Number(trimmed);
+    if (trimmed === "" || !Number.isFinite(parsed) || parsed < 0) {
+      setCostEdits((prev) => ({
+        ...prev,
+        [lineId]: { ...prev[lineId], error: "Cost must be ≥ 0" },
+      }));
+      return;
+    }
+    setCostEdits((prev) => ({
+      ...prev,
+      [lineId]: { value: trimmed, saving: true, error: null },
+    }));
+    try {
+      const pb = getPocketBase();
+      await pb.collection("stock_purchase_items").update(lineId, {
+        cost_price: parsed,
+      });
+      toast.success("Cost updated");
+      setCostEdits((prev) => {
+        const next = { ...prev };
+        delete next[lineId];
+        return next;
+      });
+      await fetchAll();
+    } catch (err: unknown) {
+      const msg =
+        err instanceof Error ? err.message : "Failed to save cost";
+      toast.error(msg);
+      setCostEdits((prev) => ({
+        ...prev,
+        [lineId]: { ...prev[lineId], saving: false, error: msg },
+      }));
+    }
+  };
+
+  const cancelCostEdit = (lineId: string) => {
+    setCostEdits((prev) => {
+      const next = { ...prev };
+      delete next[lineId];
+      return next;
+    });
+  };
+
   if (loading) {
     return (
       <div className="flex items-center justify-center py-12">
@@ -888,7 +960,81 @@ export default function StockOrderDetailPage({
                         {ordered}
                       </TableCell>
                       <TableCell className="text-right tabular-nums">
-                        {formatCurrency(Number(line.cost_price ?? 0))}
+                        {costEdits[line.id] ? (
+                          <div className="flex flex-col items-end gap-1">
+                            <Input
+                              type="number"
+                              min="0"
+                              step="0.01"
+                              autoFocus
+                              value={costEdits[line.id].value}
+                              onChange={(e) =>
+                                setCostEdits((prev) => ({
+                                  ...prev,
+                                  [line.id]: {
+                                    value: e.target.value.replace(
+                                      /[^\d.]/g,
+                                      "",
+                                    ),
+                                    saving: false,
+                                    error: null,
+                                  },
+                                }))
+                              }
+                              onBlur={() => void saveCostEdit(line.id)}
+                              onKeyDown={(e) => {
+                                if (e.key === "Enter")
+                                  void saveCostEdit(line.id);
+                                if (e.key === "Escape")
+                                  cancelCostEdit(line.id);
+                              }}
+                              className="h-8 w-24 px-2 py-1 text-right"
+                              aria-label={`Edit cost for ${line.description}`}
+                              data-testid={`stock-order-cost-input-${line.id}`}
+                              aria-invalid={
+                                costEdits[line.id].error ? true : undefined
+                              }
+                            />
+                            {costEdits[line.id].error ? (
+                              <span
+                                className="text-xs text-destructive"
+                                data-testid={`stock-order-cost-error-${line.id}`}
+                              >
+                                {costEdits[line.id].error}
+                              </span>
+                            ) : (
+                              <span className="text-xs text-muted-foreground">
+                                Enter to save, Esc to cancel
+                              </span>
+                            )}
+                          </div>
+                        ) : (
+                          <div className="flex items-center justify-end gap-1">
+                            <span data-cost-value={String(line.cost_price ?? 0)}>
+                              {formatCurrency(Number(line.cost_price ?? 0))}
+                            </span>
+                            {purchase.status === "sent" ? (
+                              <button
+                                type="button"
+                                onClick={() =>
+                                  setCostEdits((prev) => ({
+                                    ...prev,
+                                    [line.id]: {
+                                      value: String(line.cost_price ?? 0),
+                                      saving: false,
+                                      error: null,
+                                    },
+                                  }))
+                                }
+                                aria-label={`Edit cost for ${line.description}`}
+                                data-testid={`stock-order-cost-edit-${line.id}`}
+                                className="inline-flex h-6 w-6 items-center justify-center rounded text-muted-foreground hover:bg-accent hover:text-foreground"
+                              >
+                                <Pencil className="h-3 w-3" />
+                              </button>
+                            ) : null}
+                          </div>
+                        )}
                       </TableCell>
                       <TableCell className="text-right tabular-nums">
                         {already}