diff --git a/src/components/ui/mark-ready-dialog.test.tsx b/src/components/ui/mark-ready-dialog.test.tsx
new file mode 100644
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+++ b/src/components/ui/mark-ready-dialog.test.tsx
@@ -0,0 +1,126 @@
+import { afterEach, beforeEach, describe, expect, it, vi } from "vitest";
+import { cleanup, fireEvent, render, screen } from "@testing-library/react";
+
+import { MarkReadyDialog } from "./mark-ready-dialog";
+
+const PO_LABEL = "PO-2026-07-009";
+const ITEM_LABEL = "Magnetic drill";
+
+describe("MarkReadyDialog", () => {
+  beforeEach(() => {
+    class ResizeObserverMock {
+      observe() {}
+      unobserve() {}
+      disconnect() {}
+    }
+    vi.stubGlobal("ResizeObserver", ResizeObserverMock);
+    window.HTMLElement.prototype.scrollIntoView = vi.fn();
+  });
+
+  afterEach(() => {
+    cleanup();
+  });
+
+  it("renders the PO and item in the title", () => {
+    render(
+      <MarkReadyDialog
+        open
+        onOpenChange={vi.fn()}
+        onSubmit={vi.fn()}
+        poLabel={PO_LABEL}
+        itemLabel={ITEM_LABEL}
+      />,
+    );
+    expect(
+      screen.getByText(`Mark ready: ${PO_LABEL} · ${ITEM_LABEL}`),
+    ).toBeInTheDocument();
+  });
+
+  it("disables Save when the textarea is empty", () => {
+    render(
+      <MarkReadyDialog
+        open
+        onOpenChange={vi.fn()}
+        onSubmit={vi.fn()}
+        poLabel={PO_LABEL}
+        itemLabel={ITEM_LABEL}
+      />,
+    );
+    expect(screen.getByRole("button", { name: /save & mark ready/i })).toBeDisabled();
+  });
+
+  it("disables Save when the textarea is whitespace only", () => {
+    render(
+      <MarkReadyDialog
+        open
+        onOpenChange={vi.fn()}
+        onSubmit={vi.fn()}
+        poLabel={PO_LABEL}
+        itemLabel={ITEM_LABEL}
+      />,
+    );
+    fireEvent.change(screen.getByRole("textbox"), {
+      target: { value: "   \n  " },
+    });
+    expect(screen.getByRole("button", { name: /save & mark ready/i })).toBeDisabled();
+  });
+
+  it("enables Save and submits the trimmed note", () => {
+    const onSubmit = vi.fn();
+    render(
+      <MarkReadyDialog
+        open
+        onOpenChange={vi.fn()}
+        onSubmit={onSubmit}
+        poLabel={PO_LABEL}
+        itemLabel={ITEM_LABEL}
+      />,
+    );
+    fireEvent.change(screen.getByRole("textbox"), {
+      target: { value: "  spoke to David  " },
+    });
+    const save = screen.getByRole("button", { name: /save & mark ready/i });
+    expect(save).not.toBeDisabled();
+    fireEvent.click(save);
+    expect(onSubmit).toHaveBeenCalledWith("spoke to David");
+  });
+
+  it("renders a server error without closing the modal", () => {
+    render(
+      <MarkReadyDialog
+        open
+        onOpenChange={vi.fn()}
+        onSubmit={vi.fn()}
+        poLabel={PO_LABEL}
+        itemLabel={ITEM_LABEL}
+        serverError="A note is required when marking ready."
+      />,
+    );
+    expect(
+      screen.getByText(/a note is required when marking ready/i),
+    ).toBeInTheDocument();
+    expect(screen.getByRole("alert")).toBeInTheDocument();
+    expect(
+      screen.getByRole("dialog", {
+        name: `Mark ready: ${PO_LABEL} · ${ITEM_LABEL}`,
+      }),
+    ).toBeInTheDocument();
+  });
+
+  it("does not call onSubmit when the dialog is closed", () => {
+    const onSubmit = vi.fn();
+    const onOpenChange = vi.fn();
+    render(
+      <MarkReadyDialog
+        open={false}
+        onOpenChange={onOpenChange}
+        onSubmit={onSubmit}
+        poLabel={PO_LABEL}
+        itemLabel={ITEM_LABEL}
+      />,
+    );
+    const save = screen.queryByRole("button", { name: /save & mark ready/i });
+    expect(save).toBeNull();
+    expect(onSubmit).not.toHaveBeenCalled();
+  });
+});
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