diff --git a/panel/src/components/tasks/__tests__/create-task-dialog.test.tsx b/panel/src/components/tasks/__tests__/create-task-dialog.test.tsx
new file mode 100644
index 00000000..577bb855
--- /dev/null
+++ b/panel/src/components/tasks/__tests__/create-task-dialog.test.tsx
@@ -0,0 +1,206 @@
+import { describe, it, expect, vi, beforeEach, afterEach } from "vitest";
+import { fireEvent, render, screen, waitFor } from "@testing-library/react";
+
+const { mutateAsync } = vi.hoisted(() => ({
+  mutateAsync: vi.fn().mockResolvedValue(undefined),
+}));
+
+// useCreateTask: just needs a mutateAsync the test can assert on.
+vi.mock("@/hooks/use-tasks", () => ({
+  useCreateTask: () => ({ mutateAsync, isPending: false }),
+}));
+
+// useProducts: one product so the Product <Select> has a pickable option.
+vi.mock("@/hooks/use-products", () => ({
+  useProducts: () => ({
+    data: [{ id: "prod-1", name: "Fan-out Prod" }],
+  }),
+}));
+
+// ProjectSelector: a button that sets the project, so the test can populate
+// projectId without driving a data-fetching combobox.
+vi.mock("@/components/projects/project-selector", () => ({
+  ProjectSelector: ({ onChange }: { onChange: (v: string | null) => void }) => (
+    <button type="button" onClick={() => onChange("proj-1")}>
+      Set Project
+    </button>
+  ),
+}));
+
+// AcceptanceCriteriaEditor: a button that supplies one criterion, bypassing
+// the multi-input editor so the AC validation passes.
+vi.mock("../acceptance-criteria-editor", () => ({
+  AcceptanceCriteriaEditor: ({
+    onChange,
+  }: {
+    onChange: (c: string[]) => void;
+  }) => (
+    <button type="button" onClick={() => onChange(["criterion one"])}>
+      Add Criterion
+    </button>
+  ),
+}));
+
+// MarkdownEditor: a controlled textarea so the test can type a >=20-char desc.
+vi.mock("../markdown-editor", () => ({
+  MarkdownEditor: ({
+    value,
+    onChange,
+    label,
+  }: {
+    value: string;
+    onChange: (v: string) => void;
+    label: string;
+  }) => (
+    <label>
+      {label}
+      <textarea
+        aria-label={label}
+        value={value}
+        onChange={(e) => onChange(e.target.value)}
+      />
+    </label>
+  ),
+}));
+
+// DependencySelector / TaskSelector / AgentSelector: optional, render nothing.
+vi.mock("../dependency-selector", () => ({
+  DependencySelector: () => null,
+}));
+vi.mock("../task-selector", () => ({ TaskSelector: () => null }));
+vi.mock("@/components/agents/agent-selector", () => ({
+  AgentSelector: () => null,
+}));
+
+// Collapsible: render the advanced section open so the Project/Product fields
+// are reachable without a pointer-driven toggle.
+vi.mock("@/components/ui/collapsible", () => ({
+  Collapsible: ({ children }: { children: React.ReactNode }) => <>{children}</>,
+  CollapsibleTrigger: ({ children }: { children: React.ReactNode }) => (
+    <>{children}</>
+  ),
+  CollapsibleContent: ({ children }: { children: React.ReactNode }) => (
+    <>{children}</>
+  ),
+}));
+
+// Select: a native <select> stub so the Product picker (a radix Select) can be
+// driven with fireEvent.change. All the dialog's selects become native ones;
+// they carry their own default values, so only the Product select needs action.
+vi.mock("@/components/ui/select", () => ({
+  Select: ({
+    value,
+    onValueChange,
+    children,
+  }: {
+    value: string;
+    onValueChange?: (v: string) => void;
+    children: React.ReactNode;
+  }) => (
+    <select value={value} onChange={(e) => onValueChange?.(e.target.value)}>
+      {children}
+    </select>
+  ),
+  SelectTrigger: ({ children }: { children: React.ReactNode }) => (
+    <>{children}</>
+  ),
+  SelectValue: () => null,
+  SelectContent: ({ children }: { children: React.ReactNode }) => (
+    <>{children}</>
+  ),
+  SelectItem: ({
+    value,
+    children,
+  }: {
+    value: string;
+    children: React.ReactNode;
+  }) => <option value={value}>{children}</option>,
+}));
+
+import { CreateTaskDialog } from "../create-task-dialog";
+
+function fillBasics() {
+  fireEvent.change(screen.getByPlaceholderText(/Enter task title/i), {
+    target: { value: "A task title" },
+  });
+  fireEvent.change(screen.getByLabelText("Description"), {
+    target: {
+      value: "A description long enough to pass the twenty char minimum.",
+    },
+  });
+  fireEvent.click(screen.getByRole("button", { name: "Add Criterion" }));
+}
+
+// The Product select is the only native <select> offering the prod-1 option.
+// radix Dialog portals content to document.body, so query the document (not
+// the render container).
+function productSelect() {
+  const selects = Array.from(document.querySelectorAll("select"));
+  return selects.find((s) => s.querySelector('option[value="prod-1"]'))!;
+}
+
+function submit() {
+  fireEvent.submit(document.querySelector("form")!);
+}
+
+describe("CreateTaskDialog — project/product mutual exclusivity (F085)", () => {
+  beforeEach(() => {
+    mutateAsync.mockClear();
+  });
+  afterEach(() => {
+    vi.clearAllMocks();
+  });
+
+  it("rejects submitting both a Project and a Product", async () => {
+    render(<CreateTaskDialog />);
+    // Open the dialog (DialogTrigger renders a "New Task" button).
+    fireEvent.click(screen.getByRole("button", { name: /New Task/i }));
+
+    fillBasics();
+    fireEvent.click(screen.getByRole("button", { name: "Set Project" }));
+    fireEvent.change(productSelect(), { target: { value: "prod-1" } });
+
+    submit();
+
+    // A task targets ONE repo OR fans out via a Product — never both. The
+    // server silently lets product_id win and drops project_id, so the panel
+    // must refuse the ambiguous submit. Before the fix validate() only checked
+    // "at least one", so both sailed through to mutateAsync.
+    await waitFor(() => {
+      expect(mutateAsync).not.toHaveBeenCalled();
+    });
+    expect(
+      screen.getByText(/either a project or a product/i),
+    ).toBeInTheDocument();
+  });
+
+  it("submits with project_id only when just a Project is picked", async () => {
+    render(<CreateTaskDialog />);
+    fireEvent.click(screen.getByRole("button", { name: /New Task/i }));
+
+    fillBasics();
+    fireEvent.click(screen.getByRole("button", { name: "Set Project" }));
+
+    submit();
+
+    await waitFor(() => expect(mutateAsync).toHaveBeenCalledTimes(1));
+    const payload = mutateAsync.mock.calls[0][0] as Record<string, unknown>;
+    expect(payload.project_id).toBe("proj-1");
+    expect(payload.product_id).toBeUndefined();
+  });
+
+  it("submits with product_id only when just a Product is picked", async () => {
+    render(<CreateTaskDialog />);
+    fireEvent.click(screen.getByRole("button", { name: /New Task/i }));
+
+    fillBasics();
+    fireEvent.change(productSelect(), { target: { value: "prod-1" } });
+
+    submit();
+
+    await waitFor(() => expect(mutateAsync).toHaveBeenCalledTimes(1));
+    const payload = mutateAsync.mock.calls[0][0] as Record<string, unknown>;
+    expect(payload.product_id).toBe("prod-1");
+    expect(payload.project_id).toBeUndefined();
+  });
+});
diff --git a/panel/src/components/tasks/create-task-dialog.tsx b/panel/src/components/tasks/create-task-dialog.tsx
index 807fe7ce..a750cb65 100644
--- a/panel/src/components/tasks/create-task-dialog.tsx
+++ b/panel/src/components/tasks/create-task-dialog.tsx
@@ -126,6 +126,16 @@ export function CreateTaskDialog() {
         "Pick a Project (the repo) or a Product (cell→project map for a fan-out task)";
     }
 
+    // A task targets exactly one repo OR fans out via a Product — never both.
+    // The server silently lets product_id win at routing and drops project_id,
+    // so submitting both would record a misleading, never-used project_id. The
+    // selectors clear each other on pick (below); this validator is the
+    // backstop for any path that sets both.
+    if (projectId && productId) {
+      newErrors.project_id =
+        "Pick either a Project or a Product, not both — a task targets one repo or fans out via a Product";
+    }
+
     setErrors(newErrors);
     return Object.keys(newErrors).length === 0;
   };