diff --git a/apps/web/components/task-create-dialog-extra-repos.test.tsx b/apps/web/components/task-create-dialog-extra-repos.test.tsx
new file mode 100644
index 0000000000..20630c0a65
--- /dev/null
+++ b/apps/web/components/task-create-dialog-extra-repos.test.tsx
@@ -0,0 +1,154 @@
+import { describe, it, expect, vi, afterEach } from "vitest";
+import { render, screen, fireEvent, cleanup } from "@testing-library/react";
+
+afterEach(cleanup);
+import type { Repository } from "@/lib/types/http";
+import type { DialogFormState, ExtraRepositoryRow } from "./task-create-dialog-types";
+
+// Mock branches hook so the BranchSelector inside the row doesn't blow up.
+vi.mock("@/hooks/domains/workspace/use-repository-branches", () => ({
+  useRepositoryBranches: () => ({ branches: [], isLoading: false }),
+}));
+
+// Defer the import until after mocks are set up.
+async function loadComponent() {
+  return (await import("./task-create-dialog-extra-repos")).ExtraRepositoryRows;
+}
+
+const REPO_FRONT_ID = "repo-front";
+const REPO_BACK_ID = "repo-back";
+
+function makeRepo(id: string, name: string): Repository {
+  return {
+    id,
+    workspace_id: "ws-1",
+    name,
+    source_type: "local",
+    local_path: `/repos/${name}`,
+    default_branch: "main",
+    created_at: new Date().toISOString(),
+    updated_at: new Date().toISOString(),
+  } as Repository;
+}
+
+function makeFs(overrides: Partial<DialogFormState>): DialogFormState {
+  // Only the fields ExtraRepositoryRows actually reads need to be real.
+  return {
+    repositoryId: REPO_FRONT_ID,
+    extraRepositories: [] as ExtraRepositoryRow[],
+    addExtraRepository: vi.fn(),
+    removeExtraRepository: vi.fn(),
+    updateExtraRepository: vi.fn(),
+    ...overrides,
+  } as unknown as DialogFormState;
+}
+
+describe("ExtraRepositoryRows", () => {
+  it("disables the Add button until a primary repo is selected", async () => {
+    const ExtraRepositoryRows = await loadComponent();
+    render(
+      <ExtraRepositoryRows
+        fs={makeFs({})}
+        repositories={[makeRepo(REPO_FRONT_ID, "frontend")]}
+        isTaskStarted={false}
+        primarySelected={false}
+      />,
+    );
+    const btn = screen.getByTestId("add-extra-repository") as HTMLButtonElement;
+    expect(btn.disabled).toBe(true);
+  });
+
+  it("calls addExtraRepository when the Add button is clicked", async () => {
+    const ExtraRepositoryRows = await loadComponent();
+    const fs = makeFs({});
+    render(
+      <ExtraRepositoryRows
+        fs={fs}
+        // Need a second repo so the Add button is enabled (the primary
+        // would otherwise be the only choice).
+        repositories={[makeRepo(REPO_FRONT_ID, "frontend"), makeRepo(REPO_BACK_ID, "backend")]}
+        isTaskStarted={false}
+        primarySelected
+      />,
+    );
+    fireEvent.click(screen.getByTestId("add-extra-repository"));
+    expect(fs.addExtraRepository).toHaveBeenCalledOnce();
+  });
+
+  it("renders one editable row per extra repository", async () => {
+    const ExtraRepositoryRows = await loadComponent();
+    render(
+      <ExtraRepositoryRows
+        fs={makeFs({
+          extraRepositories: [
+            { key: "extra-1", repositoryId: "", branch: "" },
+            { key: "extra-2", repositoryId: "", branch: "" },
+          ],
+        })}
+        repositories={[
+          makeRepo(REPO_FRONT_ID, "frontend"),
+          makeRepo(REPO_BACK_ID, "backend"),
+          makeRepo("repo-shared", "shared"),
+        ]}
+        isTaskStarted={false}
+        primarySelected
+      />,
+    );
+    expect(screen.getAllByTestId("extra-repository-row")).toHaveLength(2);
+  });
+
+  it("hides the section when the task is already started", async () => {
+    const ExtraRepositoryRows = await loadComponent();
+    const { container } = render(
+      <ExtraRepositoryRows
+        fs={makeFs({
+          extraRepositories: [{ key: "extra-1", repositoryId: "", branch: "" }],
+        })}
+        repositories={[makeRepo(REPO_FRONT_ID, "frontend")]}
+        isTaskStarted
+        primarySelected
+      />,
+    );
+    expect(container.querySelector("[data-testid='extra-repositories']")).toBeNull();
+  });
+
+  it("opening a row's dropdown shows the unused workspace repos", async () => {
+    const ExtraRepositoryRows = await loadComponent();
+    render(
+      <ExtraRepositoryRows
+        fs={makeFs({
+          repositoryId: REPO_FRONT_ID,
+          extraRepositories: [{ key: "extra-1", repositoryId: "", branch: "" }],
+        })}
+        repositories={[
+          makeRepo(REPO_FRONT_ID, "frontend"),
+          makeRepo(REPO_BACK_ID, "backend"),
+          makeRepo("repo-shared", "shared"),
+        ]}
+        isTaskStarted={false}
+        primarySelected
+      />,
+    );
+    // Open the row's repository combobox.
+    const triggers = screen.getAllByTestId("repository-selector");
+    fireEvent.click(triggers[0]);
+    // The primary repo (frontend) is excluded; the other two should be offered.
+    expect(screen.queryByText("backend")).not.toBeNull();
+    expect(screen.queryByText("shared")).not.toBeNull();
+    expect(screen.queryByText("frontend")).toBeNull();
+  });
+
+  it("disables the Add button when no other workspace repos are available", async () => {
+    const ExtraRepositoryRows = await loadComponent();
+    render(
+      <ExtraRepositoryRows
+        fs={makeFs({ repositoryId: "repo-only" })}
+        repositories={[makeRepo("repo-only", "only")]}
+        isTaskStarted={false}
+        primarySelected
+      />,
+    );
+    const btn = screen.getByTestId("add-extra-repository") as HTMLButtonElement;
+    expect(btn.disabled).toBe(true);
+  });
+});
diff --git a/apps/web/components/task-create-dialog-extra-repos.tsx b/apps/web/components/task-create-dialog-extra-repos.tsx
index eab7b8e524..bbcc480d58 100644
--- a/apps/web/components/task-create-dialog-extra-repos.tsx
+++ b/apps/web/components/task-create-dialog-extra-repos.tsx
@@ -41,6 +41,14 @@ export function ExtraRepositoryRows({
   // Hide the section in started/edit modes where the repo set is fixed.
   if (isTaskStarted) return null;
 
+  // Count workspace repositories the user could still add: total minus primary
+  // minus all already-chosen extras. When zero, we disable the Add button so
+  // the user gets a clear signal instead of a row with an empty dropdown.
+  const usedIds = collectUsedRepoIds(fs, "");
+  const remainingCount = repositories.filter((r) => !usedIds.has(r.id)).length;
+  const canAddMore = primarySelected && remainingCount > 0;
+  const addButtonHint = computeAddButtonHint(primarySelected, remainingCount);
+
   return (
     <div className="space-y-2" data-testid="extra-repositories">
       {fs.extraRepositories.map((row) => (
@@ -60,7 +68,8 @@ export function ExtraRepositoryRows({
         variant="ghost"
         size="sm"
         onClick={fs.addExtraRepository}
-        disabled={!primarySelected}
+        disabled={!canAddMore}
+        title={addButtonHint}
         className="text-xs cursor-pointer text-muted-foreground hover:text-foreground"
         data-testid="add-extra-repository"
       >
@@ -71,6 +80,15 @@ export function ExtraRepositoryRows({
   );
 }
 
+function computeAddButtonHint(
+  primarySelected: boolean,
+  remainingCount: number,
+): string | undefined {
+  if (!primarySelected) return "Select the primary repository first";
+  if (remainingCount === 0) return "All workspace repositories are already added";
+  return undefined;
+}
+
 function computeBranchPlaceholder(
   repoId: string,
   branchesLoading: boolean,