diff --git a/app/src/components/ui/Button.test.tsx b/app/src/components/ui/Button.test.tsx
new file mode 100644
index 0000000..0a85c43
--- /dev/null
+++ b/app/src/components/ui/Button.test.tsx
@@ -0,0 +1,77 @@
+import { describe, it, expect, vi, beforeEach } from "vitest";
+import { render, screen, fireEvent } from "@testing-library/react";
+import Button from "./Button";
+
+const onClick = vi.fn();
+const onKeyDown = vi.fn();
+
+describe("Button", () => {
+  beforeEach(() => {
+    vi.clearAllMocks();
+  });
+
+  it("still supports the native disabled attribute", () => {
+    render(
+      <Button disabled onClick={onClick}>
+        Save
+      </Button>,
+    );
+    expect(screen.getByRole("button", { name: "Save" })).toBeDisabled();
+  });
+
+  it("stays in the accessibility tree when unavailable, and says why", () => {
+    render(
+      <Button unavailable unavailableReason="Stop the container first.">
+        Save
+      </Button>,
+    );
+    const button = screen.getByRole("button", { name: "Save" });
+    expect(button).not.toBeDisabled();
+    expect(button).toHaveAttribute("aria-disabled", "true");
+    expect(button).toHaveAccessibleDescription("Stop the container first.");
+    // The reason is a description, not part of the name.
+    expect(button).toHaveAccessibleName("Save");
+  });
+
+  it("guards clicks and Enter/Space while unavailable", () => {
+    render(
+      <Button unavailable unavailableReason="Stop the container first." onClick={onClick}>
+        Save
+      </Button>,
+    );
+    const button = screen.getByRole("button", { name: "Save" });
+    fireEvent.click(button);
+    fireEvent.keyDown(button, { key: "Enter" });
+    fireEvent.keyDown(button, { key: " " });
+    expect(onClick).not.toHaveBeenCalled();
+  });
+
+  it("still forwards keys that are not activation keys", () => {
+    render(
+      <Button
+        unavailable
+        unavailableReason="Stop the container first."
+        onKeyDown={onKeyDown}
+      >
+        Save
+      </Button>,
+    );
+    fireEvent.keyDown(screen.getByRole("button", { name: "Save" }), {
+      key: "Escape",
+    });
+    expect(onKeyDown).toHaveBeenCalled();
+  });
+
+  it("behaves like an ordinary button when available", () => {
+    render(
+      <Button unavailable={false} unavailableReason="Stop the container first." onClick={onClick}>
+        Save
+      </Button>,
+    );
+    const button = screen.getByRole("button", { name: "Save" });
+    expect(button).not.toHaveAttribute("aria-disabled");
+    expect(button).toHaveAccessibleDescription("");
+    fireEvent.click(button);
+    expect(onClick).toHaveBeenCalledTimes(1);
+  });
+});
diff --git a/app/src/components/ui/Button.tsx b/app/src/components/ui/Button.tsx
index 79b8655..be41f79 100644
--- a/app/src/components/ui/Button.tsx
+++ b/app/src/components/ui/Button.tsx
@@ -1,4 +1,5 @@
 import type { ButtonHTMLAttributes, ReactNode } from "react";
+import { useUnavailable } from "./unavailable";
 
 export type ButtonVariant = "primary" | "secondary" | "danger" | "ghost";
 export type ButtonSize = "sm" | "md";
@@ -7,22 +8,37 @@ interface Props extends ButtonHTMLAttributes<HTMLButtonElement> {
   variant?: ButtonVariant;
   size?: ButtonSize;
   children: ReactNode;
+  /**
+   * Unavailable, but still announced. Renders `aria-disabled` and wires
+   * `unavailableReason` to `aria-describedby` instead of using the native
+   * `disabled` attribute, which would take the button out of the tab order and
+   * out of the accessibility tree — reason and all. Clicks and Enter/Space are
+   * guarded for you. Prefer this over `disabled` whenever there is a reason
+   * worth telling the user.
+   */
+  unavailable?: boolean;
+  /** Why the button cannot be used. Required for `unavailable` to say anything. */
+  unavailableReason?: string;
 }
 
 /**
  * Real buttons with visible bounds and a ≥24px hit target.
  * Filled variants use the *-emphasis tokens so white text clears WCAG AA;
  * `--accent` stays reserved for foreground/link use.
+ *
+ * The `aria-disabled:` class mirrors below exist because Tailwind's
+ * `disabled:` variant only matches the native attribute, which `unavailable`
+ * deliberately does not set. Keep the two lists in step.
  */
 const VARIANTS: Record<ButtonVariant, string> = {
   primary:
-    "bg-[var(--accent-emphasis)] text-white border border-transparent hover:bg-[var(--accent-emphasis-hover)] disabled:bg-[var(--bg-tertiary)] disabled:text-[var(--text-disabled)] disabled:border-[var(--border-color)]",
+    "bg-[var(--accent-emphasis)] text-white border border-transparent hover:bg-[var(--accent-emphasis-hover)] disabled:bg-[var(--bg-tertiary)] disabled:text-[var(--text-disabled)] disabled:border-[var(--border-color)] aria-disabled:bg-[var(--bg-tertiary)] aria-disabled:text-[var(--text-disabled)] aria-disabled:border-[var(--border-color)] aria-disabled:hover:bg-[var(--bg-tertiary)]",
   secondary:
-    "bg-[var(--bg-tertiary)] text-[var(--text-primary)] border border-[var(--border-color)] hover:bg-[var(--border-color)] disabled:text-[var(--text-disabled)] disabled:hover:bg-[var(--bg-tertiary)]",
+    "bg-[var(--bg-tertiary)] text-[var(--text-primary)] border border-[var(--border-color)] hover:bg-[var(--border-color)] disabled:text-[var(--text-disabled)] disabled:hover:bg-[var(--bg-tertiary)] aria-disabled:text-[var(--text-disabled)] aria-disabled:hover:bg-[var(--bg-tertiary)]",
   danger:
-    "bg-transparent text-[var(--error)] border border-[var(--error)]/40 hover:bg-[var(--error-muted)] disabled:text-[var(--text-disabled)] disabled:border-[var(--border-color)] disabled:hover:bg-transparent",
+    "bg-transparent text-[var(--error)] border border-[var(--error)]/40 hover:bg-[var(--error-muted)] disabled:text-[var(--text-disabled)] disabled:border-[var(--border-color)] disabled:hover:bg-transparent aria-disabled:text-[var(--text-disabled)] aria-disabled:border-[var(--border-color)] aria-disabled:hover:bg-transparent",
   ghost:
-    "bg-transparent text-[var(--text-secondary)] border border-transparent hover:text-[var(--text-primary)] hover:bg-[var(--bg-tertiary)] disabled:text-[var(--text-disabled)] disabled:hover:bg-transparent",
+    "bg-transparent text-[var(--text-secondary)] border border-transparent hover:text-[var(--text-primary)] hover:bg-[var(--bg-tertiary)] disabled:text-[var(--text-disabled)] disabled:hover:bg-transparent aria-disabled:text-[var(--text-disabled)] aria-disabled:hover:text-[var(--text-disabled)] aria-disabled:hover:bg-transparent",
 };
 
 const SIZES: Record<ButtonSize, string> = {
@@ -35,16 +51,30 @@ export default function Button({
   size = "sm",
   className = "",
   type = "button",
+  unavailable = false,
+  unavailableReason = "",
   children,
   ...rest
 }: Props) {
+  const { controlProps, reasonNode } = useUnavailable({
+    unavailable,
+    reason: unavailableReason,
+    onClick: rest.onClick,
+    onKeyDown: rest.onKeyDown,
+  });
+
   return (
-    <button
-      type={type}
-      {...rest}
-      className={`inline-flex items-center justify-center whitespace-nowrap rounded-[var(--radius-control)] font-medium transition-colors disabled:cursor-not-allowed ${SIZES[size]} ${VARIANTS[variant]} ${className}`}
-    >
-      {children}
-    </button>
+    <>
+      <button
+        type={type}
+        {...rest}
+        {...controlProps}
+        className={`inline-flex items-center justify-center whitespace-nowrap rounded-[var(--radius-control)] font-medium transition-colors disabled:cursor-not-allowed aria-disabled:cursor-not-allowed ${SIZES[size]} ${VARIANTS[variant]} ${className}`}
+      >
+        {children}
+      </button>
+      {/* Outside the button: inside, the reason would join its accessible name. */}
+      {reasonNode}
+    </>
   );
 }
diff --git a/app/src/components/ui/unavailable.tsx b/app/src/components/ui/unavailable.tsx
new file mode 100644
index 0000000..b25e7bc
--- /dev/null
+++ b/app/src/components/ui/unavailable.tsx
@@ -0,0 +1,87 @@
+import {
+  useId,
+  type KeyboardEventHandler,
+  type MouseEventHandler,
+  type ReactNode,
+} from "react";
+
+/** Keys a native `<button>` turns into a click. */
+const ACTIVATION_KEYS = new Set([" ", "Spacebar", "Enter"]);
+
+export interface UnavailableControlProps {
+  "aria-disabled"?: true;
+  "aria-describedby"?: string;
+  onClick?: MouseEventHandler<HTMLButtonElement>;
+  onKeyDown?: KeyboardEventHandler<HTMLButtonElement>;
+}
+
+export interface UnavailableControl {
+  /** Spread onto the control. Carries the guarded handlers. */
+  controlProps: UnavailableControlProps;
+  /**
+   * Render as a *sibling* of the control — inside it the reason would be
+   * appended to the accessible name instead of the description.
+   */
+  reasonNode: ReactNode;
+}
+
+/**
+ * Makes a control unavailable without hiding it from assistive technology.
+ *
+ * `disabled` takes an element out of the tab order *and* out of the
+ * accessibility tree, so the `title` explaining why it cannot be used is
+ * announced to nobody and shown only to a sighted user with a mouse. That is
+ * backwards: the people who most need the reason are the ones who never get
+ * it. `aria-disabled` keeps the control focusable and announced, and
+ * `aria-describedby` hands over the reason.
+ *
+ * The catch is that `aria-disabled` is advisory — it does not block clicks or
+ * Enter/Space the way `disabled` does. This hook therefore returns the guards
+ * along with the attributes, so a call site cannot take the announcement
+ * without the guard. Handlers that a form can reach without going through the
+ * control (Enter inside a text field submits the form) still have to guard
+ * themselves.
+ */
+export function useUnavailable({
+  unavailable,
+  reason,
+  onClick,
+  onKeyDown,
+}: {
+  unavailable: boolean;
+  reason: string;
+  onClick?: MouseEventHandler<HTMLButtonElement>;
+  onKeyDown?: KeyboardEventHandler<HTMLButtonElement>;
+}): UnavailableControl {
+  const reasonId = `${useId()}unavailable`;
+
+  if (!unavailable) {
+    return { controlProps: { onClick, onKeyDown }, reasonNode: null };
+  }
+
+  return {
+    controlProps: {
+      "aria-disabled": true,
+      "aria-describedby": reasonId,
+      onClick: (e) => {
+        e.preventDefault();
+        e.stopPropagation();
+      },
+      onKeyDown: (e) => {
+        if (!ACTIVATION_KEYS.has(e.key)) {
+          onKeyDown?.(e);
+          return;
+        }
+        // Suppress the default action before it can become a click, submit a
+        // form, or scroll the page.
+        e.preventDefault();
+        e.stopPropagation();
+      },
+    },
+    reasonNode: (
+      <span id={reasonId} className="sr-only">
+        {reason}
+      </span>
+    ),
+  };
+}