diff --git a/src/components/PlanBadge.test.tsx b/src/components/PlanBadge.test.tsx
new file mode 100644
index 0000000..1a6a5d1
--- /dev/null
+++ b/src/components/PlanBadge.test.tsx
@@ -0,0 +1,29 @@
+// @vitest-environment jsdom
+
+import { cleanup, fireEvent, render, screen } from "@testing-library/react";
+import { afterEach, describe, expect, it } from "vitest";
+import PlanBadge from "./PlanBadge";
+
+describe("PlanBadge", () => {
+  afterEach(() => cleanup());
+
+  it("renders the tier label", () => {
+    render(<PlanBadge tier="pro" />);
+    expect(screen.getByText("Pro")).toBeTruthy();
+  });
+
+  it("exposes rate-limit details in the accessible name", () => {
+    render(<PlanBadge tier="free" />);
+    expect(
+      screen.getByLabelText(/free plan\. rate limit: 60 requests \/ minute\./i),
+    ).toBeTruthy();
+  });
+
+  it("reveals a tooltip with rate-limit details on focus", () => {
+    render(<PlanBadge tier="enterprise" />);
+    fireEvent.focus(screen.getByText("Enterprise"));
+    const tip = screen.getByRole("tooltip");
+    expect(tip.textContent).toContain("Enterprise plan");
+    expect(tip.textContent).toContain("Custom / unmetered");
+  });
+});
diff --git a/src/components/PlanBadge.tsx b/src/components/PlanBadge.tsx
new file mode 100644
index 0000000..dba5066
--- /dev/null
+++ b/src/components/PlanBadge.tsx
@@ -0,0 +1,92 @@
+import Tooltip from "./Tooltip";
+
+/**
+ * PlanBadge — a small pill that labels an API/account plan tier and, on
+ * hover / focus / long-press, explains the tier with its rate-limit details
+ * via an accessible {@link Tooltip} (issue #283).
+ */
+
+export type PlanTier = "free" | "pro" | "enterprise";
+
+type PlanMeta = {
+  label: string;
+  /** Human-readable rate limit shown in the tooltip. */
+  rateLimit: string;
+  description: string;
+  /** Token names that resolve to theme-aware colours. */
+  bg: string;
+  fg: string;
+};
+
+const PLAN_META: Record<PlanTier, PlanMeta> = {
+  free: {
+    label: "Free",
+    rateLimit: "60 requests / minute",
+    description: "Best for prototyping and evaluation.",
+    bg: "var(--plan-free-bg, #e5e7eb)",
+    fg: "var(--plan-free-fg, #374151)",
+  },
+  pro: {
+    label: "Pro",
+    rateLimit: "1,000 requests / minute",
+    description: "Higher throughput and priority support for production apps.",
+    bg: "var(--plan-pro-bg, #dbeafe)",
+    fg: "var(--plan-pro-fg, #1e40af)",
+  },
+  enterprise: {
+    label: "Enterprise",
+    rateLimit: "Custom / unmetered",
+    description: "Dedicated capacity, SLAs, and custom rate limits.",
+    bg: "var(--plan-enterprise-bg, #ede9fe)",
+    fg: "var(--plan-enterprise-fg, #5b21b6)",
+  },
+};
+
+type PlanBadgeProps = {
+  tier: PlanTier;
+  className?: string;
+};
+
+export default function PlanBadge({
+  tier,
+  className,
+}: PlanBadgeProps): JSX.Element {
+  const meta = PLAN_META[tier];
+
+  const tooltipContent = (
+    <span>
+      <strong style={{ display: "block", marginBottom: "0.25rem" }}>
+        {meta.label} plan
+      </strong>
+      <span style={{ display: "block" }}>{meta.description}</span>
+      <span style={{ display: "block", marginTop: "0.25rem", opacity: 0.85 }}>
+        Rate limit: {meta.rateLimit}
+      </span>
+    </span>
+  );
+
+  return (
+    <Tooltip content={tooltipContent}>
+      <span
+        className={className}
+        tabIndex={0}
+        aria-label={`${meta.label} plan. Rate limit: ${meta.rateLimit}.`}
+        style={{
+          display: "inline-flex",
+          alignItems: "center",
+          padding: "0.125rem 0.5rem",
+          borderRadius: "999px",
+          fontSize: "0.6875rem",
+          fontWeight: 600,
+          textTransform: "uppercase",
+          letterSpacing: "0.03em",
+          cursor: "default",
+          background: meta.bg,
+          color: meta.fg,
+        }}
+      >
+        {meta.label}
+      </span>
+    </Tooltip>
+  );
+}
diff --git a/src/components/Tooltip.test.tsx b/src/components/Tooltip.test.tsx
new file mode 100644
index 0000000..e953042
--- /dev/null
+++ b/src/components/Tooltip.test.tsx
@@ -0,0 +1,55 @@
+// @vitest-environment jsdom
+
+import { cleanup, fireEvent, render, screen } from "@testing-library/react";
+import { afterEach, describe, expect, it } from "vitest";
+import Tooltip from "./Tooltip";
+
+describe("Tooltip", () => {
+  afterEach(() => cleanup());
+
+  it("is hidden by default", () => {
+    render(
+      <Tooltip content="Helpful text">
+        <button>Trigger</button>
+      </Tooltip>,
+    );
+    expect(screen.queryByRole("tooltip")).toBeNull();
+  });
+
+  it("shows on hover and hides on leave", () => {
+    render(
+      <Tooltip content="Helpful text">
+        <button>Trigger</button>
+      </Tooltip>,
+    );
+    const trigger = screen.getByText("Trigger");
+    fireEvent.mouseEnter(trigger);
+    expect(screen.getByRole("tooltip")).toBeTruthy();
+    fireEvent.mouseLeave(trigger);
+    expect(screen.queryByRole("tooltip")).toBeNull();
+  });
+
+  it("shows on keyboard focus and links via aria-describedby", () => {
+    render(
+      <Tooltip content="Helpful text">
+        <button>Trigger</button>
+      </Tooltip>,
+    );
+    const trigger = screen.getByText("Trigger");
+    fireEvent.focus(trigger);
+    const tip = screen.getByRole("tooltip");
+    expect(trigger.getAttribute("aria-describedby")).toBe(tip.id);
+  });
+
+  it("dismisses on Escape", () => {
+    render(
+      <Tooltip content="Helpful text">
+        <button>Trigger</button>
+      </Tooltip>,
+    );
+    fireEvent.mouseEnter(screen.getByText("Trigger"));
+    expect(screen.getByRole("tooltip")).toBeTruthy();
+    fireEvent.keyDown(document, { key: "Escape" });
+    expect(screen.queryByRole("tooltip")).toBeNull();
+  });
+});
diff --git a/src/components/Tooltip.tsx b/src/components/Tooltip.tsx
new file mode 100644
index 0000000..24445a7
--- /dev/null
+++ b/src/components/Tooltip.tsx
@@ -0,0 +1,120 @@
+import {
+  cloneElement,
+  isValidElement,
+  useEffect,
+  useId,
+  useRef,
+  useState,
+  type ReactElement,
+  type ReactNode,
+} from "react";
+
+/**
+ * Tooltip — an accessible tooltip that opens on hover, keyboard focus, and
+ * touch long-press (issue #283).
+ *
+ * Accessibility (WCAG 2.1 AA):
+ * - The trigger gets `aria-describedby` pointing at the tooltip content.
+ * - Content has `role="tooltip"`.
+ * - Escape dismisses an open tooltip.
+ * - Colours come from design tokens so it reads in light and dark mode.
+ */
+
+type TooltipProps = {
+  /** Tooltip body. Plain text or rich nodes. */
+  content: ReactNode;
+  /** Single focusable trigger element (e.g. a <span> or <button>). */
+  children: ReactElement;
+  /** Long-press duration in ms before the tooltip opens on touch. */
+  longPressMs?: number;
+};
+
+export default function Tooltip({
+  content,
+  children,
+  longPressMs = 500,
+}: TooltipProps): JSX.Element {
+  const [open, setOpen] = useState(false);
+  const tooltipId = useId();
+  const pressTimer = useRef<number | null>(null);
+
+  const clearPressTimer = () => {
+    if (pressTimer.current !== null) {
+      window.clearTimeout(pressTimer.current);
+      pressTimer.current = null;
+    }
+  };
+
+  useEffect(() => clearPressTimer, []);
+
+  useEffect(() => {
+    if (!open) return;
+    const onKey = (e: KeyboardEvent) => {
+      if (e.key === "Escape") setOpen(false);
+    };
+    document.addEventListener("keydown", onKey);
+    return () => document.removeEventListener("keydown", onKey);
+  }, [open]);
+
+  const show = () => setOpen(true);
+  const hide = () => {
+    clearPressTimer();
+    setOpen(false);
+  };
+
+  const handleTouchStart = () => {
+    clearPressTimer();
+    pressTimer.current = window.setTimeout(() => setOpen(true), longPressMs);
+  };
+
+  const trigger = isValidElement(children) ? (
+    cloneElement(children, {
+      "aria-describedby": open ? tooltipId : undefined,
+      onMouseEnter: show,
+      onMouseLeave: hide,
+      onFocus: show,
+      onBlur: hide,
+      onTouchStart: handleTouchStart,
+      onTouchEnd: clearPressTimer,
+      onTouchCancel: clearPressTimer,
+    } as Record<string, unknown>)
+  ) : (
+    children
+  );
+
+  return (
+    <span
+      style={{ position: "relative", display: "inline-flex" }}
+      onMouseLeave={hide}
+    >
+      {trigger}
+      {open && (
+        <span
+          role="tooltip"
+          id={tooltipId}
+          style={{
+            position: "absolute",
+            bottom: "calc(100% + 6px)",
+            left: "50%",
+            transform: "translateX(-50%)",
+            zIndex: 50,
+            maxWidth: "240px",
+            width: "max-content",
+            padding: "0.5rem 0.625rem",
+            borderRadius: "0.375rem",
+            fontSize: "0.75rem",
+            lineHeight: 1.4,
+            textAlign: "left",
+            color: "var(--tooltip-text, #ffffff)",
+            background: "var(--tooltip-bg, #1f2937)",
+            border: "1px solid var(--border-subtle, rgba(255,255,255,0.1))",
+            boxShadow: "0 4px 12px rgba(0,0,0,0.18)",
+            pointerEvents: "none",
+          }}
+        >
+          {content}
+        </span>
+      )}
+    </span>
+  );
+}