diff --git a/README.md b/README.md
index 114efae8..475b1ab4 100644
--- a/README.md
+++ b/README.md
@@ -23,6 +23,8 @@ This is a Next.js-based frontend skeleton that provides the UI structure for all
 - [UI PR review checklist for reviewers](./docs/CODE_REVIEW.md)
 - [Period lifecycle and state machine](./docs/PERIOD_LIFECYCLE.md)
 - [Internal jargon glossary (contributors)](./docs/GLOSSARY.md)
+- [Error handling strategy for contributors](./docs/error-handling.md)
+- [Routing errors — how they surface to the user](./docs/ROUTING_ERRORS.md)
 - [Hydration mismatch patterns and fixes](./docs/HYDRATION_MISMATCH.md)
 - [Search UX: instant debounce, URL-submit, and recent results](./docs/SEARCH_UX.md)
 - [Transaction detail (receipt) page data flow for contributors](./docs/transaction-detail-receipt-page.md)
diff --git a/components/ui/PrimaryButton.test.tsx b/components/ui/PrimaryButton.test.tsx
new file mode 100644
index 00000000..dc0ff464
--- /dev/null
+++ b/components/ui/PrimaryButton.test.tsx
@@ -0,0 +1,175 @@
+import { describe, it, expect } from 'vitest'
+import { render, screen } from '@testing-library/react'
+import userEvent from '@testing-library/user-event'
+import PrimaryButton from './PrimaryButton'
+
+describe('PrimaryButton', () => {
+  // ──────────────────────────────────────────────────────────────────────────
+  // Rendering
+  // ──────────────────────────────────────────────────────────────────────────
+
+  it('renders with children', () => {
+    render(<PrimaryButton>Submit</PrimaryButton>)
+    expect(screen.getByRole('button', { name: 'Submit' })).toBeInTheDocument()
+  })
+
+  it('renders with complex children', () => {
+    render(
+      <PrimaryButton>
+        <span data-testid="icon" />
+        Pay Now
+      </PrimaryButton>,
+    )
+    expect(screen.getByRole('button')).toBeInTheDocument()
+    expect(screen.getByTestId('icon')).toBeInTheDocument()
+    expect(screen.getByText('Pay Now')).toBeInTheDocument()
+  })
+
+  // ──────────────────────────────────────────────────────────────────────────
+  // Default styling
+  // ──────────────────────────────────────────────────────────────────────────
+
+  it('applies base visual classes', () => {
+    render(<PrimaryButton>Send</PrimaryButton>)
+    const button = screen.getByRole('button')
+    // Base layout classes
+    expect(button.className).toContain('inline-flex')
+    expect(button.className).toContain('items-center')
+    expect(button.className).toContain('justify-center')
+    // Brand colour
+    expect(button.className).toContain('bg-brand.red')
+    expect(button.className).toContain('text-white')
+    // Sizing
+    expect(button.className).toContain('px-6')
+    expect(button.className).toContain('py-3')
+    expect(button.className).toContain('rounded-lg')
+    expect(button.className).toContain('font-semibold')
+    // Transition
+    expect(button.className).toContain('transition-colors')
+    expect(button.className).toContain('duration-150')
+  })
+
+  it('applies hover classes', () => {
+    render(<PrimaryButton>Send</PrimaryButton>)
+    const button = screen.getByRole('button')
+    expect(button.className).toContain('hover:bg-brand.redHover')
+  })
+
+  it('applies focus classes', () => {
+    render(<PrimaryButton>Send</PrimaryButton>)
+    const button = screen.getByRole('button')
+    expect(button.className).toContain('focus:outline-none')
+    expect(button.className).toContain('focus:ring-2')
+    expect(button.className).toContain('focus:ring-brand.red')
+    expect(button.className).toContain('focus:ring-offset-2')
+  })
+
+  // ──────────────────────────────────────────────────────────────────────────
+  // Disabled state
+  // ──────────────────────────────────────────────────────────────────────────
+
+  it('renders disabled when disabled prop is true', () => {
+    render(<PrimaryButton disabled>Send</PrimaryButton>)
+    const button = screen.getByRole('button')
+    expect(button).toBeDisabled()
+  })
+
+  it('sets aria-disabled when disabled', () => {
+    render(<PrimaryButton disabled>Send</PrimaryButton>)
+    const button = screen.getByRole('button')
+    expect(button).toHaveAttribute('aria-disabled', 'true')
+  })
+
+  it('does not set aria-disabled when enabled', () => {
+    render(<PrimaryButton>Send</PrimaryButton>)
+    const button = screen.getByRole('button')
+    expect(button).not.toHaveAttribute('aria-disabled')
+  })
+
+  it('applies disabled visual classes', () => {
+    render(<PrimaryButton disabled>Send</PrimaryButton>)
+    const button = screen.getByRole('button')
+    expect(button.className).toContain('disabled:opacity-50')
+    expect(button.className).toContain('disabled:cursor-not-allowed')
+    expect(button.className).toContain('disabled:hover:bg-brand.red')
+  })
+
+  it('does not trigger onClick when disabled', async () => {
+    const user = userEvent.setup()
+    let clicked = false
+    render(
+      <PrimaryButton disabled onClick={() => { clicked = true }}>
+        Send
+      </PrimaryButton>,
+    )
+    await user.click(screen.getByRole('button'))
+    expect(clicked).toBe(false)
+  })
+
+  // ──────────────────────────────────────────────────────────────────────────
+  // Custom className merging
+  // ──────────────────────────────────────────────────────────────────────────
+
+  it('merges custom className with default classes', () => {
+    render(<PrimaryButton className="extra-class">Send</PrimaryButton>)
+    const button = screen.getByRole('button')
+    expect(button.className).toContain('extra-class')
+    // Default classes should still be present
+    expect(button.className).toContain('bg-brand.red')
+    expect(button.className).toContain('inline-flex')
+  })
+
+  // ──────────────────────────────────────────────────────────────────────────
+  // forwardRef support
+  // ──────────────────────────────────────────────────────────────────────────
+
+  it('forwards ref to the button element', () => {
+    const ref = { current: null as HTMLButtonElement | null }
+    render(<PrimaryButton ref={ref}>Send</PrimaryButton>)
+    expect(ref.current).toBeInstanceOf(HTMLButtonElement)
+    expect(ref.current?.tagName).toBe('BUTTON')
+  })
+
+  it('has correct displayName', () => {
+    expect(PrimaryButton.displayName).toBe('PrimaryButton')
+  })
+
+  // ──────────────────────────────────────────────────────────────────────────
+  // Additional HTML button attributes
+  // ──────────────────────────────────────────────────────────────────────────
+
+  it('supports type attribute', () => {
+    render(<PrimaryButton type="submit">Submit</PrimaryButton>)
+    expect(screen.getByRole('button')).toHaveAttribute('type', 'submit')
+  })
+
+  it('supports aria-label', () => {
+    render(<PrimaryButton aria-label="Close dialog">X</PrimaryButton>)
+    expect(screen.getByRole('button', { name: 'Close dialog' })).toBeInTheDocument()
+  })
+
+  it('supports data-* attributes', () => {
+    render(<PrimaryButton data-testid="submit-btn">Send</PrimaryButton>)
+    expect(screen.getByTestId('submit-btn')).toBeInTheDocument()
+  })
+
+  // ──────────────────────────────────────────────────────────────────────────
+  // Interaction parity: hover then focus
+  // ──────────────────────────────────────────────────────────────────────────
+
+  it('maintains hover class when focused', async () => {
+    const user = userEvent.setup()
+    render(<PrimaryButton>Send</PrimaryButton>)
+    const button = screen.getByRole('button')
+
+    await user.hover(button)
+    // Hover classes are present
+    expect(button.className).toContain('hover:bg-brand.redHover')
+
+    // Focus does not remove hover
+    button.focus()
+    expect(button.className).toContain('hover:bg-brand.redHover')
+    // Focus classes are also present
+    expect(button.className).toContain('focus:ring-2')
+  })
+})
\ No newline at end of file
diff --git a/docs/ROUTING_ERRORS.md b/docs/ROUTING_ERRORS.md
new file mode 100644
index 00000000..123068d9
--- /dev/null
+++ b/docs/ROUTING_ERRORS.md
@@ -0,0 +1,97 @@
+# Routing Errors — How They Surface to the User
+
+> **Audience:** contributor. This doc explains the different ways a routing error can
+> reach a user in the RemitWise UI, which file handles each case, and what the user
+> actually sees. It is the routing-specific companion to
+> [docs/error-handling.md](error-handling.md) (which covers the general error-boundary
+> strategy).
+
+## Why this matters
+
+Routing errors are the most visible failure surface in any Next.js app: a user lands
+on a URL that doesn't exist, a route fails to render, or a dynamic segment references
+a record that was deleted. Each of these has a distinct, intentional UX. Knowing which
+file owns which case prevents contributors from "fixing" a 404 by editing the wrong
+component, or from accidentally shipping a raw stack trace to end users.
+
+## The four routing-error surfaces
+
+### 1. Unknown / unmatched URL → `app/not-found.tsx`
+
+When a user navigates to a path that matches **no route** in the app, Next.js renders
+`app/not-found.tsx` (the global 404 page).
+
+- **File:** `app/not-found.tsx`
+- **What the user sees:** a branded "404 – Page Not Found" badge, a "Lost in the
+  transfer?" heading, links to the primary destinations (Dashboard, Send, Bills,
+  Insurance, Family, Settings), and "Go to Home" / "Open Dashboard" CTAs.
+- **Metadata:** `title: "Page Not Found – RemitWise"`, with a short description, is set
+  for search engines and screen readers.
+- **When it runs:** any unmatched top-level or nested path. It is the last-resort
+  fallback for URLs that don't exist.
+
+### 2. Route handler explicitly calls `notFound()` → `app/not-found.tsx`
+
+Routes that look up a dynamic resource (e.g. `/receipt/[txHash]`) call Next's
+`notFound()` from `next/navigation` when the resource is missing.
+
+- **File(s):** `app/receipt/[txHash]/page.tsx`, `app/debug/page.tsx`
+- **Behaviour:** `notFound()` throws a special `NEXT_NOT_FOUND` error that Next.js
+  catches and forwards to the nearest `not-found` boundary — for the app shell that is
+  `app/not-found.tsx`.
+- **What the user sees:** the same branded 404 page as case 1, so a missing receipt
+  and a mistyped URL are visually consistent.
+- **Item-ID leak note:** the page sets a local `notFound` flag and only renders the 404
+  UI — it does **not** echo the requested `txHash` back onto the page, so a failed
+  lookup doesn't leak internal identifiers.
+
+### 3. Client render failure → `app/error.tsx` → `RootErrorFallback`
+
+When a route **exists** but throws while rendering on the client (outside a widget
+boundary), Next.js renders the nearest `error.tsx`.
+
+- **File:** `app/error.tsx` (root) → `components/RootErrorFallback.tsx`
+- **Pipeline:**
+  1. `app/error.tsx` reports the exception to Sentry via
+     `errorReporter.captureException(error)` (PII scrubbed).
+  2. It renders `RootErrorFallback`, passing the `reset` callback.
+  3. `RootErrorFallback` reads the current `pathname` and picks a route-specific
+     message from `lib/config/route-errors.ts`.
+- **What the user sees:** a focused, per-route fallback (e.g. "Dashboard unavailable",
+  "Transfer unavailable", "Transactions unavailable") with a retry button that calls
+  `onReset` to remount the subtree from a clean React tree. The heading is focused on
+  mount for screen-reader users.
+- **Route messaging:** `lib/config/route-errors.ts` maps route prefixes to
+  `{ titleKey, defaultTitle, descriptionKey, defaultDescription }`. Unknown paths fall
+  back to `DEFAULT_ERROR_MESSAGE` ("Something went wrong" / "We hit an unexpected
+  problem, but your session is still safe…").
+
+### 4. API route hits a missing resource → HTTP 404 JSON
+
+API routes return a **404 HTTP status** (not a rendered page) when a resource is
+missing. The client then surfaces it via the shared `useFormAction` hook or `apiClient`.
+
+- **File(s):** `app/api/bills/route.ts`, `app/api/bills/[id]/route.ts`
+- **Pattern:** route handlers catch a `not-found` error and respond with a typed 404
+  JSON body rather than a generic 500.
+- **What the user sees:** the form/hook sets an error message near the relevant control
+  (e.g. "Bill not found") instead of a generic "Request failed". See
+  [docs/use-form-action.md](use-form-action.md) for the error-message pipeline.
+
+## Choosing the right boundary
+
+| Scenario | File to touch | User sees |
+|:---|:---|:---|
+| URL doesn't match any route | `app/not-found.tsx` | Branded 404 page |
+| Dynamic route lookup fails | `notFound()` in the page → `app/not-found.tsx` | Branded 404 page |
+| Route renders but throws on client | `app/error.tsx` + `RootErrorFallback` + `route-errors.ts` | Per-route fallback + retry |
+| API call for a missing record | API route handler → 404 JSON | Inline field error via `useFormAction` |
+
+## Cross-links
+
+- [docs/error-handling.md](error-handling.md) — general boundary strategy and the
+  `FeatureBoundary` / `WidgetErrorBoundary` wrappers.
+- [docs/use-form-action.md](use-form-action.md) — how client-side API errors surface in
+  forms.
+- [docs/session-expiry-design.md](session-expiry-design.md) — the related session-expiry
+  redirect flow.
\ No newline at end of file
diff --git a/docs/error-handling.md b/docs/error-handling.md
index 66eaeade..50b07c52 100644
--- a/docs/error-handling.md
+++ b/docs/error-handling.md
@@ -18,3 +18,7 @@ and component loading states in a single, reusable wrapper.
 
 For more details on implementing standard default, hover, focus, error, disabled, and loading states across components, see the [Frontend Component States Guide](COMPONENT_STATES.md).
 
+For a routing-specific breakdown of how each error type reaches the user (404 pages,
+route-level error.tsx, per-route fallback messages, and API 404s), see
+[docs/ROUTING_ERRORS.md](ROUTING_ERRORS.md).
+
diff --git a/lib/utils.ts b/lib/utils.ts
index 365058ce..607cb86e 100644
--- a/lib/utils.ts
+++ b/lib/utils.ts
@@ -1,6 +1 @@
-import { type ClassValue, clsx } from "clsx";
-import { twMerge } from "tailwind-merge";
-
-export function cn(...inputs: ClassValue[]) {
-  return twMerge(clsx(inputs));
-}
+export { cn } from "./utils/cn";
diff --git a/lib/utils/index.ts b/lib/utils/index.ts
index 37295e1c..c25627d5 100644
--- a/lib/utils/index.ts
+++ b/lib/utils/index.ts
@@ -1,6 +1 @@
-import { clsx } from "clsx";
-import { twMerge } from "tailwind-merge";
-
-export function cn(...inputs: Parameters<typeof clsx>) {
-  return twMerge(clsx(inputs));
-}
+export { cn } from "./cn";