How to fix: Missing URLs return real 404 (no soft-404)

Use `error_page 404 /404.html;` and mark the target internal so it can't be requested directly:

  error_page 404 /404.html;
  location = /404.html { internal; }

Common trap: a catch-all `try_files $uri $uri/ /index.html;` for SPA frameworks makes every missing URL return the SPA shell at 200. Replace with `try_files $uri $uri/ =404;` for static routes and route SPA fallbacks only under a specific prefix (e.g. `location /app/ { try_files ... /app/index.html; }`).

  ErrorDocument 404 /404.html
  <Files "404.html">
    Require all granted
  </Files>

Ensure the ErrorDocument response actually carries the 404 status — a rewrite of the form `RewriteRule . /index.html [L]` will return 200 for missing URLs; add [R=404] or route missing URLs through a script that sets `http_response_code(404)`.

No native Cloudflare setting for this — soft-404s originate at the origin. Options: (1) Fix at origin (preferred). (2) Cloudflare Workers can intercept missing-URL patterns and return a 404 before the origin: `return new Response('Not found', {status: 404})`. (3) A Custom Error Page with a real 404 status can be set under Rules → Error Pages, but only fires when the origin returns 5xx / 522 / 523.

WordPress serves a real 404 for missing posts if the theme's 404.php template exists AND doesn't send its own `http_response_code(200)`. Verify with: `curl -I https://your-site/does-not-exist-abc` — should return `HTTP/1.1 404 Not Found`. Common causes of soft-404s: (a) a redirect plugin (e.g. Redirection, Rank Math redirects) sending every missing URL to the homepage — remove the 404 → / catch-all rule. (b) A page-builder theme returning 200 for the default 404 page — switch to a theme with a proper 404.php or add `status_header(404); nocache_headers();` at the top of the template.

Flask returns 404 automatically for unmatched routes. If you're seeing a soft-404 it's usually a catch-all: `@app.route('/<path:p>')` that renders a template with a 200 status. Fix by returning a proper 404:

  @app.errorhandler(404)
  def not_found(e):
      return render_template('404.html'), 404

SPA hosting: only serve the SPA shell under specific prefixes, not for every unmatched URL, so real 404s reach the errorhandler.

Add a 404 middleware AFTER all your routes:

  app.use((req, res) => {
    res.status(404).render('404');
  });

Never `res.sendFile(indexHtml)` unconditionally — that's the classic SPA soft-404. Match SPA fallbacks to a specific route (`app.get('/app/*', spaHandler)`) so everything else falls through to the 404 middleware.

Rails routes fall through to a 404 by default when nothing matches. Soft-404s usually come from a catch-all like `get '*path' => 'pages#show'` where the controller renders a template with default status 200. Either return the correct status:

  def show
    @page = Page.find_by(slug: params[:path])
    return render 'errors/not_found', status: :not_found if @page.nil?
    render :show
  end

Or drop the catch-all and let Rails' `public/404.html` serve missing URLs.