cpp-httplib/docs-src/pages/en/cookbook/s13-error-handler.md

title, order, status

title	order	status
S13. Return a Custom Error Page	32	draft

To customize the response for 4xx or 5xx errors, use set_error_handler(). You can replace the plain default error page with your own HTML or JSON.

Basic usage

svr.set_error_handler([](const httplib::Request &req, httplib::Response &res) {
  auto body = "<h1>Error " + std::to_string(res.status) + "</h1>";
  res.set_content(body, "text/html");
});

The error handler runs right before an error response is sent — any time res.status is 4xx or 5xx. Replace the body with res.set_content() and every error response uses the same template.

Branch by status code

svr.set_error_handler([](const httplib::Request &req, httplib::Response &res) {
  if (res.status == 404) {
    res.set_content("<h1>Not Found</h1><p>" + req.path + "</p>", "text/html");
  } else if (res.status >= 500) {
    res.set_content("<h1>Server Error</h1>", "text/html");
  }
});

Checking res.status lets you show a custom message for 404s and a "contact support" link for 5xx errors.

JSON error responses

For an API server, you probably want errors as JSON.

svr.set_error_handler([](const httplib::Request &req, httplib::Response &res) {
  nlohmann::json j = {
    {"error", true},
    {"status", res.status},
    {"path", req.path},
  };
  res.set_content(j.dump(), "application/json");
});

Now every error comes back in a consistent JSON shape.

Note: set_error_handler() also fires for 500 responses caused by exceptions thrown from a route handler. To get at the exception itself, combine it with set_exception_handler(). See S14. Catch exceptions.