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+;;; test-system-utils-identify-external-open-command.el --- Tests for cj/identify-external-open-command -*- lexical-binding: t; -*-
+
+;;; Commentary:
+;; Unit tests for `cj/identify-external-open-command' in system-utils.el.
+;; The function dispatches on host-environment predicates to return the
+;; appropriate "open" command: xdg-open on Linux, open on macOS,
+;; start on Windows.  Anything else is a fatal error.
+
+;;; Code:
+
+(require 'ert)
+(require 'cl-lib)
+
+(add-to-list 'load-path (expand-file-name "modules" user-emacs-directory))
+(require 'system-utils)
+
+(defmacro test-siuec--with-host (linux macos windows &rest body)
+  "Run BODY with env-*-p predicates stubbed to LINUX, MACOS, WINDOWS."
+  (declare (indent 3))
+  `(cl-letf (((symbol-function 'env-linux-p)   (lambda () ,linux))
+			 ((symbol-function 'env-macos-p)   (lambda () ,macos))
+			 ((symbol-function 'env-windows-p) (lambda () ,windows)))
+	 ,@body))
+
+;;; Normal cases
+
+(ert-deftest test-siuec-linux-returns-xdg-open ()
+  "Normal: Linux host returns \"xdg-open\"."
+  (test-siuec--with-host t nil nil
+	(should (string= "xdg-open" (cj/identify-external-open-command)))))
+
+(ert-deftest test-siuec-macos-returns-open ()
+  "Normal: macOS host returns \"open\"."
+  (test-siuec--with-host nil t nil
+	(should (string= "open" (cj/identify-external-open-command)))))
+
+(ert-deftest test-siuec-windows-returns-start ()
+  "Normal: Windows host returns \"start\"."
+  (test-siuec--with-host nil nil t
+	(should (string= "start" (cj/identify-external-open-command)))))
+
+;;; Boundary cases
+
+(ert-deftest test-siuec-dispatch-order-linux-wins ()
+  "Boundary: Linux check runs first; wins when predicates disagree.
+Documents the dispatch order.  A real host only returns t from one
+of these anyway — but if something goes wrong, Linux takes priority."
+  (test-siuec--with-host t t t
+	(should (string= "xdg-open" (cj/identify-external-open-command)))))
+
+;;; Error cases
+
+(ert-deftest test-siuec-unsupported-host-signals-error ()
+  "Error: when no platform predicate returns non-nil, signals an error."
+  (test-siuec--with-host nil nil nil
+	(should-error (cj/identify-external-open-command))))
+
+(provide 'test-system-utils-identify-external-open-command)
+;;; test-system-utils-identify-external-open-command.el ends here