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diff --git a/tests/test-system-lib--completion-file-annotator.el b/tests/test-system-lib--completion-file-annotator.el
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+++ b/tests/test-system-lib--completion-file-annotator.el
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+;;; test-system-lib--completion-file-annotator.el --- Tests for cj/completion-file-annotator -*- lexical-binding: t; -*-
+
+;;; Commentary:
+;; Unit tests for `cj/completion-file-annotator', the annotation-function
+;; factory used to annotate file-basename completion pickers with size and
+;; modification date.
+
+;;; Code:
+
+(require 'ert)
+(require 'system-lib)
+
+(ert-deftest test-system-lib-completion-file-annotator-normal-file-shows-size-and-date ()
+  "Normal: a regular file is annotated with a size and an ISO date."
+  (let ((file (make-temp-file "cfa-test-" nil ".txt" "hello world")))
+    (unwind-protect
+        (let* ((annotate (cj/completion-file-annotator
+                          (lambda (_cand) file)))
+               (result (funcall annotate "anything")))
+          (should (stringp result))
+          ;; file-size-human-readable of 11 bytes is "11"
+          (should (string-match-p "11" result))
+          ;; ISO date for the file's mtime
+          (should (string-match-p
+                   (format-time-string "%Y-%m-%d"
+                                       (file-attribute-modification-time
+                                        (file-attributes file)))
+                   result)))
+      (delete-file file))))
+
+(ert-deftest test-system-lib-completion-file-annotator-boundary-directory-marked-dir ()
+  "Boundary: a directory candidate is annotated with the `dir' marker."
+  (let ((dir (make-temp-file "cfa-dir-" t)))
+    (unwind-protect
+        (let* ((annotate (cj/completion-file-annotator (lambda (_c) dir)))
+               (result (funcall annotate "d")))
+          (should (stringp result))
+          (should (string-match-p "dir" result)))
+      (delete-directory dir t))))
+
+(ert-deftest test-system-lib-completion-file-annotator-error-nil-path-returns-nil ()
+  "Error: a candidate whose path-resolver returns nil yields no annotation."
+  (let ((annotate (cj/completion-file-annotator (lambda (_c) nil))))
+    (should (null (funcall annotate "missing")))))
+
+(ert-deftest test-system-lib-completion-file-annotator-error-missing-file-returns-nil ()
+  "Error: a path that does not exist yields no annotation."
+  (let* ((path (expand-file-name "definitely-not-here-12345.txt"
+                                 temporary-file-directory))
+         (annotate (cj/completion-file-annotator (lambda (_c) path))))
+    (should (null (funcall annotate "gone")))))
+
+(provide 'test-system-lib--completion-file-annotator)
+;;; test-system-lib--completion-file-annotator.el ends here