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diff --git a/tests/test-system-lib--format-region-with-program.el b/tests/test-system-lib--format-region-with-program.el
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+;;; test-system-lib--format-region-with-program.el --- Tests for cj/format-region-with-program -*- lexical-binding: t; -*-
+
+;;; Commentary:
+;; `cj/format-region-with-program' runs an external formatter over the whole
+;; buffer via `call-process-region' (argv, no shell) and replaces the buffer
+;; only when the program exits zero.  Extracted from the byte-identical
+;; per-language helpers in prog-json.el / prog-yaml.el, so this is the first
+;; direct unit coverage of the logic.  call-process-region is mocked at the
+;; boundary (the established pattern in test-prog-json--json-format-buffer.el).
+
+;;; Code:
+
+(require 'ert)
+(require 'cl-lib)
+
+(add-to-list 'load-path (expand-file-name "modules" user-emacs-directory))
+(require 'system-lib)
+
+(ert-deftest test-system-lib-format-region-with-program-replaces-on-success ()
+  "Normal: on exit 0 the buffer is replaced with the program's output, returns t."
+  (cl-letf (((symbol-function 'call-process-region)
+             (lambda (_start _end _prog &rest rest)
+               (with-current-buffer (nth 1 rest) (insert "FORMATTED"))
+               0)))
+    (with-temp-buffer
+      (insert "raw")
+      (should (eq t (cj/format-region-with-program "fmt")))
+      (should (equal "FORMATTED" (buffer-string))))))
+
+(ert-deftest test-system-lib-format-region-with-program-forwards-argv ()
+  "Normal: PROGRAM and ARGS reach call-process-region as argv (no shell)."
+  (let (got-prog got-args)
+    (cl-letf (((symbol-function 'call-process-region)
+               (lambda (_start _end prog &rest rest)
+                 (setq got-prog prog
+                       got-args (nthcdr 3 rest))
+                 (with-current-buffer (nth 1 rest) (insert "x"))
+                 0)))
+      (with-temp-buffer
+        (cj/format-region-with-program "jq" "--sort-keys" ".")))
+    (should (equal "jq" got-prog))
+    (should (equal '("--sort-keys" ".") got-args))))
+
+(ert-deftest test-system-lib-format-region-with-program-empty-output ()
+  "Boundary: empty program output empties the buffer and still returns t."
+  (cl-letf (((symbol-function 'call-process-region)
+             (lambda (_start _end _prog &rest _rest) 0)))   ; writes nothing
+    (with-temp-buffer
+      (insert "raw")
+      (should (eq t (cj/format-region-with-program "fmt")))
+      (should (equal "" (buffer-string))))))
+
+(ert-deftest test-system-lib-format-region-with-program-nonzero-untouched ()
+  "Error: a non-zero exit leaves the buffer untouched and signals user-error
+carrying the program's stderr text."
+  (cl-letf (((symbol-function 'call-process-region)
+             (lambda (_start _end _prog &rest rest)
+               (with-current-buffer (nth 1 rest) (insert "boom: bad input"))
+               1)))
+    (with-temp-buffer
+      (insert "raw")
+      (let ((err (should-error (cj/format-region-with-program "fmt")
+                               :type 'user-error)))
+        (should (string-match-p "boom: bad input" (error-message-string err))))
+      (should (equal "raw" (buffer-string))))))
+
+(provide 'test-system-lib--format-region-with-program)
+;;; test-system-lib--format-region-with-program.el ends here