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diff --git a/tests/test-gptel-tools-git-diff.el b/tests/test-gptel-tools-git-diff.el
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+;;; test-gptel-tools-git-diff.el --- Tests for git_diff gptel tool -*- lexical-binding: t; -*-
+
+;;; Commentary:
+;; Tests run against real temp git repos under HOME via `process-file'.
+
+;;; Code:
+
+(require 'ert)
+(require 'cl-lib)
+
+(eval-and-compile
+  (add-to-list 'load-path (expand-file-name "tests" user-emacs-directory))
+  (add-to-list 'load-path (expand-file-name "gptel-tools" user-emacs-directory))
+  (setq load-prefer-newer t)
+  (unless (featurep 'gptel)
+    (defvar gptel-tools nil)
+    (defun gptel-make-tool (&rest _args) nil)
+    (defun gptel-get-tool (&rest _args) nil)
+    (provide 'gptel)))
+
+(require 'git_diff)
+
+;; ---------- helpers
+
+(defun test-gptel-tools-git-diff--with-repo (fn)
+  "Create a temp git repo under HOME with one committed file, call FN."
+  (let* ((name (format ".test-gptel-tools-git-diff-%s"
+                       (format-time-string "%s%N")))
+         (dir (expand-file-name name "~")))
+    (unwind-protect
+        (progn
+          (make-directory dir)
+          (let ((default-directory dir))
+            (call-process "git" nil nil nil "init" "--quiet")
+            (call-process "git" nil nil nil "config" "user.email" "test@x")
+            (call-process "git" nil nil nil "config" "user.name" "Test")
+            (with-temp-file (expand-file-name "f.txt" dir)
+              (insert "original\n"))
+            (call-process "git" nil nil nil "add" "f.txt")
+            (call-process "git" nil nil nil "commit" "--quiet" "-m" "initial"))
+          (funcall fn dir))
+      (when (file-exists-p dir) (delete-directory dir t)))))
+
+;; ---------- build-args
+
+(ert-deftest test-gptel-tools-git-diff-build-args-no-refs ()
+  "Normal: no refs / no file → bare diff args."
+  (should (equal (cj/gptel-git-diff--build-args nil nil nil)
+                 '("-c" "color.ui=false" "diff"))))
+
+(ert-deftest test-gptel-tools-git-diff-build-args-with-ref1 ()
+  "Normal: REF1 appended."
+  (should (equal (cj/gptel-git-diff--build-args "HEAD~1" nil nil)
+                 '("-c" "color.ui=false" "diff" "HEAD~1"))))
+
+(ert-deftest test-gptel-tools-git-diff-build-args-with-both-refs ()
+  "Normal: REF1 and REF2 both appended."
+  (should (equal (cj/gptel-git-diff--build-args "HEAD~1" "HEAD" nil)
+                 '("-c" "color.ui=false" "diff" "HEAD~1" "HEAD"))))
+
+(ert-deftest test-gptel-tools-git-diff-build-args-with-file ()
+  "Normal: FILE appended after `--'."
+  (should (equal (cj/gptel-git-diff--build-args nil nil "foo.txt")
+                 '("-c" "color.ui=false" "diff" "--" "foo.txt"))))
+
+(ert-deftest test-gptel-tools-git-diff-build-args-boundary-empty-strings ()
+  "Boundary: empty-string REF/FILE values are ignored."
+  (should (equal (cj/gptel-git-diff--build-args "" "" "")
+                 '("-c" "color.ui=false" "diff"))))
+
+;; ---------- truncate
+
+(ert-deftest test-gptel-tools-git-diff-truncate-under-cap ()
+  "Normal: short input returns unchanged."
+  (should (equal (cj/gptel-git-diff--truncate "small diff") "small diff")))
+
+(ert-deftest test-gptel-tools-git-diff-truncate-over-cap ()
+  "Boundary: output exceeding the cap is truncated with a marker."
+  (let* ((cap cj/gptel-git-diff--max-output-bytes)
+         (huge (make-string (+ cap 1000) ?x))
+         (out (cj/gptel-git-diff--truncate huge)))
+    (should (string-match-p "\\[truncated:" out))
+    (should (> (length huge) (length out)))))
+
+;; ---------- validate-path
+
+(ert-deftest test-gptel-tools-git-diff-validate-path-normal ()
+  "Normal: validator accepts a git working tree."
+  (test-gptel-tools-git-diff--with-repo
+   (lambda (dir)
+     (should (equal (cj/gptel-git-diff--validate-path dir) dir)))))
+
+(ert-deftest test-gptel-tools-git-diff-validate-path-error-outside-home ()
+  "Error: path outside HOME signals."
+  (should-error (cj/gptel-git-diff--validate-path "/etc")))
+
+(ert-deftest test-gptel-tools-git-diff-validate-path-error-not-a-repo ()
+  "Error: non-git directory signals."
+  (let ((dir (make-temp-file
+              (expand-file-name ".test-gptel-tools-git-diff-" "~") t)))
+    (unwind-protect
+        (should-error (cj/gptel-git-diff--validate-path dir))
+      (when (file-exists-p dir) (delete-directory dir t)))))
+
+;; ---------- run
+
+(ert-deftest test-gptel-tools-git-diff-run-no-changes ()
+  "Boundary: a clean tree with no refs returns the no-diff marker."
+  (test-gptel-tools-git-diff--with-repo
+   (lambda (dir)
+     (let ((out (cj/gptel-git-diff--run dir)))
+       (should (string-match-p "No diff" out))))))
+
+(ert-deftest test-gptel-tools-git-diff-run-unstaged-change ()
+  "Normal: an unstaged edit appears as a real diff."
+  (test-gptel-tools-git-diff--with-repo
+   (lambda (dir)
+     (with-temp-file (expand-file-name "f.txt" dir)
+       (insert "changed\n"))
+     (let ((out (cj/gptel-git-diff--run dir)))
+       (should (string-match-p "^-original" out))
+       (should (string-match-p "^\\+changed" out))))))
+
+(ert-deftest test-gptel-tools-git-diff-run-narrow-to-file ()
+  "Normal: FILE argument narrows the diff."
+  (test-gptel-tools-git-diff--with-repo
+   (lambda (dir)
+     (with-temp-file (expand-file-name "f.txt" dir)
+       (insert "changed\n"))
+     (with-temp-file (expand-file-name "g.txt" dir)
+       (insert "second file\n"))
+     (let ((out (cj/gptel-git-diff--run dir nil nil "f.txt")))
+       (should (string-match-p "f.txt" out))
+       (should-not (string-match-p "g.txt" out))))))
+
+(provide 'test-gptel-tools-git-diff)
+;;; test-gptel-tools-git-diff.el ends here