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+;;; test-ai-config-backend-and-model.el --- Tests for cj/gptel-backend-and-model -*- lexical-binding: t; -*-
+
+;;; Commentary:
+;; Tests for cj/gptel-backend-and-model from ai-config.el.
+;;
+;; Returns a formatted string "backend: model [timestamp]" for use in
+;; org headings marking AI responses.  Uses pcase to extract the display
+;; name from vector backends, falling back to "AI" otherwise.
+
+;;; Code:
+
+(require 'ert)
+
+(add-to-list 'load-path (expand-file-name "tests" user-emacs-directory))
+(add-to-list 'load-path (expand-file-name "modules" user-emacs-directory))
+(require 'testutil-ai-config)
+(require 'ai-config)
+
+;;; Normal Cases
+
+(ert-deftest test-ai-config-backend-and-model-normal-vector-backend-extracts-name ()
+  "Vector backend should use element at index 1 as display name."
+  (let ((gptel-backend (vector 'cl-struct "Claude"))
+        (gptel-model "claude-opus-4-6"))
+    (let ((result (cj/gptel-backend-and-model)))
+      (should (string-match-p "^Claude:" result))
+      (should (string-match-p "claude-opus-4-6" result)))))
+
+(ert-deftest test-ai-config-backend-and-model-normal-contains-timestamp ()
+  "Result should contain a bracketed timestamp."
+  (let ((gptel-backend nil)
+        (gptel-model nil))
+    (should (string-match-p "\\[[-0-9]+ [0-9]+:[0-9]+:[0-9]+\\]"
+                            (cj/gptel-backend-and-model)))))
+
+(ert-deftest test-ai-config-backend-and-model-normal-format-structure ()
+  "Result should follow 'backend: model [timestamp]' format."
+  (let ((gptel-backend (vector 'cl-struct "TestBackend"))
+        (gptel-model "test-model"))
+    (should (string-match-p "^TestBackend: test-model \\["
+                            (cj/gptel-backend-and-model)))))
+
+;;; Boundary Cases
+
+(ert-deftest test-ai-config-backend-and-model-boundary-nil-backend-shows-ai ()
+  "Nil backend should fall back to \"AI\" display name."
+  (let ((gptel-backend nil)
+        (gptel-model "some-model"))
+    (should (string-match-p "^AI:" (cj/gptel-backend-and-model)))))
+
+(ert-deftest test-ai-config-backend-and-model-boundary-nil-model-shows-empty ()
+  "Nil model should produce empty string in model position."
+  (let ((gptel-backend nil)
+        (gptel-model nil))
+    (should (string-match-p "^AI:  \\[" (cj/gptel-backend-and-model)))))
+
+(ert-deftest test-ai-config-backend-and-model-boundary-string-backend-shows-ai ()
+  "String backend (not vector) should fall back to \"AI\"."
+  (let ((gptel-backend "just-a-string")
+        (gptel-model "model"))
+    (should (string-match-p "^AI:" (cj/gptel-backend-and-model)))))
+
+(ert-deftest test-ai-config-backend-and-model-boundary-symbol-model-formatted ()
+  "Symbol model should be formatted as its print representation."
+  (let ((gptel-backend nil)
+        (gptel-model 'some-model))
+    (should (string-match-p "some-model" (cj/gptel-backend-and-model)))))
+
+(ert-deftest test-ai-config-backend-and-model-boundary-timestamp-reflects-today ()
+  "Timestamp should contain today's date."
+  (let ((gptel-backend nil)
+        (gptel-model nil)
+        (today (format-time-string "%Y-%m-%d")))
+    (should (string-match-p (regexp-quote today)
+                            (cj/gptel-backend-and-model)))))
+
+(provide 'test-ai-config-backend-and-model)
+;;; test-ai-config-backend-and-model.el ends here