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diff --git a/tests/test-wttrin--make-cache-key.el b/tests/test-wttrin--make-cache-key.el
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+;;; test-wttrin--make-cache-key.el --- Tests for wttrin--make-cache-key -*- lexical-binding: t; -*-
+
+;; Copyright (C) 2025 Craig Jennings
+
+;;; Commentary:
+
+;; Unit tests for wttrin--make-cache-key function.
+;; Tests cache key generation from location and unit system.
+
+;;; Code:
+
+(require 'ert)
+(require 'wttrin)
+(require 'testutil-wttrin)
+
+;;; Normal Cases
+
+(ert-deftest test-wttrin--make-cache-key-normal-location-with-metric-returns-key ()
+  "Test that location with metric unit system creates correct cache key."
+  (testutil-wttrin-with-unit-system "m"
+    (should (equal "Paris|m" (wttrin--make-cache-key "Paris")))))
+
+(ert-deftest test-wttrin--make-cache-key-normal-location-with-uscs-returns-key ()
+  "Test that location with USCS unit system creates correct cache key."
+  (testutil-wttrin-with-unit-system "u"
+    (should (equal "New York|u" (wttrin--make-cache-key "New York")))))
+
+(ert-deftest test-wttrin--make-cache-key-normal-location-no-unit-returns-default ()
+  "Test that location with no unit system uses default in cache key."
+  (testutil-wttrin-with-unit-system nil
+    (should (equal "London|default" (wttrin--make-cache-key "London")))))
+
+;;; Boundary Cases
+
+(ert-deftest test-wttrin--make-cache-key-boundary-empty-location-returns-key ()
+  "Test that empty location string creates cache key."
+  (testutil-wttrin-with-unit-system "m"
+    (should (equal "|m" (wttrin--make-cache-key "")))))
+
+(ert-deftest test-wttrin--make-cache-key-boundary-location-with-spaces-returns-key ()
+  "Test that location with spaces creates correct cache key."
+  (testutil-wttrin-with-unit-system "m"
+    (should (equal "New York, NY|m" (wttrin--make-cache-key "New York, NY")))))
+
+(ert-deftest test-wttrin--make-cache-key-boundary-location-with-commas-returns-key ()
+  "Test that location with commas creates correct cache key."
+  (testutil-wttrin-with-unit-system nil
+    (should (equal "Berlin, DE|default" (wttrin--make-cache-key "Berlin, DE")))))
+
+(ert-deftest test-wttrin--make-cache-key-boundary-unicode-location-returns-key ()
+  "Test that Unicode location creates correct cache key."
+  (testutil-wttrin-with-unit-system "m"
+    (should (equal "東京|m" (wttrin--make-cache-key "東京")))))
+
+(ert-deftest test-wttrin--make-cache-key-boundary-location-with-special-chars-returns-key ()
+  "Test that location with special characters creates cache key."
+  (testutil-wttrin-with-unit-system "m"
+    (should (equal "~Eiffel+Tower|m" (wttrin--make-cache-key "~Eiffel+Tower")))))
+
+;;; Error Cases
+
+(ert-deftest test-wttrin--make-cache-key-error-nil-location-returns-key ()
+  "Test that nil location creates cache key with empty string."
+  ;; Note: concat converts nil to empty string, this documents current behavior
+  (testutil-wttrin-with-unit-system "m"
+    (should (equal "|m" (wttrin--make-cache-key nil)))))
+
+(provide 'test-wttrin--make-cache-key)
+;;; test-wttrin--make-cache-key.el ends here