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diff --git a/tests/test-wttrin--build-url.el b/tests/test-wttrin--build-url.el
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+;;; test-wttrin--build-url.el --- Tests for wttrin--build-url -*- lexical-binding: t; -*-
+
+;; Copyright (C) 2025 Craig Jennings
+
+;;; Commentary:
+
+;; Unit tests for wttrin--build-url function.
+;; Tests URL construction with proper encoding and parameters.
+
+;;; Code:
+
+(require 'ert)
+(require 'wttrin)
+(require 'testutil-wttrin)
+
+;;; Normal Cases
+
+(ert-deftest test-wttrin--build-url-normal-simple-location-returns-url ()
+  "Test that simple location builds correct URL."
+  (testutil-wttrin-with-unit-system nil
+    (should (equal "https://wttr.in/Paris?A"
+                   (wttrin--build-url "Paris")))))
+
+(ert-deftest test-wttrin--build-url-normal-location-with-metric-returns-url ()
+  "Test that location with metric unit system builds correct URL."
+  (testutil-wttrin-with-unit-system "m"
+    (should (equal "https://wttr.in/London?mA"
+                   (wttrin--build-url "London")))))
+
+(ert-deftest test-wttrin--build-url-normal-location-with-uscs-returns-url ()
+  "Test that location with USCS unit system builds correct URL."
+  (testutil-wttrin-with-unit-system "u"
+    (should (equal "https://wttr.in/Berlin?uA"
+                   (wttrin--build-url "Berlin")))))
+
+;;; Boundary Cases
+
+(ert-deftest test-wttrin--build-url-boundary-location-with-spaces-encodes-url ()
+  "Test that location with spaces is properly URL-encoded."
+  (testutil-wttrin-with-unit-system nil
+    (should (equal "https://wttr.in/New%20York?A"
+                   (wttrin--build-url "New York")))))
+
+(ert-deftest test-wttrin--build-url-boundary-location-with-comma-encodes-url ()
+  "Test that location with comma is properly URL-encoded."
+  (testutil-wttrin-with-unit-system nil
+    (should (equal "https://wttr.in/New%20York%2C%20NY?A"
+                   (wttrin--build-url "New York, NY")))))
+
+(ert-deftest test-wttrin--build-url-boundary-location-with-special-chars-encodes-url ()
+  "Test that location with special characters is properly URL-encoded."
+  (testutil-wttrin-with-unit-system "m"
+    ;; ~Eiffel+Tower format is used by wttr.in for landmarks
+    ;; ~ is an unreserved character (RFC 3986) and is not encoded
+    ;; + is encoded as %2B
+    (should (equal "https://wttr.in/~Eiffel%2BTower?mA"
+                   (wttrin--build-url "~Eiffel+Tower")))))
+
+(ert-deftest test-wttrin--build-url-boundary-unicode-location-encodes-url ()
+  "Test that Unicode location is properly URL-encoded."
+  (testutil-wttrin-with-unit-system nil
+    ;; Unicode should be properly encoded
+    (let ((result (wttrin--build-url "東京")))
+      (should (string-prefix-p "https://wttr.in/" result))
+      (should (string-suffix-p "?A" result))
+      ;; Should contain URL-encoded Unicode
+      (should (string-match-p "%[0-9A-F][0-9A-F]" result)))))
+
+(ert-deftest test-wttrin--build-url-boundary-empty-location-returns-url ()
+  "Test that empty location builds URL with empty query."
+  (testutil-wttrin-with-unit-system nil
+    (should (equal "https://wttr.in/?A"
+                   (wttrin--build-url "")))))
+
+(ert-deftest test-wttrin--build-url-boundary-gps-coordinates-encodes-url ()
+  "Test that GPS coordinates are properly URL-encoded."
+  (testutil-wttrin-with-unit-system nil
+    ;; Format: -78.46,106.79
+    (should (equal "https://wttr.in/-78.46%2C106.79?A"
+                   (wttrin--build-url "-78.46,106.79")))))
+
+;;; Error Cases
+
+(ert-deftest test-wttrin--build-url-error-nil-location-signals-error ()
+  "Test that nil location signals an error."
+  (testutil-wttrin-with-unit-system nil
+    (should-error (wttrin--build-url nil)
+                  :type 'error)))
+
+(provide 'test-wttrin--build-url)
+;;; test-wttrin--build-url.el ends here