diff --git a/tests/operations/tests/ExtendedGCD.mjs b/tests/operations/tests/ExtendedGCD.mjs
new file mode 100644
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+++ b/tests/operations/tests/ExtendedGCD.mjs
@@ -0,0 +1,126 @@
+/**
+ * Extended GCD tests.
+ *
+ * @author p-leriche [philip.leriche@cantab.net]
+ *
+ * @copyright Crown Copyright 2025
+ * @license Apache-2.0
+ */
+import TestRegister from "../../lib/TestRegister.mjs";
+
+TestRegister.addTests([
+    {
+        name: "Extended GCD: coprime numbers (3, 11)",
+        input: "",
+        expectedOutput: "Extended Euclidean Algorithm Results:\n" +
+            "=====================================\n\n" +
+            "gcd(a, b) = 1\n\n" +
+            "Béut coefficients:\n" +
+            "  x = 4\n" +
+            "  y = -1\n\n" +
+            "Verification:\n" +
+            "  a·x + b·y = gcd(a, b)\n" +
+            "  (3) ×(4) + (11) ×(-1) = 1",
+        recipeConfig: [
+            {
+                op: "Extended GCD",
+                args: ["3", "11"],
+            },
+        ],
+    },
+    {
+        name: "Extended GCD: non-coprime numbers (240, 46)",
+        input: "",
+        expectedOutput: "Extended Euclidean Algorithm Results:\n" +
+            "=====================================\n\n" +
+            "gcd(a, b) = 2\n\n" +
+            "Béut coefficients:\n" +
+            "  x = -9\n" +
+            "  y = 47\n\n" +
+            "Verification:\n" +
+            "  a·x + b·y = gcd(a, b)\n" +
+            "  (240) ×(-9) + (46) ×(47) = 2",
+        recipeConfig: [
+            {
+                op: "Extended GCD",
+                args: ["240", "46"],
+            },
+        ],
+    },
+    {
+        name: "Extended GCD: with zero (17, 0)",
+        input: "",
+        expectedOutput: "Extended Euclidean Algorithm Results:\n" +
+            "=====================================\n\n" +
+            "gcd(a, b) = 17\n\n" +
+            "Béut coefficients:\n" +
+            "  x = 1\n" +
+            "  y = 0\n\n" +
+            "Verification:\n" +
+            "  a·x + b·y = gcd(a, b)\n" +
+            "  (17) ×(1) + (0) ×(0) = 17",
+        recipeConfig: [
+            {
+                op: "Extended GCD",
+                args: ["17", "0"],
+            },
+        ],
+    },
+    {
+        name: "Extended GCD: hexadecimal input (0xFF, 0x11)",
+        input: "",
+        expectedOutput: "Extended Euclidean Algorithm Results:\n" +
+            "=====================================\n\n" +
+            "gcd(a, b) = 17\n\n" +
+            "Béut coefficients:\n" +
+            "  x = 1\n" +
+            "  y = -15\n\n" +
+            "Verification:\n" +
+            "  a·x + b·y = gcd(a, b)\n" +
+            "  (255) ×(1) + (17) ×(-15) = 17",
+        recipeConfig: [
+            {
+                op: "Extended GCD",
+                args: ["0xFF", "0x11"],
+            },
+        ],
+    },
+    {
+        name: "Extended GCD: using input field for value a",
+        input: "42",
+        expectedOutput: "Extended Euclidean Algorithm Results:\n" +
+            "=====================================\n\n" +
+            "gcd(a, b) = 7\n\n" +
+            "Béut coefficients:\n" +
+            "  x = -2\n" +
+            "  y = 3\n\n" +
+            "Verification:\n" +
+            "  a·x + b·y = gcd(a, b)\n" +
+            "  (42) ×(-2) + (35) ×(3) = 7",
+        recipeConfig: [
+            {
+                op: "Extended GCD",
+                args: ["", "35"],
+            },
+        ],
+    },
+    {
+        name: "Extended GCD: large numbers",
+        input: "",
+        expectedOutput: "Extended Euclidean Algorithm Results:\n" +
+            "=====================================\n\n" +
+            "gcd(a, b) = 1\n\n" +
+            "Béut coefficients:\n" +
+            "  x = -80538738812075595\n" +
+            "  y = 10000000000000000\n\n" +
+            "Verification:\n" +
+            "  a·x + b·y = gcd(a, b)\n" +
+            "  (123456789012345678901234567890) ×(-80538738812075595) + (994064509324197316) ×(10000000000000000) = 1",
+        recipeConfig: [
+            {
+                op: "Extended GCD",
+                args: ["123456789012345678901234567890", "994064509324197316"],
+            },
+        ],
+    },
+]);