diff --git a/deno.json b/deno.json
index b61140f..cca59f8 100644
--- a/deno.json
+++ b/deno.json
@@ -1,5 +1,6 @@
 {
   "imports": {
+    "@std/assert": "jsr:@std/assert@^1.0.15",
     "@std/cli": "jsr:@std/cli@^1.0.27",
     "@std/fs": "jsr:@std/fs@^1.0.22",
     "@std/path": "jsr:@std/path@^1.1.4",
diff --git a/src/lib_test.ts b/src/lib_test.ts
new file mode 100644
index 0000000..06040d4
--- /dev/null
+++ b/src/lib_test.ts
@@ -0,0 +1,84 @@
+import { assertEquals } from "jsr:@std/assert";
+import { getOutputPath } from "./lib.ts";
+
+Deno.test("getOutputPath - default output without custom path", () => {
+  const result = getOutputPath({
+    input: "Checker.png",
+    rationalAngle: { degrees: 45 },
+  });
+  assertEquals(result, "./Checker-tile-45.png");
+});
+
+Deno.test("getOutputPath - with full path", () => {
+  const result = getOutputPath({
+    input: "/path/to/Checker.png",
+    rationalAngle: { degrees: 45 },
+  });
+  assertEquals(result, "/path/to/Checker-tile-45.png");
+});
+
+Deno.test("getOutputPath - with relative path", () => {
+  const result = getOutputPath({
+    input: "./images/test.jpg",
+    rationalAngle: { degrees: 90 },
+  });
+  assertEquals(result, "./images/test-tile-90.jpg");
+});
+
+Deno.test("getOutputPath - with custom output path", () => {
+  const result = getOutputPath({
+    input: "Checker.png",
+    rationalAngle: { degrees: 45 },
+    output: "custom-output.png",
+  });
+  assertEquals(result, "custom-output.png");
+});
+
+Deno.test("getOutputPath - with negative degrees", () => {
+  const result = getOutputPath({
+    input: "test.png",
+    rationalAngle: { degrees: -45 },
+  });
+  assertEquals(result, "./test-tile--45.png");
+});
+
+Deno.test("getOutputPath - with decimal degrees", () => {
+  const result = getOutputPath({
+    input: "/home/user/image.png",
+    rationalAngle: { degrees: 26.565 },
+  });
+  assertEquals(result, "/home/user/image-tile-26.565.png");
+});
+
+Deno.test("getOutputPath - with different file extension", () => {
+  const result = getOutputPath({
+    input: "photo.jpeg",
+    rationalAngle: { degrees: 90 },
+  });
+  assertEquals(result, "./photo-tile-90.jpeg");
+});
+
+Deno.test("getOutputPath - zero degrees", () => {
+  const result = getOutputPath({
+    input: "./assets/background.png",
+    rationalAngle: { degrees: 0 },
+  });
+  assertEquals(result, "./assets/background-tile-0.png");
+});
+
+Deno.test("getOutputPath - preserves directory structure", () => {
+  const result = getOutputPath({
+    input: "../../parent/child/file.png",
+    rationalAngle: { degrees: 45 },
+  });
+  assertEquals(result, "../../parent/child/file-tile-45.png");
+});
+
+Deno.test("getOutputPath - custom output ignores input path structure", () => {
+  const result = getOutputPath({
+    input: "/some/long/path/to/image.png",
+    rationalAngle: { degrees: 63.435 },
+    output: "./output.png",
+  });
+  assertEquals(result, "./output.png");
+});