diff --git a/tests/args.test.mjs b/tests/args.test.mjs
new file mode 100644
index 0000000..68c67ef
--- /dev/null
+++ b/tests/args.test.mjs
@@ -0,0 +1,70 @@
+import test from "node:test";
+import assert from "node:assert/strict";
+
+import { parseArgs, splitRawArgumentString } from "../plugins/codex/scripts/lib/args.mjs";
+
+// --- parseArgs ---
+
+test("parseArgs: boolean flag --flag=true sets true, --flag=false sets false", () => {
+  const configTrue = parseArgs(["--verbose=true"], { booleanOptions: ["verbose"] });
+  assert.equal(configTrue.options.verbose, true);
+
+  const configFalse = parseArgs(["--verbose=false"], { booleanOptions: ["verbose"] });
+  assert.equal(configFalse.options.verbose, false);
+});
+
+test("parseArgs: value option --output consumes next token", () => {
+  const { options } = parseArgs(["--output", "/tmp/out.txt"], { valueOptions: ["output"] });
+  assert.equal(options.output, "/tmp/out.txt");
+});
+
+test("parseArgs: inline value --output=path uses inline value", () => {
+  const { options } = parseArgs(["--output=/tmp/out.txt"], { valueOptions: ["output"] });
+  assert.equal(options.output, "/tmp/out.txt");
+});
+
+test("parseArgs: short alias -o resolved via aliasMap", () => {
+  const { options } = parseArgs(["-o", "/tmp/out.txt"], {
+    valueOptions: ["output"],
+    aliasMap: { o: "output" },
+  });
+  assert.equal(options.output, "/tmp/out.txt");
+});
+
+test("parseArgs: positionals after -- land in positionals array", () => {
+  const { options, positionals } = parseArgs(
+    ["--verbose", "--", "--not-a-flag", "file.txt"],
+    { booleanOptions: ["verbose"] }
+  );
+  assert.equal(options.verbose, true);
+  assert.deepEqual(positionals, ["--not-a-flag", "file.txt"]);
+});
+
+test("parseArgs: missing value for value option throws Error", () => {
+  assert.throws(
+    () => parseArgs(["--output"], { valueOptions: ["output"] }),
+    { message: "Missing value for --output" }
+  );
+});
+
+// --- splitRawArgumentString ---
+
+test("splitRawArgumentString: space-separated tokens", () => {
+  assert.deepEqual(splitRawArgumentString("foo bar baz"), ["foo", "bar", "baz"]);
+});
+
+test("splitRawArgumentString: single-quoted string with spaces becomes one token", () => {
+  assert.deepEqual(splitRawArgumentString("hello 'foo bar' world"), ["hello", "foo bar", "world"]);
+});
+
+test("splitRawArgumentString: double-quoted string with spaces becomes one token", () => {
+  assert.deepEqual(splitRawArgumentString('hello "foo bar" world'), ["hello", "foo bar", "world"]);
+});
+
+test("splitRawArgumentString: backslash escape preserves next char", () => {
+  assert.deepEqual(splitRawArgumentString("foo\\ bar baz"), ["foo bar", "baz"]);
+});
+
+test("splitRawArgumentString: trailing backslash appended literally", () => {
+  assert.deepEqual(splitRawArgumentString("foo\\"), ["foo\\"]);
+});
diff --git a/tests/prompts.test.mjs b/tests/prompts.test.mjs
new file mode 100644
index 0000000..3fb7fba
--- /dev/null
+++ b/tests/prompts.test.mjs
@@ -0,0 +1,25 @@
+import test from "node:test";
+import assert from "node:assert/strict";
+
+import { interpolateTemplate } from "../plugins/codex/scripts/lib/prompts.mjs";
+
+test("interpolateTemplate: replaces {{KEY}} with provided variable", () => {
+  assert.equal(interpolateTemplate("Hello {{NAME}}", { NAME: "World" }), "Hello World");
+});
+
+test("interpolateTemplate: replaces multiple different keys in one pass", () => {
+  const result = interpolateTemplate("{{GREETING}}, {{NAME}}!", { GREETING: "Hi", NAME: "Alice" });
+  assert.equal(result, "Hi, Alice!");
+});
+
+test("interpolateTemplate: unknown key is replaced with empty string", () => {
+  assert.equal(interpolateTemplate("Hello {{MISSING}}", {}), "Hello ");
+});
+
+test("interpolateTemplate: template with no placeholders is returned unchanged", () => {
+  assert.equal(interpolateTemplate("no placeholders here", { KEY: "val" }), "no placeholders here");
+});
+
+test("interpolateTemplate: key appearing twice is replaced both times", () => {
+  assert.equal(interpolateTemplate("{{X}} and {{X}}", { X: "ok" }), "ok and ok");
+});