diff --git a/tests/index.js b/tests/index.js
index fd8dbd0..5d48164 100644
--- a/tests/index.js
+++ b/tests/index.js
@@ -7,7 +7,12 @@ const tests = readdirSync("tests").filter((file) => file.endsWith(".test.js"));
 
 for (const file of tests) {
   test(file, async (t) => {
-    const { stderr, stdout } = await execa("node", [`tests/${file}`]);
+    // Override Actions environment variables that change `core`’s behavior
+    const env = {
+      GITHUB_OUTPUT: undefined,
+      GITHUB_STATE: undefined,
+    };
+    const { stderr, stdout } = await execa("node", [`tests/${file}`], { env });
     t.snapshot(stderr, "stderr");
     t.snapshot(stdout, "stdout");
   });
diff --git a/tests/main-missing-owner.test.js b/tests/main-missing-owner.test.js
new file mode 100644
index 0000000..e19542b
--- /dev/null
+++ b/tests/main-missing-owner.test.js
@@ -0,0 +1,9 @@
+process.env.GITHUB_REPOSITORY = "actions/create-github-app-token";
+delete process.env.GITHUB_REPOSITORY_OWNER;
+
+// Verify `main` exits with an error when `GITHUB_REPOSITORY_OWNER` is missing.
+try {
+  await import("../main.js");
+} catch (error) {
+  console.error(error.message);
+}
diff --git a/tests/main-missing-repository.test.js b/tests/main-missing-repository.test.js
new file mode 100644
index 0000000..8496eab
--- /dev/null
+++ b/tests/main-missing-repository.test.js
@@ -0,0 +1,8 @@
+delete process.env.GITHUB_REPOSITORY;
+
+// Verify `main` exits with an error when `GITHUB_REPOSITORY` is missing.
+try {
+  await import("../main.js");
+} catch (error) {
+  console.error(error.message);
+}
diff --git a/tests/main-token-get-owner-set-repo-set-to-many.test.js b/tests/main-token-get-owner-set-repo-set-to-many.test.js
new file mode 100644
index 0000000..fa18b1a
--- /dev/null
+++ b/tests/main-token-get-owner-set-repo-set-to-many.test.js
@@ -0,0 +1,7 @@
+import { test } from "./main.js";
+
+// Verify `main` successfully obtains a token when the `owner` and `repositories` inputs are set (and the latter is a list of repos).
+await test(() => {
+  process.env.INPUT_OWNER = process.env.GITHUB_REPOSITORY_OWNER;
+  process.env.INPUT_REPOSITORIES = `${process.env.GITHUB_REPOSITORY},actions/toolkit`;
+});
diff --git a/tests/main-token-get-owner-set-repo-set-to-one.test.js b/tests/main-token-get-owner-set-repo-set-to-one.test.js
new file mode 100644
index 0000000..3e0f733
--- /dev/null
+++ b/tests/main-token-get-owner-set-repo-set-to-one.test.js
@@ -0,0 +1,7 @@
+import { test } from "./main.js";
+
+// Verify `main` successfully obtains a token when the `owner` and `repositories` inputs are set (and the latter is a single repo).
+await test(() => {
+  process.env.INPUT_OWNER = process.env.GITHUB_REPOSITORY_OWNER;
+  process.env.INPUT_REPOSITORIES = process.env.GITHUB_REPOSITORY;
+});
diff --git a/tests/main-token-get-owner-set-to-org-repo-unset.test.js b/tests/main-token-get-owner-set-to-org-repo-unset.test.js
new file mode 100644
index 0000000..1fa0e1f
--- /dev/null
+++ b/tests/main-token-get-owner-set-to-org-repo-unset.test.js
@@ -0,0 +1,25 @@
+import { test } from "./main.js";
+
+// Verify `main` successfully obtains a token when the `owner` input is set (to an org), but the `repositories` input isn’t set.
+await test((mockPool) => {
+  process.env.INPUT_OWNER = process.env.GITHUB_REPOSITORY_OWNER;
+  delete process.env.INPUT_REPOSITORIES;
+
+  // Mock installation id request
+  const mockInstallationId = "123456";
+  mockPool
+    .intercept({
+      path: `/orgs/${process.env.INPUT_OWNER}/installation`,
+      method: "GET",
+      headers: {
+        accept: "application/vnd.github.v3+json",
+        "user-agent": "actions/create-github-app-token",
+        // Intentionally omitting the `authorization` header, since JWT creation is not idempotent.
+      },
+    })
+    .reply(
+      200,
+      { id: mockInstallationId },
+      { headers: { "content-type": "application/json" } }
+    );
+});
diff --git a/tests/main-token-get-owner-set-to-user-repo-unset.test.js b/tests/main-token-get-owner-set-to-user-repo-unset.test.js
new file mode 100644
index 0000000..bbb802c
--- /dev/null
+++ b/tests/main-token-get-owner-set-to-user-repo-unset.test.js
@@ -0,0 +1,36 @@
+import { test } from "./main.js";
+
+// Verify `main` successfully obtains a token when the `owner` input is set (to a user), but the `repositories` input isn’t set.
+await test((mockPool) => {
+  process.env.INPUT_OWNER = "smockle";
+  delete process.env.INPUT_REPOSITORIES;
+
+  // Mock installation id request
+  const mockInstallationId = "123456";
+  mockPool
+    .intercept({
+      path: `/orgs/${process.env.INPUT_OWNER}/installation`,
+      method: "GET",
+      headers: {
+        accept: "application/vnd.github.v3+json",
+        "user-agent": "actions/create-github-app-token",
+        // Intentionally omitting the `authorization` header, since JWT creation is not idempotent.
+      },
+    })
+    .reply(404);
+  mockPool
+    .intercept({
+      path: `/users/${process.env.INPUT_OWNER}/installation`,
+      method: "GET",
+      headers: {
+        accept: "application/vnd.github.v3+json",
+        "user-agent": "actions/create-github-app-token",
+        // Intentionally omitting the `authorization` header, since JWT creation is not idempotent.
+      },
+    })
+    .reply(
+      200,
+      { id: mockInstallationId },
+      { headers: { "content-type": "application/json" } }
+    );
+});
diff --git a/tests/main-token-get-owner-unset-repo-set.test.js b/tests/main-token-get-owner-unset-repo-set.test.js
new file mode 100644
index 0000000..89d6a85
--- /dev/null
+++ b/tests/main-token-get-owner-unset-repo-set.test.js
@@ -0,0 +1,7 @@
+import { test } from "./main.js";
+
+// Verify `main` successfully obtains a token when the `owner` input is not set, but the `repositories` input is set.
+await test(() => {
+  delete process.env.INPUT_OWNER;
+  process.env.INPUT_REPOSITORIES = process.env.GITHUB_REPOSITORY;
+});
diff --git a/tests/main-token-get-owner-unset-repo-unset.test.js b/tests/main-token-get-owner-unset-repo-unset.test.js
new file mode 100644
index 0000000..ccc20dd
--- /dev/null
+++ b/tests/main-token-get-owner-unset-repo-unset.test.js
@@ -0,0 +1,25 @@
+import { test } from "./main.js";
+
+// Verify `main` successfully obtains a token when neither the `owner` nor `repositories` input is set.
+await test((mockPool) => {
+  delete process.env.INPUT_OWNER;
+  delete process.env.INPUT_REPOSITORIES;
+
+  // Mock installation id request
+  const mockInstallationId = "123456";
+  mockPool
+    .intercept({
+      path: `/repos/${process.env.GITHUB_REPOSITORY}/installation`,
+      method: "GET",
+      headers: {
+        accept: "application/vnd.github.v3+json",
+        "user-agent": "actions/create-github-app-token",
+        // Intentionally omitting the `authorization` header, since JWT creation is not idempotent.
+      },
+    })
+    .reply(
+      200,
+      { id: mockInstallationId },
+      { headers: { "content-type": "application/json" } }
+    );
+});
diff --git a/tests/main.js b/tests/main.js
new file mode 100644
index 0000000..9e62af8
--- /dev/null
+++ b/tests/main.js
@@ -0,0 +1,96 @@
+// Base for all `main` tests.
+// @ts-check
+import { MockAgent, setGlobalDispatcher } from "undici";
+
+export async function test(cb = (_mockPool) => {}) {
+  // Set required environment variables and inputs
+  process.env.GITHUB_REPOSITORY_OWNER = "actions";
+  process.env.GITHUB_REPOSITORY = "actions/create-github-app-token";
+  // inputs are set as environment variables with the prefix INPUT_
+  // https://docs.github.com/en/actions/creating-actions/metadata-syntax-for-github-actions#example-specifying-inputs
+  process.env.INPUT_APP_ID = "123456";
+  process.env.INPUT_PRIVATE_KEY = `-----BEGIN RSA PRIVATE KEY-----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+-----END RSA PRIVATE KEY-----`; // This key is invalidated. It’s from https://github.com/octokit/auth-app.js/issues/465#issuecomment-1564998327.
+
+  // Set up mocking
+  const mockAgent = new MockAgent();
+  mockAgent.disableNetConnect();
+  setGlobalDispatcher(mockAgent);
+  const mockPool = mockAgent.get("https://api.github.com");
+
+  // Calling `auth({ type: "app" })` to obtain a JWT doesn’t make network requests, so no need to intercept.
+
+  // Mock installation id request
+  const mockInstallationId = "123456";
+  const owner = process.env.INPUT_OWNER ?? process.env.GITHUB_REPOSITORY_OWNER;
+  const repo = encodeURIComponent(
+    (process.env.INPUT_REPOSITORIES ?? process.env.GITHUB_REPOSITORY).split(
+      ","
+    )[0]
+  );
+  mockPool
+    .intercept({
+      path: `/repos/${owner}/${repo}/installation`,
+      method: "GET",
+      headers: {
+        accept: "application/vnd.github.v3+json",
+        "user-agent": "actions/create-github-app-token",
+        // Intentionally omitting the `authorization` header, since JWT creation is not idempotent.
+      },
+    })
+    .reply(
+      200,
+      { id: mockInstallationId },
+      { headers: { "content-type": "application/json" } }
+    );
+
+  // Mock installation access token request
+  const mockInstallationAccessToken =
+    "ghs_16C7e42F292c6912E7710c838347Ae178B4a"; // This token is invalidated. It’s from https://docs.github.com/en/rest/apps/apps?apiVersion=2022-11-28#create-an-installation-access-token-for-an-app.
+  mockPool
+    .intercept({
+      path: `/app/installations/${mockInstallationId}/access_tokens`,
+      method: "POST",
+      headers: {
+        accept: "application/vnd.github.v3+json",
+        "user-agent": "actions/create-github-app-token",
+        // Note: Intentionally omitting the `authorization` header, since JWT creation is not idempotent.
+      },
+    })
+    .reply(
+      201,
+      { token: mockInstallationAccessToken },
+      { headers: { "content-type": "application/json" } }
+    );
+
+  // Run the callback
+  cb(mockPool);
+
+  // Run the main script
+  await import("../main.js");
+}
diff --git a/tests/snapshots/index.js.md b/tests/snapshots/index.js.md
index 1f0debc..4ab4701 100644
--- a/tests/snapshots/index.js.md
+++ b/tests/snapshots/index.js.md
@@ -4,6 +4,110 @@ The actual snapshot is saved in `index.js.snap`.
 
 Generated by [AVA](https://avajs.dev).
 
+## main-missing-owner.test.js
+
+> stderr
+
+    'GITHUB_REPOSITORY_OWNER missing, must be set to \'<owner>\''
+
+> stdout
+
+    ''
+
+## main-missing-repository.test.js
+
+> stderr
+
+    'GITHUB_REPOSITORY missing, must be set to \'<owner>/<repo>\''
+
+> stdout
+
+    ''
+
+## main-token-get-owner-set-repo-set-to-many.test.js
+
+> stderr
+
+    ''
+
+> stdout
+
+    `owner and repositories set, creating token for repositories "actions/create-github-app-token,actions/toolkit" owned by "actions"␊
+    ::add-mask::ghs_16C7e42F292c6912E7710c838347Ae178B4a␊
+    ␊
+    ::set-output name=token::ghs_16C7e42F292c6912E7710c838347Ae178B4a␊
+    ::save-state name=token::ghs_16C7e42F292c6912E7710c838347Ae178B4a`
+
+## main-token-get-owner-set-repo-set-to-one.test.js
+
+> stderr
+
+    ''
+
+> stdout
+
+    `owner and repositories set, creating token for repositories "actions/create-github-app-token" owned by "actions"␊
+    ::add-mask::ghs_16C7e42F292c6912E7710c838347Ae178B4a␊
+    ␊
+    ::set-output name=token::ghs_16C7e42F292c6912E7710c838347Ae178B4a␊
+    ::save-state name=token::ghs_16C7e42F292c6912E7710c838347Ae178B4a`
+
+## main-token-get-owner-set-to-org-repo-unset.test.js
+
+> stderr
+
+    ''
+
+> stdout
+
+    `repositories not set, creating token for all repositories for given owner "actions"␊
+    ::add-mask::ghs_16C7e42F292c6912E7710c838347Ae178B4a␊
+    ␊
+    ::set-output name=token::ghs_16C7e42F292c6912E7710c838347Ae178B4a␊
+    ::save-state name=token::ghs_16C7e42F292c6912E7710c838347Ae178B4a`
+
+## main-token-get-owner-set-to-user-repo-unset.test.js
+
+> stderr
+
+    ''
+
+> stdout
+
+    `repositories not set, creating token for all repositories for given owner "smockle"␊
+    ::add-mask::ghs_16C7e42F292c6912E7710c838347Ae178B4a␊
+    ␊
+    ::set-output name=token::ghs_16C7e42F292c6912E7710c838347Ae178B4a␊
+    ::save-state name=token::ghs_16C7e42F292c6912E7710c838347Ae178B4a`
+
+## main-token-get-owner-unset-repo-set.test.js
+
+> stderr
+
+    ''
+
+> stdout
+
+    `owner not set, creating owner for given repositories "actions/create-github-app-token" in current owner ("actions")␊
+    ::add-mask::ghs_16C7e42F292c6912E7710c838347Ae178B4a␊
+    ␊
+    ::set-output name=token::ghs_16C7e42F292c6912E7710c838347Ae178B4a␊
+    ::save-state name=token::ghs_16C7e42F292c6912E7710c838347Ae178B4a`
+
+## main-token-get-owner-unset-repo-unset.test.js
+
+> stderr
+
+    ''
+
+> stdout
+
+    `owner and repositories not set, creating token for the current repository ("create-github-app-token")␊
+    ::add-mask::ghs_16C7e42F292c6912E7710c838347Ae178B4a␊
+    ␊
+    ::set-output name=token::ghs_16C7e42F292c6912E7710c838347Ae178B4a␊
+    ::save-state name=token::ghs_16C7e42F292c6912E7710c838347Ae178B4a`
+
 ## post-token-set.test.js
 
 > stderr
diff --git a/tests/snapshots/index.js.snap b/tests/snapshots/index.js.snap
index 954c16d..fb12409 100644
Binary files a/tests/snapshots/index.js.snap and b/tests/snapshots/index.js.snap differ