diff --git a/README.md b/README.md
index b57c702..c75538a 100644
--- a/README.md
+++ b/README.md
@@ -9,8 +9,8 @@ GitHub Action for creating a GitHub App installation access token.
 In order to use this action, you need to:
 
 1. [Register new GitHub App](https://docs.github.com/apps/creating-github-apps/setting-up-a-github-app/creating-a-github-app).
-2. [Store the App's ID or Client ID in your repository environment variables](https://docs.github.com/actions/learn-github-actions/variables#defining-configuration-variables-for-multiple-workflows) (example: `APP_ID`).
-3. [Store the App's private key in your repository secrets](https://docs.github.com/actions/security-guides/encrypted-secrets?tool=webui#creating-encrypted-secrets-for-a-repository) (example: `PRIVATE_KEY`).
+2. [Store the App's Client ID in your repository environment variables](https://docs.github.com/actions/how-tos/write-workflows/choose-what-workflows-do/use-variables#defining-configuration-variables-for-multiple-workflows) (example: `GITHUB_APP_CLIENT_ID`).
+3. [Store the App's private key in your repository secrets](https://docs.github.com/actions/how-tos/write-workflows/choose-what-workflows-do/use-secrets?tool=webui#creating-secrets-for-a-repository) (example: `GITHUB_APP_PRIVATE_KEY`).
 
 > [!IMPORTANT]  
 > An installation access token expires after 1 hour. Please [see this comment](https://github.com/actions/create-github-app-token/issues/121#issuecomment-2043214796) for alternative approaches if you have long-running processes.
@@ -31,8 +31,8 @@ jobs:
       - uses: actions/create-github-app-token@v3
         id: app-token
         with:
-          app-id: ${{ vars.APP_ID }}
-          private-key: ${{ secrets.PRIVATE_KEY }}
+          client-id: ${{ vars.GITHUB_APP_CLIENT_ID }}
+          private-key: ${{ secrets.GITHUB_APP_PRIVATE_KEY }}
       - uses: ./actions/staging-tests
         with:
           token: ${{ steps.app-token.outputs.token }}
@@ -51,8 +51,8 @@ jobs:
         id: app-token
         with:
           # required
-          app-id: ${{ vars.APP_ID }}
-          private-key: ${{ secrets.PRIVATE_KEY }}
+          client-id: ${{ vars.GITHUB_APP_CLIENT_ID }}
+          private-key: ${{ secrets.GITHUB_APP_PRIVATE_KEY }}
       - uses: actions/checkout@v6
         with:
           token: ${{ steps.app-token.outputs.token }}
@@ -77,8 +77,8 @@ jobs:
         id: app-token
         with:
           # required
-          app-id: ${{ vars.APP_ID }}
-          private-key: ${{ secrets.PRIVATE_KEY }}
+          client-id: ${{ vars.GITHUB_APP_CLIENT_ID }}
+          private-key: ${{ secrets.GITHUB_APP_PRIVATE_KEY }}
       - name: Get GitHub App User ID
         id: get-user-id
         run: echo "user-id=$(gh api "/users/${{ steps.app-token.outputs.app-slug }}[bot]" --jq .id)" >> "$GITHUB_OUTPUT"
@@ -102,8 +102,8 @@ jobs:
         id: app-token
         with:
           # required
-          app-id: ${{ vars.APP_ID }}
-          private-key: ${{ secrets.PRIVATE_KEY }}
+          client-id: ${{ vars.GITHUB_APP_CLIENT_ID }}
+          private-key: ${{ secrets.GITHUB_APP_PRIVATE_KEY }}
       - name: Get GitHub App User ID
         id: get-user-id
         run: echo "user-id=$(gh api "/users/${{ steps.app-token.outputs.app-slug }}[bot]" --jq .id)" >> "$GITHUB_OUTPUT"
@@ -138,8 +138,8 @@ jobs:
       - uses: actions/create-github-app-token@v3
         id: app-token
         with:
-          app-id: ${{ vars.APP_ID }}
-          private-key: ${{ secrets.PRIVATE_KEY }}
+          client-id: ${{ vars.GITHUB_APP_CLIENT_ID }}
+          private-key: ${{ secrets.GITHUB_APP_PRIVATE_KEY }}
           owner: ${{ github.repository_owner }}
       - uses: peter-evans/create-or-update-comment@v4
         with:
@@ -160,8 +160,8 @@ jobs:
       - uses: actions/create-github-app-token@v3
         id: app-token
         with:
-          app-id: ${{ vars.APP_ID }}
-          private-key: ${{ secrets.PRIVATE_KEY }}
+          client-id: ${{ vars.GITHUB_APP_CLIENT_ID }}
+          private-key: ${{ secrets.GITHUB_APP_PRIVATE_KEY }}
           owner: ${{ github.repository_owner }}
           repositories: |
             repo1
@@ -185,8 +185,8 @@ jobs:
       - uses: actions/create-github-app-token@v3
         id: app-token
         with:
-          app-id: ${{ vars.APP_ID }}
-          private-key: ${{ secrets.PRIVATE_KEY }}
+          client-id: ${{ vars.GITHUB_APP_CLIENT_ID }}
+          private-key: ${{ secrets.GITHUB_APP_PRIVATE_KEY }}
           owner: another-owner
       - uses: peter-evans/create-or-update-comment@v4
         with:
@@ -210,8 +210,8 @@ jobs:
       - uses: actions/create-github-app-token@v3
         id: app-token
         with:
-          app-id: ${{ vars.APP_ID }}
-          private-key: ${{ secrets.PRIVATE_KEY }}
+          client-id: ${{ vars.GITHUB_APP_CLIENT_ID }}
+          private-key: ${{ secrets.GITHUB_APP_PRIVATE_KEY }}
           owner: ${{ github.repository_owner }}
           permission-issues: write
       - uses: peter-evans/create-or-update-comment@v4
@@ -252,8 +252,8 @@ jobs:
       - uses: actions/create-github-app-token@v3
         id: app-token
         with:
-          app-id: ${{ vars.APP_ID }}
-          private-key: ${{ secrets.PRIVATE_KEY }}
+          client-id: ${{ vars.GITHUB_APP_CLIENT_ID }}
+          private-key: ${{ secrets.GITHUB_APP_PRIVATE_KEY }}
           owner: ${{ matrix.owners-and-repos.owner }}
           repositories: ${{ join(matrix.owners-and-repos.repos) }}
       - uses: octokit/request-action@v2.x
@@ -281,7 +281,7 @@ jobs:
         id: create_token
         uses: actions/create-github-app-token@v3
         with:
-          app-id: ${{ vars.GHES_APP_ID }}
+          client-id: ${{ vars.GHES_APP_CLIENT_ID }}
           private-key: ${{ secrets.GHES_APP_PRIVATE_KEY }}
           owner: ${{ vars.GHES_INSTALLATION_ORG }}
           github-api-url: ${{ vars.GITHUB_API_URL }}
@@ -310,15 +310,18 @@ If you set `HTTP_PROXY` or `HTTPS_PROXY`, also set `NODE_USE_ENV_PROXY: "1"` on
     NO_PROXY: github.example.com
     NODE_USE_ENV_PROXY: "1"
   with:
-    app-id: ${{ vars.APP_ID }}
-    private-key: ${{ secrets.PRIVATE_KEY }}
+    client-id: ${{ vars.GITHUB_APP_CLIENT_ID }}
+    private-key: ${{ secrets.GITHUB_APP_PRIVATE_KEY }}
 ```
 
 ## Inputs
 
-### `app-id`
+### `client-id` or `app-id`
 
-**Required:** GitHub App ID.
+**Required:** GitHub App Client ID.
+
+> [!NOTE]
+> The legacy `app-id` input is also accepted, but `client-id` is recommended.
 
 ### `private-key`
 
@@ -331,14 +334,14 @@ steps:
   - name: Decode the GitHub App Private Key
     id: decode
     run: |
-      private_key=$(echo "${{ secrets.PRIVATE_KEY }}" | base64 -d | awk 'BEGIN {ORS="\\n"} {print}' | head -c -2) &> /dev/null
+      private_key=$(echo "${{ secrets.GITHUB_APP_PRIVATE_KEY }}" | base64 -d | awk 'BEGIN {ORS="\\n"} {print}' | head -c -2) &> /dev/null
       echo "::add-mask::$private_key"
       echo "private-key=$private_key" >> "$GITHUB_OUTPUT"
   - name: Generate GitHub App Token
     id: app-token
     uses: actions/create-github-app-token@v3
     with:
-      app-id: ${{ vars.APP_ID }}
+      client-id: ${{ vars.GITHUB_APP_CLIENT_ID }}
       private-key: ${{ steps.decode.outputs.private-key }}
 ```
 
diff --git a/action.yml b/action.yml
index ba4e915..e5b2ddd 100644
--- a/action.yml
+++ b/action.yml
@@ -5,9 +5,13 @@ branding:
   icon: "lock"
   color: "gray-dark"
 inputs:
+  client-id:
+    description: "GitHub App Client ID"
+    required: false
   app-id:
     description: "GitHub App ID"
-    required: true
+    required: false
+    deprecationMessage: "Use 'client-id' instead."
   private-key:
     description: "GitHub App private key"
     required: true
diff --git a/dist/main.cjs b/dist/main.cjs
index e2674f2..683df24 100644
--- a/dist/main.cjs
+++ b/dist/main.cjs
@@ -23153,7 +23153,7 @@ async function pRetry(input, options = {}) {
 }
 
 // lib/main.js
-async function main(appId, privateKey, owner, repositories, permissions, core, createAppAuth2, request2, skipTokenRevoke) {
+async function main(clientId, privateKey, owner, repositories, permissions, core, createAppAuth2, request2, skipTokenRevoke) {
   let parsedOwner = "";
   let parsedRepositoryNames = [];
   if (!owner && repositories.length === 0) {
@@ -23188,7 +23188,7 @@ async function main(appId, privateKey, owner, repositories, permissions, core, c
     );
   }
   const auth5 = createAppAuth2({
-    appId,
+    appId: clientId,
     privateKey,
     request: request2
   });
@@ -23307,14 +23307,17 @@ if (!process.env.GITHUB_REPOSITORY_OWNER) {
 }
 async function run() {
   ensureNativeProxySupport();
-  const appId = getInput("app-id");
+  const clientId = getInput("client-id") || getInput("app-id");
+  if (!clientId) {
+    throw new Error("Either 'client-id' or 'app-id' input must be set");
+  }
   const privateKey = getInput("private-key");
   const owner = getInput("owner");
   const repositories = getInput("repositories").split(/[\n,]+/).map((s) => s.trim()).filter((x) => x !== "");
   const skipTokenRevoke = getBooleanInput("skip-token-revoke");
   const permissions = getPermissionsFromInputs(process.env);
   return main(
-    appId,
+    clientId,
     privateKey,
     owner,
     repositories,
diff --git a/lib/main.js b/lib/main.js
index 9ae9d78..8f5ef9a 100644
--- a/lib/main.js
+++ b/lib/main.js
@@ -2,7 +2,7 @@ import pRetry from "p-retry";
 // @ts-check
 
 /**
- * @param {string} appId
+ * @param {string} clientId
  * @param {string} privateKey
  * @param {string} owner
  * @param {string[]} repositories
@@ -13,7 +13,7 @@ import pRetry from "p-retry";
  * @param {boolean} skipTokenRevoke
  */
 export async function main(
-  appId,
+  clientId,
   privateKey,
   owner,
   repositories,
@@ -70,7 +70,7 @@ export async function main(
   }
 
   const auth = createAppAuth({
-    appId,
+    appId: clientId,
     privateKey,
     request,
   });
diff --git a/main.js b/main.js
index d8ebee4..b685126 100644
--- a/main.js
+++ b/main.js
@@ -18,7 +18,10 @@ if (!process.env.GITHUB_REPOSITORY_OWNER) {
 async function run() {
   ensureNativeProxySupport();
 
-  const appId = core.getInput("app-id");
+  const clientId = core.getInput("client-id") || core.getInput("app-id");
+  if (!clientId) {
+    throw new Error("Either 'client-id' or 'app-id' input must be set");
+  }
   const privateKey = core.getInput("private-key");
   const owner = core.getInput("owner");
   const repositories = core
@@ -32,7 +35,7 @@ async function run() {
   const permissions = getPermissionsFromInputs(process.env);
 
   return main(
-    appId,
+    clientId,
     privateKey,
     owner,
     repositories,
diff --git a/tests/README.md b/tests/README.md
index 2eeeb7d..d3ce05c 100644
--- a/tests/README.md
+++ b/tests/README.md
@@ -32,5 +32,5 @@ node --test --test-update-snapshots tests/index.js
 
 We have tests both for the `main.js` and `post.js` scripts.
 
-- If you do not expect an error, take [main-token-permissions-set.test.js](tests/main-token-permissions-set.test.js) as a starting point.
-- If your test has an expected error, take [main-missing-app-id.test.js](tests/main-missing-app-id.test.js) as a starting point.
+- If you do not expect an error, take [main-token-permissions-set.test.js](main-token-permissions-set.test.js) as a starting point.
+- If your test has an expected error, take [main-missing-client-and-app-id.test.js](main-missing-client-and-app-id.test.js) as a starting point.
diff --git a/tests/index.js.snapshot b/tests/index.js.snapshot
index 06cac80..d5d5f9b 100644
--- a/tests/index.js.snapshot
+++ b/tests/index.js.snapshot
@@ -1,3 +1,41 @@
+exports[`action-deprecated-inputs.test.js > stdout 1`] = `
+app-id — Use 'client-id' instead.
+`;
+
+exports[`main-app-id-fallback.test.js > stdout 1`] = `
+Inputs 'owner' and 'repositories' are not set. Creating token for this repository (actions/create-github-app-token).
+::add-mask::ghs_16C7e42F292c6912E7710c838347Ae178B4a
+
+::set-output name=token::ghs_16C7e42F292c6912E7710c838347Ae178B4a
+
+::set-output name=installation-id::123456
+
+::set-output name=app-slug::github-actions
+::save-state name=token::ghs_16C7e42F292c6912E7710c838347Ae178B4a
+::save-state name=expiresAt::2016-07-11T22:14:10Z
+--- REQUESTS ---
+GET /repos/actions/create-github-app-token/installation
+POST /app/installations/123456/access_tokens
+{"repositories":["create-github-app-token"]}
+`;
+
+exports[`main-client-id-precedence.test.js > stdout 1`] = `
+Inputs 'owner' and 'repositories' are not set. Creating token for this repository (actions/create-github-app-token).
+::add-mask::ghs_16C7e42F292c6912E7710c838347Ae178B4a
+
+::set-output name=token::ghs_16C7e42F292c6912E7710c838347Ae178B4a
+
+::set-output name=installation-id::123456
+
+::set-output name=app-slug::github-actions
+::save-state name=token::ghs_16C7e42F292c6912E7710c838347Ae178B4a
+::save-state name=expiresAt::2016-07-11T22:14:10Z
+--- REQUESTS ---
+GET /repos/actions/create-github-app-token/installation
+POST /app/installations/123456/access_tokens
+{"repositories":["create-github-app-token"]}
+`;
+
 exports[`main-custom-github-api-url.test.js > stdout 1`] = `
 Inputs 'owner' and 'repositories' are set. Creating token for the following repositories:
       
@@ -17,6 +55,14 @@ POST /api/v3/app/installations/123456/access_tokens
 {"repositories":["create-github-app-token"]}
 `;
 
+exports[`main-missing-client-and-app-id.test.js > stderr 1`] = `
+Either 'client-id' or 'app-id' input must be set
+`;
+
+exports[`main-missing-client-and-app-id.test.js > stdout 1`] = `
+::error::Either 'client-id' or 'app-id' input must be set
+`;
+
 exports[`main-missing-owner.test.js > stderr 1`] = `
 GITHUB_REPOSITORY_OWNER missing, must be set to '<owner>'
 `;
diff --git a/tests/main-app-id-fallback.test.js b/tests/main-app-id-fallback.test.js
new file mode 100644
index 0000000..21c6852
--- /dev/null
+++ b/tests/main-app-id-fallback.test.js
@@ -0,0 +1,11 @@
+import { DEFAULT_ENV, test } from "./main.js";
+
+// Verify `main` falls back to `app-id` when `client-id` is not set
+await test(
+  () => {},
+  {
+    ...DEFAULT_ENV,
+    "INPUT_CLIENT-ID": "",
+    "INPUT_APP-ID": "123456",
+  }
+);
diff --git a/tests/main-client-id-precedence.test.js b/tests/main-client-id-precedence.test.js
new file mode 100644
index 0000000..f3cf78a
--- /dev/null
+++ b/tests/main-client-id-precedence.test.js
@@ -0,0 +1,11 @@
+import { DEFAULT_ENV, test } from "./main.js";
+
+// Verify `client-id` takes precedence when both `client-id` and `app-id` are set
+await test(
+  () => {},
+  {
+    ...DEFAULT_ENV,
+    "INPUT_CLIENT-ID": "Iv1.0123456789abcdef",
+    "INPUT_APP-ID": "123456",
+  }
+);
diff --git a/tests/main-missing-client-and-app-id.test.js b/tests/main-missing-client-and-app-id.test.js
new file mode 100644
index 0000000..06e2b35
--- /dev/null
+++ b/tests/main-missing-client-and-app-id.test.js
@@ -0,0 +1,20 @@
+import { DEFAULT_ENV } from "./main.js";
+
+for (const [key, value] of Object.entries({
+  ...DEFAULT_ENV,
+  "INPUT_CLIENT-ID": "",
+  "INPUT_APP-ID": "",
+})) {
+  process.env[key] = value;
+}
+
+// Log only the error message, not the full stack trace, because the stack
+// trace contains environment-specific paths and ANSI codes that differ
+// between local and CI environments.
+const _error = console.error;
+console.error = (err) => _error(err?.message ?? err);
+
+// Verify `main` exits with an error when neither `client-id` nor `app-id` is set.
+const { default: promise } = await import("../main.js");
+await promise;
+process.exitCode = 0;
diff --git a/tests/main.js b/tests/main.js
index 5466529..a76c39a 100644
--- a/tests/main.js
+++ b/tests/main.js
@@ -9,7 +9,7 @@ export const DEFAULT_ENV = {
   // https://docs.github.com/actions/creating-actions/metadata-syntax-for-github-actions#example-specifying-inputs
   "INPUT_GITHUB-API-URL": "https://api.github.com",
   "INPUT_SKIP-TOKEN-REVOKE": "false",
-  "INPUT_APP-ID": "123456",
+  "INPUT_CLIENT-ID": "Iv1.0123456789abcdef",
   // This key is invalidated. It’s from https://github.com/octokit/auth-app.js/issues/465#issuecomment-1564998327.
   "INPUT_PRIVATE-KEY": `-----BEGIN RSA PRIVATE KEY-----
 MIIEowIBAAKCAQEA280nfuUM9w00Ib9E2rvZJ6Qu3Ua3IqR34ZlK53vn/Iobn2EL