diff --git a/dist/main.cjs b/dist/main.cjs
index c7cf562..b9dc621 100644
--- a/dist/main.cjs
+++ b/dist/main.cjs
@@ -23352,7 +23352,7 @@ async function run() {
   ensureNativeProxySupport();
   const clientId = getInput("client-id") || getInput("app-id");
   if (!clientId) {
-    throw new Error("Either 'client-id' or 'app-id' input must be set");
+    throw new Error("The 'client-id' (or deprecated 'app-id') input must be set to a non-empty string. If using a secret or variable, ensure it is available in this workflow context.");
   }
   const privateKey = getInput("private-key");
   const owner = getInput("owner");
diff --git a/main.js b/main.js
index b685126..32696dd 100644
--- a/main.js
+++ b/main.js
@@ -20,7 +20,7 @@ async function run() {
 
   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");
+    throw new Error("The 'client-id' (or deprecated 'app-id') input must be set to a non-empty string. If using a secret or variable, ensure it is available in this workflow context.");
   }
   const privateKey = core.getInput("private-key");
   const owner = core.getInput("owner");
diff --git a/tests/index.js.snapshot b/tests/index.js.snapshot
index d5d5f9b..4789f44 100644
--- a/tests/index.js.snapshot
+++ b/tests/index.js.snapshot
@@ -56,11 +56,11 @@ POST /api/v3/app/installations/123456/access_tokens
 `;
 
 exports[`main-missing-client-and-app-id.test.js > stderr 1`] = `
-Either 'client-id' or 'app-id' input must be set
+The 'client-id' (or deprecated 'app-id') input must be set to a non-empty string. If using a secret or variable, ensure it is available in this workflow context.
 `;
 
 exports[`main-missing-client-and-app-id.test.js > stdout 1`] = `
-::error::Either 'client-id' or 'app-id' input must be set
+::error::The 'client-id' (or deprecated 'app-id') input must be set to a non-empty string. If using a secret or variable, ensure it is available in this workflow context.
 `;
 
 exports[`main-missing-owner.test.js > stderr 1`] = `