diff --git a/ai/custom-func-ai-agent/AiVertex.js b/ai/custom-func-ai-agent/AiVertex.js
new file mode 100644
index 000000000..aa58d0836
--- /dev/null
+++ b/ai/custom-func-ai-agent/AiVertex.js
@@ -0,0 +1,111 @@
+/*
+Copyright 2025 Google LLC
+
+Licensed under the Apache License, Version 2.0 (the "License");
+you may not use this file except in compliance with the License.
+You may obtain a copy of the License at
+
+    https://www.apache.org/licenses/LICENSE-2.0
+
+Unless required by applicable law or agreed to in writing, software
+distributed under the License is distributed on an "AS IS" BASIS,
+WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+See the License for the specific language governing permissions and
+limitations under the License.
+*/
+
+const LOCATION = PropertiesService.getScriptProperties().getProperty('LOCATION');
+const GEMINI_MODEL_ID = PropertiesService.getScriptProperties().getProperty('GEMINI_MODEL_ID');
+const REASONING_ENGINE_ID = PropertiesService.getScriptProperties().getProperty('REASONING_ENGINE_ID');
+const SERVICE_ACCOUNT_KEY = PropertiesService.getScriptProperties().getProperty('SERVICE_ACCOUNT_KEY');
+
+const credentials = credentialsForVertexAI();
+
+/**
+ * @param {string} statement The statement to fact-check.
+ */
+function requestLlmAuditorAdkAiAgent(statement) {
+  return UrlFetchApp.fetch(
+    `https://${LOCATION}-aiplatform.googleapis.com/v1/projects/${credentials.projectId}/locations/${LOCATION}/reasoningEngines/${REASONING_ENGINE_ID}:streamQuery?alt=sse`,
+    {
+      method: 'post',
+      headers: { 'Authorization': `Bearer ${credentials.accessToken}` },
+      contentType: 'application/json',
+      muteHttpExceptions: true,
+      payload: JSON.stringify({
+        "class_method": "async_stream_query",
+        "input": {
+          "user_id": "google_sheets_custom_function_fact_check",
+          "message": statement,
+        }
+      })
+    }
+  ).getContentText();
+}
+
+/**
+ * @param {string} prompt The Gemini prompt to use.
+ */
+function requestOutputFormatting(prompt) {
+  const response = UrlFetchApp.fetch(
+    `https://${LOCATION}-aiplatform.googleapis.com/v1/projects/${credentials.projectId}/locations/${LOCATION}/publishers/google/models/${GEMINI_MODEL_ID}:generateContent`,
+    {
+      method: 'post',
+      headers: { 'Authorization': `Bearer ${credentials.accessToken}` },
+      contentType: 'application/json',
+      muteHttpExceptions: true,
+      payload: JSON.stringify({
+        "contents": [{
+          "role": "user",
+          "parts": [{ "text": prompt }]
+        }],
+        "generationConfig": { "temperature": 0.1, "maxOutputTokens": 2048 },
+        "safetySettings": [
+          {
+            "category": "HARM_CATEGORY_HARASSMENT",
+            "threshold": "BLOCK_NONE"
+          },
+          {
+            "category": "HARM_CATEGORY_HATE_SPEECH",
+            "threshold": "BLOCK_NONE"
+          },
+          {
+            "category": "HARM_CATEGORY_SEXUALLY_EXPLICIT",
+            "threshold": "BLOCK_NONE"
+          },
+          {
+            "category": "HARM_CATEGORY_DANGEROUS_CONTENT",
+            "threshold": "BLOCK_NONE"
+          }
+        ]
+      })
+    }
+  );
+  return JSON.parse(response).candidates[0].content.parts[0].text
+}
+
+/**
+ * Gets credentials required to call Vertex API using a Service Account.
+ * Requires use of Service Account Key stored with project.
+ * 
+ * @return {!Object} Containing the Google Cloud project ID and the access token.
+ */
+function credentialsForVertexAI() {
+  const credentials = SERVICE_ACCOUNT_KEY;
+  if (!credentials) {
+    throw new Error("service_account_key script property must be set.");
+  }
+
+  const parsedCredentials = JSON.parse(credentials);
+
+  const service = OAuth2.createService("Vertex")
+    .setTokenUrl('https://oauth2.googleapis.com/token')
+    .setPrivateKey(parsedCredentials['private_key'])
+    .setIssuer(parsedCredentials['client_email'])
+    .setPropertyStore(PropertiesService.getScriptProperties())
+    .setScope("https://www.googleapis.com/auth/cloud-platform");
+  return {
+    projectId: parsedCredentials['project_id'],
+    accessToken: service.getAccessToken(),
+  }
+}
diff --git a/ai/custom-func-ai-agent/Code.js b/ai/custom-func-ai-agent/Code.js
new file mode 100644
index 000000000..d18674f6a
--- /dev/null
+++ b/ai/custom-func-ai-agent/Code.js
@@ -0,0 +1,36 @@
+/*
+Copyright 2025 Google LLC
+
+Licensed under the Apache License, Version 2.0 (the "License");
+you may not use this file except in compliance with the License.
+You may obtain a copy of the License at
+
+    https://www.apache.org/licenses/LICENSE-2.0
+
+Unless required by applicable law or agreed to in writing, software
+distributed under the License is distributed on an "AS IS" BASIS,
+WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+See the License for the specific language governing permissions and
+limitations under the License.
+*/
+
+/**
+ * Passes a statement to fact-check and, optionally, output formatting instructions.
+ * 
+ * @param {string} statement The statement to fact-check as a string or single cell
+ *   reference (data ranges are not supported).
+ * @param {string} outputFormat The instructions as a string or single cell reference
+ *   (data ranges are not supported).
+ * 
+ * @return The generated and formatted verdict
+ * @customfunction
+ */
+function FACT_CHECK(statement, outputFormat) {
+  if (!outputFormat || outputFormat == "") {
+    outputFormat = 'Summarize it. Only keep the verdict result and main arguments. '
+      + 'Do not reiterate the fact being checked. Remove all markdown. '
+      + 'State the verdit result in a first paragraph in a few words and the rest of the summary in a second paragraph.';
+  }
+
+  return requestOutputFormatting(`Here is a fact checking result: ${requestLlmAuditorAdkAiAgent(statement)}.\n\n${outputFormat}`);
+}
diff --git a/ai/custom-func-ai-agent/README.md b/ai/custom-func-ai-agent/README.md
new file mode 100644
index 000000000..43fe32494
--- /dev/null
+++ b/ai/custom-func-ai-agent/README.md
@@ -0,0 +1,38 @@
+# Google Sheets Custom Function relying on ADK AI Agent and Gemini model
+
+A [Vertex AI](https://cloud.google.com/vertex-ai) agent-powered **fact checker** custom function for Google Sheets to be used as a bound Apps Script project.
+
+![](./images/showcase.png)
+
+## Overview
+
+The **Google Sheets custom function** named `FACT_CHECK` integrates the sophisticated, multi-tool, multi-step reasoning capabilities of a **Vertex AI Agent Engine (ADK Agent)** directly into your Google Sheets spreadsheets.
+
+It operates as an end-to-end solution. It analyzes a statement, grounds its response using the latest web information, and returns the result in the format you need:
+
+  * Usage: `=FACT_CHECK("Your statement here")` for a concise and summarized output. `=FACT_CHECK("Your statement here", "Your output formatting instructions here")` for a specific output format.
+  * Reasoning: [**LLM Auditor ADK AI Agent (Python sample)**](https://github.com/google/adk-samples/tree/main/python/agents/llm-auditor).
+  * Output formatting: [**Gemini model**](https://cloud.google.com/vertex-ai/generative-ai/docs/models).
+
+## Prerequisites
+
+* Google Cloud Project with billing enabled.
+
+## Set up your environment
+
+1. Configure the Google Cloud project
+   1. Enable the Vertex AI API
+   1. Create a Service Account and grant the role `Vertex AI User`
+   1. Create a private key with type JSON. This will download the JSON file.
+1. Setup, install, and deploy the LLM Auditor ADK AI Agent sample
+   1. Use Vertex AI
+   1. Use the same Google Cloud project
+   1. Use the location `us-central1`
+   1. Use the Vertex AI Agent Engine
+1. Open an Apps Script project bound to a Google Sheets spreadsheet
+   1. Add a Script Property. Enter `LOCATION` as the property name and `us-central1` as the value. 
+   1. Add a Script Property. Enter `GEMINI_MODEL_ID` as the property name and `gemini-2.5-flash-lite` as the value. 
+   1. Add a Script Property. Enter `REASONING_ENGINE_ID` as the property name and the ID of the deployed LLM Auditor ADK AI Agent as the value. 
+   1. Add a Script Property. Enter `SERVICE_ACCOUNT_KEY` as the property name and paste the JSON key from the service account as the value. 
+   1. Add OAuth2 v43 Apps Script Library using the ID `1B7FSrk5Zi6L1rSxxTDgDEUsPzlukDsi4KGuTMorsTQHhGBzBkMun4iDF`
+   1. Set the script files `Code.gs` and `AiVertex.gs` in the Apps Script project with the JS file contents in this project
diff --git a/ai/custom-func-ai-agent/appsscript.json b/ai/custom-func-ai-agent/appsscript.json
new file mode 100644
index 000000000..d1a41c7c1
--- /dev/null
+++ b/ai/custom-func-ai-agent/appsscript.json
@@ -0,0 +1,12 @@
+{
+  "timeZone": "America/Los_Angeles",
+  "dependencies": {
+    "libraries": [{
+      "userSymbol": "OAuth2",
+      "libraryId": "1B7FSrk5Zi6L1rSxxTDgDEUsPzlukDsi4KGuTMorsTQHhGBzBkMun4iDF",
+      "version": "43"
+    }]
+  },
+  "exceptionLogging": "STACKDRIVER",
+  "runtimeVersion": "V8"
+}
\ No newline at end of file
diff --git a/ai/custom-func-ai-agent/images/showcase.png b/ai/custom-func-ai-agent/images/showcase.png
new file mode 100644
index 000000000..4b0314f15
Binary files /dev/null and b/ai/custom-func-ai-agent/images/showcase.png differ