diff --git a/src/main/resources/mock-idp/idp.js b/src/main/resources/mock-idp/idp.js
index 1501495..23b908b 100644
--- a/src/main/resources/mock-idp/idp.js
+++ b/src/main/resources/mock-idp/idp.js
@@ -7,6 +7,7 @@ const path = require('path');
 const bodyParser = require('body-parser');
 const zlib = require('zlib');
 const xml2js = require('xml2js');
+const xpath = require('xpath');
 
 const app = express();
 const port = 3000;
@@ -34,12 +35,15 @@ app.get('/saml/sso', async (req, res) => {
   storedSamlRequest = samlRequest;
   storedRelayState = relayState;
 
+  const authnRequestId = await extractAuthnRequestId(storedSamlRequest);
+  console.log('SAML2 AuthnRequestId:', authnRequestId); 
+
   // Show simple login form
   res.send(createLoginForm());
 });
 
 // Step 2: Handle login form and send SAMLResponse
-app.post('/saml/login', (req, res) => {
+app.post('/saml/login', async (req, res) => {
   const { username, email, orgCode, securityGroup, roles } = req.body;
   if (!storedSamlRequest) {
     return res.status(400).send('No SAMLRequest stored');
@@ -60,7 +64,8 @@ app.post('/saml/login', (req, res) => {
   console.log('ServiceProvider URL :', serviceProviderUrl); 
   console.log('ServiceProvider ACS :', acsUrl); 
 
-  const samlResponse = createFakeSamlResponse(username, email, orgCode, securityGroup, roles, serviceProviderUrl, issuerUrl);
+  const inResponseTo = await extractAuthnRequestId(storedSamlRequest);
+  const samlResponse = createFakeSamlResponse(username, email, orgCode, securityGroup, roles, serviceProviderUrl, issuerUrl, inResponseTo);
   const relayState = storedRelayState;
 
   res.send(`
@@ -75,7 +80,41 @@ app.post('/saml/login', (req, res) => {
   `);
 });
 
-function createFakeSamlResponse(nameId, email, orgCode, securityGroupCode, roles, serviceProviderUrl, issuerUrl) {
+/**
+ * Extracts the AuthnRequest ID from a base64-encoded SAMLRequest.
+ * @param {string} samlRequest - Base64-encoded SAMLRequest parameter from the URL.
+ * @returns {Promise<string|null>} - Promise resolving to the AuthnRequest ID, or null if not found.
+ */
+function extractAuthnRequestId(samlRequest) {
+  return new Promise((resolve, reject) => {
+    try {
+      const decoded = Buffer.from(samlRequest, 'base64');
+
+      zlib.inflateRaw(decoded, (err, inflated) => {
+        if (err) return reject(new Error('Failed to inflate SAMLRequest: ' + err.message));
+
+        const xml = inflated.toString('utf8');
+        const doc = new DOMParser().parseFromString(xml, 'text/xml');
+
+        // Define namespace mappings for XPath
+        const select = xpath.useNamespaces({
+          saml2p: 'urn:oasis:names:tc:SAML:2.0:protocol',
+        });
+
+        const node = select('//saml2p:AuthnRequest', doc)[0];
+        if (node) {
+          resolve(node.getAttribute('ID'));
+        } else {
+          resolve(null);
+        }
+      });
+    } catch (error) {
+      reject(error);
+    }
+  });
+}
+
+function createFakeSamlResponse(nameId, email, orgCode, securityGroupCode, roles, serviceProviderUrl, issuerUrl, inResponseTo) {
   const issuer = issuerUrl;
   // const audience = "https://localhost:8443/saml2/service-provider-metadata/mock-idp";
   const audience = serviceProviderUrl + "/saml2/service-provider-metadata/mock-idp";
@@ -152,7 +191,7 @@ function createFakeSamlResponse(nameId, email, orgCode, securityGroupCode, roles
   });
 
   const signedAssertion = sig.getSignedXml();
-  const samlResponse = `<?xml version="1.0" encoding="UTF-8"?><samlp:Response xmlns:samlp="urn:oasis:names:tc:SAML:2.0:protocol" xmlns:saml="urn:oasis:names:tc:SAML:2.0:assertion" ID="${responseId}" Version="2.0" IssueInstant="${now}" Destination="${destination}"><saml:Issuer>${issuer}</saml:Issuer><samlp:Status><samlp:StatusCode Value="urn:oasis:names:tc:SAML:2.0:status:Success"/></samlp:Status>${signedAssertion}</samlp:Response>`;
+  const samlResponse = `<?xml version="1.0" encoding="UTF-8"?><samlp:Response xmlns:samlp="urn:oasis:names:tc:SAML:2.0:protocol" xmlns:saml="urn:oasis:names:tc:SAML:2.0:assertion" ID="${responseId}" Version="2.0" IssueInstant="${now}" Destination="${destination}" InResponseTo="${inResponseTo}"><saml:Issuer>${issuer}</saml:Issuer><samlp:Status><samlp:StatusCode Value="urn:oasis:names:tc:SAML:2.0:status:Success"/></samlp:Status>${signedAssertion}</samlp:Response>`;
 
   const samlResponseToUse = samlResponse.trim().replace(/^\uFEFF/, '');
   return Buffer.from(samlResponseToUse, 'utf-8').toString('base64');