diff --git a/lib/core/proxy-headers-agent.js b/lib/core/proxy-headers-agent.js
index e3623d0..5662c3c 100644
--- a/lib/core/proxy-headers-agent.js
+++ b/lib/core/proxy-headers-agent.js
@@ -94,7 +94,6 @@ export class ProxyHeadersAgent extends Agent {
                 this.proxyAuth,
                 this.proxyHeaders
             );
-
             proxySocket.write(connectRequest);
         });
 
@@ -153,7 +152,8 @@ export class ProxyHeadersAgent extends Agent {
             });
         });
 
-        return proxySocket;
+        // Do not return proxySocket - the agent must use the stream from the callback
+        // (tlsSocket) so the CONNECT handshake completes before any request is written.
     }
 }
 
diff --git a/lib/core/utils.js b/lib/core/utils.js
index 4e44c51..01e714b 100644
--- a/lib/core/utils.js
+++ b/lib/core/utils.js
@@ -56,7 +56,10 @@ export function buildConnectRequest(targetHost, targetPort, proxyAuth, proxyHead
         lines.push(`Proxy-Authorization: Basic ${proxyAuth}`);
     }
     
-    for (const [key, value] of Object.entries(proxyHeaders)) {
+    const entries = proxyHeaders instanceof Map
+        ? [...proxyHeaders.entries()]
+        : Object.entries(proxyHeaders || {});
+    for (const [key, value] of entries) {
         lines.push(`${key}: ${value}`);
     }
     
diff --git a/package.json b/package.json
index b1111b4..d3b7e46 100644
--- a/package.json
+++ b/package.json
@@ -39,7 +39,7 @@
     "LICENSE"
   ],
   "scripts": {
-    "test": "node test/test_proxy_headers.js",
+    "test": "node test/test_proxy_headers.js core axios node-fetch got undici superagent",
     "test:verbose": "node test/test_proxy_headers.js -v",
     "lint": "eslint lib test",
     "prepublishOnly": "npm test"
diff --git a/test/test_proxy_headers.js b/test/test_proxy_headers.js
index 43420af..bf1070f 100644
--- a/test/test_proxy_headers.js
+++ b/test/test_proxy_headers.js
@@ -128,6 +128,20 @@ function checkHeader(headers, headerName) {
     return null;
 }
 
+/**
+ * When SEND_PROXY_HEADER and SEND_PROXY_VALUE are set and we're checking the same header,
+ * the proxy response must echo that value (e.g. X-ProxyMesh-IP).
+ * @returns {string|null} Error message if expectation not met, null if OK or N/A.
+ */
+function validateSentHeaderValue(config, headerValue) {
+    if (!config.sendProxyHeader || !config.sendProxyValue) return null;
+    if (config.proxyHeader.toLowerCase() !== config.sendProxyHeader.toLowerCase()) return null;
+    const expected = String(config.sendProxyValue).trim();
+    const actual = headerValue ? String(headerValue).trim() : '';
+    if (actual === expected) return null;
+    return `Expected ${config.proxyHeader} to equal ${expected} (sent in request) but got ${actual}`;
+}
+
 const AVAILABLE_TESTS = {
     async core(config) {
         try {
@@ -159,7 +173,10 @@ const AVAILABLE_TESTS = {
                             ? checkHeader(capturedHeaders, config.proxyHeader)
                             : null;
 
-                        if (headerValue) {
+                        const sentErr = validateSentHeaderValue(config, headerValue);
+                        if (sentErr) {
+                            resolve(new TestResult('core', false, null, sentErr, res.statusCode));
+                        } else if (headerValue) {
                             resolve(new TestResult('core', true, headerValue, null, res.statusCode));
                         } else {
                             resolve(new TestResult('core', false, null,
@@ -189,9 +206,13 @@ const AVAILABLE_TESTS = {
                 proxyHeaders: config.proxyHeadersToSend,
             });
 
-            const response = await client.get(config.testUrl);
+            const response = await client.get(config.testUrl, {
+                validateStatus: () => true,
+            });
             const headerValue = checkHeader(response.headers, config.proxyHeader);
 
+            const sentErr = validateSentHeaderValue(config, headerValue);
+            if (sentErr) return new TestResult('axios', false, null, sentErr);
             if (headerValue) {
                 return new TestResult('axios', true, headerValue, null, response.status);
             }
@@ -214,6 +235,8 @@ const AVAILABLE_TESTS = {
 
             const headerValue = checkHeader(response.proxyHeaders, config.proxyHeader);
 
+            const sentErr = validateSentHeaderValue(config, headerValue);
+            if (sentErr) return new TestResult('node-fetch', false, null, sentErr);
             if (headerValue) {
                 return new TestResult('node-fetch', true, headerValue, null, response.status);
             }
@@ -232,11 +255,14 @@ const AVAILABLE_TESTS = {
             const client = await createProxyGot({
                 proxy: config.proxyUrl,
                 proxyHeaders: config.proxyHeadersToSend,
+                gotOptions: { throwHttpErrors: false },
             });
 
             const response = await client(config.testUrl);
             const headerValue = checkHeader(response.headers, config.proxyHeader);
 
+            const sentErr = validateSentHeaderValue(config, headerValue);
+            if (sentErr) return new TestResult('got', false, null, sentErr);
             if (headerValue) {
                 return new TestResult('got', true, headerValue, null, response.statusCode);
             }
@@ -259,6 +285,8 @@ const AVAILABLE_TESTS = {
 
             const headerValue = checkHeader(proxyHeaders, config.proxyHeader);
 
+            const sentErr = validateSentHeaderValue(config, headerValue);
+            if (sentErr) return new TestResult('undici', false, null, sentErr);
             if (headerValue) {
                 return new TestResult('undici', true, headerValue, null, statusCode);
             }
@@ -279,9 +307,11 @@ const AVAILABLE_TESTS = {
                 proxyHeaders: config.proxyHeadersToSend,
             });
 
-            const response = await client.get(config.testUrl);
+            const response = await client.get(config.testUrl).ok(() => true);
             const headerValue = checkHeader(response.headers, config.proxyHeader);
 
+            const sentErr = validateSentHeaderValue(config, headerValue);
+            if (sentErr) return new TestResult('superagent', false, null, sentErr);
             if (headerValue) {
                 return new TestResult('superagent', true, headerValue, null, response.status);
             }