diff --git a/lib/_http_agent.js b/lib/_http_agent.js
index 3871c5b56a2..daaee3d8fdd 100644
--- a/lib/_http_agent.js
+++ b/lib/_http_agent.js
@@ -654,7 +654,7 @@ Agent.prototype.keepSocketAlive = function keepSocketAlive(socket) {
           // Cannot safely reuse the socket because the server timeout is
           // too short
           canKeepSocketAlive = false;
-        } else if (serverHintTimeout < agentTimeout) {
+        } else if (agentTimeout === 0 || serverHintTimeout < agentTimeout) {
           agentTimeout = serverHintTimeout;
         }
       }
diff --git a/test/parallel/test-http-agent-keep-alive-server-hint.js b/test/parallel/test-http-agent-keep-alive-server-hint.js
new file mode 100644
index 00000000000..0394d5d60bf
--- /dev/null
+++ b/test/parallel/test-http-agent-keep-alive-server-hint.js
@@ -0,0 +1,162 @@
+'use strict';
+
+// Regression test for https://github.com/nodejs/node/issues/66734
+//
+// When http.Agent is created with keepAlive: true but no explicit timeout,
+// the server's Keep-Alive: timeout=N hint must be applied to the idle socket.
+// Previously, agentTimeout defaulted to 0 and the condition
+// `serverHintTimeout < agentTimeout` was never true, so setTimeout(0) was
+// called, disabling the timer and leaving stale sockets in freeSockets.
+
+const common = require('../common');
+const assert = require('assert');
+const http = require('http');
+
+// Helper: returns the first idle socket for the agent, after the 'free'
+// event has been processed (one setImmediate tick after response end).
+function getIdleSocket(agent) {
+  const key = Object.keys(agent.freeSockets)[0];
+  return key ? agent.freeSockets[key][0] : undefined;
+}
+
+// Case 1: Agent has no explicit timeout.
+// Server sends Keep-Alive: timeout=5.
+// Expected: socket.timeout === 5000 - 1000 (default buffer) === 4000.
+{
+  const SERVER_HINT_S = 5;
+  const EXPECTED_TIMEOUT = (SERVER_HINT_S * 1000) - 1000; // 4000ms
+
+  const server = http.createServer(common.mustCall((req, res) => {
+    res.writeHead(200, { 'Keep-Alive': `timeout=${SERVER_HINT_S}` });
+    res.end('ok');
+  }));
+
+  server.listen(0, common.mustCall(() => {
+    const agent = new http.Agent({ keepAlive: true });
+
+    http.get({ port: server.address().port, agent }, common.mustCall((res) => {
+      res.resume();
+      res.on('end', common.mustCall(() => {
+        setImmediate(common.mustCall(() => {
+          const socket = getIdleSocket(agent);
+          assert.ok(socket, 'socket should be in freeSockets');
+          assert.strictEqual(
+            socket.timeout,
+            EXPECTED_TIMEOUT,
+            `Case 1: expected socket.timeout ${EXPECTED_TIMEOUT}, ` +
+            `got ${socket.timeout}`
+          );
+          agent.destroy();
+          server.close();
+        }));
+      }));
+    }));
+  }));
+}
+
+// Case 2: Agent has an explicit timeout longer than the server hint.
+// Server sends Keep-Alive: timeout=5 (4000ms after buffer).
+// Agent timeout is 10000ms.
+// Expected: server hint wins → socket.timeout === 4000.
+{
+  const SERVER_HINT_S = 5;
+  const AGENT_TIMEOUT = 10000;
+  const EXPECTED_TIMEOUT = (SERVER_HINT_S * 1000) - 1000; // 4000ms
+
+  const server = http.createServer(common.mustCall((req, res) => {
+    res.writeHead(200, { 'Keep-Alive': `timeout=${SERVER_HINT_S}` });
+    res.end('ok');
+  }));
+
+  server.listen(0, common.mustCall(() => {
+    const agent = new http.Agent({ keepAlive: true, timeout: AGENT_TIMEOUT });
+
+    http.get({ port: server.address().port, agent }, common.mustCall((res) => {
+      res.resume();
+      res.on('end', common.mustCall(() => {
+        setImmediate(common.mustCall(() => {
+          const socket = getIdleSocket(agent);
+          assert.ok(socket, 'socket should be in freeSockets');
+          assert.strictEqual(
+            socket.timeout,
+            EXPECTED_TIMEOUT,
+            `Case 2: expected socket.timeout ${EXPECTED_TIMEOUT}, ` +
+            `got ${socket.timeout}`
+          );
+          agent.destroy();
+          server.close();
+        }));
+      }));
+    }));
+  }));
+}
+
+// Case 3: Agent has an explicit timeout shorter than the server hint.
+// Server sends Keep-Alive: timeout=10 (9000ms after buffer).
+// Agent timeout is 3000ms.
+// Expected: agent timeout wins → socket.timeout === 3000.
+{
+  const SERVER_HINT_S = 10;
+  const AGENT_TIMEOUT = 3000;
+  const EXPECTED_TIMEOUT = AGENT_TIMEOUT; // 3000ms
+
+  const server = http.createServer(common.mustCall((req, res) => {
+    res.writeHead(200, { 'Keep-Alive': `timeout=${SERVER_HINT_S}` });
+    res.end('ok');
+  }));
+
+  server.listen(0, common.mustCall(() => {
+    const agent = new http.Agent({ keepAlive: true, timeout: AGENT_TIMEOUT });
+
+    http.get({ port: server.address().port, agent }, common.mustCall((res) => {
+      res.resume();
+      res.on('end', common.mustCall(() => {
+        setImmediate(common.mustCall(() => {
+          const socket = getIdleSocket(agent);
+          assert.ok(socket, 'socket should be in freeSockets');
+          assert.strictEqual(
+            socket.timeout,
+            EXPECTED_TIMEOUT,
+            `Case 3: expected socket.timeout ${EXPECTED_TIMEOUT}, ` +
+            `got ${socket.timeout}`
+          );
+          agent.destroy();
+          server.close();
+        }));
+      }));
+    }));
+  }));
+}
+
+// Case 4: Server sends no Keep-Alive hint (keepAliveTimeout: 0 suppresses the
+// automatic header), no agent timeout.
+// Expected: socket.timeout === 0 (no timer set — existing behaviour unchanged).
+{
+  const server = http.createServer(
+    { keepAliveTimeout: 0 },
+    common.mustCall((req, res) => {
+      res.end('ok');
+    })
+  );
+
+  server.listen(0, common.mustCall(() => {
+    const agent = new http.Agent({ keepAlive: true });
+
+    http.get({ port: server.address().port, agent }, common.mustCall((res) => {
+      res.resume();
+      res.on('end', common.mustCall(() => {
+        setImmediate(common.mustCall(() => {
+          const socket = getIdleSocket(agent);
+          assert.ok(socket, 'socket should be in freeSockets');
+          assert.strictEqual(
+            socket.timeout,
+            0,
+            `Case 4: expected socket.timeout 0, got ${socket.timeout}`
+          );
+          agent.destroy();
+          server.close();
+        }));
+      }));
+    }));
+  }));
+}