diff --git a/rust/py_test/test_launch_server.py b/rust/py_test/test_launch_server.py
index daa0b821ed6..2528a72e1c5 100644
--- a/rust/py_test/test_launch_server.py
+++ b/rust/py_test/test_launch_server.py
@@ -1,3 +1,4 @@
+import json
 import socket
 import subprocess
 import time
@@ -21,6 +22,7 @@ def popen_launch_router(
     dp_size: int,
     timeout: float,
     policy: str = "cache_aware",
+    max_payload_size: int = None,
 ):
     """
     Launch the router server process.
@@ -31,6 +33,7 @@ def popen_launch_router(
         dp_size: Data parallel size
         timeout: Server launch timeout
         policy: Router policy, one of "cache_aware", "round_robin", "random"
+        max_payload_size: Maximum payload size in bytes
     """
     _, host, port = base_url.split(":")
     host = host[2:]
@@ -53,6 +56,9 @@ def popen_launch_router(
         policy,
     ]
 
+    if max_payload_size is not None:
+        command.extend(["--router-max-payload-size", str(max_payload_size)])
+
     process = subprocess.Popen(command, stdout=None, stderr=None)
 
     start_time = time.time()
@@ -280,6 +286,79 @@ def kill_worker():
         msg = f"MMLU test {'passed' if passed else 'failed'} with score {score:.3f} (threshold: {THRESHOLD})"
         self.assertGreaterEqual(score, THRESHOLD, msg)
 
+    def test_4_payload_size(self):
+        print("Running test_4_payload_size...")
+        # Start router with default 4MB limit
+        self.process = popen_launch_router(
+            self.model,
+            self.base_url,
+            dp_size=1,
+            timeout=DEFAULT_TIMEOUT_FOR_SERVER_LAUNCH,
+            policy="round_robin",
+        )
+
+        # Test case 1: Payload just under 4MB should succeed
+        payload_3mb = {
+            "text": "x" * (3 * 1024 * 1024),  # 3MB of text
+            "temperature": 0.0,
+        }
+
+        with requests.Session() as session:
+            response = session.post(
+                f"{self.base_url}/generate",
+                json=payload_3mb,
+                headers={"Content-Type": "application/json"},
+            )
+            self.assertEqual(
+                response.status_code,
+                200,
+                f"3MB payload should succeed but got status {response.status_code}",
+            )
+
+        # Test case 2: Payload over 4MB should fail
+        payload_5mb = {
+            "text": "x" * (5 * 1024 * 1024),  # 5MB of text
+            "temperature": 0.0,
+        }
+
+        with requests.Session() as session:
+            response = session.post(
+                f"{self.base_url}/generate",
+                json=payload_5mb,
+                headers={"Content-Type": "application/json"},
+            )
+            self.assertEqual(
+                response.status_code,
+                413,  # Payload Too Large
+                f"5MB payload should fail with 413 but got status {response.status_code}",
+            )
+
+        # Test case 3: Start router with custom 8MB limit
+        if self.process:
+            terminate_and_wait(self.process)
+
+        self.process = popen_launch_router(
+            self.model,
+            self.base_url,
+            dp_size=1,
+            timeout=DEFAULT_TIMEOUT_FOR_SERVER_LAUNCH,
+            policy="round_robin",
+            max_payload_size=8 * 1024 * 1024,  # 8MB limit
+        )
+
+        # Now 5MB payload should succeed with 8MB limit
+        with requests.Session() as session:
+            response = session.post(
+                f"{self.base_url}/generate",
+                json=payload_5mb,
+                headers={"Content-Type": "application/json"},
+            )
+            self.assertEqual(
+                response.status_code,
+                200,
+                f"5MB payload should succeed with 8MB limit but got status {response.status_code}",
+            )
+
 
 if __name__ == "__main__":
     unittest.main()