diff --git a/jupyter_server/gateway/gateway_client.py b/jupyter_server/gateway/gateway_client.py
index 43373fd7e4..53b46fc3a2 100644
--- a/jupyter_server/gateway/gateway_client.py
+++ b/jupyter_server/gateway/gateway_client.py
@@ -416,7 +416,14 @@ def load_connection_args(self, **kwargs):
         if len(self._static_args) == 0:
             self.init_static_args()
 
-        kwargs.update(self._static_args)
+        for arg, static_value in self._static_args.items():
+            if arg == "headers":
+                given_value = kwargs.setdefault(arg, {})
+                if isinstance(given_value, dict):
+                    given_value.update(static_value)
+            else:
+                kwargs[arg] = static_value
+
         return kwargs
 
 
diff --git a/jupyter_server/gateway/managers.py b/jupyter_server/gateway/managers.py
index 48e43a92df..33c079e348 100644
--- a/jupyter_server/gateway/managers.py
+++ b/jupyter_server/gateway/managers.py
@@ -436,7 +436,12 @@ async def start_kernel(self, **kwargs):
 
             json_body = json_encode({"name": kernel_name, "env": kernel_env})
 
-            response = await gateway_request(self.kernels_url, method="POST", body=json_body)
+            response = await gateway_request(
+                self.kernels_url,
+                method="POST",
+                headers={"Content-Type": "application/json"},
+                body=json_body,
+            )
             self.kernel = json_decode(response.body)
             self.kernel_id = self.kernel["id"]
             self.kernel_url = url_path_join(self.kernels_url, url_escape(str(self.kernel_id)))
@@ -467,7 +472,12 @@ async def restart_kernel(self, **kw):
             assert self.kernel_url is not None
             kernel_url = self.kernel_url + "/restart"
             self.log.debug("Request restart kernel at: %s", kernel_url)
-            response = await gateway_request(kernel_url, method="POST", body=json_encode({}))
+            response = await gateway_request(
+                kernel_url,
+                method="POST",
+                headers={"Content-Type": "application/json"},
+                body=json_encode({}),
+            )
             self.log.debug("Restart kernel response: %d %s", response.code, response.reason)
 
     async def interrupt_kernel(self):
@@ -476,7 +486,12 @@ async def interrupt_kernel(self):
             assert self.kernel_url is not None
             kernel_url = self.kernel_url + "/interrupt"
             self.log.debug("Request interrupt kernel at: %s", kernel_url)
-            response = await gateway_request(kernel_url, method="POST", body=json_encode({}))
+            response = await gateway_request(
+                kernel_url,
+                method="POST",
+                headers={"Content-Type": "application/json"},
+                body=json_encode({}),
+            )
             self.log.debug("Interrupt kernel response: %d %s", response.code, response.reason)
 
     async def is_alive(self):