diff --git a/runpod/endpoint/helpers.py b/runpod/endpoint/helpers.py
index cbdb83f9..acef8b4e 100644
--- a/runpod/endpoint/helpers.py
+++ b/runpod/endpoint/helpers.py
@@ -1,6 +1,6 @@
 """ Helper functions for the Runpod Endpoint API. """
 
-FINAL_STATES = ["COMPLETED", "FAILED", "TIMED_OUT"]
+FINAL_STATES = ["COMPLETED", "FAILED", "TIMED_OUT", "CANCELLED"]
 
 # Exception Messages
 UNAUTHORIZED_MSG = "401 Unauthorized | Make sure Runpod API key is set and valid."
@@ -14,4 +14,4 @@
 
 def is_completed(status: str) -> bool:
     """Returns true if status is one of the possible final states for a serverless request."""
-    return status in ["COMPLETED", "FAILED", "TIMED_OUT", "CANCELLED"]
+    return status in FINAL_STATES
diff --git a/runpod/endpoint/runner.py b/runpod/endpoint/runner.py
index d8c48759..a3127edd 100644
--- a/runpod/endpoint/runner.py
+++ b/runpod/endpoint/runner.py
@@ -44,8 +44,17 @@ def __init__(self, api_key: Optional[str] = None):
             raise RuntimeError(API_KEY_NOT_SET_MSG)
 
         self.rp_session = requests.Session()
-        retries = Retry(total=5, backoff_factor=1, status_forcelist=[408, 429])
-        self.rp_session.mount("http://", HTTPAdapter(max_retries=retries))
+        retries = Retry(
+            total=5,
+            backoff_factor=1,
+            status_forcelist=[408, 429, 500, 502, 503, 504],
+            allowed_methods=frozenset(["GET", "POST"]),
+        )
+        adapter = HTTPAdapter(max_retries=retries)
+        # The production API is served over https; mount on both schemes so the
+        # retry/backoff policy actually applies to real traffic.
+        self.rp_session.mount("https://", adapter)
+        self.rp_session.mount("http://", adapter)
 
         self.headers = {
             "Content-Type": "application/json",
diff --git a/runpod/serverless/modules/rp_http.py b/runpod/serverless/modules/rp_http.py
index 3d82d35b..b326fa9a 100644
--- a/runpod/serverless/modules/rp_http.py
+++ b/runpod/serverless/modules/rp_http.py
@@ -26,7 +26,7 @@
 log = RunPodLogger()
 
 
-async def _transmit(client_session: ClientSession, url, job_data):
+async def _transmit(client_session: ClientSession, url, job_data, request_id=None):
     """
     Wrapper for transmitting results via POST.
     """
@@ -35,12 +35,18 @@ async def _transmit(client_session: ClientSession, url, job_data):
         client_session=client_session, retry_options=retry_options
     )
 
+    headers = {
+        "charset": "utf-8",
+        "Content-Type": "application/x-www-form-urlencoded",
+    }
+    # Pass the request id per-request rather than mutating the shared session's
+    # headers, which would race across concurrently handled jobs.
+    if request_id is not None:
+        headers["X-Request-ID"] = request_id
+
     kwargs = {
         "data": job_data,
-        "headers": {
-            "charset": "utf-8",
-            "Content-Type": "application/x-www-form-urlencoded",
-        },
+        "headers": headers,
         "raise_for_status": True,
     }
 
@@ -55,14 +61,12 @@ async def _handle_result(
     A helper function to handle the result, either for sending or streaming.
     """
     try:
-        session.headers["X-Request-ID"] = job["id"]
-
         serialized_job_data = json.dumps(job_data, ensure_ascii=False)
 
         is_stream = "true" if is_stream else "false"
         url = url_template.replace("$ID", job["id"]) + f"&isStream={is_stream}"
 
-        await _transmit(session, url, serialized_job_data)
+        await _transmit(session, url, serialized_job_data, request_id=job["id"])
         log.debug(f"{log_message}", job["id"])
 
     except ClientError as err:
diff --git a/tests/test_serverless/test_modules/test_http.py b/tests/test_serverless/test_modules/test_http.py
index c7462245..62249907 100644
--- a/tests/test_serverless/test_modules/test_http.py
+++ b/tests/test_serverless/test_modules/test_http.py
@@ -65,6 +65,7 @@ async def test_send_result(self):
                 headers={
                     "charset": "utf-8",
                     "Content-Type": "application/x-www-form-urlencoded",
+                    "X-Request-ID": self.job["id"],
                 },
                 raise_for_status=True,
             )
@@ -159,6 +160,7 @@ async def test_stream_result(self):
                 headers={
                     "charset": "utf-8",
                     "Content-Type": "application/x-www-form-urlencoded",
+                    "X-Request-ID": self.job["id"],
                 },
                 raise_for_status=True,
             )