diff --git a/substack/api.py b/substack/api.py
index 6e1dd53..af4a35a 100644
--- a/substack/api.py
+++ b/substack/api.py
@@ -10,7 +10,7 @@
 import os
 from datetime import datetime
 from urllib.parse import urljoin
-
+from substack.cookie_session import CookieSession
 import requests
 
 from substack.exceptions import SubstackAPIException, SubstackRequestException
@@ -64,9 +64,7 @@ def __init__(
         # Load cookies from file if provided
         # Helps with Captcha errors by reusing cookies from "local" auth, then switching to running code in the cloud
         if cookies_path is not None:
-            with open(cookies_path) as f:
-                cookies = json.load(f)
-            self._session.cookies.update(cookies)
+            self._session = CookieSession(cookies_path)
 
         elif email is not None and password is not None:
             self.login(email, password)
@@ -189,11 +187,11 @@ def get_publication_url(publication: dict) -> str:
         Args:
             publication:
         """
-        custom_domain = publication["custom_domain"]
-        if not custom_domain:
-            publication_url = f"https://{publication['subdomain']}.substack.com"
-        else:
+        if publication['custom_domain_optional']:
+            custom_domain = publication["custom_domain"]
             publication_url = f"https://{custom_domain}"
+        else:
+            publication_url = f"https://{publication['subdomain']}.substack.com"
 
         return publication_url
 
diff --git a/substack/cookie_session.py b/substack/cookie_session.py
new file mode 100644
index 0000000..aa21305
--- /dev/null
+++ b/substack/cookie_session.py
@@ -0,0 +1,16 @@
+# substack/cookie_session.py
+import json, os, requests
+from requests.utils import cookiejar_from_dict
+
+class CookieSession(requests.Session):
+    """
+    A requests.Session that loads cookies from Chrome/Firefox export
+    (JSON list of {name,value,domain,path,...}).  If a CSRF token is
+    present it is copied to the required headers so every POST works.
+    """
+    def __init__(self, cookie_file: str | os.PathLike, verify_ssl: bool = True):
+        super().__init__()
+        with open(cookie_file, "r", encoding="utf-8") as f:
+            raw = json.load(f)
+        self.cookies = cookiejar_from_dict({c["name"]: c["value"] for c in raw})
+        self.verify = verify_ssl