diff --git a/src/aioweb/session.py b/src/aioweb/session.py
index bc0cdf4..e0501f7 100644
--- a/src/aioweb/session.py
+++ b/src/aioweb/session.py
@@ -9,6 +9,7 @@ headers, domain rewriting, previews, retry/backoff. See README for usage and con
 """
 
 import asyncio
+import ipaddress
 import logging
 import warnings
 from http.cookies import SimpleCookie
@@ -28,6 +29,17 @@ def _route_body(data):
     return data, None
 
 
+def _is_ip_host(host) -> bool:
+    """whether host is a literal IPv4/IPv6 address rather than a hostname"""
+    if not host:
+        return False
+    try:
+        ipaddress.ip_address(host)
+    except ValueError:
+        return False
+    return True
+
+
 class _RetryStatus(Exception):
     """internal signal: a retryable HTTP status; carries the real Response through aretry"""
 
@@ -174,7 +186,19 @@ class ExtendedSession:
     #   domain overwrites
 
     def overwrite_domain(self, target, replacement):
-        """register a host-substring rewrite (target -> replacement)"""
+        """register a host-substring rewrite (target -> replacement)
+
+        raises:
+            ValueError: if replacement contains a port (e.g. 'host:1234') - yarl's
+                with_host() rejects any ':' in a host at request time, deep inside
+                yarl with a stack trace far from this call; caught here instead
+        """
+        if ":" in replacement:
+            raise ValueError(
+                f"overwrite_domain replacement {replacement!r} contains a port - "
+                "yarl's URL.with_host() cannot take a host:port string; rewrite the "
+                "port separately or via a full URL override, not domain_overwrites"
+            )
         self.domain_overwrites[target] = replacement
 
     def _apply_domain_overwrites(self, url: str) -> str:
@@ -229,6 +253,12 @@ class ExtendedSession:
         (scheme optional, defaulted to http://) scopes it to that host - a bare hostname
         is normalized into a URL so the jar binds by host instead of silently storing
         another domain-less shared cookie. `path` is honored via the morsel itself.
+
+        raises:
+            ValueError: if domain is a bare IP host (e.g. '127.0.0.1') and the jar is
+                the default aiohttp.CookieJar(unsafe=False), which drops cookies bound
+                to IP hosts with no store, no send, and no log - construct the session
+                with cookie_jar=aiohttp.CookieJar(unsafe=True) to allow IP-host cookies
         """
         cookie = SimpleCookie()
         cookie[name] = value
@@ -238,7 +268,17 @@ class ExtendedSession:
             return
         if "://" not in domain:
             domain = "http://" + domain
-        self.session.cookie_jar.update_cookies(cookie, response_url=URL(domain))
+        url = URL(domain)
+        jar = self.session.cookie_jar
+        if _is_ip_host(url.raw_host) and not getattr(jar, "unsafe", False):
+            raise ValueError(
+                f"set_cookie domain {domain!r} resolves to a bare IP host, but "
+                "aiohttp's cookie jar rejects IP-address domains under unsafe=False "
+                "(the default) - the cookie would be silently dropped; construct "
+                "ExtendedSession with cookie_jar=aiohttp.CookieJar(unsafe=True) to "
+                "allow IP-host cookies"
+            )
+        jar.update_cookies(cookie, response_url=url)
 
     def clear_cookies(self):
         """clear the session cookie jar"""