diff --git a/api/auth.py b/api/auth.py
index 048b0f40..7960e954 100644
--- a/api/auth.py
+++ b/api/auth.py
@@ -9,6 +9,7 @@ import http.cookies
 import json
 import logging
 import os
+import re
 import secrets
 import tempfile
 import threading
@@ -57,6 +58,33 @@ PUBLIC_PATHS = frozenset({
 COOKIE_NAME = 'hermes_session'
 CSRF_HEADER_NAME = 'X-Hermes-CSRF-Token'
 
+
+# RFC 6265 cookie-name token: a non-empty run of token chars
+# (no controls, whitespace, or separators such as ';', '=', ',').
+_COOKIE_NAME_RE = re.compile(r"^[-!#$%&'*+.^_`|~0-9A-Za-z]+$")
+
+
+def _resolve_cookie_name() -> str:
+    """Resolve the auth session cookie name from env > default.
+
+    Honours ``HERMES_WEBUI_COOKIE_NAME`` so multiple WebUI instances sharing a
+    hostname (different ports) can use distinct cookie names instead of
+    trampling each other's session — browsers scope cookies by host, not
+    host+port (RFC 6265). Falls back to ``COOKIE_NAME`` when the env var is
+    unset, empty, or not a valid RFC 6265 token.
+    """
+    name = os.getenv('HERMES_WEBUI_COOKIE_NAME', '').strip()
+    if not name:
+        return COOKIE_NAME
+    if _COOKIE_NAME_RE.match(name):
+        return name
+    logger.warning(
+        'Ignoring invalid HERMES_WEBUI_COOKIE_NAME=%r; falling back to %r '
+        '(name must be a valid RFC 6265 token)', name, COOKIE_NAME,
+    )
+    return COOKIE_NAME
+
+
 _SESSIONS_FILE = STATE_DIR / '.sessions.json'
 
 
@@ -482,7 +510,7 @@ def parse_cookie(handler) -> str | None:
         cookie.load(cookie_header)
     except http.cookies.CookieError:
         return None
-    morsel = cookie.get(COOKIE_NAME)
+    morsel = cookie.get(_resolve_cookie_name())
     return morsel.value if morsel else None
 
 
@@ -589,21 +617,23 @@ def _is_secure_context(handler=None) -> bool:
 def set_auth_cookie(handler, cookie_value) -> None:
     """Set the auth cookie on the response."""
     cookie = http.cookies.SimpleCookie()
-    cookie[COOKIE_NAME] = cookie_value
-    cookie[COOKIE_NAME]['httponly'] = True
-    cookie[COOKIE_NAME]['samesite'] = 'Lax'
-    cookie[COOKIE_NAME]['path'] = '/'
-    cookie[COOKIE_NAME]['max-age'] = str(_resolve_session_ttl())
+    name = _resolve_cookie_name()
+    cookie[name] = cookie_value
+    cookie[name]['httponly'] = True
+    cookie[name]['samesite'] = 'Lax'
+    cookie[name]['path'] = '/'
+    cookie[name]['max-age'] = str(_resolve_session_ttl())
     if _is_secure_context(handler):
-        cookie[COOKIE_NAME]['secure'] = True
-    handler.send_header('Set-Cookie', cookie[COOKIE_NAME].OutputString())
+        cookie[name]['secure'] = True
+    handler.send_header('Set-Cookie', cookie[name].OutputString())
 
 
 def clear_auth_cookie(handler) -> None:
     """Clear the auth cookie on the response."""
     cookie = http.cookies.SimpleCookie()
-    cookie[COOKIE_NAME] = ''
-    cookie[COOKIE_NAME]['httponly'] = True
-    cookie[COOKIE_NAME]['path'] = '/'
-    cookie[COOKIE_NAME]['max-age'] = '0'
-    handler.send_header('Set-Cookie', cookie[COOKIE_NAME].OutputString())
+    name = _resolve_cookie_name()
+    cookie[name] = ''
+    cookie[name]['httponly'] = True
+    cookie[name]['path'] = '/'
+    cookie[name]['max-age'] = '0'
+    handler.send_header('Set-Cookie', cookie[name].OutputString())
diff --git a/tests/test_auth_sessions.py b/tests/test_auth_sessions.py
index 725e336b..81da0d9c 100644
--- a/tests/test_auth_sessions.py
+++ b/tests/test_auth_sessions.py
@@ -254,3 +254,55 @@ class TestSessionTtlResolution(unittest.TestCase):
                 break
         else:
             self.fail("Session token not found in _sessions")
+
+
+class TestCookieNameResolution(unittest.TestCase):
+    """Verify the auth cookie name resolution (env > default)."""
+
+    def setUp(self):
+        self._saved = os.environ.get("HERMES_WEBUI_COOKIE_NAME")
+        os.environ.pop("HERMES_WEBUI_COOKIE_NAME", None)
+
+    def tearDown(self):
+        if self._saved is None:
+            os.environ.pop("HERMES_WEBUI_COOKIE_NAME", None)
+        else:
+            os.environ["HERMES_WEBUI_COOKIE_NAME"] = self._saved
+
+    def test_default_when_unset(self):
+        """With the env var unset the legacy default name is used."""
+        self.assertEqual(auth._resolve_cookie_name(), auth.COOKIE_NAME)
+
+    def test_env_var_overrides_default(self):
+        """A valid env var name overrides the default."""
+        os.environ["HERMES_WEBUI_COOKIE_NAME"] = "hermes_session_alt"
+        self.assertEqual(auth._resolve_cookie_name(), "hermes_session_alt")
+
+    def test_empty_env_falls_back(self):
+        """Whitespace-only env var falls back to the default."""
+        os.environ["HERMES_WEBUI_COOKIE_NAME"] = "   "
+        self.assertEqual(auth._resolve_cookie_name(), auth.COOKIE_NAME)
+
+    def test_invalid_name_falls_back(self):
+        """A name with characters illegal in an RFC 6265 token is rejected."""
+        os.environ["HERMES_WEBUI_COOKIE_NAME"] = "bad name=x"
+        self.assertEqual(auth._resolve_cookie_name(), auth.COOKIE_NAME)
+
+    def test_set_cookie_uses_resolved_name(self):
+        """set_auth_cookie emits the resolved name in the Set-Cookie header."""
+        os.environ["HERMES_WEBUI_COOKIE_NAME"] = "hermes_session_alt"
+        sent = {}
+
+        class _H:
+            request = object()  # no getpeercert -> not a TLS socket
+            headers: dict = {}
+
+            def send_header(self, k, v):
+                sent[k] = v
+
+        auth.set_auth_cookie(_H(), "tok.sig")
+        self.assertIn("hermes_session_alt=", sent["Set-Cookie"])
+
+
+if __name__ == "__main__":
+    unittest.main()