diff --git a/src/aioweb/session.py b/src/aioweb/session.py
index c7bfd77..5286f0f 100644
--- a/src/aioweb/session.py
+++ b/src/aioweb/session.py
@@ -15,6 +15,7 @@ import warnings
 from http.cookies import SimpleCookie
 
 import aiohttp
+from multidict import CIMultiDict
 from yarl import URL
 from commons import aretry
 
@@ -85,12 +86,15 @@ class ExtendedSession:
             kwargs: passed through to aiohttp.ClientSession
         """
         self.inject = inject
-        self.header_overwrites = overwrites or {}
-        self.domain_overwrites = domain_overwrites or {}
+        # copied, not aliased: a shared config dict must not be mutated by (or leak a
+        # clear_proxies() into) a sibling session
+        self.header_overwrites = dict(overwrites or {})
+        self.domain_overwrites = dict(domain_overwrites or {})
         self.ephemeral_headers = {}
-        self.proxies = proxies or {}
-        # own header layer, not aiohttp privates, so update_headers/clear_headers work
-        self._default_headers = dict(headers or {})
+        self.proxies = dict(proxies or {})
+        # own header layer, not aiohttp privates, so update_headers/clear_headers work;
+        # case-insensitive like HTTP header names
+        self._default_headers = CIMultiDict(headers or {})
         self._session_timeout = timeout
         self._session_kwargs = kwargs
         # aiohttp>=3.14 needs a running loop to build ClientSession; built lazily instead
@@ -139,8 +143,12 @@ class ExtendedSession:
         self.header_overwrites[header] = value
 
     def _apply_overwrites(self, request_headers):
-        """apply static overwrites and ephemeral headers to a request's headers"""
-        request_headers = dict(request_headers or {})
+        """apply static overwrites and ephemeral headers to a request's headers
+
+        header names match case-insensitively, so an overwrite replaces a `user-agent`
+        rather than sending it alongside a second `User-Agent`
+        """
+        request_headers = CIMultiDict(request_headers or {})
         # list()-snapshot so a concurrent mutation of the shared dicts can't raise
         # "dict changed size during iteration"
         for header, value in list(self.header_overwrites.items()):
@@ -153,7 +161,7 @@ class ExtendedSession:
                     request_headers.update(value)
                 else:
                     request_headers[header] = value
-        return request_headers
+        return dict(request_headers)
 
     def set_ephemeral(self, header, value_callable):
         """register a header generated fresh (via callable) for each request"""
@@ -262,7 +270,7 @@ class ExtendedSession:
         falling back to them.
         """
         proxies = self.proxies if proxies is None else proxies
-        scheme = url.split("://")[0]
+        scheme = URL(url).scheme
         return proxies.get(scheme)
 
     # -------------------------------------------------------------------------
@@ -353,7 +361,8 @@ class ExtendedSession:
         """
         url = self._apply_domain_overwrites(url)
         proxy = self._get_proxy(url, kwargs.pop("proxies", None))
-        merged = {**self._default_headers, **(kwargs.pop("headers", None) or {})}
+        merged = CIMultiDict(self._default_headers)
+        merged.update(kwargs.pop("headers", None) or {})
         headers = self._apply_overwrites(merged)
 
         timeout = kwargs.get("timeout")
@@ -421,22 +430,19 @@ class ExtendedSession:
             kwargs["proxy"] = resolved_proxy
         debug = kwargs.pop("debug", False)
 
-        merged = {**self._default_headers, **(kwargs.get("headers") or {})}
+        merged = CIMultiDict(self._default_headers)
+        merged.update(kwargs.get("headers") or {})
         kwargs["headers"] = self._apply_overwrites(merged)
         kwargs["headers"] = {str(k): str(v) for k, v in kwargs["headers"].items()}
 
         timeout = kwargs.get("timeout")
-        # the hang-guard deadline, captured as a bare number BEFORE the ClientTimeout
-        # conversion below consumes it: a per-call numeric timeout, else the session
-        # default. a timeout=None (omitted, or the explicit None request_with_retries
-        # passes) means "use the session bound" - so it guards on self._session_timeout,
-        # NOT unguarded. the ONLY unguarded case is a session deliberately constructed
-        # unbounded (Session(timeout=None) -> self._session_timeout is None): the
-        # documented opt-out for a genuinely unbounded call (long-poll/streaming).
-        if isinstance(timeout, (int, float)):
-            guard_secs = timeout
-        else:
-            guard_secs = self._session_timeout
+        # the hang-guard deadline: a per-call timeout (a number, or a ClientTimeout's
+        # .total), else the session default. a timeout=None (omitted, or the explicit None
+        # request_with_retries passes) means "use the session bound" - so it guards on
+        # self._session_timeout, NOT unguarded. unguarded only when the caller asked for no
+        # bound: a session constructed with timeout=None (the documented long-poll/streaming
+        # opt-out), or a per-call ClientTimeout(total=None).
+        guard_secs = self._session_timeout if timeout is None else getattr(timeout, "total", timeout)
         if isinstance(timeout, (int, float)):
             kwargs["timeout"] = aiohttp.ClientTimeout(total=timeout)
         elif timeout is None and "timeout" in kwargs: