diff --git a/src/authgate/__init__.py b/src/authgate/__init__.py
index 9706f03..ef0ec38 100644
--- a/src/authgate/__init__.py
+++ b/src/authgate/__init__.py
@@ -2,6 +2,7 @@
 
 from __future__ import annotations
 
+import asyncio
 import enum
 
 from authgate._version import __version__
@@ -12,7 +13,7 @@
 from authgate.credstore import default_token_secure_store
 from authgate.discovery.async_client import AsyncDiscoveryClient
 from authgate.discovery.client import DiscoveryClient
-from authgate.exceptions import AuthGateError
+from authgate.exceptions import AuthFlowError, AuthGateError, NotFoundError, OAuthError
 from authgate.oauth.async_client import AsyncOAuthClient
 from authgate.oauth.client import OAuthClient
 from authgate.oauth.models import Token
@@ -63,8 +64,13 @@ def authenticate(
     try:
         token = ts.token()
         return client, token
-    except Exception:
+    except (NotFoundError, AuthFlowError):
         pass
+    except OAuthError as exc:
+        # Only swallow errors indicating an invalid/expired refresh token.
+        # Re-raise unexpected OAuth errors (e.g., server_error) so they surface.
+        if exc.code not in ("invalid_grant", "invalid_token"):
+            raise
 
     # 5. No valid token — run the appropriate authentication flow
     if flow_mode == FlowMode.BROWSER:
@@ -111,20 +117,37 @@ async def async_authenticate(
     # 2. Create async OAuth client
     client = AsyncOAuthClient(client_id, meta.to_endpoints())
 
-    # 3. Check stored token (sync store, run in thread)
+    # 3. Check stored token and attempt refresh if expired
     store = default_token_secure_store(service_name, store_path)
-    ts = TokenSource(OAuthClient(client_id, meta.to_endpoints()), store=store)
     try:
-        token = ts.token()
-        return client, token
-    except Exception:
+        stored = await asyncio.to_thread(store.load, client_id)
+        from authgate.authflow.token_source import credstore_to_oauth, oauth_to_credstore
+
+        if stored.is_valid():
+            token = credstore_to_oauth(stored)
+            return client, token
+
+        # Try refreshing with the stored refresh token
+        if stored.refresh_token:
+            try:
+                token = await client.refresh_token(stored.refresh_token)
+                await asyncio.to_thread(store.save, client_id, oauth_to_credstore(token, client_id))
+                return client, token
+            except OAuthError as exc:
+                # Only swallow errors indicating an invalid/expired refresh token.
+                # Re-raise unexpected OAuth errors (e.g., server_error).
+                if exc.code not in ("invalid_grant", "invalid_token"):
+                    raise
+    except NotFoundError:
         pass
 
     # 4. Run device flow (always, since auth code needs sync HTTP server)
     token = await async_run_device_flow(client, _scopes)
 
     # 5. Persist
-    ts.save_token(token)
+    from authgate.authflow.token_source import oauth_to_credstore
+
+    await asyncio.to_thread(store.save, client_id, oauth_to_credstore(token, client_id))
 
     return client, token
 
diff --git a/src/authgate/authflow/__init__.py b/src/authgate/authflow/__init__.py
index fc2757c..e93ea73 100644
--- a/src/authgate/authflow/__init__.py
+++ b/src/authgate/authflow/__init__.py
@@ -4,14 +4,16 @@
 from authgate.authflow.browser import check_browser_availability, open_browser
 from authgate.authflow.device import async_run_device_flow, run_device_flow
 from authgate.authflow.pkce import PKCE, generate_pkce
-from authgate.authflow.token_source import TokenSource
+from authgate.authflow.token_source import TokenSource, credstore_to_oauth, oauth_to_credstore
 
 __all__ = [
     "PKCE",
     "TokenSource",
     "async_run_device_flow",
     "check_browser_availability",
+    "credstore_to_oauth",
     "generate_pkce",
+    "oauth_to_credstore",
     "open_browser",
     "run_auth_code_flow",
     "run_device_flow",
diff --git a/src/authgate/authflow/authcode.py b/src/authgate/authflow/authcode.py
index e879343..096e016 100644
--- a/src/authgate/authflow/authcode.py
+++ b/src/authgate/authflow/authcode.py
@@ -5,79 +5,86 @@
 import os
 import threading
 from http.server import BaseHTTPRequestHandler, HTTPServer
-from urllib.parse import parse_qs, urlparse
+from urllib.parse import parse_qs, urlencode, urlparse
 
 from authgate.authflow.browser import open_browser
 from authgate.authflow.pkce import generate_pkce
-from authgate.exceptions import OAuthError
+from authgate.exceptions import AuthFlowError, OAuthError
 from authgate.oauth.client import OAuthClient
 from authgate.oauth.models import Token
 
+_CALLBACK_TIMEOUT = 300.0  # 5 minutes
+
 
 def _generate_state() -> str:
     """Generate a cryptographically random state string for CSRF protection."""
     return os.urandom(16).hex()
 
 
-class _CallbackHandler(BaseHTTPRequestHandler):
-    """HTTP handler for the OAuth callback."""
-
-    result: dict[str, str]  # shared via class attribute set by factory
-    event: threading.Event
-
-    def do_GET(self) -> None:
-        parsed = urlparse(self.path)
-        if parsed.path != "/callback":
-            self.send_response(404)
-            self.end_headers()
-            return
-
-        params = parse_qs(parsed.query)
-
-        # Only process the first callback
-        if self.event.is_set():
-            self.send_response(200)
-            self.end_headers()
-            self.wfile.write(b"<html><body><h1>Already processed</h1></body></html>")
-            return
-
-        state = params.get("state", [""])[0]
-        if state != self.result.get("expected_state"):
-            self.result["error"] = "invalid_state"
-            self.result["error_description"] = "State parameter mismatch"
-            self.event.set()
-            self.send_response(200)
-            self.end_headers()
-            self.wfile.write(
-                b"<html><body><h1>Authentication failed</h1>"
-                b"<p>State mismatch. You can close this window.</p></body></html>"
-            )
-            return
-
-        code = params.get("code", [""])[0]
-        if not code:
-            self.result["error"] = params.get("error", ["no code received"])[0]
-            self.result["error_description"] = params.get("error_description", [""])[0]
-            self.event.set()
+def _make_callback_handler(
+    result: dict[str, str],
+    event: threading.Event,
+) -> type[BaseHTTPRequestHandler]:
+    """Create a handler class with per-flow state to avoid class-attribute sharing."""
+
+    class _CallbackHandler(BaseHTTPRequestHandler):
+        """HTTP handler for the OAuth callback."""
+
+        def do_GET(self) -> None:
+            parsed = urlparse(self.path)
+            if parsed.path != "/callback":
+                self.send_response(404)
+                self.end_headers()
+                return
+
+            params = parse_qs(parsed.query)
+
+            # Only process the first callback
+            if event.is_set():
+                self.send_response(200)
+                self.end_headers()
+                self.wfile.write(b"<html><body><h1>Already processed</h1></body></html>")
+                return
+
+            state = params.get("state", [""])[0]
+            if state != result.get("expected_state"):
+                result["error"] = "invalid_state"
+                result["error_description"] = "State parameter mismatch"
+                event.set()
+                self.send_response(200)
+                self.end_headers()
+                self.wfile.write(
+                    b"<html><body><h1>Authentication failed</h1>"
+                    b"<p>State mismatch. You can close this window.</p></body></html>"
+                )
+                return
+
+            code = params.get("code", [""])[0]
+            if not code:
+                result["error"] = params.get("error", ["no code received"])[0]
+                result["error_description"] = params.get("error_description", [""])[0]
+                event.set()
+                self.send_response(200)
+                self.end_headers()
+                self.wfile.write(
+                    b"<html><body><h1>Authentication failed</h1>"
+                    b"<p>You can close this window.</p></body></html>"
+                )
+                return
+
+            result["code"] = code
+            event.set()
             self.send_response(200)
             self.end_headers()
             self.wfile.write(
-                b"<html><body><h1>Authentication failed</h1>"
+                b"<html><body><h1>Authentication successful</h1>"
                 b"<p>You can close this window.</p></body></html>"
             )
-            return
-
-        self.result["code"] = code
-        self.event.set()
-        self.send_response(200)
-        self.end_headers()
-        self.wfile.write(
-            b"<html><body><h1>Authentication successful</h1>"
-            b"<p>You can close this window.</p></body></html>"
-        )
 
-    def log_message(self, fmt: str, *args: object) -> None:
-        pass  # Suppress HTTP server logs
+        def log_message(self, fmt: str, *args: object) -> None:
+            pass  # Suppress HTTP server logs
+
+    return _CallbackHandler
 
 
 def run_auth_code_flow(
@@ -90,39 +97,43 @@ def run_auth_code_flow(
     pkce = generate_pkce()
     state = _generate_state()
 
+    # Set up per-flow shared state
+    result: dict[str, str] = {"expected_state": state}
+    event = threading.Event()
+    handler_cls = _make_callback_handler(result, event)
+
     # Start the callback server
-    server = HTTPServer(("127.0.0.1", local_port), _CallbackHandler)
+    server = HTTPServer(("127.0.0.1", local_port), handler_cls)
     port = server.server_address[1]
     redirect_uri = f"http://127.0.0.1:{port}/callback"
 
-    # Set up shared state via handler class attributes
-    result: dict[str, str] = {"expected_state": state}
-    event = threading.Event()
-    _CallbackHandler.result = result
-    _CallbackHandler.event = event
-
     server_thread = threading.Thread(target=server.serve_forever, daemon=True)
     server_thread.start()
 
     try:
-        # Build authorization URL
+        # Build authorization URL with proper encoding
         endpoints = client.endpoints
-        params = (
-            f"response_type=code"
-            f"&client_id={client.client_id}"
-            f"&redirect_uri={redirect_uri}"
-            f"&scope={'+'.join(scopes or [])}"
-            f"&state={state}"
-            f"&code_challenge={pkce.challenge}"
-            f"&code_challenge_method={pkce.method}"
+        query = urlencode(
+            {
+                "response_type": "code",
+                "client_id": client.client_id,
+                "redirect_uri": redirect_uri,
+                "scope": " ".join(scopes or []),
+                "state": state,
+                "code_challenge": pkce.challenge,
+                "code_challenge_method": pkce.method,
+            }
         )
-        auth_url = f"{endpoints.authorize_url}?{params}"
+        auth_url = f"{endpoints.authorize_url}?{query}"
 
         if not open_browser(auth_url):
             print(f"Open this URL in your browser:\n{auth_url}")
 
-        # Wait for callback
-        event.wait()
+        # Wait for callback with timeout
+        if not event.wait(timeout=_CALLBACK_TIMEOUT):
+            raise AuthFlowError(
+                f"auth code flow timed out after {_CALLBACK_TIMEOUT:.0f}s waiting for callback"
+            )
 
         if "error" in result:
             raise OAuthError(
@@ -133,6 +144,7 @@ def run_auth_code_flow(
         code = result["code"]
     finally:
         server.shutdown()
+        server.server_close()
         server_thread.join(timeout=5)
 
     return client.exchange_auth_code(code, redirect_uri, pkce.verifier)
diff --git a/src/authgate/authflow/token_source.py b/src/authgate/authflow/token_source.py
index 58ee185..0e0e9a8 100644
--- a/src/authgate/authflow/token_source.py
+++ b/src/authgate/authflow/token_source.py
@@ -11,7 +11,7 @@
 from authgate.oauth.models import Token
 
 
-def _credstore_to_oauth(t: StoredToken) -> Token:
+def credstore_to_oauth(t: StoredToken) -> Token:
     return Token(
         access_token=t.access_token,
         refresh_token=t.refresh_token,
@@ -20,7 +20,7 @@ def _credstore_to_oauth(t: StoredToken) -> Token:
     )
 
 
-def _oauth_to_credstore(t: Token, client_id: str) -> StoredToken:
+def oauth_to_credstore(t: Token, client_id: str) -> StoredToken:
     return StoredToken(
         access_token=t.access_token,
         refresh_token=t.refresh_token,
@@ -62,7 +62,7 @@ def token(self) -> Token:
             try:
                 stored = self._store.load(self._client.client_id)
                 if stored.is_valid():
-                    tok = _credstore_to_oauth(stored)
+                    tok = credstore_to_oauth(stored)
                     self._cached = tok
                     return tok
             except NotFoundError:
@@ -107,7 +107,7 @@ def _do_refresh(self) -> Token:
             try:
                 stored = self._store.load(self._client.client_id)
                 if stored.is_valid():
-                    tok = _credstore_to_oauth(stored)
+                    tok = credstore_to_oauth(stored)
                     self._cached = tok
                     return tok
 
@@ -133,5 +133,5 @@ def _save_token(self, token: Token) -> None:
             return
         self._store.save(
             self._client.client_id,
-            _oauth_to_credstore(token, self._client.client_id),
+            oauth_to_credstore(token, self._client.client_id),
         )
diff --git a/src/authgate/exceptions.py b/src/authgate/exceptions.py
index bdca498..8d1e7b9 100644
--- a/src/authgate/exceptions.py
+++ b/src/authgate/exceptions.py
@@ -1,5 +1,7 @@
 """AuthGate exception hierarchy."""
 
+from __future__ import annotations
+
 
 class AuthGateError(Exception):
     """Base exception for all AuthGate errors."""
diff --git a/src/authgate/middleware/fastapi.py b/src/authgate/middleware/fastapi.py
index 3493b3b..7f5afef 100644
--- a/src/authgate/middleware/fastapi.py
+++ b/src/authgate/middleware/fastapi.py
@@ -2,15 +2,13 @@
 
 from __future__ import annotations
 
-from collections.abc import Callable
-
 from authgate.exceptions import OAuthError
 from authgate.middleware.core import ValidationMode, extract_bearer_token, validate_token
 from authgate.middleware.models import TokenInfo
 from authgate.oauth.client import OAuthClient
 
 try:
-    from fastapi import Depends, HTTPException, Request
+    from fastapi import HTTPException, Request
     from fastapi.security import HTTPBearer
 except ImportError as exc:
     raise ImportError(
@@ -69,27 +67,3 @@ async def __call__(self, request: Request) -> TokenInfo:
                 )
 
         return info
-
-
-def require_scope(*scopes: str) -> Callable[..., object]:
-    """FastAPI dependency that checks for additional scopes.
-
-    Must be used after BearerAuth.
-    """
-
-    _default = Depends()
-
-    async def dependency(info: TokenInfo = _default) -> TokenInfo:
-        for scope in scopes:
-            if not info.has_scope(scope):
-                raise HTTPException(
-                    status_code=403,
-                    detail={
-                        "error": "insufficient_scope",
-                        "error_description": f"Token does not have required scope: {scope}",
-                    },
-                    headers={"WWW-Authenticate": 'Bearer error="insufficient_scope"'},
-                )
-        return info
-
-    return dependency
diff --git a/src/authgate/oauth/async_client.py b/src/authgate/oauth/async_client.py
index 1e4d16b..da5b2a9 100644
--- a/src/authgate/oauth/async_client.py
+++ b/src/authgate/oauth/async_client.py
@@ -123,9 +123,15 @@ async def revoke(self, token: str) -> None:
         """Revoke a token (RFC 7009)."""
         if not self._endpoints.revocation_url:
             raise OAuthError("invalid_request", "revocation endpoint not configured")
+        data: dict[str, str] = {
+            "token": token,
+            "client_id": self._client_id,
+        }
+        if self._client_secret:
+            data["client_secret"] = self._client_secret
         resp = await self._http.post(
             self._endpoints.revocation_url,
-            data={"token": token},
+            data=data,
         )
         if resp.status_code != 200:
             raise _parse_error_response(resp)
diff --git a/src/authgate/oauth/client.py b/src/authgate/oauth/client.py
index eafbf51..7707c66 100644
--- a/src/authgate/oauth/client.py
+++ b/src/authgate/oauth/client.py
@@ -123,9 +123,15 @@ def revoke(self, token: str) -> None:
         """Revoke a token (RFC 7009)."""
         if not self._endpoints.revocation_url:
             raise OAuthError("invalid_request", "revocation endpoint not configured")
+        data: dict[str, str] = {
+            "token": token,
+            "client_id": self._client_id,
+        }
+        if self._client_secret:
+            data["client_secret"] = self._client_secret
         resp = self._http.post(
             self._endpoints.revocation_url,
-            data={"token": token},
+            data=data,
         )
         if resp.status_code != 200:
             raise _parse_error_response(resp)