modelcontextprotocol · beraterkanelcelik · Jun 15, 2026
diff --git a/src/mcp/client/auth/oauth2.py b/src/mcp/client/auth/oauth2.py
@@ -583,6 +583,21 @@ async def async_auth_flow(self, request: httpx.Request) -> AsyncGenerator[httpx.
                         self.context.client_metadata.grant_types,
                     )
 
+                    # Step 3.5: Use a stored refresh_token before full re-auth.
+                    # A provider reloaded from storage never restores
+                    # token_expiry_time, so is_token_valid() treats the stale token
+                    # as valid and the proactive-refresh branch above is skipped.
+                    # Without this, every access-token expiry forces an interactive
+                    # re-authorization instead of a silent refresh.
+                    if self.context.can_refresh_token():
+                        refresh_request = await self._refresh_token()
+                        refresh_response = yield refresh_request
+                        if await self._handle_refresh_response(refresh_response):
+                            self._add_auth_header(request)
+                            yield request
+                            return
+                        # refresh failed -> fall through to full re-authorization
+
                     # Step 4: Register client or use URL-based client ID (CIMD)
                     if not self.context.client_info:
                         if should_use_client_metadata_url(

diff --git a/tests/client/test_auth.py b/tests/client/test_auth.py
@@ -45,13 +45,13 @@ def __init__(self):
         self._client_info: OAuthClientInformationFull | None = None
 
     async def get_tokens(self) -> OAuthToken | None:
-        return self._tokens  # pragma: no cover
+        return self._tokens
 
     async def set_tokens(self, tokens: OAuthToken) -> None:
         self._tokens = tokens
 
     async def get_client_info(self) -> OAuthClientInformationFull | None:
-        return self._client_info  # pragma: no cover
+        return self._client_info
 
     async def set_client_info(self, client_info: OAuthClientInformationFull) -> None:
         self._client_info = client_info
@@ -2636,3 +2636,161 @@ async def callback_handler() -> tuple[str, str | None]:
             await auth_flow.asend(final_response)
         except StopAsyncIteration:
             pass
+
+
+def _reloaded_client_info() -> OAuthClientInformationFull:
+    return OAuthClientInformationFull(
+        client_id="test_client_id",
+        client_secret="test_client_secret",
+        redirect_uris=[AnyUrl("http://localhost:3030/callback")],
+    )
+
+
+@pytest.mark.anyio
+async def test_initialize_does_not_restore_token_expiry(
+    oauth_provider: OAuthClientProvider, mock_storage: MockTokenStorage, valid_tokens: OAuthToken
+):
+    """After _initialize() loads stored tokens, token_expiry_time stays None.
+
+    OAuthToken.expires_in is relative, so a reloaded provider cannot know the real
+    expiry; is_token_valid() then reports the loaded token as valid and the
+    proactive-refresh branch in async_auth_flow is skipped.
+    """
+    await mock_storage.set_tokens(valid_tokens)
+    await mock_storage.set_client_info(_reloaded_client_info())
+
+    await oauth_provider._initialize()
+
+    assert oauth_provider.context.token_expiry_time is None
+    assert oauth_provider.context.is_token_valid() is True
+    assert oauth_provider.context.can_refresh_token() is True
+
+
+@pytest.mark.anyio
+async def test_reloaded_provider_uses_refresh_token_on_401(
+    oauth_provider: OAuthClientProvider, mock_storage: MockTokenStorage, valid_tokens: OAuthToken
+):
+    """A fresh provider that reloaded a stored refresh_token must perform a
+    refresh_token grant on 401 instead of a full re-authorization."""
+    await mock_storage.set_tokens(valid_tokens)
+    await mock_storage.set_client_info(_reloaded_client_info())
+    oauth_provider._perform_authorization_code_grant = mock.AsyncMock(return_value=("unused_code", "unused_verifier"))
+
+    request = httpx.Request("GET", "https://api.example.com/mcp")
+    auth_flow = oauth_provider.async_auth_flow(request)
+
+    # _initialize() loads the stale token and (wrongly) treats it as valid
+    first_request = await auth_flow.__anext__()
+    assert first_request.headers["Authorization"] == "Bearer test_access_token"
+
+    # server rejects the stale token -> discovery -> refresh
+    resp_401 = httpx.Response(
+        401,
+        headers={
+            "WWW-Authenticate": 'Bearer resource_metadata="https://api.example.com/.well-known/oauth-protected-resource"'
+        },
+        request=request,
+    )
+    prm_request = await auth_flow.asend(resp_401)
+    prm_response = httpx.Response(
+        200,
+        content=b'{"resource": "https://api.example.com/v1/mcp", "authorization_servers": ["https://auth.example.com"]}',
+        request=prm_request,
+    )
+    asm_request = await auth_flow.asend(prm_response)
+    asm_response = httpx.Response(
+        200,
+        content=(
+            b'{"issuer": "https://auth.example.com", '
+            b'"authorization_endpoint": "https://auth.example.com/authorize", '
+            b'"token_endpoint": "https://auth.example.com/token", '
+            b'"registration_endpoint": "https://auth.example.com/register"}'
+        ),
+        request=asm_request,
+    )
+
+    refresh_request = await auth_flow.asend(asm_response)
+    body = refresh_request.content.decode()
+    assert refresh_request.method == "POST"
+    assert str(refresh_request.url) == "https://auth.example.com/token"
+    assert "grant_type=refresh_token" in body
+    assert "refresh_token=test_refresh_token" in body
+    oauth_provider._perform_authorization_code_grant.assert_not_called()
+
+    refresh_response = httpx.Response(
+        200,
+        content=(
+            b'{"access_token": "new_access_token", "token_type": "Bearer", '
+            b'"expires_in": 3600, "refresh_token": "new_rt"}'
+        ),
+        request=refresh_request,
+    )
+    final_request = await auth_flow.asend(refresh_response)
+    assert final_request.headers["Authorization"] == "Bearer new_access_token"
+
+    try:
+        await auth_flow.asend(httpx.Response(200, request=final_request))
+    except StopAsyncIteration:
+        pass
+
+
+@pytest.mark.anyio
+async def test_reloaded_provider_falls_back_to_reauth_when_refresh_fails(
+    oauth_provider: OAuthClientProvider, mock_storage: MockTokenStorage, valid_tokens: OAuthToken
+):
+    """If the refresh_token grant fails, the 401 flow falls back to full
+    re-authorization rather than erroring out."""
+    await mock_storage.set_tokens(valid_tokens)
+    await mock_storage.set_client_info(_reloaded_client_info())
+    oauth_provider._perform_authorization_code_grant = mock.AsyncMock(return_value=("reauth_code", "reauth_verifier"))
+
+    request = httpx.Request("GET", "https://api.example.com/mcp")
+    auth_flow = oauth_provider.async_auth_flow(request)
+    await auth_flow.__anext__()
+
+    resp_401 = httpx.Response(
+        401,
+        headers={
+            "WWW-Authenticate": 'Bearer resource_metadata="https://api.example.com/.well-known/oauth-protected-resource"'
+        },
+        request=request,
+    )
+    prm_request = await auth_flow.asend(resp_401)
+    prm_response = httpx.Response(
+        200,
+        content=b'{"resource": "https://api.example.com/v1/mcp", "authorization_servers": ["https://auth.example.com"]}',
+        request=prm_request,
+    )
+    asm_request = await auth_flow.asend(prm_response)
+    asm_response = httpx.Response(
+        200,
+        content=(
+            b'{"issuer": "https://auth.example.com", '
+            b'"authorization_endpoint": "https://auth.example.com/authorize", '
+            b'"token_endpoint": "https://auth.example.com/token", '
+            b'"registration_endpoint": "https://auth.example.com/register"}'
+        ),
+        request=asm_request,
+    )
+
+    refresh_request = await auth_flow.asend(asm_response)
+    assert "grant_type=refresh_token" in refresh_request.content.decode()
+
+    # refresh fails -> fall through to full authorization-code grant
+    refresh_failure = httpx.Response(400, content=b'{"error": "invalid_grant"}', request=refresh_request)
+    token_request = await auth_flow.asend(refresh_failure)
+    assert "grant_type=authorization_code" in token_request.content.decode()
+    oauth_provider._perform_authorization_code_grant.assert_called_once()
+
+    token_response = httpx.Response(
+        200,
+        content=b'{"access_token": "reauth_access_token", "token_type": "Bearer", "expires_in": 3600}',
+        request=token_request,
+    )
+    final_request = await auth_flow.asend(token_response)
+    assert final_request.headers["Authorization"] == "Bearer reauth_access_token"
+
+    try:
+        await auth_flow.asend(httpx.Response(200, request=final_request))
+    except StopAsyncIteration:
+        pass