diff --git a/src/mcp/server/auth/provider.py b/src/mcp/server/auth/provider.py
index b84db89a2..a7b108602 100644
--- a/src/mcp/server/auth/provider.py
+++ b/src/mcp/server/auth/provider.py
@@ -281,7 +281,7 @@ async def revoke_token(
 
 def construct_redirect_uri(redirect_uri_base: str, **params: str | None) -> str:
     parsed_uri = urlparse(redirect_uri_base)
-    query_params = [(k, v) for k, vs in parse_qs(parsed_uri.query) for v in vs]
+    query_params = [(k, v) for k, vs in parse_qs(parsed_uri.query).items() for v in vs]
     for k, v in params.items():
         if v is not None:
             query_params.append((k, v))
diff --git a/tests/server/auth/test_provider.py b/tests/server/auth/test_provider.py
new file mode 100644
index 000000000..7fe621349
--- /dev/null
+++ b/tests/server/auth/test_provider.py
@@ -0,0 +1,77 @@
+"""
+Tests for mcp.server.auth.provider module.
+"""
+
+from mcp.server.auth.provider import construct_redirect_uri
+
+
+class TestConstructRedirectUri:
+    """Tests for the construct_redirect_uri function."""
+
+    def test_construct_redirect_uri_no_existing_params(self):
+        """Test construct_redirect_uri with no existing query parameters."""
+        base_uri = "http://localhost:8000/callback"
+        result = construct_redirect_uri(base_uri, code="auth_code", state="test_state")
+
+        assert "http://localhost:8000/callback?code=auth_code&state=test_state" == result
+
+    def test_construct_redirect_uri_with_existing_params(self):
+        """Test construct_redirect_uri with existing query parameters (regression test for #1279)."""
+        base_uri = "http://localhost:8000/callback?session_id=1234"
+        result = construct_redirect_uri(base_uri, code="auth_code", state="test_state")
+
+        # Should preserve existing params and add new ones
+        assert "session_id=1234" in result
+        assert "code=auth_code" in result
+        assert "state=test_state" in result
+        assert result.startswith("http://localhost:8000/callback?")
+
+    def test_construct_redirect_uri_multiple_existing_params(self):
+        """Test construct_redirect_uri with multiple existing query parameters."""
+        base_uri = "http://localhost:8000/callback?session_id=1234&user=test"
+        result = construct_redirect_uri(base_uri, code="auth_code")
+
+        assert "session_id=1234" in result
+        assert "user=test" in result
+        assert "code=auth_code" in result
+
+    def test_construct_redirect_uri_with_none_values(self):
+        """Test construct_redirect_uri filters out None values."""
+        base_uri = "http://localhost:8000/callback"
+        result = construct_redirect_uri(base_uri, code="auth_code", state=None)
+
+        assert result == "http://localhost:8000/callback?code=auth_code"
+        assert "state" not in result
+
+    def test_construct_redirect_uri_empty_params(self):
+        """Test construct_redirect_uri with no additional parameters."""
+        base_uri = "http://localhost:8000/callback?existing=param"
+        result = construct_redirect_uri(base_uri)
+
+        assert result == "http://localhost:8000/callback?existing=param"
+
+    def test_construct_redirect_uri_duplicate_param_names(self):
+        """Test construct_redirect_uri when adding param that already exists."""
+        base_uri = "http://localhost:8000/callback?code=existing"
+        result = construct_redirect_uri(base_uri, code="new_code")
+
+        # Should contain both values (this is expected behavior of parse_qs/urlencode)
+        assert "code=existing" in result
+        assert "code=new_code" in result
+
+    def test_construct_redirect_uri_multivalued_existing_params(self):
+        """Test construct_redirect_uri with existing multi-valued parameters."""
+        base_uri = "http://localhost:8000/callback?scope=read&scope=write"
+        result = construct_redirect_uri(base_uri, code="auth_code")
+
+        assert "scope=read" in result
+        assert "scope=write" in result
+        assert "code=auth_code" in result
+
+    def test_construct_redirect_uri_encoded_values(self):
+        """Test construct_redirect_uri handles URL encoding properly."""
+        base_uri = "http://localhost:8000/callback"
+        result = construct_redirect_uri(base_uri, state="test state with spaces")
+
+        # urlencode uses + for spaces by default
+        assert "state=test+state+with+spaces" in result