diff --git a/sqlit/domains/connections/providers/oracle/adapter.py b/sqlit/domains/connections/providers/oracle/adapter.py
index d2a03b74..76edc895 100644
--- a/sqlit/domains/connections/providers/oracle/adapter.py
+++ b/sqlit/domains/connections/providers/oracle/adapter.py
@@ -77,12 +77,12 @@ def connect(self, config: ConnectionConfig) -> Any:
         connection_type = config.get_option("oracle_connection_type", "service_name")
 
         if connection_type == "sid":
-            # SID format: host:port:sid (uses colon separator)
-            # SID is stored in oracle_sid field, fall back to database for backward compat
+            # Thin-mode Easy Connect doesn't accept the legacy host:port:SID form;
+            # it tries to resolve the string as a TNS alias and fails with DPY-4027.
+            # makedsn emits a full TNS descriptor that thin mode handles directly.
             sid = config.get_option("oracle_sid") or endpoint.database
-            dsn = f"{endpoint.host}:{port}:{sid}"
+            dsn = oracledb.makedsn(endpoint.host, port, sid=sid)
         else:
-            # Service Name format: host:port/service_name (uses slash separator)
             dsn = f"{endpoint.host}:{port}/{endpoint.database}"
 
         # Determine connection mode based on oracle_role
diff --git a/tests/unit/test_oracle_adapter.py b/tests/unit/test_oracle_adapter.py
index b1fca942..7693b76e 100644
--- a/tests/unit/test_oracle_adapter.py
+++ b/tests/unit/test_oracle_adapter.py
@@ -4,6 +4,8 @@
 
 from unittest.mock import MagicMock, patch
 
+import pytest
+
 from tests.helpers import ConnectionConfig
 
 
@@ -160,7 +162,11 @@ def test_connect_service_name_format(self):
             assert call_kwargs["dsn"] == "localhost:1521/XEPDB1"
 
     def test_connect_sid_format(self):
-        """Test that sid connection type uses colon separator with oracle_sid field."""
+        """SID connection type must go through oracledb.makedsn — see issue #106.
+
+        The legacy host:port:SID Easy-Connect form is rejected by thin-mode with
+        DPY-4027, so the adapter must use makedsn to emit a TNS descriptor.
+        """
         mock_oracledb = MagicMock()
         mock_oracledb.AUTH_MODE_SYSDBA = 2
         mock_oracledb.AUTH_MODE_SYSOPER = 4
@@ -181,10 +187,9 @@ def test_connect_sid_format(self):
 
             adapter.connect(config)
 
-            mock_oracledb.connect.assert_called_once()
+            mock_oracledb.makedsn.assert_called_once_with("localhost", 1521, sid="ORCL")
             call_kwargs = mock_oracledb.connect.call_args.kwargs
-            # SID uses colon separator: host:port:sid
-            assert call_kwargs["dsn"] == "localhost:1521:ORCL"
+            assert call_kwargs["dsn"] is mock_oracledb.makedsn.return_value
 
     def test_connect_sid_backward_compat_uses_database_field(self):
         """Test that SID falls back to database field for backward compatibility."""
@@ -210,10 +215,7 @@ def test_connect_sid_backward_compat_uses_database_field(self):
 
             adapter.connect(config)
 
-            mock_oracledb.connect.assert_called_once()
-            call_kwargs = mock_oracledb.connect.call_args.kwargs
-            # Should use database field as fallback
-            assert call_kwargs["dsn"] == "localhost:1521:LEGACY_SID"
+            mock_oracledb.makedsn.assert_called_once_with("localhost", 1521, sid="LEGACY_SID")
 
     def test_connect_default_connection_type_is_service_name(self):
         """Test that missing oracle_connection_type defaults to service_name format."""
@@ -265,6 +267,34 @@ def test_connect_sid_with_custom_port(self):
 
             adapter.connect(config)
 
-            mock_oracledb.connect.assert_called_once()
-            call_kwargs = mock_oracledb.connect.call_args.kwargs
-            assert call_kwargs["dsn"] == "db.example.com:1522:PROD"
+            mock_oracledb.makedsn.assert_called_once_with("db.example.com", 1522, sid="PROD")
+
+    def test_sid_dsn_parses_in_real_driver(self):
+        """Issue #106 regression: adapter must produce a DSN that thin-mode accepts.
+
+        Calls the adapter against a port that is guaranteed closed so the driver
+        is forced to parse the DSN but cannot complete a network connection.
+        If parsing fails (DPY-4027), we've regressed.
+        """
+        oracledb = pytest.importorskip("oracledb")
+
+        from sqlit.domains.connections.providers.oracle.adapter import OracleAdapter
+
+        adapter = OracleAdapter()
+        config = ConnectionConfig(
+            name="test",
+            db_type="oracle",
+            server="127.0.0.1",
+            port="1",  # reserved port — guaranteed no Oracle listener
+            username="x",
+            password="x",
+            options={"oracle_connection_type": "sid", "oracle_sid": "FREE"},
+        )
+
+        with pytest.raises(oracledb.DatabaseError) as exc_info:
+            adapter.connect(config)
+
+        message = str(exc_info.value)
+        assert "DPY-4027" not in message, (
+            f"issue #106 regression: SID DSN rejected at parse step: {message}"
+        )