diff --git a/HISTORY.rst b/HISTORY.rst
index 191e613..78eed42 100644
--- a/HISTORY.rst
+++ b/HISTORY.rst
@@ -14,6 +14,13 @@ History
 * ``RunInfo`` and ``UnfinishedRun`` gain a required ``project`` field:
   the run's project, taken from its ruleset's project when the run starts.
 * Added ``connect_timeout_seconds`` and ``read_timeout_seconds`` to the instance config models.
+* Added an optional ``instance`` field to ``DatabaseConnectionConfig``: the SQL Server named instance,
+  taken from the Instance input in the connection form's advanced settings.
+  It round-trips through ``list_connections`` and ``create_or_update_connection``,
+  and is sent only for ``mssql`` and ``mssql_linked``, the two engines that have it on the server.
+* ``DatabaseConnectionConfig.port`` is now optional.
+  A SQL Server connection that names an instance is reached through the SQL Server Browser and stores no port,
+  so a connection saved that way previously failed to deserialize.
 
 Requires server version 3.26.20
 
diff --git a/datamasque/client/models/connection.py b/datamasque/client/models/connection.py
index 5bec97b..9ddbddc 100644
--- a/datamasque/client/models/connection.py
+++ b/datamasque/client/models/connection.py
@@ -376,7 +376,9 @@ class DatabaseConnectionConfig(ConnectionConfig):
     """
 
     host: str
-    port: int
+    # The server leaves the port unset when the connection reaches its database another way: a SQL
+    # Server named instance resolved by the SQL Server Browser, or an engine with a default port.
+    port: Optional[int] = None
     database: str
     user: str
     password: Optional[str] = None
@@ -384,6 +386,9 @@ class DatabaseConnectionConfig(ConnectionConfig):
     engine_options: Optional[dict] = None
     db_schema: Optional[str] = Field(default=None, alias="schema")
     data_encoding: Optional[str] = None
+    # The SQL Server named instance to connect to, in place of a port. Only `mssql` and
+    # `mssql_linked` carry it on the server, so it is pruned for the rest.
+    instance: Optional[str] = None
     is_read_only: bool = False
     s3_bucket_name: Optional[str] = None
     s3_redshift_iam_role: Optional[str] = None
@@ -443,6 +448,8 @@ def _serialize(self, handler: Callable) -> dict:
             d.pop("s3_redshift_iam_role", None)
         if db_type not in _AWS_TAGGABLE_DATABASE_TYPES:
             d.pop("iam_role_arn", None)
+        if db_type not in {DatabaseType.mssql, DatabaseType.mssql_linked}:
+            d.pop("instance", None)
         if not d.get("engine_options"):
             d.pop("engine_options", None)
         return d
diff --git a/tests/test_connections.py b/tests/test_connections.py
index cbc3f7d..9469bdc 100644
--- a/tests/test_connections.py
+++ b/tests/test_connections.py
@@ -1594,6 +1594,117 @@ def test_redshift_connection_keeps_its_cluster_s3_role_separate_from_its_tagging
     assert api_dict["iam_role_arn"] == "arn:aws:iam::119836602066:role/tagger"
 
 
+def _sql_server_connection_json(**extra) -> dict:
+    """A SQL Server connection as `GET /api/connections/` returns it, with no port of its own."""
+    return {
+        "id": "9f1d2c4e-5b6a-4f8d-9c0e-1a2b3c4d5e6f",
+        "name": "mssql_named_instance",
+        "mask_type": "database",
+        "db_type": "mssql",
+        "host": "sql-server.example.com",
+        "database": "mssqldb",
+        "user": "mssqluser",
+        "schema": "dbo",
+        "is_read_only": False,
+        **extra,
+    }
+
+
+def test_mssql_connection_read_from_the_api_keeps_its_named_instance_and_blank_port():
+    """A named instance is reached through the SQL Server Browser, so the server stores no port."""
+    conn = DatabaseConnectionConfig.model_validate(_sql_server_connection_json(instance="SQLEXPRESS"))
+
+    assert conn.instance == "SQLEXPRESS"
+    assert conn.port is None
+
+
+def test_mssql_connection_read_from_the_api_without_an_instance_has_none():
+    conn = DatabaseConnectionConfig.model_validate(_sql_server_connection_json(port=1433))
+
+    assert conn.instance is None
+    assert conn.port == 1433
+
+
+def test_mssql_connection_round_trips_its_named_instance_back_to_the_api():
+    conn = DatabaseConnectionConfig.model_validate(_sql_server_connection_json(instance="SQLEXPRESS"))
+
+    api_dict = conn.model_dump(exclude_none=True, by_alias=True, mode="json")
+
+    assert api_dict["instance"] == "SQLEXPRESS"
+    assert "port" not in api_dict
+
+
+def test_create_mssql_connection_sends_the_named_instance(client):
+    config = DatabaseConnectionConfig(
+        name="mssql_named_instance",
+        host="sql-server.example.com",
+        database="mssqldb",
+        user="mssqluser",
+        password="secret",
+        database_type=DatabaseType.mssql,
+        instance="SQLEXPRESS",
+    )
+
+    with requests_mock.Mocker() as m:
+        m.get("http://test-server/api/connections/", json=[], status_code=200)
+        m.post("http://test-server/api/connections/", json={"id": "mssql-id-1"}, status_code=201)
+
+        result = client.create_or_update_connection(config)
+
+    assert result.id == ConnectionId("mssql-id-1")
+    sent = m.last_request.json()
+    assert sent["instance"] == "SQLEXPRESS"
+    assert sent["db_type"] == "mssql"
+
+
+def test_mssql_linked_server_connection_keeps_its_named_instance():
+    """A linked-server connection names the instance that holds the linked server."""
+    conn = MssqlLinkedServerConnectionConfig(
+        name="mssql_linked",
+        host="sql-server.example.com",
+        database="mssqldb",
+        user="mssqluser",
+        password="secret",
+        database_type=DatabaseType.mssql_linked,
+        linked_server="other.db.dbo",
+        instance="SQLEXPRESS",
+    )
+
+    api_dict = conn.model_dump(exclude_none=True, by_alias=True, mode="json")
+
+    assert api_dict["instance"] == "SQLEXPRESS"
+
+
+@pytest.mark.parametrize(
+    "database_type",
+    [
+        DatabaseType.postgres,
+        DatabaseType.mysql,
+        DatabaseType.oracle,
+        DatabaseType.db2_luw,
+        DatabaseType.redshift,
+    ],
+)
+def test_database_connection_omits_the_named_instance_for_engines_without_one(database_type):
+    """These engines have no `instance` field on the server, so sending one would be rejected."""
+    extra = (
+        {"s3_bucket_name": "bucket", "s3_redshift_iam_role": "arn:aws:iam::123456789012:role/redshift-s3"}
+        if database_type is DatabaseType.redshift
+        else {}
+    )
+    conn = _database_connection(database_type, instance="SQLEXPRESS", **extra)
+
+    api_dict = conn.model_dump(exclude_none=True, by_alias=True, mode="json")
+
+    assert "instance" not in api_dict
+
+
+def test_database_connection_omits_the_named_instance_when_unset():
+    conn = _database_connection(DatabaseType.mssql)
+
+    assert "instance" not in conn.model_dump(exclude_none=True, by_alias=True, mode="json")
+
+
 @pytest.mark.parametrize(
     ("database_type", "message"),
     [