feat: updating merge behaviour of databricks_tags

Signed-off-by: Can Bekleyici <can.bekleyici@deepl.com>

Author: canbekley

CHANGELOG.md

@@ -13,6 +13,11 @@
 
 - Fix catalog names with special characters (e.g., hyphens) not being quoted in `SHOW SCHEMAS` commands, causing `INVALID_IDENTIFIER` errors ([#1325](https://github.com/databricks/dbt-databricks/issues/1325))
 
+### Under the Hood
+
+- **BREAKING:** `databricks_tags` defined at different hierarchy levels (e.g. project-level and model-level) now merge additively instead of the child config completely replacing the parent.
+
+
 ## dbt-databricks 1.11.5 (Feb 19, 2026)
 
 ### Fixes

dbt/adapters/databricks/impl.py

@@ -5,7 +5,7 @@
 from collections.abc import Iterable, Iterator
 from concurrent.futures import Future
 from contextlib import contextmanager
-from dataclasses import dataclass
+from dataclasses import dataclass, field
 from importlib import metadata
 from multiprocessing.context import SpawnContext
 from typing import TYPE_CHECKING, Any, ClassVar, Generic, NamedTuple, Optional, Union, cast
@@ -29,7 +29,7 @@
     SparkAdapter,
 )
 from dbt_common.behavior_flags import BehaviorFlag
-from dbt_common.contracts.config.base import BaseConfig
+from dbt_common.contracts.config.base import BaseConfig, MergeBehavior
 from dbt_common.exceptions import DbtConfigError, DbtInternalError
 from dbt_common.utils import executor
 from dbt_common.utils.dict import AttrDict
@@ -175,7 +175,7 @@ class DatabricksConfig(AdapterConfig):
     options: Optional[dict[str, str]] = None
     merge_update_columns: Optional[str] = None
     merge_exclude_columns: Optional[str] = None
-    databricks_tags: Optional[dict[str, str]] = None
+    databricks_tags: Optional[dict[str, str]] = field(default=None, metadata=MergeBehavior.Update.meta())
     query_tags: Optional[str] = None
     tblproperties: Optional[dict[str, str]] = None
     zorder: Optional[Union[list[str], str]] = None

tests/functional/adapter/tags/test_databricks_tags.py

@@ -15,6 +15,17 @@ def models(self):
             "tags.sql": fixtures.tags_sql.replace("table", self.materialized),
         }
 
+    @pytest.fixture(scope="class")
+    def project_config_update(self):
+        return {
+            "models": {
+                "+databricks_tags": {
+                    "a": "TO_BE_REPLACED_AT_LOWER_LEVEL",
+                    "x": "y",
+                }
+            }
+        }
+
     def test_tags(self, project):
         _ = util.run_dbt(["run", "--models", "tags"])
         _ = util.run_dbt(["run", "--models", "tags"])
@@ -23,8 +34,8 @@ def test_tags(self, project):
             " where schema_name = '{schema}' and table_name='tags'",
             fetch="all",
         )
-        assert len(results) == 3
-        expected_tags = {("a", "b"), ("c", "d"), ("k", "")}
+        assert len(results) == 4
+        expected_tags = {("a", "b"), ("c", "d"), ("k", ""), ("x", "y")}
         actual_tags = set((row[0], row[1]) for row in results)
         assert actual_tags == expected_tags