diff --git a/converters/dbt/src/ossie_dbt/msi_to_osi.py b/converters/dbt/src/ossie_dbt/msi_to_osi.py
index db9e1315..30b7d06d 100644
--- a/converters/dbt/src/ossie_dbt/msi_to_osi.py
+++ b/converters/dbt/src/ossie_dbt/msi_to_osi.py
@@ -122,7 +122,6 @@ def convert(
         return ConverterResult(
             output=OSIDocument(
                 version="0.2.0.dev0",
-                dialects=[self._dialect],
                 semantic_model=[
                     OSISemanticModel(
                         name=osi_model_name,
diff --git a/converters/dbt/tests/__snapshots__/test_msi_to_osi.ambr b/converters/dbt/tests/__snapshots__/test_msi_to_osi.ambr
index facd135e..d88444ad 100644
--- a/converters/dbt/tests/__snapshots__/test_msi_to_osi.ambr
+++ b/converters/dbt/tests/__snapshots__/test_msi_to_osi.ambr
@@ -1,25 +1,7 @@
 # serializer version: 1
-# Licensed to the Apache Software Foundation (ASF) under one
-# or more contributor license agreements.  See the NOTICE file
-# distributed with this work for additional information
-# regarding copyright ownership.  The ASF licenses this file
-# to you under the Apache License, Version 2.0 (the
-# "License"); you may not use this file except in compliance
-# with the License.  You may obtain a copy of the License at
-#
-#   http://www.apache.org/licenses/LICENSE-2.0
-#
-# Unless required by applicable law or agreed to in writing,
-# software distributed under the License is distributed on an
-# "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
-# KIND, either express or implied.  See the License for the
-# specific language governing permissions and limitations
-# under the License.
 # name: TestMetricConversion.test_derived_metric_nested
   '''
   version: 0.2.0.dev0
-  dialects:
-  - ANSI_SQL
   semantic_model:
   - name: semantic_model
     datasets:
@@ -73,8 +55,6 @@
 # name: TestMetricConversion.test_ratio_metric_inlines_sub_expressions
   '''
   version: 0.2.0.dev0
-  dialects:
-  - ANSI_SQL
   semantic_model:
   - name: semantic_model
     datasets:
@@ -114,8 +94,6 @@
 # name: TestMetricFilterFlattening.test_metric_and_measure_filters_combined_with_and
   '''
   version: 0.2.0.dev0
-  dialects:
-  - ANSI_SQL
   semantic_model:
   - name: semantic_model
     datasets:
@@ -140,8 +118,6 @@
 # name: TestRelationshipConversion.test_three_datasets_produce_all_pairs
   '''
   version: 0.2.0.dev0
-  dialects:
-  - ANSI_SQL
   semantic_model:
   - name: semantic_model
     datasets:
diff --git a/converters/dbt/tests/__snapshots__/test_osi_to_msi.ambr b/converters/dbt/tests/__snapshots__/test_osi_to_msi.ambr
index e886e0b4..08ac9e4a 100644
--- a/converters/dbt/tests/__snapshots__/test_osi_to_msi.ambr
+++ b/converters/dbt/tests/__snapshots__/test_osi_to_msi.ambr
@@ -1,25 +1,7 @@
 # serializer version: 1
-# Licensed to the Apache Software Foundation (ASF) under one
-# or more contributor license agreements.  See the NOTICE file
-# distributed with this work for additional information
-# regarding copyright ownership.  The ASF licenses this file
-# to you under the Apache License, Version 2.0 (the
-# "License"); you may not use this file except in compliance
-# with the License.  You may obtain a copy of the License at
-#
-#   http://www.apache.org/licenses/LICENSE-2.0
-#
-# Unless required by applicable law or agreed to in writing,
-# software distributed under the License is distributed on an
-# "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
-# KIND, either express or implied.  See the License for the
-# specific language governing permissions and limitations
-# under the License.
 # name: TestOSIToMSIRoundTrip.test_osi_to_msi_to_osi_preserves_structure
   '''
   version: 0.2.0.dev0
-  dialects:
-  - ANSI_SQL
   semantic_model:
   - name: semantic_model
     datasets:
diff --git a/converters/dbt/tests/test_msi_to_osi.py b/converters/dbt/tests/test_msi_to_osi.py
index a0aa9f8a..9284ec4d 100644
--- a/converters/dbt/tests/test_msi_to_osi.py
+++ b/converters/dbt/tests/test_msi_to_osi.py
@@ -19,6 +19,7 @@
 from typing import List, Optional
 
 import pytest
+import yaml
 from syrupy.assertion import SnapshotAssertion
 
 from ossie_dbt.converter_issues import ConverterIssueType
@@ -126,6 +127,19 @@ def test_multiple_semantic_models_become_multiple_datasets(self) -> None:
         names = [ds.name for ds in result.semantic_model[0].datasets]
         assert names == ["orders", "users"]
 
+    def test_serialized_output_omits_schema_invalid_root_advertisement_fields(self) -> None:
+        sm = semantic_model_with_guaranteed_meta(
+            name="orders",
+            dimensions=[_dimension("status")],
+        )
+        result = MSIToOSIConverter().convert(_manifest(semantic_models=[sm])).output
+
+        data = yaml.safe_load(result.to_osi_yaml())
+
+        assert "dialects" not in data
+        assert "vendors" not in data
+        assert _fields(result)[0].expression.dialects[0].dialect == OSIDialect.ANSI_SQL
+
 
 class TestDimensionConversion:
     def test_categorical_dimension_has_is_time_false(self) -> None:
@@ -302,7 +316,6 @@ def test_default_dialect_is_ansi_sql(self) -> None:
         )
         result = MSIToOSIConverter().convert(_manifest(semantic_models=[sm])).output
 
-        assert result.dialects == [OSIDialect.ANSI_SQL]
         assert _fields(result)[0].expression.dialects[0].dialect == OSIDialect.ANSI_SQL
 
     def test_configurable_dialect(self) -> None:
@@ -312,7 +325,6 @@ def test_configurable_dialect(self) -> None:
         )
         result = MSIToOSIConverter(dialect=OSIDialect.SNOWFLAKE).convert(_manifest(semantic_models=[sm])).output
 
-        assert result.dialects == [OSIDialect.SNOWFLAKE]
         assert _fields(result)[0].expression.dialects[0].dialect == OSIDialect.SNOWFLAKE
 
 
diff --git a/converters/honeydew/src/honeydew_osi/converter.py b/converters/honeydew/src/honeydew_osi/converter.py
index e3f78f4f..1e55cf6a 100644
--- a/converters/honeydew/src/honeydew_osi/converter.py
+++ b/converters/honeydew/src/honeydew_osi/converter.py
@@ -88,6 +88,13 @@ def convert_osi_to_honeydew(osi_yaml_str: str) -> dict[str, str]:
     if not isinstance(root, dict):
         raise HoneydewConversionError("Invalid OSI YAML: expected a mapping at the root")
 
+    extra_root_fields = sorted(set(root) - {"version", "semantic_model"})
+    if extra_root_fields:
+        raise HoneydewConversionError(
+            "Invalid OSI YAML: unsupported root field(s): "
+            + ", ".join(extra_root_fields)
+        )
+
     version_str = str(root.get("version", ""))
     if version_str != SUPPORTED_OSI_VERSION:
         raise HoneydewConversionError(
@@ -104,11 +111,10 @@ def convert_osi_to_honeydew(osi_yaml_str: str) -> dict[str, str]:
             "only the first will be converted"
         )
 
-    vendors = [v for v in (root.get("vendors") or []) if v != HONEYDEW_VENDOR]
-    return _model_to_files(semantic_models[0], extra_vendors=vendors)
+    return _model_to_files(semantic_models[0])
 
 
-def _model_to_files(sm: dict[str, Any], *, extra_vendors: list[str] | None = None) -> dict[str, str]:
+def _model_to_files(sm: dict[str, Any]) -> dict[str, str]:
     name = sm.get("name")
     if not name:
         raise HoneydewConversionError("Missing 'name' in semantic model")
@@ -119,13 +125,12 @@ def _model_to_files(sm: dict[str, Any], *, extra_vendors: list[str] | None = Non
     if sm.get("description"):
         workspace["description"] = sm["description"]
 
-    # Preserve model-level ai_context, non-HONEYDEW custom_extensions, and extra vendors
+    # Preserve model-level ai_context and non-HONEYDEW custom_extensions.
     model_ai_ctx = sm.get("ai_context")
     model_ext = [e for e in (sm.get("custom_extensions") or []) if e.get("vendor_name") != HONEYDEW_VENDOR]
     ws_meta = _build_osi_metadata(
         ai_context=model_ai_ctx,
         custom_extensions=model_ext or None,
-        extra_vendors=extra_vendors or None,
     )
     if ws_meta:
         workspace["metadata"] = [ws_meta]
@@ -602,11 +607,8 @@ def convert_honeydew_to_osi(workspace_dir: str) -> str:
     if osi_metrics:
         sm["metrics"] = osi_metrics
 
-    extra_vendors = ws_osi_meta.get("vendors") or []
-    vendors = [HONEYDEW_VENDOR] + [v for v in extra_vendors if v != HONEYDEW_VENDOR]
     root: dict[str, Any] = {
         "version": SUPPORTED_OSI_VERSION,
-        "vendors": vendors,
         "semantic_model": [sm],
     }
     return _dump(root)
@@ -930,7 +932,6 @@ def _build_osi_metadata(
     label: str | None = None,
     unique_keys: Any = None,
     custom_extensions: list | None = None,
-    extra_vendors: list[str] | None = None,
 ) -> dict[str, Any] | None:
     """Build a Honeydew metadata entry that stores OSI-only fields for round-tripping."""
     items: list[dict[str, Any]] = []
@@ -944,8 +945,6 @@ def _build_osi_metadata(
         items.append({"name": "unique_keys", "value": json.dumps(unique_keys)})
     if custom_extensions:
         items.append({"name": "custom_extensions", "value": json.dumps(custom_extensions)})
-    if extra_vendors:
-        items.append({"name": "vendors", "value": json.dumps(extra_vendors)})
 
     if not items:
         return None
@@ -968,7 +967,7 @@ def _read_osi_metadata(obj: dict[str, Any]) -> dict[str, Any]:
                     result[key] = raw
             elif key == "label":
                 result[key] = raw
-            elif key in ("unique_keys", "custom_extensions", "vendors"):
+            elif key in ("unique_keys", "custom_extensions"):
                 try:
                     result[key] = json.loads(raw)
                 except (json.JSONDecodeError, TypeError):
@@ -1065,4 +1064,4 @@ def main() -> None:
 
 
 if __name__ == "__main__":
-    main()
\ No newline at end of file
+    main()
diff --git a/converters/honeydew/tests/test_honeydew_osi_converter.py b/converters/honeydew/tests/test_honeydew_osi_converter.py
index c3aaa75c..f89a62ff 100644
--- a/converters/honeydew/tests/test_honeydew_osi_converter.py
+++ b/converters/honeydew/tests/test_honeydew_osi_converter.py
@@ -608,13 +608,29 @@ def test_osi_to_honeydew_multiple_models_warns():
 # ─────────────────────────────────────────────────────────────────────────────
 
 def _hd_root(sm):
-    return {"version": OSI_VERSION, "vendors": ["HONEYDEW"], "semantic_model": [sm]}
+    return {"version": OSI_VERSION, "semantic_model": [sm]}
 
 
 def _ansi(expr):
     return {"dialects": [{"dialect": "ANSI_SQL", "expression": expr}]}
 
 
+def test_honeydew_to_osi_omits_schema_invalid_root_advertisement_fields(tmp_path):
+    _write_workspace(str(tmp_path), "ws", [{
+        "name": "orders",
+        "keys": ["id"],
+        "key_dataset": "orders",
+        "sql": "db.s.orders",
+        "dataset_attrs": [],
+    }])
+
+    result = yaml.safe_load(convert_honeydew_to_osi(str(tmp_path)))
+
+    assert "vendors" not in result
+    assert "dialects" not in result
+    assert result["semantic_model"][0]["datasets"][0]["name"] == "orders"
+
+
 @pytest.mark.parametrize("ws_name,entities,expected_root", [
     # ── basic entity with two dataset attributes ──────────────────────────────
     pytest.param(
@@ -800,8 +816,7 @@ def test_honeydew_to_osi_missing_workspace_raises(tmp_path):
 def test_honeydew_to_osi_missing_schema_dir_empty_model(tmp_path):
     (tmp_path / "workspace.yml").write_text(yaml.dump({"type": "workspace", "name": "ws"}))
     result = yaml.safe_load(convert_honeydew_to_osi(str(tmp_path)))
-    assert result == {"version": OSI_VERSION, "vendors": ["HONEYDEW"],
-                      "semantic_model": [{"name": "ws", "datasets": []}]}
+    assert result == {"version": OSI_VERSION, "semantic_model": [{"name": "ws", "datasets": []}]}
 
 
 def test_honeydew_to_osi_empty_metric_sql_skipped(tmp_path):
@@ -1405,28 +1420,22 @@ def test_connectionless_relation_warns():
 
 
 # ─────────────────────────────────────────────────────────────────────────────
-# Vendors round-trip
+# Root advertisement fields are schema-invalid
 # ─────────────────────────────────────────────────────────────────────────────
 
-@pytest.mark.parametrize("input_vendors,expected_vendors", [
-    (["SNOWFLAKE", "HONEYDEW"], ["HONEYDEW", "SNOWFLAKE"]),
-    (["SNOWFLAKE"], ["HONEYDEW", "SNOWFLAKE"]),
-    (["HONEYDEW"], ["HONEYDEW"]),
+@pytest.mark.parametrize("field,value", [
+    ("vendors", ["SNOWFLAKE", "HONEYDEW"]),
+    ("dialects", ["ANSI_SQL"]),
 ])
-def test_vendors_roundtrip(tmp_path, input_vendors, expected_vendors):
+def test_osi_to_honeydew_rejects_root_advertisement_fields(field, value):
     doc = yaml.dump({
         "version": OSI_VERSION,
-        "vendors": input_vendors,
+        field: value,
         "semantic_model": [{"name": "m", "datasets": []}],
     })
-    files = convert_osi_to_honeydew(doc)
-    for rel_path, content in files.items():
-        p = tmp_path / rel_path
-        p.parent.mkdir(parents=True, exist_ok=True)
-        p.write_text(content)
-    result = yaml.safe_load(convert_honeydew_to_osi(str(tmp_path)))
-    assert result == {"version": OSI_VERSION, "vendors": expected_vendors,
-                      "semantic_model": [{"name": "m", "datasets": []}]}
+
+    with pytest.raises(HoneydewConversionError, match=field):
+        convert_osi_to_honeydew(doc)
 
 
 # ─────────────────────────────────────────────────────────────────────────────
@@ -1469,7 +1478,6 @@ def test_main_honeydew_to_osi(tmp_path):
     assert result.returncode == 0
     assert yaml.safe_load(output_file.read_text()) == {
         "version": OSI_VERSION,
-        "vendors": ["HONEYDEW"],
         "semantic_model": [{"name": "ws", "datasets": [
             {"name": "orders", "source": "DB.S.ORDERS", "primary_key": ["id"],
              "unique_keys": [["id"]]},
diff --git a/python/src/ossie/models.py b/python/src/ossie/models.py
index 5406a743..87d56c34 100644
--- a/python/src/ossie/models.py
+++ b/python/src/ossie/models.py
@@ -206,11 +206,9 @@ class OSISemanticModel(BaseModel):
 class OSIDocument(BaseModel):
     """Root Ossie document."""
 
-    model_config = ConfigDict(frozen=True)
+    model_config = ConfigDict(frozen=True, extra="forbid")
 
     version: str = "0.2.0.dev0"
-    dialects: Optional[list[OSIDialect]] = None
-    vendors: Optional[list[OSIVendor]] = None
     semantic_model: list[OSISemanticModel]
 
     def to_osi_yaml(self, **kwargs: Any) -> str:
diff --git a/python/tests/test_models.py b/python/tests/test_models.py
index 749a226c..9ac2d1e6 100644
--- a/python/tests/test_models.py
+++ b/python/tests/test_models.py
@@ -111,6 +111,14 @@ def test_invalid_datatype_is_rejected() -> None:
         OSIDocument.model_validate(document)
 
 
+def test_root_dialects_field_is_rejected() -> None:
+    document = _document()
+    document["dialects"] = ["ANSI_SQL"]
+
+    with pytest.raises(ValidationError):
+        OSIDocument.model_validate(document)
+
+
 @pytest.mark.parametrize(
     ("dimension", "datatype", "expected"),
     [