diff --git a/CHANGELOG.md b/CHANGELOG.md
index bbab44cbe..fef952ffd 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -6,6 +6,10 @@
 
 - MarkdownLoader (experimental): added a Markdown loader to support `.md` and `.markdown` files.
 
+### Fixed
+
+- `VertexAILLM`: passing a Pydantic model with `extra="forbid"` as `response_format` no longer raises a `ParseError`. Unsupported JSON Schema fields (e.g. `additionalProperties`) are now stripped before the schema is forwarded to VertexAI's protobuf `Schema` type.
+
 ### Changed
 
 - SimpleKG pipeline (experimental): the `from_pdf` parameter is deprecated in favor of `from_file` (PDF and Markdown inputs). `from_pdf` still works but emits a deprecation warning and will be removed in a future version.
diff --git a/src/neo4j_graphrag/llm/vertexai_llm.py b/src/neo4j_graphrag/llm/vertexai_llm.py
index b49f3634e..23fd5e156 100644
--- a/src/neo4j_graphrag/llm/vertexai_llm.py
+++ b/src/neo4j_graphrag/llm/vertexai_llm.py
@@ -69,6 +69,30 @@
 _GENERATION_CONFIG_SCHEMA_PARAMS = {"response_schema", "response_mime_type"}
 
 
+def _strip_unsupported_schema_fields(schema: dict[str, Any]) -> dict[str, Any]:
+    """Recursively remove JSON Schema fields not supported by VertexAI's Schema proto.
+
+    For example, Pydantic adds ``additionalProperties: false`` when a model uses
+    ``extra="forbid"``, but the VertexAI protobuf Schema type does not have that
+    field and raises a ``ParseError`` when it encounters it.
+    """
+    _UNSUPPORTED = {"additionalProperties", "$defs", "$schema"}
+    result = {k: v for k, v in schema.items() if k not in _UNSUPPORTED}
+    if "properties" in result and isinstance(result["properties"], dict):
+        result["properties"] = {
+            k: _strip_unsupported_schema_fields(v)
+            for k, v in result["properties"].items()
+        }
+    if "items" in result and isinstance(result["items"], dict):
+        result["items"] = _strip_unsupported_schema_fields(result["items"])
+    if "anyOf" in result and isinstance(result["anyOf"], list):
+        result["anyOf"] = [
+            _strip_unsupported_schema_fields(s) if isinstance(s, dict) else s
+            for s in result["anyOf"]
+        ]
+    return result
+
+
 def _extract_generation_config_params(
     config: Any, exclude_schema: bool = True
 ) -> dict[str, Any]:
@@ -574,7 +598,9 @@ def _get_call_params_v2(
                         response_format, BaseModel
                     ):
                         # if we migrate to new google-genai-sdk, Pydantic models can be passed directly
-                        schema = response_format.model_json_schema()
+                        schema = _strip_unsupported_schema_fields(
+                            response_format.model_json_schema()
+                        )
                     else:
                         schema = response_format
                     params["response_mime_type"] = "application/json"