diff --git a/semantica/mcp_server/__init__.py b/semantica/mcp_server/__init__.py
index ebb4cc5c..248a5687 100644
--- a/semantica/mcp_server/__init__.py
+++ b/semantica/mcp_server/__init__.py
@@ -267,9 +267,16 @@ def _tool_get_graph_analytics(args: dict) -> dict:
         return {"error": str(exc)}
 
 
+_EXPORT_GRAPH_FORMATS = ("turtle", "ttl", "nt", "xml", "json-ld", "json")
+
+
 def _tool_export_graph(args: dict) -> dict:
     """Export the current knowledge graph to a serialised format."""
     fmt = args.get("format", "json-ld")
+    if fmt not in _EXPORT_GRAPH_FORMATS:
+        return {
+            "error": f"Unsupported format '{fmt}'. Supported: {', '.join(_EXPORT_GRAPH_FORMATS)}"
+        }
     graph = _get_graph()
     try:
         from semantica.export import RDFExporter
@@ -453,7 +460,7 @@ TOOLS = [
             "properties": {
                 "format": {
                     "type": "string",
-                    "enum": ["turtle", "ttl", "nt", "xml", "json-ld", "json"],
+                    "enum": list(_EXPORT_GRAPH_FORMATS),
                     "description": "Export format (default: json-ld)",
                 }
             },
diff --git a/tests/test_mcp_server_export_graph.py b/tests/test_mcp_server_export_graph.py
index ca29f7c4..09179fd8 100644
--- a/tests/test_mcp_server_export_graph.py
+++ b/tests/test_mcp_server_export_graph.py
@@ -70,6 +70,21 @@ class TestExportGraphTool(unittest.TestCase):
     def test_progress_is_disabled_for_the_server_process(self):
         self.assertEqual(os.environ.get("SEMANTICA_DISABLE_PROGRESS"), "1")
 
+    def test_unsupported_format_returns_error_not_mislabeled_json(self):
+        """A format outside the declared enum (typo, unsupported value, or a
+        client that skips schema validation) must error, not silently return
+        JSON data mislabeled with the requested format string."""
+        result = mcp_server._tool_export_graph({"format": "yaml"})
+        self.assertIn("error", result)
+        self.assertIn("yaml", result["error"])
+
+    def test_export_graph_schema_enum_matches_handled_formats(self):
+        """The tool's declared inputSchema enum must not drift from the set
+        of formats the handler actually accepts."""
+        tool = next(t for t in mcp_server.TOOLS if t["name"] == "export_graph")
+        schema_enum = set(tool["inputSchema"]["properties"]["format"]["enum"])
+        self.assertEqual(schema_enum, set(mcp_server._EXPORT_GRAPH_FORMATS))
+
 
 if __name__ == "__main__":
     unittest.main()