diff --git a/extensions/configuration-editing/schemas/attachContainer.schema.json b/extensions/configuration-editing/schemas/attachContainer.schema.json
index cebd0f2bc3e..58e59d72ee0 100644
--- a/extensions/configuration-editing/schemas/attachContainer.schema.json
+++ b/extensions/configuration-editing/schemas/attachContainer.schema.json
@@ -40,7 +40,9 @@
 					"type": "array",
 					"description": "An array of extensions that should be installed into the container.",
 					"items": {
-						"type": "string"
+						"type": "string",
+						"pattern": "^([a-z0-9A-Z][a-z0-9\\-A-Z]*)\\.([a-z0-9A-Z][a-z0-9\\-A-Z]*)$",
+						"errorMessage": "Expected format '${publisher}.${name}'. Example: 'vscode.csharp'."
 					}
 				}
 			}
diff --git a/extensions/configuration-editing/schemas/devContainer.schema.json b/extensions/configuration-editing/schemas/devContainer.schema.json
index aed58455d01..c9c34af63fa 100644
--- a/extensions/configuration-editing/schemas/devContainer.schema.json
+++ b/extensions/configuration-editing/schemas/devContainer.schema.json
@@ -15,7 +15,9 @@
 					"type": "array",
 					"description": "An array of extensions that should be installed into the container.",
 					"items": {
-						"type": "string"
+						"type": "string",
+						"pattern": "^([a-z0-9A-Z][a-z0-9\\-A-Z]*)\\.([a-z0-9A-Z][a-z0-9\\-A-Z]*)$",
+						"errorMessage": "Expected format '${publisher}.${name}'. Example: 'vscode.csharp'."
 					}
 				},
 				"settings": {