Add tool input validation guidelines to AGENTS.md

AGENTS.md

@@ -31,3 +31,14 @@ The project is organized as a monorepo with workspaces:
 - `client/`: React frontend with Vite, TypeScript and Tailwind
 - `server/`: Express backend with TypeScript
 - `cli/`: Command-line interface for testing and invoking MCP server methods directly
+
+## Tool Input Validation Guidelines
+
+When handling tool input parameters and form fields:
+
+- **Optional fields with empty values should be omitted entirely** - Do not send empty strings or null values for optional parameters, UNLESS the field has an explicit default value in the schema that matches the current value
+- **Fields with explicit defaults should preserve their default values** - If a field has an explicit default in its schema (e.g., `default: null`), and the current value matches that default, include it in the request. This is a meaningful value the tool expects
+- **Required fields should preserve their values even when empty** - This allows the server to properly validate and return appropriate error messages
+- **Deeper validation should be handled by the server** - Inspector should focus on basic field presence, while the MCP server handles parameter validation according to its schema
+
+These guidelines ensure clean parameter passing and proper separation of concerns between the Inspector client and MCP servers.