diff --git a/CLAUDE.md b/CLAUDE.md
index 5eba3bb..835d3a8 100644
--- a/CLAUDE.md
+++ b/CLAUDE.md
@@ -16,6 +16,7 @@ These are the non-negotiable principles of this project. Every decision — arch
 ### Methodology
 
 - Follow Research → Spec → Test → Implement → Review. No shortcuts.
+- API-first development: the OpenAPI spec (`backend/src/main/resources/openapi/api.yaml`) is the single source of truth for the REST API contract. Define endpoints and schemas in the spec first, then generate backend interfaces and frontend types before writing any implementation code.
 - Never write implementation code without a specification.
 - Always write tests before implementation (TDD). Red → Green → Refactor.
 - Refactoring is permitted freely as long as it does not alter the fundamental architecture.