diff --git a/AGENTS.md b/AGENTS.md
index 50d5cc8..91bac29 100644
--- a/AGENTS.md
+++ b/AGENTS.md
@@ -42,6 +42,11 @@ provide [specific document or clarification]." Never fill gaps with invented beh
 
 - Build a lightweight MVP first.
 - Prefer a working end-to-end flow over broad incomplete features.
+- The entire project follows TDD (Test-Driven Development). For every feature,
+  bug fix, data transformation, service, or behavior change, write the relevant
+  failing test first, implement the smallest code to pass it, then refactor only
+  after tests pass. If a task cannot reasonably be test-first, state the reason
+  before implementation and add verification coverage as close to the change as possible.
 - Keep the architecture aligned with the current plan: FastAPI, SQLite, SQLAlchemy, Jinja2 templates, simple CSS or Bootstrap, native JavaScript, and Docker Compose.
 - Do not introduce a frontend SPA framework, Redis, Celery, PostgreSQL, login system, scheduled jobs, or distributed workers unless the user explicitly changes the scope.
 - Do not commit real API keys, tokens, cookies, or private credentials.