diff --git a/AGENTS.md b/AGENTS.md
index 459e6fd..099485d 100644
--- a/AGENTS.md
+++ b/AGENTS.md
@@ -40,6 +40,19 @@ If two rules conflict, use the stricter one.
 - Never push remote changes unless the user explicitly requests it.
 - Keep AGENTS chain lean: put domain details in sub-AGENTS, avoid duplicate rules across layers.
 
+## Protocol as Source of Truth
+
+`docs/protocols/` is the single source of truth for data formats and protocols.
+
+- All data schemas, API contracts, and UI schema definitions MUST be documented in `docs/protocols/`.
+- Frontend and backend implementations MUST conform to the protocols defined in `docs/protocols/`.
+- Before modifying any data format or protocol:
+  1. Update the corresponding document in `docs/protocols/` first
+  2. Verify the change is backward compatible or plan migration
+  3. Then implement the code changes
+
+This ensures frontend and backend stay synchronized and prevents drift.
+
 ## Skills Index
 
 - `ag-ui`: AG-UI protocol implementation guidance.