diff --git a/backend/llm.txt b/backend/llm.txt
index 8df481d..9219100 100644
--- a/backend/llm.txt
+++ b/backend/llm.txt
@@ -10,6 +10,7 @@ This file condenses `backend/docs/dev/http_api.md` + `backend/docs/dev/model.md`
 - MUST: HTTP module folder name MUST be `snake_case` (e.g. `tenant_public`), not `camelCase`/`mixedCase`.
 - DO keep controller methods thin: parse/bind → call `services.*` → return result/error.
 - DO regenerate code after changes (routes/docs/models).
+- MUST: in `backend/app/services`, prefer the generated GORM-Gen DAO (`backend/database/models/*`) for DB access; treat raw `*gorm.DB` usage as a last resort.
 - MUST: after adding/removing/renaming any files under `backend/app/services/`, run `atomctl gen service --path ./app/services` to regenerate `backend/app/services/services.gen.go`; DO NOT edit `services.gen.go` manually.
 - MUST: a single service's methods MUST live in a single file; do NOT split one service across multiple files (e.g. `type user struct{}` in `user.go` but methods in `user_admin.go`), because `atomctl gen service` uses filenames to infer services and will generate incorrect `services.gen.go`.
 - DO add `// @provider` above every controller/service `struct` declaration.
@@ -155,6 +156,42 @@ Models live in:
 
 - `atomctl gen model`
 
+---
+
+## 3) Service-layer DB access (GORM Gen)
+
+This project uses a PostgreSQL-focused GORM-Gen variant (`go.ipao.vip/gen` + generated `backend/database/models/*`).
+Reference: `backend/llm.gorm_gen.txt`.
+
+### 3.1 Query style (preferred)
+
+- MUST: in services, build queries via:
+  - `tbl, q := models.<Table>Query.QueryContext(ctx)`
+  - Use type-safe conditions (`tbl.ID.Eq(...)`, `tbl.TenantID.Eq(...)`, `tbl.DeletedAt.IsNull()`, etc).
+- DO NOT: use string SQL in `Where("...")` unless absolutely necessary.
+
+### 3.2 Transactions
+
+- MUST: use Gen transaction wrapper so all queries share the same tx connection:
+  - `models.Q.Transaction(func(tx *models.Query) error { ... })`
+  - Inside tx, use `tx.<Table>.QueryContext(ctx)` / `tx.<Table>.WithContext(ctx)`
+- DO NOT: use `_db.WithContext(ctx).Transaction(...)` in services unless Gen cannot express a required operation.
+
+### 3.3 Updates
+
+- Prefer `UpdateSimple(...)` with typed assign expressions when possible.
+- Otherwise use `Updates(map[string]any{...})`, but MUST:
+  - include tenant boundary conditions (`tenant_id`) in the WHERE,
+  - avoid updating columns by concatenating user input.
+
+### 3.4 Columns not in generated models (temporary escape hatch)
+
+If migrations add columns but `atomctl gen model` has not been re-run yet, the typed `models.<Struct>` will not contain those fields.
+In this case:
+- Use `q.UnderlyingDB()` (from Gen DO) to do a narrow query/update (single table, explicit columns).
+- Add a short Chinese comment explaining why, and that `atomctl gen model` should be run when DB is reachable.
+- Avoid spreading this pattern: keep it localized to one function.
+
 ### 2.2 Enum strategy
 
 - DO NOT use native DB ENUM.