diff --git a/apps/api/src/models.rs b/apps/api/src/models.rs
index 1cb5c88..c88c266 100644
--- a/apps/api/src/models.rs
+++ b/apps/api/src/models.rs
@@ -86,24 +86,6 @@ pub enum ProductStatus {
     Unpublished,
 }
 
-#[derive(Debug, Serialize, sqlx::FromRow)]
-pub struct Category {
-    pub id: Uuid,
-    pub parent_id: Option<Uuid>,
-    pub name: serde_json::Value,
-    pub slug: String,
-    pub position: i32,
-}
-
-#[derive(Debug, Serialize, sqlx::FromRow)]
-pub struct Brand {
-    pub id: Uuid,
-    pub name: serde_json::Value,
-    pub slug: String,
-    pub position: i32,
-    pub active: bool,
-}
-
 #[derive(Debug, Clone, Serialize, sqlx::FromRow)]
 pub struct Product {
     pub id: Uuid,
diff --git a/apps/api/src/modules/brand/dto.rs b/apps/api/src/modules/brand/dto.rs
new file mode 100644
index 0000000..07faac0
--- /dev/null
+++ b/apps/api/src/modules/brand/dto.rs
@@ -0,0 +1,11 @@
+use serde::Serialize;
+use uuid::Uuid;
+
+#[derive(Debug, Serialize, sqlx::FromRow)]
+pub struct Brand {
+    pub id: Uuid,
+    pub name: serde_json::Value,
+    pub slug: String,
+    pub position: i32,
+    pub active: bool,
+}
diff --git a/apps/api/src/modules/brand/handlers.rs b/apps/api/src/modules/brand/handlers.rs
new file mode 100644
index 0000000..f0ce29f
--- /dev/null
+++ b/apps/api/src/modules/brand/handlers.rs
@@ -0,0 +1,27 @@
+use axum::{routing::{get, put}, extract::State, Json, Router};
+
+use crate::auth::AuthUser;
+use crate::error::ApiResult;
+use crate::state::AppState;
+
+use super::dto::Brand;
+use super::service::{self, BrandInput};
+
+pub fn router() -> Router<AppState> {
+    Router::new()
+        .route("/brands", get(list_brands))
+        .route("/admin/brands", put(replace_brands))
+}
+
+async fn list_brands(State(state): State<AppState>) -> ApiResult<Json<Vec<Brand>>> {
+    Ok(Json(service::list_brands(&state).await?))
+}
+
+async fn replace_brands(
+    State(state): State<AppState>,
+    auth: AuthUser,
+    Json(body): Json<Vec<BrandInput>>,
+) -> ApiResult<Json<Vec<Brand>>> {
+    auth.require_admin()?;
+    Ok(Json(service::replace_brands(&state, body).await?))
+}
diff --git a/apps/api/src/modules/brand/mod.rs b/apps/api/src/modules/brand/mod.rs
new file mode 100644
index 0000000..7892b06
--- /dev/null
+++ b/apps/api/src/modules/brand/mod.rs
@@ -0,0 +1,11 @@
+mod dto;
+mod handlers;
+pub mod service;
+
+use axum::Router;
+
+use crate::state::AppState;
+
+pub fn router() -> Router<AppState> {
+    handlers::router()
+}
diff --git a/apps/api/src/modules/brand/service.rs b/apps/api/src/modules/brand/service.rs
new file mode 100644
index 0000000..1866f7e
--- /dev/null
+++ b/apps/api/src/modules/brand/service.rs
@@ -0,0 +1,68 @@
+use crate::error::{unique_conflict, ApiError, ApiResult};
+use crate::state::AppState;
+
+use super::dto::Brand;
+
+pub async fn list_brands(state: &AppState) -> ApiResult<Vec<Brand>> {
+    Ok(sqlx::query_as::<_, Brand>(
+        "SELECT id, name, slug, position, active FROM brands WHERE active = TRUE ORDER BY position, created_at",
+    )
+    .fetch_all(&state.db)
+    .await?)
+}
+
+#[derive(serde::Deserialize)]
+pub struct BrandInput {
+    pub slug: String,
+    pub name: serde_json::Value,
+    #[serde(default = "default_active")]
+    pub active: bool,
+}
+
+fn default_active() -> bool {
+    true
+}
+
+pub async fn replace_brands(state: &AppState, body: Vec<BrandInput>) -> ApiResult<Vec<Brand>> {
+    for brand in &body {
+        if brand.slug.trim().is_empty()
+            || !brand
+                .slug
+                .chars()
+                .all(|c| c.is_ascii_alphanumeric() || c == '-')
+        {
+            return Err(ApiError::BadRequest(format!(
+                "invalid brand slug: {}",
+                brand.slug
+            )));
+        }
+        let ok = ["en", "zh"].iter().all(|code| {
+            brand
+                .name
+                .get(code)
+                .and_then(serde_json::Value::as_str)
+                .is_some_and(|s| !s.trim().is_empty())
+        });
+        if !ok {
+            return Err(ApiError::BadRequest(format!(
+                "brand {} needs non-empty en and zh names",
+                brand.slug
+            )));
+        }
+    }
+
+    let mut tx = state.db.begin().await?;
+    sqlx::query("DELETE FROM brands").execute(&mut *tx).await?;
+    for (i, brand) in body.iter().enumerate() {
+        sqlx::query("INSERT INTO brands (name, slug, position, active) VALUES ($1, $2, $3, $4)")
+            .bind(&brand.name)
+            .bind(brand.slug.trim())
+            .bind(i as i32)
+            .bind(brand.active)
+            .execute(&mut *tx)
+            .await
+            .map_err(|e| unique_conflict(e, format!("duplicate brand slug: {}", brand.slug)))?;
+    }
+    tx.commit().await?;
+    list_brands(state).await
+}
diff --git a/apps/api/src/modules/category/dto.rs b/apps/api/src/modules/category/dto.rs
new file mode 100644
index 0000000..01d5137
--- /dev/null
+++ b/apps/api/src/modules/category/dto.rs
@@ -0,0 +1,11 @@
+use serde::Serialize;
+use uuid::Uuid;
+
+#[derive(Debug, Serialize, sqlx::FromRow)]
+pub struct Category {
+    pub id: Uuid,
+    pub parent_id: Option<Uuid>,
+    pub name: serde_json::Value,
+    pub slug: String,
+    pub position: i32,
+}
diff --git a/apps/api/src/modules/category/handlers.rs b/apps/api/src/modules/category/handlers.rs
new file mode 100644
index 0000000..37596dc
--- /dev/null
+++ b/apps/api/src/modules/category/handlers.rs
@@ -0,0 +1,15 @@
+use axum::{extract::State, routing::get, Json, Router};
+
+use crate::error::ApiResult;
+use crate::state::AppState;
+
+use super::dto::Category;
+use super::service;
+
+pub fn router() -> Router<AppState> {
+    Router::new().route("/categories", get(list_categories))
+}
+
+async fn list_categories(State(state): State<AppState>) -> ApiResult<Json<Vec<Category>>> {
+    Ok(Json(service::list_categories(&state).await?))
+}
diff --git a/apps/api/src/modules/category/mod.rs b/apps/api/src/modules/category/mod.rs
new file mode 100644
index 0000000..7892b06
--- /dev/null
+++ b/apps/api/src/modules/category/mod.rs
@@ -0,0 +1,11 @@
+mod dto;
+mod handlers;
+pub mod service;
+
+use axum::Router;
+
+use crate::state::AppState;
+
+pub fn router() -> Router<AppState> {
+    handlers::router()
+}
diff --git a/apps/api/src/modules/category/service.rs b/apps/api/src/modules/category/service.rs
new file mode 100644
index 0000000..a33bbb1
--- /dev/null
+++ b/apps/api/src/modules/category/service.rs
@@ -0,0 +1,18 @@
+use crate::error::ApiResult;
+use crate::state::AppState;
+
+use super::dto::Category;
+
+pub(crate) const SUBTREE_CTE: &str = "WITH RECURSIVE subtree AS (
+         SELECT id FROM categories WHERE id = $1::uuid
+         UNION ALL
+         SELECT c.id FROM categories c JOIN subtree st ON c.parent_id = st.id
+     ) ";
+
+pub async fn list_categories(state: &AppState) -> ApiResult<Vec<Category>> {
+    Ok(sqlx::query_as::<_, Category>(
+        "SELECT id, parent_id, name, slug, position FROM categories ORDER BY position, slug",
+    )
+    .fetch_all(&state.db)
+    .await?)
+}
diff --git a/apps/api/src/modules/mod.rs b/apps/api/src/modules/mod.rs
index a553a53..a76c530 100644
--- a/apps/api/src/modules/mod.rs
+++ b/apps/api/src/modules/mod.rs
@@ -1,8 +1,9 @@
 pub mod account;
 pub mod address;
 pub mod billing;
+pub mod brand;
 pub mod cart;
-pub mod catalog;
+pub mod category;
 pub mod content;
 pub mod coupon;
 pub mod currency;
@@ -14,6 +15,7 @@ pub mod health;
 pub mod identity;
 pub mod order;
 pub mod points;
+pub mod product;
 pub mod shop;
 
 use axum::Router;
@@ -27,7 +29,9 @@ pub fn api_router() -> Router<AppState> {
         .merge(address::router())
         .merge(identity::router())
         .merge(currency::router())
-        .merge(catalog::router())
+        .merge(product::router())
+        .merge(category::router())
+        .merge(brand::router())
         .merge(content::router())
         .merge(cart::router())
         .merge(coupon::router())
diff --git a/apps/api/src/modules/catalog/dto.rs b/apps/api/src/modules/product/dto.rs
similarity index 100%
rename from apps/api/src/modules/catalog/dto.rs
rename to apps/api/src/modules/product/dto.rs
diff --git a/apps/api/src/modules/catalog/handlers.rs b/apps/api/src/modules/product/handlers.rs
similarity index 80%
rename from apps/api/src/modules/catalog/handlers.rs
rename to apps/api/src/modules/product/handlers.rs
index b6c4a0a..058b7e7 100644
--- a/apps/api/src/modules/catalog/handlers.rs
+++ b/apps/api/src/modules/product/handlers.rs
@@ -1,7 +1,7 @@
 use axum::{
     extract::{Path, Query, State},
     http::StatusCode,
-    routing::{get, post, put},
+    routing::{get, post},
     Json, Router,
 };
 use serde::Deserialize;
@@ -10,19 +10,16 @@ use uuid::Uuid;
 use crate::auth::AuthUser;
 use crate::error::ApiResult;
 use crate::http::Paged;
-use crate::models::{Brand, Category, ProductStatus, Sku};
+use crate::models::{ProductStatus, Sku};
 use crate::state::AppState;
 
 use super::dto::{ProductBody, ProductWithSkus, PublicListQuery, SkuBody};
-use super::service::{self, BrandInput};
+use super::service;
 
 pub fn router() -> Router<AppState> {
     Router::new()
         .route("/products", get(list_products))
         .route("/products/{id_or_slug}", get(get_product))
-        .route("/categories", get(list_categories))
-        .route("/brands", get(list_brands))
-        .route("/admin/brands", put(replace_brands))
         .route("/shop/products", get(shop_list).post(create_product))
         .route("/shop/products/{id}", get(shop_get).put(update_product))
         .route("/shop/products/{id}/publish", post(publish))
@@ -44,23 +41,6 @@ async fn get_product(
     Ok(Json(service::get_public(&state, &id_or_slug).await?))
 }
 
-async fn list_categories(State(state): State<AppState>) -> ApiResult<Json<Vec<Category>>> {
-    Ok(Json(service::list_categories(&state).await?))
-}
-
-async fn list_brands(State(state): State<AppState>) -> ApiResult<Json<Vec<Brand>>> {
-    Ok(Json(service::list_brands(&state).await?))
-}
-
-async fn replace_brands(
-    State(state): State<AppState>,
-    auth: AuthUser,
-    Json(body): Json<Vec<BrandInput>>,
-) -> ApiResult<Json<Vec<Brand>>> {
-    auth.require_admin()?;
-    Ok(Json(service::replace_brands(&state, body).await?))
-}
-
 #[derive(Deserialize)]
 struct ShopListQuery {
     page: Option<i64>,
diff --git a/apps/api/src/modules/catalog/mod.rs b/apps/api/src/modules/product/mod.rs
similarity index 100%
rename from apps/api/src/modules/catalog/mod.rs
rename to apps/api/src/modules/product/mod.rs
diff --git a/apps/api/src/modules/catalog/repo.rs b/apps/api/src/modules/product/repo.rs
similarity index 100%
rename from apps/api/src/modules/catalog/repo.rs
rename to apps/api/src/modules/product/repo.rs
diff --git a/apps/api/src/modules/catalog/service.rs b/apps/api/src/modules/product/service.rs
similarity index 81%
rename from apps/api/src/modules/catalog/service.rs
rename to apps/api/src/modules/product/service.rs
index 8af69bf..62a4f59 100644
--- a/apps/api/src/modules/catalog/service.rs
+++ b/apps/api/src/modules/product/service.rs
@@ -1,6 +1,7 @@
 use crate::error::{unique_conflict, ApiError, ApiResult};
 use crate::http::{clamp_page, clamp_per_page, Paged};
-use crate::models::{Brand, Category, Product, ProductStatus, Sku};
+use crate::models::{Product, ProductStatus, Sku};
+use crate::modules::category::service::SUBTREE_CTE;
 use crate::state::AppState;
 use uuid::Uuid;
 
@@ -14,12 +15,6 @@ const SOLD_UNITS: &str = "(SELECT COALESCE(SUM(oi.qty), 0)
       WHERE sk.product_id = p.id
         AND o.status IN ('paid', 'fulfilling', 'shipped', 'completed'))";
 
-const SUBTREE_CTE: &str = "WITH RECURSIVE subtree AS (
-         SELECT id FROM categories WHERE id = $1::uuid
-         UNION ALL
-         SELECT c.id FROM categories c JOIN subtree st ON c.parent_id = st.id
-     ) ";
-
 const MIN_PRICE: &str =
     "(SELECT MIN(price_minor) FROM skus WHERE product_id = p.id AND active = TRUE)";
 
@@ -111,78 +106,6 @@ pub async fn get_public(state: &AppState, id_or_slug: &str) -> ApiResult<Product
     Ok(items.remove(0))
 }
 
-pub async fn list_categories(state: &AppState) -> ApiResult<Vec<Category>> {
-    Ok(sqlx::query_as::<_, Category>(
-        "SELECT id, parent_id, name, slug, position FROM categories ORDER BY position, slug",
-    )
-    .fetch_all(&state.db)
-    .await?)
-}
-
-pub async fn list_brands(state: &AppState) -> ApiResult<Vec<Brand>> {
-    Ok(sqlx::query_as::<_, Brand>(
-        "SELECT id, name, slug, position, active FROM brands WHERE active = TRUE ORDER BY position, created_at",
-    )
-    .fetch_all(&state.db)
-    .await?)
-}
-
-#[derive(serde::Deserialize)]
-pub struct BrandInput {
-    pub slug: String,
-    pub name: serde_json::Value,
-    #[serde(default = "default_active")]
-    pub active: bool,
-}
-
-fn default_active() -> bool {
-    true
-}
-
-pub async fn replace_brands(state: &AppState, body: Vec<BrandInput>) -> ApiResult<Vec<Brand>> {
-    for brand in &body {
-        if brand.slug.trim().is_empty()
-            || !brand
-                .slug
-                .chars()
-                .all(|c| c.is_ascii_alphanumeric() || c == '-')
-        {
-            return Err(ApiError::BadRequest(format!(
-                "invalid brand slug: {}",
-                brand.slug
-            )));
-        }
-        let ok = ["en", "zh"].iter().all(|code| {
-            brand
-                .name
-                .get(code)
-                .and_then(serde_json::Value::as_str)
-                .is_some_and(|s| !s.trim().is_empty())
-        });
-        if !ok {
-            return Err(ApiError::BadRequest(format!(
-                "brand {} needs non-empty en and zh names",
-                brand.slug
-            )));
-        }
-    }
-
-    let mut tx = state.db.begin().await?;
-    sqlx::query("DELETE FROM brands").execute(&mut *tx).await?;
-    for (i, brand) in body.iter().enumerate() {
-        sqlx::query("INSERT INTO brands (name, slug, position, active) VALUES ($1, $2, $3, $4)")
-            .bind(&brand.name)
-            .bind(brand.slug.trim())
-            .bind(i as i32)
-            .bind(brand.active)
-            .execute(&mut *tx)
-            .await
-            .map_err(|e| unique_conflict(e, format!("duplicate brand slug: {}", brand.slug)))?;
-    }
-    tx.commit().await?;
-    list_brands(state).await
-}
-
 async fn load_own_product(state: &AppState, shop_id: Uuid, id: Uuid) -> ApiResult<Product> {
     sqlx::query_as::<_, Product>(
         "SELECT id, shop_id, category_id, brand_id, slug, name, description, images, status, created_at, updated_at
diff --git a/openspec/changes/split-catalog-into-product-category-brand/.openspec.yaml b/openspec/changes/split-catalog-into-product-category-brand/.openspec.yaml
new file mode 100644
index 0000000..1b9acb7
--- /dev/null
+++ b/openspec/changes/split-catalog-into-product-category-brand/.openspec.yaml
@@ -0,0 +1,2 @@
+schema: spec-driven
+created: 2026-09-22
diff --git a/openspec/changes/split-catalog-into-product-category-brand/design.md b/openspec/changes/split-catalog-into-product-category-brand/design.md
new file mode 100644
index 0000000..3fd8cdb
--- /dev/null
+++ b/openspec/changes/split-catalog-into-product-category-brand/design.md
@@ -0,0 +1,40 @@
+## Context
+
+`apps/api/src/modules/catalog` (dto/handlers/repo/service, 693 lines) currently owns four concepts: `Product`, `Sku`, `Category`, `Brand`. Routes are already flat resource paths (`/api/products`, `/api/categories`, `/api/brands`, `/api/admin/brands`, `/api/shop/products*`) rather than nested under `/api/catalog/*`, so the module boundary is purely internal — no client or `packages/shared` type depends on the Rust module name. `Category` has no other module dependents (checked: `content`, `points`, `group_buying`, `flash_sale` don't reference it). `Brand` is referenced only from `catalog::handlers`/`service`. `Product`/`Sku` are referenced by `favorite`, `flash_sale`, `group_buying` via `crate::models`, so those two stay in the shared `models.rs`.
+
+## Goals / Non-Goals
+
+**Goals:**
+- Give Product/SKU, Category, and Brand each their own top-level module under `apps/api/src/modules/`, following the existing `dto.rs`/`handlers.rs`/`repo.rs`/`service.rs` shape used by every other module.
+- Zero behavior change: identical routes, request/response JSON, SQL, and error codes.
+- Keep `Product`/`Sku` in `crate::models` (cross-module dependents); move `Category`/`Brand` out of `models.rs` into their new modules since nothing outside references them.
+
+**Non-Goals:**
+- No API versioning, route renaming, or DB schema/migration changes.
+- No behavior changes to sorting, filtering, or the subtree category-matching CTE.
+- Not folding Brand into Category — they're kept as separate modules per the chosen split (Option B from exploration), since they have independent admin-replace vs. tree-query shapes.
+
+## Decisions
+
+- **Three modules, not two.** Category and Brand are both small, platform-owned lookup tables with no lifecycle, but they don't share logic (recursive CTE vs. flat replace-all transaction) — merging them into one `taxonomy` module would just recreate a smaller version of the same problem. Alternative considered: `product` + `taxonomy` (category+brand together) — rejected per explicit user choice.
+- **`Product`/`Sku` stay in `models.rs`; `Category`/`Brand` move out.** Determined by actual cross-module usage (grepped `crate::models::{...}` across all modules): `Sku`/`Product` are imported by `flash_sale`, `group_buying`, `favorite`; `Category`/`Brand` are imported nowhere outside the old `catalog` module. This matches the existing pattern where module-local types (e.g. `Shop`, `Coupon`, `AddressBookEntry`) already live in `models.rs` alongside genuinely shared ones — moving `Category`/`Brand` doesn't fully break that existing pattern, but reduces unnecessary sharing surface for types nothing else touches.
+- **`repo.rs`'s `attach_skus` moves to `product/repo.rs` unchanged.** It only touches `Product`/`Sku`/order tables — no category/brand coupling.
+- **No spec deltas.** Per `openspec/specs/catalog/spec.md` and `openspec/specs/brand/spec.md`, this change alters no requirement text — those specs describe HTTP-level behavior which is untouched. `docs/tech-specs/rust-api.md` (if it names `modules/catalog`) gets a doc update as part of tasks, not a spec change.
+
+## Risks / Trade-offs
+
+- **Import churn** → every file that did `use crate::models::{Brand, Category, Product, ...}` needs updating; mitigated by compiler errors making every missed reference a hard build failure, not a silent bug.
+- **Merge/rebase conflicts** for anyone with in-flight branches touching `modules/catalog` → mitigated by doing the move as one atomic commit and merging promptly.
+- **sqlx query cache / compile-time verification** (if `sqlx::query!` macros or `.sqlx/` cache are used) → verified `catalog/service.rs` uses `sqlx::query_as::<_, T>(&format!(...))` (runtime-checked, not compile-time macros), so no `cargo sqlx prepare` step is needed.
+
+## Migration Plan
+
+1. Create `modules/product`, `modules/category`, `modules/brand` with the moved code (see tasks.md for the file-by-file split).
+2. Update `modules/mod.rs` router merge list and `models.rs`.
+3. `cargo build` and `cargo test` (integration tests hit HTTP routes, unaffected by internal module names) to confirm zero behavior drift.
+4. Delete `modules/catalog/`.
+5. Single commit/PR — no incremental rollout needed since this is a same-process, same-deploy internal refactor. Rollback is a plain `git revert`.
+
+## Open Questions
+
+- Should `apps/api/tests/catalog.rs` be renamed/split into `product.rs`/`category.rs`/`brand.rs` to mirror the new module boundaries, or left as-is since it already tests via HTTP and doesn't reference Rust module paths? (Default: leave as-is unless the user wants test-file parity too — captured as an optional task.)
diff --git a/openspec/changes/split-catalog-into-product-category-brand/proposal.md b/openspec/changes/split-catalog-into-product-category-brand/proposal.md
new file mode 100644
index 0000000..f64f430
--- /dev/null
+++ b/openspec/changes/split-catalog-into-product-category-brand/proposal.md
@@ -0,0 +1,29 @@
+## Why
+
+`apps/api/src/modules/catalog` currently bundles four distinct concepts — Product, SKU, Category, and Brand — behind one module, one router, and one 392-line service file. Every other module in the codebase (`favorite`, `coupon`, `flash_sale`, `address`, ...) owns exactly one bounded aggregate. `catalog` is the outlier, and it mixes a shop-owned, stateful aggregate (Product/SKU with a publish lifecycle) with platform-owned, admin-curated lookup data (Category tree, Brand list) that has no lifecycle of its own. Splitting now, while the module is still small, avoids compounding the mismatch as more product-side logic (variants, attributes) gets added.
+
+## What Changes
+
+- Split `modules/catalog` into three top-level modules: `modules/product` (Product + Sku, publish lifecycle, shop-scoped CRUD), `modules/category` (Category tree, subtree query), and `modules/brand` (Brand list, admin replace-all).
+- Move `Category` and `Brand` structs out of the shared `models.rs` into their owning modules' `dto.rs`/models, following the pattern already used by `Shop`, `Coupon`, etc. staying in `models.rs` only where genuinely cross-module (Product/Sku stay in `models.rs` since `flash_sale`/`group_buying`/`favorite` reference them).
+- Register the three new modules in `modules/mod.rs`, removing `catalog` from the router merge list.
+- No route paths, request/response shapes, or database schema change — `/api/products`, `/api/categories`, `/api/brands`, `/api/admin/brands`, and `/api/shop/products*` are unaffected.
+- No SQL query text changes beyond moving them to new files.
+
+## Capabilities
+
+No system behavior changes. To keep spec capability boundaries aligned with the new Rust module boundaries, the existing `catalog` capability is re-filed (not behaviorally changed) into two capabilities along the same line the code splits on:
+
+### New Capabilities
+- `product`: product/SKU content, publish lifecycle, shop isolation, SKU pricing, and public product browse/filter/sort — carried over verbatim from `catalog`.
+- `category`: the category tree and its use as a browse filter (including subtree matching) — carried over verbatim from `catalog`.
+
+### Modified Capabilities
+- `catalog`: requirements removed (re-filed into `product` and `category` above with identical text/scenarios; no behavior change). `brand` is unaffected and stays a separate capability as it already was.
+
+## Impact
+
+- **Code**: `apps/api/src/modules/catalog/*` deleted; replaced by `apps/api/src/modules/product/*`, `apps/api/src/modules/category/*`, `apps/api/src/modules/brand/*`. `apps/api/src/modules/mod.rs` and `apps/api/src/models.rs` updated.
+- **Tests**: `apps/api/tests/catalog.rs` stays as-is (integration tests hit HTTP routes, which are unchanged) or is optionally renamed/split to mirror the new module boundaries.
+- **API/DB**: none — purely internal Rust restructuring.
+- **Frontends / `packages/shared`**: none — no contract change.
diff --git a/openspec/changes/split-catalog-into-product-category-brand/specs/catalog/spec.md b/openspec/changes/split-catalog-into-product-category-brand/specs/catalog/spec.md
new file mode 100644
index 0000000..a11919f
--- /dev/null
+++ b/openspec/changes/split-catalog-into-product-category-brand/specs/catalog/spec.md
@@ -0,0 +1,77 @@
+## REMOVED Requirements
+
+### Requirement: Localized product content
+Product and category names/descriptions SHALL be stored as JSONB maps keyed by locale (`en`, `zh`). The API MUST return the full map; clients pick the display locale.
+
+#### Scenario: bilingual round-trip
+- **WHEN** a shop owner creates a product with name `{"en": "Mug", "zh": "马克杯"}`
+- **THEN** both public detail and shop-admin GET return the identical map
+
+**Reason**: The `catalog` capability is split so its requirements are filed under the new `product` and `category` capabilities, mirroring the Rust module split (`modules/product`, `modules/category`). No behavior changes.
+**Migration**: See `specs/product/spec.md` ("Localized product content") for the identical requirement and scenario.
+
+### Requirement: Publish lifecycle
+Products SHALL have status `draft | published | unpublished`. Only `published` products appear in public list/detail.
+
+#### Scenario: publish then unpublish
+- **WHEN** a product is published
+- **THEN** it appears in `GET /api/products`
+- **WHEN** it is unpublished
+- **THEN** public detail returns 404 and it disappears from listings
+
+#### Scenario: publish requires sellable SKU
+- **WHEN** publishing a product with no active SKU having price > 0
+- **THEN** the API returns 400
+
+**Reason**: Re-filed under `product` capability; no behavior change.
+**Migration**: See `specs/product/spec.md` ("Publish lifecycle").
+
+### Requirement: Shop isolation
+Shop-role users SHALL only see and mutate their own shop's products under `/api/shop/products`.
+
+#### Scenario: cross-shop access denied
+- **WHEN** shop owner A requests `/api/shop/products/{id}` of shop B
+- **THEN** the API returns 404
+
+**Reason**: Re-filed under `product` capability; no behavior change.
+**Migration**: See `specs/product/spec.md` ("Shop isolation").
+
+### Requirement: SKU pricing
+Each SKU SHALL carry `price_minor` (integer minor units) and an ISO `currency` code; stock is a non-negative integer.
+
+#### Scenario: negative stock rejected
+- **WHEN** upserting a SKU with stock < 0
+- **THEN** the API returns 400
+
+**Reason**: Re-filed under `product` capability; no behavior change.
+**Migration**: See `specs/product/spec.md` ("SKU pricing").
+
+### Requirement: Public product browse
+Public `GET /api/products` SHALL return only `published` products whose shop is active, and SHALL remain readable without authentication. When `category_id` is supplied, the filter SHALL match that category **and every category beneath it**, so requesting a parent category returns products assigned to its child and grandchild categories. When `brand_id` is supplied the filter SHALL match that brand and compose with the other filters. The listing SHALL accept an optional `sort` of `price` or `sales`: `price` orders by each product's lowest active SKU price, and `sales` orders by units sold across orders that reached payment, which SHALL also be reported per product as `sold_count`. Any other `sort` value SHALL be rejected with a 400 `ApiError` rather than silently ignored. An unsorted listing SHALL order newest first. Paging SHALL keep returning `page` and `per_page` alongside the filtered `total`.
+
+#### Scenario: parent category includes descendant products
+- **WHEN** a shopper requests products for a category that has child categories holding published products
+- **THEN** the response contains the products assigned to those descendant categories, not only those assigned directly to the requested category
+
+#### Scenario: sort by lowest active SKU price
+- **WHEN** a shopper requests the product list with `sort=price` and `order=asc`
+- **THEN** products come back ordered by their lowest active SKU price ascending
+
+#### Scenario: sort by units sold
+- **WHEN** a shopper requests the product list with `sort=sales` and `order=desc`
+- **THEN** products come back ordered by their `sold_count` descending, and a product with no paid orders reports zero rather than being omitted
+
+#### Scenario: filter by brand
+- **WHEN** a shopper requests products with a `brand_id` alongside a `category_id`
+- **THEN** only products matching both filters are returned, and `total` reflects the combined filter
+
+#### Scenario: unsupported sort is rejected
+- **WHEN** a client requests a `sort` value that is neither `price` nor `sales`
+- **THEN** the API responds 400 with an `ApiError` body instead of ignoring the parameter
+
+#### Scenario: unpublished products never appear
+- **WHEN** any public listing or filter is applied
+- **THEN** products that are not `published`, or whose shop is not active, are absent from both `items` and `total`
+
+**Reason**: Re-filed under `product` capability (the listing endpoint itself, its lifecycle/shop-isolation/sort/paging contract); the category-subtree matching behavior it depends on is separately documented under the new `category` capability. No behavior change.
+**Migration**: See `specs/product/spec.md` ("Public product browse") and `specs/category/spec.md` ("Category subtree browse filtering").
diff --git a/openspec/changes/split-catalog-into-product-category-brand/specs/category/spec.md b/openspec/changes/split-catalog-into-product-category-brand/specs/category/spec.md
new file mode 100644
index 0000000..5ddae8b
--- /dev/null
+++ b/openspec/changes/split-catalog-into-product-category-brand/specs/category/spec.md
@@ -0,0 +1,15 @@
+## ADDED Requirements
+
+### Requirement: Localized category content
+Category names SHALL be stored as JSONB maps keyed by locale (`en`, `zh`). The API MUST return the full map; clients pick the display locale.
+
+#### Scenario: bilingual round-trip
+- **WHEN** an admin creates a category with name `{"en": "Mugs", "zh": "马克杯"}`
+- **THEN** `GET /api/categories` returns the identical map for that category
+
+### Requirement: Category subtree browse filtering
+`GET /api/categories` SHALL return the full category tree as a flat list, each with `id`, `parent_id`, `name`, `slug`, and `position`, ordered by `position` then `slug`. When a product listing is filtered by `category_id`, the filter SHALL match that category **and every category beneath it** in the tree, so requesting a parent category returns products assigned to its child and grandchild categories.
+
+#### Scenario: parent category includes descendant products
+- **WHEN** a shopper requests products for a category that has child categories holding published products
+- **THEN** the response contains the products assigned to those descendant categories, not only those assigned directly to the requested category
diff --git a/openspec/changes/split-catalog-into-product-category-brand/specs/product/spec.md b/openspec/changes/split-catalog-into-product-category-brand/specs/product/spec.md
new file mode 100644
index 0000000..4587c6f
--- /dev/null
+++ b/openspec/changes/split-catalog-into-product-category-brand/specs/product/spec.md
@@ -0,0 +1,62 @@
+## ADDED Requirements
+
+### Requirement: Localized product content
+Product names/descriptions SHALL be stored as JSONB maps keyed by locale (`en`, `zh`). The API MUST return the full map; clients pick the display locale.
+
+#### Scenario: bilingual round-trip
+- **WHEN** a shop owner creates a product with name `{"en": "Mug", "zh": "马克杯"}`
+- **THEN** both public detail and shop-admin GET return the identical map
+
+### Requirement: Publish lifecycle
+Products SHALL have status `draft | published | unpublished`. Only `published` products appear in public list/detail.
+
+#### Scenario: publish then unpublish
+- **WHEN** a product is published
+- **THEN** it appears in `GET /api/products`
+- **WHEN** it is unpublished
+- **THEN** public detail returns 404 and it disappears from listings
+
+#### Scenario: publish requires sellable SKU
+- **WHEN** publishing a product with no active SKU having price > 0
+- **THEN** the API returns 400
+
+### Requirement: Shop isolation
+Shop-role users SHALL only see and mutate their own shop's products under `/api/shop/products`.
+
+#### Scenario: cross-shop access denied
+- **WHEN** shop owner A requests `/api/shop/products/{id}` of shop B
+- **THEN** the API returns 404
+
+### Requirement: SKU pricing
+Each SKU SHALL carry `price_minor` (integer minor units) and an ISO `currency` code; stock is a non-negative integer.
+
+#### Scenario: negative stock rejected
+- **WHEN** upserting a SKU with stock < 0
+- **THEN** the API returns 400
+
+### Requirement: Public product browse
+Public `GET /api/products` SHALL return only `published` products whose shop is active, and SHALL remain readable without authentication. When `category_id` is supplied, the filter SHALL match that category and every category beneath it (per the `category` capability's subtree matching). When `brand_id` is supplied the filter SHALL match that brand and compose with the other filters. The listing SHALL accept an optional `sort` of `price` or `sales`: `price` orders by each product's lowest active SKU price, and `sales` orders by units sold across orders that reached payment, which SHALL also be reported per product as `sold_count`. Any other `sort` value SHALL be rejected with a 400 `ApiError` rather than silently ignored. An unsorted listing SHALL order newest first. Paging SHALL keep returning `page` and `per_page` alongside the filtered `total`.
+
+#### Scenario: parent category includes descendant products
+- **WHEN** a shopper requests products for a category that has child categories holding published products
+- **THEN** the response contains the products assigned to those descendant categories, not only those assigned directly to the requested category
+
+#### Scenario: sort by lowest active SKU price
+- **WHEN** a shopper requests the product list with `sort=price` and `order=asc`
+- **THEN** products come back ordered by their lowest active SKU price ascending
+
+#### Scenario: sort by units sold
+- **WHEN** a shopper requests the product list with `sort=sales` and `order=desc`
+- **THEN** products come back ordered by their `sold_count` descending, and a product with no paid orders reports zero rather than being omitted
+
+#### Scenario: filter by brand
+- **WHEN** a shopper requests products with a `brand_id` alongside a `category_id`
+- **THEN** only products matching both filters are returned, and `total` reflects the combined filter
+
+#### Scenario: unsupported sort is rejected
+- **WHEN** a client requests a `sort` value that is neither `price` nor `sales`
+- **THEN** the API responds 400 with an `ApiError` body instead of ignoring the parameter
+
+#### Scenario: unpublished products never appear
+- **WHEN** any public listing or filter is applied
+- **THEN** products that are not `published`, or whose shop is not active, are absent from both `items` and `total`
diff --git a/openspec/changes/split-catalog-into-product-category-brand/tasks.md b/openspec/changes/split-catalog-into-product-category-brand/tasks.md
new file mode 100644
index 0000000..311c6e6
--- /dev/null
+++ b/openspec/changes/split-catalog-into-product-category-brand/tasks.md
@@ -0,0 +1,42 @@
+## 1. Create `modules/category`
+
+- [x] 1.1 Create `apps/api/src/modules/category/mod.rs` (mirror `catalog/mod.rs` shape: `mod dto; mod handlers; pub mod service;` + `router()`).
+- [x] 1.2 Move `Category` struct from `models.rs` into `category/dto.rs` (or a `category/models.rs` if the module needs more than one type later).
+- [x] 1.3 Move `list_categories` (the `SELECT id, parent_id, name, slug, position FROM categories ORDER BY position, slug` query) from `catalog/service.rs` into `category/service.rs`.
+- [x] 1.4 Move the `SUBTREE_CTE` constant (`WITH RECURSIVE subtree AS (...)`) into `category` — expose it as `pub(crate) const SUBTREE_CTE` (or a small helper fn) so `product/service.rs` can still build the category-subtree-filtered product query.
+- [x] 1.5 Move the `GET /categories` route + handler from `catalog/handlers.rs` into `category/handlers.rs`.
+
+## 2. Create `modules/brand`
+
+- [x] 2.1 Create `apps/api/src/modules/brand/mod.rs` (same shape as above).
+- [x] 2.2 Move `Brand` struct from `models.rs` into `brand/dto.rs`.
+- [x] 2.3 Move `list_brands`, `BrandInput`, `default_active`, and `replace_brands` (including its slug/bilingual-name validation and the transactional delete+reinsert) from `catalog/service.rs` into `brand/service.rs`.
+- [x] 2.4 Move the `GET /brands` and `PUT /admin/brands` routes + handlers (`list_brands`, `replace_brands`) from `catalog/handlers.rs` into `brand/handlers.rs`.
+
+## 3. Create `modules/product`
+
+- [x] 3.1 Create `apps/api/src/modules/product/mod.rs` (same shape as above).
+- [x] 3.2 Move `product/dto.rs`: `ProductWithSkus`, `PublicListQuery`, `SortBy` (+ `sort_by()`/`ascending()`), `ProductBody`, `SkuBody` from `catalog/dto.rs` unchanged.
+- [x] 3.3 Move `product/repo.rs`: `attach_skus` and `SoldRow` from `catalog/repo.rs` unchanged (only touches `Product`/`Sku`/order tables, no category/brand coupling).
+- [x] 3.4 Move `product/service.rs`: `list_public`, `get_public`, `load_own_product`, `list_shop_products`, `get_shop_product`, `validate_product_body`, `create_product`, `update_product`, `transition`, `upsert_sku`, plus the `SOLD_UNITS`, `MIN_PRICE`, `PRODUCT_COLS` constants — import `category`'s `SUBTREE_CTE` instead of redefining it locally.
+- [x] 3.5 Move `product/handlers.rs`: `/products`, `/products/{id_or_slug}`, `/shop/products`, `/shop/products/{id}`, `/shop/products/{id}/publish`, `/shop/products/{id}/unpublish`, `/shop/products/{id}/skus` routes + handlers, and the `ShopListQuery` struct.
+- [x] 3.6 Keep `Product` and `Sku` structs in `crate::models` (do not move) — confirmed cross-module dependents in `favorite`, `flash_sale`, `group_buying`.
+
+## 4. Wire up and remove the old module
+
+- [x] 4.1 In `apps/api/src/modules/mod.rs`: replace `pub mod catalog;` with `pub mod product; pub mod category; pub mod brand;`, and replace `.merge(catalog::router())` with `.merge(product::router()).merge(category::router()).merge(brand::router())`.
+- [x] 4.2 Update `apps/api/src/models.rs` to remove the (now-moved) `Category` and `Brand` structs.
+- [x] 4.3 `rg -n "modules::catalog|catalog::"` across `apps/api/src` and fix any remaining references.
+- [x] 4.4 Delete `apps/api/src/modules/catalog/`.
+
+## 5. Verify
+
+- [x] 5.1 `cargo build` (or `cargo check`) in `apps/api` — zero errors.
+- [x] 5.2 `cargo clippy` — zero new warnings.
+- [x] 5.3 `cargo test` — `apps/api/tests/catalog.rs` and the full suite pass unchanged (routes/behavior are identical).
+- [x] 5.4 Manually diff route list (`rg "\.route\(" apps/api/src/modules/{product,category,brand}`) against the original `catalog/handlers.rs` router to confirm no route was dropped or duplicated.
+- [x] 5.5 `rg -n "modules/catalog|modules::catalog"` across `docs/` (e.g. `docs/tech-specs/rust-api.md`) and update any references to the old module name.
+
+## 6. Optional test-file parity
+
+- [ ] 6.1 (Optional, per design.md open question) Rename/split `apps/api/tests/catalog.rs` into `tests/product.rs`, `tests/category.rs`, `tests/brand.rs` to mirror the new module boundaries — only if desired; not required since these are HTTP-level integration tests unaffected by internal module names.