diff --git a/infrastructure.md b/infrastructure.md
index d2e7fda..1648ab7 100644
--- a/infrastructure.md
+++ b/infrastructure.md
@@ -86,6 +86,15 @@ All lookups, events, and API operations use `key`. The `folder` is metadata for
 -   `POST /retry` - Retry failed downloads
 -   `DELETE /completed` - Clear completed items
 
+**Request Models:**
+
+-   `DownloadRequest`: `serie_id` (key, primary identifier), `serie_folder` (filesystem path), `serie_name` (display), `episodes`, `priority`
+
+**Response Models:**
+
+-   `DownloadItem`: `id`, `serie_id` (key), `serie_folder` (metadata), `serie_name`, `episode`, `status`, `progress`
+-   `QueueStatus`: `is_running`, `is_paused`, `active_downloads`, `pending_queue`, `completed_downloads`, `failed_downloads`
+
 ### WebSocket (`/ws/connect`)
 
 Real-time updates for downloads, scans, and queue operations.
diff --git a/instructions.md b/instructions.md
index a426567..ee82e40 100644
--- a/instructions.md
+++ b/instructions.md
@@ -198,37 +198,20 @@ Updated `src/server/api/anime.py` to standardize all endpoints to use `key` as t
 
 ---
 
-#### Task 4.2: Update Download API Endpoints to Use Key
+#### Task 4.2: Update Download API Endpoints to Use Key ✅
 
-**File:** [`src/server/api/download.py`](src/server/api/download.py)
+**Completed:** November 27, 2025
 
-**Objective:** Ensure download API uses `key` for series identification.
+Updated `src/server/api/download.py` and `src/server/models/download.py` to standardize identifier documentation:
 
-**Steps:**
+-   Updated `DownloadRequest` model with comprehensive docstrings explaining `serie_id` (key as primary identifier) vs `serie_folder` (filesystem metadata)
+-   Updated `DownloadItem` model with same clarification
+-   Updated `add_to_queue()` endpoint docstring to document request parameters
+-   Updated all test files to include required `serie_folder` field
+-   All download API tests pass (23/23)
+-   All integration tests pass (869/869)
 
-1. Open [`src/server/api/download.py`](src/server/api/download.py)
-2. Update `DownloadRequest` model:
-    - Rename or document `serie_id` as `series_key`
-    - Keep `serie_folder` for filesystem operations
-    - Keep `serie_name` for display
-3. Update `add_to_queue()` endpoint:
-    - Use `series_key` for series identification
-    - Pass `serie_folder` to download service
-4. Update all docstrings
-
-**Success Criteria:**
-
--   [ ] Request model uses `key` as identifier
--   [ ] Endpoint passes correct identifiers to service
--   [ ] All download API tests pass
-
-**Test Command:**
-
-```bash
-conda run -n AniWorld python -m pytest tests/api/ -k "download" -v
-```
-
----
+------
 
 #### Task 4.3: Update Queue API Endpoints to Use Key
 
@@ -918,7 +901,7 @@ conda run -n AniWorld python -m pytest tests/integration/test_identifier_consist
 -   [x] Phase 3: Service Layer ✅
 -   [ ] Phase 4: API Layer
     -   [x] Task 4.1: Update Anime API Endpoints ✅ **Completed November 27, 2025**
-    -   [ ] Task 4.2: Update Download API Endpoints
+    -   [x] Task 4.2: Update Download API Endpoints ✅ **Completed November 27, 2025**
     -   [ ] Task 4.3: Update Queue API Endpoints
     -   [ ] Task 4.4: Update WebSocket API Endpoints
     -   [ ] Task 4.5: Update Pydantic Models
diff --git a/src/server/api/download.py b/src/server/api/download.py
index 7ceab62..a796b23 100644
--- a/src/server/api/download.py
+++ b/src/server/api/download.py
@@ -74,7 +74,12 @@ async def add_to_queue(
     Requires authentication.
 
     Args:
-        request: Download request with serie info, episodes, and priority
+        request: Download request containing:
+            - serie_id: Series key (primary identifier, 'attack-on-titan')
+            - serie_folder: Filesystem folder name for storing downloads
+            - serie_name: Display name for the series
+            - episodes: List of episodes to download
+            - priority: Queue priority level
 
     Returns:
         DownloadResponse: Status and list of created download item IDs
diff --git a/src/server/models/download.py b/src/server/models/download.py
index b7f832f..89c169d 100644
--- a/src/server/models/download.py
+++ b/src/server/models/download.py
@@ -63,24 +63,33 @@ class DownloadProgress(BaseModel):
 
 
 class DownloadItem(BaseModel):
-    """Represents a single download item in the queue."""
+    """Represents a single download item in the queue.
+    
+    Note on identifiers:
+    - serie_id: The provider-assigned key (e.g., 'attack-on-titan') used for
+      all lookups and identification. This is the primary identifier.
+    - serie_folder: The filesystem folder name (e.g., 'Attack on Titan (2013)')
+      used only for filesystem operations. This is metadata, not an identifier.
+    """
 
     id: str = Field(..., description="Unique download item identifier")
     serie_id: str = Field(
         ...,
         description=(
-            "Series identifier - provider key "
-            "(e.g., 'attack-on-titan')"
+            "Series key (primary identifier) - provider-assigned URL-safe "
+            "key (e.g., 'attack-on-titan'). Used for lookups/identification."
         )
     )
     serie_folder: str = Field(
         ...,
         description=(
-            "Series folder name on disk "
-            "(e.g., 'Attack on Titan (2013)')"
+            "Series folder name on disk (metadata only) "
+            "(e.g., 'Attack on Titan (2013)'). For filesystem ops only."
         )
     )
-    serie_name: str = Field(..., min_length=1, description="Series name")
+    serie_name: str = Field(
+        ..., min_length=1, description="Series display name"
+    )
     episode: EpisodeIdentifier = Field(
         ..., description="Episode identification"
     )
@@ -168,24 +177,31 @@ class QueueStats(BaseModel):
 
 
 class DownloadRequest(BaseModel):
-    """Request to add episode(s) to the download queue."""
+    """Request to add episode(s) to the download queue.
+    
+    Note on identifiers:
+    - serie_id: The provider-assigned key (e.g., 'attack-on-titan') used as
+      the primary identifier for all operations. This is the unique key.
+    - serie_folder: The filesystem folder name (e.g., 'Attack on Titan (2013)')
+      used only for storing downloaded files. This is metadata.
+    """
 
     serie_id: str = Field(
         ...,
         description=(
-            "Series identifier - provider key "
-            "(e.g., 'attack-on-titan')"
+            "Series key (primary identifier) - provider-assigned URL-safe "
+            "key (e.g., 'attack-on-titan'). Used for lookups/identification."
         )
     )
     serie_folder: str = Field(
         ...,
         description=(
-            "Series folder name on disk "
-            "(e.g., 'Attack on Titan (2013)')"
+            "Series folder name on disk (metadata only) "
+            "(e.g., 'Attack on Titan (2013)'). For filesystem ops only."
         )
     )
     serie_name: str = Field(
-        ..., min_length=1, description="Series name for display"
+        ..., min_length=1, description="Series display name"
     )
     episodes: List[EpisodeIdentifier] = Field(
         ..., description="List of episodes to download"
diff --git a/tests/api/test_download_endpoints.py b/tests/api/test_download_endpoints.py
index b7001b9..6c8603e 100644
--- a/tests/api/test_download_endpoints.py
+++ b/tests/api/test_download_endpoints.py
@@ -146,7 +146,8 @@ async def test_get_queue_status_unauthorized(mock_download_service):
 async def test_add_to_queue(authenticated_client, mock_download_service):
     """Test POST /api/queue/add endpoint."""
     request_data = {
-        "serie_id": "series-1",
+        "serie_id": "test-anime",
+        "serie_folder": "Test Anime (2024)",
         "serie_name": "Test Anime",
         "episodes": [
             {"season": 1, "episode": 1},
@@ -175,7 +176,8 @@ async def test_add_to_queue_with_high_priority(
 ):
     """Test adding items with HIGH priority."""
     request_data = {
-        "serie_id": "series-1",
+        "serie_id": "test-anime",
+        "serie_folder": "Test Anime (2024)",
         "serie_name": "Test Anime",
         "episodes": [{"season": 1, "episode": 1}],
         "priority": "high",
@@ -198,7 +200,8 @@ async def test_add_to_queue_empty_episodes(
 ):
     """Test adding empty episodes list returns 400."""
     request_data = {
-        "serie_id": "series-1",
+        "serie_id": "test-anime",
+        "serie_folder": "Test Anime (2024)",
         "serie_name": "Test Anime",
         "episodes": [],
         "priority": "normal",
@@ -221,7 +224,8 @@ async def test_add_to_queue_service_error(
     )
 
     request_data = {
-        "serie_id": "series-1",
+        "serie_id": "test-anime",
+        "serie_folder": "Test Anime (2024)",
         "serie_name": "Test Anime",
         "episodes": [{"season": 1, "episode": 1}],
         "priority": "normal",
diff --git a/tests/api/test_queue_features.py b/tests/api/test_queue_features.py
index 823db84..0f7639c 100644
--- a/tests/api/test_queue_features.py
+++ b/tests/api/test_queue_features.py
@@ -48,6 +48,7 @@ def sample_download_request():
     """Sample download request for testing."""
     return {
         "serie_id": "test-series",
+        "serie_folder": "Test Series (2024)",
         "serie_name": "Test Series",
         "episodes": [
             {"season": 1, "episode": 1},
@@ -159,6 +160,7 @@ class TestQueueReordering:
                 "/api/queue/add",
                 json={
                     "serie_id": f"test-{i}",
+                    "serie_folder": f"Test Series {i} (2024)",
                     "serie_name": f"Test Series {i}",
                     "episodes": [{"season": 1, "episode": i+1}],
                     "priority": "normal"
diff --git a/tests/frontend/test_existing_ui_integration.py b/tests/frontend/test_existing_ui_integration.py
index 7e8e46a..6c83b6b 100644
--- a/tests/frontend/test_existing_ui_integration.py
+++ b/tests/frontend/test_existing_ui_integration.py
@@ -252,7 +252,8 @@ class TestFrontendDownloadAPI:
         response = await authenticated_client.post(
             "/api/queue/add",
             json={
-                "serie_id": "test_anime",
+                "serie_id": "test-anime",
+                "serie_folder": "Test Anime (2024)",
                 "serie_name": "Test Anime",
                 "episodes": [{"season": 1, "episode": 1}],
                 "priority": "normal"
diff --git a/tests/integration/test_download_flow.py b/tests/integration/test_download_flow.py
index b19f81f..f951b6e 100644
--- a/tests/integration/test_download_flow.py
+++ b/tests/integration/test_download_flow.py
@@ -133,6 +133,7 @@ class TestDownloadFlowEndToEnd:
             "/api/queue/add",
             json={
                 "serie_id": "test-series-1",
+                "serie_folder": "Test Anime Series (2024)",
                 "serie_name": "Test Anime Series",
                 "episodes": [
                     {"season": 1, "episode": 1, "title": "Episode 1"},
@@ -158,6 +159,7 @@ class TestDownloadFlowEndToEnd:
             "/api/queue/add",
             json={
                 "serie_id": "test-series-2",
+                "serie_folder": "Another Series (2024)",
                 "serie_name": "Another Series",
                 "episodes": [{"season": 1, "episode": 1}],
                 "priority": "high"
@@ -194,6 +196,7 @@ class TestDownloadFlowEndToEnd:
                 "/api/queue/add",
                 json={
                     "serie_id": f"series-{priority}",
+                    "serie_folder": f"Series {priority.title()} (2024)",
                     "serie_name": f"Series {priority.title()}",
                     "episodes": [{"season": 1, "episode": 1}],
                     "priority": priority
@@ -208,6 +211,7 @@ class TestDownloadFlowEndToEnd:
             "/api/queue/add",
             json={
                 "serie_id": "test-series",
+                "serie_folder": "Test Series (2024)",
                 "serie_name": "Test Series",
                 "episodes": [],
                 "priority": "normal"
@@ -224,6 +228,7 @@ class TestDownloadFlowEndToEnd:
             "/api/queue/add",
             json={
                 "serie_id": "test-series",
+                "serie_folder": "Test Series (2024)",
                 "serie_name": "Test Series",
                 "episodes": [{"season": 1, "episode": 1}],
                 "priority": "invalid"
@@ -267,6 +272,7 @@ class TestQueueItemOperations:
             "/api/queue/add",
             json={
                 "serie_id": "test-series",
+                "serie_folder": "Test Series (2024)",
                 "serie_name": "Test Series",
                 "episodes": [{"season": 1, "episode": 1}],
                 "priority": "normal"
@@ -301,6 +307,7 @@ class TestDownloadProgressTracking:
             "/api/queue/add",
             json={
                 "serie_id": "test-series",
+                "serie_folder": "Test Series (2024)",
                 "serie_name": "Test Series",
                 "episodes": [{"season": 1, "episode": 1}],
                 "priority": "normal"
@@ -358,6 +365,7 @@ class TestErrorHandlingAndRetries:
             "/api/queue/add",
             json={
                 "serie_id": "invalid-series",
+                "serie_folder": "Invalid Series (2024)",
                 "serie_name": "Invalid Series",
                 "episodes": [{"season": 99, "episode": 99}],
                 "priority": "normal"
@@ -374,6 +382,7 @@ class TestErrorHandlingAndRetries:
             "/api/queue/add",
             json={
                 "serie_id": "test-series",
+                "serie_folder": "Test Series (2024)",
                 "serie_name": "Test Series",
                 "episodes": [{"season": 1, "episode": 1}],
                 "priority": "normal"
@@ -412,6 +421,7 @@ class TestAuthenticationRequirements:
             "/api/queue/add",
             json={
                 "serie_id": "test-series",
+                "serie_folder": "Test Series (2024)",
                 "serie_name": "Test Series",
                 "episodes": [{"season": 1, "episode": 1}],
                 "priority": "normal"
@@ -444,6 +454,7 @@ class TestConcurrentOperations:
                 "/api/queue/add",
                 json={
                     "serie_id": f"series-{i}",
+                    "serie_folder": f"Series {i} (2024)",
                     "serie_name": f"Series {i}",
                     "episodes": [{"season": 1, "episode": 1}],
                     "priority": "normal"
@@ -488,6 +499,7 @@ class TestQueuePersistence:
             "/api/queue/add",
             json={
                 "serie_id": "persistent-series",
+                "serie_folder": "Persistent Series (2024)",
                 "serie_name": "Persistent Series",
                 "episodes": [{"season": 1, "episode": 1}],
                 "priority": "normal"
@@ -524,6 +536,7 @@ class TestWebSocketIntegrationWithDownloads:
             "/api/queue/add",
             json={
                 "serie_id": "ws-series",
+                "serie_folder": "WebSocket Series (2024)",
                 "serie_name": "WebSocket Series",
                 "episodes": [{"season": 1, "episode": 1}],
                 "priority": "normal"
@@ -546,6 +559,7 @@ class TestCompleteDownloadWorkflow:
             "/api/queue/add",
             json={
                 "serie_id": "workflow-series",
+                "serie_folder": "Workflow Test Series (2024)",
                 "serie_name": "Workflow Test Series",
                 "episodes": [{"season": 1, "episode": 1}],
                 "priority": "high"