fix(download): restore aria2 resume lifecycle

locales/de.json

@@ -263,6 +263,19 @@
                 "red": "civitai.red (uneingeschränkt)"
             }
         },
+        "downloadBackend": {
+            "label": "Download-Backend",
+            "help": "Wähle aus, wie Modelldateien heruntergeladen werden. Python verwendet den eingebauten Downloader. aria2 verwendet den experimentellen externen Downloader-Prozess.",
+            "options": {
+                "python": "Python (integriert)",
+                "aria2": "aria2 (experimentell)"
+            }
+        },
+        "aria2cPath": {
+            "label": "aria2c-Pfad",
+            "help": "Optionaler Pfad zur ausführbaren aria2c-Datei. Leer lassen, um aria2c aus dem System-PATH zu verwenden.",
+            "placeholder": "Leer lassen, um aria2c aus dem PATH zu verwenden"
+        },
         "civitaiHostBanner": {
             "title": "Civitai-Host-Einstellung verfügbar",
             "content": "Civitai verwendet jetzt civitai.com für SFW-Inhalte und civitai.red für uneingeschränkte Inhalte. In den Einstellungen können Sie ändern, welche Seite standardmäßig geöffnet wird.",
@@ -278,6 +291,7 @@
         },
         "sections": {
             "contentFiltering": "Inhaltsfilterung",
+            "downloads": "Downloads",
             "videoSettings": "Video-Einstellungen",
             "layoutSettings": "Layout-Einstellungen",
             "misc": "Verschiedenes",

locales/en.json

@@ -263,6 +263,19 @@
                 "red": "civitai.red (unrestricted)"
             }
         },
+        "downloadBackend": {
+            "label": "Download backend",
+            "help": "Choose how model files are downloaded. Python uses the built-in downloader. aria2 uses the experimental external downloader process.",
+            "options": {
+                "python": "Python (built-in)",
+                "aria2": "aria2 (experimental)"
+            }
+        },
+        "aria2cPath": {
+            "label": "aria2c path",
+            "help": "Optional path to the aria2c executable. Leave empty to use aria2c from your system PATH.",
+            "placeholder": "Leave empty to use aria2c from PATH"
+        },
         "civitaiHostBanner": {
             "title": "Civitai host preference available",
             "content": "Civitai now uses civitai.com for SFW content and civitai.red for unrestricted content. You can change which site opens by default in Settings.",
@@ -278,6 +291,7 @@
         },
         "sections": {
             "contentFiltering": "Content Filtering",
+            "downloads": "Downloads",
             "videoSettings": "Video Settings",
             "layoutSettings": "Layout Settings",
             "misc": "Miscellaneous",

locales/es.json

@@ -263,6 +263,19 @@
                 "red": "civitai.red (sin restricciones)"
             }
         },
+        "downloadBackend": {
+            "label": "Backend de descarga",
+            "help": "Elige cómo se descargan los archivos del modelo. Python usa el descargador integrado. aria2 usa el proceso externo experimental de descarga.",
+            "options": {
+                "python": "Python (integrado)",
+                "aria2": "aria2 (experimental)"
+            }
+        },
+        "aria2cPath": {
+            "label": "Ruta de aria2c",
+            "help": "Ruta opcional al ejecutable aria2c. Déjalo vacío para usar aria2c desde el PATH del sistema.",
+            "placeholder": "Déjalo vacío para usar aria2c desde el PATH"
+        },
         "civitaiHostBanner": {
             "title": "Preferencia de host de Civitai disponible",
             "content": "Civitai ahora usa civitai.com para contenido SFW y civitai.red para contenido sin restricciones. Puedes cambiar en Ajustes qué sitio se abre por defecto.",
@@ -278,6 +291,7 @@
         },
         "sections": {
             "contentFiltering": "Filtrado de contenido",
+            "downloads": "Descargas",
             "videoSettings": "Configuración de video",
             "layoutSettings": "Configuración de diseño",
             "misc": "Varios",

locales/fr.json

@@ -263,6 +263,19 @@
                 "red": "civitai.red (sans restriction)"
             }
         },
+        "downloadBackend": {
+            "label": "Moteur de téléchargement",
+            "help": "Choisissez comment les fichiers de modèles sont téléchargés. Python utilise le téléchargeur intégré. aria2 utilise le processus externe expérimental de téléchargement.",
+            "options": {
+                "python": "Python (intégré)",
+                "aria2": "aria2 (expérimental)"
+            }
+        },
+        "aria2cPath": {
+            "label": "Chemin vers aria2c",
+            "help": "Chemin facultatif vers l’exécutable aria2c. Laissez vide pour utiliser aria2c depuis le PATH système.",
+            "placeholder": "Laisser vide pour utiliser aria2c depuis le PATH"
+        },
         "civitaiHostBanner": {
             "title": "Préférence d’hôte Civitai disponible",
             "content": "Civitai utilise désormais civitai.com pour le contenu SFW et civitai.red pour le contenu sans restriction. Vous pouvez modifier dans les paramètres le site ouvert par défaut.",
@@ -278,6 +291,7 @@
         },
         "sections": {
             "contentFiltering": "Filtrage du contenu",
+            "downloads": "Téléchargements",
             "videoSettings": "Paramètres vidéo",
             "layoutSettings": "Paramètres d'affichage",
             "misc": "Divers",

locales/he.json

@@ -263,6 +263,19 @@
                 "red": "civitai.red (ללא הגבלות)"
             }
         },
+        "downloadBackend": {
+            "label": "מנגנון הורדה",
+            "help": "בחר כיצד יורדים קבצי המודל. Python משתמש במוריד המובנה. aria2 משתמש בתהליך הורדה חיצוני ניסיוני.",
+            "options": {
+                "python": "Python (מובנה)",
+                "aria2": "aria2 (ניסיוני)"
+            }
+        },
+        "aria2cPath": {
+            "label": "נתיב aria2c",
+            "help": "נתיב אופציונלי לקובץ ההפעלה aria2c. השאר ריק כדי להשתמש ב-aria2c מתוך ה-PATH של המערכת.",
+            "placeholder": "השאר ריק כדי להשתמש ב-aria2c מתוך ה-PATH"
+        },
         "civitaiHostBanner": {
             "title": "העדפת מארח Civitai זמינה",
             "content": "Civitai משתמש כעת ב-civitai.com עבור תוכן SFW וב-civitai.red עבור תוכן ללא הגבלות. ניתן לשנות בהגדרות איזה אתר ייפתח כברירת מחדל.",
@@ -278,6 +291,7 @@
         },
         "sections": {
             "contentFiltering": "סינון תוכן",
+            "downloads": "הורדות",
             "videoSettings": "הגדרות וידאו",
             "layoutSettings": "הגדרות פריסה",
             "misc": "שונות",

locales/ja.json

@@ -263,6 +263,19 @@
                 "red": "civitai.red（制限なし）"
             }
         },
+        "downloadBackend": {
+            "label": "ダウンロードバックエンド",
+            "help": "モデルファイルのダウンロード方法を選択します。Python は内蔵ダウンローダーを使用し、aria2 は実験的な外部ダウンローダープロセスを使用します。",
+            "options": {
+                "python": "Python（内蔵）",
+                "aria2": "aria2（実験的）"
+            }
+        },
+        "aria2cPath": {
+            "label": "aria2c のパス",
+            "help": "aria2c 実行ファイルへの任意のパスです。空欄のままにすると、システム PATH 上の aria2c を使用します。",
+            "placeholder": "空欄のままにすると PATH 上の aria2c を使用します"
+        },
         "civitaiHostBanner": {
             "title": "Civitai ホスト設定を利用できます",
             "content": "Civitai は現在、SFW コンテンツには civitai.com、制限なしコンテンツには civitai.red を使用しています。設定で既定で開くサイトを変更できます。",
@@ -278,6 +291,7 @@
         },
         "sections": {
             "contentFiltering": "コンテンツフィルタリング",
+            "downloads": "ダウンロード",
             "videoSettings": "動画設定",
             "layoutSettings": "レイアウト設定",
             "misc": "その他",

locales/ko.json

@@ -263,6 +263,19 @@
                 "red": "civitai.red(무제한)"
             }
         },
+        "downloadBackend": {
+            "label": "다운로드 백엔드",
+            "help": "모델 파일을 다운로드하는 방식을 선택합니다. Python은 내장 다운로더를 사용하고, aria2는 실험적인 외부 다운로더 프로세스를 사용합니다.",
+            "options": {
+                "python": "Python(내장)",
+                "aria2": "aria2(실험적)"
+            }
+        },
+        "aria2cPath": {
+            "label": "aria2c 경로",
+            "help": "aria2c 실행 파일의 선택적 경로입니다. 비워 두면 시스템 PATH의 aria2c를 사용합니다.",
+            "placeholder": "비워 두면 PATH의 aria2c를 사용합니다"
+        },
         "civitaiHostBanner": {
             "title": "Civitai 호스트 기본 설정 사용 가능",
             "content": "이제 Civitai는 SFW 콘텐츠에 civitai.com을, 무제한 콘텐츠에 civitai.red를 사용합니다. 설정에서 기본으로 열 사이트를 변경할 수 있습니다.",
@@ -278,6 +291,7 @@
         },
         "sections": {
             "contentFiltering": "콘텐츠 필터링",
+            "downloads": "다운로드",
             "videoSettings": "비디오 설정",
             "layoutSettings": "레이아웃 설정",
             "misc": "기타",

locales/ru.json

@@ -263,6 +263,19 @@
                 "red": "civitai.red (без ограничений)"
             }
         },
+        "downloadBackend": {
+            "label": "Бэкенд загрузки",
+            "help": "Выберите способ загрузки файлов моделей. Python использует встроенный загрузчик. aria2 использует экспериментальный внешний процесс загрузки.",
+            "options": {
+                "python": "Python (встроенный)",
+                "aria2": "aria2 (экспериментальный)"
+            }
+        },
+        "aria2cPath": {
+            "label": "Путь к aria2c",
+            "help": "Необязательный путь к исполняемому файлу aria2c. Оставьте пустым, чтобы использовать aria2c из системного PATH.",
+            "placeholder": "Оставьте пустым, чтобы использовать aria2c из PATH"
+        },
         "civitaiHostBanner": {
             "title": "Доступна настройка хоста Civitai",
             "content": "Теперь Civitai использует civitai.com для контента SFW и civitai.red для контента без ограничений. В настройках можно изменить, какой сайт открывать по умолчанию.",
@@ -278,6 +291,7 @@
         },
         "sections": {
             "contentFiltering": "Фильтрация контента",
+            "downloads": "Загрузки",
             "videoSettings": "Настройки видео",
             "layoutSettings": "Настройки макета",
             "misc": "Разное",

locales/zh-CN.json

@@ -263,6 +263,19 @@
                 "red": "civitai.red（无限制）"
             }
         },
+        "downloadBackend": {
+            "label": "下载后端",
+            "help": "选择模型文件的下载方式。Python 使用内置下载器。aria2 使用实验性的外部下载进程。",
+            "options": {
+                "python": "Python（内置）",
+                "aria2": "aria2（实验性）"
+            }
+        },
+        "aria2cPath": {
+            "label": "aria2c 路径",
+            "help": "可选的 aria2c 可执行文件路径。留空则使用系统 PATH 中的 aria2c。",
+            "placeholder": "留空则使用 PATH 中的 aria2c"
+        },
         "civitaiHostBanner": {
             "title": "已提供 Civitai 站点偏好设置",
             "content": "Civitai 现在使用 civitai.com 提供 SFW 内容，使用 civitai.red 提供无限制内容。你可以在设置中更改默认打开的站点。",
@@ -278,6 +291,7 @@
         },
         "sections": {
             "contentFiltering": "内容过滤",
+            "downloads": "下载",
             "videoSettings": "视频设置",
             "layoutSettings": "布局设置",
             "misc": "其他",

locales/zh-TW.json

@@ -263,6 +263,19 @@
                 "red": "civitai.red（無限制）"
             }
         },
+        "downloadBackend": {
+            "label": "下載後端",
+            "help": "選擇模型檔案的下載方式。Python 使用內建下載器。aria2 使用實驗性的外部下載程序。",
+            "options": {
+                "python": "Python（內建）",
+                "aria2": "aria2（實驗性）"
+            }
+        },
+        "aria2cPath": {
+            "label": "aria2c 路徑",
+            "help": "可選的 aria2c 可執行檔路徑。留空則使用系統 PATH 中的 aria2c。",
+            "placeholder": "留空則使用 PATH 中的 aria2c"
+        },
         "civitaiHostBanner": {
             "title": "已提供 Civitai 站點偏好設定",
             "content": "Civitai 現在使用 civitai.com 提供 SFW 內容，使用 civitai.red 提供無限制內容。你可以在設定中變更預設開啟的站點。",
@@ -278,6 +291,7 @@
         },
         "sections": {
             "contentFiltering": "內容過濾",
+            "downloads": "下載",
             "videoSettings": "影片設定",
             "layoutSettings": "版面設定",
             "misc": "其他",

py/services/aria2_downloader.py

@@ -0,0 +1,570 @@
+from __future__ import annotations
+
+import asyncio
+import json
+import logging
+import os
+import secrets
+import shutil
+import socket
+from dataclasses import dataclass
+from datetime import datetime
+from pathlib import Path
+from typing import Any, Dict, Optional, Tuple
+
+import aiohttp
+
+from .downloader import DownloadProgress, get_downloader
+from .aria2_transfer_state import Aria2TransferStateStore
+from .settings_manager import get_settings_manager
+
+logger = logging.getLogger(__name__)
+
+CIVITAI_DOWNLOAD_URL_PREFIXES = (
+    "https://civitai.com/api/download/",
+    "https://civitai.red/api/download/",
+)
+
+
+class Aria2Error(RuntimeError):
+    """Raised when aria2 integration fails."""
+
+
+@dataclass
+class Aria2Transfer:
+    """Track an aria2 download registered by the Python coordinator."""
+
+    gid: str
+    save_path: str
+
+
+class Aria2Downloader:
+    """Manage an aria2 RPC daemon for experimental model downloads."""
+
+    _instance = None
+    _lock = asyncio.Lock()
+
+    @classmethod
+    async def get_instance(cls) -> "Aria2Downloader":
+        async with cls._lock:
+            if cls._instance is None:
+                cls._instance = cls()
+            return cls._instance
+
+    def __init__(self) -> None:
+        if hasattr(self, "_initialized"):
+            return
+
+        self._initialized = True
+        self._process: Optional[asyncio.subprocess.Process] = None
+        self._rpc_port: Optional[int] = None
+        self._rpc_secret = ""
+        self._rpc_url = ""
+        self._rpc_session: Optional[aiohttp.ClientSession] = None
+        self._rpc_session_lock = asyncio.Lock()
+        self._process_lock = asyncio.Lock()
+        self._transfers: Dict[str, Aria2Transfer] = {}
+        self._poll_interval = 0.5
+        self._state_store = Aria2TransferStateStore()
+
+    @property
+    def is_running(self) -> bool:
+        return self._process is not None and self._process.returncode is None
+
+    async def download_file(
+        self,
+        url: str,
+        save_path: str,
+        *,
+        download_id: str,
+        progress_callback=None,
+        headers: Optional[Dict[str, str]] = None,
+    ) -> Tuple[bool, str]:
+        """Download a file using aria2 RPC and wait for completion."""
+
+        await self._ensure_process()
+        save_path = os.path.abspath(save_path)
+        transfer = self._transfers.get(download_id)
+        if transfer is None or os.path.abspath(transfer.save_path) != save_path:
+            gid = await self._schedule_download(
+                url,
+                save_path,
+                download_id=download_id,
+                headers=headers,
+            )
+            transfer = Aria2Transfer(gid=gid, save_path=save_path)
+            self._transfers[download_id] = transfer
+
+        try:
+            while True:
+                status = await self.get_status(download_id)
+                if status is None:
+                    return False, "aria2 download not found"
+
+                snapshot = self._build_progress_snapshot(status)
+                if progress_callback is not None:
+                    await self._dispatch_progress(progress_callback, snapshot)
+
+                state = status.get("status", "")
+                if state == "complete":
+                    completed_path = self._resolve_completed_path(status, save_path)
+                    return True, completed_path
+                if state == "error":
+                    return False, status.get("errorMessage") or "aria2 download failed"
+                if state == "removed":
+                    return False, "Download was cancelled"
+
+                await asyncio.sleep(self._poll_interval)
+        finally:
+            self._transfers.pop(download_id, None)
+
+    async def _schedule_download(
+        self,
+        url: str,
+        save_path: str,
+        *,
+        download_id: str,
+        headers: Optional[Dict[str, str]] = None,
+    ) -> str:
+        save_dir = os.path.dirname(save_path)
+        out_name = os.path.basename(save_path)
+
+        Path(save_dir).mkdir(parents=True, exist_ok=True)
+
+        resolved_url = url
+        request_headers = headers
+        if headers and url.startswith(CIVITAI_DOWNLOAD_URL_PREFIXES):
+            resolved_url = await self._resolve_authenticated_redirect_url(url, headers)
+            if resolved_url != url:
+                request_headers = None
+                logger.debug(
+                    "Resolved Civitai download %s to signed URL for aria2",
+                    download_id,
+                )
+
+        options: Dict[str, str] = {
+            "dir": save_dir,
+            "out": out_name,
+            "continue": "true",
+            "max-connection-per-server": "4",
+            "split": "4",
+            "min-split-size": "1M",
+            "allow-overwrite": "true",
+            "auto-file-renaming": "false",
+            "file-allocation": "none",
+        }
+        if request_headers:
+            options["header"] = [
+                f"{key}: {value}" for key, value in request_headers.items()
+            ]
+
+        logger.debug(
+            "Submitting aria2 download %s -> %s (auth=%s, civitai_signed=%s)",
+            download_id,
+            save_path,
+            bool(request_headers),
+            resolved_url != url,
+        )
+
+        try:
+            gid = await self._rpc_call("aria2.addUri", [[resolved_url], options])
+        except Exception as exc:
+            raise Aria2Error(f"Failed to schedule aria2 download: {exc}") from exc
+
+        logger.debug("aria2 accepted download %s with gid %s", download_id, gid)
+        await self._state_store.upsert(
+            download_id,
+            {
+                "gid": gid,
+                "save_path": save_path,
+                "status": "downloading",
+                "url": url,
+            },
+        )
+        return gid
+
+    async def get_status(self, download_id: str) -> Optional[Dict[str, Any]]:
+        """Return the raw aria2 status payload for a known download."""
+
+        transfer = self._transfers.get(download_id)
+        if transfer is None:
+            return None
+
+        keys = [
+            "gid",
+            "status",
+            "totalLength",
+            "completedLength",
+            "downloadSpeed",
+            "errorMessage",
+            "files",
+        ]
+        try:
+            status = await self._rpc_call("aria2.tellStatus", [transfer.gid, keys])
+        except Exception as exc:
+            raise Aria2Error(f"Failed to query aria2 download status: {exc}") from exc
+
+        if isinstance(status, dict):
+            return status
+        return None
+
+    async def get_status_by_gid(self, gid: str) -> Optional[Dict[str, Any]]:
+        keys = [
+            "gid",
+            "status",
+            "totalLength",
+            "completedLength",
+            "downloadSpeed",
+            "errorMessage",
+            "files",
+        ]
+        try:
+            status = await self._rpc_call("aria2.tellStatus", [gid, keys])
+        except Exception as exc:
+            message = str(exc)
+            if "cannot be found" in message.lower() or "not found" in message.lower():
+                return None
+            raise Aria2Error(f"Failed to query aria2 download status: {exc}") from exc
+
+        if isinstance(status, dict):
+            return status
+        return None
+
+    async def restore_transfer(self, download_id: str, gid: str, save_path: str) -> None:
+        await self._ensure_process()
+        self._transfers[download_id] = Aria2Transfer(
+            gid=gid,
+            save_path=os.path.abspath(save_path),
+        )
+
+    async def reassign_transfer(
+        self, from_download_id: str, to_download_id: str
+    ) -> Optional[Aria2Transfer]:
+        transfer = self._transfers.get(from_download_id)
+        if transfer is None:
+            return None
+
+        self._transfers[to_download_id] = transfer
+        if from_download_id != to_download_id:
+            self._transfers.pop(from_download_id, None)
+        return transfer
+
+    async def has_transfer(self, download_id: str) -> bool:
+        return download_id in self._transfers
+
+    async def pause_download(self, download_id: str) -> Dict[str, Any]:
+        transfer = self._transfers.get(download_id)
+        if transfer is None:
+            return {"success": False, "error": "Download task not found"}
+
+        try:
+            await self._rpc_call("aria2.forcePause", [transfer.gid])
+        except Exception as exc:
+            return {"success": False, "error": str(exc)}
+
+        await self._state_store.upsert(download_id, {"status": "paused"})
+        return {"success": True, "message": "Download paused successfully"}
+
+    async def resume_download(self, download_id: str) -> Dict[str, Any]:
+        transfer = self._transfers.get(download_id)
+        if transfer is None:
+            return {"success": False, "error": "Download task not found"}
+
+        try:
+            await self._rpc_call("aria2.unpause", [transfer.gid])
+        except Exception as exc:
+            return {"success": False, "error": str(exc)}
+
+        await self._state_store.upsert(download_id, {"status": "downloading"})
+        return {"success": True, "message": "Download resumed successfully"}
+
+    async def cancel_download(self, download_id: str) -> Dict[str, Any]:
+        transfer = self._transfers.get(download_id)
+        if transfer is None:
+            return {"success": False, "error": "Download task not found"}
+
+        try:
+            await self._rpc_call("aria2.forceRemove", [transfer.gid])
+        except Exception as exc:
+            return {"success": False, "error": str(exc)}
+
+        await self._state_store.remove(download_id)
+        return {"success": True, "message": "Download cancelled successfully"}
+
+    async def close(self) -> None:
+        """Shut down the RPC process and session."""
+
+        if self._rpc_session is not None:
+            await self._rpc_session.close()
+            self._rpc_session = None
+
+        process = self._process
+        self._process = None
+        self._transfers.clear()
+
+        if process is None:
+            return
+
+        if process.returncode is None:
+            process.terminate()
+            try:
+                await asyncio.wait_for(process.wait(), timeout=5.0)
+            except asyncio.TimeoutError:
+                process.kill()
+                await process.wait()
+
+    async def _dispatch_progress(self, callback, snapshot: DownloadProgress) -> None:
+        try:
+            result = callback(snapshot, snapshot)
+        except TypeError:
+            result = callback(snapshot.percent_complete)
+
+        if asyncio.iscoroutine(result):
+            await result
+        elif hasattr(result, "__await__"):
+            await result
+
+    def _build_progress_snapshot(self, status: Dict[str, Any]) -> DownloadProgress:
+        completed = self._parse_int(status.get("completedLength"))
+        total = self._parse_int(status.get("totalLength"))
+        speed = float(self._parse_int(status.get("downloadSpeed")))
+        percent = 0.0
+        if total > 0:
+            percent = (completed / total) * 100.0
+
+        return DownloadProgress(
+            percent_complete=max(0.0, min(percent, 100.0)),
+            bytes_downloaded=completed,
+            total_bytes=total or None,
+            bytes_per_second=speed,
+            timestamp=datetime.now().timestamp(),
+        )
+
+    def _resolve_completed_path(self, status: Dict[str, Any], default_path: str) -> str:
+        files = status.get("files")
+        if isinstance(files, list) and files:
+            first = files[0]
+            if isinstance(first, dict):
+                candidate = first.get("path")
+                if isinstance(candidate, str) and candidate:
+                    return candidate
+        return default_path
+
+    @staticmethod
+    def _parse_int(value: Any) -> int:
+        try:
+            return int(value)
+        except (TypeError, ValueError):
+            return 0
+
+    async def _resolve_authenticated_redirect_url(
+        self,
+        url: str,
+        headers: Dict[str, str],
+    ) -> str:
+        downloader = await get_downloader()
+        session = await downloader.session
+        request_headers = dict(downloader.default_headers)
+        request_headers.update(headers)
+        request_headers["Accept-Encoding"] = "identity"
+
+        try:
+            async with session.get(
+                url,
+                headers=request_headers,
+                allow_redirects=False,
+                proxy=downloader.proxy_url,
+            ) as response:
+                if response.status in {301, 302, 303, 307, 308}:
+                    location = response.headers.get("Location")
+                    if location:
+                        return location
+                    raise Aria2Error(
+                        "Authenticated Civitai redirect did not include a Location header"
+                    )
+
+                if response.status == 200:
+                    return url
+
+                body = await response.text()
+                raise Aria2Error(
+                    f"Failed to resolve authenticated Civitai redirect: status={response.status} body={body[:300]}"
+                )
+        except aiohttp.ClientError as exc:
+            raise Aria2Error(
+                f"Failed to resolve authenticated Civitai redirect: {exc}"
+            ) from exc
+
+    async def _ensure_process(self) -> None:
+        async with self._process_lock:
+            if self.is_running and await self._ping():
+                return
+
+            await self.close()
+
+            executable = self._resolve_executable()
+            self._rpc_port = self._find_free_port()
+            self._rpc_secret = secrets.token_hex(16)
+            self._rpc_url = f"http://127.0.0.1:{self._rpc_port}/jsonrpc"
+
+            command = [
+                executable,
+                "--enable-rpc=true",
+                "--rpc-listen-all=false",
+                f"--rpc-listen-port={self._rpc_port}",
+                f"--rpc-secret={self._rpc_secret}",
+                "--check-certificate=true",
+                "--allow-overwrite=true",
+                "--auto-file-renaming=false",
+                "--file-allocation=none",
+                "--max-concurrent-downloads=5",
+                "--continue=true",
+                "--daemon=false",
+                "--quiet=true",
+                f"--stop-with-process={os.getpid()}",
+            ]
+
+            logger.info("Starting aria2 RPC daemon from %s", executable)
+            self._process = await asyncio.create_subprocess_exec(
+                *command,
+                stdout=asyncio.subprocess.DEVNULL,
+                stderr=asyncio.subprocess.PIPE,
+            )
+
+            await self._wait_until_ready()
+
+    def _resolve_executable(self) -> str:
+        settings = get_settings_manager()
+        configured_path = (settings.get("aria2c_path") or "").strip()
+        candidate = configured_path or "aria2c"
+
+        resolved = shutil.which(candidate)
+        if resolved:
+            return resolved
+
+        if configured_path and os.path.isfile(configured_path) and os.access(
+            configured_path, os.X_OK
+        ):
+            return configured_path
+
+        raise Aria2Error(
+            "aria2c executable was not found. Install aria2 or configure aria2c_path."
+        )
+
+    async def _wait_until_ready(self) -> None:
+        assert self._process is not None
+
+        start_time = asyncio.get_running_loop().time()
+        last_error = ""
+        while asyncio.get_running_loop().time() - start_time < 10.0:
+            if self._process.returncode is not None:
+                stderr_output = ""
+                if self._process.stderr is not None:
+                    try:
+                        stderr_output = (
+                            await asyncio.wait_for(self._process.stderr.read(), timeout=0.2)
+                        ).decode("utf-8", errors="replace")
+                    except Exception:
+                        stderr_output = ""
+                raise Aria2Error(
+                    f"aria2 RPC process exited early with code {self._process.returncode}: {stderr_output.strip()}"
+                )
+
+            try:
+                if await self._ping():
+                    return
+            except Exception as exc:  # pragma: no cover - startup race
+                last_error = str(exc)
+
+            await asyncio.sleep(0.2)
+
+        raise Aria2Error(
+            f"Timed out waiting for aria2 RPC to become ready{': ' + last_error if last_error else ''}"
+        )
+
+    async def _ping(self) -> bool:
+        try:
+            result = await self._rpc_call("aria2.getVersion", [])
+        except Exception:
+            return False
+
+        return isinstance(result, dict)
+
+    async def _rpc_call(self, method: str, params: list[Any]) -> Any:
+        if not self._rpc_url:
+            raise Aria2Error("aria2 RPC endpoint is not initialized")
+
+        session = await self._get_rpc_session()
+        payload = {
+            "jsonrpc": "2.0",
+            "id": secrets.token_hex(8),
+            "method": method,
+            "params": [f"token:{self._rpc_secret}", *params],
+        }
+
+        async with session.post(self._rpc_url, json=payload) as response:
+            text = await response.text()
+
+        try:
+            body = json.loads(text)
+        except json.JSONDecodeError:
+            body = None
+
+        if body is None:
+            if response.status != 200:
+                raise Aria2Error(
+                    f"aria2 RPC returned status {response.status} with non-JSON body: {text}"
+                )
+            raise Aria2Error(f"Invalid aria2 RPC response: {text}")
+
+        if "error" in body:
+            error = body["error"] or {}
+            code = error.get("code") if isinstance(error, dict) else None
+            message = error.get("message") if isinstance(error, dict) else str(error)
+            logger.error(
+                "aria2 RPC %s failed with HTTP %s, code=%s, message=%s",
+                method,
+                response.status,
+                code,
+                message,
+            )
+            status_message = (
+                f"aria2 RPC {method} failed with status {response.status}: {message}"
+                if response.status != 200
+                else message
+            )
+            raise Aria2Error(status_message or "Unknown aria2 RPC error")
+
+        if response.status != 200:
+            logger.error(
+                "aria2 RPC %s returned unexpected HTTP status %s without error payload: %s",
+                method,
+                response.status,
+                body,
+            )
+            raise Aria2Error(
+                f"aria2 RPC {method} returned unexpected status {response.status}"
+            )
+
+        return body.get("result")
+
+    async def _get_rpc_session(self) -> aiohttp.ClientSession:
+        if self._rpc_session is None or self._rpc_session.closed:
+            async with self._rpc_session_lock:
+                if self._rpc_session is None or self._rpc_session.closed:
+                    timeout = aiohttp.ClientTimeout(total=30)
+                    self._rpc_session = aiohttp.ClientSession(timeout=timeout)
+        return self._rpc_session
+
+    @staticmethod
+    def _find_free_port() -> int:
+        with socket.socket(socket.AF_INET, socket.SOCK_STREAM) as sock:
+            sock.bind(("127.0.0.1", 0))
+            sock.listen(1)
+            return int(sock.getsockname()[1])
+
+
+async def get_aria2_downloader() -> Aria2Downloader:
+    """Get the singleton aria2 downloader."""
+
+    return await Aria2Downloader.get_instance()

py/services/aria2_transfer_state.py

@@ -0,0 +1,108 @@
+from __future__ import annotations
+
+import asyncio
+import json
+import os
+from copy import deepcopy
+from typing import Any, Dict, Optional
+
+from ..utils.cache_paths import get_cache_base_dir
+
+
+def get_aria2_state_path() -> str:
+    base_dir = get_cache_base_dir(create=True)
+    state_dir = os.path.join(base_dir, "aria2")
+    os.makedirs(state_dir, exist_ok=True)
+    return os.path.join(state_dir, "downloads.json")
+
+
+class Aria2TransferStateStore:
+    """Persist aria2 transfer metadata needed for restart recovery."""
+
+    _locks_by_path: Dict[str, asyncio.Lock] = {}
+
+    def __init__(self, state_path: Optional[str] = None) -> None:
+        self._state_path = os.path.abspath(state_path or get_aria2_state_path())
+        self._lock = self._locks_by_path.setdefault(self._state_path, asyncio.Lock())
+
+    def _read_all_unlocked(self) -> Dict[str, Dict[str, Any]]:
+        try:
+            with open(self._state_path, "r", encoding="utf-8") as handle:
+                data = json.load(handle)
+        except FileNotFoundError:
+            return {}
+        except json.JSONDecodeError:
+            return {}
+
+        if not isinstance(data, dict):
+            return {}
+
+        normalized: Dict[str, Dict[str, Any]] = {}
+        for download_id, entry in data.items():
+            if isinstance(download_id, str) and isinstance(entry, dict):
+                normalized[download_id] = entry
+        return normalized
+
+    def _write_all_unlocked(self, data: Dict[str, Dict[str, Any]]) -> None:
+        directory = os.path.dirname(self._state_path)
+        if directory:
+            os.makedirs(directory, exist_ok=True)
+
+        temp_path = f"{self._state_path}.tmp"
+        with open(temp_path, "w", encoding="utf-8") as handle:
+            json.dump(data, handle, ensure_ascii=True, indent=2, sort_keys=True)
+        os.replace(temp_path, self._state_path)
+
+    async def load_all(self) -> Dict[str, Dict[str, Any]]:
+        async with self._lock:
+            return deepcopy(self._read_all_unlocked())
+
+    async def get(self, download_id: str) -> Optional[Dict[str, Any]]:
+        async with self._lock:
+            return deepcopy(self._read_all_unlocked().get(download_id))
+
+    async def upsert(self, download_id: str, payload: Dict[str, Any]) -> Dict[str, Any]:
+        async with self._lock:
+            data = self._read_all_unlocked()
+            current = data.get(download_id, {})
+            current.update(payload)
+            data[download_id] = current
+            self._write_all_unlocked(data)
+            return deepcopy(current)
+
+    async def remove(self, download_id: str) -> None:
+        async with self._lock:
+            data = self._read_all_unlocked()
+            if download_id in data:
+                del data[download_id]
+                self._write_all_unlocked(data)
+
+    async def find_by_save_path(
+        self, save_path: str, *, exclude_download_id: Optional[str] = None
+    ) -> Optional[Dict[str, Any]]:
+        normalized_target = os.path.abspath(save_path)
+        async with self._lock:
+            data = self._read_all_unlocked()
+            for download_id, entry in data.items():
+                if exclude_download_id and download_id == exclude_download_id:
+                    continue
+                candidate = entry.get("save_path")
+                if isinstance(candidate, str) and os.path.abspath(candidate) == normalized_target:
+                    result = dict(entry)
+                    result["download_id"] = download_id
+                    return result
+        return None
+
+    async def reassign(self, from_download_id: str, to_download_id: str) -> Optional[Dict[str, Any]]:
+        async with self._lock:
+            data = self._read_all_unlocked()
+            existing = data.get(from_download_id)
+            if existing is None:
+                return None
+            updated = dict(existing)
+            updated["download_id"] = to_download_id
+            data[to_download_id] = updated
+            if from_download_id != to_download_id:
+                data.pop(from_download_id, None)
+            self._write_all_unlocked(data)
+            return deepcopy(updated)

py/services/settings_manager.py

@@ -55,6 +55,8 @@ DEFAULT_KEYS_CLEANUP_THRESHOLD = 10
 DEFAULT_SETTINGS: Dict[str, Any] = {
     "civitai_api_key": "",
     "civitai_host": "civitai.com",
+    "download_backend": "python",
+    "aria2c_path": "",
     "use_portable_settings": False,
     "hash_chunk_size_mb": DEFAULT_HASH_CHUNK_SIZE_MB,
     "language": "en",

static/css/components/modal/settings-modal.css

@@ -346,11 +346,13 @@
 .api-key-input input {
     width: 100%;
     padding: 6px 40px 6px 10px; /* Add left padding */
-    height: 20px;
+    height: 32px;
+    box-sizing: border-box;
     border-radius: var(--border-radius-xs);
     border: 1px solid var(--border-color);
     background-color: var(--lora-surface);
     color: var(--text-color);
+    font-size: 0.95em;
 }
 
 .api-key-input .toggle-visibility {
@@ -379,7 +381,8 @@
 .text-input-wrapper input {
     width: 100%;
     padding: 6px 10px;
-    height: 20px;
+    height: 32px;
+    box-sizing: border-box;
     border-radius: var(--border-radius-xs);
     border: 1px solid var(--border-color);
     background-color: var(--lora-surface);
@@ -760,10 +763,12 @@
 }
 
 .setting-control {
-    width: 60%; /* Decreased slightly from 65% */
+    flex: 0 0 60%;
+    max-width: 60%;
     margin-bottom: 0;
     display: flex;
     justify-content: flex-end; /* Right-align all controls */
+    min-width: 0;
 }
 
 /* Select Control Styles */
@@ -773,6 +778,13 @@
     justify-content: flex-end;
 }
 
+.setting-control select,
+.setting-control input[type="text"],
+.setting-control input[type="password"],
+.setting-control input[type="number"] {
+    font-size: 0.95em;
+}
+
 .select-control select {
     width: 100%;
     max-width: 100%; /* Increased from 200px */
@@ -781,8 +793,8 @@
     border: 1px solid var(--border-color);
     background-color: var(--lora-surface);
     color: var(--text-color);
-    font-size: 0.95em;
     height: 32px;
+    box-sizing: border-box;
 }
 
 /* Fix dark theme select dropdown text color */
@@ -888,8 +900,8 @@ input:checked + .toggle-slider:before {
     border: 1px solid var(--border-color);
     background-color: var(--lora-surface);
     color: var(--text-color);
-    font-size: 0.95em;
     height: 32px;
+    box-sizing: border-box;
 }
 
 /* Add warning text style for settings */

static/js/managers/SettingsManager.js

@@ -807,6 +807,16 @@ export class SettingsManager {
             civitaiHostSelect.value = state.global.settings.civitai_host || 'civitai.com';
         }
 
+        const downloadBackendSelect = document.getElementById('downloadBackend');
+        if (downloadBackendSelect) {
+            downloadBackendSelect.value = state.global.settings.download_backend || 'python';
+        }
+
+        const aria2cPathInput = document.getElementById('aria2cPath');
+        if (aria2cPathInput) {
+            aria2cPathInput.value = state.global.settings.aria2c_path || '';
+        }
+
         const recipesPathInput = document.getElementById('recipesPath');
         if (recipesPathInput) {
             recipesPathInput.value = state.global.settings.recipes_path || '';
@@ -950,9 +960,36 @@ export class SettingsManager {
             languageSelect.value = currentLanguage;
         }
 
+        this.loadDownloadBackendSettings();
         this.loadProxySettings();
     }
 
+    loadDownloadBackendSettings() {
+        const downloadBackendSelect = document.getElementById('downloadBackend');
+        const aria2PathSetting = document.getElementById('aria2PathSetting');
+        const updateVisibility = () => {
+            if (!aria2PathSetting || !downloadBackendSelect) {
+                return;
+            }
+            aria2PathSetting.style.display = downloadBackendSelect.value === 'aria2' ? 'block' : 'none';
+        };
+
+        if (downloadBackendSelect) {
+            downloadBackendSelect.value = state.global.settings.download_backend || 'python';
+            downloadBackendSelect.onchange = () => {
+                updateVisibility();
+                this.saveSelectSetting('downloadBackend', 'download_backend');
+            };
+        }
+
+        const aria2cPathInput = document.getElementById('aria2cPath');
+        if (aria2cPathInput) {
+            aria2cPathInput.value = state.global.settings.aria2c_path || '';
+        }
+
+        updateVisibility();
+    }
+
     setupPriorityTagInputs() {
         ['lora', 'checkpoint', 'embedding'].forEach((modelType) => {
             const textarea = document.getElementById(`${modelType}PriorityTagsInput`);

static/js/state/index.js

@@ -6,6 +6,8 @@ import { DEFAULT_PATH_TEMPLATES, DEFAULT_PRIORITY_TAG_CONFIG } from '../utils/co
 const DEFAULT_SETTINGS_BASE = Object.freeze({
     civitai_api_key: '',
     civitai_host: 'civitai.com',
+    download_backend: 'python',
+    aria2c_path: '',
     use_portable_settings: false,
     language: 'en',
     show_only_sfw: false,

templates/components/modals/settings_modal.html

@@ -129,6 +129,43 @@
                             </div>
                         </div>
 
+                        <div class="settings-subsection">
+                            <div class="settings-subsection-header">
+                                <h4>{{ t('settings.sections.downloads') }}</h4>
+                            </div>
+                            <div class="setting-item">
+                                <div class="setting-row">
+                                    <div class="setting-info">
+                                        <label for="downloadBackend">{{ t('settings.downloadBackend.label') }}</label>
+                                        <i class="fas fa-info-circle info-icon" data-tooltip="{{ t('settings.downloadBackend.help') }}"></i>
+                                    </div>
+                                    <div class="setting-control select-control">
+                                        <select id="downloadBackend" onchange="settingsManager.saveSelectSetting('downloadBackend', 'download_backend')">
+                                            <option value="python">{{ t('settings.downloadBackend.options.python') }}</option>
+                                            <option value="aria2">{{ t('settings.downloadBackend.options.aria2') }}</option>
+                                        </select>
+                                    </div>
+                                </div>
+                            </div>
+                            <div class="setting-item" id="aria2PathSetting" style="display: none;">
+                                <div class="setting-row">
+                                    <div class="setting-info">
+                                        <label for="aria2cPath">{{ t('settings.aria2cPath.label') }}</label>
+                                        <i class="fas fa-info-circle info-icon" data-tooltip="{{ t('settings.aria2cPath.help') }}"></i>
+                                    </div>
+                                    <div class="setting-control">
+                                        <div class="text-input-wrapper">
+                                            <input type="text"
+                                                   id="aria2cPath"
+                                                   placeholder="{{ t('settings.aria2cPath.placeholder') }}"
+                                                   onblur="settingsManager.saveInputSetting('aria2cPath', 'aria2c_path')"
+                                                   onkeydown="if(event.key === 'Enter') { this.blur(); }" />
+                                        </div>
+                                    </div>
+                                </div>
+                            </div>
+                        </div>
+
                         <!-- Backup -->
                         <div class="settings-subsection">
                             <div class="settings-subsection-header">

tests/frontend/managers/settingsManager.library.test.js

@@ -305,4 +305,39 @@ describe('SettingsManager library controls', () => {
             'success',
         );
     });
+
+    it('loads download backend settings and toggles the aria2 path field', () => {
+        const manager = createManager();
+        document.body.innerHTML = `
+            <select id="downloadBackend">
+                <option value="python">Python</option>
+                <option value="aria2">aria2</option>
+            </select>
+            <div id="aria2PathSetting" style="display: none;"></div>
+            <input id="aria2cPath" />
+        `;
+
+        state.global.settings = {
+            download_backend: 'aria2',
+            aria2c_path: '/usr/bin/aria2c',
+        };
+
+        const saveSpy = vi.spyOn(manager, 'saveSelectSetting').mockResolvedValue();
+
+        manager.loadDownloadBackendSettings();
+
+        const backendSelect = document.getElementById('downloadBackend');
+        const aria2PathSetting = document.getElementById('aria2PathSetting');
+        const aria2cPath = document.getElementById('aria2cPath');
+
+        expect(backendSelect.value).toBe('aria2');
+        expect(aria2cPath.value).toBe('/usr/bin/aria2c');
+        expect(aria2PathSetting.style.display).toBe('block');
+
+        backendSelect.value = 'python';
+        backendSelect.onchange();
+
+        expect(aria2PathSetting.style.display).toBe('none');
+        expect(saveSpy).toHaveBeenCalledWith('downloadBackend', 'download_backend');
+    });
 });

tests/services/test_aria2_downloader.py

@@ -0,0 +1,354 @@
+from __future__ import annotations
+
+import asyncio
+from pathlib import Path
+from unittest.mock import AsyncMock
+
+import pytest
+
+from py.services.aria2_downloader import Aria2Downloader, Aria2Error
+from py.services.aria2_transfer_state import Aria2TransferStateStore
+from py.services import aria2_transfer_state
+
+
+@pytest.fixture(autouse=True)
+def isolate_aria2_state(monkeypatch, tmp_path):
+    state_path = tmp_path / "cache" / "aria2" / "downloads.json"
+    monkeypatch.setattr(
+        aria2_transfer_state,
+        "get_aria2_state_path",
+        lambda: str(state_path),
+    )
+
+
+@pytest.mark.asyncio
+async def test_download_file_polls_until_complete(tmp_path, monkeypatch):
+    downloader = Aria2Downloader()
+    downloader._rpc_url = "http://127.0.0.1/jsonrpc"
+    downloader._rpc_secret = "secret"
+
+    save_path = tmp_path / "downloads" / "model.safetensors"
+    progress_events = []
+    rpc_calls = []
+    statuses = iter(
+        [
+            {
+                "gid": "gid-1",
+                "status": "active",
+                "completedLength": "5",
+                "totalLength": "10",
+                "downloadSpeed": "25",
+            },
+            {
+                "gid": "gid-1",
+                "status": "complete",
+                "completedLength": "10",
+                "totalLength": "10",
+                "downloadSpeed": "0",
+                "files": [{"path": str(save_path)}],
+            },
+        ]
+    )
+
+    async def fake_rpc_call(method, params):
+        rpc_calls.append((method, params))
+        if method == "aria2.addUri":
+            return "gid-1"
+        if method == "aria2.tellStatus":
+            return next(statuses)
+        raise AssertionError(f"Unexpected RPC method: {method}")
+
+    monkeypatch.setattr(downloader, "_ensure_process", AsyncMock())
+    monkeypatch.setattr(
+        downloader,
+        "_resolve_authenticated_redirect_url",
+        AsyncMock(
+            return_value="https://signed.example.com/model.safetensors?token=abc"
+        ),
+    )
+    monkeypatch.setattr(downloader, "_rpc_call", fake_rpc_call)
+    monkeypatch.setattr("py.services.aria2_downloader.asyncio.sleep", AsyncMock())
+
+    async def progress_callback(progress, snapshot=None):
+        progress_events.append(snapshot.percent_complete if snapshot else progress)
+
+    success, result = await downloader.download_file(
+        "https://civitai.com/api/download/models/123",
+        str(save_path),
+        download_id="download-1",
+        progress_callback=progress_callback,
+        headers={"Authorization": "Bearer token"},
+    )
+
+    assert success is True
+    assert result == str(save_path)
+    assert progress_events == [50.0, 100.0]
+    assert downloader._transfers == {}
+    assert rpc_calls[0][0] == "aria2.addUri"
+    assert rpc_calls[0][1][0] == [
+        "https://signed.example.com/model.safetensors?token=abc"
+    ]
+    assert rpc_calls[0][1][1]["out"] == "model.safetensors"
+    assert "header" not in rpc_calls[0][1][1]
+
+
+@pytest.mark.asyncio
+async def test_transfer_state_store_shares_lock_and_preserves_concurrent_updates(tmp_path):
+    state_path = tmp_path / "cache" / "aria2" / "downloads.json"
+    store_a = Aria2TransferStateStore(str(state_path))
+    store_b = Aria2TransferStateStore(str(state_path))
+
+    assert store_a._lock is store_b._lock
+
+    await asyncio.gather(
+        store_a.upsert("download-1", {"status": "downloading", "gid": "gid-1"}),
+        store_b.upsert("download-2", {"status": "paused", "gid": "gid-2"}),
+    )
+
+    assert await store_a.get("download-1") == {"status": "downloading", "gid": "gid-1"}
+    assert await store_b.get("download-2") == {"status": "paused", "gid": "gid-2"}
+
+
+@pytest.mark.asyncio
+async def test_download_file_keeps_auth_headers_when_civitai_does_not_redirect(
+    tmp_path, monkeypatch
+):
+    downloader = Aria2Downloader()
+    downloader._rpc_url = "http://127.0.0.1/jsonrpc"
+    downloader._rpc_secret = "secret"
+
+    save_path = tmp_path / "downloads" / "model.safetensors"
+    rpc_calls = []
+    statuses = iter(
+        [
+            {
+                "gid": "gid-1",
+                "status": "complete",
+                "completedLength": "10",
+                "totalLength": "10",
+                "downloadSpeed": "0",
+                "files": [{"path": str(save_path)}],
+            },
+        ]
+    )
+
+    async def fake_rpc_call(method, params):
+        rpc_calls.append((method, params))
+        if method == "aria2.addUri":
+            return "gid-1"
+        if method == "aria2.tellStatus":
+            return next(statuses)
+        raise AssertionError(f"Unexpected RPC method: {method}")
+
+    monkeypatch.setattr(downloader, "_ensure_process", AsyncMock())
+    monkeypatch.setattr(
+        downloader,
+        "_resolve_authenticated_redirect_url",
+        AsyncMock(return_value="https://civitai.com/api/download/models/123"),
+    )
+    monkeypatch.setattr(downloader, "_rpc_call", fake_rpc_call)
+    monkeypatch.setattr("py.services.aria2_downloader.asyncio.sleep", AsyncMock())
+
+    success, result = await downloader.download_file(
+        "https://civitai.com/api/download/models/123",
+        str(save_path),
+        download_id="download-1",
+        headers={"Authorization": "Bearer token"},
+    )
+
+    assert success is True
+    assert result == str(save_path)
+    assert rpc_calls[0][1][0] == ["https://civitai.com/api/download/models/123"]
+    assert rpc_calls[0][1][1]["header"] == ["Authorization: Bearer token"]
+
+
+@pytest.mark.asyncio
+async def test_pause_resume_cancel_forward_to_rpc(monkeypatch):
+    downloader = Aria2Downloader()
+    downloader._transfers["download-1"] = type(
+        "Transfer", (), {"gid": "gid-1", "save_path": "/tmp/model.safetensors"}
+    )()
+
+    calls = []
+
+    async def fake_rpc_call(method, params):
+        calls.append((method, params))
+        return "gid-1"
+
+    monkeypatch.setattr(downloader, "_rpc_call", fake_rpc_call)
+
+    pause_result = await downloader.pause_download("download-1")
+    resume_result = await downloader.resume_download("download-1")
+    cancel_result = await downloader.cancel_download("download-1")
+
+    assert pause_result["success"] is True
+    assert resume_result["success"] is True
+    assert cancel_result["success"] is True
+    assert calls == [
+        ("aria2.forcePause", ["gid-1"]),
+        ("aria2.unpause", ["gid-1"]),
+        ("aria2.forceRemove", ["gid-1"]),
+    ]
+
+
+@pytest.mark.asyncio
+async def test_download_file_reuses_existing_transfer_without_add_uri(
+    tmp_path, monkeypatch
+):
+    downloader = Aria2Downloader()
+    downloader._rpc_url = "http://127.0.0.1/jsonrpc"
+    downloader._rpc_secret = "secret"
+
+    save_path = tmp_path / "downloads" / "model.safetensors"
+    downloader._transfers["download-1"] = type(
+        "Transfer", (), {"gid": "gid-1", "save_path": str(save_path)}
+    )()
+
+    rpc_calls = []
+    statuses = iter(
+        [
+            {
+                "gid": "gid-1",
+                "status": "active",
+                "completedLength": "5",
+                "totalLength": "10",
+                "downloadSpeed": "25",
+            },
+            {
+                "gid": "gid-1",
+                "status": "complete",
+                "completedLength": "10",
+                "totalLength": "10",
+                "downloadSpeed": "0",
+                "files": [{"path": str(save_path)}],
+            },
+        ]
+    )
+
+    async def fake_rpc_call(method, params):
+        rpc_calls.append((method, params))
+        if method == "aria2.tellStatus":
+            return next(statuses)
+        raise AssertionError(f"Unexpected RPC method: {method}")
+
+    monkeypatch.setattr(downloader, "_ensure_process", AsyncMock())
+    monkeypatch.setattr(downloader, "_rpc_call", fake_rpc_call)
+    monkeypatch.setattr("py.services.aria2_downloader.asyncio.sleep", AsyncMock())
+
+    success, result = await downloader.download_file(
+        "https://example.com/model.safetensors",
+        str(save_path),
+        download_id="download-1",
+    )
+
+    assert success is True
+    assert result == str(save_path)
+    assert [call[0] for call in rpc_calls] == ["aria2.tellStatus", "aria2.tellStatus"]
+
+
+def test_build_progress_snapshot_normalizes_numeric_fields():
+    downloader = Aria2Downloader()
+
+    snapshot = downloader._build_progress_snapshot(
+        {
+            "completedLength": "75",
+            "totalLength": "100",
+            "downloadSpeed": "512",
+        }
+    )
+
+    assert snapshot.percent_complete == 75.0
+    assert snapshot.bytes_downloaded == 75
+    assert snapshot.total_bytes == 100
+    assert snapshot.bytes_per_second == 512.0
+
+
+def test_resolve_executable_raises_when_binary_missing(monkeypatch):
+    downloader = Aria2Downloader()
+    settings = type("Settings", (), {"get": lambda self, key, default=None: ""})()
+
+    monkeypatch.setattr("py.services.aria2_downloader.get_settings_manager", lambda: settings)
+    monkeypatch.setattr("py.services.aria2_downloader.shutil.which", lambda _: None)
+
+    with pytest.raises(Aria2Error):
+        downloader._resolve_executable()
+
+
+@pytest.mark.asyncio
+async def test_rpc_call_surfaces_json_error_on_non_200(monkeypatch):
+    downloader = Aria2Downloader()
+    downloader._rpc_url = "http://127.0.0.1:6800/jsonrpc"
+    downloader._rpc_secret = "secret"
+
+    class FakeResponse:
+        status = 400
+
+        async def text(self):
+            return (
+                '{"jsonrpc":"2.0","id":"x","error":{"code":1,"message":"Unauthorized"}}'
+            )
+
+        async def __aenter__(self):
+            return self
+
+        async def __aexit__(self, exc_type, exc, tb):
+            return False
+
+    class FakeSession:
+        def post(self, _url, json=None):
+            return FakeResponse()
+
+    monkeypatch.setattr(downloader, "_get_rpc_session", AsyncMock(return_value=FakeSession()))
+
+    with pytest.raises(Aria2Error) as exc_info:
+        await downloader._rpc_call("aria2.addUri", [["https://example.com/file"]])
+
+    assert "Unauthorized" in str(exc_info.value)
+    assert "aria2.addUri" in str(exc_info.value)
+
+
+@pytest.mark.asyncio
+async def test_resolve_authenticated_redirect_url_returns_location(monkeypatch):
+    downloader = Aria2Downloader()
+
+    class FakeResponse:
+        status = 307
+        headers = {"Location": "https://signed.example.com/file.safetensors"}
+
+        async def text(self):
+            return ""
+
+        async def __aenter__(self):
+            return self
+
+        async def __aexit__(self, exc_type, exc, tb):
+            return False
+
+    class FakeSession:
+        def get(self, _url, headers=None, allow_redirects=False, proxy=None):
+            return FakeResponse()
+
+    class FakeDownloader:
+        default_headers = {"User-Agent": "ComfyUI-LoRA-Manager/1.0"}
+        proxy_url = None
+
+        @property
+        def session(self):
+            async def _session():
+                return FakeSession()
+
+            return _session()
+
+    fake_downloader = FakeDownloader()
+
+    monkeypatch.setattr(
+        "py.services.aria2_downloader.get_downloader",
+        AsyncMock(return_value=fake_downloader),
+    )
+
+    result = await downloader._resolve_authenticated_redirect_url(
+        "https://civitai.com/api/download/models/123",
+        {"Authorization": "Bearer token"},
+    )
+
+    assert result == "https://signed.example.com/file.safetensors"

tests/services/test_download_manager_basic.py

@@ -10,6 +10,7 @@ import pytest
 
 from py.services.download_manager import DownloadManager
 from py.services import download_manager
+from py.services import aria2_transfer_state
 from py.services.service_registry import ServiceRegistry
 from py.services.settings_manager import SettingsManager, get_settings_manager
 
@@ -46,6 +47,16 @@ def isolate_settings(monkeypatch, tmp_path):
     monkeypatch.setattr(SettingsManager, "_save_settings", lambda self: None)
 
 
+@pytest.fixture(autouse=True)
+def isolate_aria2_state(monkeypatch, tmp_path):
+    state_path = tmp_path / "cache" / "aria2" / "downloads.json"
+    monkeypatch.setattr(
+        aria2_transfer_state,
+        "get_aria2_state_path",
+        lambda: str(state_path),
+    )
+
+
 @pytest.fixture(autouse=True)
 def stub_metadata(monkeypatch):
     class _StubMetadata:
@@ -179,6 +190,7 @@ async def test_successful_download_uses_defaults(
         progress_callback,
         model_type,
         download_id,
+        transfer_backend=None,
     ):
         captured.update(
             {
@@ -268,6 +280,7 @@ async def test_download_uses_active_mirrors(
         progress_callback,
         model_type,
         download_id,
+        transfer_backend=None,
     ):
         captured["download_urls"] = download_urls
         return {"success": True}
@@ -288,6 +301,644 @@ async def test_download_uses_active_mirrors(
     assert captured["download_urls"] == ["https://mirror.example/file.safetensors"]
 
 
+@pytest.mark.asyncio
+async def test_pause_resume_cancel_delegate_to_aria2_backend(monkeypatch):
+    manager = DownloadManager()
+
+    task = asyncio.create_task(asyncio.sleep(60))
+    manager._download_tasks["download-1"] = task
+    manager._pause_events["download-1"] = download_manager.DownloadStreamControl()
+    manager._active_downloads["download-1"] = {
+        "transfer_backend": "aria2",
+        "status": "downloading",
+    }
+
+    class DummyAria2Downloader:
+        def __init__(self):
+            self.calls = []
+
+        async def pause_download(self, download_id):
+            self.calls.append(("pause", download_id))
+            return {"success": True, "message": "paused"}
+
+        async def resume_download(self, download_id):
+            self.calls.append(("resume", download_id))
+            return {"success": True, "message": "resumed"}
+
+        async def cancel_download(self, download_id):
+            self.calls.append(("cancel", download_id))
+            return {"success": True, "message": "cancelled"}
+
+        async def has_transfer(self, download_id):
+            self.calls.append(("has_transfer", download_id))
+            return True
+
+    dummy_aria2 = DummyAria2Downloader()
+    monkeypatch.setattr(
+        download_manager,
+        "get_aria2_downloader",
+        AsyncMock(return_value=dummy_aria2),
+    )
+
+    pause_result = await manager.pause_download("download-1")
+    assert pause_result["success"] is True
+    assert manager._active_downloads["download-1"]["status"] == "paused"
+
+    resume_result = await manager.resume_download("download-1")
+    assert resume_result["success"] is True
+    assert manager._active_downloads["download-1"]["status"] == "downloading"
+
+    cancel_result = await manager.cancel_download("download-1")
+    assert cancel_result["success"] is True
+    assert task.cancelled() or task.done()
+    assert dummy_aria2.calls == [
+        ("has_transfer", "download-1"),
+        ("pause", "download-1"),
+        ("has_transfer", "download-1"),
+        ("resume", "download-1"),
+        ("cancel", "download-1"),
+    ]
+
+
+@pytest.mark.asyncio
+async def test_cancel_allows_queued_aria2_task_without_transfer(monkeypatch):
+    manager = DownloadManager()
+
+    started = asyncio.Event()
+
+    async def blocked_task():
+        started.set()
+        await asyncio.sleep(60)
+
+    task = asyncio.create_task(blocked_task())
+    await started.wait()
+
+    manager._download_tasks["download-queued"] = task
+    manager._active_downloads["download-queued"] = {
+        "transfer_backend": "aria2",
+        "status": "queued",
+    }
+
+    class DummyAria2Downloader:
+        async def cancel_download(self, download_id):
+            return {"success": False, "error": "Download task not found"}
+
+    monkeypatch.setattr(
+        download_manager,
+        "get_aria2_downloader",
+        AsyncMock(return_value=DummyAria2Downloader()),
+    )
+
+    result = await manager.cancel_download("download-queued")
+
+    assert result["success"] is True
+    assert task.cancelled() or task.done()
+
+
+@pytest.mark.asyncio
+async def test_pause_resume_queued_aria2_task_without_transfer(monkeypatch):
+    manager = DownloadManager()
+
+    task = asyncio.create_task(asyncio.sleep(60))
+    manager._download_tasks["download-queued"] = task
+    manager._pause_events["download-queued"] = download_manager.DownloadStreamControl()
+    manager._active_downloads["download-queued"] = {
+        "transfer_backend": "aria2",
+        "status": "waiting",
+        "bytes_per_second": 12.0,
+    }
+
+    class DummyAria2Downloader:
+        def __init__(self):
+            self.calls = []
+
+        async def has_transfer(self, download_id):
+            self.calls.append(("has_transfer", download_id))
+            return False
+
+        async def pause_download(self, download_id):
+            self.calls.append(("pause", download_id))
+            return {"success": True, "message": "paused"}
+
+        async def resume_download(self, download_id):
+            self.calls.append(("resume", download_id))
+            return {"success": True, "message": "resumed"}
+
+    dummy_aria2 = DummyAria2Downloader()
+    monkeypatch.setattr(
+        download_manager,
+        "get_aria2_downloader",
+        AsyncMock(return_value=dummy_aria2),
+    )
+
+    pause_result = await manager.pause_download("download-queued")
+    assert pause_result == {"success": True, "message": "Download paused successfully"}
+    assert manager._active_downloads["download-queued"]["status"] == "paused"
+    assert manager._pause_events["download-queued"].is_paused() is True
+
+    resume_result = await manager.resume_download("download-queued")
+    assert resume_result == {"success": True, "message": "Download resumed successfully"}
+    assert manager._active_downloads["download-queued"]["status"] == "downloading"
+    assert manager._pause_events["download-queued"].is_set() is True
+    assert dummy_aria2.calls == [
+        ("has_transfer", "download-queued"),
+        ("has_transfer", "download-queued"),
+    ]
+
+    task.cancel()
+    with pytest.raises(asyncio.CancelledError):
+        await task
+
+
+@pytest.mark.asyncio
+async def test_resume_download_restores_persisted_aria2_task(monkeypatch, tmp_path):
+    manager = DownloadManager()
+    save_dir = tmp_path / "downloads"
+    save_dir.mkdir()
+    save_path = save_dir / "file.safetensors"
+    save_path.write_text("partial")
+    (save_dir / "file.safetensors.aria2").write_text("control")
+
+    await manager._aria2_state_store.upsert(
+        "download-1",
+        {
+            "download_id": "download-1",
+            "transfer_backend": "aria2",
+            "status": "paused",
+            "save_dir": str(save_dir),
+            "relative_path": "",
+            "use_default_paths": False,
+            "save_path": str(save_path),
+            "file_path": str(save_path),
+            "model_id": 12,
+            "model_version_id": 34,
+        },
+    )
+
+    created = {}
+
+    async def fake_download_with_semaphore(
+        self,
+        task_id,
+        model_id,
+        model_version_id,
+        save_dir,
+        relative_path,
+        progress_callback=None,
+        use_default_paths=False,
+        source=None,
+        file_params=None,
+    ):
+        created.update(
+            {
+                "task_id": task_id,
+                "model_id": model_id,
+                "model_version_id": model_version_id,
+                "save_dir": save_dir,
+            }
+        )
+        return {"success": True}
+
+    class DummyAria2Downloader:
+        def __init__(self):
+            self.calls = []
+
+        async def get_status_by_gid(self, gid):
+            return None
+
+        async def has_transfer(self, download_id):
+            self.calls.append(("has_transfer", download_id))
+            return False
+
+        async def resume_download(self, download_id):
+            self.calls.append(("resume", download_id))
+            return {"success": True, "message": "resumed"}
+
+        async def restore_transfer(self, download_id, gid, save_path):
+            self.calls.append(("restore_transfer", download_id, gid, save_path))
+
+    dummy_aria2 = DummyAria2Downloader()
+    monkeypatch.setattr(
+        download_manager, "_download_with_semaphore", None, raising=False
+    )
+    monkeypatch.setattr(
+        DownloadManager,
+        "_download_with_semaphore",
+        fake_download_with_semaphore,
+    )
+    monkeypatch.setattr(
+        download_manager,
+        "get_aria2_downloader",
+        AsyncMock(return_value=dummy_aria2),
+    )
+
+    result = await manager.resume_download("download-1")
+    await asyncio.sleep(0)
+
+    assert result == {"success": True, "message": "Download resumed successfully"}
+    assert created["task_id"] == "download-1"
+    assert created["model_version_id"] == 34
+    assert manager._active_downloads["download-1"]["status"] == "downloading"
+    assert manager._pause_events["download-1"].is_set() is True
+
+
+@pytest.mark.asyncio
+async def test_get_active_downloads_restores_persisted_aria2_entries(monkeypatch, tmp_path):
+    manager = DownloadManager()
+    save_dir = tmp_path / "downloads"
+    save_dir.mkdir()
+    save_path = save_dir / "file.safetensors"
+    save_path.write_text("partial")
+    (save_dir / "file.safetensors.aria2").write_text("control")
+
+    await manager._aria2_state_store.upsert(
+        "download-1",
+        {
+            "download_id": "download-1",
+            "transfer_backend": "aria2",
+            "status": "paused",
+            "save_path": str(save_path),
+            "file_path": str(save_path),
+            "model_id": 12,
+            "model_version_id": 34,
+        },
+    )
+
+    class DummyAria2Downloader:
+        async def get_status_by_gid(self, gid):
+            return None
+
+    monkeypatch.setattr(
+        download_manager,
+        "get_aria2_downloader",
+        AsyncMock(return_value=DummyAria2Downloader()),
+    )
+
+    downloads = await manager.get_active_downloads()
+
+    assert downloads["downloads"] == [
+        {
+            "download_id": "download-1",
+            "model_id": 12,
+            "model_version_id": 34,
+            "progress": 0,
+            "status": "paused",
+            "error": None,
+            "bytes_downloaded": 0,
+            "total_bytes": None,
+            "bytes_per_second": 0.0,
+        }
+    ]
+
+
+@pytest.mark.asyncio
+async def test_get_active_downloads_restores_orphaned_aria2_partial_as_paused(
+    monkeypatch, tmp_path
+):
+    manager = DownloadManager()
+    save_dir = tmp_path / "downloads"
+    save_dir.mkdir()
+    save_path = save_dir / "file.safetensors"
+    save_path.write_text("partial")
+    (save_dir / "file.safetensors.aria2").write_text("control")
+
+    await manager._aria2_state_store.upsert(
+        "download-1",
+        {
+            "download_id": "download-1",
+            "transfer_backend": "aria2",
+            "status": "downloading",
+            "save_path": str(save_path),
+            "file_path": str(save_path),
+            "model_id": 12,
+            "model_version_id": 34,
+            "gid": "missing-gid",
+        },
+    )
+
+    class DummyAria2Downloader:
+        async def get_status_by_gid(self, gid):
+            return None
+
+    monkeypatch.setattr(
+        download_manager,
+        "get_aria2_downloader",
+        AsyncMock(return_value=DummyAria2Downloader()),
+    )
+
+    downloads = await manager.get_active_downloads()
+    persisted = await manager._aria2_state_store.get("download-1")
+
+    assert downloads["downloads"] == [
+        {
+            "download_id": "download-1",
+            "model_id": 12,
+            "model_version_id": 34,
+            "progress": 0,
+            "status": "paused",
+            "error": None,
+            "bytes_downloaded": 0,
+            "total_bytes": None,
+            "bytes_per_second": 0.0,
+        }
+    ]
+    assert manager._pause_events["download-1"].is_paused() is True
+    assert persisted["status"] == "paused"
+
+
+@pytest.mark.asyncio
+async def test_get_active_downloads_restarts_from_resume_context_for_active_restored_aria2(
+    monkeypatch, tmp_path
+):
+    manager = DownloadManager()
+    save_dir = tmp_path / "downloads"
+    save_dir.mkdir()
+    save_path = save_dir / "file.safetensors"
+    save_path.write_text("partial")
+
+    await manager._aria2_state_store.upsert(
+        "download-1",
+        {
+            "download_id": "download-1",
+            "transfer_backend": "aria2",
+            "status": "downloading",
+            "save_path": str(save_path),
+            "file_path": str(save_path),
+            "model_id": 12,
+            "model_version_id": 34,
+            "gid": "gid-1",
+            "resume_context": {
+                "version_info": {
+                    "id": 34,
+                    "modelId": 12,
+                    "model": {"id": 12, "type": "LoRA", "tags": ["fantasy"]},
+                    "images": [],
+                },
+                "file_info": {
+                    "name": "file.safetensors",
+                    "type": "Model",
+                    "primary": True,
+                    "downloadUrl": "https://example.com/file.safetensors",
+                },
+                "model_type": "lora",
+                "relative_path": "",
+                "save_dir": str(save_dir),
+                "download_urls": ["https://example.com/file.safetensors"],
+            },
+        },
+    )
+
+    restarted = {}
+
+    class DummyAria2Downloader:
+        async def get_status_by_gid(self, gid):
+            return {"gid": gid, "status": "active"}
+
+        async def restore_transfer(self, download_id, gid, restored_path):
+            return None
+
+    monkeypatch.setattr(
+        download_manager,
+        "get_aria2_downloader",
+        AsyncMock(return_value=DummyAria2Downloader()),
+    )
+
+    async def fake_resume_restored_aria2_download(self, download_id, record):
+        restarted.update(
+            {
+                "download_id": download_id,
+                "model_id": record.get("model_id"),
+                "model_version_id": record.get("model_version_id"),
+                "save_dir": record.get("save_dir"),
+                "resume_context": record.get("resume_context"),
+            }
+        )
+        return {"success": True}
+
+    monkeypatch.setattr(
+        DownloadManager,
+        "_resume_restored_aria2_download",
+        fake_resume_restored_aria2_download,
+    )
+    execute_original = AsyncMock(side_effect=AssertionError("should not refetch metadata"))
+    monkeypatch.setattr(
+        DownloadManager,
+        "_execute_original_download",
+        execute_original,
+    )
+
+    downloads = await manager.get_active_downloads()
+    assert downloads["downloads"][0]["status"] == "downloading"
+    restarted_task = manager._download_tasks["download-1"]
+    await restarted_task
+
+    assert restarted["download_id"] == "download-1"
+    assert restarted["model_id"] == 12
+    assert restarted["model_version_id"] == 34
+    assert restarted["save_dir"] is None
+    assert restarted["resume_context"]["model_type"] == "lora"
+    assert execute_original.await_count == 0
+
+
+@pytest.mark.asyncio
+async def test_get_active_downloads_restores_persisted_aria2_without_initial_save_path(
+    monkeypatch, tmp_path
+):
+    manager = DownloadManager()
+    save_dir = tmp_path / "downloads"
+    save_dir.mkdir()
+    save_path = save_dir / "file.safetensors"
+    save_path.write_text("partial")
+    (save_dir / "file.safetensors.aria2").write_text("control")
+
+    await manager._aria2_state_store.upsert(
+        "download-1",
+        {
+            "download_id": "download-1",
+            "transfer_backend": "aria2",
+            "status": "paused",
+            "model_id": 12,
+            "model_version_id": 34,
+            "resume_context": {
+                "version_info": {
+                    "id": 34,
+                    "modelId": 12,
+                    "model": {"id": 12, "type": "LoRA"},
+                    "images": [],
+                },
+                "file_info": {
+                    "name": "file.safetensors",
+                    "type": "Model",
+                    "primary": True,
+                    "downloadUrl": "https://example.com/file.safetensors",
+                },
+                "model_type": "lora",
+                "relative_path": "",
+                "save_dir": str(save_dir),
+                "download_urls": ["https://example.com/file.safetensors"],
+            },
+        },
+    )
+
+    class DummyAria2Downloader:
+        async def get_status_by_gid(self, gid):
+            return None
+
+    monkeypatch.setattr(
+        download_manager,
+        "get_aria2_downloader",
+        AsyncMock(return_value=DummyAria2Downloader()),
+    )
+
+    downloads = await manager.get_active_downloads()
+    persisted = await manager._aria2_state_store.get("download-1")
+
+    assert downloads["downloads"] == [
+        {
+            "download_id": "download-1",
+            "model_id": 12,
+            "model_version_id": 34,
+            "progress": 0,
+            "status": "paused",
+            "error": None,
+            "bytes_downloaded": 0,
+            "total_bytes": None,
+            "bytes_per_second": 0.0,
+        }
+    ]
+    assert manager._active_downloads["download-1"]["file_path"] == str(save_path)
+    assert persisted["save_path"] == str(save_path)
+    assert persisted["file_path"] == str(save_path)
+
+
+@pytest.mark.asyncio
+async def test_resume_restored_aria2_download_updates_terminal_status_and_cleanup(monkeypatch):
+    manager = DownloadManager()
+    manager._active_downloads["download-1"] = {
+        "transfer_backend": "aria2",
+        "status": "paused",
+        "model_id": 12,
+        "model_version_id": 34,
+        "bytes_per_second": 10.0,
+    }
+
+    persist_state = AsyncMock()
+    cleanup_record = AsyncMock(return_value=None)
+    execute_download = AsyncMock(return_value={"success": True})
+    record_history = AsyncMock(return_value=None)
+    sync_version = AsyncMock(return_value=None)
+
+    monkeypatch.setattr(manager, "_persist_aria2_state", persist_state)
+    monkeypatch.setattr(manager, "_cleanup_download_record", cleanup_record)
+    monkeypatch.setattr(manager, "_execute_download", execute_download)
+    monkeypatch.setattr(manager, "_record_downloaded_version_history", record_history)
+    monkeypatch.setattr(manager, "_sync_downloaded_version", sync_version)
+
+    scheduled_tasks = []
+    original_create_task = asyncio.create_task
+
+    def tracking_create_task(coro):
+        task = original_create_task(coro)
+        scheduled_tasks.append(task)
+        return task
+
+    monkeypatch.setattr(download_manager.asyncio, "create_task", tracking_create_task)
+
+    result = await manager._resume_restored_aria2_download(
+        "download-1",
+        {
+            "download_id": "download-1",
+            "save_path": "/tmp/file.safetensors",
+            "file_path": "/tmp/file.safetensors",
+            "model_id": 12,
+            "model_version_id": 34,
+            "resume_context": {
+                "version_info": {
+                    "id": 34,
+                    "modelId": 12,
+                    "model": {"id": 12},
+                    "images": [],
+                },
+                "file_info": {
+                    "name": "file.safetensors",
+                    "downloadUrl": "https://example.com/file.safetensors",
+                },
+                "model_type": "lora",
+                "relative_path": "",
+                "save_dir": "/tmp",
+                "download_urls": ["https://example.com/file.safetensors"],
+            },
+        },
+    )
+
+    assert result == {"success": True}
+    assert manager._active_downloads["download-1"]["status"] == "completed"
+    assert manager._active_downloads["download-1"]["bytes_per_second"] == 0.0
+    assert persist_state.await_count == 2
+    assert len(scheduled_tasks) == 1
+    await asyncio.gather(*scheduled_tasks)
+    cleanup_record.assert_awaited_once_with("download-1")
+
+
+@pytest.mark.asyncio
+async def test_download_uses_captured_backend_when_settings_change(
+    monkeypatch, scanners, metadata_provider, tmp_path
+):
+    manager = DownloadManager()
+    settings = get_settings_manager()
+    settings.settings["download_backend"] = "aria2"
+
+    semaphore = asyncio.Semaphore(0)
+    manager._download_semaphore = semaphore
+
+    captured = {}
+
+    async def fake_execute_original_download(
+        self,
+        model_id,
+        model_version_id,
+        save_dir,
+        relative_path,
+        progress_callback,
+        use_default_paths,
+        download_id=None,
+        transfer_backend="python",
+        source=None,
+        file_params=None,
+    ):
+        captured["transfer_backend"] = transfer_backend
+        return {"success": True}
+
+    monkeypatch.setattr(
+        DownloadManager,
+        "_execute_original_download",
+        fake_execute_original_download,
+    )
+
+    download_task = asyncio.create_task(
+        manager.download_from_civitai(
+            model_version_id=99,
+            save_dir=str(tmp_path),
+            use_default_paths=True,
+            progress_callback=None,
+            source=None,
+        )
+    )
+
+    await asyncio.sleep(0)
+    assert len(manager._active_downloads) == 1
+    download_id = next(iter(manager._active_downloads))
+    assert manager._active_downloads[download_id]["transfer_backend"] == "aria2"
+
+    settings.settings["download_backend"] = "python"
+    semaphore.release()
+
+    result = await download_task
+
+    assert result["success"] is True
+    assert captured["transfer_backend"] == "aria2"
+
+
 @pytest.mark.asyncio
 async def test_download_aborts_when_version_exists(
     monkeypatch, scanners, metadata_provider

tests/services/test_settings_manager.py

@@ -147,6 +147,11 @@ def test_environment_variable_overrides_settings(tmp_path, monkeypatch):
     assert mgr.get("civitai_api_key") == "secret"
 
 
+def test_default_download_backend_is_python(manager):
+    assert manager.get("download_backend") == "python"
+    assert manager.get("aria2c_path") == ""
+
+
 def _create_manager_with_settings(
     tmp_path, monkeypatch, initial_settings, *, save_spy=None
 ):