diff --git a/configure_model_root.py b/configure_model_root.py
new file mode 100644
index 0000000..31c646a
--- /dev/null
+++ b/configure_model_root.py
@@ -0,0 +1,48 @@
+#    Substitute BackEnd - backend liaison services for SugarSubstitute and ComfyUI
+#    Copyright (C) 2026  Artificial Sweetener and contributors
+#
+#    This program is free software: you can redistribute it and/or modify
+#    it under the terms of the GNU Affero General Public License as published by
+#    the Free Software Foundation, either version 3 of the License, or
+#    (at your option) any later version.
+#
+#    This program is distributed in the hope that it will be useful,
+#    but WITHOUT ANY WARRANTY; without even the implied warranty of
+#    MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+#    GNU Affero General Public License for more details.
+#
+#    You should have received a copy of the GNU Affero General Public License
+#    along with this program.  If not, see <https://www.gnu.org/licenses/>.
+"""Configure BackEnd model-root state before a newly installed Comfy can start."""
+
+from __future__ import annotations
+
+import argparse
+import sys
+from pathlib import Path
+
+_EXTENSION_ROOT = Path(__file__).resolve().parent
+if str(_EXTENSION_ROOT) not in sys.path:
+    sys.path.insert(0, str(_EXTENSION_ROOT))
+
+from substitute_backend.features.environment_management.infrastructure.model_root_store import (  # noqa: E402
+    ModelRootStore,
+)
+
+
+def main(argv: list[str] | None = None) -> int:
+    """Persist a model-root selection for an offline Comfy installation."""
+
+    parser = argparse.ArgumentParser()
+    parser.add_argument("--comfy-root", required=True, type=Path)
+    selection = parser.add_mutually_exclusive_group(required=True)
+    selection.add_argument("--default", action="store_true")
+    selection.add_argument("--path", type=Path)
+    arguments = parser.parse_args(argv)
+    store = ModelRootStore(arguments.comfy_root)
+    store.save(None if arguments.default else arguments.path)
+    return 0
+
+
+if __name__ == "__main__":
+    raise SystemExit(main())
diff --git a/package-lock.json b/package-lock.json
index 98e9933..3f6bedf 100644
--- a/package-lock.json
+++ b/package-lock.json
@@ -1,12 +1,12 @@
 {
   "name": "substitute-backend",
-  "version": "1.6.2",
+  "version": "1.7.0",
   "lockfileVersion": 3,
   "requires": true,
   "packages": {
     "": {
       "name": "substitute-backend",
-      "version": "1.6.2",
+      "version": "1.7.0",
       "license": "AGPL-3.0-or-later",
       "devDependencies": {
         "@semantic-release/changelog": "^6.0.3",
diff --git a/package.json b/package.json
index 9587af9..598c973 100644
--- a/package.json
+++ b/package.json
@@ -1,7 +1,7 @@
 {
   "name": "substitute-backend",
   "private": true,
-  "version": "1.6.2",
+  "version": "1.7.0",
   "license": "AGPL-3.0-or-later",
   "type": "module",
   "scripts": {
diff --git a/prestartup_script.py b/prestartup_script.py
new file mode 100644
index 0000000..d955197
--- /dev/null
+++ b/prestartup_script.py
@@ -0,0 +1,45 @@
+#    Substitute BackEnd - backend liaison services for SugarSubstitute and ComfyUI
+#    Copyright (C) 2026  Artificial Sweetener and contributors
+#
+#    This program is free software: you can redistribute it and/or modify
+#    it under the terms of the GNU Affero General Public License as published by
+#    the Free Software Foundation, either version 3 of the License, or
+#    (at your option) any later version.
+#
+#    This program is distributed in the hope that it will be useful,
+#    but WITHOUT ANY WARRANTY; without even the implied warranty of
+#    MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+#    GNU Affero General Public License for more details.
+#
+#    You should have received a copy of the GNU Affero General Public License
+#    along with this program.  If not, see <https://www.gnu.org/licenses/>.
+"""Run BackEnd model-root configuration in ComfyUI's prestartup phase."""
+
+from __future__ import annotations
+
+import importlib
+import importlib.util
+from pathlib import Path
+from types import ModuleType
+
+
+def _load_prestartup_support(extension_root: Path) -> ModuleType:
+    """Load the isolated support module without importing the node package."""
+
+    module_path = extension_root / "substitute_backend_prestartup.py"
+    spec = importlib.util.spec_from_file_location(
+        "substitute_backend_prestartup_support", module_path
+    )
+    if spec is None or spec.loader is None:
+        raise RuntimeError("Substitute BackEnd prestartup support could not be loaded.")
+    module = importlib.util.module_from_spec(spec)
+    spec.loader.exec_module(module)
+    return module
+
+
+_EXTENSION_ROOT = Path(__file__).resolve().parent
+_SUPPORT = _load_prestartup_support(_EXTENSION_ROOT)
+_SUPPORT.apply_model_root(
+    _EXTENSION_ROOT.parents[1],
+    importlib.import_module("folder_paths"),
+)
diff --git a/pyproject.toml b/pyproject.toml
index d6bfb1a..03afb83 100644
--- a/pyproject.toml
+++ b/pyproject.toml
@@ -5,7 +5,7 @@ build-backend = "setuptools.build_meta"
 [project]
 name = "substitute-backend"
 description = "Backend liaison services for SugarSubstitute and ComfyUI."
-version = "1.6.2"
+version = "1.7.0"
 license = "AGPL-3.0-or-later"
 license-files = ["LICENSE"]
 requires-python = ">=3.12"
@@ -31,6 +31,9 @@ dev = [
 Repository = "https://github.com/Artificial-Sweetener/Substitute-BackEnd"
 "Bug Tracker" = "https://github.com/Artificial-Sweetener/Substitute-BackEnd/issues"
 
+[tool.setuptools]
+py-modules = ["substitute_backend_prestartup"]
+
 [tool.comfy]
 PublisherId = "artificialsweetener"
 DisplayName = "Substitute BackEnd"
diff --git a/substitute_backend/__init__.py b/substitute_backend/__init__.py
index 653c944..ff1a5c8 100644
--- a/substitute_backend/__init__.py
+++ b/substitute_backend/__init__.py
@@ -17,7 +17,7 @@
 
 from __future__ import annotations
 
-__version__ = "1.6.2"
+__version__ = "1.7.0"
 
 EXTENSION_NAME = "Substitute BackEnd"
 API_VERSION = 1
diff --git a/substitute_backend/features/environment_management/api/routes.py b/substitute_backend/features/environment_management/api/routes.py
index 8f3e936..8a4533a 100644
--- a/substitute_backend/features/environment_management/api/routes.py
+++ b/substitute_backend/features/environment_management/api/routes.py
@@ -27,6 +27,7 @@ from substitute_backend.api.errors import BackendHttpError, json_error
 from substitute_backend.features.environment_management.application.services import (
     EnvironmentManagementServices,
 )
+from substitute_backend.features.environment_management.domain.model_root import ModelRootMode
 
 RouteHandler = Callable[[web.Request], Awaitable[web.StreamResponse]]
 
@@ -37,6 +38,8 @@ class EnvironmentRouteHandlers:
 
     capabilities: RouteHandler
     status: RouteHandler
+    get_model_root: RouteHandler
+    update_model_root: RouteHandler
     list_packages: RouteHandler
     list_components: RouteHandler
     plan_operation: RouteHandler
@@ -85,6 +88,44 @@ def build_environment_route_handlers(
                 )
             )
 
+    async def get_model_root(request: web.Request) -> web.Response:
+        """Return persisted and active Comfy model-root state."""
+
+        _ = request
+        try:
+            return web.json_response(services.model_root.get_status().to_payload())
+        except (OSError, ValueError) as exc:
+            logger.warning(
+                "model-root status unavailable",
+                extra={"operation": "model-root-status", "error": repr(exc)},
+            )
+            return json_error(
+                BackendHttpError(
+                    message="Model-root configuration could not be read.",
+                    status=500,
+                    code="model-root-status-unavailable",
+                )
+            )
+
+    async def update_model_root(request: web.Request) -> web.Response:
+        """Persist the model root that ComfyUI will use after restart."""
+
+        try:
+            body = await _json_object_body(request)
+            mode, raw_path = _parse_model_root_update(body)
+            status = services.model_root.configure(mode, raw_path)
+            return web.json_response(status.to_payload())
+        except BackendHttpError as exc:
+            return json_error(exc)
+        except (OSError, ValueError) as exc:
+            return json_error(
+                BackendHttpError(
+                    message=str(exc),
+                    status=400,
+                    code="invalid-model-root",
+                )
+            )
+
     async def restart(request: web.Request) -> web.Response:
         """Queue a Comfy restart operation."""
 
@@ -302,6 +343,8 @@ def build_environment_route_handlers(
     return EnvironmentRouteHandlers(
         capabilities=capabilities,
         status=status,
+        get_model_root=get_model_root,
+        update_model_root=update_model_root,
         list_packages=list_packages,
         list_components=list_components,
         plan_operation=plan_operation,
@@ -330,6 +373,36 @@ async def _json_object_body(request: web.Request) -> dict[str, object]:
     return body
 
 
+def _parse_model_root_update(
+    body: dict[str, object],
+) -> tuple[ModelRootMode, str | None]:
+    """Parse one model-root update without leaking untyped request data."""
+
+    raw_mode = body.get("mode")
+    if not isinstance(raw_mode, str):
+        raise BackendHttpError(
+            message="Model-root mode is required.",
+            status=400,
+            code="invalid-model-root",
+        )
+    try:
+        mode = ModelRootMode(raw_mode)
+    except ValueError as exc:
+        raise BackendHttpError(
+            message="Model-root mode must be 'default' or 'custom'.",
+            status=400,
+            code="invalid-model-root",
+        ) from exc
+    raw_path = body.get("path")
+    if raw_path is not None and not isinstance(raw_path, str):
+        raise BackendHttpError(
+            message="Model-root path must be a string.",
+            status=400,
+            code="invalid-model-root",
+        )
+    return mode, raw_path
+
+
 def _required_int(data: dict[str, object], key: str) -> int:
     """Read one required integer request field."""
 
diff --git a/substitute_backend/features/environment_management/application/environment_service.py b/substitute_backend/features/environment_management/application/environment_service.py
index 982e44d..0e57a43 100644
--- a/substitute_backend/features/environment_management/application/environment_service.py
+++ b/substitute_backend/features/environment_management/application/environment_service.py
@@ -50,6 +50,7 @@ class EnvironmentService:
             EnvironmentFeature.PACKAGE_INVENTORY,
             EnvironmentFeature.COMPONENT_INVENTORY,
             EnvironmentFeature.OPERATION_PLANNING,
+            EnvironmentFeature.MODEL_ROOT_MANAGEMENT,
         ]
         if restart_support.supported:
             supported_features.append(EnvironmentFeature.RESTART)
@@ -59,6 +60,7 @@ class EnvironmentService:
             restart_supported=restart_support.supported,
             package_mutation_supported=False,
             operation_planning_supported=True,
+            model_root_management_supported=True,
             restart_unavailable_reason=restart_support.unavailable_reason,
         )
 
diff --git a/substitute_backend/features/environment_management/application/model_root_service.py b/substitute_backend/features/environment_management/application/model_root_service.py
new file mode 100644
index 0000000..63c8758
--- /dev/null
+++ b/substitute_backend/features/environment_management/application/model_root_service.py
@@ -0,0 +1,74 @@
+#    Substitute BackEnd - backend liaison services for SugarSubstitute and ComfyUI
+#    Copyright (C) 2026  Artificial Sweetener and contributors
+#
+#    This program is free software: you can redistribute it and/or modify
+#    it under the terms of the GNU Affero General Public License as published by
+#    the Free Software Foundation, either version 3 of the License, or
+#    (at your option) any later version.
+#
+#    This program is distributed in the hope that it will be useful,
+#    but WITHOUT ANY WARRANTY; without even the implied warranty of
+#    MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+#    GNU Affero General Public License for more details.
+#
+#    You should have received a copy of the GNU Affero General Public License
+#    along with this program.  If not, see <https://www.gnu.org/licenses/>.
+"""Coordinate persisted and active Comfy model-root state."""
+
+from __future__ import annotations
+
+from dataclasses import dataclass
+from pathlib import Path
+
+from substitute_backend.features.environment_management.domain.model_root import (
+    ModelRootMode,
+    ModelRootStatus,
+)
+from substitute_backend.features.environment_management.infrastructure.model_root_runtime import (
+    ModelRootRuntime,
+)
+from substitute_backend.features.environment_management.infrastructure.model_root_store import (
+    ModelRootStore,
+)
+
+_MODEL_ROOT_SCHEMA_VERSION = 1
+
+
+@dataclass(frozen=True)
+class ModelRootService:
+    """Own model-root queries and configuration changes for one Comfy host."""
+
+    comfy_root: Path
+    store: ModelRootStore
+    runtime: ModelRootRuntime
+
+    def get_status(self) -> ModelRootStatus:
+        """Return the persisted selection alongside the running process state."""
+
+        default_root = (self.comfy_root / "models").resolve()
+        configured_root = self.store.load()
+        active_root = self.runtime.active_model_root()
+        desired_root = configured_root or default_root
+        return ModelRootStatus(
+            schema_version=_MODEL_ROOT_SCHEMA_VERSION,
+            default_model_root=default_root,
+            configured_model_root=configured_root,
+            active_model_root=active_root,
+            restart_required=desired_root != active_root,
+        )
+
+    def configure(self, mode: ModelRootMode, path: str | None) -> ModelRootStatus:
+        """Persist an explicit default or custom selection for the next launch."""
+
+        if mode is ModelRootMode.DEFAULT:
+            if path is not None:
+                raise ValueError("Default model-root mode does not accept a path.")
+            self.store.save(None)
+        else:
+            if path is None or not path.strip():
+                raise ValueError("Custom model-root mode requires a path.")
+            self.store.save(ModelRootStore.resolve_custom_root(path))
+        return self.get_status()
+
+
+__all__ = ["ModelRootService"]
diff --git a/substitute_backend/features/environment_management/application/services.py b/substitute_backend/features/environment_management/application/services.py
index 28ba3a0..b0e12e1 100644
--- a/substitute_backend/features/environment_management/application/services.py
+++ b/substitute_backend/features/environment_management/application/services.py
@@ -32,6 +32,9 @@ from substitute_backend.features.environment_management.application.inventory_se
 from substitute_backend.features.environment_management.application.job_service import (
     JobService,
 )
+from substitute_backend.features.environment_management.application.model_root_service import (
+    ModelRootService,
+)
 from substitute_backend.features.environment_management.application.restart_service import (
     RestartService,
 )
@@ -47,3 +50,4 @@ class EnvironmentManagementServices:
     maintenance_plan: MaintenancePlanService
     operation_planning: OperationPlanningService
     restart: RestartService
+    model_root: ModelRootService
diff --git a/substitute_backend/features/environment_management/domain/capabilities.py b/substitute_backend/features/environment_management/domain/capabilities.py
index 7ba7edc..ac308ba 100644
--- a/substitute_backend/features/environment_management/domain/capabilities.py
+++ b/substitute_backend/features/environment_management/domain/capabilities.py
@@ -31,6 +31,7 @@ class EnvironmentFeature(StrEnum):
     RESTART = "restart"
     OPERATION_PLANNING = "operation-planning"
     PACKAGE_MUTATION = "package-mutation"
+    MODEL_ROOT_MANAGEMENT = "model-root-management"
 
 
 @dataclass(frozen=True)
@@ -42,6 +43,7 @@ class EnvironmentCapabilities:
     restart_supported: bool
     package_mutation_supported: bool
     operation_planning_supported: bool
+    model_root_management_supported: bool
     restart_unavailable_reason: str | None = None
 
     def to_payload(self) -> JsonObject:
@@ -53,6 +55,7 @@ class EnvironmentCapabilities:
             "restartSupported": self.restart_supported,
             "packageMutationSupported": self.package_mutation_supported,
             "operationPlanningSupported": self.operation_planning_supported,
+            "modelRootManagementSupported": self.model_root_management_supported,
         }
         if self.restart_unavailable_reason is not None:
             payload["restartUnavailableReason"] = self.restart_unavailable_reason
diff --git a/substitute_backend/features/environment_management/domain/model_root.py b/substitute_backend/features/environment_management/domain/model_root.py
new file mode 100644
index 0000000..22c0f23
--- /dev/null
+++ b/substitute_backend/features/environment_management/domain/model_root.py
@@ -0,0 +1,65 @@
+#    Substitute BackEnd - backend liaison services for SugarSubstitute and ComfyUI
+#    Copyright (C) 2026  Artificial Sweetener and contributors
+#
+#    This program is free software: you can redistribute it and/or modify
+#    it under the terms of the GNU Affero General Public License as published by
+#    the Free Software Foundation, either version 3 of the License, or
+#    (at your option) any later version.
+#
+#    This program is distributed in the hope that it will be useful,
+#    but WITHOUT ANY WARRANTY; without even the implied warranty of
+#    MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+#    GNU Affero General Public License for more details.
+#
+#    You should have received a copy of the GNU Affero General Public License
+#    along with this program.  If not, see <https://www.gnu.org/licenses/>.
+"""Model-root contracts owned by the Comfy environment."""
+
+from __future__ import annotations
+
+from dataclasses import dataclass
+from enum import StrEnum
+from pathlib import Path
+
+from substitute_backend.api.serialization import JsonObject
+
+
+class ModelRootMode(StrEnum):
+    """Identify whether Comfy uses its default or a configured model root."""
+
+    DEFAULT = "default"
+    CUSTOM = "custom"
+
+
+@dataclass(frozen=True)
+class ModelRootStatus:
+    """Describe persisted and active model-root state for one Comfy host."""
+
+    schema_version: int
+    default_model_root: Path
+    configured_model_root: Path | None
+    active_model_root: Path
+    restart_required: bool
+
+    @property
+    def uses_default(self) -> bool:
+        """Return whether the persisted configuration selects Comfy's default."""
+
+        return self.configured_model_root is None
+
+    def to_payload(self) -> JsonObject:
+        """Return a stable API payload for external environment clients."""
+
+        return {
+            "schemaVersion": self.schema_version,
+            "defaultModelRoot": str(self.default_model_root),
+            "configuredModelRoot": (
+                str(self.configured_model_root) if self.configured_model_root is not None else None
+            ),
+            "activeModelRoot": str(self.active_model_root),
+            "usesDefault": self.uses_default,
+            "restartRequired": self.restart_required,
+        }
+
+
+__all__ = ["ModelRootMode", "ModelRootStatus"]
diff --git a/substitute_backend/features/environment_management/infrastructure/model_root_runtime.py b/substitute_backend/features/environment_management/infrastructure/model_root_runtime.py
new file mode 100644
index 0000000..bc02012
--- /dev/null
+++ b/substitute_backend/features/environment_management/infrastructure/model_root_runtime.py
@@ -0,0 +1,48 @@
+#    Substitute BackEnd - backend liaison services for SugarSubstitute and ComfyUI
+#    Copyright (C) 2026  Artificial Sweetener and contributors
+#
+#    This program is free software: you can redistribute it and/or modify
+#    it under the terms of the GNU Affero General Public License as published by
+#    the Free Software Foundation, either version 3 of the License, or
+#    (at your option) any later version.
+#
+#    This program is distributed in the hope that it will be useful,
+#    but WITHOUT ANY WARRANTY; without even the implied warranty of
+#    MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+#    GNU Affero General Public License for more details.
+#
+#    You should have received a copy of the GNU Affero General Public License
+#    along with this program.  If not, see <https://www.gnu.org/licenses/>.
+"""Read active model-root state from ComfyUI at the host boundary."""
+
+from __future__ import annotations
+
+import importlib
+from pathlib import Path
+from typing import Protocol, cast
+
+
+class ModelRootRuntime(Protocol):
+    """Expose the model root active in the current Comfy process."""
+
+    def active_model_root(self) -> Path:
+        """Return the model root currently used by ComfyUI."""
+
+
+class _FolderPathsModule(Protocol):
+    """Describe the ComfyUI state required by the runtime adapter."""
+
+    models_dir: str
+
+
+class ComfyModelRootRuntime:
+    """Read the active root from ComfyUI's initialized folder registry."""
+
+    def active_model_root(self) -> Path:
+        """Return ComfyUI's normalized active model root."""
+
+        folder_paths = cast("_FolderPathsModule", importlib.import_module("folder_paths"))
+        return Path(folder_paths.models_dir).resolve()
+
+
+__all__ = ["ComfyModelRootRuntime", "ModelRootRuntime"]
diff --git a/substitute_backend/features/environment_management/infrastructure/model_root_store.py b/substitute_backend/features/environment_management/infrastructure/model_root_store.py
new file mode 100644
index 0000000..f38fc4a
--- /dev/null
+++ b/substitute_backend/features/environment_management/infrastructure/model_root_store.py
@@ -0,0 +1,110 @@
+#    Substitute BackEnd - backend liaison services for SugarSubstitute and ComfyUI
+#    Copyright (C) 2026  Artificial Sweetener and contributors
+#
+#    This program is free software: you can redistribute it and/or modify
+#    it under the terms of the GNU Affero General Public License as published by
+#    the Free Software Foundation, either version 3 of the License, or
+#    (at your option) any later version.
+#
+#    This program is distributed in the hope that it will be useful,
+#    but WITHOUT ANY WARRANTY; without even the implied warranty of
+#    MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+#    GNU Affero General Public License for more details.
+#
+#    You should have received a copy of the GNU Affero General Public License
+#    along with this program.  If not, see <https://www.gnu.org/licenses/>.
+"""Persist the Comfy installation's authoritative model-root selection."""
+
+from __future__ import annotations
+
+import json
+import os
+from pathlib import Path
+from uuid import uuid4
+
+_CONFIG_DIRECTORY = ".substitute"
+_CONFIG_FILE = "model_root.json"
+_SCHEMA_VERSION = 1
+
+
+class ModelRootStore:
+    """Read and atomically replace model-root configuration for one Comfy root."""
+
+    def __init__(self, comfy_root: Path) -> None:
+        """Initialize persistence beneath the supplied Comfy installation."""
+
+        self._comfy_root = comfy_root.resolve()
+        self._config_path = self._comfy_root / _CONFIG_DIRECTORY / _CONFIG_FILE
+
+    @property
+    def config_path(self) -> Path:
+        """Return the canonical persisted configuration path."""
+
+        return self._config_path
+
+    def load(self) -> Path | None:
+        """Return the configured custom root, or ``None`` for Comfy's default."""
+
+        if not self._config_path.exists():
+            return None
+        payload = json.loads(self._config_path.read_text(encoding="utf-8"))
+        if payload.get("schemaVersion") != _SCHEMA_VERSION:
+            raise ValueError("Unsupported model-root configuration schema.")
+        value = payload.get("modelRoot")
+        if not isinstance(value, str) or not value.strip():
+            raise ValueError("Model-root configuration does not contain a path.")
+        return self.resolve_custom_root(value)
+
+    def save(self, model_root: Path | None) -> Path | None:
+        """Persist a custom root, or remove configuration for Comfy's default."""
+
+        if model_root is None:
+            self._remove_config()
+            return None
+        resolved_root = self.resolve_custom_root(str(model_root))
+        resolved_root.mkdir(parents=True, exist_ok=True)
+        if not resolved_root.is_dir():
+            raise ValueError("Model root must be a directory.")
+        self._config_path.parent.mkdir(parents=True, exist_ok=True)
+        temporary_path = self._config_path.with_name(f".{self._config_path.name}.{uuid4().hex}.tmp")
+        try:
+            temporary_path.write_text(
+                json.dumps(
+                    {
+                        "schemaVersion": _SCHEMA_VERSION,
+                        "modelRoot": str(resolved_root),
+                    },
+                    indent=2,
+                )
+                + "\n",
+                encoding="utf-8",
+            )
+            os.replace(temporary_path, self._config_path)
+        finally:
+            if temporary_path.exists():
+                temporary_path.unlink()
+        return resolved_root
+
+    @staticmethod
+    def resolve_custom_root(value: str) -> Path:
+        """Validate and normalize one host-side custom model root."""
+
+        expanded = Path(os.path.expandvars(value)).expanduser()
+        if not expanded.is_absolute():
+            raise ValueError("Model root must be an absolute path.")
+        resolved = expanded.resolve()
+        if resolved.exists() and not resolved.is_dir():
+            raise ValueError("Model root must be a directory.")
+        return resolved
+
+    def _remove_config(self) -> None:
+        """Remove the custom selection while preserving unrelated host state."""
+
+        if self._config_path.exists():
+            self._config_path.unlink()
+        directory = self._config_path.parent
+        if directory.exists() and not any(directory.iterdir()):
+            directory.rmdir()
+
+
+__all__ = ["ModelRootStore"]
diff --git a/substitute_backend/host/extension.py b/substitute_backend/host/extension.py
index 0339a87..a4de828 100644
--- a/substitute_backend/host/extension.py
+++ b/substitute_backend/host/extension.py
@@ -62,6 +62,9 @@ from substitute_backend.features.environment_management.application.inventory_se
 from substitute_backend.features.environment_management.application.job_service import (
     JobService,
 )
+from substitute_backend.features.environment_management.application.model_root_service import (
+    ModelRootService,
+)
 from substitute_backend.features.environment_management.application.restart_service import (
     RestartService,
 )
@@ -75,6 +78,12 @@ from substitute_backend.features.environment_management.infrastructure import (
     PipInspector,
 )
 from substitute_backend.features.environment_management.infrastructure.job_store import JobStore
+from substitute_backend.features.environment_management.infrastructure.model_root_runtime import (
+    ComfyModelRootRuntime,
+)
+from substitute_backend.features.environment_management.infrastructure.model_root_store import (
+    ModelRootStore,
+)
 from substitute_backend.features.environment_management.infrastructure.python_environment import (
     PythonEnvironmentInspector,
 )
@@ -367,6 +376,11 @@ def build_environment_management_services(extension_root: Path) -> EnvironmentMa
             coordinator=restart_coordinator,
             logger=get_logger("environment.restart"),
         ),
+        model_root=ModelRootService(
+            comfy_root=comfy_root,
+            store=ModelRootStore(comfy_root),
+            runtime=ComfyModelRootRuntime(),
+        ),
     )
 
 
diff --git a/substitute_backend/host/routes.py b/substitute_backend/host/routes.py
index ee03f0e..9c654f1 100644
--- a/substitute_backend/host/routes.py
+++ b/substitute_backend/host/routes.py
@@ -91,6 +91,9 @@ class RouteRegistrar(Protocol):
     def post(self, path: str) -> Callable[[_RouteHandler], _RouteHandler]:
         """Return a decorator registering a POST handler."""
 
+    def put(self, path: str) -> Callable[[_RouteHandler], _RouteHandler]:
+        """Return a decorator registering a PUT handler."""
+
     def delete(self, path: str) -> Callable[[_RouteHandler], _RouteHandler]:
         """Return a decorator registering a DELETE handler."""
 
@@ -239,6 +242,8 @@ def register_routes(
     routes.post("/substitute/v1/cube-library/sync-and-check")(cube_library_handlers.sync_and_check)
     routes.get("/substitute/v1/environment/capabilities")(environment_handlers.capabilities)
     routes.get("/substitute/v1/environment/status")(environment_handlers.status)
+    routes.get("/substitute/v1/environment/model-root")(environment_handlers.get_model_root)
+    routes.put("/substitute/v1/environment/model-root")(environment_handlers.update_model_root)
     routes.get("/substitute/v1/environment/packages")(environment_handlers.list_packages)
     routes.get("/substitute/v1/environment/components")(environment_handlers.list_components)
     routes.post("/substitute/v1/environment/operations/plan")(environment_handlers.plan_operation)
diff --git a/substitute_backend_prestartup.py b/substitute_backend_prestartup.py
new file mode 100644
index 0000000..d9eac50
--- /dev/null
+++ b/substitute_backend_prestartup.py
@@ -0,0 +1,165 @@
+#    Substitute BackEnd - backend liaison services for SugarSubstitute and ComfyUI
+#    Copyright (C) 2026  Artificial Sweetener and contributors
+#
+#    This program is free software: you can redistribute it and/or modify
+#    it under the terms of the GNU Affero General Public License as published by
+#    the Free Software Foundation, either version 3 of the License, or
+#    (at your option) any later version.
+#
+#    This program is distributed in the hope that it will be useful,
+#    but WITHOUT ANY WARRANTY; without even the implied warranty of
+#    MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+#    GNU Affero General Public License for more details.
+#
+#    You should have received a copy of the GNU Affero General Public License
+#    along with this program.  If not, see <https://www.gnu.org/licenses/>.
+"""Apply BackEnd-owned model configuration before normal custom-node imports."""
+
+from __future__ import annotations
+
+import json
+import logging
+import os
+import shutil
+from pathlib import Path
+from typing import Any
+from uuid import uuid4
+
+_CONFIG_RELATIVE_PATH = Path(".substitute") / "model_root.json"
+_LEGACY_CONFIG_RELATIVE_PATH = Path(".substitute") / "managed_model_root.json"
+_LEGACY_HOOK_DIRECTORY = "SubstituteManagedModelRoot"
+_SCHEMA_VERSION = 1
+_LOGGER = logging.getLogger("substitute_backend.prestartup")
+
+
+def apply_model_root(comfy_root: Path, folder_paths: Any) -> Path:
+    """Apply the persisted root and return the model root active after startup."""
+
+    _migrate_legacy_configuration(comfy_root)
+    old_root = Path(str(folder_paths.models_dir)).resolve()
+    configured_root = _read_configured_root(comfy_root)
+    new_root = configured_root or (comfy_root / "models").resolve()
+    new_root.mkdir(parents=True, exist_ok=True)
+    if new_root != old_root:
+        _redirect_registered_paths(folder_paths, old_root, new_root)
+        folder_paths.models_dir = str(new_root)
+        _clear_caches(folder_paths)
+        _LOGGER.info("Substitute BackEnd configured ComfyUI model root: %s", new_root)
+    _remove_legacy_hook(comfy_root)
+    return new_root
+
+
+def _migrate_legacy_configuration(comfy_root: Path) -> None:
+    """Adopt desktop-owned persisted state before the normal startup read."""
+
+    canonical_path = comfy_root / _CONFIG_RELATIVE_PATH
+    legacy_path = comfy_root / _LEGACY_CONFIG_RELATIVE_PATH
+    if not legacy_path.exists():
+        return
+    if canonical_path.exists():
+        legacy_path.unlink()
+        _LOGGER.info("Removed superseded legacy Substitute model-root configuration.")
+        return
+    payload = json.loads(legacy_path.read_text(encoding="utf-8"))
+    value = payload.get("model_root")
+    if not isinstance(value, str) or not value.strip():
+        raise ValueError("Legacy Substitute model-root configuration is invalid.")
+    model_root = Path(value).expanduser()
+    if not model_root.is_absolute():
+        raise ValueError("Legacy Substitute model root must be absolute.")
+    _write_configuration(canonical_path, model_root.resolve())
+    legacy_path.unlink()
+    _LOGGER.info("Migrated legacy Substitute model-root configuration.")
+
+
+def _write_configuration(config_path: Path, model_root: Path) -> None:
+    """Atomically write the lightweight prestartup configuration shape."""
+
+    config_path.parent.mkdir(parents=True, exist_ok=True)
+    temporary_path = config_path.with_name(f".{config_path.name}.{uuid4().hex}.tmp")
+    try:
+        temporary_path.write_text(
+            json.dumps(
+                {"schemaVersion": _SCHEMA_VERSION, "modelRoot": str(model_root)},
+                indent=2,
+            )
+            + "\n",
+            encoding="utf-8",
+        )
+        os.replace(temporary_path, config_path)
+    finally:
+        if temporary_path.exists():
+            temporary_path.unlink()
+
+
+def _remove_legacy_hook(comfy_root: Path) -> None:
+    """Remove the superseded desktop-generated custom node after migration."""
+
+    hook_directory = comfy_root / "custom_nodes" / _LEGACY_HOOK_DIRECTORY
+    if not hook_directory.exists():
+        return
+    try:
+        shutil.rmtree(hook_directory)
+    except OSError as exc:
+        _LOGGER.warning(
+            "Could not remove legacy Substitute model-root hook: %s",
+            exc,
+        )
+        return
+    _LOGGER.info("Removed legacy Substitute model-root startup hook.")
+
+
+def _read_configured_root(comfy_root: Path) -> Path | None:
+    """Read the lightweight configuration without importing BackEnd services."""
+
+    config_path = comfy_root / _CONFIG_RELATIVE_PATH
+    if not config_path.exists():
+        return None
+    payload = json.loads(config_path.read_text(encoding="utf-8"))
+    if payload.get("schemaVersion") != _SCHEMA_VERSION:
+        raise ValueError("Unsupported Substitute BackEnd model-root schema.")
+    value = payload.get("modelRoot")
+    if not isinstance(value, str) or not value.strip():
+        raise ValueError("Substitute BackEnd model-root configuration is invalid.")
+    configured_root = Path(value).expanduser()
+    if not configured_root.is_absolute():
+        raise ValueError("Substitute BackEnd model root must be absolute.")
+    return configured_root.resolve()
+
+
+def _redirect_registered_paths(
+    folder_paths: Any,
+    old_root: Path,
+    new_root: Path,
+) -> None:
+    """Move default-root registry entries while preserving external additions."""
+
+    registry = getattr(folder_paths, "folder_names_and_paths", {})
+    for folder_name, values in list(registry.items()):
+        paths, extensions = values
+        rewritten: list[str] = []
+        changed = False
+        for raw_path in paths:
+            path = Path(str(raw_path)).resolve()
+            try:
+                relative = path.relative_to(old_root)
+            except ValueError:
+                rewritten.append(str(raw_path))
+                continue
+            rewritten.append(str(new_root / relative))
+            changed = True
+        if changed:
+            registry[folder_name] = (rewritten, extensions)
+
+
+def _clear_caches(folder_paths: Any) -> None:
+    """Clear supported Comfy filename caches after registry redirection."""
+
+    for attribute in ("filename_list_cache", "cache_helper"):
+        cache = getattr(folder_paths, attribute, None)
+        clear = getattr(cache, "clear", None)
+        if callable(clear):
+            clear()
+
+
+__all__ = ["apply_model_root"]
diff --git a/tests/test_capabilities_and_routes.py b/tests/test_capabilities_and_routes.py
index f3a81aa..58a181a 100644
--- a/tests/test_capabilities_and_routes.py
+++ b/tests/test_capabilities_and_routes.py
@@ -68,6 +68,11 @@ class FakeRoutes:
 
         return self._record("POST", path)
 
+    def put(self, path: str) -> Callable[[_RouteHandler], _RouteHandler]:
+        """Record a PUT route registration."""
+
+        return self._record("PUT", path)
+
     def delete(self, path: str) -> Callable[[_RouteHandler], _RouteHandler]:
         """Record a DELETE route registration."""
 
@@ -152,6 +157,8 @@ def test_register_routes_uses_expected_surface(tmp_path: Path) -> None:
         ("POST", "/substitute/v1/cube-library/sync-and-check"),
         ("GET", "/substitute/v1/environment/capabilities"),
         ("GET", "/substitute/v1/environment/status"),
+        ("GET", "/substitute/v1/environment/model-root"),
+        ("PUT", "/substitute/v1/environment/model-root"),
         ("GET", "/substitute/v1/environment/packages"),
         ("GET", "/substitute/v1/environment/components"),
         ("POST", "/substitute/v1/environment/operations/plan"),
diff --git a/tests/test_download_telemetry.py b/tests/test_download_telemetry.py
index 21afa91..3f1106c 100644
--- a/tests/test_download_telemetry.py
+++ b/tests/test_download_telemetry.py
@@ -637,6 +637,11 @@ class _FakeRoutes:
 
         return self._record("POST", path)
 
+    def put(self, path: str) -> Any:
+        """Record a PUT handler."""
+
+        return self._record("PUT", path)
+
     def delete(self, path: str) -> Any:
         """Record a DELETE handler."""
 
diff --git a/tests/test_environment_management.py b/tests/test_environment_management.py
index d52ead3..52cf3b1 100644
--- a/tests/test_environment_management.py
+++ b/tests/test_environment_management.py
@@ -46,6 +46,9 @@ from substitute_backend.features.environment_management.application.inventory_se
 from substitute_backend.features.environment_management.application.job_service import (
     JobService,
 )
+from substitute_backend.features.environment_management.application.model_root_service import (
+    ModelRootService,
+)
 from substitute_backend.features.environment_management.application.restart_service import (
     RestartService,
 )
@@ -79,6 +82,12 @@ from substitute_backend.features.environment_management.infrastructure import (
 from substitute_backend.features.environment_management.infrastructure.job_store import (
     JobStore,
 )
+from substitute_backend.features.environment_management.infrastructure.model_root_runtime import (
+    ModelRootRuntime,
+)
+from substitute_backend.features.environment_management.infrastructure.model_root_store import (
+    ModelRootStore,
+)
 from substitute_backend.features.environment_management.infrastructure.python_environment import (
     PythonEnvironmentInspector,
 )
@@ -129,6 +138,20 @@ class UnsupportedRestartCoordinator(RestartCoordinator):
         )
 
 
+class StaticModelRootRuntime(ModelRootRuntime):
+    """Return a fixed active model root for service tests."""
+
+    def __init__(self, active_root: Path) -> None:
+        """Store the active root."""
+
+        self._active_root = active_root
+
+    def active_model_root(self) -> Path:
+        """Return the configured active root."""
+
+        return self._active_root.resolve()
+
+
 class StaticPipInspector(PipInspector):
     """Return fixed pip packages for inventory tests."""
 
@@ -1007,6 +1030,47 @@ def _maintenance_plan_service(
     )
 
 
+def test_model_root_routes_report_and_persist_host_state(tmp_path: Path) -> None:
+    """Environment routes expose a typed model-root mutation contract."""
+
+    services = _environment_services(tmp_path, StaticPipInspector())
+    handlers = build_environment_route_handlers(
+        services,
+        get_logger("tests.environment.routes"),
+    )
+    custom_root = tmp_path / "shared-models"
+
+    async def run_routes() -> None:
+        """Exercise model-root GET and update handlers."""
+
+        initial = await handlers.get_model_root(cast("web.Request", object()))
+        initial_payload = _response_payload(initial)
+        assert initial_payload["usesDefault"] is True
+        assert initial_payload["restartRequired"] is False
+
+        updated = await handlers.update_model_root(
+            cast(
+                "web.Request",
+                FakeJsonRequest({"mode": "custom", "path": str(custom_root)}),
+            )
+        )
+        updated_payload = _response_payload(updated)
+        assert updated_payload["configuredModelRoot"] == str(custom_root.resolve())
+        assert updated_payload["restartRequired"] is True
+
+        invalid = await handlers.update_model_root(
+            cast(
+                "web.Request",
+                FakeJsonRequest({"mode": "custom", "path": "relative/models"}),
+            )
+        )
+        assert invalid.status == 400
+        error = cast("JsonObject", _response_payload(invalid)["error"])
+        assert error["code"] == "invalid-model-root"
+
+    asyncio.run(run_routes())
+
+
 def _expected_triton_requirement() -> str:
     """Return the platform-specific Triton requirement expected by the service."""
 
@@ -1048,6 +1112,11 @@ def _environment_services(
             coordinator=SupportedRestartCoordinator(),
             logger=get_logger("tests.environment.restart"),
         ),
+        model_root=ModelRootService(
+            comfy_root=tmp_path,
+            store=ModelRootStore(tmp_path),
+            runtime=StaticModelRootRuntime(tmp_path / "models"),
+        ),
     )
 
 
diff --git a/tests/test_model_root_management.py b/tests/test_model_root_management.py
new file mode 100644
index 0000000..41203e1
--- /dev/null
+++ b/tests/test_model_root_management.py
@@ -0,0 +1,227 @@
+#    Substitute BackEnd - backend liaison services for SugarSubstitute and ComfyUI
+#    Copyright (C) 2026  Artificial Sweetener and contributors
+#
+#    This program is free software: you can redistribute it and/or modify
+#    it under the terms of the GNU Affero General Public License as published by
+#    the Free Software Foundation, either version 3 of the License, or
+#    (at your option) any later version.
+#
+#    This program is distributed in the hope that it will be useful,
+#    but WITHOUT ANY WARRANTY; without even the implied warranty of
+#    MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+#    GNU Affero General Public License for more details.
+#
+#    You should have received a copy of the GNU Affero General Public License
+#    along with this program.  If not, see <https://www.gnu.org/licenses/>.
+"""Tests for authoritative Comfy model-root management."""
+
+from __future__ import annotations
+
+import json
+from pathlib import Path
+from types import ModuleType
+from typing import Any, cast
+
+import pytest
+
+from substitute_backend.features.environment_management.application.model_root_service import (
+    ModelRootService,
+)
+from substitute_backend.features.environment_management.domain.model_root import ModelRootMode
+from substitute_backend.features.environment_management.infrastructure.model_root_runtime import (
+    ModelRootRuntime,
+)
+from substitute_backend.features.environment_management.infrastructure.model_root_store import (
+    ModelRootStore,
+)
+from substitute_backend_prestartup import apply_model_root
+
+
+class MutableModelRootRuntime(ModelRootRuntime):
+    """Expose a mutable active root for restart-transition tests."""
+
+    def __init__(self, active_root: Path) -> None:
+        """Initialize the active root."""
+
+        self.active_root = active_root
+
+    def active_model_root(self) -> Path:
+        """Return the current active root."""
+
+        return self.active_root.resolve()
+
+
+class ClearTracker:
+    """Record Comfy cache invalidation calls."""
+
+    def __init__(self) -> None:
+        """Initialize an uncleared tracker."""
+
+        self.cleared = False
+
+    def clear(self) -> None:
+        """Record cache invalidation."""
+
+        self.cleared = True
+
+
+def test_store_defaults_to_comfy_models_without_configuration(tmp_path: Path) -> None:
+    """An unconfigured host uses its own models directory."""
+
+    runtime = MutableModelRootRuntime(tmp_path / "models")
+    service = ModelRootService(tmp_path, ModelRootStore(tmp_path), runtime)
+
+    status = service.get_status()
+
+    assert status.uses_default is True
+    assert status.default_model_root == (tmp_path / "models").resolve()
+    assert status.active_model_root == (tmp_path / "models").resolve()
+    assert status.restart_required is False
+
+
+def test_custom_root_is_persisted_atomically_and_requires_restart(tmp_path: Path) -> None:
+    """A custom selection is durable while active state remains process-owned."""
+
+    runtime = MutableModelRootRuntime(tmp_path / "models")
+    store = ModelRootStore(tmp_path)
+    service = ModelRootService(tmp_path, store, runtime)
+    custom_root = tmp_path / "shared models"
+
+    status = service.configure(ModelRootMode.CUSTOM, str(custom_root))
+
+    assert status.configured_model_root == custom_root.resolve()
+    assert status.active_model_root == (tmp_path / "models").resolve()
+    assert status.restart_required is True
+    assert store.load() == custom_root.resolve()
+    assert not tuple(store.config_path.parent.glob("*.tmp"))
+
+    runtime.active_root = custom_root
+    assert service.get_status().restart_required is False
+
+
+def test_default_mode_removes_only_owned_configuration(tmp_path: Path) -> None:
+    """Resetting to default preserves unrelated Substitute host state."""
+
+    store = ModelRootStore(tmp_path)
+    store.save(tmp_path / "shared")
+    unrelated = store.config_path.parent / "other.json"
+    unrelated.write_text("{}", encoding="utf-8")
+    service = ModelRootService(
+        tmp_path,
+        store,
+        MutableModelRootRuntime(tmp_path / "shared"),
+    )
+
+    status = service.configure(ModelRootMode.DEFAULT, None)
+
+    assert status.uses_default is True
+    assert status.restart_required is True
+    assert not store.config_path.exists()
+    assert unrelated.exists()
+
+
+@pytest.mark.parametrize("value", ["models", "", "."])
+def test_custom_root_rejects_relative_paths(tmp_path: Path, value: str) -> None:
+    """Host configuration never interprets relative paths ambiguously."""
+
+    service = ModelRootService(
+        tmp_path,
+        ModelRootStore(tmp_path),
+        MutableModelRootRuntime(tmp_path / "models"),
+    )
+
+    with pytest.raises(ValueError, match=r"absolute|requires"):
+        service.configure(ModelRootMode.CUSTOM, value)
+
+
+def test_prestartup_redirects_default_categories_and_preserves_external_paths(
+    tmp_path: Path,
+) -> None:
+    """Prestartup moves Comfy defaults without consuming extra model paths."""
+
+    comfy_root = tmp_path / "ComfyUI"
+    old_root = comfy_root / "models"
+    new_root = tmp_path / "shared-models"
+    external_root = tmp_path / "external-loras"
+    config_path = comfy_root / ".substitute" / "model_root.json"
+    config_path.parent.mkdir(parents=True)
+    config_path.write_text(
+        json.dumps({"schemaVersion": 1, "modelRoot": str(new_root)}),
+        encoding="utf-8",
+    )
+    folder_paths = cast(Any, ModuleType("folder_paths"))
+    folder_paths.models_dir = str(old_root)
+    folder_paths.folder_names_and_paths = {
+        "checkpoints": ([str(old_root / "checkpoints")], {".safetensors"}),
+        "loras": (
+            [str(old_root / "loras"), str(external_root)],
+            {".safetensors"},
+        ),
+    }
+    filename_cache = ClearTracker()
+    helper_cache = ClearTracker()
+    folder_paths.filename_list_cache = filename_cache
+    folder_paths.cache_helper = helper_cache
+
+    active_root = apply_model_root(comfy_root, folder_paths)
+
+    assert active_root == new_root.resolve()
+    assert Path(folder_paths.models_dir) == new_root.resolve()
+    assert folder_paths.folder_names_and_paths["checkpoints"][0] == [str(new_root / "checkpoints")]
+    assert folder_paths.folder_names_and_paths["loras"][0] == [
+        str(new_root / "loras"),
+        str(external_root),
+    ]
+    assert filename_cache.cleared is True
+    assert helper_cache.cleared is True
+
+
+def test_prestartup_uses_default_without_configuration(tmp_path: Path) -> None:
+    """The BackEnd hook does not require SugarSubstitute launch state."""
+
+    comfy_root = tmp_path / "ComfyUI"
+    folder_paths = cast(Any, ModuleType("folder_paths"))
+    folder_paths.models_dir = str(comfy_root / "models")
+    folder_paths.folder_names_and_paths = {}
+
+    assert apply_model_root(comfy_root, folder_paths) == (comfy_root / "models").resolve()
+
+
+def test_prestartup_migrates_legacy_desktop_state_and_hook(tmp_path: Path) -> None:
+    """BackEnd adopts the previous owner before removing its generated hook."""
+
+    comfy_root = tmp_path / "ComfyUI"
+    legacy_config = comfy_root / ".substitute" / "managed_model_root.json"
+    legacy_hook = comfy_root / "custom_nodes" / "SubstituteManagedModelRoot"
+    custom_root = tmp_path / "legacy-models"
+    legacy_config.parent.mkdir(parents=True)
+    legacy_config.write_text(
+        json.dumps({"schema_version": 1, "model_root": str(custom_root)}),
+        encoding="utf-8",
+    )
+    legacy_hook.mkdir(parents=True)
+    (legacy_hook / "prestartup_script.py").write_text("", encoding="utf-8")
+    folder_paths = cast(Any, ModuleType("folder_paths"))
+    folder_paths.models_dir = str(comfy_root / "models")
+    folder_paths.folder_names_and_paths = {}
+
+    assert apply_model_root(comfy_root, folder_paths) == custom_root.resolve()
+    assert not legacy_config.exists()
+    assert not legacy_hook.exists()
+    assert ModelRootStore(comfy_root).load() == custom_root.resolve()
+
+
+def test_prestartup_keeps_canonical_selection_over_legacy_state(tmp_path: Path) -> None:
+    """Canonical BackEnd state should win and remove a leftover desktop file."""
+
+    comfy_root = tmp_path / "ComfyUI"
+    canonical_root = tmp_path / "canonical-models"
+    ModelRootStore(comfy_root).save(canonical_root)
+    legacy_config = comfy_root / ".substitute" / "managed_model_root.json"
+    legacy_config.write_text("not valid anymore", encoding="utf-8")
+    folder_paths = cast(Any, ModuleType("folder_paths"))
+    folder_paths.models_dir = str(comfy_root / "models")
+    folder_paths.folder_names_and_paths = {}
+
+    assert apply_model_root(comfy_root, folder_paths) == canonical_root.resolve()
+    assert not legacy_config.exists()