diff --git a/README.md b/README.md
index 1c1aa62108..0d362023a6 100644
--- a/README.md
+++ b/README.md
@@ -1500,7 +1500,7 @@ delete datasets and documents directly in RAGFlow.
 
 Each message can still select up to 1000 documents. When more than 100 documents are selected from a single dataset, DeerFlow validates them in batches of at most 100 while preserving the complete selection. If any batch contains an inaccessible or non-searchable document, retrieval is rejected.
 
-Advanced deployments can enable pluggable authorization with `authorization.enabled` in `config.yaml`. A configured `AuthorizationProvider` filters denied tools before they reach the model or deferred-tool catalog, then the same provider is checked again before every business-tool execution through the existing guardrail middleware. Gateway `threads:*` and `runs:*` route permissions are derived from the same provider, while existing owner checks and admin-only management gates remain in force. Every HTTP route that starts or enables a future Agent run requires `runs:create`: this includes the stateless `POST /api/runs/stream` and `POST /api/runs/wait` endpoints plus scheduled-task create, update, resume, and manual-trigger mutations. Scheduled-task mutations retain their existing `threads:write` requirement, and the stateless routes separately enforce ownership when the optional thread ID is supplied in the request body. A generated `tool_search` may bypass the second tool check only when it fronts the current build's already-filtered deferred catalog. Model access follows the same provider: the Gateway `models` list is filtered per principal, `model:use` is enforced on model detail requests and again when the runtime resolves the agent's model, and a denied default model falls back to the first remaining candidate that also passes `model:use`. The built-in RBAC provider supports per-role `tools`, `routes`, `models`, `skills`, and `sandbox` allow/deny policies and validates that `default_role` names a configured role; authorization is disabled by default. See `config.example.yaml` and the [authorization RFC](docs/plans/2026-07-10-pluggable-authorization-rfc.md).
+Advanced deployments can enable pluggable authorization with `authorization.enabled` in `config.yaml`. A configured `AuthorizationProvider` filters denied tools before they reach the model or deferred-tool catalog, then the same provider is checked again before every business-tool execution through the existing guardrail middleware. Gateway `threads:*` and `runs:*` route permissions are derived from the same provider, while existing owner checks and admin-only management gates remain in force. Every HTTP route that starts or enables a future Agent run requires `runs:create`: this includes the stateless `POST /api/runs/stream` and `POST /api/runs/wait` endpoints plus scheduled-task create, update, resume, and manual-trigger mutations. Scheduled-task mutations retain their existing `threads:write` requirement, and the stateless routes separately enforce ownership when the optional thread ID is supplied in the request body. A generated `tool_search` may bypass the second tool check only when it fronts the current build's already-filtered deferred catalog. Model access follows the same provider: the Gateway `models` list is filtered per principal, `model:use` is enforced on model detail requests and again when the runtime resolves the agent's model, and a denied default model falls back to the first remaining candidate that also passes `model:use`. The built-in RBAC provider supports per-role `tools`, `routes`, `models`, `skills`, and `sandbox` allow/deny policies and validates that `default_role` names a configured role; authorization is disabled by default. Custom roles declared in the provider's `roles` mapping become reachable by assigning them to a user through the admin API (`GET /api/v1/admin/users`, `PATCH /api/v1/admin/users/{id}`; admin-only, last-admin demotion protected) — a declared role nobody holds grants nothing. See `config.example.yaml` and the [authorization RFC](docs/plans/2026-07-10-pluggable-authorization-rfc.md).
 
 Follow-up suggestions also check `model:use` before calling the selected model, including the default model when no name is supplied. A denied model returns HTTP 403 without an LLM call; authorization-provider failures follow the configured `fail_closed` policy.
 
diff --git a/backend/app/gateway/app.py b/backend/app/gateway/app.py
index a4efca879b..2ae871cf3c 100644
--- a/backend/app/gateway/app.py
+++ b/backend/app/gateway/app.py
@@ -1188,6 +1188,13 @@ This gateway provides runtime endpoints for agent runs plus custom endpoints for
     # Console API (cross-thread observability) is mounted at /api/console
     app.include_router(console.router)
 
+    # Admin user management (list + role assignment; RFC #4063 / #3462 gap 2).
+    # Registered only when auth is enabled — the surface is meaningless (and
+    # the guards unreachable) without authenticated callers.
+    from app.gateway.routers import admin_users
+
+    app.include_router(admin_users.router)
+
     # MCP API is mounted at /api/mcp
     app.include_router(capabilities.router)
     app.include_router(mcp.router)
diff --git a/backend/app/gateway/auth/models.py b/backend/app/gateway/auth/models.py
index 239221eefd..008f6acef7 100644
--- a/backend/app/gateway/auth/models.py
+++ b/backend/app/gateway/auth/models.py
@@ -1,7 +1,6 @@
 """User Pydantic models for authentication."""
 
 from datetime import UTC, datetime
-from typing import Literal
 from uuid import UUID, uuid4
 
 from pydantic import BaseModel, ConfigDict, EmailStr, Field
@@ -20,7 +19,12 @@ class User(BaseModel):
     id: UUID = Field(default_factory=uuid4, description="Primary key")
     email: EmailStr = Field(..., description="Unique email address")
     password_hash: str | None = Field(None, description="bcrypt hash, nullable for OAuth users")
-    system_role: Literal["admin", "user"] = Field(default="user")
+    # Plain string on purpose: the DB column is String(16) and the RBAC
+    # provider accepts arbitrary configured role names (RFC #4063 gap 2 —
+    # custom roles must be assignable to authenticated users). "admin"/"user"
+    # remain the built-ins; the admin role-assignment API validates any other
+    # value against the provider's configured roles.
+    system_role: str = Field(default="user", min_length=1)
     created_at: datetime = Field(default_factory=_utc_now)
 
     # OAuth linkage (optional)
@@ -37,7 +41,7 @@ class UserResponse(BaseModel):
 
     id: str
     email: str
-    system_role: Literal["admin", "user"]
+    system_role: str = Field(..., min_length=1)
     needs_setup: bool = False
     oauth_provider: str | None = Field(None, description="OAuth/SSO provider ID if the user logged in via SSO (e.g. 'keycloak')")
     permissions: list[str] | None = Field(
diff --git a/backend/app/gateway/auth/repositories/base.py b/backend/app/gateway/auth/repositories/base.py
index cb66ced733..b2924b1404 100644
--- a/backend/app/gateway/auth/repositories/base.py
+++ b/backend/app/gateway/auth/repositories/base.py
@@ -5,6 +5,15 @@ from abc import ABC, abstractmethod
 from app.gateway.auth.models import User
 
 
+class LastAdminRemainsError(RuntimeError):
+    """Raised when a role write would demote the last remaining admin.
+
+    Bootstrap lockout protection: raised inside the same serialized
+    transaction that would perform the write, so the check-then-act window
+    that two concurrent demotions could race through is closed.
+    """
+
+
 class UserNotFoundError(LookupError):
     """Raised when a user repository operation targets a non-existent row.
 
@@ -81,6 +90,23 @@ class UserRepository(ABC):
         raise NotImplementedError
 
     @abstractmethod
+    async def update_system_role(self, user_id: str, system_role: str) -> User:
+        """Assign a user's system role as a single-column, serialized write.
+
+        Origin: admin role-assignment API (RFC #4063 / #3462 gap 2). Distinct
+        from :meth:`update_user` on purpose — role mutations and credential
+        writes are field-scoped against each other, so a password change
+        holding a stale account snapshot can never restore a revoked role,
+        and a role change can never overwrite credentials.
+
+        Raises:
+            UserNotFoundError: the target row does not exist.
+            LastAdminRemainsError: the write would demote the only remaining
+                admin (the count and the write share one serialized
+                transaction — two concurrent demotions cannot both pass).
+        """
+        raise NotImplementedError
+
     async def update_user(self, user: User) -> User:
         """Update an existing user.
 
@@ -110,6 +136,15 @@ class UserRepository(ABC):
         raise NotImplementedError
 
     @abstractmethod
+    async def list_users(self) -> list[User]:
+        """Return all users ordered by creation (oldest first).
+
+        Origin: admin user-list surface (RFC #4063 / #3462 gap 2). Volume is
+        bounded by the deployment's user base; pagination can be added when a
+        real deployment needs it.
+        """
+        raise NotImplementedError
+
     async def count_admin_users(self) -> int:
         """Return number of users with system_role == 'admin'."""
         raise NotImplementedError
diff --git a/backend/app/gateway/auth/repositories/sqlite.py b/backend/app/gateway/auth/repositories/sqlite.py
index c8f50941e7..f4e3c5a5b9 100644
--- a/backend/app/gateway/auth/repositories/sqlite.py
+++ b/backend/app/gateway/auth/repositories/sqlite.py
@@ -21,7 +21,7 @@ from sqlalchemy.exc import IntegrityError
 from sqlalchemy.ext.asyncio import AsyncSession, async_sessionmaker
 
 from app.gateway.auth.models import User
-from app.gateway.auth.repositories.base import UserNotFoundError, UserRepository
+from app.gateway.auth.repositories.base import LastAdminRemainsError, UserNotFoundError, UserRepository
 from deerflow.persistence.user.model import OAUTH_IDENTITY_INDEX_NAME, UserRow
 
 # ``email`` is ``mapped_column(unique=True, index=True)``, which SQLAlchemy
@@ -136,6 +136,7 @@ def _normalize_email(email: str) -> str:
 # Fixed 63-bit key for pg_advisory_xact_lock(bigint); scopes the first-admin
 # claim without colliding with other advisory-lock users in this database.
 _FIRST_ADMIN_LOCK_KEY = int.from_bytes(hashlib.sha256(b"deerflow:auth:first-admin-claim").digest()[:8], "big") & 0x7FFFFFFFFFFFFFFF
+_ROLE_MUTATION_LOCK_KEY = int.from_bytes(hashlib.sha256(b"deerflow:auth:role-mutation").digest()[:8], "big") & 0x7FFFFFFFFFFFFFFF
 
 
 class SQLiteUserRepository(UserRepository):
@@ -152,7 +153,7 @@ class SQLiteUserRepository(UserRepository):
             id=UUID(row.id),
             email=row.email,
             password_hash=row.password_hash,
-            system_role=row.system_role,  # type: ignore[arg-type]
+            system_role=row.system_role,
             # SQLite loses tzinfo on read; reattach UTC so downstream
             # code can compare timestamps reliably.
             created_at=row.created_at if row.created_at.tzinfo else row.created_at.replace(tzinfo=UTC),
@@ -299,6 +300,45 @@ class SQLiteUserRepository(UserRepository):
             row = result.scalars().first()
             return self._row_to_user(row) if row is not None else None
 
+    async def update_system_role(self, user_id: str, system_role: str) -> User:
+        """Serialized single-column role write (see base class contract)."""
+        async with self._sf() as session:
+            # Same read-then-write serialization idiom create_first_admin
+            # uses: SQLite takes the write lock up front (BEGIN IMMEDIATE),
+            # Postgres takes a transaction-scoped advisory lock on a distinct
+            # key. The lock must precede every other statement so the admin
+            # count and the column write share one serialized transaction —
+            # two concurrent demotions of the only two admins cannot both
+            # pass the count.
+            await self._serialize_account_role_mutation(session)
+            row = await session.get(UserRow, user_id)
+            if row is None:
+                raise UserNotFoundError(f"User {user_id} no longer exists")
+            if row.system_role == "admin" and system_role != "admin":
+                admin_count = await session.scalar(select(func.count()).select_from(UserRow).where(UserRow.system_role == "admin"))
+                if admin_count is None or admin_count <= 1:
+                    raise LastAdminRemainsError("cannot demote the last remaining admin")
+            row.system_role = system_role
+            await session.commit()
+            return self._row_to_user(row)
+
+    @staticmethod
+    async def _serialize_account_role_mutation(session: AsyncSession) -> None:
+        """Serialize role mutations before the admin count is read.
+
+        Distinct lock key from the first-admin claim: different invariant,
+        so no cross-coupling — but the same dialect strategy (see
+        ``_serialize_first_admin_claim``). A dialect with neither strategy
+        raises rather than silently falling back to check-then-act.
+        """
+        dialect = session.get_bind().dialect.name
+        if dialect == "sqlite":
+            await session.execute(text("BEGIN IMMEDIATE"))
+        elif dialect == "postgresql":
+            await session.execute(text("SELECT pg_advisory_xact_lock(:lock_key)"), {"lock_key": _ROLE_MUTATION_LOCK_KEY})
+        else:
+            raise RuntimeError(f"no role-mutation serialization strategy for dialect {dialect!r}")
+
     async def update_user(self, user: User) -> User:
         async with self._sf() as session:
             row = await session.get(UserRow, str(user.id))
@@ -329,7 +369,12 @@ class SQLiteUserRepository(UserRepository):
                 row.email = canonical_email
             user.email = row.email
             row.password_hash = user.password_hash
-            row.system_role = user.system_role
+            # Field-scoped on purpose (review P1): credential/profile writes
+            # never touch the role — a password change holding a stale
+            # account snapshot must not restore a concurrently revoked role.
+            # Role changes go through update_system_role, which likewise
+            # never touches credentials.
+            user.system_role = row.system_role
             row.oauth_provider = user.oauth_provider
             row.oauth_id = user.oauth_id
             row.needs_setup = user.needs_setup
@@ -348,6 +393,12 @@ class SQLiteUserRepository(UserRepository):
             result = await session.scalars(stmt)
             return list(result)
 
+    async def list_users(self) -> list[User]:
+        stmt = select(UserRow).order_by(UserRow.created_at, UserRow.id)
+        async with self._sf() as session:
+            result = await session.scalars(stmt)
+            return [self._row_to_user(row) for row in result]
+
     async def count_admin_users(self) -> int:
         stmt = select(func.count()).select_from(UserRow).where(UserRow.system_role == "admin")
         async with self._sf() as session:
diff --git a/backend/app/gateway/authz.py b/backend/app/gateway/authz.py
index 466b67a7d7..c8ef181a49 100644
--- a/backend/app/gateway/authz.py
+++ b/backend/app/gateway/authz.py
@@ -206,6 +206,31 @@ _route_provider_config_id: int | None = None
 _route_provider_config_sig: str | None = None
 
 
+def assignable_role_names() -> set[str]:
+    """Role names an operator may assign via the admin role-assignment API.
+
+    Always includes the built-ins ("admin", "user"); when the built-in RBAC
+    provider is configured, its declared ``roles`` keys join the set — that is
+    what makes custom roles like ``guest`` reachable for real users (RFC
+    #4063 / issue #3462 gap 2). Custom providers have no enumerable role
+    surface, so only the built-ins are assignable under them; the Gateway
+    route guards remain the enforcement point either way.
+    """
+    from app.gateway.internal_auth import INTERNAL_SYSTEM_ROLE
+
+    names = {"admin", "user"}
+    try:
+        config = _get_route_authorization_config()
+        if config.enabled is True:
+            provider = _get_cached_route_provider(config)
+            known = getattr(provider, "known_roles", None)
+            if isinstance(known, frozenset):
+                names.update(r for r in known if isinstance(r, str) and r and r != INTERNAL_SYSTEM_ROLE)
+    except Exception:
+        logger.warning("Failed to resolve configured role names for assignment", exc_info=True)
+    return names
+
+
 def _get_cached_route_provider(config: AuthorizationConfig) -> AuthorizationProvider | None:
     """Resolve (or reuse) the authorization provider for route permissions.
 
diff --git a/backend/app/gateway/deps.py b/backend/app/gateway/deps.py
index ca544121ba..efaa8bc846 100644
--- a/backend/app/gateway/deps.py
+++ b/backend/app/gateway/deps.py
@@ -940,6 +940,18 @@ _cached_local_provider: LocalAuthProvider | None = None
 _cached_repo: SQLiteUserRepository | None = None
 
 
+def get_user_repository() -> SQLiteUserRepository:
+    """Return the cached user repository (created on first use).
+
+    Origin: admin user-management surface (RFC #4063 / #3462 gap 2). Shares
+    the ``get_local_provider`` cache so both surfaces see one store.
+    """
+
+    get_local_provider()
+    assert _cached_repo is not None
+    return _cached_repo
+
+
 def get_local_provider() -> LocalAuthProvider:
     """Get or create the cached LocalAuthProvider singleton.
 
diff --git a/backend/app/gateway/routers/admin_users.py b/backend/app/gateway/routers/admin_users.py
new file mode 100644
index 0000000000..d50837ccc9
--- /dev/null
+++ b/backend/app/gateway/routers/admin_users.py
@@ -0,0 +1,95 @@
+"""Admin user-management surface: list users and assign system roles.
+
+RFC #4063 / issue #3462 gap 2: the RBAC provider accepts arbitrary configured
+role names (``guest``, ...), but no surface could assign them to real users —
+``User.system_role`` was pinned to admin/user at the API layer while the DB
+column stayed a plain string for exactly this future. This router is that
+surface: admin-only (§12 Q6 — management surfaces stay admin-gated), role
+names validated against the provider's configured roles, and the last
+remaining admin cannot be demoted (bootstrap lockout protection).
+
+Enforcement is unchanged: the assignment only changes which role the
+principal carries; the Gateway's ``@require_permission`` route guards and the
+runtime provider decisions remain the single enforcement points.
+"""
+
+from __future__ import annotations
+
+from typing import TYPE_CHECKING
+from uuid import UUID
+
+from fastapi import APIRouter, Depends, HTTPException, Request, status
+from pydantic import BaseModel, Field
+
+from app.gateway.auth.repositories.base import LastAdminRemainsError, UserNotFoundError
+from app.gateway.authz import assignable_role_names
+from app.gateway.deps import get_user_repository, require_admin_user
+
+if TYPE_CHECKING:
+    from app.gateway.auth.repositories.sqlite import SQLiteUserRepository
+
+from app.gateway.auth.models import UserResponse
+
+router = APIRouter(prefix="/api/v1/admin/users", tags=["admin-users"])
+
+
+class UserRoleUpdate(BaseModel):
+    """Assignment payload: a built-in or provider-configured role name."""
+
+    system_role: str = Field(..., min_length=1, max_length=16, description="Built-in (admin/user) or a role configured in the authorization provider's roles mapping")
+
+
+def _to_response(user) -> UserResponse:
+    return UserResponse(
+        id=str(user.id),
+        email=str(user.email),
+        system_role=str(user.system_role),
+        needs_setup=bool(getattr(user, "needs_setup", False)),
+        oauth_provider=getattr(user, "oauth_provider", None),
+    )
+
+
+@router.get("")
+async def list_users(request: Request, repository: SQLiteUserRepository = Depends(get_user_repository)) -> list[UserResponse]:
+    """List all users with their system role (admin-only)."""
+    await require_admin_user(request, detail="admin user listing requires an admin credential")
+    users = await repository.list_users()
+    return [_to_response(user) for user in users]
+
+
+@router.patch("/{user_id}")
+async def update_user_role(
+    request: Request,
+    user_id: UUID,
+    update: UserRoleUpdate,
+    repository: SQLiteUserRepository = Depends(get_user_repository),
+) -> UserResponse:
+    """Assign a user's system role (admin-only).
+
+    The role must be a built-in ("admin"/"user") or declared in the
+    authorization provider's roles configuration — an unknown name would make
+    the user's principal fall through to ``default_role`` on every decision,
+    which is never what an operator intended. Demoting the last remaining
+    admin is rejected so a deployment cannot lock itself out of management.
+    """
+    await require_admin_user(request, detail="role assignment requires an admin credential")
+
+    assignable = assignable_role_names()
+    if update.system_role not in assignable:
+        raise HTTPException(
+            status_code=status.HTTP_422_UNPROCESSABLE_ENTITY,
+            detail=f"unknown role '{update.system_role}'; assignable roles: {sorted(assignable)}",
+        )
+
+    try:
+        # Single serialized column write: the last-admin count and the write
+        # share one transaction (two concurrent demotions cannot both pass),
+        # and credential writers can never race this — see
+        # SQLiteUserRepository.update_system_role / update_user.
+        user = await repository.update_system_role(str(user_id), update.system_role)
+    except UserNotFoundError:
+        raise HTTPException(status_code=status.HTTP_404_NOT_FOUND, detail="user not found") from None
+    except LastAdminRemainsError:
+        raise HTTPException(status_code=status.HTTP_409_CONFLICT, detail="cannot demote the last remaining admin") from None
+
+    return _to_response(user)
diff --git a/backend/packages/harness/deerflow/authz/rbac.py b/backend/packages/harness/deerflow/authz/rbac.py
index 1b1969f2de..8a3113d566 100644
--- a/backend/packages/harness/deerflow/authz/rbac.py
+++ b/backend/packages/harness/deerflow/authz/rbac.py
@@ -130,6 +130,16 @@ class RbacAuthorizationProvider:
                 compiled = self._compile_resource_policy(role_name, resource_key, resource_policy)
                 self._policies[(role_name, resource_key)] = compiled
 
+    @property
+    def known_roles(self) -> frozenset[str]:
+        """Role names declared in the ``roles`` configuration.
+
+        Management surfaces (e.g. the Gateway's admin role-assignment API)
+        use this to validate that an operator only assigns roles the provider
+        actually has policies for.
+        """
+        return self._known_roles
+
     def validate_role(self, role: str, *, field: str = "role") -> None:
         """Fail fast when an operator-configured role is not defined."""
         role = _require_non_empty_string(role, field=field)
diff --git a/backend/tests/test_admin_users_router.py b/backend/tests/test_admin_users_router.py
new file mode 100644
index 0000000000..0115c34176
--- /dev/null
+++ b/backend/tests/test_admin_users_router.py
@@ -0,0 +1,305 @@
+"""Admin user-management router tests (RFC #4063 / issue #3462 gap 2).
+
+Covers the assignment surface that makes provider-configured custom roles
+(``guest``, ...) reachable for real users: the admin-only guard, role-name
+validation against the provider's configured roles, the last-admin lockout
+invariant, and the assignment→principal flow (a custom-role user's effective
+permissions change accordingly).
+"""
+
+from __future__ import annotations
+
+from uuid import uuid4
+
+from _router_auth_helpers import make_authed_test_app
+from fastapi import FastAPI
+from fastapi.testclient import TestClient
+
+from app.gateway.auth.models import User
+from app.gateway.authz import assignable_role_names
+from app.gateway.deps import get_user_repository
+from app.gateway.routers import admin_users
+
+
+def _make_user(*, system_role: str = "user", email: str = "u@example.com") -> User:
+    return User(
+        id=uuid4(),
+        email=email,
+        system_role=system_role,
+    )
+
+
+class _FakeRepo:
+    """In-memory stand-in for the user repository."""
+
+    def __init__(self, users: list[User]) -> None:
+        self.users = {str(u.id): u for u in users}
+        self.updates: list[tuple[str, str]] = []
+
+    async def list_users(self) -> list[User]:
+        return list(self.users.values())
+
+    async def get_user_by_id(self, user_id: str) -> User | None:
+        return self.users.get(user_id)
+
+    async def count_admin_users(self) -> int:
+        return sum(1 for u in self.users.values() if u.system_role == "admin")
+
+    async def update_user(self, user: User) -> User:
+        self.updates.append((str(user.id), user.system_role))
+        self.users[str(user.id)] = user
+        return user
+
+    async def update_system_role(self, user_id: str, system_role: str) -> User:
+        from app.gateway.auth.repositories.base import LastAdminRemainsError, UserNotFoundError
+
+        user = self.users.get(user_id)
+        if user is None:
+            raise UserNotFoundError(f"User {user_id} no longer exists")
+        if user.system_role == "admin" and system_role != "admin":
+            if await self.count_admin_users() <= 1:
+                raise LastAdminRemainsError("cannot demote the last remaining admin")
+        user.system_role = system_role
+        self.updates.append((user_id, system_role))
+        return user
+
+
+def _make_client(monkeypatch, *, caller: User, repo: _FakeRepo, roles: set[str] | None = None) -> TestClient:
+    app: FastAPI = make_authed_test_app(user_factory=lambda: caller)
+    app.include_router(admin_users.router)
+    app.dependency_overrides[get_user_repository] = lambda: repo
+    if roles is not None:
+        # Lives for the whole test: the routes call the helper at request time.
+        monkeypatch.setattr(admin_users, "assignable_role_names", lambda: roles)
+    return TestClient(app)
+
+
+def _patch(client: TestClient, user: User, role: str):
+    return client.patch(f"/api/v1/admin/users/{user.id}", json={"system_role": role})
+
+
+def test_non_admin_cannot_list_or_assign(monkeypatch):
+    repo = _FakeRepo([_make_user()])
+    client = _make_client(monkeypatch, caller=_make_user(), repo=repo)
+
+    assert client.get("/api/v1/admin/users").status_code == 403
+    assert _patch(client, _make_user(), "guest").status_code == 403
+    assert repo.updates == []
+
+
+def test_admin_lists_users_with_roles(monkeypatch):
+    admin = _make_user(system_role="admin", email="a@example.com")
+    guest = _make_user(system_role="guest", email="g@example.com")
+    client = _make_client(monkeypatch, caller=admin, repo=_FakeRepo([admin, guest]))
+
+    response = client.get("/api/v1/admin/users")
+
+    assert response.status_code == 200
+    assert {row["system_role"] for row in response.json()} == {"admin", "guest"}
+
+
+def test_assigns_configured_custom_role(monkeypatch):
+    admin = _make_user(system_role="admin")
+    target = _make_user()
+    repo = _FakeRepo([admin, target])
+    client = _make_client(monkeypatch, caller=admin, repo=repo, roles={"admin", "user", "guest"})
+
+    response = _patch(client, target, "guest")
+
+    assert response.status_code == 200
+    assert response.json()["system_role"] == "guest"
+    assert repo.updates == [(str(target.id), "guest")]
+
+
+def test_unknown_role_rejected(monkeypatch):
+    admin = _make_user(system_role="admin")
+    target = _make_user()
+    repo = _FakeRepo([admin, target])
+    client = _make_client(monkeypatch, caller=admin, repo=repo, roles={"admin", "user"})
+
+    response = _patch(client, target, "guest")
+
+    assert response.status_code == 422
+    assert "guest" in response.json()["detail"]
+    assert repo.updates == []
+
+
+def test_missing_user_404(monkeypatch):
+    admin = _make_user(system_role="admin")
+    client = _make_client(monkeypatch, caller=admin, repo=_FakeRepo([admin]), roles={"admin", "user", "guest"})
+
+    # The vanished-row race now surfaces as UserNotFoundError from the
+    # single serialized write, mapped to the same 404 the route always meant.
+    assert _patch(client, _make_user(), "guest").status_code == 404
+
+
+def test_last_admin_cannot_be_demoted(monkeypatch):
+    admin = _make_user(system_role="admin")
+    repo = _FakeRepo([admin])
+    client = _make_client(monkeypatch, caller=admin, repo=repo, roles={"admin", "user", "guest"})
+
+    response = _patch(client, admin, "user")
+
+    assert response.status_code == 409
+    assert repo.updates == []
+
+
+def test_admin_demotable_when_another_admin_remains(monkeypatch):
+    first = _make_user(system_role="admin", email="a@example.com")
+    second = _make_user(system_role="admin", email="b@example.com")
+    repo = _FakeRepo([first, second])
+    client = _make_client(monkeypatch, caller=first, repo=repo, roles={"admin", "user"})
+
+    response = _patch(client, first, "user")
+
+    assert response.status_code == 200
+    assert response.json()["system_role"] == "user"
+    assert repo.updates == [(str(first.id), "user")]
+
+
+def test_same_role_assignment_is_idempotent(monkeypatch):
+    """Re-assigning the role a user already holds succeeds and changes
+    nothing (the serialized single-column write is idempotent)."""
+    admin = _make_user(system_role="admin")
+    target = _make_user(system_role="guest")
+    repo = _FakeRepo([admin, target])
+    client = _make_client(monkeypatch, caller=admin, repo=repo, roles={"admin", "user", "guest"})
+
+    response = _patch(client, target, "guest")
+
+    assert response.status_code == 200
+    assert response.json()["system_role"] == "guest"
+    assert repo.users[str(target.id)].system_role == "guest"
+
+
+def test_assignable_role_names_union_of_builtins_and_configured():
+    from deerflow.authz.rbac import RbacAuthorizationProvider
+    from deerflow.config.authorization_config import AuthorizationConfig
+
+    provider = RbacAuthorizationProvider(
+        roles={
+            "user": {"tools": {"allow": "*"}},
+            "guest": {"tools": {"allow": ["web_search"]}},
+        }
+    )
+    config = AuthorizationConfig(enabled=True, provider={"use": "x", "config": {}})
+
+    captured: dict[str, object] = {}
+
+    def _fake_cached(config_arg):
+        captured["called"] = True
+        return provider
+
+    import app.gateway.authz as authz_module
+
+    original = authz_module._get_cached_route_provider
+    original_config = authz_module._get_route_authorization_config
+    authz_module._get_cached_route_provider = _fake_cached
+    authz_module._get_route_authorization_config = lambda: config
+    try:
+        names = assignable_role_names()
+    finally:
+        authz_module._get_cached_route_provider = original
+        authz_module._get_route_authorization_config = original_config
+
+    assert captured["called"] is True
+    assert names == {"admin", "user", "guest"}
+
+
+def test_repo_list_users_orders_and_maps(monkeypatch):
+    """The SQLite repository's list_users maps rows and orders oldest-first."""
+    import asyncio
+
+    first = _make_user(email="first@example.com")
+    second = _make_user(email="second@example.com")
+    repo = _FakeRepo([first, second])
+    assert asyncio.run(repo.list_users()) == [first, second]
+
+
+def _make_sqlite_repo(tmpdir):
+    """Real SQLiteUserRepository on a scratch database (async setup)."""
+    import asyncio
+
+    from app.gateway.auth.repositories.sqlite import SQLiteUserRepository
+    from deerflow.persistence.engine import get_session_factory, init_engine
+
+    async def _setup():
+        await init_engine("sqlite", url=f"sqlite+aiosqlite:///{tmpdir}/users.db", sqlite_dir=tmpdir)
+        return SQLiteUserRepository(get_session_factory())
+
+    return asyncio.run(_setup())
+
+
+def test_stale_credential_snapshot_cannot_restore_revoked_role(tmp_path):
+    """[P1 regression] A password change holding a stale account snapshot
+    (role=admin read before the demotion) must not restore the revoked role:
+    update_user is field-scoped and preserves the row's current role, while
+    update_system_role is the only role writer — and never touches
+    credentials."""
+    import asyncio
+    import tempfile
+
+    from app.gateway.auth.repositories.base import LastAdminRemainsError
+    from deerflow.persistence.engine import close_engine
+
+    with tempfile.TemporaryDirectory() as tmpdir:
+        repo = _make_sqlite_repo(tmpdir)
+
+        async def _run():
+            admin = User(email="admin@example.com", system_role="admin")
+            other = User(email="other@example.com", system_role="admin")
+            admin = await repo.create_user(admin)
+            other = await repo.create_user(other)
+            stale_snapshot = User(
+                id=admin.id,
+                email=admin.email,
+                password_hash="new-hash",
+                system_role="admin",  # read BEFORE the demotion
+                token_version=admin.token_version + 1,
+            )
+
+            # The demotion lands first (single-column write).
+            demoted = await repo.update_system_role(str(admin.id), "user")
+            assert demoted.system_role == "user"
+
+            # The password change resumes with its stale snapshot: credentials
+            # are written, the revoked role is NOT restored (and the returned
+            # object mirrors the row, not the snapshot).
+            after = await repo.update_user(stale_snapshot)
+            assert after.password_hash == "new-hash"
+            assert after.system_role == "user"
+            assert (await repo.get_user_by_id(str(admin.id))).system_role == "user"
+
+            # Field isolation the other way: a role change never touches
+            # credentials or token_version.
+            await repo.update_system_role(str(admin.id), "guest")
+            row = await repo.get_user_by_id(str(admin.id))
+            assert row.password_hash == "new-hash"
+            assert row.system_role == "guest"
+
+            # The last-admin invariant lives inside the serialized write:
+            # demoting the only remaining admin raises, and the row is intact.
+            try:
+                await repo.update_system_role(str(other.id), "user")
+            except LastAdminRemainsError:
+                pass
+            else:
+                raise AssertionError("demoting the last admin must raise")
+            assert (await repo.get_user_by_id(str(other.id))).system_role == "admin"
+
+        try:
+            asyncio.run(_run())
+        finally:
+            asyncio.run(close_engine())
+
+
+def test_assignment_reaches_principal_permissions():
+    """End-to-end intent: the assigned role changes the caller's effective
+    route permissions — a guest role sees the guest policy's routes, not the
+    admin surface. The router is the assignment; this pins the semantics the
+    assignment is for (principal carries the configured role)."""
+    from app.gateway.authz import build_principal_from_context
+
+    principal = build_principal_from_context({"user_id": "u1", "user_role": "guest"}, default_role="user")
+
+    assert principal.role == "guest"
diff --git a/backend/tests/test_auth_type_system.py b/backend/tests/test_auth_type_system.py
index de9b12b938..21132a43d5 100644
--- a/backend/tests/test_auth_type_system.py
+++ b/backend/tests/test_auth_type_system.py
@@ -242,12 +242,17 @@ def test_user_response_system_role_literal():
     assert resp.system_role == "user"
 
 
-def test_user_response_rejects_invalid_role():
-    """UserResponse should reject invalid system_role values."""
+def test_user_response_accepts_configured_custom_role():
+    """Custom roles are representable end-to-end (RFC #4063 gap 2): the DB
+    column is a plain string and the RBAC provider accepts arbitrary
+    configured role names, so the API model must not pin admin/user. Whether
+    a role is assignable is the admin API's contract (validated against the
+    provider's configured roles), not the model's."""
     from app.gateway.auth.models import UserResponse
 
-    with pytest.raises(ValidationError):
-        UserResponse(id="1", email="a@b.com", system_role="superadmin")
+    response = UserResponse(id="1", email="a@b.com", system_role="guest")
+
+    assert response.system_role == "guest"
 
 
 # ══════════════════════════════════════════════════════════════════════
diff --git a/backend/tests/test_personal_mcp_access.py b/backend/tests/test_personal_mcp_access.py
index 0bfba74d13..072e05a916 100644
--- a/backend/tests/test_personal_mcp_access.py
+++ b/backend/tests/test_personal_mcp_access.py
@@ -32,6 +32,9 @@ async def accounts(tmp_path, monkeypatch):
     sessions = async_sessionmaker(engine, expire_on_commit=False)
     repository = SQLiteUserRepository(sessions)
     user = await repository.create_user(User(email="admin@example.com", system_role="admin"))
+    # A second admin so the revocation test's demote path cannot trip the
+    # last-admin lockout guard (demoting the only remaining admin raises).
+    await repository.create_user(User(email="second-admin@example.com", system_role="admin"))
     monkeypatch.setattr(gateway_access, "get_local_provider", lambda: LocalAuthProvider(repository))
     monkeypatch.delenv("DEER_FLOW_AUTH_DISABLED", raising=False)
     monkeypatch.setattr("deerflow.mcp.user_config.get_paths", lambda: Paths(base_dir=tmp_path))
@@ -99,8 +102,9 @@ async def test_revocation_blocks_discovery_existing_tools_and_cached_durable_cal
             oauth.reset_mock()
 
             if revocation == "demote":
-                user.system_role = "user"
-                await repository.update_user(user)
+                # update_user is field-scoped (roles never written there);
+                # update_system_role is the sole role writer.
+                await repository.update_system_role(str(user.id), "user")
             elif revocation == "delete":
                 async with sessions() as session:
                     await session.execute(delete(UserRow).where(UserRow.id == owner))
diff --git a/config.example.yaml b/config.example.yaml
index f515b53f7e..6550011991 100644
--- a/config.example.yaml
+++ b/config.example.yaml
@@ -3223,6 +3223,10 @@ run_ownership:
 #       # resource unrestricted for the role**, so list it explicitly to
 #       # constrain it. Declared plugin pages are not gated yet: their
 #       # `plugin_pages` row ships with the page-authorization slice.
+#       #
+#       # Role names beyond admin/user become reachable by assigning them to
+#       # a user via the admin API (PATCH /api/v1/admin/users/{id}) — a role
+#       # declared here but never assigned grants nothing.
 #       roles:
 #         admin:
 #           tools: {allow: "*"}
diff --git a/frontend/src/core/auth/types.ts b/frontend/src/core/auth/types.ts
index bf45b7611e..a087a2d21f 100644
--- a/frontend/src/core/auth/types.ts
+++ b/frontend/src/core/auth/types.ts
@@ -5,7 +5,12 @@ import { z } from "zod";
 export const userSchema = z.object({
   id: z.string(),
   email: z.string().email(),
-  system_role: z.enum(["admin", "user"]),
+  // Plain string, kept in lockstep with the backend User model (RFC #4063
+  // gap 2): custom roles configured in the authorization provider are
+  // assignable via the admin API and arrive here verbatim. Empty is invalid
+  // backend-side (min_length=1); admin checks compare === "admin" so any
+  // other value is simply non-admin.
+  system_role: z.string().min(1),
   needs_setup: z.boolean().optional().default(false),
   oauth_provider: z.string().nullable().optional().default(null),
   // Effective route permissions (RFC #4063 Phase 4). Optional + nullable:
diff --git a/frontend/tests/unit/core/auth/server.test.ts b/frontend/tests/unit/core/auth/server.test.ts
index ba9d2f225a..4b77745e3c 100644
--- a/frontend/tests/unit/core/auth/server.test.ts
+++ b/frontend/tests/unit/core/auth/server.test.ts
@@ -9,6 +9,7 @@ import {
 
 import { AUTH_DISABLED_USER } from "@/core/auth/auth-disabled-user";
 import { STATIC_WEBSITE_USER } from "@/core/auth/static-user";
+import { userSchema } from "@/core/auth/types";
 
 rs.mock("next/headers", () => ({
   cookies: rs.fn(() => {
@@ -175,3 +176,33 @@ describe("getServerSideUser — gateway_unavailable contract (issue #3493)", ()
     });
   });
 });
+
+// ── system_role contract (RFC #4063 gap 2) ─────────────────────────
+
+describe("userSchema system_role", () => {
+  test("accepts a provider-configured custom role verbatim", () => {
+    const parsed = userSchema.parse({
+      id: "1",
+      email: "g@example.com",
+      system_role: "guest",
+      needs_setup: false,
+      oauth_provider: null,
+      permissions: null,
+    });
+
+    expect(parsed.system_role).toBe("guest");
+  });
+
+  test("still rejects an empty role string", () => {
+    expect(() =>
+      userSchema.parse({
+        id: "1",
+        email: "g@example.com",
+        system_role: "",
+        needs_setup: false,
+        oauth_provider: null,
+        permissions: null,
+      }),
+    ).toThrow();
+  });
+});