diff --git a/core/auth.py b/core/auth.py
index 4bc9a70dd..66fb6b753 100644
--- a/core/auth.py
+++ b/core/auth.py
@@ -20,7 +20,6 @@ logger = logging.getLogger(__name__)
 
 
 from core.atomic_io import atomic_write_json as _atomic_write_json  # noqa: E402
-from core.middleware import INTERNAL_TOOL_USER  # noqa: E402
 
 DEFAULT_PRIVILEGES = {
     "can_use_agent": True,
@@ -49,24 +48,18 @@ ADMIN_PRIVILEGES["allowed_models_restricted"] = False
 ADMIN_PRIVILEGES["block_all_models"] = False
 
 from src.constants import AUTH_FILE, PASSWORD_MIN_LENGTH
+from src.owner_identity import RESERVED_AUTH_USERNAMES
 DEFAULT_AUTH_PATH = AUTH_FILE
 TOKEN_TTL = 60 * 60 * 24 * 7  # 7 days
 
-# Usernames the auth + middleware layer reserve as internal "synthetic owner"
-# sentinels; they must never belong to a real account. The most dangerous is
-# "internal-tool": `core.middleware.require_admin` treats any request whose
-# `current_user == "internal-tool"` as the in-process tool loopback and grants
-# admin, and because the cookie auth path sets `current_user` to the raw
-# username, an account literally named "internal-tool" would be silently
-# treated as an admin by every `require_admin`-gated route. "api" collides with
-# the bearer-token owner-attribution sentinel. "demo"/"system" round out the
-# synthetic-owner set the rest of the codebase already special-cases (see
-# `_SYNTHETIC_OWNERS` in routes/assistant_routes.py and the matching guards in
-# src/task_scheduler.py / routes/research_routes.py) — a real account with one
-# of those names would be denied an assistant and inconsistently owner-scoped.
-# Refuse to create or rename into any of them so the sentinels can't be
-# impersonated. (Keep this in sync with that synthetic-owner set.)
-RESERVED_USERNAMES = frozenset({INTERNAL_TOOL_USER, "api", "demo", "system"})
+# Usernames the auth + middleware layer reserves for request sentinels and
+# internal storage owners; they must never belong to a real login account.
+# "internal-tool" is the most dangerous because `core.middleware.require_admin`
+# treats it as the in-process tool loopback. "api" collides with bearer-token
+# attribution. "demo"/"system" are synthetic owners already special-cased by
+# scheduler/assistant/research paths. The Default/Local owner is a storage
+# bucket for explicit auth-disabled no-login mode, not a login username.
+RESERVED_USERNAMES = frozenset(RESERVED_AUTH_USERNAMES)
 
 
 def normalize_known_username(users: Dict[str, Any], username: str | None) -> Optional[str]:
diff --git a/core/middleware.py b/core/middleware.py
index 0e164e35a..51b1f97ca 100644
--- a/core/middleware.py
+++ b/core/middleware.py
@@ -8,6 +8,8 @@ from fastapi import HTTPException, Request
 from starlette.middleware.base import BaseHTTPMiddleware
 from starlette.responses import Response
 
+from src.owner_identity import INTERNAL_TOOL_USER, auth_disabled
+
 
 # Per-process token that lets the in-app tool layer hit admin-gated
 # routes via HTTP loopback (the agent's tool calls don't carry the
@@ -15,8 +17,6 @@ from starlette.responses import Response
 # same value from this module. Never persisted or exposed externally.
 INTERNAL_TOOL_TOKEN = os.environ.get("ODYSSEUS_INTERNAL_TOKEN") or secrets.token_hex(32)
 INTERNAL_TOOL_HEADER = "X-Odysseus-Internal-Token"
-# Pseudo-username on in-process tool-loopback requests; require_admin trusts it and it is reserved.
-INTERNAL_TOOL_USER = "internal-tool"
 
 
 def is_cors_preflight(method: str, headers) -> bool:
@@ -47,7 +47,7 @@ def require_admin(request: Request):
         pass
 
     auth_mgr = getattr(request.app.state, "auth_manager", None)
-    if os.getenv("AUTH_ENABLED", "true").lower() == "false":
+    if auth_disabled():
         return
     if not auth_mgr or not auth_mgr.is_configured:
         raise HTTPException(403, "Admin only")
diff --git a/src/auth_helpers.py b/src/auth_helpers.py
index 49f3f01be..d290396c2 100644
--- a/src/auth_helpers.py
+++ b/src/auth_helpers.py
@@ -4,6 +4,8 @@ import os
 from typing import Optional
 from fastapi import Request, HTTPException
 
+from src.owner_identity import auth_disabled, effective_storage_owner
+
 
 def get_current_user(request: Request) -> Optional[str]:
     """Get current username from request state (set by auth middleware)."""
@@ -56,7 +58,17 @@ def _auth_disabled() -> bool:
     """True when the operator has explicitly turned off auth via .env.
     Mirrors the AUTH_ENABLED parse in app.py / core/middleware.py so the
     three call sites agree on what "off" means."""
-    return os.getenv("AUTH_ENABLED", "true").lower() == "false"
+    return auth_disabled()
+
+
+def storage_owner_for_request(request: Request) -> Optional[str]:
+    """Resolve the storage owner for code paths that need an owner bucket.
+
+    This does not replace route authentication. It only gives auth-disabled
+    no-login mode a stable storage identity instead of writing new data as
+    legacy NULL/ownerless state.
+    """
+    return effective_storage_owner(effective_user(request))
 
 
 def require_user(request: Request) -> str:
diff --git a/src/owner_identity.py b/src/owner_identity.py
new file mode 100644
index 000000000..3eec83e42
--- /dev/null
+++ b/src/owner_identity.py
@@ -0,0 +1,56 @@
+"""Shared owner identity constants and helpers."""
+
+from __future__ import annotations
+
+import os
+from typing import Optional
+
+
+DEFAULT_LOCAL_OWNER = "__odysseus_local__"
+DEFAULT_LOCAL_OWNER_LABEL = "Local"
+INTERNAL_TOOL_USER = "internal-tool"
+
+REQUEST_SENTINEL_OWNERS = frozenset({INTERNAL_TOOL_USER, "api", "demo", "system"})
+RESERVED_AUTH_USERNAMES = REQUEST_SENTINEL_OWNERS | {DEFAULT_LOCAL_OWNER}
+
+
+def auth_disabled() -> bool:
+    """Return True only when auth is explicitly disabled by configuration."""
+    return os.getenv("AUTH_ENABLED", "true").strip().lower() == "false"
+
+
+def normalize_owner(owner: str | None) -> Optional[str]:
+    """Normalize an owner-like value without inventing a fallback identity."""
+    value = str(owner or "").strip()
+    return value or None
+
+
+def owner_key(owner: str | None) -> Optional[str]:
+    normalized = normalize_owner(owner)
+    return normalized.lower() if normalized else None
+
+
+def is_request_sentinel_owner(owner: str | None) -> bool:
+    return owner_key(owner) in REQUEST_SENTINEL_OWNERS
+
+
+def effective_storage_owner(owner: str | None, *, auth_is_disabled: bool | None = None) -> Optional[str]:
+    """Resolve the owner used for storage writes that need a real bucket.
+
+    ``None`` still means no authenticated owner when auth is enabled. In the
+    explicit no-login mode, it resolves to the reserved local owner instead of
+    conflating local-operator writes with legacy NULL/ownerless rows.
+    """
+    normalized = normalize_owner(owner)
+    if normalized:
+        if is_request_sentinel_owner(normalized):
+            return None
+        return normalized
+    disabled = auth_disabled() if auth_is_disabled is None else auth_is_disabled
+    if disabled:
+        return DEFAULT_LOCAL_OWNER
+    return None
+
+
+def is_default_local_owner(owner: str | None) -> bool:
+    return owner_key(owner) == DEFAULT_LOCAL_OWNER
diff --git a/tests/test_owner_identity.py b/tests/test_owner_identity.py
new file mode 100644
index 000000000..1f3d159ef
--- /dev/null
+++ b/tests/test_owner_identity.py
@@ -0,0 +1,63 @@
+from types import SimpleNamespace
+
+import pytest
+
+
+def test_effective_storage_owner_matrix(monkeypatch):
+    from src.owner_identity import DEFAULT_LOCAL_OWNER, effective_storage_owner
+
+    monkeypatch.delenv("AUTH_ENABLED", raising=False)
+    assert effective_storage_owner(None) is None
+    assert effective_storage_owner("") is None
+    assert effective_storage_owner("alice") == "alice"
+    for sentinel in ("api", "demo", "system", "internal-tool"):
+        assert effective_storage_owner(sentinel) is None
+        assert effective_storage_owner(f" {sentinel.upper()} ") is None
+
+    monkeypatch.setenv("AUTH_ENABLED", "false")
+    assert effective_storage_owner(None) == DEFAULT_LOCAL_OWNER
+    assert effective_storage_owner("") == DEFAULT_LOCAL_OWNER
+    assert effective_storage_owner("admin") == "admin"
+    for sentinel in ("api", "demo", "system", "internal-tool"):
+        assert effective_storage_owner(sentinel) is None
+
+
+def test_storage_owner_for_request_uses_api_token_owner(monkeypatch):
+    from src.auth_helpers import storage_owner_for_request
+
+    monkeypatch.delenv("AUTH_ENABLED", raising=False)
+    request = SimpleNamespace(
+        state=SimpleNamespace(
+            current_user="api",
+            api_token=True,
+            api_token_owner="alice",
+        )
+    )
+
+    assert storage_owner_for_request(request) == "alice"
+
+
+@pytest.mark.parametrize("sentinel", ["api", "demo", "system", "internal-tool"])
+def test_storage_owner_for_request_rejects_request_sentinel(monkeypatch, sentinel):
+    from src.auth_helpers import storage_owner_for_request
+
+    monkeypatch.delenv("AUTH_ENABLED", raising=False)
+    request = SimpleNamespace(
+        state=SimpleNamespace(
+            current_user=sentinel,
+            api_token=sentinel == "api",
+            api_token_owner=None,
+        )
+    )
+
+    assert storage_owner_for_request(request) is None
+
+
+def test_storage_owner_for_request_uses_default_local_when_auth_disabled(monkeypatch):
+    from src.auth_helpers import storage_owner_for_request
+    from src.owner_identity import DEFAULT_LOCAL_OWNER
+
+    monkeypatch.setenv("AUTH_ENABLED", "false")
+    request = SimpleNamespace(state=SimpleNamespace(current_user=None))
+
+    assert storage_owner_for_request(request) == DEFAULT_LOCAL_OWNER
diff --git a/tests/test_reserved_username_admin_escalation.py b/tests/test_reserved_username_admin_escalation.py
index eab5e4539..d70062581 100644
--- a/tests/test_reserved_username_admin_escalation.py
+++ b/tests/test_reserved_username_admin_escalation.py
@@ -16,8 +16,11 @@ from types import SimpleNamespace
 import pytest
 from fastapi import HTTPException
 
+from src.owner_identity import DEFAULT_LOCAL_OWNER
 from tests.helpers.import_state import clear_module
 
+_RESERVED_NAMES = ["internal-tool", "api", "demo", "system", DEFAULT_LOCAL_OWNER]
+
 
 def _fresh_auth_manager(tmp_path):
     # Same import dance as test_security_regressions: drop any cached stub so
@@ -30,7 +33,17 @@ def _fresh_auth_manager(tmp_path):
 
 @pytest.mark.parametrize(
     "name",
-    ["internal-tool", "api", "demo", "system", "INTERNAL-TOOL", " Internal-Tool ", "Api", "SYSTEM"],
+    [
+        "internal-tool",
+        "api",
+        "demo",
+        "system",
+        DEFAULT_LOCAL_OWNER,
+        "INTERNAL-TOOL",
+        " Internal-Tool ",
+        "Api",
+        "SYSTEM",
+    ],
 )
 def test_create_user_rejects_reserved_usernames(tmp_path, name):
     mgr = _fresh_auth_manager(tmp_path)
@@ -45,34 +58,37 @@ def test_create_user_rejects_empty_username(tmp_path):
     assert "" not in mgr.users
 
 
-def test_setup_rejects_reserved_admin_username(tmp_path):
+@pytest.mark.parametrize("name", _RESERVED_NAMES)
+def test_setup_rejects_reserved_admin_username(tmp_path, name):
     mgr = _fresh_auth_manager(tmp_path)
     # First-run admin setup funnels through create_user, so it's covered too.
-    assert mgr.setup("internal-tool", "pw-123456") is False
+    assert mgr.setup(name, "pw-123456") is False
     assert mgr.is_configured is False
 
 
-def test_rename_into_reserved_username_is_blocked(tmp_path):
+@pytest.mark.parametrize("name", _RESERVED_NAMES)
+def test_rename_into_reserved_username_is_blocked(tmp_path, name):
     mgr = _fresh_auth_manager(tmp_path)
     assert mgr.create_user("admin", "pw-123456", is_admin=True) is True
     assert mgr.create_user("bob", "pw-123456") is True
-    assert mgr.rename_user("bob", "internal-tool", "admin") is False
-    assert "internal-tool" not in mgr.users
+    assert mgr.rename_user("bob", name, "admin") is False
+    assert name not in mgr.users
     assert "bob" in mgr.users
 
 
-def test_legacy_reserved_username_is_removed_on_load(tmp_path):
+@pytest.mark.parametrize("name", _RESERVED_NAMES)
+def test_legacy_reserved_username_is_removed_on_load(tmp_path, name):
     auth_path = tmp_path / "auth.json"
     auth_path.write_text(
-        '{"users": {"internal-tool": {"password_hash": "unused", "is_admin": false}, '
-        '"admin": {"password_hash": "unused", "is_admin": true}}}',
+        '{"users": {"%s": {"password_hash": "unused", "is_admin": false}, '
+        '"admin": {"password_hash": "unused", "is_admin": true}}}' % name,
         encoding="utf-8",
     )
     mgr = _fresh_auth_manager(tmp_path)
 
-    assert "internal-tool" not in mgr.users
+    assert name not in mgr.users
     assert "admin" in mgr.users
-    assert "internal-tool" not in auth_path.read_text(encoding="utf-8")
+    assert name not in auth_path.read_text(encoding="utf-8")
 
 
 def test_legacy_reserved_username_session_cannot_authenticate(tmp_path):
@@ -121,15 +137,16 @@ def test_legacy_reserved_username_session_cannot_pass_admin_gate(tmp_path, monke
     assert exc.value.status_code == 403
 
 
-def test_legacy_reserved_single_user_migrates_to_admin(tmp_path):
+@pytest.mark.parametrize("name", _RESERVED_NAMES)
+def test_legacy_reserved_single_user_migrates_to_admin(tmp_path, name):
     auth_path = tmp_path / "auth.json"
     auth_path.write_text(
-        '{"username": "internal-tool", "password_hash": "unused"}',
+        '{"username": "%s", "password_hash": "unused"}' % name,
         encoding="utf-8",
     )
     mgr = _fresh_auth_manager(tmp_path)
 
-    assert "internal-tool" not in mgr.users
+    assert name not in mgr.users
     assert "admin" in mgr.users
     assert mgr.is_admin("admin") is True
 
@@ -141,8 +158,8 @@ def test_token_cache_owner_normalization_requires_current_user():
     users = {"alice": {}, "admin": {}}
 
     assert normalize_known_username(users, " Alice ") == "alice"
-    assert normalize_known_username(users, "internal-tool") is None
-    assert normalize_known_username(users, "api") is None
+    for name in _RESERVED_NAMES:
+        assert normalize_known_username(users, name) is None
     assert normalize_known_username(users, "") is None