diff --git a/backend/app/core/config.py b/backend/app/core/config.py
index 5f215cda..96a364e3 100644
--- a/backend/app/core/config.py
+++ b/backend/app/core/config.py
@@ -211,6 +211,10 @@ class Settings(BaseSettings):
         # concrete rendered script so a draft_template can be proposed.
         # Creates a persistent library artifact on accept, so Sonnet.
         "template_extraction": "standard",
+        # L1 AI tree builder (Phase 2A): per-node generation is latency-sensitive
+        # on a live call → Sonnet; classification is a short label task → Haiku.
+        "l1_realtime_build": "standard",
+        "l1_classify": "fast",
     }
 
     def get_model_for_action(self, action_type: str) -> str:
diff --git a/backend/app/models/account.py b/backend/app/models/account.py
index 6a081215..361a3818 100644
--- a/backend/app/models/account.py
+++ b/backend/app/models/account.py
@@ -67,7 +67,9 @@ class Account(Base):
     sso_provider: Mapped[Optional[str]] = mapped_column(String(20), nullable=True)  # "saml" | "oidc"
     sso_config: Mapped[Optional[dict]] = mapped_column(JSONB, nullable=True)
 
-    # L1 AI tree builder — per-account allowlist of problem categories
+    # L1 AI tree builder — per-account allowlist of problem categories.
+    # Keep this server_default in sync with DEFAULT_L1_CATEGORIES in
+    # app/services/l1_category_service.py when adding/removing categories.
     enabled_l1_categories: Mapped[list[str]] = mapped_column(
         JSONB(), nullable=False,
         server_default=sa_text(
diff --git a/backend/app/services/l1_category_service.py b/backend/app/services/l1_category_service.py
new file mode 100644
index 00000000..cc6afdb5
--- /dev/null
+++ b/backend/app/services/l1_category_service.py
@@ -0,0 +1,69 @@
+"""L1 category allowlist + the always-forbidden hard floor.
+
+DEFAULT_L1_CATEGORIES seeds an account's enabled set. HARD_FLOOR_FORBIDDEN is a
+category-independent safety floor the AI tree builder must never emit and admins
+cannot enable. See spec §5.1/§5.2.
+"""
+from uuid import UUID
+
+from sqlalchemy import select
+from sqlalchemy.ext.asyncio import AsyncSession
+
+from app.models.account import Account
+
+# WARNING: keep in sync with Account.enabled_l1_categories server_default in
+# app/models/account.py. The migration default (cb9e282267d2) is intentionally
+# a frozen copy and is NOT updated when this list changes.
+DEFAULT_L1_CATEGORIES: list[str] = [
+    "password_reset", "account_lockout", "printer", "email_outlook_client",
+    "wifi_network_basics", "vpn_connect", "teams_zoom_av",
+    "browser_cache_cookies", "peripheral_reconnect", "os_restart_update",
+]
+
+# Always-forbidden action classes (keys are stable identifiers; the human-readable
+# phrasing lives in the builder system prompt). Admins cannot enable these.
+HARD_FLOOR_FORBIDDEN: list[str] = [
+    "registry_edit", "system_file_or_boot_edit", "data_or_disk_deletion",
+    "credential_or_mfa_change", "security_or_av_or_firewall_change",
+    "elevated_or_admin_script", "domain_dns_dhcp_change",
+    "server_or_production_config", "billing_or_license_change",
+]
+
+# Substrings that, if present in a generated node's text, indicate a hard-floor
+# violation. Used by ai_tree_builder per-node validation (defense in depth).
+HARD_FLOOR_TEXT_PATTERNS: list[str] = [
+    "regedit", "registry", "format ", "delete partition", "diskpart",
+    "reset password for", "disable firewall", "disable antivirus", "disable defender",
+    "run as administrator", "sudo ", "domain controller", "dns record", "dhcp scope",
+    "uninstall security", "bitlocker",
+]
+
+
+def is_category_enabled(category: str, enabled: list[str]) -> bool:
+    """A category is buildable only if explicitly enabled and not hard-floored."""
+    if category in HARD_FLOOR_FORBIDDEN:
+        return False
+    return category in enabled
+
+
+async def get_enabled_categories(account_id: UUID, db: AsyncSession) -> list[str]:
+    """Return the account's enabled L1 categories (``or []`` guards pre-default rows)."""
+    acct = (await db.execute(select(Account).where(Account.id == account_id))).scalar_one()
+    return list(acct.enabled_l1_categories or [])
+
+
+async def set_enabled_categories(
+    account_id: UUID, categories: list[str], db: AsyncSession
+) -> list[str]:
+    """Persist the enabled set, dropping anything unknown or hard-floored.
+
+    Hard-floored keys (HARD_FLOOR_FORBIDDEN) are by design never present in
+    DEFAULT_L1_CATEGORIES, so the DEFAULT membership filter already excludes them.
+    If you ever add a key to DEFAULT_L1_CATEGORIES, verify it is not also in
+    HARD_FLOOR_FORBIDDEN. dict.fromkeys dedupes while preserving first-seen order.
+    """
+    cleaned = list(dict.fromkeys(c for c in categories if c in DEFAULT_L1_CATEGORIES))
+    acct = (await db.execute(select(Account).where(Account.id == account_id))).scalar_one()
+    acct.enabled_l1_categories = cleaned
+    await db.flush()
+    return cleaned
diff --git a/backend/tests/test_l1_category_service.py b/backend/tests/test_l1_category_service.py
new file mode 100644
index 00000000..b198b365
--- /dev/null
+++ b/backend/tests/test_l1_category_service.py
@@ -0,0 +1,16 @@
+from app.services.l1_category_service import (
+    DEFAULT_L1_CATEGORIES, HARD_FLOOR_FORBIDDEN, is_category_enabled,
+)
+
+
+def test_defaults_and_hard_floor_present():
+    assert "password_reset" in DEFAULT_L1_CATEGORIES
+    assert "registry_edit" in HARD_FLOOR_FORBIDDEN  # representative forbidden action key
+    assert len(DEFAULT_L1_CATEGORIES) == 10
+
+
+def test_is_category_enabled():
+    enabled = ["printer", "vpn_connect"]
+    assert is_category_enabled("printer", enabled) is True
+    assert is_category_enabled("registry_edit", enabled) is False
+    assert is_category_enabled("unknown", enabled) is False