diff --git a/autobot-slm-backend/main.py b/autobot-slm-backend/main.py
index ede5c7419..28b0a4b28 100644
--- a/autobot-slm-backend/main.py
+++ b/autobot-slm-backend/main.py
@@ -76,6 +76,59 @@
 logger = logging.getLogger(__name__)
 
 
+def _check_tablename_collisions() -> None:
+    """Detect shared tablenames across both SQLAlchemy Base MetaData objects (#1878).
+
+    The SLM backend uses two independent ``DeclarativeBase`` subclasses that each
+    target a **different** PostgreSQL database:
+
+    * ``models.database.Base`` — SLM admin models (nodes, deployments, …)
+      → connects to the main SLM database (``settings.database_url``)
+    * ``user_management.models.base.Base`` — User management models (users, roles, …)
+      → connects to the ``slm_users`` database (``SLM_USERS_DATABASE_URL``)
+
+    Because each Base has its own ``MetaData`` object, SQLAlchemy cannot enforce
+    cross-Base uniqueness.  A developer who accidentally assigns the same
+    ``__tablename__`` to models from both bases will get no compile-time error; the
+    table will simply be created in the wrong database and FK references will silently
+    break, exactly as happened with the ``users`` / ``slm_users`` incident (#1854).
+
+    This function logs a WARNING for every overlapping tablename so that such
+    regressions are surfaced immediately at startup rather than discovered later via
+    mysterious query failures.
+
+    Note: This does NOT raise because several names (e.g. ``roles``, ``audit_logs``)
+    are intentionally shared between the two databases for independent domain purposes.
+    The warning is the signal; renaming is the developer's responsibility.
+    """
+    # Import after path is set up so this function is safe to call early in lifespan.
+    import user_management.models  # noqa: F401 — registers all UM models with UMBase
+    from models.database import Base as SLMBase
+    from user_management.models.base import Base as UMBase
+
+    slm_tables: set[str] = set(SLMBase.metadata.tables.keys())
+    um_tables: set[str] = set(UMBase.metadata.tables.keys())
+    collisions: set[str] = slm_tables & um_tables
+
+    if collisions:
+        sorted_names = sorted(collisions)
+        logger.warning(
+            "Tablename overlap detected between SLM Base and UserManagement Base — "
+            "%d shared name(s): %s. "
+            "These names refer to tables in different databases, but sharing names "
+            "increases the risk of future model misplacement. "
+            "See GitHub issue #1878.",
+            len(sorted_names),
+            sorted_names,
+        )
+    else:
+        logger.info(
+            "Tablename collision check passed — %d SLM tables, %d UM tables, 0 shared names",
+            len(slm_tables),
+            len(um_tables),
+        )
+
+
 async def _init_user_management_tables() -> None:
     """Create user management tables (SSO, organizations, users, etc.) (#921).
 
@@ -131,6 +184,10 @@ async def lifespan(app: FastAPI):
     logger.info("Starting SLM Backend v1.0.0")
     logger.info("Debug mode: %s", settings.debug)
 
+    # Validate that the two Base MetaData objects share no tablenames (#1878).
+    # Must run before create_all / migrations so conflicts are caught immediately.
+    _check_tablename_collisions()
+
     # Create base tables first, then apply incremental migrations
     await db_service.initialize()
     await _init_user_management_tables()