fix(mcp): surface DB-unavailable as a real error on per-user env var lookup

When prisma_client is None, _load_user_env_vars returned an empty dict,
which on the tool-call path was indistinguishable from "user has no
stored values" and produced a misleading 412 directing the user to set
up credentials they can never store without a database. Raise instead so
the tool-call path fails with a clear error and the listing path stays
best-effort via its existing catch.

litellm/proxy/_experimental/mcp_server/mcp_server_manager.py

@@ -1697,9 +1697,11 @@ class MCPServerManager:
         window; the cache is invalidated when the user stores or clears values.
         Pass ``force_refresh`` to bypass the cache read and re-fetch from the DB
         (used before raising a "missing credentials" error so a process-local
-        stale entry cannot mask values stored on another worker). DB errors
-        propagate so the caller can decide between failing the request (tool-call
-        path) and staying best-effort (listing path).
+        stale entry cannot mask values stored on another worker). A missing DB
+        connection and any other DB error propagate so the caller can decide
+        between failing the request (tool-call path) and staying best-effort
+        (listing path); they must never be mistaken for "user has no values",
+        which would send the user a misleading "set up your credentials" 412.
         """
         if user_api_key_auth is None:
             return {}
@@ -1718,7 +1720,11 @@ class MCPServerManager:
         from litellm.proxy.proxy_server import prisma_client  # noqa: PLC0415
 
         if prisma_client is None:
-            return {}
+            raise RuntimeError(
+                "MCP per-user env vars require a database connection, but none "
+                "is configured. Connect a database to your proxy to use per-user "
+                "MCP env vars."
+            )
         from litellm.proxy._experimental.mcp_server.db import (  # noqa: PLC0415
             get_user_env_vars,
         )

tests/test_litellm/proxy/_experimental/mcp_server/test_mcp_env_vars.py

@@ -484,8 +484,11 @@ async def test_load_user_env_vars_returns_empty_without_user_id():
 
 
 @pytest.mark.asyncio
-async def test_load_user_env_vars_returns_empty_when_db_unavailable(monkeypatch):
-    """If prisma_client is None, the lookup short-circuits rather than crashing."""
+async def test_load_user_env_vars_raises_when_db_unavailable(monkeypatch):
+    """A missing DB connection must raise, not return ``{}``. Returning ``{}``
+    would be indistinguishable from "user has no values" and would mislead the
+    tool-call path into a "set up your credentials" 412 the user can never
+    satisfy (per-user env vars are unusable without a DB)."""
     from unittest.mock import MagicMock
 
     from litellm.proxy._experimental.mcp_server.mcp_server_manager import (
@@ -500,7 +503,33 @@ async def test_load_user_env_vars_returns_empty_when_db_unavailable(monkeypatch)
     fake_auth = MagicMock()
     fake_auth.user_id = "alice"
     monkeypatch.setattr("litellm.proxy.proxy_server.prisma_client", None)
-    assert await manager._load_user_env_vars(server, fake_auth) == {}
+    with pytest.raises(RuntimeError, match="database connection"):
+        await manager._load_user_env_vars(server, fake_auth)
+
+
+@pytest.mark.asyncio
+async def test_resolve_static_headers_db_unavailable_is_not_missing_412(
+    mock_server, monkeypatch
+):
+    """On the tool-call path, an unavailable DB must surface as a real error
+    rather than a misleading MCPMissingUserEnvVarsError (412). This guards the
+    regression where ``_load_user_env_vars`` returned ``{}`` when prisma_client
+    was None, making a DB outage look like "user has no credentials"."""
+    from unittest.mock import MagicMock
+
+    from litellm.proxy._experimental.mcp_server.mcp_server_manager import (
+        MCPServerManager,
+    )
+
+    manager = MCPServerManager()
+    fake_auth = MagicMock()
+    fake_auth.user_id = "alice"
+    monkeypatch.setattr("litellm.proxy.proxy_server.prisma_client", None)
+
+    with pytest.raises(RuntimeError, match="database connection"):
+        await manager._resolve_static_headers_with_env_vars(
+            mock_server, user_api_key_auth=fake_auth
+        )
 
 
 @pytest.mark.asyncio