diff --git a/src/agents/librarian/client.py b/src/agents/librarian/client.py
index 3d0a98d..7ae6b53 100644
--- a/src/agents/librarian/client.py
+++ b/src/agents/librarian/client.py
@@ -17,7 +17,7 @@ import httpx
 from pydantic import BaseModel, Field
 
 from src.core.config import config
-from src.core.context import get_user
+from src.core.context import apply_tenant_guard, get_user
 from src.core.logging_config import get_logger
 
 logger = get_logger(__name__)
@@ -271,14 +271,19 @@ class LibraryDeskClient:
         request must carry an explicit tenant (a missing user will 422).
         An empty tenant is a programming or configuration error - fail
         loudly here, before any bytes hit the wire.
+
+        Explicit user arguments are stripped and routed through the same
+        tenant guard as context resolution (get_user() already applies
+        it), so a dev environment can never send the production tenant -
+        or a sanitization-collision variant of it - to library-desk.
         """
-        effective = user if user is not None else get_user()
-        if not effective or not effective.strip():
+        effective = (user if user is not None else get_user()).strip()
+        if not effective:
             raise ValueError(
                 "library-desk request requires a non-empty user (tenant); "
                 "got an empty value from the caller or request context"
             )
-        return effective
+        return apply_tenant_guard(effective)
 
     async def _request_with_retry(
         self,
diff --git a/tests/agents/librarian/test_client.py b/tests/agents/librarian/test_client.py
index 7c8109a..5d82c9c 100644
--- a/tests/agents/librarian/test_client.py
+++ b/tests/agents/librarian/test_client.py
@@ -671,6 +671,59 @@ class TestExplicitUserContract:
 
         mock_httpx.post.assert_not_called()
 
+    @pytest.mark.asyncio
+    async def test_explicit_padded_user_is_stripped_on_the_wire(self):
+        """Padded explicit users are stripped, not sent verbatim."""
+        client, mock_httpx = self._wire_client()
+
+        await client.hybrid_search("q", user=" llm_tester ")
+
+        assert self._sent_user(mock_httpx) == "llm_tester"
+
+    @pytest.mark.asyncio
+    @pytest.mark.parametrize(
+        "explicit_user",
+        ["jpmschweitzer", "JPMSchweitzer", "jpmschweitzer.", " jpmschweitzer"],
+    )
+    @pytest.mark.parametrize("method_name,kwargs", TENANT_SCOPED_METHODS)
+    async def test_explicit_production_tenant_is_guarded_in_dev(
+        self, monkeypatch, method_name, kwargs, explicit_user
+    ):
+        """
+        An explicit production-tenant argument (or a sanitization-collision
+        variant) never reaches library-desk from a non-production
+        environment - the client applies the same tenant guard as
+        context resolution.
+        """
+        from src.core import config as config_module
+        from src.core.config import Environment
+
+        monkeypatch.setattr(
+            config_module.config, "ENVIRONMENT", Environment.DEVELOPMENT
+        )
+        client, mock_httpx = self._wire_client()
+
+        await getattr(client, method_name)(user=explicit_user, **kwargs)
+
+        assert self._sent_user(mock_httpx) == "llm_tester"
+
+    @pytest.mark.asyncio
+    async def test_explicit_production_tenant_passes_through_in_prod(
+        self, monkeypatch
+    ):
+        """In production the production tenant is sent unchanged."""
+        from src.core import config as config_module
+        from src.core.config import Environment
+
+        monkeypatch.setattr(
+            config_module.config, "ENVIRONMENT", Environment.PRODUCTION
+        )
+        client, mock_httpx = self._wire_client()
+
+        await client.hybrid_search("q", user="jpmschweitzer")
+
+        assert self._sent_user(mock_httpx) == "jpmschweitzer"
+
 
 @pytest.mark.unit
 class TestNewResponseModels: