diff --git a/campus_python/auth/v1/logins.py b/campus_python/auth/v1/logins.py
index 9d9fee6..6ec46fa 100644
--- a/campus_python/auth/v1/logins.py
+++ b/campus_python/auth/v1/logins.py
@@ -143,12 +143,32 @@ def token(self) -> campus.model.OAuthToken:
             nothing (C5). The response omits the refresh token —
             campus refresh material never leaves the auth service in
             this lane.
+
+            The call presents Basic client credentials, never the
+            ambient bearer: the route denies user principals outright
+            (#963/A10), and with_user_session() nested inside another
+            with_user_session() (e.g. an auth-broker release reached
+            from a route that already holds the user session) would
+            otherwise present the outer session's user token and be
+            refused with 403. Any prior Authorization header is
+            restored afterwards, so an enclosing session keeps its
+            authorization until it re-runs use_token() with the fresh
+            bearer.
             """
             # Canonical path has no trailing slash (route rule is
             # /<session_id>/token, unlike the CRUD subresources).
-            resp = self.client.post(
-                self.make_path("token")
-            )
+            prior_authorization = self.client.authorization
+            self.client.reset_authorization()
+            try:
+                resp = self.client.post(
+                    self.make_path("token")
+                )
+            finally:
+                if prior_authorization is not None and prior_authorization.startswith(
+                        "Bearer "):
+                    self.client.set_bearer_authorization(
+                        prior_authorization[len("Bearer "):]
+                    )
             # Raise error if status code is not 2XX or 3XX
             resp.raise_for_status()
             return campus.model.OAuthToken.from_resource(resp.json())
diff --git a/campus_python/json_client/__init__.py b/campus_python/json_client/__init__.py
index c7904c8..2ae7cab 100644
--- a/campus_python/json_client/__init__.py
+++ b/campus_python/json_client/__init__.py
@@ -130,6 +130,11 @@ def reset_authorization(self) -> None:
             secret=env.CLIENT_SECRET
         )
 
+    @property
+    def authorization(self) -> str | None:
+        """The current Authorization header value, or None when unset."""
+        return self._session.headers.get("Authorization")
+
     def set_basic_authorization(self, client_id: str, secret: str) -> None:
         """Set Basic Authorization header using a pre-encoded token.
 
diff --git a/campus_python/json_client/interface.py b/campus_python/json_client/interface.py
index e23b150..4a7e500 100644
--- a/campus_python/json_client/interface.py
+++ b/campus_python/json_client/interface.py
@@ -131,6 +131,11 @@ def __init__(
     def reset_authorization(self) -> None:
         """Reset authorization back to client credentials from env."""
 
+    @property
+    @abstractmethod
+    def authorization(self) -> str | None:
+        """The current Authorization header value, or None when unset."""
+
     @abstractmethod
     def set_basic_authorization(self, client_id: str, secret: str) -> None:
         """Set Basic Authorization header using a pre-encoded token.
diff --git a/tests/unit/test_token_refresh.py b/tests/unit/test_token_refresh.py
index b00d0ea..cd0bc3f 100644
--- a/tests/unit/test_token_refresh.py
+++ b/tests/unit/test_token_refresh.py
@@ -43,8 +43,14 @@
 
 
 def make_auth() -> tuple[AuthRoot, Mock]:
-    """Create an AuthRoot backed by a mock JSON client."""
+    """Create an AuthRoot backed by a mock JSON client.
+
+    headers.get defaults to None (no ambient Authorization header) —
+    token() restores whatever header it finds, so tests that exercise
+    the ambient-bearer path override this.
+    """
     client = Mock()
+    client.authorization = None  # property read in token()
     return AuthRoot(json_client=client), client
 
 
@@ -155,6 +161,63 @@ def test_invalid_grant_surfaces_oauth_error(self):
         self.assertEqual(ctx.exception.oauth_error, "invalid_grant")
 
 
+class TestLoginTokenPresentsBasicOnly(unittest.TestCase):
+    """The session-token route serves client principals only (#963/A10):
+    token() must present Basic credentials even when an ambient bearer
+    is set — a with_user_session() nested inside another one otherwise
+    presents the outer session's user token and is refused with 403.
+    The prior header is restored afterwards."""
+
+    def setUp(self):
+        self.auth, self.client = make_auth()
+        self.client.post.return_value = make_response(
+            200, dict(LOGIN_TOKEN_PAYLOAD)
+        )
+
+    def _call(self):
+        with session_context():
+            flask.session["logins_login_id"] = "login-1"
+            return self.auth.logins.token()
+
+    def test_forces_basic_when_bearer_is_set(self):
+        self.client.authorization = "Bearer outer-user-token"
+        self._call()
+        calls = [name for name, _, _ in self.client.mock_calls
+                 if name in ("reset_authorization", "post",
+                             "set_bearer_authorization")]
+        # The exact ordering contract: drop the ambient bearer (Basic),
+        # make the call, then restore it.
+        self.assertEqual(
+            ["reset_authorization", "post", "set_bearer_authorization"], calls
+        )
+
+    def test_restores_ambient_bearer_after_the_call(self):
+        self.client.authorization = "Bearer outer-user-token"
+        self._call()
+        self.client.set_bearer_authorization.assert_called_once_with(
+            "outer-user-token"
+        )
+
+    def test_failure_still_restores_ambient_bearer(self):
+        self.client.authorization = "Bearer outer-user-token"
+        self.client.post.return_value = make_response(403, {
+            "error": {"code": "FORBIDDEN", "message": "denied"}
+        })
+        with session_context():
+            flask.session["logins_login_id"] = "login-1"
+            with self.assertRaises(errors.APIError):
+                self.auth.logins.token()
+        self.client.set_bearer_authorization.assert_called_once_with(
+            "outer-user-token"
+        )
+
+    def test_no_ambient_header_leaves_basic_in_place(self):
+        self.client.headers.get.return_value = None
+        self._call()
+        self.client.reset_authorization.assert_called_once()
+        self.client.set_bearer_authorization.assert_not_called()
+
+
 class TestPublicClientRefresh(unittest.TestCase):
     """auth.refresh(stored) presents the stored refresh token with the
     issuing client_id and returns the rotated pair (#87)."""