diff --git a/agent/secret_scope.py b/agent/secret_scope.py
index 26022ca9b0ef..d8730db2bd6e 100644
--- a/agent/secret_scope.py
+++ b/agent/secret_scope.py
@@ -127,10 +127,16 @@ def get_secret(name: str, default: Optional[str] = None) -> Optional[str]:
 
     1. Genuinely-global vars (``_is_global_env``) always read ``os.environ`` —
        they are deployment settings, not profile secrets.
-    2. When a secret scope is installed (multiplexed turn), read from it; an
-       absent key returns ``default``. The scope is authoritative — we do NOT
-       fall through to ``os.environ``, because in a multiplexer ``os.environ``
-       may hold another profile's value.
+    2. When a secret scope is installed (multiplexed turn), read from it. Under
+       multiplexing the scope is authoritative — an absent key returns
+       ``default`` and we do NOT fall through to ``os.environ``, because in a
+       multiplexer ``os.environ`` may hold another profile's value. When
+       multiplexing is OFF, a scope miss falls through to ``os.environ``:
+       single-profile deployments legitimately provide credentials via the
+       process environment (systemd ``Environment=``, secret-manager wrappers
+       like ``pass-cli run`` / ``op run``, plain shell exports) rather than
+       ``<home>/.env``, and the scope — installed unconditionally around e.g.
+       every cron job — must stay a ``.env`` overlay, not a blindfold.
     3. No scope installed:
        - multiplex INACTIVE (default deployment): read ``os.environ`` —
          identical to the legacy ``os.getenv`` behavior every caller had before.
@@ -144,6 +150,17 @@ def get_secret(name: str, default: Optional[str] = None) -> Optional[str]:
     scope = _SECRET_SCOPE.get()
     if scope is not None:
         val = scope.get(name)
+        if val is not None:
+            return val
+        if _MULTIPLEX_ACTIVE:
+            return default
+        # Multiplex off: the scope is an overlay over the process environment,
+        # not an isolation boundary — there is no other profile to leak from.
+        # Without this fallthrough, credentials injected only into the process
+        # environment vanish inside any set_secret_scope(...) block (the cron
+        # scheduler installs one around every job), so cron jobs send a
+        # placeholder API key and 401 while interactive turns keep working.
+        val = os.environ.get(name)
         return val if val is not None else default
 
     if _MULTIPLEX_ACTIVE:
diff --git a/tests/agent/test_secret_scope.py b/tests/agent/test_secret_scope.py
index 1b8a1cace40e..43dae703c4f5 100644
--- a/tests/agent/test_secret_scope.py
+++ b/tests/agent/test_secret_scope.py
@@ -72,6 +72,53 @@ class TestMultiplexActiveFailClosed:
         assert ss.get_secret("HERMES_KANBAN_DB") == "/x/kanban.db"
 
 
+class TestScopedSingleProfile:
+    """Multiplex OFF with a scope installed: the scope is an overlay, not a
+    blindfold. The cron scheduler installs a ``<home>/.env`` scope around every
+    job unconditionally, and single-profile deployments legitimately supply
+    credentials via the process environment only (systemd ``Environment=``,
+    ``pass-cli run`` / ``op run`` wrappers) — those must keep resolving."""
+
+    def test_scope_hit_wins_over_environ(self, monkeypatch):
+        monkeypatch.setenv("ANTHROPIC_API_KEY", "sk-from-environ")
+        token = ss.set_secret_scope({"ANTHROPIC_API_KEY": "sk-from-env-file"})
+        try:
+            assert ss.get_secret("ANTHROPIC_API_KEY") == "sk-from-env-file"
+        finally:
+            ss.reset_secret_scope(token)
+
+    def test_scope_miss_falls_back_to_environ(self, monkeypatch):
+        # Regression: provider key injected into the process env but absent
+        # from <home>/.env made every cron job send a placeholder API key
+        # (401) while interactive turns kept working.
+        monkeypatch.setenv("GLM_VOOY_API_KEY", "sk-from-process-env")
+        token = ss.set_secret_scope({"UNRELATED_KEY": "x"})
+        try:
+            assert ss.get_secret("GLM_VOOY_API_KEY") == "sk-from-process-env"
+        finally:
+            ss.reset_secret_scope(token)
+
+    def test_scope_miss_absent_everywhere_returns_default(self, monkeypatch):
+        monkeypatch.delenv("NOPE_KEY", raising=False)
+        token = ss.set_secret_scope({})
+        try:
+            assert ss.get_secret("NOPE_KEY") is None
+            assert ss.get_secret("NOPE_KEY", "d") == "d"
+        finally:
+            ss.reset_secret_scope(token)
+
+    def test_multiplex_on_still_authoritative(self, monkeypatch):
+        # The fallthrough is strictly multiplex-off behavior: turning
+        # multiplexing on must restore scope-authoritative semantics.
+        monkeypatch.setenv("OPENAI_API_KEY", "sk-other-profile")
+        ss.set_multiplex_active(True)
+        token = ss.set_secret_scope({})
+        try:
+            assert ss.get_secret("OPENAI_API_KEY") is None
+        finally:
+            ss.reset_secret_scope(token)
+
+
 class TestScopeIsolation:
     """Two scopes never see each other's secrets."""