diff --git a/api/models.py b/api/models.py
index 84a91b9c..99226cb4 100644
--- a/api/models.py
+++ b/api/models.py
@@ -48,7 +48,11 @@ from api.agent_sessions import (
 from api.process_event_utils import stamp_message_source
 
 logger = logging.getLogger(__name__)
-CLI_VISIBLE_SESSION_LIMIT = 20
+# Size of the interactive sidebar recency window. Also bounds how many
+# delegated subagent children can be rendered at once, since a child only
+# nests when it wins a slot in this window. Override with
+# HERMES_WEBUI_VISIBLE_SESSION_LIMIT.
+CLI_VISIBLE_SESSION_LIMIT = _cfg._env_int("HERMES_WEBUI_VISIBLE_SESSION_LIMIT", 20)
 # How many messageful cron sessions to surface in the project-chip layer.
 # Needs to exceed CLI_VISIBLE_SESSION_LIMIT so older cron runs stay
 # addressable even when many newer non-cron sessions dominate the default
diff --git a/tests/test_visible_session_limit_env.py b/tests/test_visible_session_limit_env.py
new file mode 100644
index 00000000..178465f5
--- /dev/null
+++ b/tests/test_visible_session_limit_env.py
@@ -0,0 +1,35 @@
+"""CLI_VISIBLE_SESSION_LIMIT is overridable via HERMES_WEBUI_VISIBLE_SESSION_LIMIT.
+
+The sidebar recency window also bounds how many delegated subagent children can
+render at once, since a child only nests when its row wins a slot in the same
+payload. Operators running wide fan-outs need to raise it without editing code.
+"""
+
+import importlib
+
+
+def _reload_limit(monkeypatch, value):
+    if value is None:
+        monkeypatch.delenv("HERMES_WEBUI_VISIBLE_SESSION_LIMIT", raising=False)
+    else:
+        monkeypatch.setenv("HERMES_WEBUI_VISIBLE_SESSION_LIMIT", value)
+    import api.models as models
+
+    return importlib.reload(models).CLI_VISIBLE_SESSION_LIMIT
+
+
+def test_defaults_to_20_when_unset(monkeypatch):
+    assert _reload_limit(monkeypatch, None) == 20
+
+
+def test_env_override_raises_the_window(monkeypatch):
+    assert _reload_limit(monkeypatch, "64") == 64
+
+
+def test_invalid_value_falls_back_to_default(monkeypatch):
+    assert _reload_limit(monkeypatch, "bogus") == 20
+
+
+def test_zero_and_negative_fall_back_to_default(monkeypatch):
+    assert _reload_limit(monkeypatch, "0") == 20
+    assert _reload_limit(monkeypatch, "-5") == 20