diff --git a/api/routes.py b/api/routes.py
index 877f8a8c..f55aeefe 100644
--- a/api/routes.py
+++ b/api/routes.py
@@ -12746,6 +12746,8 @@ def handle_post(handler, parsed) -> bool:
         if "name" not in body:
             return bad(handler, "Missing required field: name")
         sid = body["session_id"]
+        if _session_is_subagent_view_only(sid):
+            return bad(handler, "Subagent sessions are view-only and cannot be modified from WebUI", 400)
         name = body["name"].strip()
         try:
             s = get_session(sid)
@@ -12832,6 +12834,8 @@ def handle_post(handler, parsed) -> bool:
         except ValueError as e:
             return bad(handler, str(e))
         sid = body["session_id"]
+        if _session_is_subagent_view_only(sid):
+            return bad(handler, "Subagent sessions are view-only and cannot store a draft from WebUI", 400)
         text = body.get("text")
         files = body.get("files")
         # Stage-326 hardening (per Opus advisor): size + type validation on
@@ -19222,6 +19226,8 @@ def _normalize_chat_attachments(raw_attachments):
 
 def _handle_chat_sync(handler, body):
     """Fallback synchronous chat endpoint (POST /api/chat). Not used by frontend."""
+    if _session_is_subagent_view_only(str(body.get("session_id") or "")):
+        return bad(handler, "Subagent sessions are view-only and cannot be written from WebUI", 400)
     s = get_session(body["session_id"])
     msg = str(body.get("message", "")).strip()
     if not msg:
@@ -21658,6 +21664,8 @@ def _handle_handoff_summary(handler, body):
 
     from api.models import get_cli_session_messages, count_conversation_rounds, CONVERSATION_ROUND_THRESHOLD
 
+    if _session_is_subagent_view_only(sid):
+        return bad(handler, "Subagent sessions are view-only and cannot be summarized from WebUI", 400)
     rounds = count_conversation_rounds(sid, since=since)
     if rounds < CONVERSATION_ROUND_THRESHOLD:
         return bad(handler, "Not enough conversation rounds to generate a summary.", 400)