diff --git a/static/messages.js b/static/messages.js
index 75758f7c..5d43d344 100644
--- a/static/messages.js
+++ b/static/messages.js
@@ -210,6 +210,7 @@ async function send(){
   startClarifyPolling(activeSid);
   _fetchYoloState(activeSid);  // sync YOLO pill with backend state
   S.activeStreamId = null;  // will be set after stream starts
+  if(typeof updateSendBtn==='function') updateSendBtn();
 
   // Set provisional title from user message immediately so session appears
   // in the sidebar right away with a meaningful name (server may refine later)
@@ -243,6 +244,7 @@ async function send(){
       profile:S.activeProfile||S.session.profile||'default',
       attachments:uploaded.length?uploaded:undefined
     })});
+
     if(startData.effective_model && S.session){
       S.session.model=startData.effective_model;
       S.session.model_provider=startData.effective_model_provider||S.session.model_provider||null;
@@ -259,6 +261,9 @@ async function send(){
     }
     streamId=startData.stream_id;
     S.activeStreamId = streamId;
+    // setBusy(true) already ran with activeStreamId=null; refresh now that we
+    // have a stream id so the primary button can switch to Stop (see getComposerPrimaryAction).
+    if(typeof updateSendBtn==='function') updateSendBtn();
     if(S.session&&typeof startData.pending_started_at==='number'){
       S.session.pending_started_at=startData.pending_started_at;
     }
diff --git a/tests/test_1062_busy_input_modes.py b/tests/test_1062_busy_input_modes.py
index 7e70ca5b..bb7514d2 100644
--- a/tests/test_1062_busy_input_modes.py
+++ b/tests/test_1062_busy_input_modes.py
@@ -207,6 +207,51 @@ class TestBusySendButton:
             "boot.js should wire btnSend to handleComposerPrimaryAction(), not directly to send()"
         )
 
+    def test_send_refreshes_primary_button_after_clearing_active_stream_id(self):
+        """send() must call updateSendBtn after resetting activeStreamId for a new turn.
+
+        getComposerPrimaryAction maps to Stop only when S.activeStreamId is set; after
+        nulling the id, btnSend must refresh so a stale Stop icon cannot linger until
+        the next composer input event.
+        """
+        send_start = MESSAGES_JS.find("async function send(")
+        assert send_start >= 0, "send() not found in messages.js"
+        send_end = MESSAGES_JS.find("const LIVE_STREAMS={}", send_start)
+        assert send_end > send_start, "could not find end of send() body"
+        send_body = MESSAGES_JS[send_start:send_end]
+        marker = "S.activeStreamId = null;  // will be set after stream starts"
+        mpos = send_body.find(marker)
+        assert mpos >= 0, "send() must reset activeStreamId before chat/start"
+        window = send_body[mpos : mpos + 200]
+        assert "updateSendBtn" in window, (
+            "send() must call updateSendBtn() after clearing activeStreamId "
+            "so btnSend state matches the pending-start phase"
+        )
+
+    def test_send_refreshes_primary_button_after_chat_start_stream_id(self):
+        """send() must call updateSendBtn in the chat/start try block after assigning streamId.
+
+        setBusy(true) already ran updateSendBtn while activeStreamId was still null, so the
+        Stop affordance did not appear until something else (e.g. typing) called
+        updateSendBtn again.
+        """
+        send_start = MESSAGES_JS.find("async function send(")
+        assert send_start >= 0, "send() not found in messages.js"
+        send_end = MESSAGES_JS.find("const LIVE_STREAMS={}", send_start)
+        assert send_end > send_start, "could not find end of send() body"
+        send_body = MESSAGES_JS[send_start:send_end]
+        assign = "S.activeStreamId = streamId;"
+        apos = send_body.find(assign)
+        assert apos >= 0, "send() must assign S.activeStreamId from startData"
+        after_assign = send_body[apos:]
+        end_try = after_assign.find("  }catch(e){")
+        assert end_try > 0, "send() outer try/catch not found after stream id assign"
+        try_after_assign = after_assign[:end_try]
+        assert "updateSendBtn" in try_after_assign, (
+            "send() must call updateSendBtn() in the chat/start try block after assigning "
+            "streamId so the primary button switches to Stop without waiting for composer input"
+        )
+
 
 class TestSendBusyBranchDispatch:
     """send()'s busy block must read window._busyInputMode and branch accordingly."""