diff --git a/src/cli/App.tsx b/src/cli/App.tsx
index 4054450..663bc36 100644
--- a/src/cli/App.tsx
+++ b/src/cli/App.tsx
@@ -3132,9 +3132,13 @@ export default function App({
       // Send cancel request to backend asynchronously (fire-and-forget)
       // Don't wait for it or show errors since user already got feedback
       getClient()
-        .then((client) =>
-          client.conversations.cancel(conversationIdRef.current),
-        )
+        .then((client) => {
+          // Use agents API for "default" conversation (primary message history)
+          if (conversationIdRef.current === "default") {
+            return client.agents.messages.cancel(agentIdRef.current);
+          }
+          return client.conversations.cancel(conversationIdRef.current);
+        })
         .catch(() => {
           // Silently ignore - cancellation already happened client-side
         });
@@ -3153,7 +3157,12 @@ export default function App({
       setInterruptRequested(true);
       try {
         const client = await getClient();
-        await client.conversations.cancel(conversationIdRef.current);
+        // Use agents API for "default" conversation (primary message history)
+        if (conversationIdRef.current === "default") {
+          await client.agents.messages.cancel(agentIdRef.current);
+        } else {
+          await client.conversations.cancel(conversationIdRef.current);
+        }
 
         if (abortControllerRef.current) {
           abortControllerRef.current.abort();
@@ -3799,9 +3808,13 @@ export default function App({
 
             // Send cancel request to backend (fire-and-forget)
             getClient()
-              .then((client) =>
-                client.conversations.cancel(conversationIdRef.current),
-              )
+              .then((client) => {
+                // Use agents API for "default" conversation (primary message history)
+                if (conversationIdRef.current === "default") {
+                  return client.agents.messages.cancel(agentIdRef.current);
+                }
+                return client.conversations.cancel(conversationIdRef.current);
+              })
               .then(() => {})
               .catch(() => {
                 // Reset flag if cancel fails