fix(cli): verify agent block exists before nesting tool calls

Prevents tool calls from attempting to nest under agent blocks that
don't exist yet. Adds recursive search to confirm agent block presence
before nesting, falling back to top-level placement when needed.

🤖 Generated with Codebuff
Co-Authored-By: Codebuff <noreply@codebuff.com>

Author: brandonkachen

cli/src/hooks/use-send-message.ts

@@ -24,6 +24,24 @@ const hiddenToolNames = new Set<ToolName | 'spawn_agent_inline'>([
   'spawn_agents',
 ])
 
+// Helper function to check if an agent block exists
+const findAgentBlock = (
+  blocks: ContentBlock[],
+  targetAgentId: string,
+): boolean => {
+  for (const block of blocks) {
+    if (block.type === 'agent' && block.agentId === targetAgentId) {
+      return true
+    }
+    if (block.type === 'agent' && block.blocks) {
+      if (findAgentBlock(block.blocks, targetAgentId)) {
+        return true
+      }
+    }
+  }
+  return false
+}
+
 // Helper function to recursively update blocks
 const updateBlocksRecursively = (
   blocks: ContentBlock[],
@@ -714,8 +732,22 @@ export const useSendMessage = ({
                 agentId: agentId || 'none',
               })
 
-              // If this tool call belongs to a subagent, add it to that agent's blocks
-              if (agentId) {
+              // Check if this tool call should be nested in a subagent
+              // Only nest if the agent block actually exists
+              const shouldNestInAgent = agentId && setMessages.length > 0
+              let agentBlockExists = false
+
+              if (shouldNestInAgent) {
+                setMessages((prev) => {
+                  const msg = prev.find((m) => m.id === aiMessageId)
+                  if (msg?.blocks) {
+                    agentBlockExists = findAgentBlock(msg.blocks, agentId)
+                  }
+                  return prev
+                })
+              }
+
+              if (agentBlockExists) {
                 logger.info('setMessages: tool_call for subagent', {
                   agentId,
                   toolName,
@@ -754,7 +786,7 @@ export const useSendMessage = ({
                   }),
                 )
               } else {
-                // Top-level tool call
+                // Top-level tool call (or agent block doesn't exist yet)
                 setMessages((prev) =>
                   prev.map((msg) => {
                     if (msg.id !== aiMessageId) {