Claude Code: Complete A2A routing - add target_agent validation with 400 error

- Modify POST /api/say to accept optional target_agent string
- Return 400 error if target_agent not found in registry
- Default to broadcast when target_agent is null/missing
- Log routing decision (Broadcast vs Direct) to console

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

app.py

@@ -282,64 +282,67 @@ async def api_say(request_data: dict):
     Payload:
     {
         "message": "Hello, Agent!",
-        "target_agent": "AgentName",  // Optional - defaults to "all" if missing
+        "target_agent": "AgentName",  // Optional - if null/missing, broadcast to all
         "sender": "sender_name"       // Optional - for logging
     }
     """
     global _registered_agents, _agent_name_routing
 
     message = request_data.get("message", "")
-    target_agent = request_data.get("target_agent", "")
+    target_agent = request_data.get("target_agent")
     sender = request_data.get("sender", "unknown")
 
     if not message:
         raise HTTPException(status_code=400, detail="message field is required")
 
-    # Handle missing target_agent with warning log for observability
-    if not target_agent:
-        logger.warning(f"[A2A] No target_agent specified by '{sender}', defaulting to broadcast (all). "
-                       f"This may cause unnecessary network traffic.")
-        target_agent = "all"
-
-    results = {"delivered": [], "failed": [], "target_type": target_agent}
+    results = {"delivered": [], "failed": [], "routing_type": ""}
 
-    if target_agent.lower() == "all":
-        # Broadcast to all registered agents using dict iteration (still O(n) but explicit)
-        logger.info(f"[A2A] Broadcasting message from '{sender}' to all agents")
+    # Handle missing or null target_agent - default to broadcast
+    if not target_agent:
+        logger.info(f"[A2A] Routing: BROADCAST (no target specified, sending to all agents)")
+        results["routing_type"] = "Broadcast"
         for agent_id, agent_info in _registered_agents.items():
             agent_name = agent_info["agentName"]
             results["delivered"].append({
                 "agentId": agent_id,
                 "agentName": agent_name
             })
-            # In a real implementation, you'd push to each agent here
         results["broadcast_count"] = len(results["delivered"])
+        return {
+            "success": True,
+            "results": results
+        }
 
+    # Direct routing to specific agent
+    logger.info(f"[A2A] Routing: DIRECT to '{target_agent}'")
+    results["routing_type"] = "Direct"
+
+    # Look up agent in routing table - return 400 if not found
+    if target_agent not in _agent_name_routing:
+        logger.warning(f"[A2A] Agent '{target_agent}' not found in registry")
+        raise HTTPException(
+            status_code=400,
+            detail=f"Agent '{target_agent}' not found in registry. Available agents: {list(_agent_name_routing.keys())}"
+        )
+
+    target_id = _agent_name_routing[target_agent]
+    agent_info = _registered_agents.get(target_id)
+    if agent_info:
+        results["delivered"].append({
+            "agentId": target_id,
+            "agentName": target_agent
+        })
+        logger.info(f"[A2A] Message from '{sender}' delivered to '{target_agent}'")
     else:
-        # Dictionary-based routing - O(1) lookup prevents broadcast storms
-        if target_agent in _agent_name_routing:
-            target_id = _agent_name_routing[target_agent]
-            agent_info = _registered_agents.get(target_id)
-            if agent_info:
-                results["delivered"].append({
-                    "agentId": target_id,
-                    "agentName": target_agent
-                })
-                logger.info(f"[A2A] Message from '{sender}' delivered to '{target_agent}'")
-            else:
-                results["failed"].append({
-                    "agentName": target_agent,
-                    "error": "Agent registered in routing but not in registry"
-                })
-        else:
-            results["failed"].append({
-                "agentName": target_agent,
-                "error": "Agent not found in routing table"
-            })
-            logger.warning(f"[A2A] Target agent '{target_agent}' not found in routing table")
+        # This should never happen if routing table is consistent
+        logger.error(f"[A2A] Inconsistent state: '{target_agent}' in routing but not in registry")
+        raise HTTPException(
+            status_code=500,
+            detail=f"Internal error: Agent '{target_agent}' found in routing table but not in registry"
+        )
 
     return {
-        "success": len(results["failed"]) == 0,
+        "success": True,
         "results": results
     }