router block error path decision

Author: icecrasher321

apps/docs/content/docs/en/blocks/router.mdx

@@ -2,7 +2,6 @@
 title: Router
 ---
 
-import { Callout } from 'fumadocs-ui/components/callout'
 import { Tab, Tabs } from 'fumadocs-ui/components/tabs'
 import { Image } from '@/components/ui/image'
 
@@ -90,11 +89,18 @@ Input (Lead) → Router → Agent (Enterprise Sales) or Workflow (Self-serve)
 ```
 
 
+## Error Handling
+
+When the Router cannot determine an appropriate route for the given context, it will route to the **error path** instead of arbitrarily selecting a route. This happens when:
+
+- The context doesn't clearly match any of the defined route descriptions
+- The AI determines that none of the available routes are appropriate
+
 ## Best Practices
 
 - **Provide clear target descriptions**: Help the Router understand when to select each destination with specific, detailed descriptions
 - **Use specific routing criteria**: Define clear conditions and examples for each path to improve accuracy
-- **Implement fallback paths**: Connect a default destination for when no specific path is appropriate
+- **Connect an error path**: Handle cases where no route matches by connecting an error handler for graceful fallback behavior
 - **Test with diverse inputs**: Ensure the Router handles various input types, edge cases, and unexpected content
 - **Monitor routing performance**: Review routing decisions regularly and refine criteria based on actual usage patterns
 - **Choose appropriate models**: Use models with strong reasoning capabilities for complex routing decisions

apps/sim/blocks/blocks/router.ts

@@ -115,29 +115,31 @@ Description: ${route.value || 'No description provided'}
     )
     .join('\n')
 
-  return `You are a DETERMINISTIC routing agent. You MUST select exactly ONE route. There is NO other option.
+  return `You are a DETERMINISTIC routing agent. You MUST select exactly ONE option.
 
 CRITICAL RULES:
-- You MUST output a route ID. Period.
+- You MUST output ONLY a route ID or "NO_MATCH" - copy it EXACTLY as shown
 - You CANNOT say "I need more information"
-- You CANNOT refuse to choose
 - You CANNOT explain your reasoning
 - You CANNOT ask questions
-- You CANNOT output anything except a route ID
-- There is ALWAYS a best option. Pick it.
+- You CANNOT output anything except the exact route ID or "NO_MATCH"
 
 Available Routes:
 ${routesInfo}
 
+Special Option:
+- If the context does NOT clearly match ANY route description, output exactly: NO_MATCH
+
 Context:
 ${context}
 
 MANDATORY BEHAVIOR:
 1. Analyze the context against each route description
-2. ALWAYS pick the BEST matching route - one route is always better than the others
-3. Output ONLY the route ID - nothing else
+2. If one route clearly matches the context, output that route's ID exactly as shown
+3. If NO route is appropriate for the context, output exactly: NO_MATCH
+4. Output ONLY the route ID or NO_MATCH - nothing else
 
-Your response must be EXACTLY one route ID from the list above. No punctuation. No explanation. No refusal. Just the ID.`
+Your response must be EXACTLY one route ID from the list above OR "NO_MATCH". No punctuation. No explanation. Just the ID or NO_MATCH.`
 }
 
 /**

apps/sim/executor/handlers/router/router-handler.ts

@@ -273,15 +273,24 @@ export class RouterBlockHandler implements BlockHandler {
       const result = await response.json()
 
       const chosenRouteId = result.content.trim()
-      let chosenRoute = routes.find((r) => r.id === chosenRouteId)
 
-      // Fallback to first route if LLM returns invalid route ID
+      if (chosenRouteId === 'NO_MATCH' || chosenRouteId.toUpperCase() === 'NO_MATCH') {
+        logger.info('Router determined no route matches the context, routing to error path')
+        throw new Error('Router could not determine a matching route for the given context')
+      }
+
+      const chosenRoute = routes.find((r) => r.id === chosenRouteId)
+
+      // Throw error if LLM returns invalid route ID - this routes through error path
       if (!chosenRoute) {
-        logger.warn(
-          `Invalid routing decision. Response content: "${result.content}", falling back to first route. Available routes:`,
-          routes.map((r) => ({ id: r.id, title: r.title }))
+        const availableRoutes = routes.map((r) => ({ id: r.id, title: r.title }))
+        logger.error(
+          `Invalid routing decision. Response content: "${result.content}". Available routes:`,
+          availableRoutes
+        )
+        throw new Error(
+          `Router could not determine a valid route. LLM response: "${result.content}". Available route IDs: ${routes.map((r) => r.id).join(', ')}`
         )
-        chosenRoute = routes[0]
       }
 
       // Find the target block connected to this route's handle