[runtime] Add CORS headers and request handling to health check check response

.changeset/twenty-parents-type.md

@@ -0,0 +1,5 @@
+---
+"@workflow/core": patch
+---
+
+Add CORS headers to endpoints health check response

packages/core/src/runtime/helpers.ts

@@ -34,6 +34,16 @@ export async function getAllWorkflowRunEvents(runId: string): Promise<Event[]> {
   return allEvents;
 }
 
+/**
+ * CORS headers for health check responses.
+ * Allows the observability UI to check endpoint health from a different origin.
+ */
+const HEALTH_CHECK_CORS_HEADERS = {
+  'Access-Control-Allow-Origin': '*',
+  'Access-Control-Allow-Methods': 'POST, OPTIONS, GET, HEAD',
+  'Access-Control-Allow-Headers': 'Content-Type',
+};
+
 /**
  * Wraps a request/response handler and adds a health check "mode"
  * based on the presence of a `__health` query parameter.
@@ -45,9 +55,22 @@ export function withHealthCheck(
     const url = new URL(req.url);
     const isHealthCheck = url.searchParams.has('__health');
     if (isHealthCheck) {
+      // Handle CORS preflight for health check
+      if (req.method === 'OPTIONS') {
+        return new Response(null, {
+          status: 204,
+          headers: HEALTH_CHECK_CORS_HEADERS,
+        });
+      }
       return new Response(
         `Workflow DevKit "${url.pathname}" endpoint is healthy`,
-        { status: 200, headers: { 'Content-Type': 'text/plain' } }
+        {
+          status: 200,
+          headers: {
+            'Content-Type': 'text/plain',
+            ...HEALTH_CHECK_CORS_HEADERS,
+          },
+        }
       );
     }
     return await handler(req);