What is the reason the /api/v2/wfm/scheduling/users endpoint is throwing a 400 Bad Request when I increase the JMeter thread count to just 25 concurrent requests? I am trying to simulate a bulk schedule update scenario for a mid-sized team in the Asia/Singapore region, but the API seems to choke on the payload structure even though it works perfectly fine with 1 or 2 threads. The error response body is minimal, just {"errorCode":"bad.request","message":"Invalid input"}, which doesn’t give me much to go on. I am using Genesys Cloud SDK version 12.5.0 for the initial token generation, but the actual load test hits the REST API directly via JMeter 5.6.2 to avoid any SDK overhead. The JSON payload I am sending is identical to the example in the developer docs, including the userId, date, and scheduleEntries array with startTime and endTime in ISO 8601 format. I have double-checked the timezone offsets, and they are correct for Asia/Singapore. When I run this through Postman, it succeeds every time. The issue only appears under concurrent load. I suspect there might be a hidden rate limit or a mutex lock on the scheduling engine that isn’t documented, or perhaps the If-Match header handling is failing under stress. I have tried adding a 500ms think time between requests, but the 400 errors persist. I also noticed that the x-correlation-id header is sometimes missing from the error response, making it hard to trace in the Genesys Cloud logs. Is this a known limitation of the WFM API during high-concurrency writes? Should I be batching these requests differently or using a different endpoint for bulk updates? I am relatively new to the platform, so any guidance on best practices for load testing WFM endpoints would be appreciated. I want to ensure our onboarding process can handle bulk schedule changes without breaking the API contract.
It’s worth reviewing at the payload serialization format when scaling up thread counts. The WFM Scheduling API often rejects requests if the JSON body contains null values for optional fields like timezone or userGroupId during bulk operations. Even if the schema allows nulls, the validation logic tightens under load.
In JMeter, ensure the Content-Type header is strictly application/json and not application/json; charset=utf-8, as some GC endpoints are sensitive to charset declarations in the header. Also, check if the request body is being truncated. With 25 threads, memory pressure can cause the HTTP Request sampler to send incomplete JSON.
Try adding a Debug Sampler to capture the exact request sent. If the payload looks correct, the issue might be rate limiting masquerading as a 400 error. Check the Retry-After header. If missing, add a Constant Timer of 500ms between requests to smooth out the burst.