Long-running requests spike concurrency

Your app calls a vendor LLM API. The vendor enforces both a requests-per-minute limit AND a max-concurrent-requests limit. At 15:00 you start getting intermittent 429s with the message 'concurrency limit exceeded', not RPM. Your requests-per-minute is comfortably under the documented RPM cap. Dashboards: your average concurrent in-flight requests to the vendor is fine, but you see brief concurrency spikes whenever a few long-running (high-token) generations overlap with normal short ones. No traffic change; you shipped a feature yesterday that allows much longer outputs. How do you triage and mitigate?