AWS Lambda adds public preview runtimes, starting with Node.js 26 and Python 3.15

Sources: AWS Compute Blog, HN discussion
Summary: Preview functions use the same runtime identifiers the GA runtimes will use, nodejs26.x and python3.15, and graduate with no configuration change. They carry no Lambda SLA and no AWS Support, may take breaking changes during the preview, and emit a warning to CloudWatch Logs on every cold start. Both upstream languages are expected to reach stable release in October 2026, with Lambda GA targeted within two months after that.
Why it matters: Teams can test workloads on the next runtime before GA without a later configuration change, against no SLA and no support.