AWS Lambda introduces public preview runtimes with Node.js 26 and Python 3.15

AWS Lambda introduced 'public preview runtimes,' giving developers an official, supported way to test upcoming language versions before those runtimes reach general availability. The first two available today are Node.js 26 and Python 3.15, and AWS is explicitly asking developers to build against them, report issues, and help shape runtime quality before GA.

Mechanically, this closes a long-standing gap: previously teams either waited for GA runtimes or hacked together custom runtimes and container images to test new language versions on Lambda. A first-class preview channel lets them validate dependency compatibility, cold-start behavior, and performance regressions early, and gives AWS a feedback loop before locking in the GA runtime.

The timing matters for the agentic-AI workloads dominating this week's AWS output — faster language runtimes and earlier access to new interpreter optimizations feed directly into the serverless glue code orchestrating agents. AWS separately raised Lambda function bandwidth to 3,000 Mbps and shipped guidance on dynamic feature flags with AppConfig on Lambda, part of a broader push to make Lambda the control plane for spiky, event-driven AI systems.

The caveat is standard for previews: no SLA, APIs and behavior can change, and production use is discouraged until GA. Still, for teams that want to get ahead of the Python 3.15 and Node.js 26 migrations, this removes the friction of maintaining custom runtimes. Watch for how quickly these previews graduate to GA and whether AWS extends the preview model to other runtimes like Java and .NET.