Which Google Cloud service is ideal for deploying a containerized tool as a serverless, scalable API endpoint that scales to zero when not in use?

Cloud Run is specifically designed for running containerized applications in a serverless environment, making it an optimal choice for deploying a containerized tool as a scalable API endpoint. One of its key features is the ability to scale to zero when no requests are being processed, which allows for efficient resource management and cost savings. This on-demand scaling capability means that users only pay for the resources consumed while the application is active, making it suitable for sporadic workloads or applications that do not require constant operation.

In contrast, other options like Cloud Functions, while also serverless, are typically better suited for event-driven functions rather than full containerized applications. App Engine provides a more managed platform but does not focus exclusively on containerization and may not scale to zero as efficiently as Cloud Run. Kubernetes Engine, on the other hand, offers robust orchestration for containerized applications but requires management of the underlying infrastructure, which does not align with the serverless model or the automatic scaling to zero feature.

Therefore, Cloud Run effectively meets the criteria of deploying a containerized tool as a serverless, scalable API endpoint.