The correct choice is Vertex AI Agent Builder.

Vertex AI Agent Builder is built for production generative AI agents that perform multi step reasoning and tool use. It supports function calling and extensions so the assistant can call partner and internal APIs for live inventory, discount checks, and shipping estimates. It manages multi turn state and memory so it can ask clarifying questions and then orchestrate the end to end checkout flow. It also provides enterprise features such as grounding, authentication, safety controls, evaluation, monitoring, and straightforward deployment which are essential for a reliable shopping assistant.

Dialogflow CX focuses on intent driven task bots and slot filling and while it can call webhooks it does not provide native generative planning and multi tool orchestration needed for complex multi item shopping. For this scenario the generative agent capabilities in Vertex AI Agent Builder are the better fit.

Google AI Studio is a prototyping and prompt design environment and it is not a managed runtime for production agents with connectors, tool execution, evaluation, and monitoring. It is useful for quick experiments but it does not deliver full checkout orchestration across APIs.

Gemini API with custom Python code and no agent framework would require you to build conversation state, tool routing, safety, authentication, and reliability on your own which increases effort and risk. The managed orchestration in Vertex AI Agent Builder addresses these needs out of the box.

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