License Requirement for Developing Custom Microsoft 365 Agents

Question

We would like to understand the licensing requirements for developing and testing our own Microsoft Agent 365 agents.

Could you please confirm:

Which licenses and/or add-on packs are required to develop, test, and deploy custom Agent 365 agents?

Are Microsoft 365 E5, Microsoft 365 Copilot, and Microsoft Agent 365 Frontier sufficient, or are there any additional licenses or tenant configurations required?

Is there a recommended licensing plan for development environments?

Could you also share the available monthly and yearly licensing plans (or pricing options) for Agent 365 and any required add-ons?

We want to ensure our development environment has all the necessary licenses and prerequisites to build and test custom Agent 365 agents successfully.