AI Agent Payment & Nano‑payment Platforms examines the emerging stack that lets autonomous agents and AI features transact value at micro and nano scales. As of 2026, developers and marketplaces increasingly need low‑latency, low‑fee payment rails and programmability to meter agent actions, manage subscriptions, and route revenues across partners and contributors. This topic covers both off‑chain payment APIs (e.g., fiat and stablecoin rails such as Circle’s nanotransaction offerings) and on‑chain APIs that enable composable, auditable settlements. Relevant toolchains span agent frameworks, developer platforms, and decentralized infrastructure. LangChain and LlamaIndex provide agent orchestration and retrieval/RAG primitives where payment hooks can gate capabilities or data access. Pezzo centralizes prompt/version control and observability while exposing cost metrics that map naturally to per‑action billing. Replit, Tabby, JetBrains AI Assistant, and GitHub Copilot illustrate developer and runtime environments where integrated metering and one‑click monetization influence UX and deployment patterns. Key considerations include latency and cost of payments vs. compute, privacy and compliance for fiat/stablecoin flows, interoperability between off‑chain APIs and on‑chain smart‑contract settlements, and emerging cryptographic requirements — notably post‑quantum resilience for long‑lived signatures and blockchain keys. Decentralized AI infrastructure and agent marketplaces must balance user experience with secure, auditable micropayment settlement. Practically, this means standardizing payment hooks in agent SDKs, offering hybrid off‑chain/on‑chain flows, and adopting blockchain and post‑quantum tooling when assets or reputational records require long‑term integrity. The result is a pragmatic, developer‑oriented view of how nano‑payments enable viable agent economies without sacrificing performance or security.