"Agent2Agent Protocol (A2A): Building Interoperable Multi‑Agent Systems Across Vendors" A2A is quickly becoming the pragmatic boundary line between “my agent” and “your agent”—especially when both are opaque, independently implemented, and operated under different security and governance regimes. This book is written for experienced architects and senior engineers who need multi-agent systems to interoperate across vendors without relying on shared internal prompts, tool wiring, or fragile product-specific integrations. It focuses on what must be standardized so autonomous systems can collaborate reliably across organizations. You’ll learn the protocol end-to-end: how agents are discovered and identified through Agent Cards, well-known locations, and enterprise catalogs; how A2A semantics are carried over HTTP + JSON-RPC, SSE streaming, and gRPC; and how the core domain model (tasks, messages, parts, artifacts, status) enables consistent workflows. The book drills into collaboration flows, long-running execution, idempotency and retries, routing topologies, and human/tool-in-the-loop designs. It also covers production-grade security (TLS, OAuth/OIDC/JWT, policy enforcement, auditability) and the hard work of compatibility engineering: error recovery, extensions, version negotiation, conformance testing, and protocol bridging (including MCP adapters). Readers should be comfortable with distributed systems, API design, and modern auth patterns. The emphasis is on interoperability guarantees, failure modes, and decision criteria you can apply immediately in real deployments