Chapter 5. Protocols: MCP, A2A, and Interoperability
The 2025 "protocol war" ended not with one standard winning but with layering: became the accepted "agent-to-tool" layer, took the narrow "agent-to-agent" niche at trust boundaries, and IBM ACP disappeared as a standalone protocol. Both surviving standards moved under the neutral governance of the Linux Foundation, so the risk of vendor capture is off the table. The main practical takeaway for a team building an in 2026: there is no longer any need to argue over the choice of protocol — you need to layer them correctly and lock down security, because that is exactly where real deployments break today.
MCP: The Boring Winner of the Agent-to-Tool Layer
The , introduced by Anthropic in November 2024, had by mid-2026 become an infrastructural given: about 97M monthly downloads for Python and TypeScript (a 970× rise in 18 months), more than 10,000 active servers, and client-side support in ChatGPT, Claude, Gemini, Microsoft Copilot, GitHub Copilot, Cursor, VS Code, and Zed (MCP blog, WorkOS). OpenAI adopted the protocol back in March 2025, Google DeepMind in April 2025. As Zuplo's Martin Davis put it, "MCP settled into something normal" — the protocol became a norm no one argues about anymore (Zuplo, 2026-07-03).
The question of vendor control is settled too: on December 9, 2025, MCP was handed to the AI Foundation (AAIF) — a foundation under the Linux Foundation umbrella, co-founded by Anthropic, Block, and OpenAI with backing from Google, Microsoft, AWS, Cloudflare, and Bloomberg; the platinum board has eight seats — Amazon, Anthropic, Block, Bloomberg, Cloudflare, Google, Microsoft, and OpenAI. The foundation's founding projects are MCP itself, goose from Block, and AGENTS.md from OpenAI; the maintainer structure and the SEP process did not change in the move (Anthropic; TechCrunch).
The win shows at the framework level too (the frameworks covered in that chapter): MCP is natively supported by the Claude Agent SDK (the protocol is built into the Claude Code itself), the OpenAI Agents SDK — an April 15, 2026 update added MCP and AGENTS.md support there along with native execution (Better Stack) — plus smolagents and nearly every other notable library; Google ADK is even built as a protocol-first platform around MCP and at once. For a team this means a simple thing: a tool wrapped in an MCP server once works from any stack without rewriting.
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