MCP. A2A. Apps SDK. The protocol war is real, and the answer affects how you build for the next 5 years. Plain English, honest take.
Model Context Protocol
Open protocol for connecting AI agents to external tools, data sources, and apps. The most likely-to-stick tool-connection protocol of 2026, with cross-vendor support.
If you're building agents in 2026, MCP is the most likely-to-stick way to give them tools.
Agent-to-Agent Protocol
Protocol for agents to discover and delegate to other agents. Useful for multi-agent systems and sub-task handoff.
Important if you're building multi-agent systems. Watch this space.
OpenAI Apps SDK
OpenAI's framework for building apps inside ChatGPT. It uses MCP-style servers for tools and adds ChatGPT-specific UI/widget rendering.
If your audience lives in ChatGPT, this is where you ship integrations. It builds on MCP rather than replacing it.
Open Agent Protocol
Vendor-neutral REST API for invoking agents. Less momentum than MCP/A2A.
Niche right now; could become relevant if vendor protocols fragment.
Multiple competing protocols. Multiple vendors. Each pushing their own. Familiar story.
Honest take: MCP has the most momentum, the cleanest spec, and the broadest cross-vendor support — including from Microsoft and OpenAI. Apps SDK builds on MCP-style servers rather than replacing them. If you're building agents that need tools, default to MCP and only diverge when you have a specific reason.
A2A is interesting if you're orchestrating multiple agents. Apps SDK is essential if your audience is in ChatGPT. Don't over-think it; pick the one closest to your distribution channel and ship.
Adoption ratings here are editorial, not measured. They reflect a qualitative read of vendor support, production usage, and developer tooling activity at time of review.