Every AI agent protocol guide in 2026 tells the same story: MCP for tools, A2A for messaging, problem solved. But spend a week building a multi-agent system and you'll hit a wall nobody's writing about — your agents can't remember together.
Context doesn't accumulate. Contradictions go undetected. Findings disappear between calls. This essay breaks down the third layer the agent stack is missing, why existing solutions don't fill it, and what a shared memory protocol needs to get right.
The next wave of AI products won't be defined by model quality alone. They'll be defined by continuity — how reliably systems learn, recall, and coordinate across sessions, users, and time.