The A2A (Agent2Agent) protocol announced by Google Cloud in April 2025 is the most credible 2026 candidate for an open standard for cross-vendor agent-to-agent interoperability, with backing from 50+ partners across the enterprise software ecosystem (Salesforce, SAP, ServiceNow, MongoDB, Atlassian, and others). The protocol layer covers what MCP (Model Context Protocol) does not: MCP is for agent-to-tool communication, A2A is for agent-to-agent communication. The two protocols are designed to be complementary rather than competing. A2A's adoption trajectory through 2026 will determine whether broker-mediated multi-agent patterns become the cross-vendor default; current trajectory points to deployment-grade stability in the second half of 2026, with widespread enterprise adoption following in 2027. Enterprises selecting agent platforms in 2026 should require A2A roadmap commitments from any vendor whose product will participate in cross-vendor agent workflows.

A2A protocol piece. 60-day review cadence given active protocol evolution. Watches: (1) A2A specification version updates and reference implementation maturity, (2) inflection in vendor support beyond the announcement-day partner set (e.g., Anthropic and Microsoft have not committed to A2A as of April 2026; their positioning may shift), (3) competing or parallel standards (Microsoft has hinted at alternative inter-agent primitives in their Copilot platform; Anthropic has internal context-isolation primitives that may or may not converge on A2A), (4) regulatory positioning (the EU AI Act's Article 9 risk-management requirements may begin to reference A2A or equivalent in 2026-2027 enforcement guidance).