Agent2Agent Protocol vs MCP Server Cards (SEP-2127)
They answer different questions: Agent2Agent Protocol is a step 6 (Coordinate) standard, MCP Server Cards (SEP-2127) step 1 (Discover), so they are not substitutes.
| Agent2Agent Protocol A2A | MCP Server Cards (SEP-2127) Server Card | |
|---|---|---|
| Lifecycle step | 6 Coordinate: Talk to another agent | 1 Discover: What is here, and where? |
| Answers | How do two independent agents, built on different frameworks, discover each other and delegate a task to completion? | How can a client learn what an MCP server offers before connecting to it? |
| Governed by | Linux Foundation (Technical Steering Committee: AWS, Cisco, Google, IBM Research, Microsoft, Salesforce, SAP, ServiceNow); Agentic AI Foundation (AAIF) since 17 August 2026 | Model Context Protocol project (SEP process, MCP governance) |
| Status (issuer's words) | Stable, v1.0.0 | SEP-2127 Final (2026-10-06), Extensions Track charter; the MCP Registry working group page still describes Server Cards as experimental |
| Phase | stable | candidate |
| Version | 1.0.1 (2026-05-28) | SEP-2127 (Final) (2026-10-06) |
| Form / transport | endpoint | document format |
| Found at | /.well-known/agent-card.json | GET <streamable-http-url>/server-card |
| Adoption | claimed — 150+ supporting organizations, claimed by the Linux Foundation on 9 April 2026 (https://www.linuxfoundation.org/press/a2a-protocol-surpasses-150-organizations-lands-in-major-cloud-platforms-and-sees-enterprise-production-use-in-first-year). Card-version censuses are separate measurements, kept with their frames and not averaged. Agenstry 2026-09-01 (https://agenstry.com/blog/state-of-the-agent-card): of 1,910 cards declaring protocolVersion, 65.5% (1,252) are 0.3.x and 25.5% (488) are 1.0.x. Emissar 2026-10-02 (https://emissar.ai/learn/a2a-adoption-2026): 276 cards on the Tranco top 1M (205 in v1.0 shape, 71 in v0.3 shape, 0 signed, 0 in the top 1,000 domains). APIs.io / Kin Lane 2026-09-21 (https://apievangelist.com/2026/09/21/a2a-adoption-is-421-of-27840-and-most-of-them-arrived-with-the-card/): 421 parseable cards out of 4,718 hosts probed (27,840 providers). rStation 2026-10-12 (https://rstation.ai/problems/ai_agent/papers/1): 661 cards on 2,164 x402 seller hosts (189 pass their attributed version and fail a check that assumes 1.0). | unmeasured |
| Evidence | 11 verified, 2 reported | 7 verified, 1 reported |
| Primary use case | Let agents talk to other agents | Let agents discover my API or service |
| Last verified | 2026-10-22 (current) | 2026-10-21 (current) |
| Specification | https://a2a-protocol.org/latest/specification/ | https://github.com/modelcontextprotocol/modelcontextprotocol/blob/main/seps/2127-mcp-server-cards.md |
When to use which
- Choose Agent2Agent Protocol when your question is “How do two independent agents, built on different frameworks, discover each other and delegate a task to completion?” and you accept its maturity: phase stable, adoption claimed (150+ supporting organizations, claimed by the Linux Foundation on 9 April 2026 (https://www.linuxfoundation.org/press/a2a-protocol-surpasses-150-organizations-lands-in-major-cloud-platforms-and-sees-enterprise-production-use-in-first-year). Card-version censuses are separate measurements, kept with their frames and not averaged. Agenstry 2026-09-01 (https://agenstry.com/blog/state-of-the-agent-card): of 1,910 cards declaring protocolVersion, 65.5% (1,252) are 0.3.x and 25.5% (488) are 1.0.x. Emissar 2026-10-02 (https://emissar.ai/learn/a2a-adoption-2026): 276 cards on the Tranco top 1M (205 in v1.0 shape, 71 in v0.3 shape, 0 signed, 0 in the top 1,000 domains). APIs.io / Kin Lane 2026-09-21 (https://apievangelist.com/2026/09/21/a2a-adoption-is-421-of-27840-and-most-of-them-arrived-with-the-card/): 421 parseable cards out of 4,718 hosts probed (27,840 providers). rStation 2026-10-12 (https://rstation.ai/problems/ai_agent/papers/1): 661 cards on 2,164 x402 seller hosts (189 pass their attributed version and fail a check that assumes 1.0).).
- Choose MCP Server Cards (SEP-2127) when your question is “How can a client learn what an MCP server offers before connecting to it?” and you accept its maturity: phase candidate, adoption unmeasured.
Read the full articles
- Agent2Agent Protocol: How do two independent agents, built on different frameworks, discover each other and delegate a task to completion?
- MCP Server Cards (SEP-2127): How can a client learn what an MCP server offers before connecting to it?