Protocols Feature Matrix

Table of Contents

This matrix compares the six protocols profiled in this section, A2A, ACP, AG-UI, Agent Host Protocol, AGENTS.md, and MCP, so the whole interoperability stack can be read in one table.

The six do not compete, they stack (repo-to-agent, editor-to-agent, agent-to-frontend, agent-to-tool, agent-to-agent, client-to-session), and adoption falls with every step up that stack, which is why I call AGENTS.md and MCP defaults, ACP a rising bet, AG-UI the quiet winner by raw download volume, AHP a bet underwritten by VS Code’s own distribution, and A2A an enterprise convention the coding-agent world can keep ignoring.

Legend: ✓ supported, ✗ not supported, ~ partial or conditional, ? not verified. Each column links to the full research note; every cell below traces to a source cited there or in the references.

The matrix
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Feature	A2A	ACP	AG-UI	Agent Host Protocol	AGENTS.md	MCP
Kind	wire protocol	wire protocol	wire protocol	wire protocol	file convention	wire protocol
Originated by	Google (2025-04)	Zed, with JetBrains	CopilotKit (2025-05)	Microsoft (2026-03)	OpenAI-led (2025-08)	Anthropic (2024-11)
Steward	AAIF (2026-08), TSC	vendor-neutral org	CopilotKit, no foundation	Microsoft, no foundation	AAIF	AAIF
Spec license	Apache-2.0	Apache-2.0	MIT	MIT	MIT	MIT
Maturity	v1.0.1 (2026-05)	version 1, v2 draft, remote WIP	packages 1.0.1 (2026-09-29), spec 1.0	v1.0.0 (2026-10-02)	unversioned, de facto standard	dated revisions (2026-07-28)
What it connects	agent-to-agent	editor-to-agent	agent-to-frontend	client-to-session	repo-to-agent	app-to-tools
Adoption in this section	✗ none native	~ growing (OpenCode, JetBrains, Zed)	✗ none native (CopilotKit ecosystem outside this index)	~ VS Code reference host	~ most; Claude Code shipped native support 2026-09-18	✓ near-universal
Transport or location	HTTP, gRPC, JSON-RPC	JSON-RPC over stdio	SSE, WebSockets, webhooks	URI channels on a standalone sessions server	Markdown at repo root	JSON-RPC, stdio to sse
Official SDKs	✓ six	✓ five	✓ three (TypeScript, Python, .NET)	✓ six	✗ none needed	✓ any language
Criticism recorded	redundant with MCP	sprawl, flattened UX	single-vendor origin, pre-1.0 churn	more sprawl, one vendor’s governance	weak efficacy evidence	tool poisoning, supply chain

Reading the matrix
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Governance converged faster than adoption, and every protocol that went neutral did so only after it had already won or stalled. Google donated A2A to the Linux Foundation in June 2025, Anthropic and OpenAI donated MCP and AGENTS.md to the AAIF the same day (2025-12-09), and A2A itself joined the AAIF as a Growth Stage project on 2026-08-27, so three of the six now sit in the same foundation. ACP is stewarded by Zed and JetBrains under a vendor-neutral organization with no foundation home, AHP stays inside the microsoft org with repo-first governance, and AG-UI, the newest column, is the one still closest to its corporate parent, CopilotKit, which also sells commercial support for it, so the stack now has three non-foundation columns, and ACP remains the one compounding fastest in editors. I read this as governance following adoption, not causing it.

Adoption falls as the protocol climbs the stack, and the file convention beat every wire protocol to default status. MCP is table stakes across the harness and surface matrices; AGENTS.md counts more than 60,000 carrying projects, and its one glaring holdout closed on 2026-09-18 when Claude Code shipped native support in 2.1.277; ACP rides OpenCode, JetBrains, Zed, and a Copilot CLI preview; A2A has no native speaker among this index’s harnesses, only community setups near Gemini CLI; AHP is just over six months old with 384 stars and a fresh 1.0.0 spec (2026-10-02) as of 2026-10-04, and its reference host ships inside VS Code while the VS Code team has said publicly it is rebuilding its agent infrastructure on the protocol. The SDK download ratio recorded in the A2A note, 10.9M monthly versus 257M for MCP, is the gap in one number, and AHP’s roughly 682k combined downloads (about 269.8k crates plus 412.4k npm, weekly npm downloads doubling or better through late September) show the same order-of-magnitude distance from the top. AG-UI is the anomaly that proves the rule: none of this index’s harnesses or surfaces speak it natively, yet its SDKs moved about 14.6M combined npm downloads in the month ending 2026-10-01, because the frontend layer is where end-user products live even though coding tools never touch it.

The consolidations the notes record happened in opposite corners, and neither touched the other’s territory. IBM’s Agent Communication Protocol (the other ACP, the source of the name collision) merged into A2A in August 2025 under LF AI and Data. JetBrains folded its internal Junie protocol into the Agent Client Protocol instead. Both mergers cut the rival count in 2025 while leaving the enterprise-remote and local-editor layers separate.

Each recorded criticism is a different species of doubt, and the pattern favors the incumbents. MCP is criticized for risks it creates (tool poisoning, rug pulls, a 10,000-server supply chain); AGENTS.md for whether it helps at all (an ETH Zurich study found no success-rate gain and over 20% added inference cost, against Vercel’s counter-evidence); ACP for UX flattening and protocol sprawl; AHP for being a fifth protocol to track under one vendor’s governance; A2A for whether it should exist at all; AG-UI for provenance and maturity, a CopilotKit-originated stack that only graduated to 1.0.0 packages on 2026-09-17 and whose adoption story lives in vendor blogs. The mature protocols get attacked for their risks, the young ones for their reason to exist or their provenance.

Choosing from the matrix
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Exposing tools or data to any agent: MCP, the default with a supply-chain asterisk.
Choosing harness and editor independently: ACP, the cheapest portability insurance in the stack.
Streaming an agent into a product frontend: AG-UI, accepting CopilotKit’s stewardship and dated-release churn (1.0.0 only landed 2026-09-17).
Any repository an agent touches: commit a short AGENTS.md, commands and conventions first.
Delegating work across vendors or departments: A2A, provided both sides run enterprise platforms.
Attaching a second client to a live agent session: AHP, provided v1.0.0’s short track record and Microsoft’s stewardship are acceptable.
Wiring sub-agents inside one framework: none of these, native primitives or MCP are simpler.

Changes
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2026-08-24 - Created with the stacking thesis and recorded protocol consolidations.
2026-09-16 - Extended from five to six columns with AG-UI, re-sorted alphabetically, and updated the governance, adoption, criticism, and choosing prose for the agent-to-frontend layer.
2026-09-18 - AG-UI maturity cell updated to 1.0.0 packages and a published 1.0 specification, and stars and download figures refreshed in the AG-UI and AHP prose.
2026-09-21 - AGENTS.md adoption cell updated after Claude Code shipped native support in 2.1.277 (2026-09-18), ending the holdout the cell recorded; AG-UI and AHP download and star figures refreshed in the prose.
2026-09-24 - Removed the verification preamble line on owner request.
2026-09-24 - Re-verification: refreshed star and download figures in the AG-UI and AHP prose (AG-UI about 12.0M combined monthly downloads, AHP 363 stars and roughly 369k combined downloads).
2026-09-25 - Re-sorted the columns by member title (ACP and Agent Host Protocol precede A2A under the case-insensitive title ordering the other matrices use); no cell content changed.
2026-09-27 - Re-verification: refreshed the AG-UI and AHP figures in the prose (AG-UI about 12.9M combined monthly downloads, AHP 368 stars and roughly 410k combined downloads); no cells changed.
2026-09-29 - Re-verification: refreshed the AG-UI and AHP figures in the prose (AG-UI about 12.3M combined monthly downloads, AHP 375 stars and roughly 433k combined downloads); no cells changed.
2026-09-29 - Re-sorted the columns to the section-wide case-insensitive title sort (A2A, ACP, AG-UI, Agent Host Protocol, AGENTS.md, MCP), correcting the 2026-09-25 arrangement that had placed ACP and Agent Host Protocol ahead of A2A and AG-UI; the category page list was brought to the same order in the same run; no cell content changed.
2026-10-02 - Re-verification: refreshed the AG-UI and AHP figures in the prose (AG-UI about 13.1M combined monthly downloads, AHP 382 stars and roughly 597k combined downloads); no cells changed.
2026-10-03 - AHP maturity cell moved to v1.0.0 (2026-10-02) and the AG-UI maturity cell to packages 1.0.1 (2026-09-29), with star and download figures refreshed in the prose (AG-UI about 14.6M combined monthly downloads, AHP 384 stars and roughly 681k combined downloads).