MCP and A2A: what is implemented

The two transport protocols an agent uses with the world, as agent-runtimes implements them today, in both languages, as a client and as a server; and the gaps against their latest specifications. Inventory taken on 2026-10-12 from the code (PR-01); it moves with the change that moves a row.

Versions

Package	Pin	Protocol version
Python mcp	mcp>=2,<3 (2.1.1 installed)	2026-07-28 (mcp.types.LATEST_PROTOCOL_VERSION)
Python fastmcp-slim[client]	>=4.0.0b3,<5	the transports of the MCP client
Python pydantic-ai-slim[mcp]	>=2.35.0,<3	MCPToolset, which DisplayMCPToolset extends
TypeScript @modelcontextprotocol/sdk	^1.32.1 (1.32.1 installed)	2025-11-25 (MCP_SPECIFICATION_VERSION in src/protocols/mcp)
Python fasta2a	>=2.1.1,<2.2 (extra a2a)	A2A 1.0, JSON-RPC binding
Python a2a-sdk	>=0.3.0,<0.4.0	used by tests only, to validate agent cards
TypeScript @a2a-js/sdk	^1.3.0	A2A 1.0 method names

The two MCP SDKs speak different protocol versions: Python 2026-07-28, TypeScript 2025-11-25. The Python server answers a TypeScript client at 2025-11-25 (the initialize handshake), held by test_protocols_mcp_two_languages.py.

MCP

agent-runtimes is an MCP client, in Python, for tools. It serves no MCP server of its own in either language: /api/v1/mcp/proxy/… is a REST proxy and /api/v1/mcp-ui/… an agent chat transport.

Feature	Python client	TypeScript client	Server (either)
stdio	yes (mcp/lifecycle.py)	through McpClient.connect({ transport })	no
Streamable HTTP	yes (mcp/tools.py, mcp/datalayer_gateway.py, mcp/google_workspace.py)	yes (McpClient in src/protocols/mcp/client.ts)	no
Legacy HTTP+SSE	not used	no	no
Tools: list, call	yes (mcp/lifecycle.py, mcp/displays.py)	yes (McpClient.listTools, callTool; the page still manages a runtime's servers through its REST routes)	no
A tool's display (_meta["reactor.ui/display"])	yes (mcp/displays.py)	drawn from the runtime's stream	no
Structured content, output schemas, annotations	read (protocols/mcp/reading.py, the conformance's readings); through pydantic-ai for an agent's tools	read (toolReading, callReading; a structured answer checked against its output schema by the SDK)	no
Resources, prompts	read (resource_reading, prompt_reading); not wired to an agent	read (readResource, getPrompt)	no
Sampling, elicitation, roots	no	no	no
Logging, progress, cancellation, completions	no	no	no
Tasks	library only (mcp/tasks.py: call as a task, get, cancel, list, status)	no	no
Authorization	OAuth 2.1 flow (mcp/auth/: RFC 9728 and 8414 discovery, dynamic registration, PKCE, client ID metadata documents) behind routes/mcp_auth.py; the bearer sent comes from headers (the gateway's, a configured header)	no	—
MCP UI (ui://)	—	drawn from activity messages (MCPUIPlugin); text/html and remote DOM refused by design	ui:// resources built for the mcp-ui chat transport (transports/mcp_ui.py), not an MCP server

On Datalayer, an application reaches MCP servers through the MCP gateway, which holds the credentials and decides each call (the pap and pact toolsets are served there); the runtime holds the gateway's bearer only.

A2A

agent-runtimes serves A2A in Python (fasta2a) and calls A2A agents from both languages.

Feature	Python client	Python server	TypeScript client
Binding	JSON-RPC (fasta2a.client.A2AClient, subagents/a2a.py)	JSON-RPC 2.0, POST at /api/v1/a2a/agents/{id}/ (routes/a2a.py)	JSON-RPC (@a2a-js/sdk in runtimes/browser/a2aPeer.ts; A2AAdapter.ts, useA2A.tsx by hand)
HTTP+JSON, gRPC	no	no	no
Agent card at /.well-known/agent-card.json	yes	yes	yes
SendMessage / message/send	—	yes	yes (useA2A.tsx)
SendStreamingMessage / message/stream	yes	yes (SSE)	yes (A2AAdapter.ts, a2aPeer.ts)
GetTask	yes (polling)	yes	yes
CancelTask	yes	yes	no: A2AAdapter cancels through the runtime's /api/v1/a2a/terminate
SubscribeToTask	no	yes	no
ListTasks	no	answers unsupported	no
Push notification configs	no	answers not supported; the card says pushNotifications: false	no
Authenticated extended card	no	no	no
Security	bearer	the card's Bearer scheme; an application's route gated (apps/a2a.py); signed requests verified (Web Bot Auth)	bearer
Extensions	the orchestration extension's routes	A2A-Extensions negotiated, required extensions checked; card extensions from the A2A plugin's points and the Appspec (Agent protocols)	A2A-Extensions header

A harness over A2A (SH-15)

An application whose agent is a harness (Claude Code, Codex, any ACP agent) is served by the same worker, plus: its plan as one artifact replaced as it progresses; each file it changed as a text/x-diff artifact named by its path, with the orchestration extension's artifactProvenance (sha256: of the body, when, from what); its tool calls as working statuses carrying {kind, title, status}; a permission as input-required carrying approvalRequest {toolCall, options, expiresAt}, answered on the same task by a message naming it — datalayer.approval {optionId}, or one whole word (yes, no, always, an option's name) from a caller without the extension; reasoning never an artifact. The card's skills come from the harness spec, text/x-diff among its output modes. The extension's schema describes these three fields since 15 October 2026.

Gaps

1. No MCP server in agent-runtimes; an application is reached over A2A, AG-UI or Vercel AI, never as an MCP server.
2. MCP beyond tools: resources, prompts, sampling, elicitation, roots, logging, progress, completions and cancellation are not wired, although pydantic-ai supports several. Each becomes mcp.extension.<name> when it is.
3. MCP OAuth stores a token after the flow but does not attach a stored token when connecting: on Datalayer the gateway holds credentials instead.
4. MCP tasks are library code that only tests call.
5. The TypeScript MCP client (McpClient, on @modelcontextprotocol/sdk) is a library: no page of the package uses it yet.
6. Two MCP versions across the SDKs (2026-07-28, 2025-11-25).
7. A2A server: JSON-RPC only (no HTTP+JSON binding, which PACT 1.0 requires of a Brand); push notifications and ListTasks answer unsupported; no authenticated extended card.
8. TypeScript A2A: cancels through a runtime route rather than CancelTask; no client resubscribes.