Hosting and the Capstone App — Microsoft Agent Framework in Go

Everything so far was go run against one process. A real system is agents that reach each other over the network and UIs that stream their output. This finale covers hosting in Go — A2A, MCP, AG-UI — and then the end-to-end capstone: a coupled client/server pair where one host agent discovers remote specialists and uses them as tools.

Three transports, one agent

The pattern across all three is the same: the agent is the plain foundryprovider agent from the earlier lessons — only the transport is new.

MCP lets an agent borrow tools that live in another service. Connect over streamable HTTP, list the catalog, hand it to the agent:

session, _ := mcptool.Connect(ctx, &mcp.StreamableClientTransport{
    Endpoint: "https://learn.microsoft.com/api/mcp",
})
defer session.Close()
tools, _ := mcptool.ListTools(ctx, session)   // []tool.Tool — indistinguishable from local

From the agent’s point of view, remote MCP tools are just agent.Config.Tools. The one networked step stays isolated so the offline test can build the identical agent with fake tools.

AG-UI exposes an agent over HTTP+SSE for a UI to drive. Wrapping is one call:

mux := http.NewServeMux()
mux.Handle("/", aguiprovider.NewJSONHTTPHandler(a, aguiprovider.HandlerConfig{}))
http.ListenAndServe(":8888", mux)

NewJSONHTTPHandler turns each incoming POST into an agent run and streams the run’s updates back as AG-UI events. The agent never knows it is being served over HTTP.

A2A and the capstone: agents as tools

The capstone is the payoff — the Go port of the SDK’s 05-end-to-end/a2a_client_server sample. One or more server processes each host a specialized Foundry agent over A2A; one client process discovers them and uses them as tools from a local host agent.

Each server publishes two routes — the agent card (discovery) and the JSON-RPC endpoint:

card.SupportedInterfaces = []*a2a.AgentInterface{
    a2a.NewAgentInterface(url, a2a.TransportProtocolJSONRPC),
}
handler := a2asrv.NewHandler(a2aprovider.NewExecutor(hostAgent, a2aprovider.ExecutorConfig{}),
    a2asrv.WithExtendedAgentCard(card))
mux.Handle("/", a2asrv.NewJSONRPCHandler(handler))
mux.Handle(a2asrv.WellKnownAgentCardPath, a2asrv.NewStaticAgentCardHandler(card))

The client resolves each card, builds an A2A client from it, and wraps the whole remote agent into a single tool:

card, _ := agentcard.DefaultResolver.Resolve(ctx, url)
client, _ := a2aclient.NewFromCard(ctx, card)
remote := a2aprovider.NewAgent(client, a2aprovider.AgentConfig{
    Config: agent.Config{Name: card.Name, Description: card.Description},
})
tools = append(tools, agenttool.New(remote, agenttool.Config{}))

agenttool.New(remoteAgent, …) turns a whole remote agent into one callable tool. The host model decides which specialist to call — invoice, policy, or logistics — the same way it picks any function tool.

sequenceDiagram
    autonumber
    participant U as User
    participant H as Host agent (client)
    participant S as a2a_server (invoice/policy/logistics)
    U->>H: "Dispute on TICKET-XYZ987 — fewer t-shirts than ordered"
    Note over H: card → agenttool on the host agent
    H->>S: agenttool → JSON-RPC message/send
    S-->>H: specialist's answer (tool result)
    H-->>U: final answer

The card is the contract. The server sets the card’s Name, Skills, and interface URL; the client reads exactly those fields back to name the tool and dial the endpoint. That agreement is why this is a pair, not two unrelated programs. Break it — hand newMux a different URL than the client expects — and the discovery test catches it.

# three servers, one per specialist
go run ./tutorial/05-end-to-end/a2a_client_server/a2a_server --agentType invoice   --port 5000
go run ./tutorial/05-end-to-end/a2a_client_server/a2a_server --agentType policy    --port 5001
go run ./tutorial/05-end-to-end/a2a_client_server/a2a_server --agentType logistics --port 5002
# then the client
go run ./tutorial/05-end-to-end/a2a_client_server/a2a_client

Only the servers own the hosted model; the client owns its own orchestrating host agent. The offline tests assert the wiring — cards, routes, splitURLs — with fake credentials and no port; the live paths gate behind AF_LIVE=1.

The journey, in one line

That is the whole arc across both series: a provider + config became an agent; tools let it act; sessions let it remember; workflows and orchestration let several agents coordinate; and hosting turned the result into a networked service other agents call. The consistent discipline — factor construction out of main, take dependencies as parameters, test the wiring offline before the model — is exactly what let me port these samples faithfully and fix the bugs I hit upstream in agent-framework-go.

That is the whole framework, one lesson at a time. See my upstream Microsoft Agent Framework Go contributions.

Key takeaways

• The hosting pattern is uniform: the agent is the same plain foundryprovider agent from earlier lessons — only the transport is new (MCP, AG-UI, or A2A).
• MCP lets an agent borrow tools that live in another service; connected over streamable HTTP, remote tools are indistinguishable from local agent.Config.Tools, and the one networked step stays isolated so tests can substitute fake tools.
• AG-UI wraps an agent over HTTP+SSE with a single aguiprovider.NewJSONHTTPHandler; each POST becomes an agent run streamed back as AG-UI events, and the agent never knows it’s being served over HTTP.
• The capstone shows agents-as-tools over A2A: servers publish a card + JSON-RPC endpoint, the client resolves each card and wraps the whole remote agent into one tool with agenttool.New, and the host model picks a specialist like any function tool.
• The card is the contract — server-set Name, Skills, and interface URL are exactly what the client reads back — and the consistent discipline (factor construction out of main, take dependencies as parameters, test wiring offline) is what made faithful ports and upstream bug fixes possible.

Further reading

• A2A Client
• Advanced Workflows — Microsoft Agent Framework in Go
• agent-framework-go on GitHub