Agent graph omits tools from Toolsets (MCP, skills) and draws llmagent sub-agents without edges #1726

🔴 Required Information

Describe the Bug:
The agent graph generator leaves out part of an llmagent tree:

1. Tools from Toolsets are not drawn. buildGraph only reads the agent's Tools field, so tools that come from a tool.Toolset (for example an MCP toolset or a skills toolset) never appear in the graph:
   https://github.com/google/adk-go/blob/8c7ab1baf9bc/server/adkrest/internal/services/agentgraphgenerator.go#L292
2. Sub-agents of an llmagent are drawn without an edge to their parent. buildGraph visits SubAgents() only to draw their nodes, so a coordinator and its sub-agents show up as unconnected nodes:
   https://github.com/google/adk-go/blob/8c7ab1baf9bc/server/adkrest/internal/services/agentgraphgenerator.go#L304-L309

adk-python handles both cases in agent_graph.py: it draws tools from await agent.canonical_tools(), which resolves toolsets, and it draws an edge from a non-workflow agent to each of its sub-agents (draw_edge(agent.name, sub_agent.name)).

Steps to Reproduce:

1. Save the minimal reproduction code below as main.go in a new module (go mod init graphrepro && go get google.golang.org/adk/[email protected]). It uses a fake model that is never called, so no API key is needed.
2. Run go run . web -port 8097 api -include_debug_api
3. Request the graph: curl http://localhost:8097/api/dev/apps/root/build_graph_image
4. Or open the web UI (go run . web -port 8097 api -include_debug_api webui) and look at the graph panel.

Expected Behavior:
The graph shows root connected to direct_tool, to toolset_tool (from the toolset) and to its sub-agent helper.

Observed Behavior:
Only direct_tool is connected. toolset_tool is missing and helper is a loose node:

digraph AgentGraph {
	bgcolor="#ffffff";
	rankdir=LR;
	root->direct_tool[ arrowhead=none, color="#3c4043" ];
	direct_tool [ color="#3c4043", fontcolor="#3c4043", label="🔧 direct_tool", shape=box, style=rounded ];
	helper [ color="#3c4043", fontcolor="#3c4043", label="🤖 helper", shape=ellipse, style=rounded ];
	root [ color="#3c4043", fontcolor="#3c4043", label="🤖 root", shape=ellipse, style=rounded ];

}

Environment Details:

• ADK Library Version: v2.5.0 (the code is unchanged on main at 8c7ab1b)
• OS: macOS
• Go Version: go1.26.6 darwin/arm64

Model Information:

• Which model is being used: N/A (the reproduction uses a fake model; originally found with Gemini agents using an MCP toolset and sub-agents)

🟡 Optional Information

Regression:
Unknown.

Minimal Reproduction Code:

package main

import (
	"context"
	"iter"
	"log"
	"os"

	"google.golang.org/adk/v2/agent"
	"google.golang.org/adk/v2/agent/llmagent"
	"google.golang.org/adk/v2/cmd/launcher"
	"google.golang.org/adk/v2/cmd/launcher/full"
	"google.golang.org/adk/v2/model"
	"google.golang.org/adk/v2/tool"
	"google.golang.org/adk/v2/tool/functiontool"
)

// fakeModel is never called: rendering the graph does not need a model.
type fakeModel struct{}

func (fakeModel) Name() string { return "fake" }
func (fakeModel) GenerateContent(context.Context, *model.LLMRequest, bool) iter.Seq2[*model.LLMResponse, error] {
	return func(func(*model.LLMResponse, error) bool) {}
}

type args struct{}

func newTool(name string) tool.Tool {
	t, err := functiontool.New(functiontool.Config{Name: name, Description: name},
		func(agent.Context, args) (string, error) { return "", nil })
	if err != nil {
		log.Fatal(err)
	}
	return t
}

// staticToolset stands in for an MCP toolset (mcptoolset) or a skills toolset.
type staticToolset struct{}

func (staticToolset) Name() string { return "my_toolset" }
func (staticToolset) Tools(agent.ReadonlyContext) ([]tool.Tool, error) {
	return []tool.Tool{newTool("toolset_tool")}, nil
}

func main() {
	helper, err := llmagent.New(llmagent.Config{Name: "helper", Description: "helper", Model: fakeModel{}})
	if err != nil {
		log.Fatal(err)
	}

	root, err := llmagent.New(llmagent.Config{
		Name:      "root",
		Model:     fakeModel{},
		Tools:     []tool.Tool{newTool("direct_tool")},
		Toolsets:  []tool.Toolset{staticToolset{}},
		SubAgents: []agent.Agent{helper},
	})
	if err != nil {
		log.Fatal(err)
	}

	cfg := &launcher.Config{AgentLoader: agent.NewSingleLoader(root)}
	if err := full.NewLauncher().Execute(context.Background(), cfg, os.Args[1:]); err != nil {
		log.Fatal(err)
	}
}

Additional Context:

• Resolving toolsets needs an agent.ReadonlyContext (Toolset.Tools(ctx)), and the graph endpoint has no invocation. An MCP toolset would also have to connect to its server just to list tools. If that is not desirable, drawing the toolset itself as a single node (by Toolset.Name()) would already make it visible.
• Related: Agent graph fails to render when a workflow agent is nested inside a sequential or loop agent #1723 (nested workflow agents break the graph).

How often has this issue occurred?:

• Always (100%)