feat: Agent Registry in ADK

Client library for the Agent Registry API that allows users to discover, look up, and connect to agents and MCP servers cataloged in the registry.

Co-authored-by: Kathy Wu <wukathy@google.com>
PiperOrigin-RevId: 873073675

contributing/samples/agent_registry_agent/README.md

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+# Agent Registry Sample
+
+This sample demonstrates how to use the `AgentRegistry` client to discover agents and MCP servers registered in Google Cloud.
+
+## Setup
+
+1.  Ensure you have Google Cloud credentials configured (e.g., `gcloud auth application-default login`).
+2.  Set the following environment variables:
+
+```bash
+export GOOGLE_CLOUD_PROJECT=your-project-id
+export GOOGLE_CLOUD_LOCATION=global  # or your specific region
+```
+
+3.  Obtain the full resource names for the agents and MCP servers you want to use. You can do this by running the sample script once to list them:
+
+    ```bash
+    python3 agent.py
+    ```
+
+    Alternatively, use `gcloud` to list them:
+
+    ```bash
+    # For agents
+    gcloud alpha agent-registry agents list --project=$GOOGLE_CLOUD_PROJECT --location=$GOOGLE_CLOUD_LOCATION
+
+    # For MCP servers
+    gcloud alpha agent-registry mcp-servers list --project=$GOOGLE_CLOUD_PROJECT --location=$GOOGLE_CLOUD_LOCATION
+    ```
+
+4.  Replace `AGENT_NAME` and `MCP_SERVER_NAME` in `agent.py` with the last part of the resource names (e.g., if the name is `projects/.../agents/my-agent`, use `my-agent`).
+
+## Running the Sample
+
+Run the sample script to list available agents and MCP servers:
+
+```bash
+python3 agent.py
+```
+
+## How it Works
+
+The sample uses `AgentRegistry` to:
+- List registered agents using `list_agents()`.
+- List registered MCP servers using `list_mcp_servers()`.
+
+It also shows (in comments) how to:
+- Get a `RemoteA2aAgent` instance using `get_remote_a2a_agent(name)`.
+- Get an `McpToolset` instance using `get_mcp_toolset(name)`.

contributing/samples/agent_registry_agent/__init__.py

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+# Copyright 2026 Google LLC
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#     http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+
+from . import agent

contributing/samples/agent_registry_agent/agent.py

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+# Copyright 2026 Google LLC
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#     http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+
+"""Sample agent demonstrating Agent Registry discovery."""
+
+import os
+
+from google.adk.agents.llm_agent import LlmAgent
+from google.adk.integrations.agent_registry import AgentRegistry
+
+# Project and location can be set via environment variables:
+# GOOGLE_CLOUD_PROJECT and GOOGLE_CLOUD_LOCATION
+project_id = os.environ.get("GOOGLE_CLOUD_PROJECT")
+location = os.environ.get("GOOGLE_CLOUD_LOCATION", "global")
+
+# Initialize Agent Registry client
+registry = AgentRegistry(project_id=project_id, location=location)
+
+print(f"Listing agents in {project_id}/{location}...")
+agents = registry.list_agents()
+for agent in agents.get("agents", []):
+  print(f"- Agent: {agent.get('displayName')} ({agent.get('name')})")
+
+print(f"\nListing MCP servers in {project_id}/{location}...")
+mcp_servers = registry.list_mcp_servers()
+for server in mcp_servers.get("mcpServers", []):
+  print(f"- MCP Server: {server.get('displayName')} ({server.get('name')})")
+
+# Example of using a specific agent or MCP server from the registry:
+# (Note: These names should be full resource names as returned by list methods)
+
+# 1. Using a Remote A2A Agent as a sub-agent
+# TODO: Replace AGENT_NAME with your agent name
+remote_agent = registry.get_remote_a2a_agent(
+    f"projects/{project_id}/locations/{location}/agents/AGENT_NAME"
+)
+
+# 2. Using an MCP Server in a toolset
+# TODO: Replace MCP_SERVER_NAME with your MCP server name
+mcp_toolset = registry.get_mcp_toolset(
+    f"projects/{project_id}/locations/{location}/mcpServers/MCP_SERVER_NAME"
+)
+
+root_agent = LlmAgent(
+    model="gemini-2.5-flash",
+    name="discovery_agent",
+    instruction=(
+        "You have access to tools and sub-agents discovered via Registry."
+    ),
+    tools=[mcp_toolset],
+    sub_agents=[remote_agent],
+)