chore: import upstream snapshot with attribution

contributing/samples/integrations/antigravity_agent/.gitignore

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+# Workspace the agent writes generated games into at runtime.
+game_repo/
+# Conversation trajectories persisted across turns.
+trajectories/

contributing/samples/integrations/antigravity_agent/README.md

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+# Antigravity SDK Game Developer Agent
+
+## Overview
+
+This sample wraps a pre-configured [Google Antigravity SDK](https://pypi.org/project/google-antigravity/)
+agent as a native ADK agent using `AntigravityAgent`, configured as a
+**game developer** that writes small, runnable browser games into the
+`game_repo/` workspace as single self-contained HTML files. Each turn is
+delegated to the Antigravity
+runner, and its trajectory steps (model text, tool calls, and tool responses)
+are streamed back as standard ADK events recorded in the session.
+
+`AntigravityAgent` must be used as a **standalone root agent** (the SDK currently
+only supports local mode). See the
+[package README](../../../../src/google/adk/labs/antigravity/README.md)
+for the full setup, limitations, and API details.
+
+## Prerequisites
+
+- Install the SDK: `pip install "google-adk[antigravity]"`
+- Set a Gemini API key: `export GEMINI_API_KEY="your-api-key"`
+  (required by the Antigravity SDK, which drives the model)
+
+The agent writes generated games into a `game_repo/` directory and persists
+conversation trajectories (for cross-turn resumption) into a `trajectories/`
+directory, both next to `agent.py` and created automatically on import.
+
+## Sample Inputs
+
+- `Create a playable Snake game.`
+
+  The agent writes a self-contained HTML implementation into `game_repo/` (e.g.
+  `game_repo/snake.html`, with inline CSS and JavaScript) using the built-in
+  `create_file` tool, then explains how to open it in a browser.
+
+- `Create a 2-player turn-based Artillery game with adjustable angle and power.`
+
+  The agent writes another self-contained HTML game (e.g.
+  `game_repo/artillery.html`) with canvas rendering and projectile physics.
+
+- `Create a Brick Breaker game.`
+
+  The agent writes a self-contained HTML implementation (e.g.
+  `game_repo/brick_breaker.html`) with a paddle, ball, and breakable bricks.
+
+## Graph
+
+Each turn, the wrapper delegates to the SDK agent's local Go harness and maps
+the trajectory steps it streams back into ADK events:
+
+```mermaid
+graph LR
+    Runner[ADK Runner] -->|prompt| Wrapper[AntigravityAgent]
+    Wrapper -->|send| SDK[Antigravity SDK Agent]
+    SDK -->|local mode| Harness[Go localharness]
+    Harness -->|steps| SDK
+    SDK -->|steps| Wrapper
+    Wrapper -->|ADK events| Runner
+```
+
+## How To
+
+The wrapper takes a `google.antigravity.LocalAgentConfig` via the `config`
+argument:
+
+```python
+root_agent = AntigravityAgent(
+    name="antigravity_game_developer",
+    description="...",
+    config=_sdk_config,
+)
+```
+
+The SDK agent enables its built-in file tools by default; the
+`policy.workspace_only([...])` policy keeps all file reads and writes contained
+to `game_repo/`. Internally, `AntigravityAgent._run_async_impl` deep-copies the
+config per turn (the SDK's `AsyncExitStack` is single-use), enters a fresh SDK
+`Agent`, sends the latest user prompt, and converts each streamed Step into ADK
+events.
+
+The root-only restriction is enforced at construction time: giving the agent
+`sub_agents`, or adopting it under a parent agent, raises a `ValueError`.

contributing/samples/integrations/antigravity_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.
+
+"""Game-developer agent that writes browser games as self-contained HTML.
+
+Wraps a Google Antigravity SDK agent as an ADK agent. See the package README
+for setup and details.
+"""
+
+import os
+
+from google.adk.labs.antigravity import AntigravityAgent
+from google.antigravity import LocalAgentConfig
+from google.antigravity.hooks import policy
+
+# 1. Configure the Google Antigravity SDK game-developer agent. The
+#    workspace-scoped policy lets it create and edit files inside the game_repo
+#    workspace (built-in file tools are allowed there) while keeping writes
+#    contained.
+_sample_dir = os.path.dirname(os.path.abspath(__file__))
+_workspace = os.path.join(_sample_dir, "game_repo")
+_trajectories = os.path.join(_sample_dir, "trajectories")
+os.makedirs(_workspace, exist_ok=True)
+os.makedirs(_trajectories, exist_ok=True)
+_sdk_config = LocalAgentConfig(
+    system_instructions="""\
+You are a senior web game developer. You build small, runnable games on request \
+as a single self-contained HTML file with inline CSS and JavaScript (no external \
+assets or third-party dependencies). Write the HTML file into the allowed \
+workspace using a clean absolute filesystem path.
+
+Build the file incrementally: first create it with a minimal skeleton (HTML \
+structure, canvas, and empty script), then add CSS and the game logic over a \
+few substantial edits. Group each edit around a complete feature (e.g. all \
+styling, then rendering, then input handling) rather than many tiny changes, \
+but do not attempt to write the entire game in one step.
+
+After the file is complete, briefly explain how to play it (open the .html file \
+in a browser).""",
+    workspaces=[_workspace],
+    policies=[*policy.workspace_only([_workspace])],
+    save_dir=_trajectories,
+)
+
+# 2. Wrap the SDK config as a standalone ADK root agent.
+root_agent = AntigravityAgent(
+    name="antigravity_game_developer",
+    description=(
+        "Builds small, runnable games inside the game_repo workspace via the"
+        " Antigravity SDK."
+    ),
+    config=_sdk_config,
+)