Google’s Antigravity team shipped Custom Agents on August 12, bringing first-class support for file-based agent definitions to both the Antigravity 2.0 desktop app and the Antigravity CLI, with IDE support to follow. If you’ve been juggling one bloated system prompt trying to cover testing conventions, dependency rules, and every specialized workflow your team runs, this is built specifically to solve that problem. Here’s how to define one, based directly on Google’s official announcement post.

Why Custom Agents Exist

Per the official Antigravity blog post, general-purpose coding assistants run into two recurring problems: they lack specialization (a generic assistant doesn’t know your project’s specific testing conventions or dependency rules unless you re-explain them every session), and loading a massive monolithic prompt with all your guidelines into every chat turn wastes token budget.

Custom agents are Google’s answer: specialized, file-based configurations that define a role with its own scoped instructions, tools, and constraints — keeping your active context clean and giving you a predictable partner for a specific kind of task. Google is explicit that this doesn’t replace Skills or dynamic subagents — it layers on top of them, letting you specify which subset of skills, MCP servers, and hooks are actually relevant to a given specialization, on top of customizing the system instructions and default tools directly.

Where Agent Files Live

Custom agents are defined as Markdown files with a YAML frontmatter header — the same general convention used for Skills, enabling the same kind of progressive discovery. Per the official announcement, you save these files in one of two places:

  • Project-scoped: .agents/agents/ in your local workspace. Committing these to your repo means every teammate who checks it out automatically gets the same standardized agents — no manual setup required on their end.
  • Global/user-scoped: ~/.gemini/config/agents/

A Basic Agent Definition

Google’s own announcement post includes this working example — a “dependency modernizer” agent:

---
name: dependency-modernizer
description: Helps upgrade local packages and verify that project tests pass.
model: flash
tools:
  - view_file
  - replace_file_content
  - manage_task
  - run_command
---

# Core Instructions
You are a dependency modernizer. Your job is to check configuration files,
update target dependencies, run test suites, and verify the build passes.

The frontmatter block tells Antigravity how to run the agent (which model to use, which tools it’s scoped to); the Markdown body below the frontmatter compiles directly into that agent’s system prompt. Google’s post points to their docs for the full list of fields the frontmatter supports — this article only confirms the fields shown directly in the official example above, so check the docs before assuming additional keys work the same way.

What Sets Antigravity’s Version Apart

Google frames three specific capabilities as differentiators versus how other tools in this space implement custom/subagents.

1. Execution Symmetry

In many other tools, custom agents can only run as subagents — you interact with a main default agent, and it decides when to spawn your custom agent behind the scenes; you can’t launch your custom agent directly as your primary session.

Antigravity adds two flags to the frontmatter to control this:

# Add these to the YAML frontmatter:
mainAgent: true
subagent: true

With both set, the same agent definition works two ways:

  • As a Main Agent: Select dependency-modernizer directly from the dropdown in the Antigravity 2.0 GUI, or run it via the CLI with agy --agent dependency-modernizer. Its core instructions get compiled directly into the system prompt, and you talk to it directly as your primary session.
  • As a Subagent: The same agent can also be dynamically called as a tool by a coordinator agent, the same way any standard subagent would be.

2. Scoped Safety Policies (commandExecutionPolicy)

Google calls out a specific pain point with running agents that execute command-line operations — dependency installs, test suites, and the like: an all-or-nothing permission model is either too loose (risking unverified code execution) or too strict (constant approval-prompt fatigue).

Beyond the basic all-or-nothing permission levels other tools support (like acceptEdits or bypassPermissions, which Antigravity also supports), Antigravity adds a dedicated execution filter:

# Add this to the YAML frontmatter:
permissionMode: acceptEdits
commandExecutionPolicy: auto

Setting commandExecutionPolicy: auto lets the agent run standard test and compilation commands autonomously in the background, while higher-risk commands — like file deletion — stay gated behind manual approval. For something like the dependency-modernizer example, this means it can iterate through trial-and-error test cycles without stopping to ask permission at every step, while destructive operations still require your sign-off.

3. Nested Lifecycle Hooks

Antigravity’s hooks schema goes further than the basic subagent-scoped hooks other tools support, per Google’s post. You can attach setup and verification checks at precise points in the agent’s execution:

# Add these hooks to the YAML frontmatter:
hooks:
  PreInvocation:
    - type: command
      command: scripts/setup.sh
  PreToolUse:
    - matcher: run_command
      hooks:
        - type: command
          command: scripts/verify-local-env.sh

In this example, PreInvocation runs a setup script before the agent starts working, and PreToolUse (scoped with a matcher) intercepts every time the agent tries to run a terminal command, running a verification script first. Google notes there are additional hook placement points documented at /docs/hooks — refer there for the full schema rather than assuming these two are the only available hook types.

Getting Started

To try this yourself: create a .agents/agents/ directory in your project (or ~/.gemini/config/agents/ for a global agent), add a Markdown file following the frontmatter + instructions pattern shown above, and reference Google’s Custom Agents Guide for the complete list of supported frontmatter fields before building anything more complex than the basic example. IDE support is coming “shortly” per the announcement but wasn’t available at time of writing — start with the Desktop App or CLI.

Sources

  1. Introducing Custom Agents — Antigravity Blog

Researched by Searcher → Analyzed by Analyst → Written by Writer Agent (Sonnet 4.6). Full pipeline log: subagentic-20260815-2000

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