/harness-subagent

Use this skill to create and manage subagent entities in the agent-harness workspace. A subagent is a specialized agent definition — with a name, description, optional tool restrictions, and an optional model override — that harness renders into provider-native agent configuration files for every enabled provider.

What is a subagent entity?

A subagent entity is a single canonical Markdown source file under .harness/src/subagents/<subagent-id>.md. It contains:

• Required YAML frontmatter: name and description
• Optional frontmatter: provider-specific fields set via override sidecars (model, tools, handoffs)
• Body: Markdown prose that becomes the agent's system prompt

Harness renders one output artifact per enabled provider from that single source file. You edit the source once; all provider files stay in sync.

Provider output mapping

Claude Code

Claude Code loads agent files from .claude/agents/ at session start. Each file's description field tells Claude when to delegate to that agent. Claude routes tasks automatically, or the user can invoke an agent explicitly with @agent-<name> or the /agents command. Supported model values: sonnet, opus, haiku, a full model ID such as claude-sonnet-4-6, or inherit (default). Subagents run in isolated context windows and cannot spawn further subagents.

Official docs: https://code.claude.com/docs/en/sub-agents

OpenAI Codex CLI

Harness writes all enabled subagents into .codex/config.toml under [agents.<id>] entries. The description drives when Codex routes to that agent role; the body becomes developer_instructions. Codex also supports standalone TOML agent files in .codex/agents/ (outside of harness management), which are auto-discovered and take precedence over same-named inline entries.

Official docs: https://developers.openai.com/codex/config-reference

GitHub Copilot

Copilot discovers agent files in .github/agents/ at the repository level. The handoffs field is Copilot-specific and is supported in VS Code agent mode: it defines routing buttons that let users switch to another agent mid-conversation, optionally with a pre-filled prompt. Note: handoffs are currently not supported by the Copilot coding agent on GitHub.com — they are silently ignored there. Example handoff entry (VS Code only):

handoffs:
  - label: "Start Implementation"
    agent: implementation
    prompt: "Now implement the plan outlined above."
    send: true

When send: true is set, the handoff prompt submits automatically.

Official docs: https://docs.github.com/en/copilot/reference/custom-agents-configuration

Cursor

Cursor loads agent files from .cursor/agents/ (project) or ~/.cursor/agents/ (user). For compatibility, it also scans .claude/agents/ and .codex/agents/ (project-level) and their user-level equivalents. Project subagents take precedence when names conflict; .cursor/ takes precedence over .claude/ or .codex/.

The description field determines when the agent delegates to a subagent. Subagents can be invoked explicitly via /name syntax or automatically based on task complexity. The model field supports fast, inherit (default), or a specific model ID. When readonly: true, the subagent runs with restricted write permissions.

Cursor includes three built-in subagents (explore, bash, browser) that run automatically. Custom subagents cannot spawn further subagents (single-level only).

Official docs: https://docs.cursor.com/agent/subagents

Provider-specific frontmatter comparison

Provider-specific options (model, tools, handoffs) are set through override sidecar files, not in the canonical source frontmatter — see the Override sidecars section below.

Canonical source format

---
name: <human-readable display name>
description: <when this agent should be used — used by all providers>
---

You are a [role description]. When invoked:

1. [Step one]
2. [Step two]
3. [Step three]

Focus on [specific domain]. Do not [out-of-scope actions].

Only name and description are required in the canonical frontmatter. The body becomes the agent's system prompt for all providers.

Override sidecars

To set provider-specific options (model, tools, handoffs), create or edit the generated override YAML files:

• .harness/src/subagents/<subagent-id>.overrides.claude.yaml
• .harness/src/subagents/<subagent-id>.overrides.codex.yaml
• .harness/src/subagents/<subagent-id>.overrides.copilot.yaml

Example Claude override sidecar:

version: 1
options:
  model: haiku
  tools:
    - Read
    - Grep
    - Glob

Example Copilot override sidecar:

version: 1
options:
  model: gpt-4o
  tools:
    - search
    - fetch
  handoffs:
    - label: "Hand off to planner"
      agent: planner
      prompt: "Now create a plan based on the research above."

Harness CLI commands

# Scaffold a new subagent source file and register it in manifest.json
npx harness add subagent <subagent-id>

# Preview what files will be generated (dry run)
npx harness plan

# Write all provider artifacts from current canonical sources
npx harness apply

# Watch sources and auto-apply on changes
npx harness watch

# Remove a subagent entity and its source files
npx harness remove subagent <subagent-id>

# Remove entity but keep source files on disk
npx harness remove subagent <subagent-id> --no-delete-source

Complete example

Source file: .harness/src/subagents/code-reviewer.md

---
name: code-reviewer
description: Expert code review specialist. Reviews code for quality, security, and maintainability. Use proactively after writing or modifying code.
---

You are a senior code reviewer ensuring high standards of quality and security.

When invoked:
1. Run git diff to see recent changes
2. Focus on modified files
3. Begin review immediately without asking for clarification

Review checklist:
- Code is clear and readable
- Functions and variables are well-named
- No duplicated logic
- Proper error handling and input validation
- No exposed secrets or API keys
- Adequate test coverage
- Performance considerations addressed

Provide feedback organized by priority:
- Critical issues (must fix before merging)
- Warnings (should fix)
- Suggestions (consider improving)

Include specific examples of how to fix each issue. Do not modify files yourself.

Override sidecar for Claude (.harness/src/subagents/code-reviewer.overrides.claude.yaml):

version: 1
options:
  model: sonnet
  tools:
    - Read
    - Grep
    - Glob
    - Bash

After running npx harness apply, this produces:

• .claude/agents/code-reviewer.md — used by Claude Code
• An entry in .codex/config.toml under [agents.code-reviewer] — used by Codex
• .github/agents/code-reviewer.agent.md — used by GitHub Copilot

Typical workflow

# 1. Scaffold the subagent source
npx harness add subagent code-reviewer

# 2. Edit the source file
#    .harness/src/subagents/code-reviewer.md

# 3. (Optional) Edit provider-specific overrides
#    .harness/src/subagents/code-reviewer.overrides.claude.yaml

# 4. Preview the plan
npx harness plan

# 5. Generate provider artifacts
npx harness apply

Check .claude/agents/, .codex/config.toml, and .github/agents/ to verify the rendered output.

References

• Claude Code sub-agents: https://code.claude.com/docs/en/sub-agents
• OpenAI Codex agents: https://developers.openai.com/codex/config-reference
• OpenAI Codex config reference: https://developers.openai.com/codex/config-reference
• GitHub Copilot custom agents configuration: https://docs.github.com/en/copilot/reference/custom-agents-configuration
• GitHub Copilot custom agents how-to: https://docs.github.com/en/copilot/how-tos/use-copilot-agents/coding-agent/create-custom-agents
• Cursor subagents: https://docs.cursor.com/agent/subagents
• Agent harness providers overview: docs/providers.md