Harness Skills Capabilities Reference

Source of truth for what each AI coding harness supports in terms of agent skills. Used to inform provider configs in scripts/lib/transformers/providers.js.

Last verified: 2026-04-28 (subagent landscape spot-checked 2026-06-28; Mistral Vibe row verified 2026-07-16; Grok Build skills row verified 2026-07-21; Grok Build hook stdin captured 2026-08-24)

This file is point-in-time. Capabilities move fast; verify live before relying on any "only X supports Y" claim. Notably, the subagent table below lists Impeccable's emission targets, not the support landscape (see its note).

Official Documentation

Harness	Docs URL
Claude Code	https://code.claude.com/docs/en/skills
Cursor	https://cursor.com/docs/context/skills
Gemini CLI	https://geminicli.com/docs/cli/skills/
Codex CLI	https://developers.openai.com/codex/skills
GitHub Copilot (Agents)	https://code.visualstudio.com/docs/copilot/customization/agent-skills
Kiro	https://kiro.dev/docs/skills/
OpenCode	https://opencode.ai/docs/skills/
Pi	https://github.com/badlogic/pi-mono/blob/main/packages/coding-agent/docs/skills.md
Qoder	https://docs.qoder.com/extensions/skills
Trae	TBD (no official skills docs found yet)
Rovo Dev	https://support.atlassian.com/rovo/docs/extend-rovo-dev-cli-with-agent-skills
Mistral Vibe	https://docs.mistral.ai/vibe/code/cli/skills
Grok Build	https://docs.x.ai/build/features/skills-plugins-marketplaces
Hermes Agent	https://hermes-agent.nousresearch.com/docs/
Antigravity	https://antigravity.google/docs/skills

Spec Compliance

All harnesses follow the Agent Skills specification to varying degrees. The spec defines these frontmatter fields: name, description, license, compatibility, metadata, allowed-tools.

Provider-specific extensions beyond the spec: user-invocable, argument-hint, disable-model-invocation, allowed-tools (extended syntax), model, effort, context, agent, hooks, subtask, mcp.

Frontmatter Support

Fields marked with * are spec-standard. Others are provider extensions.

Field	Claude Code	Cursor	Gemini	Codex	Copilot	Grok	Hermes	Kiro	OpenCode	Pi	Qoder	Rovo Dev	Mistral Vibe	Antigravity
name*	Yes	Yes	Yes	Yes	Yes	Yes	Yes	Yes	Yes	Yes	Yes	Yes	Yes	Yes
description*	Yes	Yes	Yes	Yes	Yes	Yes	Yes	Yes	Yes	Yes	Yes	Yes	Yes	Yes
license*	Yes	Yes	Ignored	No	Yes	Yes	Yes	Yes	Yes	Yes	Yes	Yes	Yes	Yes
compatibility*	Yes	Yes	Ignored	No	Yes	Yes	Yes	Yes	Yes	Yes	Yes	Yes	Yes	Yes
metadata*	Yes	Yes	Ignored	No	Yes	Yes	Yes	Yes	Yes	Yes	Yes	Yes	Yes	Yes
allowed-tools*	Yes	No	Ignored	No	No	Yes	No	No	Yes	Yes	Yes	Yes	Yes	Yes
user-invocable	Yes	No	No	No	Yes	Yes	No	No	Yes	No	Yes	Yes	Yes	No
argument-hint	Yes	No	No	No	Yes	Yes	No	No	Yes	No	Yes	Yes	No	No
disable-model-invocation	Yes	Yes	No	No	Yes	Yes	No	No	Yes	Yes	TBD	TBD	No	No
model	Yes	No	No	No	No	Yes	No	No	Yes	No	No	No	No	No
effort	Yes	No	No	No	No	Yes	No	No	No	No	No	No	No	No
context	Yes	No	No	No	No	No	No	No	No	No	No	No	No	No
agent	Yes	No	No	No	No	No	No	No	Yes	No	No	No	No	No
hooks	Yes	No	No	Yes	No	Yes	No	No	No	No	No	No	No	No

Notes:

• Gemini CLI validates only name and description; other spec fields are parsed but ignored.
• Codex CLI uses a separate agents/openai.yaml sidecar for skill metadata (icons, branding, MCP tools, invocation control). Codex also auto-discovers subagents bundled inside an installed skill's agents/ folder (TOML), which is how Impeccable ships its asset-producer. Standalone custom agents can still live under .codex/agents/ or ~/.codex/agents/, but Impeccable no longer installs anything there.
• Codex CLI hooks ship under [features].hooks = true (still flagged), require /hooks trust ceremony per-update, and are disabled on Windows.
• Grok Build is Claude Code compatible with zero config: it also reads .claude/skills/, .claude/settings.json hooks, and Claude plugin layouts. Native paths are .grok/skills/, .grok/hooks/*.json, and .grok/agents/. Skill frontmatter supports when-to-use in addition to the fields above. Project hooks require /hooks-trust (or --trust). See https://docs.x.ai/build/features/skills-plugins-marketplaces and https://docs.x.ai/build/features/hooks.
• Hermes Agent reads the Agent Skills spec as-is. Spec-defined fields (name, description, license, compatibility, metadata) are parsed and stored; harness-specific extensions (user-invocable, argument-hint, allowed-tools, disable-model-invocation, model, effort, context, agent, hooks) are unknown keys and silently ignored. Hermes has no hook surface, no per-skill tool ACL, and no slash-command equivalent of user-invocable (skills are loaded via /skill <name> or auto-loaded; sub-commands like /impeccable polish are routed from the skill body, not declared in frontmatter). Hermes adds two frontmatter fields not in the spec: platforms: (OS filter; default = all) and environments: (relevance gate over kanban, docker, s6). Unknown fields are silently ignored.
• Kiro recognizes user-invocable and disable-model-invocation per community reports but does not formally document them.
• Antigravity supports standard Agent Skills spec frontmatter fields (name, description, license, compatibility, metadata, allowed-tools).
• Unknown fields are silently ignored by all harnesses.

Hook surface used by Impeccable

Harness	Edit hook	Startup hook	Manifest location	Notes
Claude Code	Yes (PostToolUse)	No	.claude/settings.json	Project-local settings entry installed by npx impeccable skills install/update. Runs .claude/skills/impeccable/scripts/hook.mjs.
Codex CLI	Yes (PostToolUse)	No	.codex/hooks.json	Project-local manifest installed with the .agents/skills/impeccable payload. Runs .agents/skills/impeccable/scripts/hook.mjs from the git root. Requires normal /hooks trust approval.
Cursor	Yes (preToolUse)	No	.cursor/hooks.json	Project-level manifest installed with .cursor/skills/impeccable. Runs hook-before-edit.mjs to block bad proposed writes before they land. Reloads on save; restart Cursor if hooks do not pick up.
Grok Build	Yes (PostToolUse)	No	.grok/hooks/impeccable.json	Project-local manifest installed with .grok/skills/impeccable. Claude-compatible matchers (Edit|Write|MultiEdit) alias to Grok search_replace. PostToolUse runs the scan and warms the session cache; Grok ignores that stdout. Stop additionalContext is the user-visible pass. Ignore Grok's observe-only Stop with reason: "shutdown". Requires /hooks-trust or --trust. Plugin installs use plugin/hooks/hooks.json with ${CLAUDE_PLUGIN_ROOT} (aliased to GROK_PLUGIN_ROOT).
All other harnesses	No	No	n/a	No documented hook surface today. Skill and commands still ship.

Skill Directory Structure

Harness	Native directory	Also reads
Claude Code	.claude/skills/	-
Cursor	.cursor/skills/	.agents/skills/, .claude/skills/
Gemini CLI	.gemini/skills/	.agents/skills/
Codex CLI	.agents/skills/ (primary)	-
GitHub Copilot	.github/skills/	.agents/skills/, .claude/skills/
Kiro	.kiro/skills/	-
OpenCode	.opencode/skills/	.agents/skills/, .claude/skills/
Pi	.pi/skills/ (project), ~/.pi/agent/skills/ (global)	.agents/skills/
Qoder	.qoder/skills/	~/.qoder/skills/ (user-level)
Trae China	.trae-cn/skills/	TBD
Trae International	.trae/skills/	TBD
Rovo Dev	.rovodev/skills/	~/.rovodev/skills/ (user-level)
Mistral Vibe	.vibe/skills/ (project), ~/.vibe/skills/ (global)	.agents/skills/ (project), ~/.agents/skills/ (global)
Grok Build	.grok/skills/ (project), ~/.grok/skills/ (global)	.agents/skills/, .claude/skills/, .cursor/skills/ (Claude/Cursor compat, configurable)
Hermes Agent	.hermes/skills/ (project), ~/.hermes/skills/ (global)	skills.external_dirs config (no automatic .agents/skills/ fallback)
Antigravity	.agent/skills/ (project), ~/.gemini/config/skills/ (global)	.agents/skills/ (project), ~/.agents/skills/ (global)

All harnesses support the {skill-name}/SKILL.md directory structure with optional reference/, scripts/, and assets/ subdirectories.

Native Subagent Directory Structure (Impeccable emission targets)

Scope: this table is where Impeccable emits native subagent files, not a map of which harnesses support subagents. Subagents are broadly supported now: Cursor (auto-delegation + /name invocation, https://cursor.com/docs/subagents), GitHub Copilot, and Google Antigravity ship them too. Impeccable only writes native files where there is a stable, documented on-disk format to target.

Harness	Native directory	File format
Claude Code	.claude/agents/ (installed plugin)	Markdown with YAML frontmatter
Grok Build	.grok/agents/ (project) and plugin agents/	Markdown with YAML frontmatter (Claude-compatible)
Codex CLI	<skill>/agents/ (nested, auto-discovered)	TOML

Impeccable keeps canonical agent prompts under skill/agents/ and emits provider-native files only for harnesses with a documented on-disk subagent format. Claude reads its agents from the installed plugin; Grok reads the same markdown agents from the plugin package and from project .grok/agents/; Codex auto-discovers the TOML bundled inside the installed skill's own agents/ folder, so the normal skills install carries it with no separate sidecar.

Spawn / permission model (matters more than directory support when building skills):

Harness	Who can spawn a subagent
Claude Code	Programmatically, from within the skill/agent flow.
Grok Build	Programmatically via spawn_subagent (built-in types plus project/user agents under .grok/agents/).
Codex CLI	Only if the user has allowed sub-agents / parallel work; otherwise the skill must ask once, then stop (see skill/reference/critique.md <codex> gate).
Cursor	Agent-chosen: auto-delegated by the Agent, or user-invoked via /name. Not reliably skill-spawnable.
Others	Varies; treat as unavailable unless verified, and degrade loudly.

Placeholder / Variable Substitution

Claude Code supports runtime variable substitution directly in SKILL.md bodies: $ARGUMENTS, $0-$N, ${CLAUDE_SKILL_DIR}, ${CLAUDE_SESSION_ID}. No other harness supports substitution in skills.

Some harnesses have separate "custom commands" systems (distinct from skills) with their own substitution:

Harness	Command system	Substitution syntax
Gemini CLI	.gemini/commands/ (TOML)	{{args}}, !{shell}, @{file}
Codex CLI	.codex/prompts/	$ARGNAME
OpenCode	.opencode/commands/	$ARGUMENTS, $1-$N, !`shell`

Our build system handles cross-provider placeholders at compile time via replacePlaceholders() for {{model}}, {{config_file}}, {{ask_instruction}}, and {{available_commands}}.