Skills catalog

nexo-rs uses "skill" to mean two different things. Both are covered on this page; gating semantics for each live in Gating by env / bins.

1. Extension skills — shipped under extensions/ in the repo, discovered and spawned like any other stdio extension. 22 of them landed in Phase 13.
2. Local skills — markdown files under an agent's skills_dir/ that get injected into the system prompt at turn start.

The two overlap in name but not in mechanism:

Extension skills (Phase 13)

All shipped as stdio extensions written in Rust. _common is a shared Rust library (circuit-breaker primitives), not an extension itself.

Core utilities

Search & knowledge

Infra & ops

Media & content

Google (phase 13.18)

Single google extension covering 32 tools across Gmail, Calendar, Tasks, Drive, People, and Photos. Uses OAuth refresh-token flow. Writes gated by five independent env flags:

• GOOGLE_ALLOW_SEND — Gmail send
• GOOGLE_ALLOW_CALENDAR_WRITE
• GOOGLE_ALLOW_DRIVE_WRITE
• GOOGLE_ALLOW_TASKS_WRITE
• GOOGLE_ALLOW_PEOPLE_WRITE

See Plugins — Google for the OAuth setup and the generic google_call tool that fronts the extension.

LLM providers (phase 13.19)

anthropic and gemini are native LLM clients living under crates/llm/, not extensions. See LLM providers and children.

Templates

See Extensions — Templates.

Local skills

Local skills are markdown files loaded by SkillLoader and injected into the system prompt at turn time. Defined in the agent config:

# agents.yaml
agents:
  - id: kate
    skills_dir: ./skills
    skills:
      - weather
      - github
      - summarize
      - google-auth

Each entry resolves to <skills_dir>/<name>/SKILL.md:

---
name: "Weather"
description: "Current conditions and forecasts"
requires:
  bins: ["curl"]
  env: ["WEATHER_API_KEY"]
max_chars: 5000
---
# Weather skill

Call `weather_forecast(city)` to get a 3-day forecast.
Use metric units. Default to the user's locale when unspecified.

Loading flow

flowchart TD
    CFG[agents.yaml skills: list] --> LOOP[for each name]
    LOOP --> READ[read skills_dir/name/SKILL.md]
    READ --> FM[parse YAML frontmatter]
    FM --> GATE{bins on PATH<br/>AND env set?}
    GATE -->|no| SKIP[warn + skip<br/>not injected]
    GATE -->|yes| RENDER[render into prompt:<br/>heading + blockquote + body]
    RENDER --> TRUNC[truncate to max_chars]
    TRUNC --> INJECT[inject into system prompt]

Why local skills skip-on-miss (vs extensions warn-and-continue)

A local skill is a text instruction to the LLM describing a capability. If the backing bin/env isn't available the tool will fail — but worse, the LLM was told the capability exists and will repeatedly try to use it. Skipping the skill prevents lying to the model.

An extension is a registered tool. If the LLM invokes it and the backing bin is missing, the tool returns an error — the LLM observes and adapts. Warn-and-continue is fine.

See Gating for the full semantics.

How to pick

• Need the LLM to know how to do something (usage pattern, style rules, examples)? → local skill.
• Need the LLM to do something (make a call, return data)? → extension skill.
• Both? → ship the extension and write a local skill next to it that explains when to use it.