Compose skills with `run_skill`¶

Goal: Invoke another skill from inside a skill — either deterministically before the LLM (preprocessor) or as an LLM-driven side effect (Control IR).

Two flavors¶

Flavor	Where it runs	Control
Preprocessor `run_skill`	Before the LLM, every phase visit	Skill author decides
Control IR `run_skill`	After the LLM, when it asks	LLM decides

Pick the preprocessor when the dependency is structural (this phase always needs that skill's output). Pick Control IR when the LLM should decide whether to call.

Preprocessor pattern¶

---
type: phase
name: write_post
input: post_request
preprocessor:
  - run_skill:
      skill: recall_memory
      input:
        type: user_message
        data: { text: "what does the user prefer about post format?" }
      into: relevant_memories
---

Write the post. If `relevant_memories` includes a stated preference,
follow it; otherwise default to a 3-paragraph structure.

The sub-skill runs to completion. Its final_output artifact is bound to input.relevant_memories. The phase reads it like any other input field.

Control IR pattern¶

For LLM-driven invocation, declare nothing in the preprocessor. The OS injects run_skill into available_control_ops and the LLM emits an op when it decides:

{
  "kind": "run_skill",
  "skill": "recall_memory",
  "input": {"type": "user_message", "data": {"text": "..."}}
}

The phase instructions describe when the LLM should invoke the sub-skill, not the op's syntax (P8). Example: "If you're unsure what the user prefers, call recall_memory to check."

Sub-skill nodes in the graph¶

A graph entry can also reference a skill directly:

graph:
  prepare:        [@my_subskill]
  '@my_subskill': [aggregate]
  aggregate:      [end]

This is heavier than run_skill: the sub-skill becomes a graph node, not a one-shot side effect. Use it when the parent's flow genuinely waits on the sub-skill at a specific point.

Skill resolution¶

All three patterns resolve names the same way:

reyn/project/<name>/skill.md
reyn/local/<name>/skill.md
src/reyn/stdlib/skills/<name>/skill.md

Compose skills with run_skill¶