Summary

Fifth ADR of Phase 0 — formalizes Skill as pure declarative configuration: a system prompt + optional examples + tools/delegates by name reference. 12 invariants (S1-S12).

Builds on #267 (Adapter), #270 (Tool), #271 (Memory), #272 (Retriever).

Merge order: #267 → #270 → #271 → #272 → #273 (rebase each on main as merged; conflicts only on the ADR README index).

The load-bearing distinction

A tool is a function the model calls. A skill is a persona the model becomes.

Confusing them is the recurring failure mode in agent libraries (LangChain's "agents" half-conflate them). This contract makes the distinction explicit through three invariants:

• S10 No execute, no return value — skills are activated, not invoked
• S11 Pure declaration, no I/O at definition — skills are data, not code
• S5 Tools by reference, not inline — skills compose with whatever registry exists

Highlight invariants

• S1 Name is identity — same regex as Tool T1, shared registry namespace by convention
• S3 systemPrompt is the soul — required, non-empty; everything else supports it
• S4 Examples are few-shot, not test cases — single-turn pairs in v1
• S5 Tools by reference (array of names) — runtime resolves at activation
• S6 Delegates by reference — same shape, scales to multi-agent topologies
• S7 Cycles are runtime concerns — contract permits, runtime detects/terminates
• S9 onActivate is for dynamic tools only — narrow escape hatch for per-user/tenant credentials
• S10 No skill.run() — distinguishes skill from tool absolutely

Trade-offs accepted

• No multi-turn examples in v1 (encode in prompt prose for now; v2 may extend)
• onActivate is the only function in an otherwise pure-data contract — accepted because per-user dynamic tools are real
• Tools by reference creates a registry dependency — correct trade for shareability
• No per-skill versioning in contract — metadata.version is convention; manifest spec ([F3] #69 — Skill marketplace with versioning #182) will formalize

Unlocks

• Skill marketplace ([F3] #68 — Doc loaders (URL, PDF, GitHub, Notion, Confluence, Drive) #181) — skills are JSON-serializable, versionable, ratable
• Multi-agent topologies ([F2] #44 — Ready-made multi-agent topologies #157) — supervisor/swarm/hierarchical/blackboard as runtime patterns over delegates graphs
• MCP-aligned skill manifest ([F3] #69 — Skill marketplace with versioning #182) — at most adds a few standardized metadata keys
• agentskit ai ([F2] #31 — npx agentskit ai (NL generator) #144) — agents generating skills emit pure JSON, no codegen
• Dogfooding: AgentsKit's docs assistant, code reviewer, release agent all become SkillDefinitions

Test plan

• Doc renders on GitHub
• ADR index updated
• No code changes — pure documentation
• Reviewer: 12 invariants match packages/skills/src/* (researcher, critic, summarizer, coder, planner)?
• Reviewer: does the "skill ≠ tool" distinction land cleanly?
• Reviewer: is onActivate the right escape hatch shape?

Refs #214 #211