---
id: "concept-skills-as-contracts"
type: "concept"
source_timestamps: ["15:45:00", "15:55:00", "16:15:00"]
tags: ["architecture", "system-design"]
related: ["concept-skill-composability", "action-define-output-contracts", "concept-orchestrator-pattern"]
definition: "The design philosophy that a skill must explicitly define its inputs, outputs, and SLAs, functioning exactly like an API contract for an agent."
sources: ["s43-file-format-agreement"]
sourceVaultSlug: "s43-file-format-agreement"
originDay: 43
---
# Skills as API Contracts

## Definition

A skill must explicitly define its **inputs, outputs, and SLAs** — functioning exactly like an API contract for an agent.

## The API Analogy

To build reliable agentic systems, skills must be treated like API contracts. Just as a software developer relies on an API's documentation to know exactly what data to send and what data will be returned, an AI agent needs the same level of certainty from a skill.

The skill must be a **declarative agreement** that states:

> *"If you give me X, I will reliably produce Y in Z format."*

## Why It Matters

If a skill's output is vague or variable, it breaks the **chain of trust**. Downstream agents in a pipeline cannot reason about what they'll receive, so they cannot compose reliably (see [[concept-skill-composability]]). In the [[concept-orchestrator-pattern]], orchestrators rely on contract-shaped outputs to route work between sub-agents.

By framing the output of a skill as a strict contract, agents can confidently invoke the tool, knowing exactly what they will receive and how they can use that output to achieve their broader goals.

## Related

- [[action-define-output-contracts]] — the concrete practice
- [[concept-skill-composability]] — what contracts enable
- [[concept-methodology-body]] — output format is one of the 5 required methodology sections