The Builder's Field Manual

Three generations of AI SDK, and the one worth shipping into

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Contents · APRIL 2026

In this issue.

Preface On the Gen-3 Claim Ben Davis stood on stage in Miami and said we've been writing AI softw… p. 01

From the Editor · APRIL 2026

When the runtime is a coding agent, your program is a directory of skills.

Ben Davis named three generations of AI SDK: API wrapper, tool-schema agent loop, and full coding-agent SDK. This issue is the builder's guide to Gen 3 — OpenCode, Pi, the Claude Agent SDK — and what it means that your program is now a directory of markdown plus a few helper scripts.

The SDK is no longer an SDK. It’s a harness, and the harness is a coding agent with a filesystem attached to it.

Anatomy of Gen 3 — session primitives, event streams, skill directories
MCP vs. skills vs. code mode — the decision matrix inside a harness
The markdown program — how a directory becomes a deployable
Harness as deployment target — from local dev to cron jobs in the cloud
Field guide — picking between OpenCode, Pi, Claude Agent SDK

— Harness Editorial

Cross-References