Show Notes
Parker introduces Harness HowTo, an open-source repository that documents and compares the architectures of popular agentic coding tools like Cline, Codex, OpenCode, and Claude Code Open—helping you understand how these systems work so you can build better AI-powered workflows.
Why this repo exists
- Function calling came out in 2023 and enabled the agentic coding wave, but understanding how these tools work under the hood remained murky.
- Multi-billion dollar acquisitions (like Meta buying Cursor) show how valuable agent orchestration has become.
- Parker's own project was getting complex enough that he needed to understand these patterns at a deeper level.
What is Harness HowTo?
- A free, open-source repository with git submodules pointing to Cline, Codex, OpenCode, Claude Code Open, and other agentic tools.
- Each submodule has architecture documentation explaining how that tool handles agent loops, sessions, tool registries, and more.
- Auto-updates every 3 days via a GitHub Action that keeps the docs in sync with upstream changes.
Key questions the repo answers
- Does the tool use a mapped tool registry?
- What terminology does it use—"job", "turn", "task", or "loop"?
- How does it handle sessions, threads, and context window management?
- What are the compaction plans, memory persistence, and failure modes?
The agent loop explained
From the Codex architecture breakdown:
A thread/session receives user input, builds a prompt from an in-memory history plus instructions, asks the model to act, executes tool calls if requested, records outputs back into history, and repeats until the turn ends.
Understanding this pattern helps you see how to apply it to your own projects.
How Parker uses these patterns
- Used Codex Extra High to migrate an entire project from Postgres + Docker + Drizzle + TanStack Query to Convex in two days (144 API routes).
- Combined patterns from the Vercel AI SDK (for multi-model support) with the Convex AI Agent component (for durable retries and streaming).
- Building a "Shorts Factory" video harness with 8 sub-agents and an orchestration agent.
Tools documented in the repo
- Cline — Open-source Claude-based coding agent
- Codex — OpenAI's agentic coding tool
- OpenCode — Server-first typed SDK with plugin system
- Claude Code Open — Open-source version with ~95% pass rate compared to Claude Code
- More being added as the agentic landscape evolves
Why this matters for your projects
- Open-source projects all implement these patterns slightly differently—seeing the variations helps you pick the right approach.
- Reading through architecture docs beats reverse-engineering from UI behavior.
- You can combine patterns from multiple tools into your own custom harness.
Links
- Harness HowTo — The open-source repo documenting agentic coding architectures
- Cline — Open-source Claude-based coding agent
- OpenCode — Server-first typed SDK for building agents
- Convex AI Agent — Durable retries and streaming for AI agents
- Vibe with AI Discord — Free community for AI builders