---
title: 'Live Coding Session: Building Arena'
link: https://steipete.me/posts/2025/live-coding-session-building-arena/
source: steipete-me
published: 2025-09-06T10:00:00Z
updated: 2025-09-06T10:00:00Z
first_seen: 2026-09-27T19:29:15.293681927Z
summary: Watch me build Arena live - a real-time collaborative coding session exploring AI-powered development workflows.
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---

![](https://steipete.me/assets/img/2025/live-coding-session-building-arena/hero.png)

**tl;dr: I built and shipped a brand-new feature live (in ~1 hour), watch how I approach agentic engineering with codex**

Join me for an unfiltered look at building Arena - a live coding session where you can see my development process in action, unscripted. Thanks to Eleanor Berger for motivating me to do this video and for organizing the live event!

[YouTube video player](https://www.youtube.com/watch?v=68BS5GCRcBo)

## What we built

🤖 Heads up! This is an AI-Assisted summary.

- **Feature:** *Arena* — a new feature in my upcoming project, to see how well 2–4 users from X match
- **Input:** Twitter/X user handles
- **Pipeline:** fetch N tweets per user (shares a 1,000-tweet budget), strip to necessary fields, run profile analysis, then **score compatibility** (per pair + whole team)
- **UX:** user picker + “Analyze” button, results table, cached runs selectable under the search box
- **Infra touches:** DB migration for `arena_cache`, long-running job in the background, streaming UI, auth-guarded page

I managed to complete the feature in ~1h, and we got a **pair score of 89** for me and [@intellectronica](https://x.com/intellectronica).

## Stack & Setup

- **Model workflow:** codex (OpenAI/GPT-5) for coding sessions; it eagerly **reads the codebase** and generally “does the right thing” without handheld file lists. I keep a separate Claude-style flow for some web searching, but for repo work codex is the star.

- **Sessions:** start *fresh* for big features, run **multiple agent windows** in parallel; switch tasks while one is thinking.

- **Branching:** work **directly on `main`** with **atomic commits**. Merge conflicts + worktrees cost speed; small, well-scoped commits keep things safe.

- **Tooling:**

  - **Ghostty** for terminals, split panes with agents
  - **Better Stack** logging via a tiny [**`bslog`**](https://github.com/steipete/bslog) CLI
  - An **`xl`** CLI (`curl` wrapper) for quick X API pulls
  - Strict **biome** rules + custom codemods to normalize output
  - **Background worker** for long jobs ([Inngest](https://www.inngest.com/))
  - **Cache table** to avoid recompute
- **Docs ingestion:** pull only what’s needed, prefer **markdown** via a crawler over raw HTML to save tokens.

- **Validation:** schema validation on inputs; fail fast and surface helpful messages.

## Tactics that mattered

- **Keep the agent context clean.** Minimize tool noise and only inject docs when needed; markdown > HTML to conserve context.
- **Let codex plan on demand.** It proposes next steps that are usually solid; green-light them in sequence.
- **Cache long tasks early.** Add a table + background job queue before you polish UI; this saves you from re-running expensive analysis.
- **Copy-paste errors verbatim.** Don’t over-explain; just drop logs in and let the agent fix what broke.
- **Comments as spec.** Ask for clear intent comments near tricky code; they’re for you and for the agent on the next session.
- **Write tests afterwards, not first.** These agents are great at back-filling tests once the shape exists, and that’s usually enough to catch regressions.
- **Work on `main`, commit surgically.** You still get speed with safety. Backups + Git are your safety net.
- **Avoid local models for this workload.** Context and stability matter more than shaving milliseconds.
- **CLIs beat MCPs.** A 2-hour CLI wrapper (logs, API pulls) pays for itself and keeps context small.
- **Small “proof” projects unblock big features.** If streaming or a protocol is hard, build a tiny in-repo example that works, then transplant.

## Q&A highlights

- **Why Codex over Claude Code?** Codex actually **reads more of the repo** without handholding; requires fewer “look here, then there” hints. Claude still useful for search/web, but for coding Codex felt faster end-to-end.
- **Do you branch?** Not for features at this stage. **Main + disciplined commits** is faster and - counterintuitively - safer for rapid iteration.
- **Manual approvals for agent actions?** No. That becomes “Windows Vista prompts.” Use Git + backups; review diffs; move fast.
- **Repo prompts & MCP servers?** Nice idea, but they bloat context and introduce fragility. **Lean instructions + small CLIs** work better.
- **Compaction & long sessions?** Plan features to **fit the context**. If you expect many loops (e.g., test repairs), use the flow that compacts well - or split the task.

If you want to read more about my agent workflow, check [my AI development post here](https://steipete.me/posts/2025/optimal-ai-development-workflow).

For more advanced prompting techniques with GPT-5, also check out the [OpenAI GPT-5 Prompting Guide](https://cookbook.openai.com/examples/gpt-5/gpt-5_prompting_guide).

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