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TIL: Agent Skills

I'm learning to author and use Agent Skills in Claude and Codex. I gained some insights and I would like to share them here today.

Three types of Skill content, three levels of loading

Skill content comes in three types: instructions, scripts, and resources. Skills are organized as files/folders that Claude can draw from via a filesystem-style interface. Claude does progressive disclosure: it does not need to load the entire Skill into the context window upfront, and can fetch only the relevant files as needed. [^1]

Skill content types have different strengths:

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Claude Code 101

I've been a SWE for 7 years, between Amazon, Disney, and Capital One. The code I've shipped touches millions of users, and I built systems that couldn't afford to break. Now I'm the CTO of a startup that builds agents for enterprise, and Claude Code is my daily driver.

Here's a beginners playbook you might find useful, containing everything I've learned about Claude after using it to build robust systems that handle complex workloads from large companies. Let me know if it helps below.

Think First

Most people assume that with Claude Code and other AI tools, the first thing you need to do is type (or start talking). But that's probably one of the biggest mistakes that you can make straight off the bat. The first thing that you actually need to do is think.

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Opinion: GPT-5.2 Codex and Claude Code Opus 4.5

Just a guide. Not a prescription.

GPT-5.2 Codex works best for

• Upfront planning, especially systems design and architecture
• Hard problems that can be solved cleanly in one pass
• Code and technical design reviews
• Finishing touches, especially cleaning up gaps left by Opus 4.5

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Prompt:

Create a 3D scene of a dynamic, interactive display of Christmas-themed tarot cards arranged in a semi-circular or ring formation.

Here are the specific requirements:

**Visuals & Assets:**
*   **Environment:** A dark, moody background with subtle atmospheric effects (e.g., light fog or floating particles).
*   **Cards:** Use placeholder plane geometries that resemble tarot card dimensions (e.g., 3.5x5 units).

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Investors Think I’m Lying: Opus 4.5 Rewrote Our Backend Over Lunch

Here's what it feels like to use Opus 4.5 every day, written the way I'd tell a friend over coffee.

The launch timing was weird: Gemini 3 Pro stole the mic for a day, then everyone vanished for turkey. So the best coding model and the best agent model slipped in the side door while the Internet was sleeping. People just haven't caught up yet.

The hype around "2025 is the year of agents" finally makes sense.

Remember how GPT-4 turned "chat with a computer" into something you'd actually do? Then Sonnet 3.5 did the same for "hey, can you write this code?" Opus 4.5 is that jump again, but for "go handle this whole codebase and ping me when it's done." The first time you watch it plan, iterate, fix its own bugs, and finish a two-hour job while you play Expedition 33, the room tilts a little. After that, the old way feels like the sounds of a dial-up modem.

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ChatGPT-5 Minecraft 3D Clone Prompt

One-shot and a few multi-turn chats later, we got a simple "cinematic" level Minecraft-inspired (cube/voxel world) 3D game.

Model:

• ChatGPT-5
• ChatGPT-5 (with "think harder" prompt)
• ChatGPT-5 Thinking

User Prompts

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Beast Mode

Beast Mode is a custom chat mode for VS Code agent that adds an opinionated workflow to the agent, including use of a todo list, extensive internet research capabilities, planning, tool usage instructions and more. Designed to be used with 4.1, although it will work with any model.

Below you will find the Beast Mode prompt in various versions - starting with the most recent - 3.1

Installation Instructions

• Go to the "agent" dropdown in VS Code chat sidebar and select "Configure Modes".
• Select "Create new custom chat mode file"

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Bug Hunting with Kimi K2 and OpenCode

Previously, Kimi K2 built a Minecraft clone in just a few minutes. It's playable in my browser. But, it's hard to control and move the player around the voxel world due to a few bugs in the 3D world model. So I take this chance to stress test Kimi K2 by giving it harder tasks such as solving tough bugs.

Findings

Kimi K2 can solve complex 3D math problems in game development.

Prompt and OpenCode session below.

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Agentic Shootout: Kimi K2 vs Gemini 2.5 Pro in OpenCode

Compare Kimi K2 and Gemini 2.5 Pro agentic capabilities in OpenCode.

• Version: 0.3.48
• OS: Linux

Kimi K2 vs. Gemini 2.5 Pro

AI Model: Gemini 2.5 Pro

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Andrej Karpathy on Software 3.0, Software Engineering In The AI Era

Andrej Karpathy's concept of Software 3.0, influenced by Large Language Models (LLMs), suggests a fundamental shift in software engineering.

The evolution of software

• Software 1.0: Traditional code in languages like C++ or Python, based on explicit instructions.
• Software 2.0: Neural networks where the "code" is learned weights from data. Development involves tuning datasets and running optimizers rather than writing instructions.
• Software 3.0: The prompt provided to an LLM is the program itself, with natural language serving as the programming language.