The question everyone is actually asking
If you want to make an AI assistant better at a specific job, you have three obvious options in 2026: a Claude Skill, a ChatGPT Custom GPT, or a Gemini Gem. They look similar on the surface. You give the AI some instructions and context, and it gets better at your task. So people assume the choice is about which chatbot they already pay for.
It is not. The three are built on fundamentally different ideas, and the difference decides whether the work you put in today is still useful next year or locked to one vendor forever. This is the part most comparisons skip, so let's be direct about it.
What each one actually is
A Custom GPT is a custom version of ChatGPT. You give it instructions, upload files, and it lives inside ChatGPT. A Gemini Gem is the same idea inside Google's Gemini. Both are, in plain terms, a pre-configured assistant that runs on one company's platform.
A Claude Skill is a different kind of thing. It is not an assistant. It is a folder of instructions that tells an AI agent how to do a specific task, loaded only when that task comes up. Anthropic describes Skills as "custom onboarding materials that let you package expertise." The distinction sounds subtle and it is the whole game: a GPT or a Gem is a vendor-specific assistant, while a Skill is portable know-how that any capable agent can pick up.
The one difference that decides it: portability
Here is the fact that should drive your choice. On December 18, 2025, Anthropic published Agent Skills as an open standard for cross-platform portability. Skills are written in a plain format called SKILL.md, and because the format is open, it is not read only by Claude.
As of early 2026, the SKILL.md format is read by Claude Code, the Claude Agent SDK, Codex CLI, Gemini CLI, Cursor, JetBrains Junie, AWS Kiro, Block Goose, and a growing list of other tools. You write a skill once and it runs across all of them.
Now compare that to the other two. A Custom GPT runs inside ChatGPT and nowhere else. You cannot take a GPT and run it in Claude, in Gemini, or in Cursor. A Gemini Gem is the same story, locked to Google. If you build your workflow around a GPT and later switch tools, or the pricing changes, or the product gets deprecated, the work does not come with you. You rebuild from scratch.
That is the honest summary: Skills are portable, GPTs and Gems are vendor lock-in.
Where each one is genuinely the right call
This is not a hit piece on GPTs and Gems. They are good at what they are for.
Use a Custom GPT or a Gemini Gem when you live entirely inside ChatGPT or Gemini, you want something set up in five minutes with a friendly builder, and you are fine with it staying there. For a personal assistant that summarizes your emails inside the one app you already use all day, a GPT or a Gem is the fast, easy answer. There is no reason to overthink it.
Use a Claude Skill when the task is something you will reuse, especially across tools, or when you are building on top of AI rather than just chatting with it. If you run any kind of agent, or you work in Claude Code, Cursor, or a CLI, Skills are the format that follows you. And if you are doing real production work, agents that run on Telegram, Discord, or WhatsApp, Skills are effectively the only portable answer right now.
The practical way to think about it
Ask yourself one question: is this a throwaway helper for one app, or a piece of expertise you want to keep?
Throwaway helper, one app you love: build a GPT or a Gem, move on with your day.
Something you want to keep and reuse: put it in a Skill, because the format is open and it will still work when you change tools.
Most people underestimate how often the second case applies. The prompt you perfected for cleaning up client emails, the checklist your agent follows before it ships code, the way you want Claude to write in your voice, these are not throwaway. They are assets. Locking an asset inside one vendor's product is a decision you usually regret later, quietly, when you switch.
Where to go from here
If you have decided Skills are the format worth investing in, the next question is whether you write all of them yourself or start from ones that already work. Writing a good SKILL.md is not hard, but building a library of them is a real time sink, and most of the useful ones have already been written and tested by someone.
That is what our Skills Library is for. It is 2,390 ready-to-install Claude Code skills, one payment for lifetime access, that drop straight into ~/.claude/skills/ and work with the portable format described above. If you would rather try before you spend anything, 1,545 of them are free community skills, and the 75-page Claude guide walks through how skills work in plain English first.