8/13/2025

You Can Finally Build Custom GPTs with the New GPT-5 Engine, & It’s a Game Changer

What a time to be alive. Seriously. If you’ve been tinkering with AI, building little helpers, or just dreaming of what’s next, well, the next big thing is officially here. OpenAI dropped GPT-5 on August 7th, 2025, & it’s not just a minor update. This is a whole new level of power, & it’s now the default engine for anyone building custom GPTs.

For those of us who have been building custom GPTs since they first launched, this is the moment we've been waiting for. A Reddit user put it perfectly, calling the upgrade a "massive improvement." Custom GPTs that were once clunky or just plain unusable are now working incredibly well. The consistency, the ability to follow complex instructions, the sheer creative power—it's all been kicked up a notch. Or ten.

So, if you've been on the fence about building your own GPT, or if you tried before & felt underwhelmed, now is the time to jump back in. Let’s walk through exactly how to do it with the new GPT-5 brains behind the operation. It’s easier than you think, & honestly, a ton of fun.

What's the Big Deal with GPT-5 Anyway?

Before we dive into the "how-to," let's just appreciate what GPT-5 brings to the table. This isn't just about better conversations. OpenAI has fundamentally rebuilt the architecture. We're talking about a system that combines reasoning & non-reasoning models to deliver a faster, more accurate experience. They've even introduced different sizes in the API, like

gpt-5

gpt-5-mini

, &

gpt-5-nano

, giving developers insane flexibility.

For us, the creators of custom GPTs, this translates to:

Better Instruction Following: You can give your GPTs WAY more detailed & nuanced instructions, & they’ll actually follow them.
Enhanced Creativity: The model is better at generating creative text formats, from writing in a specific oil painting style to crafting complex code.
Fewer Hallucinations: One of the biggest frustrations with older models was their tendency to make stuff up. GPT-5 is significantly more accurate & grounded in the information you provide.
Autonomous Agent Capabilities: The new model is designed to be a "true coding collaborator" & excels at agentic tasks. This means your custom GPTs can be more than just chatbots; they can be active tools that execute tasks & automate workflows.

Basically, that little AI assistant you’ve been dreaming of creating? It's now much, MUCH more possible.

Building Your First Custom GPT with the GPT-5 Engine

Alright, let's get our hands dirty. The good news is, OpenAI has kept the process ridiculously simple. You don’t need to be a coder. You just need an idea.

Step 1: Get Access

First things first, you'll need a ChatGPT Plus, Pro, or Team account to create custom GPTs. If you have one, you're all set. If not, you’ll need to subscribe to get access to the GPT builder.

Step 2: Fire Up the GPT Builder

Once you're logged in, head over to

chat.openai.com

. On the left-hand side panel, you'll see a tab that says "Explore GPTs." Click on that.

This is your mission control for all things GPT. You’ll see featured GPTs, popular ones, & at the top right, a beautiful little button that says "+ Create". Click it. You can also just go directly to

chatgpt.com/create

Step 3: The "Create" Tab - Your Conversational Co-pilot

You’ll land in the GPT editor, which has two main sections: "Create" & "Configure."

The Create tab is where the magic starts. It’s a chat interface where you literally talk to the "GPT Builder" to bring your idea to life. It’s like having a brainstorming session with an AI that then builds the thing you’re brainstorming. Pretty meta, right?

You can start with a simple prompt. For example:

"Make a bot that helps me come up with creative, healthy dinner ideas for my family."
"Create a sarcastic marketing assistant that roasts my bad ideas & helps me write better ad copy."
"Build a coding wizard that can help me debug my Python scripts & explain complex concepts simply."

The GPT Builder will start asking you clarifying questions. It’ll suggest a name, like "Dinner Decider" or "Sarcastic Marketer." It will even use DALL-E 3 to generate a profile picture for your new GPT. You just chat with it, refine the idea, & watch it take shape in the preview panel on the right side of the screen.

This conversational approach is AMAZING for getting a solid foundation. You can test your GPT in real-time in the preview window to see how it’s behaving.

Step 4: The "Configure" Tab - Fine-Tuning Your Creation

Once you’ve got the basics down, it’s time to pop the hood & do some fine-tuning. Click over to the Configure tab. This is where you get more granular control.

Here’s what you’ll find:

Name & Description: You can change the name & description the builder suggested if you want. Make it catchy!
Instructions: This is the most important part. The instructions are the "system prompt" or the constitution for your GPT. This is where you lay down the law. Be specific!
- Bad Instruction: "Be a helpful assistant."
- GOOD Instruction: "You are the 'Investor Pitch Coach.' Your goal is to help startup founders refine their pitch decks. You should always be encouraging but firm. Ask probing questions about their business model, market size, & team. Never give financial advice. When you review a slide, provide three specific, actionable suggestions for improvement. Always end your feedback with a motivational quote about entrepreneurship."
Conversation Starters: These are the little buttons that appear for a new user to kick off the chat. Things like, "Review my pitch deck," or "Help me brainstorm a company name." It helps guide users on how to interact with your GPT.
Knowledge: This is your GPT's private library. You can upload files—PDFs, text documents, spreadsheets, etc.—that your GPT can reference. This is how you give it specialized knowledge beyond what GPT-5 was trained on. Got a 300-page manual for your company's software? Upload it here, & your GPT can become an expert support agent. This is a total game-changer.
Capabilities: Here you can toggle on/off some powerful tools:
- Web Browsing: Allow your GPT to search the web for real-time information.
- Image Generation: Let your GPT create images using DALL-E 3.
- Code Interpreter: Enable your GPT to run Python code, which is perfect for data analysis, creating charts, or solving complex math problems.
Actions: This is the most advanced feature, but also one of the most powerful. Actions allow your GPT to connect to third-party APIs. This means your GPT can interact with external services—it could check the weather, book a meeting in your Google Calendar, or pull customer data from your CRM. It’s how you turn a chatbot into a true, integrated assistant.

A Quick Word on Business & Customer Service

Here’s the thing, while building these GPTs is cool for personal use, the business applications are HUGE. Think about customer service. Most businesses struggle to provide instant, 24/7 support. It's expensive & hard to scale.

This is where a platform like Arsturn comes into play. While you can build your own GPTs on OpenAI's platform, if you're a business looking for a robust, no-code solution to engage with customers on your website, you'll want something more specialized. Arsturn helps businesses create custom AI chatbots trained on their own data. This means you can build an AI that knows your products, your policies, & your brand voice inside & out. It can provide instant customer support, answer detailed questions, & engage with website visitors around the clock, turning your site into a lead-generating machine. It’s all about building meaningful, personalized connections with your audience, & Arsturn makes that incredibly easy.

Once you've configured your GPT to perfection, it's time to release it into the wild. In the top right corner, you'll see a "Publish" (or "Update") button. When you click it, you’ll get a few options for sharing:

Only me: The GPT is private & only you can use it.
Anyone with a link: The GPT is unlisted, but anyone you share the direct link with can access it.
Public: Your GPT will be discoverable in the GPT Store for everyone to find & use.

Choose the option that fits your needs, click "Confirm," & that’s it! You've officially created a custom GPT powered by the formidable GPT-5 engine.

The Future is Custom

The launch of GPT-5 feels like a pivotal moment. The ability for anyone, regardless of technical skill, to create a specialized AI tool is profoundly democratizing. We're moving away from a one-size-fits-all model of AI to a future where we each have a suite of custom-built, personalized assistants designed for our specific needs, workflows, & creative pursuits.

Whether you're building a bot to help you plan your D&D campaigns, a research assistant to summarize academic papers, or a powerful internal tool for your business, the barrier to entry has never been lower, & the ceiling for what's possible has never been higher.

So go on, give it a shot. What will you build?

Hope this was helpful! Let me know what you think.