Custom GPT Development Service — Built for You

✓ Tested on your real tasks ✓ Your knowledge, organized ✓ API actions to your tools ✓ Team-ready usage guide

Quick answer

What is a custom GPT? 

A custom GPT is a version of ChatGPT configured for a specific purpose. It has its own instructions, uploaded knowledge files, and optional actions that call external APIs. Similar features exist as Claude Projects (Anthropic), Gems (Google Gemini) and Copilot agents (Microsoft).

What does this service include? 

Codersarts engineers define the assistant's job, write and test its instructions, prepare your knowledge files for accurate retrieval, connect API actions to your tools, and deliver a usage guide. For assistants outside ChatGPT, we build them on the OpenAI Responses API or the Claude API and embed them in your tools.

How long and how much? 

Starter from $99 (2 days, one GPT with instructions and knowledge). Standard $199 (3 days, adds API actions and a test set). Advanced $399 (5 days, an API-based assistant embedded in your tools).

Is this your problem?

• Everyone on your team prompts ChatGPT differently and gets inconsistent results

• Good prompts live in random docs and Slack messages

• The AI doesn't know your products, policies, templates or tone

• You built a GPT, but it ignores instructions or misreads your files

• You need the assistant to fetch data or take action in your tools, not just chat

• You want the assistant inside your app or intranet, not only inside ChatGPT

Why DIY custom GPTs underperform

• Instructions are vague. Short prompts can't cover your rules, edge cases, output formats and tone.

• Knowledge files aren't prepared. OpenAI recommends clear, text-forward files. Scanned PDFs, complex layouts and mixed content reduce retrieval quality.

• Platform limits. A GPT accepts up to 20 knowledge files, each up to 512 MB (OpenAI Help Center), so large knowledge bases need curating or a RAG setup.

• Actions are hard to set up. Connecting a GPT to your tools needs an OpenAPI schema, authentication and error handling.

• No testing. Most GPTs are never tested on a set of real tasks before the team relies on them.

• Outdated foundations. OpenAI's Assistants API reached end of life on August 26, 2026. New API-based assistants should be built on the Responses API.

We write, organize, connect and test, so the assistant does the job the same way every time.

Which assistant type is right for you?

We recommend the option during the scope check based on where your team works, your data and privacy needs, and your budget.

What we do

1. Scope check: define the assistant's single job, its users, inputs, outputs and what "good" looks like.

2. Instructions: write structured instructions covering role, rules, steps, output format, tone and refusal cases.

3. Knowledge: clean, convert and organize your files for accurate retrieval, and remove duplicates and outdated content.

4. Actions (Standard+): connect APIs (CRM, helpdesk, database, internal tools) with an OpenAPI schema and auth.

5. Testing: run your real example tasks, compare outputs with expectations, and refine.

6. Embed (Advanced): build the assistant on the OpenAI Responses API or the Claude API and add it to your app, Slack or intranet.

7. Handover: a usage guide for your team, an update guide for knowledge, and sharing settings.

Packages

Add-ons: Priority delivery (+50%) · Extra assistant (+$49) · Extra API action (+$29) · Team training call (+$25) · 14-day support (+$39)

Final scope and price are confirmed after a free 30-minute scope check. Subscription or API usage costs are paid to the AI provider.

Popular assistants we build

What you get

• A configured custom GPT, Claude Project, Gemini Gem or API assistant

• Structured instructions you can read and edit

• Cleaned, organized knowledge files

• API actions with schema and auth (Standard+)

• A test report on your real tasks (Standard+)

• A team usage guide and a knowledge update guide

• Proof of done: side-by-side results on your example tasks

What we need from you

Required (the delivery clock starts once we have these):

• The single job the assistant should do, and who will use it

• 10–50 example tasks or questions, with good example outputs if you have them

• Knowledge files (docs, templates, policies, FAQs)

• API docs and test credentials for any actions

• Which platform your team uses (ChatGPT, Claude, Gemini, Microsoft 365) or where to embed it

Optional:

• Brand voice or style guide

• Data privacy requirements

• Whether you need an NDA

How it works

1. Free scope check (30 min): job, platform, knowledge and package.

2. Build & test: instructions, knowledge prep, actions and testing on your tasks.

3. Deliver & prove: you see the results on your examples.

4. Review & revise: adjust within your package's revisions, then roll out to the team.

Why Codersarts Tasks

• One accountable team: a managed Codersarts engineering team, not a prompt seller.

• Tested, not guessed: every assistant is checked against your real tasks.

• Platform-neutral: ChatGPT, Claude, Gemini, Copilot or your own app, whichever fits.

• Room to grow: move from one assistant to an agent that takes action, or to a full AI product.

Related tasks

• Add AI Chatbot to Your Website: for your visitors, not your team

• Integrate OpenAI or Claude API into Your App

• Build AI Agent Workflow: when the assistant should complete tasks on its own

• Build MCP Server for Your Tools & Data: connect Claude and other clients to your systems

• Build RAG Chatbot on Your Docs: for knowledge bases too large for GPT files

Bigger than one task? Plan an AI rollout for your team →

FAQ

What is a custom GPT? A custom GPT is a version of ChatGPT set up for a specific job, with its own instructions, knowledge files and optional API actions. Claude Projects, Gemini Gems and Copilot agents offer similar setups on other platforms.

How much does it cost to build a custom GPT? From $99 for one GPT with structured instructions and up to 10 knowledge files. Standard ($199) adds up to 3 API actions and a 25-task test report. Advanced ($399) builds an API-based assistant embedded in your own tools.

How long does it take? 2 days for Starter, 3 days for Standard and 5 days for Advanced, counted from when we receive your examples and files.

Can a custom GPT connect to my CRM or internal tools? Yes, through actions that call your APIs. We create the OpenAPI schema, set up authentication and test the calls (Standard and Advanced).

How many files can a custom GPT use? OpenAI allows up to 20 knowledge files per GPT, each up to 512 MB. For larger knowledge bases, we prepare the files carefully or build an API assistant with RAG.

Who can use the GPT? You choose: only you, your workspace, anyone with the link, or the GPT Store. For private company data, we recommend sharing within your team workspace.

Should we use ChatGPT, Claude or Gemini? Use the platform your team already works in. If you need the assistant inside your own app or with full data control, we build an API assistant instead.

Is our data safe? Business plans of the major AI platforms offer workspace controls and

data-use commitments. We follow your privacy requirements, use API-based setups when stricter control is needed, and sign an NDA on request.

We built an assistant on the old OpenAI Assistants API. Can you help? Yes. OpenAI's Assistants API reached end of life on August 26, 2026. We rebuild or migrate assistants to the Responses API.

Do we own the assistant? Yes. It lives in your account or your infrastructure, and you get all instructions, files and schemas.

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Want to learn how to build GPTs yourself? Codersarts Labs → · Ask the community →