MCP vs Custom GPTs

If you have ever built a Custom GPT and uploaded files to it, you know the workflow. Export your documents. Upload them. Wait for processing. Then hope ChatGPT finds the right information when you ask.

It works. Until your documents change. Then you re-upload. And re-upload. And re-upload.

What are Custom GPTs?

Custom GPTs are customized versions of ChatGPT with instructions, optional knowledge files, and optional capabilities like actions or apps. You give your GPT a name, upload reference documents, and ChatGPT uses semantic retrieval over those files to answer your questions.

Custom GPTs are useful. They let you create focused AI assistants for specific tasks. But the knowledge files have a fundamental limitation: they are frozen in time. If your source material changes, your Custom GPT does not know.

What is MCP?

MCP (Model Context Protocol) is an open standard that lets AI assistants connect to external data sources and tools in real time.

Instead of uploading a copy of your data, you give the AI a live connection to the source. When you ask a question, the AI reads the current version of your data. Not a snapshot from last Tuesday.

MCP is supported by multiple AI clients. Claude supports it broadly. ChatGPT supports MCP connectors, though availability depends on your plan and rollout. Developer tools like Cursor also support it.

The key difference

Custom GPT knowledge files are copies. MCP works with the source.

Comparison

The re-uploading problem

This is the biggest pain point with Custom GPT knowledge files. Your knowledge changes. Meeting notes get updated. Documentation evolves. Research grows. But your Custom GPT is stuck with whatever you uploaded last.

You have two options: re-upload constantly, or accept that your AI is working with outdated information. Most people end up doing neither. They upload once, forget about it, and their Custom GPT slowly becomes useless.

MCP eliminates this entirely. Your AI reads the live source. When you update a note, the AI sees the update. No action required.

No vendor lock-in

Custom GPTs only work with ChatGPT. If you switch to Claude or start using Cursor for coding, your Custom GPT cannot come with you. Your knowledge is locked inside one vendor's ecosystem.

MCP is an open standard. One MCP server can serve multiple clients, though each client still needs its own connection and permission setup. Your knowledge is not locked to any single AI provider.

Your AI can write, not just read

Knowledge files in Custom GPTs are read-only. Actions and apps can add write capabilities separately, but that is a different mechanism from knowledge uploads.

With MCP, read and write access come through the same protocol. Ask Claude to save a summary of your conversation as a note. Ask ChatGPT to update your project documentation. The AI becomes a participant in your knowledge system, not just a consumer of it.

When to use which

Use a Custom GPT when:

• You have a small, static set of reference documents that rarely change
• You only use ChatGPT
• Your content does not need to stay current

Use MCP when:

• Your knowledge is alive and changes regularly
• You use more than one AI client
• You are tired of re-uploading files every time something changes
• You want your AI to write notes, not just read them

If your problem is stale knowledge, MCP is often a better fit than file uploads.

What if I already use Custom GPTs?

You do not have to choose one or the other. A practical migration path:

• Keep GPT instructions for behavior. Custom instructions, tone, personality. Those still work.
• Replace knowledge file uploads with live MCP access. Move the content you were uploading into a knowledge base your AI can read live.
• Use uploads for truly static content. If a document never changes, a knowledge file is fine. Use MCP for everything else.

This way you get the best of both: Custom GPT personality with live knowledge access.

How Hjarni makes this easy

Hjarni is a knowledge base with a built-in MCP server. You write notes in Markdown. Claude and ChatGPT can read them live via MCP. No uploading. No re-uploading. No infrastructure.

Sign up, write your first note, and connect ChatGPT via MCP in under five minutes. Or connect Claude. Or both. Your notes can be connected in any MCP-compatible client.

Stop re-uploading files. Give your AI a live connection to your notes.

Give your AI a memory