ChatGPT MCP setup for thirds.ai PDFs and images

With thirds.ai, ChatGPT fills a design you saved once, such as an invoice or a social post, and hands back a finished PDF or image with your logo, colours, and fonts. To use the thirds.ai MCP server from ChatGPT, add it as a Developer mode app with only the URL https://thirds.ai/mcp, then click Allow on a thirds.ai page. You can also connect through Codex in the ChatGPT desktop app. If you use a Business, Enterprise, or Edu workspace that still lets you make custom GPTs, a private GPT with Actions also works. Each successful PDF or image costs one credit.

You stop rebuilding the same layout in each chat, and every file keeps the same look. A verified account gets 50 free credits each month, so you can try it before you pay.

To make the same files without ChatGPT or any setup, start in the editor with how to make a template you can reuse. See the ChatGPT image examples for designs and prompts to try after setup. To make post images from another app, see the AI social media post generator guides for every AI tool you can connect.

Which ChatGPT setup works with thirds.ai?

Three ChatGPT setups work with the thirds.ai MCP server. A Developer mode app needs only the URL and works on Pro, Plus, Business, Enterprise, and Education accounts on the web, as OpenAI's Developer mode guide says (checked 27 September 2026). Codex in the ChatGPT desktop app works on every plan that includes Codex. A private GPT with Actions works only in a Business, Enterprise, or Edu workspace that still lets you create GPTs. The setups differ in how ChatGPT signs in and in who can use them. The thirds.ai plugin packs the same server for the ChatGPT desktop app and Codex.

OpenAI's ChatGPT release notes of 11 September 2026 say that OpenAI plans to retire custom GPTs across ChatGPT plans. OpenAI's guide to creating GPTs says that personal accounts (Free, Go, Plus, and Pro) can't create new GPTs, and that Business, Enterprise, and Edu workspaces still can when their settings allow it. Existing GPTs keep working until their retirement date, which that guide gives as 11 December 2026 for affected Enterprise workspaces. Check your account notices before you set up a new GPT.

Add thirds.ai as a ChatGPT Developer mode app

A Developer mode app connects ChatGPT to the thirds.ai MCP server with only the URL, so you never copy a key. It takes five steps. You need a thirds.ai account and ChatGPT on the web with a Pro, Plus, Business, Enterprise, or Education account. The ChatGPT steps follow OpenAI's guide to connecting an app in ChatGPT (checked 27 September 2026).

1. In ChatGPT, open Settings, select Security and login, and turn on Developer mode.
2. Go to ChatGPT Plugins and select the plus button.
3. Enter a name, such as thirds.ai, and a short description. Under Connection, enter https://thirds.ai/mcp, then create the connection.
4. When the thirds.ai page opens, sign in if you need to and click Allow.
5. Check the thirds.ai tools that ChatGPT found. In a chat, choose Developer mode from the plus menu and select the thirds.ai app.

To stop the app, open your thirds.ai Settings, find it under Connected apps, and click Disconnect. The app loses access right away.

OpenAI's Developer mode guide says the mode is meant for developers, and it warns about prompt injection and model mistakes on write actions. Ask ChatGPT to show you each action and its credit cost before it makes a file.

What the thirds.ai app can and can't do in ChatGPT

The thirds.ai app lets ChatGPT make PDFs and images from gallery templates and your saved designs, in your own logo, colours, and fonts. ChatGPT works only with the designs, brand kits, and files in the thirds.ai account that clicked Allow. It spends that account's credits, and you can ask ChatGPT to show the cost before each paid step.

The app can:

• Find gallery templates, your saved designs, and your brand kits by name.
• Make a PDF, PNG, JPEG, or WebP from a design, filled with your client's name, this week's price, or the invoice total.
• Preview a design with your details for free before you spend credits.
• Save your logo, colours, fonts, and tone as a brand kit, and add a logo or font file to it.
• Turn your own HTML, a short description, or a picture you upload into a design you can edit and use again.
• Make one file for each row of your list, fit one image design to new sizes, or export the carousel slides you pick.
• Check a file's progress and give you its download link.

The app can't:

• Make AI pictures. Use the thirds.ai editor for those, or connect Codex with an API key.
• Edit a PDF or Word file you already have.
• Make videos, post to social networks, or send email.
• See another person's thirds.ai account.

Add the thirds.ai plugin to the ChatGPT desktop app

The thirds.ai plugin adds the thirds.ai server to the ChatGPT desktop app, and you sign in with only the URL. OpenAI's plugin guide says a plugin you install in the desktop app works in Chat or Work, and in Codex (checked 30 September 2026). The plugin lives in the public thirds.ai skills repository, in the chatgpt-plugin folder, and it takes four steps to install.

1. Save the personal marketplace file from the Codex plugin steps as ~/.agents/plugins/marketplace.json.
2. Quit the ChatGPT desktop app fully and open it again.
3. Open Plugins, select Personal, find thirds.ai, and select the plus button.
4. When the thirds.ai page opens, sign in if you need to and click Allow.

Start a new chat and type @ to pick the thirds.ai plugin, or just ask for the file you want. OpenAI's plugin packaging guide says the desktop app reads a personal marketplace from that file (checked 30 September 2026).

Connect thirds.ai to Codex in the ChatGPT desktop app

You connect Codex to the thirds.ai MCP server in three steps, and then ChatGPT's Codex can make files from your saved designs. You need a thirds.ai account and Codex in the ChatGPT desktop app, the Codex CLI, or the IDE extension.

1. Create an API key on the API keys page.
2. Set THIRDS_API_KEY to that key in the environment that starts Codex.
3. Run this command, then restart Codex:

codex mcp add thirds --url https://thirds.ai/mcp \
  --bearer-token-env-var THIRDS_API_KEY

OpenAI's Codex MCP guide says that the ChatGPT desktop app, the Codex CLI, and the IDE extension share one MCP configuration (checked 25 September 2026), so you add the server once. The Codex setup page has the config.toml table, the check step, fixes for a missing token, and the way to sign in with only the URL.

Create your private GPT

A private GPT with Actions lets ChatGPT make thirds.ai images and PDFs from gallery designs in eight steps. You need a Business, Enterprise, or Edu workspace that still lets you create GPTs with Actions. Your workspace must allow the thirds.ai domain. Use a model that supports Actions. Pro mode does not.

1. Sign in to thirds.ai and create an API key.
2. Open the GPT editor in ChatGPT and create a GPT.
3. Open Actions, then click Create new action.
4. Choose Import from URL and enter https://thirds.ai/agents/chatgpt-actions.json.
5. Set Authentication to API key, then choose Bearer.
6. Paste your key into that secure authentication field and save.
7. Add the instructions below to your GPT.
8. Save the GPT for Only me and test it in Preview.

Keep this GPT private. Anyone who can use its Actions could spend your thirds.ai credits. Never put the key in a chat, GPT instructions, or the schema.

The schema contains six actions. They read gallery designs, check the connection, create files, and check file status. It cannot delete files or change your account. File creation is marked as consequential so ChatGPT asks for confirmation.

Read OpenAI's Actions setup guide and authentication guide for current controls.

Add these GPT instructions

The GPT instructions below tell ChatGPT how to use the thirds.ai Actions safely: check the connection, copy a gallery design, ask before it spends credits, and never retry a lost request by itself. Paste them into your GPT's instructions field as they are.

Use thirds.ai Actions to make files from the user's chosen gallery design.
Never ask for an API key in chat. The key belongs in Action authentication settings.
First call listTemplates to check the connection. An empty list is a valid result.
Call getGalleryTemplate with the exact gallery slug from the user's link.
Use its source as html. Start data from its full sample, then apply the user's changes.
Keep the layout, styles, and all sample fields that the user has not changed.
Ask for missing details. Never invent prices, dates, totals, or business results.
For PNG images, read the source canvas dimensions and set image.width and image.height.
For PDFs, respect the source page size. Ask if the intended size is unclear.
Show the file format, count, and cost. Each successful file costs one credit.
Get the user's approval before createImage or createPdf. Use wait=false.
Create each approved file once. Actions cannot send custom retry headers.
If creation loses its response or returns no id, stop. Ask the user to check thirds.ai file history.
Get fresh approval before another creation attempt. Never retry a lost creation response automatically.
Poll getImage or getPdf with the returned id while status is queued or running.
Stop on succeeded, failed, or cancelled. After two minutes, keep the id and offer to check again.
Do not call createImage or createPdf again just to check progress.
Only return download.url when status is succeeded and the URL is present.
If an action fails, explain the returned error. Never invent a file link.
Do not substitute ChatGPT image generation for thirds.ai.

The image action makes PNG files. This limited setup uses gallery source and sample data without saving a new reusable template.

Actions has a 45-second request timeout and a limit of 100,000 characters per request or response. The schema caps source HTML at 95,000 characters. Keep the complete request below 100,000 characters too. Use wait=false, then poll the returned ID. For a larger design, use an MCP client instead of truncating its source.

Actions does not support custom headers such as Idempotency-Key. If a creation response is lost, check your file history before approving another attempt. See OpenAI's Actions production notes.

Check the connection

You check the private GPT's connection by asking it to call listTemplates. This read-only check costs no credits. A successful response confirms that the key works, even if you have no saved designs.

Then copy a prompt from the ChatGPT page. Ask the GPT to read the linked gallery design before creating a file. Check the result before making more.

Privacy, terms, and help

When you click Allow, ChatGPT can make files and read and change your designs and brand kits with your thirds.ai account and credits. Our privacy policy says what we keep about each connected app and for how long, and the Terms of Service cover your use of thirds.ai through ChatGPT. For help with the app, the plugin, or a failed file, contact support.

FAQ

Can I use the normal ChatGPT chat box?

No. A prompt in a normal ChatGPT chat doesn't connect your thirds.ai account. Add a Developer mode app, use Codex in the ChatGPT desktop app, or use the private GPT you configured.

Can I use ChatGPT Developer mode?

Yes. Add thirds.ai as a Developer mode app with the URL https://thirds.ai/mcp, then click Allow on the thirds.ai page. You don't need an API key. OpenAI's Developer mode guide lists OAuth as a supported sign-in (checked 27 September 2026), and thirds.ai uses it for this connection. The app doesn't get the AI image tools.

What if my account cannot create GPTs with Actions?

Add a Developer mode app, use Codex in the ChatGPT desktop app, or use another supported tool such as Claude Code. A prompt cannot turn on a feature that your account or workspace blocks.

Does connecting spend credits?

No. Connecting and listing tools don't spend credits. AI template work and successful renders can. Ask ChatGPT to show the action and the cost before it starts paid work.

Does this setup guarantee every prompt works?

No. The schema follows the published API, but your account controls which Actions you can use. Test the connection in Preview and check your first finished file.