How to Use posterly with ChatGPT: Custom MCP to First Approved Schedule

You already live in ChatGPT. You do not want a second tab for captions and a third for "did that actually go out?". Founders keep asking for social to happen in the conversation they already have.

The main way to connect is the plugin listing: Add posterly in ChatGPT. The short setup page is Add posterly in ChatGPT. This post is the advanced alternative, developer mode, for people who already use a custom MCP app or cannot use the listing.

ChatGPT is the chat layer. posterly is the publish layer (18 networks, the queue, analytics). ChatGPT drafts, validates, and schedules through the connection. It does not post until you approve.

This walkthrough goes from developer mode to the first approved schedule. The long read is the ChatGPT setup guide. OpenAI's help documents write actions on this custom MCP path as Business, Enterprise, or Edu. That limit is not confirmed for the plugin listing.

Two layers, one conversation

Keep the split boring and accurate.

• ChatGPT is where you talk, where ChatGPT Work can run longer jobs, and where Codex can use local stdio. It is not a social network.
• posterly is where accounts, media, the queue, and analytics actually live. Buy Starter + API in one checkout ($10 a month). Agency API is $29 a month on Agency. That access is required before hosted tools will schedule anything. See pricing and the ChatGPT setup page.

Chat attachments never reach posterly. The paperclip in ChatGPT is not an MCP upload. For a laptop video, ChatGPT must call create_media_drop, you open https://www.poster.ly/drop/<token>, then it calls list_media. Why that exists: Solving the 4MB MCP limit.

1. Know the plan gate before you paste a URL

Custom MCP write actions (scheduling, connecting accounts, creating keys) need ChatGPT Business, Enterprise, or Edu on the web. Pro custom MCP is read and fetch only. There is no mobile MCP. Codex, inside the ChatGPT desktop app, is the exception: it can run local stdio.

If you are on Plus or Pro and you only need drafts, you can still connect and run whoami, list_accounts, and validate_post. create_post will not run until the workspace can write.

You do not need a key in the first message if you are new. The bot can start agent signup through the public flow, then you pay in the browser.

You: I want you to set up posterly as my social scheduling agent. I do not have an account yet. Do not post anything.

ChatGPT: I can start posterly's agent signup flow. Confirm the email I should use. You will pay and set a password in your own browser. I will not ask for the card or the password.

That is the contract. ChatGPT orchestrates. You stay the human in the browser for money and credentials.

2. Agent-led signup (you still pay in the browser)

Ask ChatGPT to run the documented signup tools, not to "just make an account".

1. start_signup (or POST /api/v1/signup) with your email, usually Starter, and api_addon=true.
2. It sends a checkout handoff URL. Prefer checkout_redirect_url.
3. get_signup_session while you finish Stripe and set a password.
4. It narrates status in English: checkout pending, payment confirmed, password required, then agent access required.

No API key is created or shown until billing access is active. Hosted MCP expects a pst_live_... bearer key (or OAuth), so ChatGPT cannot see your workspace until that key is in the custom app.

ChatGPT: Checkout is ready. Open this secure posterly link and pay in your browser. I will wait and tell you when payment lands.

You: Paid. I set a password.

ChatGPT: Payment is confirmed. When the dashboard shows the first key, sign in when ChatGPT opens the posterly page. I will not continue to Instagram until whoami works.

If it pastes raw curl, tell it to stop. Signup tools return human-readable next steps by default.

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3. Add the custom MCP app (short version)

ChatGPT web talks to remote HTTP MCP only. Do not paste an npx stdio block into ChatGPT. Codex can use stdio. This post is about ChatGPT web.

After the dashboard key exists:

1. In ChatGPT on the web, open Settings, then Security and login, and turn on Developer mode.
2. Go to chatgpt.com/plugins, click + next to the search box, choose Create app, then Create MCP App.
3. Name: posterly. Server URL: https://www.poster.ly/api/mcp. Authentication: OAuth.
4. Tick "I understand and want to continue", click Create, and approve the posterly sign-in. No key to paste.
5. In a new chat, turn posterly on from the + menu.

Write actions still need Business, Enterprise, or Edu. Then a read-only check:

You: Run whoami. Do not list keys or post anything.

ChatGPT: Authenticated. I can see your workspace and scopes. No social accounts are connected yet. Which network first, Instagram or LinkedIn?

If whoami fails, stop. Do not "just schedule".

4. First connect session (OAuth stays in your browser)

Connecting Instagram or LinkedIn is another browser handoff, not a password in chat.

1. Ask which account you want first.
2. Call create_connect_session for that platform.
3. Send you the secure connect_url.
4. Poll list_accounts with view: "connect_session" and tell you what is happening.

You open the link, sign into the provider, and approve scopes on Meta or LinkedIn's own page. Sessions expire after 30 minutes.

ChatGPT: I have a 30-minute Instagram connect link. Open it, approve Meta in your browser, and stay here. I will say when posterly has stored the account.

You: Approved.

ChatGPT: Instagram is connected. I can draft. I still will not schedule until you confirm a specific post.

5. Draft, validate, then say send

"Never posts without approval" is two tools and a confirm flag.

1. ChatGPT drafts the caption (and media plan) in chat.
2. It calls validate_post. That does not create a row or enqueue publish.
3. It shows the final preview: account, caption, time in your timezone, media.
4. You confirm that exact post in this turn. "Schedule that" is confirmation. "Post about the launch every Monday" is not.
5. Only then does it call create_post with confirm: true.

Laptop media: do not use the paperclip. Ask it to call create_media_drop, open the /drop/ link, then list_media and pass public_url into validate_post. Codex can use file_path instead.

You: Draft a LinkedIn post for Thursday 10:00 UK time about the webinar. Validate it. Do not create it.

ChatGPT: Draft for your LinkedIn Page, Thursday 10:00 Europe/London. validate_post passed. No row exists yet. Say schedule it if you want this one queued.

You: Schedule it.

ChatGPT: Scheduled. View in posterly: [dashboard link]

After create_post the tools return a View in posterly URL. Current-month scheduled posts open in Calendar. Broader views use Table.

6. After the first post: Codex, Work, batch

• Codex. Same key. codex mcp add posterly --env POSTERLY_API_KEY=pst_live_... -- npx -y posterly-mcp-server@latest or --url https://www.poster.ly/api/mcp. Local file_path uploads work here.
• Weekly batch. Validate each item, confirm the set, then create_posts_batch with confirm: true.
• Brand voice and analytics. list_brands (view: "profile"), list_accounts (view: "learned_voice"), generate_captions, list_analytics. Read-only until you confirm a create.

ChatGPT Work can repeat a prompt. A scheduled task is not a blanket yes. Publishing still needs validate-then-confirm.

7. Where this usually breaks

• Wrong ChatGPT plan. Plus and Pro custom MCP cannot write. You will think the connector is "broken" when it is gated.
• Paperclip. Chat attachments never become MCP arguments. Use create_media_drop then list_media, or Codex file_path.
• Stdio pasted into ChatGPT web. That is a Claude Desktop / Codex snippet. ChatGPT web needs the hosted URL.
• Mobile. There is no MCP on the ChatGPT iOS or Android apps. Use the web app.
• Skipped confirm: true. The tool refuses. Tell it to preview and wait.
• Stale connect session. 30 minutes. Ask for a new create_connect_session.

Start in Developer Mode

Turn on Developer Mode, add the hosted URL from the MCP setup guide, let ChatGPT walk agent signup if you are new, then approve one validated post. The short ChatGPT setup page has the fields. Codex users can skip the custom app and use stdio.

You keep the chat. posterly keeps the queue. Nothing publishes until you say send.