Gemini guide: Spark custom apps and posterly

This is a preview of connecting posterly as a custom app in Gemini. A successful Gemini web sign-in and tool discovery have not yet been confirmed. The human page for that setup is posterly for Gemini. This guide is the longer version of the same steps. OAuth sign-in from gemini.google.com is still being validated, so check the status before purchasing API access for Gemini web.

posterly is the publish layer. Gemini is the chat layer. After a connection exists, the useful order is whoami, list_accounts, validate_post, then create_post with confirm: true only after you clearly say yes.

Who can add a custom app

Google's custom apps help lists the gates. You need all of them:

• 18 or over, and in the US.
• A personal Google Account. Work and school accounts cannot add Spark custom apps.
• Keep Activity turned on. Custom apps are unavailable when it is off.
• Gemini in English.
• The MCP server URL. posterly's is https://www.poster.ly/api/mcp.

Connect the app on the Gemini web app. Google says that once it is connected, you can use it in the Gemini mobile app and on the web. Source: Connect and manage custom apps.

Gemini Spark itself needs a Google AI Pro or Ultra subscription, plus the age and Keep Activity rules above. Source: Use Gemini Spark.

Add the custom app

1. Check the connection status on posterly for Gemini. Do not buy API access solely for Gemini web while sign-in and tool discovery are still being validated.
2. On a computer, open gemini.google.com.
3. Open Settings, then Connected Apps, then Custom apps, then Add a custom app.
4. Paste https://www.poster.ly/api/mcp.
5. If you do not see Connected apps, open Personal Intelligence, then Connected apps. Google documents that fallback on the same help page.
6. Follow the on-screen sign-in.

posterly advertises Dynamic Client Registration, which is what Google uses when the server supports it. Sign-in from gemini.google.com is still being validated. Google documents Advanced features for servers without Dynamic Client Registration. Do not assume those fields accept a posterly API key. The origin and metadata-host allowlists are in place, but a failed sign-in or tool discovery still needs investigation. Until Spark sign-in is confirmed, use Claude, ChatGPT, or Cursor.

This is a custom app you add yourself. There is no Gemini directory listing to click.

Work and school accounts

Spark custom apps are not available if you sign in with a work or school Google Account. An administrator connects a custom MCP server in Gemini Enterprise. Google documents that admin path here: Set up your custom MCP server connection.

If you do not have an admin who can do that, use Claude, ChatGPT, or Cursor. Those clients use the same hosted endpoint.

Gemini CLI (optional)

The Gemini CLI MCP docs support local stdio servers. Use posterly's npm server with -s user for your user config and -e for the API key. Replace the placeholder key locally.

gemini mcp add -s user -e POSTERLY_API_KEY=pst_live_your_key_here \
  posterly npx -y posterly-mcp-server@latest

The hosted endpoint does not implement streamable HTTP. Do not use -t http with it.

That command configures the CLI. It does not confirm Spark web OAuth.

Laptop files

The paperclip in Gemini chat does not give posterly the file. Chat attachments never reach MCP.

For a file on your laptop, use the same path as ChatGPT web and Claude.ai:

1. Ask Gemini to call create_media_drop.
2. Open the https://www.poster.ly/drop/<token> link. No dashboard login.
3. Drop the file there.
4. Ask Gemini to call list_media with that drop_session_id.
5. Pass public_url to validate_post, then to create_post after you approve.

A public HTTPS URL can go straight to upload_media with url. Hosted base64 upload is for small images only, because the posterly API body limit is about 4MB on Vercel. Longer map: Solving the 4MB MCP limit.

First prompt

Paste this once Gemini can see the posterly tools:

Using posterly, run whoami, then list_accounts. Draft one caption for LinkedIn, X, and Instagram. Run validate_post for each, show me the drafts, and wait for my yes before create_post with confirm true.

If list_accounts is empty, connect a social account in posterly before you draft. Do not invent account IDs.

If connect fails

OAuth and dynamic client registration for gemini.google.com are still being validated. The Gemini origin and accountlinking.google.com metadata host are allowed, but that does not confirm the full connection. A failed sign-in or tool discovery needs further validation. Use Claude, ChatGPT, or Cursor until that check finishes. The hosted URL stays https://www.poster.ly/api/mcp on every client.