ActionBridge: your custom GPT Actions as an MCP server

How it works

1. Paste the schema. In the GPT editor open Configure → Actions, open the action and copy the Schema box. ActionBridge validates it and shows every operation as an MCP tool, with its arguments and safety hints.
2. Choose the authentication your Action used. API keys and client secrets are encrypted before they are stored and are only ever sent to your API's own host.
3. Get the URL. Add it to ChatGPT, Claude or Cursor. The app shows copy-paste steps for each client, filled in for your bridge.

What each tool looks like

Each operation becomes one tool named after its operationId, described from its summary and description, with a JSON Schema built from its path, query and header parameters and its JSON or form body. References are resolved and recursive schemas are cut off safely. ActionBridge speaks MCP over Streamable HTTP and answers both protocol eras: the stateless 2026-07-28 revision (per-request metadata, server/discover) and the initialize-based 2024-11-05 to 2025-11-25 versions that most clients still use.

An operation marked x-openai-isConsequential: true in your GPT schema is never marked read-only. Every tool also carries openWorldHint: true, because it calls an outside API.

Connecting ChatGPT, Claude and Cursor

ChatGPT

1. On the web, open Plugins (chatgpt.com/plugins), select +, then Add custom MCP server.
2. Enter a name, choose Public endpoint and paste your bridge URL.
3. Authentication: OAuth for a private bridge (ChatGPT opens the ActionBridge page, where you paste your access key once) or No authentication for a public one.
4. Accept the risk warning, choose Create as a plugin, install it, and turn it on in a chat.

Claude

1. Open Customize → Connectors, select + Add → Add custom connector.
2. Paste the URL. Choose Sign in now (OAuth, paste the access key on the ActionBridge page), or No sign in plus a request header Authorization: Bearer <access key>.
3. Team and Enterprise: an Owner adds it under Organization settings → Connectors.

Claude Code, Cursor and VS Code

claude mcp add --transport http my-api https://…/mcp/<id> --header "Authorization: Bearer <access key>"

Cursor (~/.cursor/mcp.json) and VS Code (.vscode/mcp.json, "type": "http") take the same URL and header. The app shows each snippet filled in for your bridge.

Pricing

Pay in the app.

Security and data

ActionBridge stores the converted tool list, your encrypted credentials, hashes of your access and management keys, and request metadata (time, tool name, outcome, status code, duration) for 30 days. It never stores tool arguments, request bodies or API responses. Upstream calls go only to public HTTPS hosts; private, loopback and cloud-metadata addresses are refused, including through redirects and DNS. Full details: security and data handling.

Alternatives

If you are comfortable running a server, open-source tools convert OpenAPI to MCP too: FastMCP's from_openapi (Python), mcpo, or our own free self-host download. Enterprise gateways such as MintMCP suit large teams. ActionBridge is for people who want the GPT's Action working again quickly without hosting, securing and adding OAuth to a server themselves.

Limits in this version

• Request bodies: JSON and form-encoded. File uploads (multipart) and cookie authentication are not supported.
• Up to 150 operations per schema; assistants work best with fewer than about 40, and you can switch tools off.
• OAuth: authorization code and client credentials flows. The implicit flow is not supported (OAuth 2.1 removed it).
• Responses over 2 MB are truncated before they reach the assistant.

Guides: how to migrate custom GPT Actions to MCP · custom GPT retirement: what to do before 11 December 2026.

Frequently asked questions

Do custom GPT Actions really stop working?

Yes. OpenAI's retirement FAQ says custom GPTs stop running on 11 December 2026 (11 February 2027 for Enterprise and FedRAMP workspaces with an approved deferral). Instructions, files and connected apps can move into a ChatGPT plugin; custom Actions cannot and must be rebuilt as an MCP server or connector.

Will the MCP server work in Claude too?

Yes. The same URL works as a Claude custom connector, in Claude Code, Cursor, VS Code and any client that supports remote MCP over Streamable HTTP.

My Action used an API key. ChatGPT has no field for it now. What happens?

ActionBridge keeps the key on the server and adds it to each API call. ChatGPT signs in to the bridge with OAuth (you paste your bridge access key once on our page), so the API key is never shared with ChatGPT.

My Action used OAuth. Does each user still sign in?

Yes. Choose OAuth with per-user sign-in, enter the same client ID, secret, authorization URL and token URL, and add the ActionBridge callback URL to your OAuth app. Each person signs in to your API once; tokens are refreshed automatically.

Can I see the code?

Yes. The free self-host download from the converter is the same MCP handler ActionBridge runs, as a Cloudflare Worker or Node.js project with a README.

Is there a ChatGPT plugin directory listing?

ActionBridge does not publish anything for you. You add your bridge to your own ChatGPT or Claude account; publishing a plugin to OpenAI's directory is your choice and needs OpenAI's review.