PromptFax started with a simple frustration: I needed to send one fax and neither ChatGPT nor Claude could reliably do it for me. After building the normal web app, I tested seven ways an AI agent could interact with the same fax workflow.
This is the short, PromptFax-specific version. For the screenshots, implementation details, and full pros-and-cons comparison, read AI Agents Can Build Apps. Why Can’t They Use Them? on cogint.ai.
What I found
- An in-app LLM is useful for questions and open-ended help, but a prompt is slower than a good form for uploading a document, entering a fax number, reviewing a quote, and sending.
- Computer use is broadly compatible, but it is slow and gives the app little control. A clear, accessible interface is about all PromptFax can do to help.
- AI browser control is more efficient than interpreting screenshots, but it still has to navigate the page and can get confused by complex interactions.
- An MCP server gives agents explicit, structured tools. It is the most important machine-facing foundation, although setup and discovery still skew toward developers and power users.
- MCP Apps are the best conceptual fit for PromptFax because they combine agent reasoning with an app-owned interface for files, previews, confirmations, and checkout. Host support is still inconsistent.
- WebMCP could let browser-side agents discover structured actions directly from the website. The idea is promising, but the end-user experience is still emerging.
- A Custom GPT was the fastest way to publish a ChatGPT-specific experience using the existing API, though it does not provide the portability or richer interface of MCP Apps.
What PromptFax kept
There is no single interface that works best everywhere. PromptFax keeps the normal website as the authoritative user experience, exposes structured workflows through MCP, and uses embedded app interfaces where the host supports them. Sensitive steps such as document review, payment authorization, and sending still require clear user participation.
The hardest implementation problem was not exposing a send_fax function. It was moving documents and workflow state safely between an LLM host, PromptFax, checkout, and the fax provider. A File ID Is Not a File covers that cross-host document problem separately.