# PromptFax Full Context PromptFax is a pay-per-use web app and remote MCP connector for preparing and sending one-off outbound faxes. It is designed for users who need to upload or assemble a document, enter a destination fax number, review a quote, authorize payment, and track delivery without creating a PromptFax account or subscription. Canonical site: https://promptfax.app/ MCP endpoint: https://promptfax.app/mcp MCP transport: streamable HTTP MCP authentication: OAuth for real workflow calls ## Core Public Pages - Homepage: https://promptfax.app/ - Pricing: https://promptfax.app/pricing - Machine-readable pricing: https://promptfax.app/pricing.json - FAQ: https://promptfax.app/faq - Machine-readable FAQ: https://promptfax.app/faq.json - MCP setup: https://promptfax.app/mcp-setup - MCP tools: https://promptfax.app/mcp-tools - MCP tools JSON: https://promptfax.app/mcp-tools.json - MCP submission manifest: https://promptfax.app/mcp-manifest.json - Official MCP registry metadata: https://promptfax.app/server.json - No-send MCP demo: https://promptfax.app/mcp-demo - Blog: https://promptfax.app/blog - ChatGPT WebMCP implementation guide: https://promptfax.app/blog/add-webmcp-support-chatgpt.md - Agent-interface summary Markdown: https://promptfax.app/blog/seven-ways-ai-agents-can-use-promptfax.md - Cross-host file-intake article Markdown: https://promptfax.app/blog/a-file-id-is-not-a-file.md - Privacy: https://promptfax.app/privacy - Terms: https://promptfax.app/terms ## Pricing and Limits PromptFax pricing is $2 for the first 5 pages plus 10 cents per page after that, capped at $4.50 for up to 35 pages. The user is charged only after successful delivery. PromptFax currently supports outbound faxing to United States numbers. It accepts PDF, JPG, JPEG, and PNG uploads and supports up to 35 total pages per fax. Each destination is a separate fax workflow and requires its own quote and payment authorization. ## FAQ Highlights PromptFax is outbound-only and does not currently provide inbound receiving fax numbers. PromptFax does not require a PromptFax account, signup, trial, or subscription. PromptFax can preserve the current assembled document for another send while the local browser session remains available, but each destination requires a separate payment authorization. PromptFax stores uploaded working documents privately and treats server-side working files as short-lived. Browser-local editable pages and delivery report artifacts can remain on the user's device for up to 24 hours after last activity unless cleared sooner. PromptFax does not claim HIPAA compliance and does not provide a Business Associate Agreement. ## MCP Setup Guidance Use `https://promptfax.app/mcp` as the remote MCP server URL. ChatGPT: create a custom MCP connector from ChatGPT app settings, use OAuth, and start a fresh chat after authorization. PromptFax first accepts official automatic `start_session.files[]` parameters and immediately imports valid temporary download URLs. If that handoff is missing or fails, the widget offers "Choose PDF from ChatGPT". Claude: add PromptFax as a custom connector from Claude's connector settings. PromptFax exposes the neutral PromptFax MCP App when the client negotiates the standard UI extension, with a compatibility fallback for older Claude clients. Stripe Checkout still opens externally. If the inline app is unavailable, use the hosted PromptFax session page. Other MCP clients: connect to the streamable HTTP endpoint, allow public discovery, and authorize before real workflow calls. Capability-advertising clients receive the neutral MCP App; text-only clients use the hosted session page. URL attachment requires a fetchable HTTPS URL. Raw IDs, attachment metadata, and local paths are not document inputs. ## MCP Tools Tool discovery is public for directory and client probes. Real workflow tool calls require OAuth. - `start_session`: create or resume a PromptFax MCP session. Interactive, non-destructive write. - `attach_document`: bind a PromptFax document or HTTPS PDF URLs to a session. Non-destructive write. - `get_quote`: create a fax quote from a session, document, or HTTPS PDF URL. Non-destructive write. - `checkout`: start external Stripe Checkout after quote acceptance. Non-destructive write. - `send_fax`: queue a paid fax for transmission. Destructive write. - `get_status`: fetch workflow, payment, and transmission status. Read-only and idempotent. - `retry_failed_fax`: prepare a retry flow after a retry-eligible failed fax. Non-destructive write. - `cancel`: cancel an active send when cancellation is still possible. Destructive write. ## Safety and Payment Notes The user reviews a quote before payment. Stripe Checkout handles payment authorization externally. PromptFax uses webhook-confirmed payment authorization as the source of truth; a browser redirect alone is not proof that a fax is authorized to send. PromptFax queues a real fax only after explicit user intent and confirmed payment authorization. Delivery status and delivery reports are available through the web app, ChatGPT widget, PromptFax MCP App, and status polling where supported. ## Technical Notes ### How to Add WebMCP Support for ChatGPT to Your Web App ChatGPT's Site tools implementation currently discovers imperative JavaScript tools registered in the top-level page, but not declarative HTML form tools or tools registered inside iframes. PromptFax wraps its existing page handlers rather than maintaining a parallel agent-only workflow. Its production test exposed additional requirements around cross-origin browser uploads, truthful asynchronous completion, persisted workflow state, human-controlled payment, idempotent cleanup, and testing in ChatGPT's actual built-in browser. - Article: https://promptfax.app/blog/add-webmcp-support-chatgpt - Clean Markdown: https://promptfax.app/blog/add-webmcp-support-chatgpt.md ### Seven Ways AI Agents Can Use PromptFax PromptFax tested seven ways an AI agent can interact with a web application: an in-app LLM, computer use, AI browser control, an MCP server, MCP Apps, WebMCP, and a Custom GPT. The normal website remains the authoritative user experience, MCP provides structured workflows, and embedded app interfaces are used when the host supports them. - PromptFax summary: https://promptfax.app/blog/seven-ways-ai-agents-can-use-promptfax - Clean Markdown: https://promptfax.app/blog/seven-ways-ai-agents-can-use-promptfax.md - Full comparison on cogint.ai: https://cogint.ai/ai-agents-can-build-apps-why-cant-they-use-them/ ### A File ID Is Not a File Chat attachment identifiers, metadata, and model-host local paths are not portable document inputs. PromptFax accepts a fetchable temporary HTTPS URL or uploaded bytes, imports them immediately into PromptFax-owned storage, and preserves the workflow when a host handoff fails. ChatGPT uses its official file-parameter contract; interactive MCP Apps hosts use the embedded PromptFax picker; text-only clients use a hosted upload or real HTTPS URL. - Article: https://promptfax.app/blog/a-file-id-is-not-a-file - Clean Markdown: https://promptfax.app/blog/a-file-id-is-not-a-file.md ## Machine-readable Metadata - MCP manifest: https://promptfax.app/mcp-manifest.json - MCP tools JSON: https://promptfax.app/mcp-tools.json - MCP registry server.json: https://promptfax.app/server.json - Square PNG logo 512: https://promptfax.app/brand/promptfax-logo-512.png - Square PNG logo 1024: https://promptfax.app/brand/promptfax-logo-1024.png - Square SVG logo: https://promptfax.app/brand/promptfax-logo.svg - OG image: https://promptfax.app/brand/promptfax-og.png