SimplePDF Copilot: Client-Side AI Tool Calling for PDF Form Filling

SimplePDF Copilot is an AI assistant that interacts with the SimplePDF editor to fill forms, answer questions, focus fields, add fields, and delete pages. The PDF never leaves the browser — parsing, rendering, and field detection run entirely client-side. The LLM only receives the text content and your messages, sent to your chosen provider (default DeepSeek V4 Flash, rate-capped; BYOK supported for cloud or local via LM Studio).

Key details

• Tool calling executed client-side: The LLM issues tool call intents, but execution happens in the browser via iframe postMessage. This enables speed (client-to-client) and data control — you can remove the tool that exposes document content and the LLM never sees it.
• Field detection uses CommonForms by Joe Barrow with extra post-processing heuristics for accurate form field identification.
• Stack: Tanstack Start, AI SDK from Vercel, Tailwind.
• Open source: The Copilot overlay and client-side tool calling logic are open source on GitHub. SimplePDF itself (the iframe) remains proprietary.
• Privacy first: Designed for healthcare customers where PII cannot leave the browser. The PDF is never uploaded.

By default the demo sends document content to the LLM, but you can sever that connection by removing the content-exposing tool. The demo is fully functional at the link below.