Developed PromptShield, a privacy-focused Chrome Extension designed to block sensitive data from being accidentally submitted to LLMs like ChatGPT. I implemented real-time scanning using JavaScript/TypeScript to detect credit cards (validated via the Luhn algorithm), SSNs, API keys, and plaintext passwords before submission. The extension features a non-intrusive UI overlay utilizing HTML/CSS to warn users of detected risks, offering options to manually edit or automatically redact sensitive information. It was engineered to run 100% locally within the browser, ensuring zero data collection, transmission, or external analytics for maximum user privacy, and was successfully deployed and published to the Google Chrome Web Store.