Building communication systems that enable seamless and symbiotic communication between humans and AI agents is increasingly important. This research advances the field of human-AI interaction by developing an innovative approach to decode imagined speech using non-invasive high-density functional near-infrared spectroscopy (fNIRS). Notably, this study introduces MindGPT, the first thought-to-LLM (large language model) system in the world. This study focuses on enhancing human-AI communication by utilising fNIRS data to develop a proprietary AI model called MindGPT capable of decoding imagined speech. Hemodynamic responses representing neural activity were collected from four participants instructed to imagine three different sentences.