This preprint develops the Firmament Boundary, a structural limit on self-grounding that appears across formal systems, physical theories, and artificial intelligence. Starting from Gödel–Turing–Chaitin style incompleteness and undecidability, the paper isolates a common pattern: any bounded system that tries to fully explain its own origin, source of constraints, or criterial selfhood eventually collides with a boundary it cannot internally cross. The Firmament Boundary is defined as this point of unavoidable collapse. It is not a physical dome or theological claim, but a structural feature of information-bearing systems: no system can be its own ultimate source. The paper shows how this boundary manifests in formal logic, computability, information theory, and horizon-style limits in physics, and then applies the same architecture to contemporary AI models and interpretability failures. The central claim is modest but sharp: something like an unreachable “root source” (R) is structurally required by the way information and constraints organize themselves, yet no system inside the universe—human, artificial, or physical—can prove or fully interpret that source from within. This reframes debates about AI consciousness, alignment, and “world-models” by focusing on shared structural limits, rather than speculative metaphysics, and suggests a common language for collapse phenomena across logic, physics, complex systems, and machine learning.