Independent Inventor Consider a concrete case: a user invokes a highly capable AI model under a declared purpose of education, but the resulting capability is in fact used for a military or terrorist end -- an illustrative example, not a claim about any real deployment. When such misuse surfaces, an unresolved question follows: is the model provider liable, is the user liable, or is the jurisdiction that permitted the deployment liable? This document does not answer that question -- liability determination remains an external legal question for the responsible court, regulator, or contracting parties -- but it addresses the technical gap that makes the question unanswerable today. Systems that collect data, or grant capability, for one stated purpose routinely permit that data or capability to be consumed for a different purpose, not because the second use was authorized, but because nothing in the protocol path was capable of refusing it or of recording what was actually authorized. The most common technical control in deployment today is a self-asserted purpose string: a "purpose" claim in a token, a field in an API request, a comment in a data-sharing agreement. A self-asserted string is evidence of intent, not proof of authority, and it fails precisely when it matters most -- when the requester lies. This architecture addresses that loophole deterministically: it makes an undeclared or purpose-switched use technically detectable and refusable at the point of use, and it produces verifiable evidence of what was actually authorized, so that any subsequent liability determination can be argued from that evidence rather than from an unverifiable self-assertion. Achieving that determinism introduces a bounded, measurable amount of evaluation latency at the point of use; this document treats that latency as an acceptable, secured trade-off for closing an otherwise unverifiable gap, not as a cost to be minimized at the expense of the guarantee. The premise is not that AI innovation should slow down, any more than cars should be built slower; it is that innovation moving this fast needs the technical equivalent of a seat belt. A runnable reference implementation accompanies this document at Purpose Execution Finality Validator -- Runnable Reference Implementation (https://github.com/sangmdas/Purpose-Execution- Finality-Validator-to-Prevent-Data-Purpose-Laundering-in-AI-Systems).