The attestation report is produced after the user sends a prompt to the LLM? I thought it was the proof the correct model weights are loaded on some machine.

The attestation report is produced ahead of time and verified on each connection (before the prompt is sent). Every time the client connects to do an inference request via one of the Tinfoil SDKs, the attestation report is checked relative to a known-good/public configuration to ensure the connection is to a server that is running the right model.