Verification should confirm that the builder identity is one of the trusted builders mapped to a level, check the signature on the provenance envelope, and match the build type and external parameters to expectations. Step 1: once, configure the verifier's roots of trust (recognised builder identities and public keys, each with the highest Build level it is trusted for); then for each artifact verify the envelope signature against those roots, check that the statement subject matches the artifact digest, check that the predicate type is the SLSA provenance v1 type, and look up the level from the recognised key and builder.id, defaulting to Build L1. L3 establishes accurate provenance assuming the platform is trusted but does not cover a malicious platform insider, so builders should be added to the roots of trust with care.
The graph holds this control, the 0 it maps to, and the evidence behind each claim, over MCP and REST.