The Forked History: Byzantine Witness and the 3-of-4 Quorum — Tested

The Forked History: Byzantine Witness and the 3-of-4 Quorum — Tested Agent Determinism Illusions (Part 19) 2026-08-20 Where this fits: Part 18 closed the runtime face of C3's boundary at capability isolation — the oracle reads from a surface the producer cannot write. Part 18's §6 named the residual this part answers: Byzantine authority . Sealing one honest oracle's history says nothing about whose view is the truth when a compromised authority can present forked views to different observers. This part maps the witness layer — the answer is not a stronger single authority, but a witness set with three separable properties, an explicit fault bound, and a governed membership surface. Part 18 ended with the oracle isolated from the producer's writable surface. Isolation answers "can the producer fake the read?" It does not answer "whose read is the truth when the authority itself equivocates?" A trusted parent that closes the verdict channel still presents the approval history. If that authority is compromised — or the CI choosing the harness is — it can show job A a signed checkpoint and job B a different fork; each job sees a locally valid tree head with an inclusion proof, and no single history exists. This part tests that shape, then the witness machinery that answers it. 1. Sealed floor ≠ global history Peter's pin-rollback reply sealed a monotonic minimum-version floor: CI could no longer resurrect an older harness with a known false-green channel. The sealed floor is honest for what it claims. It is not what it looks like at first. The split into two predicates. The sealed floor proves this job did not go backwards on the view it was shown. It does not prove the approval history itself is one global append-only log. A compromised authority can hand job A a signed checkpoint whose minimum is 2 and job B a fork whose minimum is still 1; each view carries a locally valid signature and an inclusion proof while no single history exists. cell setup result A local sea