Example. Erasing an observed bit changes the attainable frontier [ftip-00M7]

An initial observation reveals a uniform bit \(Z\). The terminal action is a bit \(a\), and quality is \(q=\mathbf 1\{a=Z\}\). Both actions are permitted at either history and have the same cost. A full-history controller chooses \(a=Z\) and attains quality one.

Now restrict the controller's entire input to a summary that is constant at the two initial histories. Its private randomness is independent of \(Z\). If it chooses one with probability \(p\), its expected quality is \(p/2+(1-p)/2=1/2\). No such summary controller can do better.

A model that averages the reward of either fixed action over the two histories obtains \(1/2\). This is the correct summary-controller value, but it underestimates the full-history frontier by \(1/2\). For the history \(Z=a\), the actual reward is one, so the averaged value fails as a reward upper bound at that history. A pointwise reward upper bound must hold at every represented history; agreement only under a tested policy's average history distribution does not establish such a certificate. The loss of an evaluator caveat in Example [ftip-00CU] illustrates why such retained distinctions can matter in a research harness.