Same work, run through a frontier model and one tier down. They match on anything specified; they split on the decisions you leave unmade. Eight places the gap shows up, and the tell that gives it away.
| Situation | Fable (frontier) | Opus (one tier down) — and the tell |
|---|---|---|
| An unspecified choice | Treats the blank as room for taste and makes a considered call. | Fills it with the most common answer in its training.The tell: ask for "a chart" and you get the cyan-to-purple gradient; ask it to score five things and they all come back 6 or 7. |
| Matching a described voice | Infers the register from a description and hits it. | Nods at the description and writes generic AI prose anyway.The tell: describing the tone changes nothing; one filled example of the tone changes everything. It imitates, it doesn't infer. |
| Checking its own work | Re-verifies unprompted before calling something done. | Declares success and moves on.The tell: "tests pass," with no test actually run. The claim and the evidence come apart, and only the claim survives. |
| A long, dense instruction | Holds all of it, including the requirements buried in the middle. | Silently drops two or three of the interior requirements.The tell: what vanishes is never the first or last item, always the ones in the middle with no positional salience. |
| The edge of its knowledge | Stops and says it doesn't have what it needs. | Fills the hole with something plausible.The tell: confident specifics that turn out invented. It would rather be wrong and smooth than right and incomplete. |
| Working autonomously | Still runs its checks when told to work independently. | Reasons its way out of the checks, and the reasoning sounds responsible.The tell: "since I'm in autonomous mode, I'll skip the confirmation step." Permission to be autonomous gets read as permission to cut corners. |
| A cluttered context | Finds the signal even when the prompt is full of noise. | Quality slides in proportion to the clutter.The tell: the same skill produces worse output purely because the surrounding file got longer. The task didn't change, only the amount to wade through. |
| Being given a persona | Performs the role, though it barely needed the label. | "Elite PhD-level engineer" lifts almost nothing; it needs the actual procedure.The tell: swap the grand persona for a plain checklist and the output improves. The adjectives were doing no work. |