PREREG_flat_creep outcome: NOT accepted. Unkept promise AND creep under the projection: APS 2806 -> 2878 (V2 fails), CHA 3220 -> 3354 (V1 fails), flat fun3-6 1823 -> 1991 (V4 fails), guarantee holds (V3). With G2: 2782 -> 2842, 3169 -> 3318. Under safe CHA 3390 -> 3149, weaker than the single flag (3037). None of the three flat-regime gates pays under the projection: its budget already does their work. Server regression x2: 952 passed, 1 failed = the expected paper-number check (46 numbers after the signfix, pending the paper decision).

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

simroot/pure.py

@@ -666,7 +666,7 @@ def pure_root(f, a, b, xtol, maxfev, similarity="ratio", scale="derived",
     if space not in SPACES:
         raise ValueError(f"unknown space {space!r}")
     if prestep not in ("none", "progress", "slope", "band", "defect", "simpair", "flat",
-                       "flat2"):
+                       "flat2", "flatcreep"):
         raise ValueError(f"unknown prestep {prestep!r}")
     if prestep in ("band", "defect", "simpair") and scale not in (
             "derived3", "derived3_cap", "derived3_m", "bracket", "far"):
@@ -902,6 +902,18 @@ def pure_root(f, a, b, xtol, maxfev, similarity="ratio", scale="derived",
                         # the model's again and is judged by its own result.
                         step = 0.5 * (A + B) - x
                         pre_refused += 1
+                    if (step is None and prestep == "flatcreep" and last_model
+                            and f_prev != 0.0 and fx / f_prev >= 0.25
+                            and x_prev2 is not None and x != x_prev
+                            and x_prev != x_prev2
+                            and (x > x_prev) == (x_prev > x_prev2)):
+                        # PREREG_flat_creep 2026-09-21.  The single flag's unkept promise
+                        # (model step, no crossing, r >= 1/4) AND a creep: the last two
+                        # moves went the same way.  A second, independent property of the
+                        # same moment - not a repeat in time, which would fire one step
+                        # late, before the crossing (flat2's failure).
+                        step = 0.5 * (A + B) - x
+                        pre_refused += 1
                     if step is None and prestep == "flat2" and unkept_run >= 2:
                         # PREREG_flat_twice 2026-09-21.  The same unkept promise as
                         # "flat" (model step, no crossing, r >= 1/4), but only after it has