fix(loops): funnel alignment — the search tie-band matches the gate under the cost objective

bench/src/flywheel-evolve.mts

@@ -90,6 +90,7 @@ async function main(): Promise<void> {
     ...(process.env.OBJECTIVE === 'cost'
       ? { objective: 'cost' as const, scoreTolerance: Number(process.env.SCORE_TOLERANCE ?? 0.05) }
       : {}),
+    ...(process.env.CHAMPION_EPSILON ? { championEpsilon: Number(process.env.CHAMPION_EPSILON) } : {}),
     ...(process.env.LOSSES_DETAIL === 'binary' ? { lossesDetail: 'binary' as const } : {}),
     ...(process.env.REPRO ? { reproducerCheck: {} } : {}),
     // Endurance: phase ledger on disk (resume skips completed phases) + the gym recycled

src/runtime/strategy-evolution.ts

@@ -368,7 +368,12 @@ export async function runStrategyEvolution(cfg: StrategyEvolutionConfig): Promis
   const populationSize = cfg.populationSize ?? 2
   const baselines = cfg.baselines ?? [sample, refine, sampleThenRefine]
   const policy = cfg.champion ?? 'costAware'
-  const epsilon = cfg.championEpsilon ?? 0.01
+  // FUNNEL ALIGNMENT: the search-side tie band must be no stricter than the promotion
+  // criterion, or the gate never sees the candidates it was designed to judge. Under the
+  // cost objective the gate accepts score within −scoreTolerance; a candidate that the
+  // gate would promote must therefore be able to DISPLACE in search at that same band.
+  const epsilon =
+    cfg.championEpsilon ?? (cfg.objective === 'cost' ? (cfg.scoreTolerance ?? 0.05) : 0.01)
   const byName = new Map<string, Strategy>(baselines.map((s) => [s.name, s]))
   const codeByName = new Map<string, string>()
 

tests/loops/strategy-evolution.test.ts

@@ -628,3 +628,27 @@ describe('checkpoint and resume', () => {
     expect(phases).toEqual(['gen0', 'gen1', 'holdout'])
   })
 })
+
+describe('funnel alignment under the cost objective', () => {
+  it('the search tie-band defaults to the gate tolerance: a near-tie cheaper candidate displaces', () => {
+    const r = {
+      n: 1,
+      excluded: 0,
+      perStrategy: {
+        incumbent: { score: 0.733, resolved: 0, usd: 0.0249, ms: 0 },
+        cheaper: { score: 0.696, resolved: 0, usd: 0.0141, ms: 0 },
+      },
+      perTask: [],
+      pareto: [],
+    }
+    // ε=1pp (the score-objective default): the incumbent holds.
+    expect(pickChampion(r.perStrategy, ['incumbent', 'cheaper'], 'costAware', 0.01).name).toBe(
+      'incumbent',
+    )
+    // ε=5pp (the cost-objective default = the gate's scoreTolerance): the cheaper
+    // within-band candidate displaces and REACHES the gate.
+    expect(pickChampion(r.perStrategy, ['incumbent', 'cheaper'], 'costAware', 0.05).name).toBe(
+      'cheaper',
+    )
+  })
+})