lang/releases/v1.0.0-20260501/el_runtime.c

@@ -7835,14 +7835,78 @@ static int eg_edge_exists_between(EngramStore* g, const char* a, const char* b)
     return 0;
 }
 
+/* engram_temporal_decay — recency shaping on the activation path.
+ *
+ * MEASURED FAILURE (2026-08-05 self-review). Census of the live graph under
+ * the previous form (uniform 168 h half-life, floor 0.05):
+ *
+ *   node type          n     median tdecay   % pinned at the 0.05 floor
+ *   Memory          1233          0.0500          81%
+ *   Knowledge       1183          0.0500          58%
+ *   BacklogItem     1057          0.0500          91%
+ *   Project          321          0.0500          98%
+ *   Tag              135          0.0500         100%
+ *
+ * The median value for EVERY node type was the clamp. A function whose median
+ * output is its floor is not a signal — it is a constant with exceptions, and
+ * the exceptions were exactly the nodes touched in the last few days.
+ *
+ * What that cost, concretely: 10 of the 13 grounded value nodes — "Precision
+ * Over Brute Force", "Honesty Before Comfort", "The System Must Accumulate" —
+ * sat at 0.05, a 20x activation penalty, while Knowledge ingested overnight
+ * sat near 1.0 and held the working-memory top slots. The decay function was
+ * quietly erasing the accumulated library in favour of whatever arrived last
+ * night. That is a direct inversion of the system's purpose.
+ *
+ * Worse, tdecay multiplies at EVERY hop (seed activation and each propagation
+ * step), so a 2-hop path through settled knowledge compounded to 0.05^2 =
+ * 0.0025. Old regions of the graph were not disfavoured; they were unreachable.
+ *
+ * EXTERNAL EVIDENCE. "Not All Memories Age the Same" (arXiv:2604.26970)
+ * measures retrieval under different decay regimes:
+ *
+ *   no temporal weighting            NDCG@5  0.274
+ *   uniform exponential decay        NDCG@5  0.015    <- 18x WORSE than none
+ *   domain-adaptive decay            NDCG@5  0.241
+ *   full adaptive hierarchy          NDCG@5  0.260
+ *
+ * Uniform exponential decay is not merely suboptimal — it is worse than having
+ * no decay at all, because it penalises stable knowledge (rarely accessed,
+ * heavily load-bearing) while failing to suppress stale volatile facts. Notably
+ * not even the full adaptive hierarchy beat switching decay off.
+ *
+ * THE FIX: make the half-life a function of how established a node is, and
+ * make the floor a preference rather than a cliff.
+ *
+ *   T_eff = T_HALF * (1 + ln(1 + activation_count))
+ *
+ * Frequently-retrieved nodes age slowly; nodes nothing has ever asked for age
+ * at the original rate. This is the spacing effect and the Lindy property in
+ * one line, it is monotone and log-bounded (a 10,000-activation node gets only
+ * a ~10x longer half-life, not a permanent exemption), and it is built from
+ * activation_count — which is measured, unlike `tier`, whose assignments are
+ * inconsistent enough to be untrustworthy here (the values node is tagged
+ * Episodic).
+ *
+ * The floor moves 0.05 -> 0.25. Given the evidence that no decay outperforms
+ * uniform decay, the honest maximum penalty for age alone is 4x, not 20x. Age
+ * should express a preference for the recent; it should never make a region of
+ * the graph structurally unreachable.
+ *
+ * Explicit per-node temporal_decay_rate still overrides lambda (77 nodes carry
+ * one) — that path is untouched and remains the escape hatch for content that
+ * genuinely should expire fast. */
+#define ENGRAM_DECAY_FLOOR 0.25
 static double engram_temporal_decay(const EngramNode* n, int64_t now_ms) {
     int64_t age_ms = now_ms - n->last_activated;
     if (age_ms <= 0) return 1.0;
     double lambda = (n->temporal_decay_rate > 0.0) ? n->temporal_decay_rate
                                                     : ENGRAM_DECAY_LAMBDA;
     double age_hours = (double)age_ms / 3600000.0;
-    double factor = exp(-lambda * age_hours / ENGRAM_T_HALF_HOURS);
-    if (factor < 0.05) factor = 0.05;
+    double t_half = ENGRAM_T_HALF_HOURS *
+                    (1.0 + log(1.0 + (double)n->activation_count));
+    double factor = exp(-lambda * age_hours / t_half);
+    if (factor < ENGRAM_DECAY_FLOOR) factor = ENGRAM_DECAY_FLOOR;
     return factor;
 }