diff --git a/scripts/lib/brief-compose.mjs b/scripts/lib/brief-compose.mjs
index 52c637666..f032de3ff 100644
--- a/scripts/lib/brief-compose.mjs
+++ b/scripts/lib/brief-compose.mjs
@@ -151,7 +151,36 @@ export function userDisplayNameFromId(userId) {
 
 // ── Compose a full brief for a single rule ──────────────────────────────────
 
-const MAX_STORIES_PER_USER = 12;
+// Cap on stories shown per user per brief.
+//
+// Default 12 — kept at the historical value because the offline sweep
+// harness (scripts/sweep-topic-thresholds.mjs) showed bumping the cap
+// to 16 against 2026-04-24 production replay data DROPPED visible
+// quality at the active 0.45 threshold (visible_quality 0.916 → 0.716;
+// positions 13-16 are mostly singletons or members of "should-separate"
+// clusters at this threshold, so they dilute without helping adjacency).
+//
+// Env-tunable via DIGEST_MAX_STORIES_PER_USER so future sweep evidence
+// (different threshold, different label set, different pool composition)
+// can be acted on with a Railway env flip without a redeploy. Any
+// invalid / non-positive value falls back to the 12 default.
+//
+// "Are we getting better" signal: re-run scripts/sweep-topic-thresholds.mjs
+// with --cap N before flipping the env, and the daily
+// scripts/brief-quality-report.mjs after.
+function readMaxStoriesPerUser() {
+  const raw = process.env.DIGEST_MAX_STORIES_PER_USER;
+  if (raw == null || raw === '') return 12;
+  const n = Number.parseInt(raw, 10);
+  return Number.isFinite(n) && n > 0 ? n : 12;
+}
+// Exported so brief-llm.mjs (buildDigestPrompt + hashDigestInput) can
+// slice to the same cap. Hard-coding `slice(0, 12)` there would mean
+// the LLM prose only references the first 12 stories even when the
+// brief envelope carries more — a quiet mismatch between what the
+// reader sees as story cards vs the AI summary above them. Reviewer
+// P1 on PR #3389.
+export const MAX_STORIES_PER_USER = readMaxStoriesPerUser();
 
 /**
  * Filter + assemble a BriefEnvelope for one alert rule from a
diff --git a/scripts/lib/brief-llm.mjs b/scripts/lib/brief-llm.mjs
index 5e49503da..0e1a680b0 100644
--- a/scripts/lib/brief-llm.mjs
+++ b/scripts/lib/brief-llm.mjs
@@ -34,6 +34,10 @@ import {
   parseWhyMatters,
 } from '../../shared/brief-llm-core.js';
 import { sanitizeForPrompt } from '../../server/_shared/llm-sanitize.js';
+// Single source of truth for the brief story cap. Both buildDigestPrompt
+// and hashDigestInput must slice to this value or the LLM prose drifts
+// from the rendered story cards (PR #3389 reviewer P1).
+import { MAX_STORIES_PER_USER } from './brief-compose.mjs';
 
 /**
  * Sanitize the story fields that flow into buildWhyMattersUserPrompt and
@@ -323,7 +327,7 @@ const DIGEST_PROSE_SYSTEM =
  * @returns {{ system: string; user: string }}
  */
 export function buildDigestPrompt(stories, sensitivity) {
-  const lines = stories.slice(0, 12).map((s, i) => {
+  const lines = stories.slice(0, MAX_STORIES_PER_USER).map((s, i) => {
     const n = String(i + 1).padStart(2, '0');
     return `${n}. [${s.threatLevel}] ${s.headline} — ${s.category} · ${s.country} · ${s.source}`;
   });
@@ -422,10 +426,11 @@ function hashDigestInput(userId, stories, sensitivity) {
   // Canonicalise as JSON of the fields the prompt actually references,
   // in the prompt's ranked order. Stable stringification via an array
   // of tuples keeps field ordering deterministic without relying on
-  // JS object-key iteration order.
+  // JS object-key iteration order. Slice MUST match buildDigestPrompt's
+  // slice or the cache key drifts from the prompt content.
   const material = JSON.stringify([
     sensitivity ?? '',
-    ...stories.slice(0, 12).map((s) => [
+    ...stories.slice(0, MAX_STORIES_PER_USER).map((s) => [
       s.headline ?? '',
       s.threatLevel ?? '',
       s.category ?? '',
diff --git a/tests/brief-from-digest-stories.test.mjs b/tests/brief-from-digest-stories.test.mjs
index 1628bbc9c..d0cc01cb5 100644
--- a/tests/brief-from-digest-stories.test.mjs
+++ b/tests/brief-from-digest-stories.test.mjs
@@ -192,7 +192,14 @@ describe('composeBriefFromDigestStories — continued', () => {
     assert.deepEqual(env.data.stories.map((s) => s.headline), ['A', 'B']);
   });
 
-  it('caps at 12 stories per brief', () => {
+  it('caps at 12 stories per brief by default (env-tunable via DIGEST_MAX_STORIES_PER_USER)', () => {
+    // Default kept at 12. Offline sweep harness against 2026-04-24
+    // production replay showed cap=16 dropped visible_quality from
+    // 0.916 → 0.716 at the active 0.45 threshold (positions 13-16
+    // are mostly singletons or "should-separate" members at this
+    // threshold, so they dilute without helping adjacency). The
+    // constant is env-tunable so a Railway flip can experiment with
+    // cap values once new sweep evidence justifies them.
     const many = Array.from({ length: 30 }, (_, i) =>
       digestStory({ hash: `h${i}`, title: `Story ${i}` }),
     );