diff --git a/content/dialogue/season1/letter-from-the-garden.ink b/content/dialogue/season1/letter-from-the-garden.ink
new file mode 100644
index 0000000..4977ee4
--- /dev/null
+++ b/content/dialogue/season1/letter-from-the-garden.ink
@@ -0,0 +1,47 @@
+// Letter from the garden — UX-02 + CONTEXT D-17 + D-18 + D-20.
+//
+// Composed from authored skeleton + templated insertions per CONTEXT D-17.
+// Slots populated at runtime from sim/offline/events.ts via the variable
+// map in src/content/ink-loader.ts.
+//
+// Per Pitfall 4: Ink VAR names are snake_case AND case-sensitive.
+// Per CONTEXT D-11: 24h offline cap is silent in voice — no numeric
+// "28h" copy in any branch.
+// Per CLAUDE.md Tone — the gardener-keeper voice. Warm. Specific.
+// Intermittent. Sometimes funny, sometimes devastating. Never a stat
+// dump (UX-02 explicitly forbids that). The skeleton holds the voice;
+// the slots fill in the specifics.
+// Per anti-fomo-doctrine.md: this letter is NOT a "you missed X — come
+// back tomorrow!" nag. It is a contemplative summary of what stayed.
+
+VAR plants_bloomed = 0
+VAR fragment_titles = ""
+VAR lura_was_here = false
+VAR fragment_count = 0
+VAR last_plant_type = ""
+
+== letter ==
+
+The garden held its breath while you were gone.
+
+{ plants_bloomed > 1:
+    {plants_bloomed} blooms came and went, each leaving the soil a little quieter than they found it.
+- else:
+  { plants_bloomed == 1:
+      One bloom came and went. The space it left feels generous, somehow.
+  - else:
+      Nothing bloomed. The wind carried something else, and the garden held that, too.
+  }
+}
+
+{ fragment_titles != "":
+    Among what stayed: {fragment_titles}.
+}
+
+{ lura_was_here:
+    Lura came by once. She did not knock. She left a folded leaf on the gate post — you'll find it when you next walk past.
+}
+
+The light is the same as when you left. The garden is older.
+
+-> END
diff --git a/src/content/ink-loader.ts b/src/content/ink-loader.ts
index b2d41a1..2fc4a08 100644
--- a/src/content/ink-loader.ts
+++ b/src/content/ink-loader.ts
@@ -31,11 +31,19 @@ const compostStoryGlob = import.meta.glob(
   { query: '?raw', import: 'default' },
 );
 
+// Plan 02-05 — letter Ink (UX-02). Lazy-loaded by the Letter overlay
+// when the boot path determines absence ≥5min and opens the overlay.
+const letterStoryGlob = import.meta.glob(
+  '/src/content/compiled-ink/season1/letter-from-the-garden.ink.json',
+  { query: '?raw', import: 'default' },
+);
+
 export type InkBeatName =
   | 'lura-arrival'
   | 'lura-mid'
   | 'lura-farewell'
-  | 'compost-acknowledgements';
+  | 'compost-acknowledgements'
+  | 'letter-from-the-garden';
 
 /**
  * INK_VARIABLE_MAP — the centralized snake_case mapping (Pitfall 4).
@@ -45,10 +53,30 @@ export type InkBeatName =
  * camelCase typo fails CI rather than silently leaving the variable
  * unbound.
  *
- * Phase 2 ships these three slots — `last_fragment_title` is reserved
- * for Plan 02-05's letter prose authoring (W4) but is exposed now so
- * the Ink files can read it without a follow-up patch.
+ * Phase 2 ships:
+ *   - fragment_count / last_plant_type / last_fragment_title (Plan 02-04)
+ *     — used by Lura's Ink files.
+ *   - plants_bloomed / fragment_titles / lura_was_here (Plan 02-05)
+ *     — used by letter-from-the-garden.ink. These read from the
+ *     SessionSlice's pendingLetterEventBlock (set by the boot path
+ *     when a returning player has been away ≥5min, per CONTEXT D-20).
  */
+
+/** Shape of pendingLetterEventBlock when the boot path populates it. */
+type PendingLetterEvents = {
+  plantsBloomedCount?: Record<string, number>;
+  harvestedFragmentIds?: string[];
+  luraBeatPending?: string | null;
+};
+
+function readPendingLetterEvents(
+  s: AppStoreShape,
+): PendingLetterEvents | null {
+  const block = s.pendingLetterEventBlock;
+  if (!block || typeof block !== 'object') return null;
+  return block as PendingLetterEvents;
+}
+
 export const INK_VARIABLE_MAP = {
   fragment_count: (s: AppStoreShape) => s.harvestedFragmentIds.length,
   last_plant_type: (s: AppStoreShape): string => {
@@ -75,6 +103,40 @@ export const INK_VARIABLE_MAP = {
     if (!frag) return '';
     return frag.body.split(/[.!?]/)[0]?.trim() ?? '';
   },
+  // Plan 02-05 — letter slots. Read from session.pendingLetterEventBlock
+  // populated by the boot path's offline catchup loop.
+  plants_bloomed: (s: AppStoreShape): number => {
+    const events = readPendingLetterEvents(s);
+    if (!events?.plantsBloomedCount) return 0;
+    return Object.values(events.plantsBloomedCount).reduce(
+      (a, b) => a + b,
+      0,
+    );
+  },
+  fragment_titles: (s: AppStoreShape): string => {
+    const events = readPendingLetterEvents(s);
+    const ids = events?.harvestedFragmentIds ?? [];
+    if (ids.length === 0) return '';
+    // Convert ids to a comma-joined human-readable list. Prefer the
+    // fragment's first-sentence body (tonal weight); fall back to a
+    // slugified id if the fragment is missing.
+    return ids
+      .map((id) => {
+        const frag = allFragments.find((f) => f.id === id);
+        if (frag) {
+          const firstLine = frag.body.split(/[.!?]/)[0]?.trim() ?? '';
+          if (firstLine.length > 0 && firstLine.length <= 60) {
+            return firstLine.toLowerCase();
+          }
+        }
+        return id.replace(/^season\d+\./, '').replace(/[._-]+/g, ' ');
+      })
+      .join('; ');
+  },
+  lura_was_here: (s: AppStoreShape): boolean => {
+    const events = readPendingLetterEvents(s);
+    return Boolean(events?.luraBeatPending);
+  },
 } as const;
 
 function compiledInkPath(name: InkBeatName): string {
@@ -96,13 +158,23 @@ function stripBom(s: string): string {
  * choosing the entry knot/path. Throws if the compiled artefact is
  * missing — runs the diagnostic message past the cause:
  *   "Did `npm run compile:ink` succeed?"
+ *
+ * Plan 02-05: dispatch extended to support the letter-from-the-garden
+ * Ink (UX-02). The three globs — luraStoryGlob, compostStoryGlob,
+ * letterStoryGlob — give Vite three independent code-split chunks so
+ * the letter doesn't enter the entry bundle until a returning player
+ * triggers it.
  */
 export async function loadInkStory(name: InkBeatName): Promise<Story> {
   const path = compiledInkPath(name);
-  const loader =
-    name === 'compost-acknowledgements'
-      ? compostStoryGlob[path]
-      : luraStoryGlob[path];
+  let loader;
+  if (name === 'compost-acknowledgements') {
+    loader = compostStoryGlob[path];
+  } else if (name === 'letter-from-the-garden') {
+    loader = letterStoryGlob[path];
+  } else {
+    loader = luraStoryGlob[path];
+  }
   if (!loader) {
     throw new Error(
       `[ink-loader] No compiled story at ${path}. Did 'npm run compile:ink' succeed?`,
diff --git a/src/sim/garden/auto-harvest.test.ts b/src/sim/garden/auto-harvest.test.ts
new file mode 100644
index 0000000..b2a4775
--- /dev/null
+++ b/src/sim/garden/auto-harvest.test.ts
@@ -0,0 +1,141 @@
+import { describe, it, expect } from 'vitest';
+import type { SimState } from '../state';
+import type { Fragment } from '../../content';
+import { autoHarvestReadyPlants } from './auto-harvest';
+import { type SimContext } from './commands';
+import { emptyTiles, type Tile } from './types';
+import { PLANT_TYPES } from './plants';
+import type { OfflineEventBlock } from '../offline/events';
+
+// Deeper warm-tag pool so multi-rosemary tests don't exhaust before the
+// auto-harvest sweep finishes. The selector is no-dup, so we need at
+// least one warm fragment per ready tile we expect to harvest.
+const fixtureFragments: Fragment[] = [
+  { id: 'season1.soil.f-warm-1', season: 1, body: 'warm-1', tags: ['warm'] },
+  { id: 'season1.soil.f-warm-2', season: 1, body: 'warm-2', tags: ['warm'] },
+  { id: 'season1.soil.f-warm-3', season: 1, body: 'warm-3', tags: ['warm'] },
+  { id: 'season1.soil.f-warm-4', season: 1, body: 'warm-4', tags: ['warm'] },
+  { id: 'season1.soil._exhaustion', season: 1, body: 'sentinel', tags: ['_meta'] },
+];
+const silentCtx: SimContext = {
+  fragments: fixtureFragments,
+  currentSeason: 1,
+  silent: true,
+};
+
+function freshSimState(overrides: Partial<SimState> = {}): SimState {
+  return {
+    garden: { tiles: emptyTiles() },
+    plants: [],
+    harvestedFragmentIds: [],
+    lastTickAt: 0,
+    tickCount: 0,
+    unlockedPlantTypes: ['rosemary'],
+    luraBeatProgress: { arrived: false, mid: false, farewell: false, pending: null },
+    offlineEvents: null,
+    settings: { musicVolume: 0.7, ambientVolume: 0.5, sfxVolume: 0.8, persistenceToastShown: false },
+    ...overrides,
+  };
+}
+
+function withReadyRosemaryAt(...indices: number[]): SimState {
+  return freshSimState({
+    garden: {
+      tiles: emptyTiles().map((t, i) =>
+        indices.includes(i)
+          ? { idx: i, plant: { plantTypeId: 'rosemary', plantedAtTick: 0 } }
+          : t,
+      ),
+    },
+  });
+}
+
+describe('autoHarvestReadyPlants (D-10 silent-mode harvest)', () => {
+  it('harvests a single ready rosemary and records offlineEvents', () => {
+    const state = withReadyRosemaryAt(0);
+    const next = autoHarvestReadyPlants(
+      state,
+      PLANT_TYPES.rosemary.durationTicks,
+      silentCtx,
+    );
+    expect((next.garden.tiles as Tile[])[0]?.plant).toBeNull();
+    expect(next.harvestedFragmentIds.length).toBe(1);
+    expect(next.offlineEvents).not.toBeNull();
+    const events = next.offlineEvents as OfflineEventBlock;
+    expect(events.plantsBloomedCount.rosemary).toBe(1);
+    expect(events.harvestedFragmentIds.length).toBe(1);
+  });
+
+  it('harvests two ready rosemaries and accumulates plantsBloomedCount.rosemary=2', () => {
+    const state = withReadyRosemaryAt(0, 5);
+    const next = autoHarvestReadyPlants(
+      state,
+      PLANT_TYPES.rosemary.durationTicks,
+      silentCtx,
+    );
+    expect((next.garden.tiles as Tile[])[0]?.plant).toBeNull();
+    expect((next.garden.tiles as Tile[])[5]?.plant).toBeNull();
+    expect(next.harvestedFragmentIds.length).toBe(2);
+    const events = next.offlineEvents as OfflineEventBlock;
+    expect(events.plantsBloomedCount.rosemary).toBe(2);
+    expect(events.harvestedFragmentIds.length).toBe(2);
+  });
+
+  it('does NOT harvest immature plants (sprout / mature stage)', () => {
+    const state = withReadyRosemaryAt(0);
+    // Tick 100 — sprout still (durationTicks = 600, mature at 33% = 198)
+    const next = autoHarvestReadyPlants(state, 100, silentCtx);
+    expect((next.garden.tiles as Tile[])[0]?.plant).not.toBeNull();
+    expect(next.harvestedFragmentIds.length).toBe(0);
+  });
+
+  it('returns the SAME state reference when there are no ready plants (empty grid)', () => {
+    const state = freshSimState();
+    const next = autoHarvestReadyPlants(state, 1000, silentCtx);
+    expect(next).toBe(state);
+  });
+
+  it('after the 1st auto-harvest crosses the threshold, offlineEvents.luraBeatPending === "arrival"', () => {
+    const state = withReadyRosemaryAt(0);
+    const next = autoHarvestReadyPlants(
+      state,
+      PLANT_TYPES.rosemary.durationTicks,
+      silentCtx,
+    );
+    expect(next.luraBeatProgress.pending).toBe('arrival');
+    const events = next.offlineEvents as OfflineEventBlock;
+    expect(events.luraBeatPending).toBe('arrival');
+  });
+
+  it('does NOT modify lastTickAt (BLOCKER 3 — saveSync owns that field)', () => {
+    const state = freshSimState({
+      lastTickAt: 99999,
+      garden: {
+        tiles: emptyTiles().map((t, i) =>
+          i === 0
+            ? { idx: i, plant: { plantTypeId: 'rosemary', plantedAtTick: 0 } }
+            : t,
+        ),
+      },
+    });
+    const next = autoHarvestReadyPlants(
+      state,
+      PLANT_TYPES.rosemary.durationTicks,
+      silentCtx,
+    );
+    expect(next.lastTickAt).toBe(99999);
+  });
+
+  it('preserves prior offlineEvents when a non-ready tile sweep yields no new harvest', () => {
+    const priorEvents: OfflineEventBlock = {
+      plantsBloomedCount: { rosemary: 3 },
+      harvestedFragmentIds: ['season1.soil.f-warm-1', 'season1.soil.f-warm-2', 'season1.soil.f-warm-3'],
+      luraBeatPending: 'arrival',
+    };
+    const state = freshSimState({ offlineEvents: priorEvents });
+    const next = autoHarvestReadyPlants(state, 500, silentCtx);
+    // Empty grid → no new harvest → state ref preserved.
+    expect(next).toBe(state);
+    expect(next.offlineEvents).toBe(priorEvents);
+  });
+});
diff --git a/src/sim/garden/auto-harvest.ts b/src/sim/garden/auto-harvest.ts
new file mode 100644
index 0000000..f51993b
--- /dev/null
+++ b/src/sim/garden/auto-harvest.ts
@@ -0,0 +1,90 @@
+import type { SimState } from '../state';
+import type { Tile } from './types';
+import { PLANT_TYPES } from './plants';
+import { advanceGrowth } from './growth';
+import { harvest, type SimContext } from './commands';
+import {
+  EMPTY_OFFLINE_EVENTS,
+  aggregateOfflineEvent,
+  type OfflineEventBlock,
+} from '../offline/events';
+
+/**
+ * autoHarvestReadyPlants — silent-mode harvest branch (CONTEXT D-10).
+ *
+ * Pure. Called from simulateOneTick when ctx.silent === true (set by the
+ * boot path's offline catchup loop in src/PhaserGame.tsx — Plan 02-05).
+ * Walks every tile, identifies plants that have reached the 'ready'
+ * stage at currentTick, and harvests them via the standard harvest()
+ * pipeline. Each successful harvest is also recorded into a fresh
+ * offlineEvents block on the returned state so the letter Ink template
+ * (UX-02) can narrate what bloomed while the player was away.
+ *
+ * BLOCKER 3 invariant preserved — this function NEVER writes lastTickAt
+ * (the wall-clock ms field is owned by saveSync; sim modules only write
+ * tickCount). The harvest() pipeline already obeys this invariant; we
+ * simply thread its return value forward.
+ *
+ * Per CLAUDE.md sim-purity rule: no Date.now, no setInterval, no DOM.
+ * The auto-harvest event log is a pure derivation of (tiles, currentTick,
+ * ctx.fragments) at call time.
+ *
+ * Note on cycle: this module imports `harvest` from './commands' AND
+ * `commands.ts` imports `autoHarvestReadyPlants` from this file. The
+ * cycle is benign in ESM because neither function references the other
+ * at module-init time — both bindings are resolved lazily at call time.
+ */
+export function autoHarvestReadyPlants(
+  state: SimState,
+  currentTick: number,
+  ctx: SimContext,
+): SimState {
+  let next = state;
+  const tiles = state.garden.tiles as Tile[];
+  // Seed the offline-events accumulator from whatever was already on the
+  // state (the boot path may chain multiple catchup ticks; the previous
+  // tick's accumulated events flow through here).
+  let events: OfflineEventBlock =
+    (next.offlineEvents as OfflineEventBlock | null) ?? EMPTY_OFFLINE_EVENTS;
+
+  for (let i = 0; i < tiles.length; i++) {
+    const tile = (next.garden.tiles as Tile[])[i];
+    if (!tile?.plant) continue;
+    const type = PLANT_TYPES[tile.plant.plantTypeId];
+    if (!type) continue;
+    const stage = advanceGrowth(tile.plant, type, currentTick);
+    if (stage !== 'ready') continue;
+
+    const harvestedBefore = next.harvestedFragmentIds.length;
+    const plantTypeId = tile.plant.plantTypeId;
+
+    // Reuse the standard harvest pipeline so the fragment selector,
+    // plant-type unlock thresholds (Pitfall 10), and Lura beat gate
+    // (STRY-10) all run identically to active-play harvests.
+    next = harvest(next, i, currentTick, ctx);
+
+    // If a fragment was actually selected (i.e. the harvest committed),
+    // record the event. selectFragment() can return null in degenerate
+    // ctx-empty fixtures; in that case harvest() returns the original
+    // state and harvestedFragmentIds.length is unchanged.
+    if (next.harvestedFragmentIds.length > harvestedBefore) {
+      const newId =
+        next.harvestedFragmentIds[next.harvestedFragmentIds.length - 1];
+      if (newId) {
+        events = aggregateOfflineEvent(
+          events,
+          plantTypeId,
+          newId,
+          next.luraBeatProgress.pending,
+        );
+      }
+    }
+  }
+
+  // Only allocate a new state object if events actually changed — keeps
+  // the no-op path === to the input for downstream identity checks.
+  if (events === ((next.offlineEvents as OfflineEventBlock | null) ?? EMPTY_OFFLINE_EVENTS)) {
+    return next;
+  }
+  return { ...next, offlineEvents: events };
+}
diff --git a/src/sim/garden/commands.ts b/src/sim/garden/commands.ts
index 709fc21..6cc064f 100644
--- a/src/sim/garden/commands.ts
+++ b/src/sim/garden/commands.ts
@@ -7,6 +7,7 @@ import { GRID_SIZE } from './types';
 import { advanceGrowth } from './growth';
 import { selectFragment } from '../memory/selector';
 import { advanceLuraBeatProgress } from '../narrative/lura-gate';
+import { autoHarvestReadyPlants } from './auto-harvest';
 
 /**
  * Pure command applications. Each returns a NEW SimState — no mutation.
@@ -56,10 +57,18 @@ function computePlantUnlocks(harvestCount: number): string[] {
  * Season. The Garden scene reads `fragments` (eager export from
  * src/content) at create() time and passes the snapshot through every
  * simulateOneTick call. Sim modules NEVER import import.meta.glob.
+ *
+ * Plan 02-05 extension: `silent` flips on during the boot path's offline
+ * catchup loop (D-10). When silent, simulateOneTick auto-harvests every
+ * ready-stage tile via autoHarvestReadyPlants — the player is away, so
+ * the sim drives harvests instead of waiting for player commands. The
+ * resulting offlineEvents block feeds the letter Ink template (UX-02).
  */
 export interface SimContext {
   fragments: readonly Fragment[];
   currentSeason: number;
+  /** Plan 02-05 — silent mode for offline catchup (D-10). */
+  silent?: boolean;
 }
 
 export function plantSeed(
@@ -222,6 +231,15 @@ export function simulateOneTick(
       next = compost(next, cmd.tileIdx, currentTick);
     }
   }
+  // Plan 02-05 — silent-mode auto-harvest (D-10). When the player is away,
+  // the boot path runs the silent catch-up loop with ctx.silent === true,
+  // so any tile that ripened during absence is harvested by the sim and
+  // recorded into next.offlineEvents (which feeds the letter UX-02).
+  // The active-play path leaves ctx.silent false/undefined so the player
+  // chooses when to harvest ready plants.
+  if (ctx.silent) {
+    next = autoHarvestReadyPlants(next, currentTick, ctx);
+  }
   return { ...next, tickCount: next.tickCount + 1 };
 }
 
diff --git a/src/sim/garden/index.ts b/src/sim/garden/index.ts
index e895265..a8d97e9 100644
--- a/src/sim/garden/index.ts
+++ b/src/sim/garden/index.ts
@@ -14,3 +14,4 @@ export {
   tileGrowthStage,
 } from './commands';
 export type { SimContext } from './commands';
+export { autoHarvestReadyPlants } from './auto-harvest';
diff --git a/src/sim/index.ts b/src/sim/index.ts
index 2590229..7d6cffe 100644
--- a/src/sim/index.ts
+++ b/src/sim/index.ts
@@ -13,4 +13,5 @@ export * from './scheduler';
 export * from './garden';
 export * from './memory';
 export * from './narrative';
+export * from './offline';
 export type { SimState } from './state';
diff --git a/src/sim/offline/events.test.ts b/src/sim/offline/events.test.ts
new file mode 100644
index 0000000..8c578d4
--- /dev/null
+++ b/src/sim/offline/events.test.ts
@@ -0,0 +1,168 @@
+import { describe, it, expect } from 'vitest';
+import {
+  OfflineEventBlockSchema,
+  EMPTY_OFFLINE_EVENTS,
+  aggregateOfflineEvent,
+  type OfflineEventBlock,
+} from './events';
+
+describe('OfflineEventBlockSchema (D-19 runtime validation)', () => {
+  it('accepts EMPTY_OFFLINE_EVENTS', () => {
+    expect(() => OfflineEventBlockSchema.parse(EMPTY_OFFLINE_EVENTS)).not.toThrow();
+  });
+
+  it('accepts a populated block', () => {
+    const block: OfflineEventBlock = {
+      plantsBloomedCount: { rosemary: 3, yarrow: 1 },
+      harvestedFragmentIds: ['season1.soil.first-bloom', 'season1.soil.the-cat'],
+      luraBeatPending: 'arrival',
+    };
+    expect(() => OfflineEventBlockSchema.parse(block)).not.toThrow();
+  });
+
+  it('rejects a missing plantsBloomedCount field', () => {
+    const bad = {
+      harvestedFragmentIds: [],
+      luraBeatPending: null,
+    };
+    expect(OfflineEventBlockSchema.safeParse(bad).success).toBe(false);
+  });
+
+  it('rejects a wrong-type plantsBloomedCount field', () => {
+    const bad = {
+      plantsBloomedCount: { rosemary: 'three' },
+      harvestedFragmentIds: [],
+      luraBeatPending: null,
+    };
+    expect(OfflineEventBlockSchema.safeParse(bad).success).toBe(false);
+  });
+
+  it('rejects a fragment id with bad regex', () => {
+    const bad = {
+      plantsBloomedCount: {},
+      harvestedFragmentIds: ['not-a-valid-id'],
+      luraBeatPending: null,
+    };
+    expect(OfflineEventBlockSchema.safeParse(bad).success).toBe(false);
+  });
+
+  it('rejects a luraBeatPending value outside the enum', () => {
+    const bad = {
+      plantsBloomedCount: {},
+      harvestedFragmentIds: [],
+      luraBeatPending: 'goodbye',
+    };
+    expect(OfflineEventBlockSchema.safeParse(bad).success).toBe(false);
+  });
+
+  it('accepts luraBeatPending: null', () => {
+    const ok = {
+      plantsBloomedCount: {},
+      harvestedFragmentIds: [],
+      luraBeatPending: null,
+    };
+    expect(OfflineEventBlockSchema.safeParse(ok).success).toBe(true);
+  });
+
+  it('rejects negative bloom counts', () => {
+    const bad = {
+      plantsBloomedCount: { rosemary: -1 },
+      harvestedFragmentIds: [],
+      luraBeatPending: null,
+    };
+    expect(OfflineEventBlockSchema.safeParse(bad).success).toBe(false);
+  });
+});
+
+describe('aggregateOfflineEvent (pure aggregator)', () => {
+  it('appends a fragment id and increments the plant count', () => {
+    const next = aggregateOfflineEvent(
+      EMPTY_OFFLINE_EVENTS,
+      'rosemary',
+      'season1.soil.first-bloom',
+      null,
+    );
+    expect(next.plantsBloomedCount).toEqual({ rosemary: 1 });
+    expect(next.harvestedFragmentIds).toEqual(['season1.soil.first-bloom']);
+    expect(next.luraBeatPending).toBeNull();
+  });
+
+  it('two consecutive aggregates increment counts correctly', () => {
+    const a = aggregateOfflineEvent(
+      EMPTY_OFFLINE_EVENTS,
+      'rosemary',
+      'season1.soil.first-bloom',
+      null,
+    );
+    const b = aggregateOfflineEvent(a, 'rosemary', 'season1.soil.the-cat', null);
+    expect(b.plantsBloomedCount).toEqual({ rosemary: 2 });
+    expect(b.harvestedFragmentIds).toEqual([
+      'season1.soil.first-bloom',
+      'season1.soil.the-cat',
+    ]);
+  });
+
+  it('counts different plant types separately', () => {
+    const a = aggregateOfflineEvent(
+      EMPTY_OFFLINE_EVENTS,
+      'rosemary',
+      'season1.soil.first-bloom',
+      null,
+    );
+    const b = aggregateOfflineEvent(
+      a,
+      'yarrow',
+      'season1.soil.what-the-wind-was-for',
+      null,
+    );
+    expect(b.plantsBloomedCount).toEqual({ rosemary: 1, yarrow: 1 });
+  });
+
+  it('luraBeatPending overwrites only when newer is non-null', () => {
+    const a = aggregateOfflineEvent(
+      EMPTY_OFFLINE_EVENTS,
+      'rosemary',
+      'season1.soil.first-bloom',
+      'arrival',
+    );
+    expect(a.luraBeatPending).toBe('arrival');
+    // Subsequent harvest with null beat preserves the prior pending value.
+    const b = aggregateOfflineEvent(a, 'rosemary', 'season1.soil.the-cat', null);
+    expect(b.luraBeatPending).toBe('arrival');
+    // A newer non-null pending overwrites.
+    const c = aggregateOfflineEvent(
+      b,
+      'rosemary',
+      'season1.soil.kettle-on-the-hob',
+      'mid',
+    );
+    expect(c.luraBeatPending).toBe('mid');
+  });
+
+  it('does NOT mutate the prev block (immutability)', () => {
+    const prev: OfflineEventBlock = {
+      plantsBloomedCount: { rosemary: 2 },
+      harvestedFragmentIds: ['season1.soil.first-bloom'],
+      luraBeatPending: null,
+    };
+    const next = aggregateOfflineEvent(
+      prev,
+      'rosemary',
+      'season1.soil.the-cat',
+      null,
+    );
+    expect(prev.plantsBloomedCount).toEqual({ rosemary: 2 });
+    expect(prev.harvestedFragmentIds).toEqual(['season1.soil.first-bloom']);
+    expect(next).not.toBe(prev);
+  });
+
+  it('output round-trips through OfflineEventBlockSchema', () => {
+    const next = aggregateOfflineEvent(
+      EMPTY_OFFLINE_EVENTS,
+      'rosemary',
+      'season1.soil.first-bloom',
+      'arrival',
+    );
+    expect(() => OfflineEventBlockSchema.parse(next)).not.toThrow();
+  });
+});
diff --git a/src/sim/offline/events.ts b/src/sim/offline/events.ts
new file mode 100644
index 0000000..7ea23da
--- /dev/null
+++ b/src/sim/offline/events.ts
@@ -0,0 +1,68 @@
+import { z } from 'zod';
+
+/**
+ * sim/offline/events — OfflineEventBlock Zod runtime validator + pure
+ * aggregator. Per CONTEXT D-19, D-10, D-11.
+ *
+ * Phase 2 ships the minimum slot vocabulary that the letter Ink template
+ * (UX-02) needs: per-plant counts of plants bloomed, the list of
+ * auto-harvested fragment ids, and a flag for any newly-unlocked Lura
+ * beat queued for first-visit. Phase 4+ may add more if playtest
+ * demands.
+ *
+ * Structurally compatible with the OfflineEventBlock interface declared
+ * in src/save/migrations.ts (Plan 02-01) — that one is the type the
+ * save layer carries; this file is the runtime validator + aggregator.
+ *
+ * Pure. Imports only zod. CORE-10 firewall + Phase-2 sim-purity rule
+ * still apply: no Date.now, no setInterval, no DOM, no fetch.
+ */
+export const OfflineEventBlockSchema = z.object({
+  plantsBloomedCount: z.record(z.string(), z.number().int().nonnegative()),
+  harvestedFragmentIds: z.array(z.string().regex(/^season\d+\.[a-z0-9._-]+$/)),
+  luraBeatPending: z.enum(['arrival', 'mid', 'farewell']).nullable(),
+});
+
+export type OfflineEventBlock = z.infer<typeof OfflineEventBlockSchema>;
+
+/**
+ * Frozen empty block. The boot path uses this as the seed for the silent
+ * catch-up loop's offlineEvents accumulator. Object.freeze prevents
+ * accidental mutation across catchup boundaries.
+ */
+export const EMPTY_OFFLINE_EVENTS: OfflineEventBlock = Object.freeze({
+  plantsBloomedCount: Object.freeze({}) as Record<string, number>,
+  harvestedFragmentIds: Object.freeze([]) as unknown as string[],
+  luraBeatPending: null,
+});
+
+/**
+ * aggregateOfflineEvent — pure combiner for a single auto-harvest event
+ * during the silent-mode catchup loop.
+ *
+ * Returns a NEW OfflineEventBlock with:
+ *   - plantsBloomedCount[plantTypeId] incremented by 1
+ *   - fragmentId appended to harvestedFragmentIds
+ *   - luraBeatPending: prev's value preserved unless the incoming
+ *     `luraBeatPending` is non-null (in which case the most recent
+ *     pending beat wins — Phase 2 has at most one pending beat at a
+ *     time per advanceLuraBeatProgress's invariant in
+ *     src/sim/narrative/lura-gate.ts).
+ *
+ * Per CONTEXT D-17/D-19 — this is the slot vocabulary the letter Ink
+ * template renders.
+ */
+export function aggregateOfflineEvent(
+  prev: OfflineEventBlock,
+  plantTypeId: string,
+  fragmentId: string,
+  luraBeatPending: OfflineEventBlock['luraBeatPending'],
+): OfflineEventBlock {
+  const counts = { ...prev.plantsBloomedCount };
+  counts[plantTypeId] = (counts[plantTypeId] ?? 0) + 1;
+  return {
+    plantsBloomedCount: counts,
+    harvestedFragmentIds: [...prev.harvestedFragmentIds, fragmentId],
+    luraBeatPending: luraBeatPending ?? prev.luraBeatPending,
+  };
+}
diff --git a/src/sim/offline/index.ts b/src/sim/offline/index.ts
new file mode 100644
index 0000000..925d1c4
--- /dev/null
+++ b/src/sim/offline/index.ts
@@ -0,0 +1,13 @@
+/**
+ * Public barrel for src/sim/offline/.
+ *
+ * Phase 2 Plan 02-05 — silent-mode offline catchup feeds the OfflineEventBlock
+ * the letter Ink template (UX-02) renders. Per CORE-10 + Phase-2 sim-purity:
+ * pure module, no Date.now / setInterval / DOM / fetch.
+ */
+export {
+  OfflineEventBlockSchema,
+  EMPTY_OFFLINE_EVENTS,
+  aggregateOfflineEvent,
+} from './events';
+export type { OfflineEventBlock } from './events';
diff --git a/src/ui/letter/letter-renderer.test.ts b/src/ui/letter/letter-renderer.test.ts
new file mode 100644
index 0000000..3d2c783
--- /dev/null
+++ b/src/ui/letter/letter-renderer.test.ts
@@ -0,0 +1,139 @@
+import { describe, it, expect } from 'vitest';
+import { buildLetterSlots } from './letter-renderer';
+import type { Fragment } from '../../content';
+import type { OfflineEventBlock } from '../../sim/offline';
+
+const fragments: Fragment[] = [
+  {
+    id: 'season1.soil.first-bloom',
+    season: 1,
+    tags: ['warm'],
+    body: 'The first thing that grew was rosemary.',
+  },
+  {
+    id: 'season1.soil.the-cat',
+    season: 1,
+    tags: ['warm'],
+    body: 'The cat is missing now too.',
+  },
+  // A fragment whose first sentence is longer than 60 chars — the
+  // helper should fall back to the slugified id.
+  {
+    id: 'season1.soil.the-very-long-one',
+    season: 1,
+    tags: ['contemplative'],
+    body:
+      'There is a kind of evening light that lasts longer than the day requires of it, and the garden seems to know.',
+  },
+];
+
+describe('buildLetterSlots (UX-02 + D-17 — pure slot builder)', () => {
+  it('returns all-empty slots when events is null', () => {
+    const slots = buildLetterSlots(null, fragments);
+    expect(slots).toEqual({
+      plants_bloomed: 0,
+      fragment_titles: '',
+      lura_was_here: false,
+    });
+  });
+
+  it('counts a single rosemary auto-harvest as plants_bloomed=1', () => {
+    const events: OfflineEventBlock = {
+      plantsBloomedCount: { rosemary: 1 },
+      harvestedFragmentIds: ['season1.soil.first-bloom'],
+      luraBeatPending: null,
+    };
+    const slots = buildLetterSlots(events, fragments);
+    expect(slots.plants_bloomed).toBe(1);
+    // First-sentence slug, lowercased.
+    expect(slots.fragment_titles).toBe('the first thing that grew was rosemary');
+    expect(slots.lura_was_here).toBe(false);
+  });
+
+  it('joins multiple fragment titles with semicolon-space', () => {
+    const events: OfflineEventBlock = {
+      plantsBloomedCount: { rosemary: 2 },
+      harvestedFragmentIds: ['season1.soil.first-bloom', 'season1.soil.the-cat'],
+      luraBeatPending: null,
+    };
+    const slots = buildLetterSlots(events, fragments);
+    expect(slots.fragment_titles).toBe(
+      'the first thing that grew was rosemary; the cat is missing now too',
+    );
+  });
+
+  it('falls back to slugified id when first sentence is too long', () => {
+    const events: OfflineEventBlock = {
+      plantsBloomedCount: { yarrow: 1 },
+      harvestedFragmentIds: ['season1.soil.the-very-long-one'],
+      luraBeatPending: null,
+    };
+    const slots = buildLetterSlots(events, fragments);
+    // Slug is the id with `season1.` stripped and dots/underscores → spaces.
+    expect(slots.fragment_titles).toBe('soil the very long one');
+  });
+
+  it('falls back to slugified id when fragment is missing from corpus', () => {
+    const events: OfflineEventBlock = {
+      plantsBloomedCount: { rosemary: 1 },
+      harvestedFragmentIds: ['season1.soil.unknown-fragment'],
+      luraBeatPending: null,
+    };
+    const slots = buildLetterSlots(events, fragments);
+    expect(slots.fragment_titles).toBe('soil unknown fragment');
+  });
+
+  it('lura_was_here flips true when luraBeatPending is non-null', () => {
+    const events: OfflineEventBlock = {
+      plantsBloomedCount: {},
+      harvestedFragmentIds: [],
+      luraBeatPending: 'arrival',
+    };
+    const slots = buildLetterSlots(events, fragments);
+    expect(slots.lura_was_here).toBe(true);
+  });
+
+  it('lura_was_here is false when luraBeatPending is null', () => {
+    const events: OfflineEventBlock = {
+      plantsBloomedCount: { rosemary: 1 },
+      harvestedFragmentIds: ['season1.soil.first-bloom'],
+      luraBeatPending: null,
+    };
+    const slots = buildLetterSlots(events, fragments);
+    expect(slots.lura_was_here).toBe(false);
+  });
+
+  it('counts multiple plant types together (D-17 plants_bloomed is total)', () => {
+    const events: OfflineEventBlock = {
+      plantsBloomedCount: { rosemary: 3, yarrow: 2 },
+      harvestedFragmentIds: [],
+      luraBeatPending: null,
+    };
+    const slots = buildLetterSlots(events, fragments);
+    expect(slots.plants_bloomed).toBe(5);
+  });
+
+  it('handles a 24h-cap edge case — 50 plants bloomed, no truncation (D-11 silent cap)', () => {
+    const events: OfflineEventBlock = {
+      plantsBloomedCount: { rosemary: 50 },
+      harvestedFragmentIds: [],
+      luraBeatPending: null,
+    };
+    const slots = buildLetterSlots(events, fragments);
+    // The Ink template handles "many" copy ('plants_bloomed > 1' branch).
+    // The helper passes the raw count through; no numeric "28h" copy
+    // appears anywhere here either.
+    expect(slots.plants_bloomed).toBe(50);
+  });
+
+  it('returns empty fragment_titles when no harvested ids (zero-bloom path)', () => {
+    const events: OfflineEventBlock = {
+      plantsBloomedCount: {},
+      harvestedFragmentIds: [],
+      luraBeatPending: null,
+    };
+    const slots = buildLetterSlots(events, fragments);
+    expect(slots.fragment_titles).toBe('');
+    expect(slots.plants_bloomed).toBe(0);
+  });
+});
diff --git a/src/ui/letter/letter-renderer.ts b/src/ui/letter/letter-renderer.ts
new file mode 100644
index 0000000..66f579b
--- /dev/null
+++ b/src/ui/letter/letter-renderer.ts
@@ -0,0 +1,62 @@
+import type { OfflineEventBlock } from '../../sim/offline';
+import type { Fragment } from '../../content';
+
+/**
+ * letter-renderer — pure helper that converts an OfflineEventBlock +
+ * the fragment corpus into the slot values for letter-from-the-garden.ink.
+ *
+ * Separated from the Letter.tsx React component so the slot-building
+ * logic is testable without spinning up happy-dom + the Ink runtime.
+ *
+ * Per CONTEXT D-17 / D-18 / UX-02 — the slots are the templated
+ * insertions; the Ink skeleton holds the voice. The fragment-titles
+ * slot prefers the first-sentence body of each fragment (tonal weight)
+ * with a slugified-id fallback for anything that fails to resolve.
+ *
+ * Per CONTEXT D-11: this helper does not surface a numeric "X hours"
+ * value — the Ink template handles "many" copy on its own. The
+ * letter-renderer never touches wall-clock time.
+ *
+ * Pure. No DOM, no Date.now, no fetch.
+ */
+export interface LetterSlots {
+  plants_bloomed: number;
+  fragment_titles: string;
+  lura_was_here: boolean;
+}
+
+const EMPTY_SLOTS: LetterSlots = Object.freeze({
+  plants_bloomed: 0,
+  fragment_titles: '',
+  lura_was_here: false,
+});
+
+export function buildLetterSlots(
+  events: OfflineEventBlock | null,
+  allFragments: readonly Fragment[],
+): LetterSlots {
+  if (!events) return EMPTY_SLOTS;
+  const total = Object.values(events.plantsBloomedCount).reduce(
+    (a, b) => a + b,
+    0,
+  );
+  const titles = events.harvestedFragmentIds
+    .map((id) => {
+      const frag = allFragments.find((f) => f.id === id);
+      // Prefer the fragment's first sentence (≤60 chars) for tonal
+      // weight; fall back to slugified id for missing fragments.
+      if (frag) {
+        const firstLine = frag.body.split(/[.!?]/)[0]?.trim() ?? '';
+        if (firstLine.length > 0 && firstLine.length <= 60) {
+          return firstLine.toLowerCase();
+        }
+      }
+      return id.replace(/^season\d+\./, '').replace(/[._-]+/g, ' ');
+    })
+    .filter((t) => t.length > 0);
+  return {
+    plants_bloomed: total,
+    fragment_titles: titles.join('; '),
+    lura_was_here: events.luraBeatPending !== null,
+  };
+}