diff --git a/src/app/api/insights/generate/route.ts b/src/app/api/insights/generate/route.ts
index 758ddb13a..7ce2885f7 100644
--- a/src/app/api/insights/generate/route.ts
+++ b/src/app/api/insights/generate/route.ts
@@ -5,7 +5,14 @@ import { extractFeatures } from "@/lib/insights/features";
 import {
   getInsightsSystemPrompt,
   buildUserPrompt,
+  type ComparisonSnapshot,
 } from "@/lib/insights/prompt";
+import { summarize, type DataPoint } from "@/lib/analytics/trends";
+import {
+  resolveDashboardLayout,
+  type ComparisonBaseline,
+} from "@/lib/dashboard-layout";
+import type { MeasurementType } from "@/generated/prisma/client";
 import { insightResultSchema, type InsightResult } from "@/lib/ai/types";
 import { resolveProvider, resolveProviderChain } from "@/lib/ai/provider";
 import {
@@ -65,6 +72,102 @@ async function evictPerStatusInsightCache(userId: string): Promise<void> {
   });
 }
 
+/**
+ * v1.4.16 phase B8 — fetch the user's persisted comparison toggle and
+ * build a `ComparisonSnapshot` for the prompt builder. Returns null
+ * when comparison is off (most users), so the prompt builder skips
+ * the context block entirely.
+ *
+ * Snapshot construction reuses the analytics `summarize()` helper
+ * (which now emits `avg30LastMonth` + `avg30LastYear`) so the numbers
+ * the LLM sees match the numbers the dashboard tile renders — no
+ * second source of truth for the comparison delta.
+ */
+async function buildComparisonSnapshotForUser(
+  userId: string,
+): Promise<ComparisonSnapshot | null> {
+  // The features payload doesn't carry prior-period values — pulling
+  // a fresh `summarize()` per metric below keeps the snapshot
+  // self-contained without retrofitting features.ts.
+  const row = await prisma.user.findUnique({
+    where: { id: userId },
+    select: { dashboardWidgetsJson: true },
+  });
+  const layout = resolveDashboardLayout(row?.dashboardWidgetsJson);
+  const baseline: ComparisonBaseline = layout.comparisonBaseline ?? "none";
+  if (baseline === "none") return null;
+
+  // Pull every measurement type once and run summarize() so the
+  // prior-period averages line up with what the dashboard analytics
+  // computes. Only include rows where the LLM can sensibly narrate
+  // a prior comparison (i.e. the metric exists in our snapshot type
+  // mapping below).
+  const typeToSnapshotKey: Record<string, string> = {
+    WEIGHT: "weight",
+    BLOOD_PRESSURE_SYS: "bloodPressureSys",
+    BLOOD_PRESSURE_DIA: "bloodPressureDia",
+    PULSE: "pulse",
+    BODY_FAT: "bodyFat",
+    SLEEP_DURATION: "sleep",
+    ACTIVITY_STEPS: "steps",
+  };
+  const typeUnits: Record<string, string> = {
+    WEIGHT: "kg",
+    BLOOD_PRESSURE_SYS: "mmHg",
+    BLOOD_PRESSURE_DIA: "mmHg",
+    PULSE: "bpm",
+    BODY_FAT: "%",
+    SLEEP_DURATION: "h",
+    ACTIVITY_STEPS: "",
+  };
+  const types = Object.keys(typeToSnapshotKey) as MeasurementType[];
+  const rows = await Promise.all(
+    types.map(async (type) => {
+      const measurements = await prisma.measurement.findMany({
+        where: { userId, type },
+        orderBy: { measuredAt: "asc" },
+        select: { measuredAt: true, value: true },
+      });
+      const summary = summarize(
+        measurements.map(
+          (m): DataPoint => ({ date: m.measuredAt, value: m.value }),
+        ),
+      );
+      const baselineAvg =
+        baseline === "lastMonth"
+          ? (summary.avg30LastMonth ?? null)
+          : (summary.avg30LastYear ?? null);
+      const currentAvg = summary.avg30 ?? null;
+      const delta =
+        currentAvg !== null && baselineAvg !== null
+          ? Math.round((currentAvg - baselineAvg) * 100) / 100
+          : null;
+      const deltaPercent =
+        delta !== null && baselineAvg !== null && baselineAvg !== 0
+          ? Math.round((delta / Math.abs(baselineAvg)) * 100 * 10) / 10
+          : null;
+      return {
+        type: typeToSnapshotKey[type] ?? type,
+        currentAvg,
+        baselineAvg,
+        delta,
+        deltaPercent,
+        unit: typeUnits[type] ?? "",
+      };
+    }),
+  );
+
+  // Drop fully-empty rows (current AND baseline missing) so the prompt
+  // doesn't pad with noise. Keep partial rows (one side null) — the
+  // block renderer flags them with "no prior-period data available"
+  // which is useful context for the model.
+  const metrics = rows.filter(
+    (row) => row.currentAvg !== null || row.baselineAvg !== null,
+  );
+
+  return { baseline, metrics };
+}
+
 export const POST = apiHandler(async (request: NextRequest) => {
   const { user } = await requireAuth();
   const userId = user.id;
@@ -155,10 +258,19 @@ export const POST = apiHandler(async (request: NextRequest) => {
   const includeRaw = dbUser?.insightsPrivacyMode === "raw";
   const features = await extractFeatures(userId, includeRaw);
   const featuresJson = JSON.stringify(features, null, 2);
+
+  // v1.4.16 phase B8 — pick up the user's persisted comparison toggle
+  // and, when active, build a compact prior-period snapshot from the
+  // `features` summaries that already include `avg30LastMonth` /
+  // `avg30LastYear` (added in the analytics summarize() helper). The
+  // narrative is default-on whenever the toggle is on per research §7
+  // Q4 — the pulldown is the single affordance.
+  const comparisonSnapshot = await buildComparisonSnapshotForUser(userId);
   const userPrompt = buildUserPrompt(
     featuresJson,
     dbUser?.insightsPrivacyMode ?? "aggregated",
     locale,
+    comparisonSnapshot ?? undefined,
   );
 
   let result;
diff --git a/src/lib/ai/__tests__/medical-reference-prompt.test.ts b/src/lib/ai/__tests__/medical-reference-prompt.test.ts
index 2414f8bd8..c7938e0d3 100644
--- a/src/lib/ai/__tests__/medical-reference-prompt.test.ts
+++ b/src/lib/ai/__tests__/medical-reference-prompt.test.ts
@@ -24,8 +24,12 @@ import {
  */
 
 describe("PROMPT_VERSION", () => {
-  it("is 4.16.0 for the v1.4.16 medical-reference grounding update", () => {
-    expect(PROMPT_VERSION).toBe("4.16.0");
+  it("is at least 4.16.0 for the v1.4.16 medical-reference grounding update", () => {
+    // v1.4.16 phase B5a anchored at 4.16.0; phase B8 bumped the
+    // patch component to 4.16.1 when the comparison-mode narrative
+    // ground rule landed. Use a string-prefix check so the test
+    // doesn't bind to a specific patch.
+    expect(PROMPT_VERSION.startsWith("4.16.")).toBe(true);
   });
 });
 
@@ -105,7 +109,8 @@ describe("buildSystemPromptWithReferences()", () => {
   it("contains the bumped PROMPT_VERSION", () => {
     const prompt = buildSystemPromptWithReferences("en", ["bp"]);
     expect(prompt).toContain(PROMPT_VERSION);
-    expect(prompt).toContain("4.16.0");
+    // v1.4.16: phase B5a was 4.16.0; phase B8 bumped to 4.16.1.
+    expect(prompt).toMatch(/4\.16\.\d+/);
   });
 });
 
@@ -115,8 +120,8 @@ describe("plain getStrictInsightsSystemPrompt() backward compatibility", () => {
     expect(en).not.toContain("SOURCES");
   });
 
-  it("contains the bumped PROMPT_VERSION 4.16.0", () => {
+  it("contains the bumped PROMPT_VERSION", () => {
     const en = getStrictInsightsSystemPrompt("en");
-    expect(en).toContain("4.16.0");
+    expect(en).toMatch(/4\.16\.\d+/);
   });
 });
diff --git a/src/lib/ai/prompts/insight-generator.ts b/src/lib/ai/prompts/insight-generator.ts
index 9e2403ef4..90482359c 100644
--- a/src/lib/ai/prompts/insight-generator.ts
+++ b/src/lib/ai/prompts/insight-generator.ts
@@ -27,7 +27,7 @@ import {
 } from "../medical-references";
 
 /** Stable identifier for the active system prompt revision. */
-export const PROMPT_VERSION = "4.16.0" as const;
+export const PROMPT_VERSION = "4.16.1" as const;
 
 const SYSTEM_PROMPT_EN = `You are a clinical-context summariser for a personal health-log app.
 Prompt version: ${PROMPT_VERSION}.
@@ -76,6 +76,13 @@ GROUND RULES — ZERO HALLUCINATIONS
    language. Reference the user's actual data trends — do not paste
    in placeholder text. The rationale.dataWindow MUST equal the
    metricSource.timeRange so the UI's mini-chart can pin to it.
+6. v1.4.16 phase B8 — when the user prompt contains a "SYSTEM
+   CONTEXT — COMPARISON MODE ACTIVE" block, narrate the comparison
+   in the summary's first sentence using the prior-period numbers
+   that block carries. Cite the deltas verbatim — do NOT invent
+   comparison numbers, and do NOT extrapolate beyond the metrics
+   listed. When the block reports "no prior-period data available"
+   for every metric, state that explicitly and skip the narration.
 
 GUIDELINE TARGETS — generic, do NOT compute precise risk scores
 - Adult resting blood pressure (ESH/ESC 2024 generic): aim < 140/90
@@ -192,6 +199,13 @@ GRUNDREGELN — NULL HALLUZINATIONEN
    Platzhaltertext. rationale.dataWindow MUSS gleich
    metricSource.timeRange sein, damit die UI das Mini-Chart auf das
    gleiche Fenster fixieren kann.
+6. v1.4.16 phase B8 — wenn der User-Prompt einen Block "SYSTEM
+   CONTEXT — VERGLEICHSMODUS AKTIV" enthält, narrative die im Block
+   gelisteten Deltas im ersten Satz der Zusammenfassung. Zitiere die
+   Werte exakt — erfinde KEINE Vergleichszahlen und extrapoliere
+   nicht über die gelisteten Metriken hinaus. Wenn der Block für
+   alle Metriken "no prior-period data available" meldet, sag das
+   explizit und lass die Narration weg.
 
 LEITLINIEN-ZIELWERTE — generisch, KEINE genauen Risiko-Scores berechnen
 - Erwachsenen-Ruheblutdruck (ESH/ESC 2024 generisch): Ziel < 140/90
diff --git a/src/lib/insights/__tests__/prompt-comparison.test.ts b/src/lib/insights/__tests__/prompt-comparison.test.ts
new file mode 100644
index 000000000..f92408986
--- /dev/null
+++ b/src/lib/insights/__tests__/prompt-comparison.test.ts
@@ -0,0 +1,133 @@
+import { describe, it, expect } from "vitest";
+import {
+  buildUserPrompt,
+  buildComparisonBlock,
+  type ComparisonSnapshot,
+} from "@/lib/insights/prompt";
+
+/**
+ * v1.4.16 phase B8 — comparison context block in the user prompt.
+ *
+ * When the dashboard's comparison toggle is active, the route layer
+ * passes a `ComparisonSnapshot` to `buildUserPrompt()` so the LLM has
+ * the prior-period numbers needed to narrate "Your average BP
+ * improved by 4 mmHg vs. last month".
+ */
+
+const monthlySnapshot: ComparisonSnapshot = {
+  baseline: "lastMonth",
+  metrics: [
+    {
+      type: "BLOOD_PRESSURE_SYS",
+      currentAvg: 128,
+      baselineAvg: 132,
+      delta: -4,
+      deltaPercent: -3,
+      unit: "mmHg",
+    },
+    {
+      type: "WEIGHT",
+      currentAvg: 80.4,
+      baselineAvg: 82.7,
+      delta: -2.3,
+      deltaPercent: -2.8,
+      unit: "kg",
+    },
+  ],
+};
+
+describe("buildUserPrompt() — comparison block (B8)", () => {
+  it("appends NO comparison block when comparison is undefined", () => {
+    const prompt = buildUserPrompt("{\"weight\":[]}", "agg", "en");
+    expect(prompt).not.toMatch(/COMPARISON MODE/i);
+    expect(prompt).not.toMatch(/VERGLEICHSMODUS/i);
+  });
+
+  it("appends an English comparison block when comparison is supplied with locale=en", () => {
+    const prompt = buildUserPrompt(
+      "{\"weight\":[]}",
+      "agg",
+      "en",
+      monthlySnapshot,
+    );
+    expect(prompt).toMatch(/COMPARISON MODE ACTIVE/);
+    expect(prompt).toMatch(/30 days ago/);
+    expect(prompt).toContain("BLOOD_PRESSURE_SYS");
+    expect(prompt).toContain("delta -4.0 mmHg");
+    expect(prompt).toContain("WEIGHT");
+    expect(prompt).toContain("delta -2.3 kg");
+  });
+
+  it("appends a German comparison block when locale=de", () => {
+    const prompt = buildUserPrompt(
+      "{\"gewicht\":[]}",
+      "agg",
+      "de",
+      monthlySnapshot,
+    );
+    expect(prompt).toMatch(/VERGLEICHSMODUS AKTIV/);
+    expect(prompt).toMatch(/30 Tage zuvor/);
+    expect(prompt).toContain("BLOOD_PRESSURE_SYS");
+  });
+
+  it("uses 365-day language for the lastYear baseline", () => {
+    const yearly: ComparisonSnapshot = {
+      baseline: "lastYear",
+      metrics: [
+        {
+          type: "WEIGHT",
+          currentAvg: 80,
+          baselineAvg: 85,
+          delta: -5,
+          deltaPercent: -5.9,
+          unit: "kg",
+        },
+      ],
+    };
+    const en = buildUserPrompt("{}", "agg", "en", yearly);
+    const de = buildUserPrompt("{}", "agg", "de", yearly);
+    expect(en).toMatch(/365 days ago/);
+    expect(de).toMatch(/365 Tage zuvor/);
+  });
+
+  it("renders an empty-metrics fallback that tells the model to say so", () => {
+    const empty: ComparisonSnapshot = {
+      baseline: "lastMonth",
+      metrics: [],
+    };
+    const block = buildComparisonBlock("en", empty);
+    expect(block).toMatch(
+      /no metric currently has enough prior-period data to compare/,
+    );
+  });
+
+  it("instructs the model that the most-significant delta must appear in the summary's first sentence", () => {
+    const block = buildComparisonBlock("en", monthlySnapshot);
+    expect(block).toMatch(/MUST appear in the summary['’]s\s+first sentence/);
+  });
+
+  it("never instructs the model to invent comparison numbers (anti-hallucination guard)", () => {
+    const en = buildComparisonBlock("en", monthlySnapshot);
+    const de = buildComparisonBlock("de", monthlySnapshot);
+    expect(en).toMatch(/Do NOT invent comparison numbers/);
+    expect(de).toMatch(/Erfinde KEINE Vergleichszahlen/);
+  });
+
+  it("flags missing-prior-period metrics with 'no prior-period data available'", () => {
+    const partial: ComparisonSnapshot = {
+      baseline: "lastMonth",
+      metrics: [
+        {
+          type: "MOOD",
+          currentAvg: 4,
+          baselineAvg: null,
+          delta: null,
+          deltaPercent: null,
+          unit: "/5",
+        },
+      ],
+    };
+    const block = buildComparisonBlock("en", partial);
+    expect(block).toMatch(/MOOD: no prior-period data available/);
+  });
+});
diff --git a/src/lib/insights/prompt.ts b/src/lib/insights/prompt.ts
index b9b4ad31c..8046632a7 100644
--- a/src/lib/insights/prompt.ts
+++ b/src/lib/insights/prompt.ts
@@ -2,6 +2,43 @@
  * System prompt and output formatting for OpenAI insights.
  */
 import type { Locale } from "@/lib/i18n/config";
+import type { ComparisonBaseline } from "@/lib/dashboard-layout";
+
+/**
+ * v1.4.16 phase B8 — comparison snapshot the prompt narrates against
+ * when the user has the dashboard's "Compare to" toggle on. The snapshot
+ * is opt-in: pass it through `buildUserPrompt`'s 4th argument when
+ * `comparisonBaseline !== "none"` so the LLM has prior-period context
+ * for narrating "Your average BP improved by 4 mmHg vs. last month".
+ *
+ * Per research §7 Q4 the narrative is default-on whenever the toggle
+ * itself is active — the pulldown is the single affordance, no
+ * secondary "include in AI" preference, so users don't see surprising
+ * mismatches between the chart they're staring at and the AI summary.
+ */
+export interface ComparisonSnapshot {
+  baseline: Exclude<ComparisonBaseline, "none">;
+  /** One row per metric the snapshot can compare. Empty array allowed
+   *  so the prompt builder is non-strict — the wrapper still adds the
+   *  context block but instructs the model that no metric had enough
+   *  prior-period data to compare. */
+  metrics: Array<{
+    /** Snapshot key matching the metricSource convention used elsewhere
+     *  in the AI pipeline ("bloodPressure", "weight", "pulse", "mood"). */
+    type: string;
+    currentAvg: number | null;
+    baselineAvg: number | null;
+    /** `currentAvg - baselineAvg`, rounded to 2dp; null when either
+     *  side missing. */
+    delta: number | null;
+    /** `(delta / baselineAvg) * 100`, rounded to 1dp; null when either
+     *  side missing or baselineAvg is 0. */
+    deltaPercent: number | null;
+    /** Free-form unit label ("mmHg", "kg", "bpm", "/5") so the model
+     *  doesn't have to guess based on metric name. */
+    unit: string;
+  }>;
+}
 
 const INSIGHTS_SYSTEM_PROMPT_DE = `Du bist ein Gesundheitsdaten-Insights-Assistent. Du analysierst aggregierte Gesundheitsdaten und gibst klare, datenbasierte Hinweise.
 
@@ -81,7 +118,10 @@ export function buildUserPrompt(
   featuresJson: string,
   privacyMode: string,
   locale: Locale,
+  comparison?: ComparisonSnapshot,
 ): string {
+  const comparisonBlock = comparison ? buildComparisonBlock(locale, comparison) : "";
+
   if (locale === "en") {
     const modeLabel =
       privacyMode === "raw"
@@ -92,7 +132,7 @@ export function buildUserPrompt(
 Data mode: ${modeLabel}
 Note: use each metric's coverage object for measurement frequency and time spans, and tailor the analysis accordingly.
 
-${featuresJson}`;
+${featuresJson}${comparisonBlock}`;
   }
 
   const modeLabel =
@@ -104,5 +144,92 @@ ${featuresJson}`;
 Datenmodus: ${modeLabel}
 Hinweis: Beachte die coverage-Objekte jeder Metrik für Informationen zu Messhäufigkeit und Zeiträumen. Passe deine Analyse entsprechend an.
 
-${featuresJson}`;
+${featuresJson}${comparisonBlock}`;
+}
+
+/**
+ * v1.4.16 phase B8 — comparison context block.
+ *
+ * Renders a dedicated SYSTEM CONTEXT section after the user data so
+ * the LLM can narrate the comparison ("Your average BP improved by
+ * 4 mmHg vs. last month"). The instruction "narrate the comparison
+ * when relevant" is in the system prompt; this block carries the
+ * actual prior-period numbers.
+ *
+ * Exported for unit tests so the integration test can assert the
+ * exact string the LLM sees.
+ */
+export function buildComparisonBlock(
+  locale: Locale,
+  snapshot: ComparisonSnapshot,
+): string {
+  const baselineLabel =
+    locale === "en"
+      ? snapshot.baseline === "lastMonth"
+        ? "the same window 30 days ago"
+        : "the same window 365 days ago"
+      : snapshot.baseline === "lastMonth"
+        ? "den gleichen Zeitraum 30 Tage zuvor"
+        : "den gleichen Zeitraum 365 Tage zuvor";
+
+  const renderRow = (
+    metric: ComparisonSnapshot["metrics"][number],
+  ): string => {
+    if (metric.delta === null || metric.baselineAvg === null) {
+      return `- ${metric.type}: no prior-period data available`;
+    }
+    const sign = metric.delta > 0 ? "+" : metric.delta < 0 ? "-" : "±";
+    const formattedDelta = `${sign}${Math.abs(metric.delta).toFixed(1)}${metric.unit ? ` ${metric.unit}` : ""}`;
+    const pct =
+      metric.deltaPercent != null
+        ? ` (${metric.deltaPercent > 0 ? "+" : metric.deltaPercent < 0 ? "-" : "±"}${Math.abs(metric.deltaPercent).toFixed(1)} %)`
+        : "";
+    const current =
+      metric.currentAvg != null
+        ? `${metric.currentAvg.toFixed(1)}${metric.unit ? ` ${metric.unit}` : ""}`
+        : "n/a";
+    const prior =
+      metric.baselineAvg != null
+        ? `${metric.baselineAvg.toFixed(1)}${metric.unit ? ` ${metric.unit}` : ""}`
+        : "n/a";
+    return `- ${metric.type}: current ${current}, baseline ${prior}, delta ${formattedDelta}${pct}`;
+  };
+
+  if (snapshot.metrics.length === 0) {
+    return locale === "en"
+      ? `
+
+SYSTEM CONTEXT — COMPARISON MODE ACTIVE
+The user has activated comparison mode against ${baselineLabel}, but no metric currently has enough prior-period data to compare. State this explicitly in the summary's first sentence.`
+      : `
+
+SYSTEM CONTEXT — VERGLEICHSMODUS AKTIV
+Der Nutzer hat den Vergleichsmodus gegen ${baselineLabel} aktiviert, aber für keine Metrik liegen ausreichend Daten aus der Vergleichsperiode vor. Erwähne das explizit im ersten Satz der Zusammenfassung.`;
+  }
+
+  if (locale === "en") {
+    return `
+
+SYSTEM CONTEXT — COMPARISON MODE ACTIVE
+The user has activated comparison mode against ${baselineLabel}.
+When narrating findings, reference the deltas below. The most
+clinically-significant non-null delta MUST appear in the summary's
+first sentence (e.g. "Your average systolic BP improved by 4 mmHg
+vs. last month"). Do NOT invent comparison numbers.
+
+Prior-period deltas:
+${snapshot.metrics.map(renderRow).join("\n")}`;
+  }
+
+  return `
+
+SYSTEM CONTEXT — VERGLEICHSMODUS AKTIV
+Der Nutzer hat den Vergleichsmodus gegen ${baselineLabel} aktiviert.
+Beziehe dich beim Narrativ auf die untenstehenden Deltas. Die
+klinisch bedeutsamste nicht-leere Abweichung MUSS im ersten Satz der
+Zusammenfassung auftauchen (z.B. "Dein systolischer Mittelwert ist
+4 mmHg besser als im Vormonat"). Erfinde KEINE Vergleichszahlen.
+
+Vorperioden-Deltas:
+${snapshot.metrics.map(renderRow).join("\n")}`;
 }