Merge work/coach-read-locale

src/lib/ai/coach/__tests__/correlations-module-gate.test.ts

@@ -45,14 +45,14 @@ beforeEach(() => {
 
 describe("readCoachCorrelations — insights module gate", () => {
   it("gates on `insights`, not on a per-domain key", async () => {
-    await readCoachCorrelations(USER);
+    await readCoachCorrelations(USER, "en");
     expect(isModuleEnabled).toHaveBeenCalledWith(USER, "insights");
   });
 
   it("omits with the module OFF, and reads nothing at all", async () => {
     vi.mocked(isModuleEnabled).mockImplementation(async () => false);
 
-    const res = await readCoachCorrelations(USER);
+    const res = await readCoachCorrelations(USER, "en");
 
     expect(res.present).toBe(false);
     // Distinct from `no_data`: the assistant is told the domain is switched
@@ -67,7 +67,7 @@ describe("readCoachCorrelations — insights module gate", () => {
   });
 
   it("proceeds to the engine with the module ON", async () => {
-    const res = await readCoachCorrelations(USER);
+    const res = await readCoachCorrelations(USER, "en");
     // The synthetic fixture has no paired data, so the honest answer is a
     // miss — but it must be the ENGINE's miss, reached after the real reads
     // ran, not the gate's.
@@ -85,7 +85,7 @@ describe("readCoachCorrelations — insights module gate", () => {
       async (_u: string, key: string) => key !== "insights",
     );
 
-    const res = await readCoachCorrelations(USER);
+    const res = await readCoachCorrelations(USER, "en");
 
     expect(res.present).toBe(false);
     expect(res.reason).toBe("module_disabled");
@@ -99,6 +99,8 @@ describe("readCoachCorrelations — insights module gate", () => {
     // loud, not silently indistinguishable from "no pattern found".
     vi.mocked(isModuleEnabled).mockRejectedValue(new Error("gate down"));
 
-    await expect(readCoachCorrelations(USER)).rejects.toThrow("gate down");
+    await expect(readCoachCorrelations(USER, "en")).rejects.toThrow(
+      "gate down",
+    );
   });
 });

src/lib/ai/coach/__tests__/correlations-read.test.ts

@@ -91,7 +91,7 @@ describe("readCoachCorrelations", () => {
       fdrQ: 0.1,
       minPairs: 20,
     });
-    const result = await readCoachCorrelations("u1");
+    const result = await readCoachCorrelations("u1", "en");
     expect(result.present).toBe(true);
     expect(result.drivers).toHaveLength(1);
     expect(result.drivers?.[0]).toMatchObject({
@@ -111,7 +111,7 @@ describe("readCoachCorrelations", () => {
       fdrQ: 0.1,
       minPairs: 20,
     });
-    const result = await readCoachCorrelations("u1");
+    const result = await readCoachCorrelations("u1", "en");
     expect(result.present).toBe(false);
     expect(result.reason).toBe("no_significant_pattern");
   });
@@ -149,7 +149,7 @@ describe("readCoachCorrelations", () => {
         computedAt: "x",
       },
     });
-    const result = await readCoachCorrelations("u1");
+    const result = await readCoachCorrelations("u1", "en");
     expect(result.present).toBe(true);
     expect(result.coincident?.fired).toBe(true);
     expect(result.coincident?.contributing).toEqual([
@@ -160,7 +160,7 @@ describe("readCoachCorrelations", () => {
 
   it("degrades to present:false on a read failure", async () => {
     loadBaselineProfile.mockRejectedValue(new Error("db down"));
-    const result = await readCoachCorrelations("u1");
+    const result = await readCoachCorrelations("u1", "en");
     expect(result.present).toBe(false);
     expect(result.reason).toBe("retrieval_failed");
   });
@@ -175,7 +175,7 @@ describe("readCoachCorrelations", () => {
       fdrQ: 0.1,
       minPairs: 20,
     });
-    await readCoachCorrelations("u1");
+    await readCoachCorrelations("u1", "en");
     const where = measurementFindMany.mock.calls[0]?.[0]?.where as {
       type: { in: string[] };
     };
@@ -198,6 +198,7 @@ describe("readCoachCorrelations", () => {
     discoverCorrelations.mockImplementation((series) =>
       real.discoverCorrelations(
         series as Parameters<typeof real.discoverCorrelations>[0],
+        { locale: "en" },
       ),
     );
 
@@ -248,7 +249,7 @@ describe("readCoachCorrelations", () => {
     ]);
     illnessDayLogFindMany.mockResolvedValue(dayLogs);
 
-    const result = await readCoachCorrelations("u1");
+    const result = await readCoachCorrelations("u1", "en");
 
     expect(result.present).toBe(true);
     const symptomDriver = result.drivers?.find(

src/lib/ai/coach/__tests__/correlations-snapshot.test.ts

@@ -47,7 +47,7 @@ describe("buildCorrelationsSnapshotBlock (D5-5)", () => {
       pairsTested: 16,
       windowDays: 180,
     });
-    const block = await buildCorrelationsSnapshotBlock("user-1");
+    const block = await buildCorrelationsSnapshotBlock("user-1", "en");
     expect(block).not.toBeNull();
     expect(block!.drivers).toHaveLength(2);
     expect(block!.drivers[0].behaviour).toBe("time in daylight");
@@ -65,7 +65,7 @@ describe("buildCorrelationsSnapshotBlock (D5-5)", () => {
       pairsTested: 30,
       windowDays: 180,
     });
-    const block = await buildCorrelationsSnapshotBlock("user-1");
+    const block = await buildCorrelationsSnapshotBlock("user-1", "en");
     expect(block!.drivers).toHaveLength(SNAPSHOT_DRIVER_CAP);
     // Order preserved from the already-ranked tool output (highest effect first).
     expect(block!.drivers[0].behaviour).toBe("behaviour 0");
@@ -76,7 +76,7 @@ describe("buildCorrelationsSnapshotBlock (D5-5)", () => {
       present: false,
       reason: "no_significant_pattern",
     });
-    expect(await buildCorrelationsSnapshotBlock("user-1")).toBeNull();
+    expect(await buildCorrelationsSnapshotBlock("user-1", "en")).toBeNull();
   });
 
   it("attaches nothing when present but the driver list is empty (coincident-only)", async () => {
@@ -89,6 +89,6 @@ describe("buildCorrelationsSnapshotBlock (D5-5)", () => {
       pairsTested: 12,
       windowDays: 180,
     });
-    expect(await buildCorrelationsSnapshotBlock("user-1")).toBeNull();
+    expect(await buildCorrelationsSnapshotBlock("user-1", "en")).toBeNull();
   });
 });

src/lib/ai/coach/__tests__/tools-executor.test.ts

@@ -56,7 +56,15 @@ function emptyAggregate() {
 
 const readCoachCorrelations = vi.fn();
 vi.mock("@/lib/ai/coach/tools/correlations-read", () => ({
-  readCoachCorrelations: (userId: string) => readCoachCorrelations(userId),
+  readCoachCorrelations: (userId: string, locale: string) =>
+    readCoachCorrelations(userId, locale),
+}));
+
+// The executor asks for the account's language before it asks for a narrated
+// correlation, so the sentences it hands the model are not silently English.
+const resolveLocaleForUser = vi.fn(async (_userId: string) => "de");
+vi.mock("@/lib/i18n/user-locale", () => ({
+  resolveLocaleForUser: (userId: string) => resolveLocaleForUser(userId),
 }));
 
 // v1.21.0 (NEW-B B-2) — the illness-recovery tool augments its snapshot blocks
@@ -357,6 +365,9 @@ describe("executeCoachTool", () => {
       drivers: [{ behaviour: "time in daylight", n: 42 }],
       pairsTested: 18,
     });
+    // The account's language reached the reader — the notes are finished
+    // sentences, and an MCP client shows some of them as they stand.
+    expect(readCoachCorrelations).toHaveBeenCalledWith("u1", "de");
   });
 
   it("returns a clean { present: false } for get_correlations on no pattern", async () => {

src/lib/ai/coach/correlations-snapshot.ts

@@ -18,6 +18,7 @@
  * Server-only — `readCoachCorrelations` reads `@/lib/db`. Fail-soft: a null
  * return (no surviving driver / read hiccup) attaches nothing.
  */
+import type { Locale } from "@/lib/i18n/config";
 import {
   readCoachCorrelations,
   type CoachCorrelationDriver,
@@ -45,8 +46,15 @@ export interface CorrelationsSnapshotBlock {
  */
 export async function buildCorrelationsSnapshotBlock(
   userId: string,
+  /**
+   * The Coach's resolved UI locale. The driver notes go into the prompt as
+   * finished sentences, and a model handed English prose for a German
+   * conversation echoes English phrasings back. Required — the snapshot knows
+   * the locale, so it says it.
+   */
+  locale: Locale,
 ): Promise<CorrelationsSnapshotBlock | null> {
-  const result = await readCoachCorrelations(userId);
+  const result = await readCoachCorrelations(userId, locale);
   if (!result.present || !result.drivers || result.drivers.length === 0) {
     return null;
   }

src/lib/ai/coach/memory-snapshot.ts

@@ -168,6 +168,7 @@ export async function buildCoachMemoryBlock(
     const ctx = await buildPeriodNarrativeContext(userId, {
       period: MEMORY_PERIOD,
       now,
+      locale,
     });
     if (ctx.status === "ready") {
       for (const transition of ctx.bandTransitions) {

src/lib/ai/coach/snapshot.ts

@@ -843,7 +843,7 @@ async function buildCoachSnapshotImpl(
   // instead of getting only the coincident flag. Gated on `derivedActive` (it
   // is the recovery / cross-metric layer) and fail-soft to null.
   const correlationsBlockPromise = derivedActive
-    ? buildCorrelationsSnapshotBlock(userId)
+    ? buildCorrelationsSnapshotBlock(userId, coachLocale)
     : null;
   const memoryBlockPromise = buildCoachMemoryBlock(
     userId,

src/lib/ai/coach/tools/correlations-read.ts

@@ -24,6 +24,7 @@
  */
 import type { MeasurementType } from "@/generated/prisma/client";
 import { prisma } from "@/lib/db";
+import type { Locale } from "@/lib/i18n/config";
 import { annotate } from "@/lib/logging/context";
 import { isModuleEnabled } from "@/lib/modules/gate";
 import { wallClockInTz } from "@/lib/tz/wall-clock";
@@ -151,6 +152,15 @@ function humanise(key: string): string {
  */
 export async function readCoachCorrelations(
   userId: string,
+  /**
+   * The reader's locale. Every `note` this returns is a finished sentence that
+   * some surface prints verbatim — the metric page's "Coach read" strip does
+   * exactly that — so the language has to be decided by whoever knows who is
+   * reading. No default: a caller that cannot name a locale is a caller that
+   * does not know, and guessing English is how the strip came to answer a
+   * German page in English.
+   */
+  locale: Locale,
 ): Promise<CoachCorrelationsResult> {
   // v1.30.22 — the `insights` gate lives HERE, at the read, not at each
   // caller. This reader is reached from four places (the Coach
@@ -271,7 +281,7 @@ export async function readCoachCorrelations(
       },
     });
 
-    const discovery = discoverCorrelations(series);
+    const discovery = discoverCorrelations(series, { locale });
     const drivers: CoachCorrelationDriver[] = discovery.discovered.map((d) => ({
       behaviour: humanise(d.behaviour),
       outcome: humanise(d.outcome),
@@ -292,6 +302,7 @@ export async function readCoachCorrelations(
     );
     const emergingResult = discoverEmergingCorrelations(series, discovery, {
       recentFromDayKey,
+      locale,
     });
     const emerging: CoachEmergingDriver[] = emergingResult.emerging.map(
       (d) => ({
@@ -307,7 +318,9 @@ export async function readCoachCorrelations(
     );
 
     // v1.22 — labs ↔ outcome pass (point-vs-window over sparse draws).
-    const labResult = discoverLabOutcomeCorrelations(labDraws, series);
+    const labResult = discoverLabOutcomeCorrelations(labDraws, series, {
+      locale,
+    });
     const labDrivers: CoachLabCorrelation[] = labResult.discovered.map((d) => ({
       lab: d.lab.startsWith("LAB:") ? d.lab.slice("LAB:".length) : d.lab,
       outcome: humanise(d.outcome),

src/lib/ai/coach/tools/executor.ts

@@ -62,6 +62,7 @@ import {
   METRIC_SERIES_EXCLUDED_SOURCES,
 } from "./source-keys";
 import { readCoachCorrelations } from "./correlations-read";
+import { resolveLocaleForUser } from "@/lib/i18n/user-locale";
 import {
   resolveEmptyRead,
   subjectForTool,
@@ -599,7 +600,12 @@ async function getCorrelations(
   // days, and the reader already reports WHY nothing survived (its own reason
   // code). "No pattern survived the correction" is an honest absence of a
   // finding, not an absence of data, and nothing older would change it.
-  const result = await readCoachCorrelations(userId);
+  // The reader's language, from their stored preference — the executor is
+  // reached from the chat loop and from MCP, neither of which carries a request
+  // of its own. The notes below are finished sentences; some MCP clients show
+  // them to the person verbatim rather than letting the model re-word them.
+  const locale = await resolveLocaleForUser(userId);
+  const result = await readCoachCorrelations(userId, locale);
   if (!result.present) {
     return { present: false, reason: result.reason ?? "no_pattern" };
   }

src/lib/cycle/__tests__/phase-crosstab.test.ts

@@ -286,7 +286,11 @@ describe("discoverPhaseCorrelations", () => {
         ),
       );
     }
-    const result = discoverPhaseCorrelations({ phaseByDay, measurements });
+    const result = discoverPhaseCorrelations({
+      phaseByDay,
+      measurements,
+      locale: "en",
+    });
     const pair = result.discovered.find(
       (p) => p.behaviour === CYCLE_PHASE_CHANNEL_KEY,
     );
@@ -315,7 +319,11 @@ describe("discoverPhaseCorrelations", () => {
       phaseByDay.set(key, phases[i % phases.length]);
       measurements.push(m("RESTING_HEART_RATE", key, 60)); // constant → no variance
     }
-    const result = discoverPhaseCorrelations({ phaseByDay, measurements });
+    const result = discoverPhaseCorrelations({
+      phaseByDay,
+      measurements,
+      locale: "en",
+    });
     expect(result.discovered).toEqual([]);
   });
 });

src/lib/cycle/phase-crosstab.ts

@@ -51,6 +51,7 @@ import {
 import { round } from "@/lib/insights/status-shared";
 import type { MeasurementType } from "@/generated/prisma/enums";
 import type { CyclePhase } from "@/lib/cycle/types";
+import type { Locale } from "@/lib/i18n/config";
 
 /** Stable channel key for the continuous CYCLE_PHASE discovery series. */
 export const CYCLE_PHASE_CHANNEL_KEY = "CYCLE_PHASE";
@@ -292,8 +293,15 @@ export function discoverPhaseCorrelations(args: {
   phaseByDay: Map<string, CyclePhase>;
   measurements: CrossMetricMeasurement[];
   userPriorityJson?: unknown;
+  /**
+   * The reader's locale. Each surviving row carries a finished `interpretation`
+   * sentence that the cycle route serves straight to the client, so it has to
+   * be written in the reader's language. Required — no default, because a
+   * default is what let the metric page's driver line ship in English.
+   */
+  locale: Locale;
 }): CorrelationDiscoveryResult {
-  const { phaseByDay, measurements } = args;
+  const { phaseByDay, measurements, locale } = args;
   const userPriorityJson = args.userPriorityJson ?? null;
 
   const phaseSeries = buildPhaseDiscoverySeries(phaseByDay);
@@ -307,7 +315,7 @@ export function discoverPhaseCorrelations(args: {
     outcomeSeries.push({ key: cfg.type, role: "outcome", points });
   }
 
-  return discoverCorrelations([phaseSeries, ...outcomeSeries]);
+  return discoverCorrelations([phaseSeries, ...outcomeSeries], { locale });
 }
 
 /**

src/lib/i18n/user-locale.ts

@@ -0,0 +1,26 @@
+/**
+ * The reader's locale for a server-internal caller that only has a user id.
+ *
+ * Route handlers resolve the locale from the request (cookie first, then the
+ * stored preference, then Accept-Language) via `resolveServerLocale`. A few
+ * server-internal fan-outs reach a prose-writing engine without a request of
+ * their own — the Coach tool executor and the MCP rich reads both take a user
+ * id and nothing else. This looks their stored preference up and hands it to
+ * the SAME resolver, so there is one place that decides what language a
+ * sentence is written in, not two.
+ *
+ * Deliberately its own module rather than a second export on `server-locale`:
+ * that file is imported by page components, and it must not start pulling
+ * Prisma along with it.
+ */
+import { prisma } from "@/lib/db";
+import type { Locale } from "./config";
+import { resolveServerLocale } from "./server-locale";
+
+export async function resolveLocaleForUser(userId: string): Promise<Locale> {
+  const row = await prisma.user.findUnique({
+    where: { id: userId },
+    select: { locale: true },
+  });
+  return resolveServerLocale({ userLocale: row?.locale ?? null });
+}

src/lib/insights/__tests__/correlation-discovery.test.ts

@@ -56,7 +56,9 @@ describe("discoverCorrelations", () => {
         points: series([400, 410, 420]),
       },
     ];
-    const result = discoverCorrelations([...behaviours, ...outcomes]);
+    const result = discoverCorrelations([...behaviours, ...outcomes], {
+      locale: "en",
+    });
     expect(result.discovered).toHaveLength(0);
     expect(result.pairsTested).toBe(0);
   });
@@ -79,7 +81,7 @@ describe("discoverCorrelations", () => {
         },
         { key: "SLEEP_DURATION", role: "outcome", points: series(outcomeVals) },
       ],
-      { fdrQ: 0.1 },
+      { fdrQ: 0.1, locale: "en" },
     );
     expect(result.pairsTested).toBe(1);
     expect(result.discovered).toHaveLength(1);
@@ -100,10 +102,13 @@ describe("discoverCorrelations", () => {
       { length: 30 },
       (_, i) => Math.cos(i * 1.7) * 10 + 50,
     );
-    const result = discoverCorrelations([
-      { key: "BLOOD_GLUCOSE", role: "behaviour", points: series(noiseA) },
-      { key: "WEIGHT", role: "outcome", points: series(noiseB) },
-    ]);
+    const result = discoverCorrelations(
+      [
+        { key: "BLOOD_GLUCOSE", role: "behaviour", points: series(noiseA) },
+        { key: "WEIGHT", role: "outcome", points: series(noiseB) },
+      ],
+      { locale: "en" },
+    );
     // It is tested (n ≥ 20) but should not survive p < 0.05 + FDR.
     expect(result.pairsTested).toBe(1);
     expect(result.discovered).toHaveLength(0);
@@ -122,7 +127,7 @@ describe("discoverCorrelations", () => {
         { key: "FACTOR:work", role: "behaviour", points: series(factorVals) },
         { key: "SLEEP_DURATION", role: "outcome", points: series(sleepVals) },
       ],
-      { fdrQ: 0.1 },
+      { fdrQ: 0.1, locale: "en" },
     );
     expect(result.discovered).toHaveLength(1);
     const pair = result.discovered[0];
@@ -153,10 +158,17 @@ describe("MOOD as outcome (F3)", () => {
       const driver = i === 0 ? 60 : 60 + ((i - 1) % 10) * 5;
       return 1 + (driver - 60) / 45 + (i % 2 === 0 ? 0.02 : -0.02);
     });
-    const result = discoverCorrelations([
-      { key: "TIME_IN_DAYLIGHT", role: "behaviour", points: series(daylight) },
-      { key: "MOOD", role: "outcome", points: series(mood) },
-    ]);
+    const result = discoverCorrelations(
+      [
+        {
+          key: "TIME_IN_DAYLIGHT",
+          role: "behaviour",
+          points: series(daylight),
+        },
+        { key: "MOOD", role: "outcome", points: series(mood) },
+      ],
+      { locale: "en" },
+    );
     const pair = result.discovered.find(
       (p) => p.behaviour === "TIME_IN_DAYLIGHT" && p.outcome === "MOOD",
     );
@@ -167,10 +179,13 @@ describe("MOOD as outcome (F3)", () => {
   it("never tests the MOOD → MOOD self-pair", () => {
     const n = 30;
     const mood = Array.from({ length: n }, (_, i) => 3 + (i % 3));
-    const result = discoverCorrelations([
-      { key: "MOOD", role: "behaviour", points: series(mood) },
-      { key: "MOOD", role: "outcome", points: series(mood) },
-    ]);
+    const result = discoverCorrelations(
+      [
+        { key: "MOOD", role: "behaviour", points: series(mood) },
+        { key: "MOOD", role: "outcome", points: series(mood) },
+      ],
+      { locale: "en" },
+    );
     expect(
       result.discovered.find(
         (p) => p.behaviour === "MOOD" && p.outcome === "MOOD",
@@ -222,10 +237,13 @@ describe("D2-1 — same-family lagged pairs are excluded from discovery", () =>
     const n = 40;
     const factor = Array.from({ length: n }, (_, i) => 1 + (i % 5));
     const mood = [3, ...factor.slice(0, n - 1).map((v) => v)];
-    const result = discoverCorrelations([
-      { key: "FACTOR:work", role: "behaviour", points: longSeries(factor) },
-      { key: "MOOD", role: "outcome", points: longSeries(mood) },
-    ]);
+    const result = discoverCorrelations(
+      [
+        { key: "FACTOR:work", role: "behaviour", points: longSeries(factor) },
+        { key: "MOOD", role: "outcome", points: longSeries(mood) },
+      ],
+      { locale: "en" },
+    );
     expect(result.pairsTested).toBe(0);
     expect(result.discovered).toHaveLength(0);
   });
@@ -283,14 +301,17 @@ describe("D2-2 — effect-size floor on discovered drivers", () => {
         .slice(0, n - 1)
         .map((v, i) => v * 0.12 + Math.cos(i * 1.3) * 40 + 100),
     ];
-    const result = discoverCorrelations([
-      {
-        key: "TIME_IN_DAYLIGHT",
-        role: "behaviour",
-        points: longSeries(behaviour),
-      },
-      { key: "SLEEP_DURATION", role: "outcome", points: longSeries(outcome) },
-    ]);
+    const result = discoverCorrelations(
+      [
+        {
+          key: "TIME_IN_DAYLIGHT",
+          role: "behaviour",
+          points: longSeries(behaviour),
+        },
+        { key: "SLEEP_DURATION", role: "outcome", points: longSeries(outcome) },
+      ],
+      { locale: "en" },
+    );
     // Either it failed significance, or it was floored out — either way no
     // confident driver row reaches the Coach.
     const surfaced = result.discovered.find(
@@ -309,14 +330,17 @@ describe("D2-2 — effect-size floor on discovered drivers", () => {
     const n = 90;
     const behaviour = Array.from({ length: n }, (_, i) => i + (i % 4));
     const outcome = [0, ...behaviour.slice(0, n - 1).map((v) => v * 2 + 5)];
-    const result = discoverCorrelations([
-      {
-        key: "TIME_IN_DAYLIGHT",
-        role: "behaviour",
-        points: longSeries(behaviour),
-      },
-      { key: "SLEEP_DURATION", role: "outcome", points: longSeries(outcome) },
-    ]);
+    const result = discoverCorrelations(
+      [
+        {
+          key: "TIME_IN_DAYLIGHT",
+          role: "behaviour",
+          points: longSeries(behaviour),
+        },
+        { key: "SLEEP_DURATION", role: "outcome", points: longSeries(outcome) },
+      ],
+      { locale: "en" },
+    );
     const pair = result.discovered.find(
       (p) =>
         p.behaviour === "TIME_IN_DAYLIGHT" && p.outcome === "SLEEP_DURATION",
@@ -343,14 +367,17 @@ describe("D2-2 — effect-size floor on discovered drivers", () => {
         .slice(0, n - 1)
         .map((v, i) => v * 0.35 + Math.cos(i * 2.1) * 9 + 100),
     ];
-    const result = discoverCorrelations([
-      {
-        key: "TIME_IN_DAYLIGHT",
-        role: "behaviour",
-        points: longSeries(behaviour),
-      },
-      { key: "SLEEP_DURATION", role: "outcome", points: longSeries(outcome) },
-    ]);
+    const result = discoverCorrelations(
+      [
+        {
+          key: "TIME_IN_DAYLIGHT",
+          role: "behaviour",
+          points: longSeries(behaviour),
+        },
+        { key: "SLEEP_DURATION", role: "outcome", points: longSeries(outcome) },
+      ],
+      { locale: "en" },
+    );
     const pair = result.discovered.find(
       (p) =>
         p.behaviour === "TIME_IN_DAYLIGHT" && p.outcome === "SLEEP_DURATION",
@@ -462,18 +489,21 @@ describe("compliance + symptom channels (FDREXTEND)", () => {
         // pair is not a perfect line (a real-world-shaped strong link).
         .map((a, i) => ((100 - a) / 20) * (i % 2 === 0 ? 1.0 : 0.98)),
     ];
-    const result = discoverCorrelations([
-      {
-        key: MEDICATION_COMPLIANCE_CHANNEL_KEY,
-        role: "behaviour",
-        points: longSeries(adherence),
-      },
-      {
-        key: SYMPTOM_SEVERITY_CHANNEL_KEY,
-        role: "outcome",
-        points: longSeries(symptom),
-      },
-    ]);
+    const result = discoverCorrelations(
+      [
+        {
+          key: MEDICATION_COMPLIANCE_CHANNEL_KEY,
+          role: "behaviour",
+          points: longSeries(adherence),
+        },
+        {
+          key: SYMPTOM_SEVERITY_CHANNEL_KEY,
+          role: "outcome",
+          points: longSeries(symptom),
+        },
+      ],
+      { locale: "en" },
+    );
     const pair = result.discovered.find(
       (p) =>
         p.behaviour === MEDICATION_COMPLIANCE_CHANNEL_KEY &&
@@ -500,18 +530,21 @@ describe("compliance + symptom channels (FDREXTEND)", () => {
       0,
       ...adherence.slice(0, n - 1).map((a) => (100 - a) / 30),
     ];
-    const result = discoverCorrelations([
-      {
-        key: MEDICATION_COMPLIANCE_CHANNEL_KEY,
-        role: "behaviour",
-        points: longSeries(adherence),
-      },
-      {
-        key: SYMPTOM_SEVERITY_CHANNEL_KEY,
-        role: "outcome",
-        points: longSeries(symptom),
-      },
-    ]);
+    const result = discoverCorrelations(
+      [
+        {
+          key: MEDICATION_COMPLIANCE_CHANNEL_KEY,
+          role: "behaviour",
+          points: longSeries(adherence),
+        },
+        {
+          key: SYMPTOM_SEVERITY_CHANNEL_KEY,
+          role: "outcome",
+          points: longSeries(symptom),
+        },
+      ],
+      { locale: "en" },
+    );
     // Below the floor the pair is never even tested.
     expect(result.pairsTested).toBe(0);
     expect(result.discovered).toHaveLength(0);
@@ -520,18 +553,21 @@ describe("compliance + symptom channels (FDREXTEND)", () => {
   it("never tests the symptom → symptom self-lag", () => {
     const n = 40;
     const symptom = Array.from({ length: n }, (_, i) => (i % 4) / 2);
-    const result = discoverCorrelations([
-      {
-        key: SYMPTOM_SEVERITY_CHANNEL_KEY,
-        role: "behaviour",
-        points: longSeries(symptom),
-      },
-      {
-        key: SYMPTOM_SEVERITY_CHANNEL_KEY,
-        role: "outcome",
-        points: longSeries(symptom),
-      },
-    ]);
+    const result = discoverCorrelations(
+      [
+        {
+          key: SYMPTOM_SEVERITY_CHANNEL_KEY,
+          role: "behaviour",
+          points: longSeries(symptom),
+        },
+        {
+          key: SYMPTOM_SEVERITY_CHANNEL_KEY,
+          role: "outcome",
+          points: longSeries(symptom),
+        },
+      ],
+      { locale: "en" },
+    );
     expect(
       result.discovered.find(
         (p) =>
@@ -555,18 +591,21 @@ describe("compliance + symptom channels (FDREXTEND)", () => {
         .slice(0, n - 1)
         .map((a, i) => 55 + (100 - a) / 6 + (i % 2 === 0 ? 0.1 : -0.1)),
     ];
-    const result = discoverCorrelations([
-      {
-        key: MEDICATION_COMPLIANCE_CHANNEL_KEY,
-        role: "behaviour",
-        points: longSeries(adherence),
-      },
-      {
-        key: "RESTING_HEART_RATE",
-        role: "outcome",
-        points: longSeries(rhr),
-      },
-    ]);
+    const result = discoverCorrelations(
+      [
+        {
+          key: MEDICATION_COMPLIANCE_CHANNEL_KEY,
+          role: "behaviour",
+          points: longSeries(adherence),
+        },
+        {
+          key: "RESTING_HEART_RATE",
+          role: "outcome",
+          points: longSeries(rhr),
+        },
+      ],
+      { locale: "en" },
+    );
     const pair = result.discovered.find(
       (p) =>
         p.behaviour === MEDICATION_COMPLIANCE_CHANNEL_KEY &&
@@ -621,17 +660,24 @@ describe("v1.22 — new curated linkages", () => {
         .slice(0, n - 1)
         .map((a, i) => 160 - a / 4 + (i % 2 ? 0.2 : -0.2)),
     ];
-    const result = discoverCorrelations([
-      {
-        key: MEDICATION_COMPLIANCE_CHANNEL_KEY,
-        role: "behaviour",
-        points: longSeries(adherence),
-      },
-      // BP is both a behaviour and an outcome in the real matrix; the
-      // same-family guard must keep BP_SYS(behaviour)→BP_SYS(outcome) out.
-      { key: "BLOOD_PRESSURE_SYS", role: "behaviour", points: longSeries(sys) },
-      { key: "BLOOD_PRESSURE_SYS", role: "outcome", points: longSeries(sys) },
-    ]);
+    const result = discoverCorrelations(
+      [
+        {
+          key: MEDICATION_COMPLIANCE_CHANNEL_KEY,
+          role: "behaviour",
+          points: longSeries(adherence),
+        },
+        // BP is both a behaviour and an outcome in the real matrix; the
+        // same-family guard must keep BP_SYS(behaviour)→BP_SYS(outcome) out.
+        {
+          key: "BLOOD_PRESSURE_SYS",
+          role: "behaviour",
+          points: longSeries(sys),
+        },
+        { key: "BLOOD_PRESSURE_SYS", role: "outcome", points: longSeries(sys) },
+      ],
+      { locale: "en" },
+    );
     expect(
       result.discovered.find(
         (p) =>
@@ -681,12 +727,13 @@ describe("discoverEmergingCorrelations (early detection)", () => {
         points: longSeries(out),
       },
     ];
-    const retrospective = discoverCorrelations(series);
+    const retrospective = discoverCorrelations(series, { locale: "en" });
     expect(retrospective.discovered).toHaveLength(0); // n too low for the floor
 
     const cutoff = longSeries(beh)[0].day; // whole (short) window
     const { emerging } = discoverEmergingCorrelations(series, retrospective, {
       recentFromDayKey: cutoff,
+      locale: "en",
       minPairs: 10,
     });
     const pair = emerging.find(
@@ -711,7 +758,7 @@ describe("discoverEmergingCorrelations (early detection)", () => {
         points: longSeries(out),
       },
     ];
-    const retrospective = discoverCorrelations(series);
+    const retrospective = discoverCorrelations(series, { locale: "en" });
     expect(
       retrospective.discovered.some(
         (p) => p.behaviour === "MOOD" && p.outcome === "HEART_RATE_VARIABILITY",
@@ -724,6 +771,7 @@ describe("discoverEmergingCorrelations (early detection)", () => {
       .slice(0, 10);
     const { emerging } = discoverEmergingCorrelations(series, retrospective, {
       recentFromDayKey: cutoff,
+      locale: "en",
       minPairs: 10,
     });
     expect(
@@ -745,10 +793,11 @@ describe("discoverEmergingCorrelations (early detection)", () => {
         points: longSeries(out),
       },
     ];
-    const retrospective = discoverCorrelations(series);
+    const retrospective = discoverCorrelations(series, { locale: "en" });
     const cutoff = longSeries(beh)[Math.max(0, n - EARLY_WINDOW_DAYS)].day;
     const { emerging } = discoverEmergingCorrelations(series, retrospective, {
       recentFromDayKey: cutoff,
+      locale: "en",
       minPairs: 10,
     });
     expect(emerging).toHaveLength(0);
@@ -778,6 +827,7 @@ describe("discoverLabOutcomeCorrelations (labs ↔ outcomes)", () => {
       draws.push({ key: "LAB:HbA1c", day, value: v });
     }
     const result = discoverLabOutcomeCorrelations(draws, [weight], {
+      locale: "en",
       minDraws: 5,
       minWindowPoints: 5,
     });
@@ -799,6 +849,7 @@ describe("discoverLabOutcomeCorrelations (labs ↔ outcomes)", () => {
       value: 5 + i,
     }));
     const result = discoverLabOutcomeCorrelations(draws, [weight], {
+      locale: "en",
       minDraws: 5,
     });
     expect(result.discovered).toHaveLength(0);
@@ -820,6 +871,7 @@ describe("discoverLabOutcomeCorrelations (labs ↔ outcomes)", () => {
       value: 5 + i,
     }));
     const result = discoverLabOutcomeCorrelations(draws, [hrv], {
+      locale: "en",
       minDraws: 5,
     });
     // HRV is not a curated lab-outcome target → no pair tested.

src/lib/insights/__tests__/correlation-series-builders.test.ts

@@ -185,7 +185,9 @@ describe("integration — built series flow through the discovery gates", () =>
       role: "outcome",
     });
 
-    const result = discoverCorrelations([compliance, symptom]);
+    const result = discoverCorrelations([compliance, symptom], {
+      locale: "en",
+    });
     expect(result.pairsTested).toBe(0);
     expect(result.discovered).toHaveLength(0);
   });
@@ -222,7 +224,9 @@ describe("integration — built series flow through the discovery gates", () =>
       role: "outcome",
     });
 
-    const result = discoverCorrelations([compliance, symptom]);
+    const result = discoverCorrelations([compliance, symptom], {
+      locale: "en",
+    });
     const pair = result.discovered.find(
       (p) =>
         p.behaviour === MEDICATION_COMPLIANCE_CHANNEL_KEY &&
@@ -256,7 +260,7 @@ describe("integration — built series flow through the discovery gates", () =>
         value: 55 + (i > 0 && (i - 1) % 4 === 0 ? 8 : 0) + (i % 2) * 0.1,
       })),
     });
-    const result = discoverCorrelations(series);
+    const result = discoverCorrelations(series, { locale: "en" });
     for (const p of result.discovered) {
       expect(p.interpretation).toMatch(/not a cause|never a cause/);
     }

src/lib/insights/__tests__/metric-correlation-context.test.ts

@@ -42,7 +42,7 @@ beforeEach(() => {
 describe("getRelevantCorrelationsForMetric", () => {
   it("returns [] without any DB read for a non-discovery metric", async () => {
     // VO2_MAX is not part of the curated discovery matrix.
-    const out = await getRelevantCorrelationsForMetric("u-1", "VO2_MAX");
+    const out = await getRelevantCorrelationsForMetric("u-1", "VO2_MAX", "en");
     expect(out).toEqual([]);
     expect(prisma.measurement.findMany).not.toHaveBeenCalled();
   });
@@ -70,6 +70,7 @@ describe("getRelevantCorrelationsForMetric", () => {
     const out = await getRelevantCorrelationsForMetric(
       "u-1",
       "RESTING_HEART_RATE",
+      "en",
     );
     expect(out.length).toBeGreaterThan(0);
     // Every surfaced relation mentions resting heart rate (the metric's channel).
@@ -87,6 +88,7 @@ describe("getRelevantCorrelationsForMetric", () => {
     const out = await getRelevantCorrelationsForMetric(
       "u-1",
       "RESTING_HEART_RATE",
+      "en",
     );
     expect(out).toEqual([]);
   });

src/lib/insights/__tests__/metric-status.test.ts

@@ -411,6 +411,7 @@ describe("generateMetricStatus — generation path", () => {
     expect(getRelevantCorrelationsForMetric).toHaveBeenCalledWith(
       "user-1",
       "RESTING_HEART_RATE",
+      "en",
     );
   });
 

src/lib/insights/blood-pressure-status.ts

@@ -678,6 +678,7 @@ export async function prepareBloodPressureStatusForUser(
   const relations = await getRelevantCorrelationsForMetric(
     userId,
     "BLOOD_PRESSURE_SYS",
+    locale,
   );
   const assessmentContextBlock = buildAssessmentContextBlock(
     {

src/lib/insights/correlation-discovery.ts

@@ -39,7 +39,7 @@
  * link. The series builders live in `correlation-series-builders.ts`.
  */
 import { pearson, MIN_PAIRED_N } from "@/lib/insights/correlations";
-import { defaultLocale, type Locale } from "@/lib/i18n/config";
+import { type Locale } from "@/lib/i18n/config";
 import { getServerTranslator } from "@/lib/i18n/server-translator";
 
 /**
@@ -447,21 +447,25 @@ export function discoverCorrelations(
     minPairs?: number;
     fdrQ?: number;
     /**
-     * Reader's locale for the narrated `interpretation`. Defaults to English
-     * so the many server callers that feed the string into an LLM prompt (which
-     * re-localises) keep their existing behaviour; the user-facing correlations
-     * route passes the resolved locale so the card renders in the reader's
-     * language.
+     * Reader's locale for the narrated `interpretation`.
+     *
+     * REQUIRED, and it must stay required. It used to be optional with an
+     * English default, and the default is what shipped the bug: a caller could
+     * say nothing and still get a finished sentence back, so the "Coach read"
+     * strip on a German metric page printed its second line in English under a
+     * German first line for months. Nobody had to write anything wrong for that
+     * to happen — silence was enough. With no default, a caller that forgets
+     * cannot compile, which is the only version of this that stays fixed.
      */
-    locale?: Locale;
-  } = {},
+    locale: Locale;
+  },
 ): CorrelationDiscoveryResult {
   const lagDays = opts.lagDays ?? 1;
   const minPairs = opts.minPairs ?? MIN_PAIRED_N;
   const fdrQ = opts.fdrQ ?? FDR_Q;
   // Named `translate` (not `t`) — the pair-mapping callbacks below bind `t` to
   // the RawPair element, so the translator must not shadow-collide with them.
-  const { t: translate } = getServerTranslator(opts.locale ?? defaultLocale);
+  const { t: translate } = getServerTranslator(opts.locale);
 
   const behaviours = series.filter((s) => s.role === "behaviour");
   const outcomes = series.filter((s) => s.role === "outcome");
@@ -717,8 +721,12 @@ export function discoverEmergingCorrelations(
     lagDays?: number;
     minPairs?: number;
     fdrQ?: number;
-    /** Reader's locale, threaded to the inner scan's narration. */
-    locale?: Locale;
+    /**
+     * Reader's locale, threaded to the inner scan's narration. Required for the
+     * same reason it is required on {@link discoverCorrelations} — an optional
+     * locale here would put the English default straight back, one call deeper.
+     */
+    locale: Locale;
   },
 ): EmergingCorrelationResult {
   const windowDays = opts.windowDays ?? EARLY_WINDOW_DAYS;
@@ -880,16 +888,20 @@ export function discoverLabOutcomeCorrelations(
     minDraws?: number;
     minWindowPoints?: number;
     fdrQ?: number;
-    /** Reader's locale for the narrated `interpretation`. Defaults to English. */
-    locale?: Locale;
-  } = {},
+    /**
+     * Reader's locale for the narrated `interpretation`. Required — see
+     * {@link discoverCorrelations}; a lab sentence leaks English just as
+     * happily as a daily one.
+     */
+    locale: Locale;
+  },
 ): LabCorrelationResult {
   const windowDays = opts.windowDays ?? LAB_OUTCOME_WINDOW_DAYS;
   const minDraws = opts.minDraws ?? LAB_MIN_DRAWS;
   const minWindowPoints = opts.minWindowPoints ?? LAB_MIN_WINDOW_POINTS;
   const fdrQ = opts.fdrQ ?? FDR_Q;
   // Named `translate` — the pair-mapping callback below binds `t` to its element.
-  const { t: translate } = getServerTranslator(opts.locale ?? defaultLocale);
+  const { t: translate } = getServerTranslator(opts.locale);
 
   // Group draws by biomarker key (newest-irrelevant; Pearson is order-free).
   const drawsByLab = new Map<string, LabDrawPoint[]>();

src/lib/insights/derived/__tests__/coach-read-locale.test.ts

@@ -0,0 +1,173 @@
+/**
+ * The "Coach read" strip's second line has to arrive in the reader's language.
+ *
+ * On a German metric page the strip used to print its own summary in German
+ * and the pattern line under it in English, one under the other. Nothing was
+ * mistranslated: the pattern line is a finished sentence the server writes,
+ * the route never said who was reading, and the engine's optional locale
+ * quietly answered in English.
+ *
+ * So this drives the REAL pipe — `buildCoachReadStrip` → `readCoachCorrelations`
+ * → `discoverCorrelations` — with only the database reads faked, and asserts the
+ * German sentence the German templates produce. Testing the engine alone would
+ * have stayed green through the whole outage: the engine could always speak
+ * German, it was simply never asked.
+ */
+import { describe, expect, it, vi, beforeEach } from "vitest";
+
+const measurementFindMany = vi.fn();
+const measurementFindFirst = vi.fn();
+const moodFindMany = vi.fn();
+const userFindUnique = vi.fn();
+const medicationFindMany = vi.fn();
+const intakeEventFindMany = vi.fn();
+const illnessEpisodeFindMany = vi.fn();
+const illnessDayLogFindMany = vi.fn();
+const labResultFindMany = vi.fn();
+
+vi.mock("@/lib/modules/gate", () => ({
+  isModuleEnabled: vi.fn(async () => true),
+}));
+vi.mock("@/lib/logging/context", () => ({ annotate: vi.fn() }));
+vi.mock("@/lib/db", () => ({
+  prisma: {
+    measurement: {
+      findMany: (a: unknown) => measurementFindMany(a),
+      findFirst: (a: unknown) => measurementFindFirst(a),
+    },
+    moodEntry: { findMany: (a: unknown) => moodFindMany(a) },
+    user: { findUnique: (a: unknown) => userFindUnique(a) },
+    medication: { findMany: (a: unknown) => medicationFindMany(a) },
+    medicationIntakeEvent: { findMany: (a: unknown) => intakeEventFindMany(a) },
+    illnessEpisode: { findMany: (a: unknown) => illnessEpisodeFindMany(a) },
+    illnessDayLog: { findMany: (a: unknown) => illnessDayLogFindMany(a) },
+    labResult: { findMany: (a: unknown) => labResultFindMany(a) },
+  },
+}));
+
+// The band engine is `baseline.test.ts`'s subject; the strip's FIRST line is
+// not what is under test here. Only the second line is, so the band degrades to
+// "still learning" and the driver line stands alone.
+const PROFILE = { ageYears: 40, sex: "MALE", heightCm: 180 };
+vi.mock("@/lib/insights/derived", async (importOriginal) => {
+  const actual =
+    await importOriginal<typeof import("@/lib/insights/derived")>();
+  return {
+    ...actual,
+    loadBaselineProfile: async () => PROFILE,
+    computeCoincidentDeviation: async () => null,
+  };
+});
+vi.mock("@/lib/insights/derived/baseline", async (importOriginal) => {
+  const actual =
+    await importOriginal<typeof import("@/lib/insights/derived/baseline")>();
+  return {
+    ...actual,
+    loadBaselineProfile: async () => PROFILE,
+    computeVitalsBaseline: async () => ({
+      status: "insufficient" as const,
+      reason: "not under test",
+    }),
+  };
+});
+
+import { buildCoachReadStrip } from "@/lib/insights/derived/coach-read";
+
+const DAYS = 45;
+const BASE = Date.UTC(2026, 4, 1, 12, 0, 0); // midday → a tz-stable day key
+
+function dayKey(offset: number): string {
+  const d = new Date(BASE + offset * 86_400_000);
+  return `${d.getUTCFullYear()}-${String(d.getUTCMonth() + 1).padStart(2, "0")}-${String(d.getUTCDate()).padStart(2, "0")}`;
+}
+
+/**
+ * 45 days where today's symptom burden runs against tomorrow's sleep, strongly
+ * enough to clear n ≥ 20, p < 0.05, the FDR control and the effect-size floor.
+ * Same fixture shape the correlations reader's own real-engine test uses.
+ */
+function seedSymptomDrivesSleep() {
+  const dayLogs: Array<{
+    date: string;
+    functionalImpact: number;
+    symptomLinks: Array<{ severity: number }>;
+  }> = [];
+  const measurements: Array<{ type: string; value: number; measuredAt: Date }> =
+    [];
+  for (let i = 0; i < DAYS; i++) {
+    const burden = i % 4;
+    dayLogs.push({
+      date: dayKey(i),
+      functionalImpact: burden,
+      symptomLinks: [],
+    });
+    measurements.push({
+      type: "SLEEP_DURATION",
+      value: 8 - burden,
+      measuredAt: new Date(BASE + (i + 1) * 86_400_000),
+    });
+  }
+  measurementFindMany.mockResolvedValue(measurements);
+  illnessEpisodeFindMany.mockResolvedValue([
+    {
+      id: "ep1",
+      onsetAt: new Date(BASE),
+      resolvedAt: new Date(BASE + DAYS * 86_400_000),
+    },
+  ]);
+  illnessDayLogFindMany.mockResolvedValue(dayLogs);
+}
+
+describe("buildCoachReadStrip — the pattern line speaks the reader's language", () => {
+  beforeEach(() => {
+    measurementFindMany.mockReset().mockResolvedValue([]);
+    measurementFindFirst.mockReset().mockResolvedValue({ value: 7 });
+    moodFindMany.mockReset().mockResolvedValue([]);
+    userFindUnique.mockReset().mockResolvedValue({ timezone: "Europe/Berlin" });
+    medicationFindMany.mockReset().mockResolvedValue([]);
+    intakeEventFindMany.mockReset().mockResolvedValue([]);
+    illnessEpisodeFindMany.mockReset().mockResolvedValue([]);
+    illnessDayLogFindMany.mockReset().mockResolvedValue([]);
+    labResultFindMany.mockReset().mockResolvedValue([]);
+    seedSymptomDrivesSleep();
+  });
+
+  it("writes the driver note in German for a German reader", async () => {
+    const strip = await buildCoachReadStrip("u1", "SLEEP_DURATION", "de");
+
+    expect(strip.driver).not.toBeNull();
+    const note = strip.driver!.note;
+
+    // The exact German sentence, not merely "different from the English one".
+    // A sentence that is different but wrong is still a broken page.
+    expect(note).toBe(
+      "Wenn die Symptomstärke höher liegt, scheint die Schlafdauer am " +
+        "Folgetag nach bisheriger Datenlage eher niedriger auszufallen — ein " +
+        "Muster, das Beachtung verdient, keine Ursache.",
+    );
+  });
+
+  it("writes the same driver note in English for an English reader", async () => {
+    const strip = await buildCoachReadStrip("u1", "SLEEP_DURATION", "en");
+
+    expect(strip.driver).not.toBeNull();
+    expect(strip.driver!.note).toBe(
+      "Higher symptom severity looks like it goes with lower next-day sleep " +
+        "duration in your data, on the evidence so far — a pattern worth " +
+        "watching, not a cause.",
+    );
+  });
+
+  it("keeps the driver's matching keys language-independent", async () => {
+    // Only the sentence is localised. `outcome` is how the strip finds the
+    // driver that belongs to THIS metric, so it has to stay the same string in
+    // every language or a German page would find no driver at all.
+    const de = await buildCoachReadStrip("u1", "SLEEP_DURATION", "de");
+    const en = await buildCoachReadStrip("u1", "SLEEP_DURATION", "en");
+
+    expect(de.driver!.outcome).toBe("sleep duration");
+    expect(en.driver!.outcome).toBe("sleep duration");
+    expect(de.driver!.behaviour).toBe(en.driver!.behaviour);
+    expect(de.driver!.note).not.toBe(en.driver!.note);
+  });
+});

src/lib/insights/derived/coach-read.ts

@@ -23,6 +23,7 @@
  */
 import type { MeasurementType } from "@/generated/prisma/client";
 import { prisma } from "@/lib/db";
+import type { Locale } from "@/lib/i18n/config";
 import {
   loadBaselineProfile,
   computeVitalsBaseline,
@@ -71,6 +72,15 @@ async function readLatestValue(
 export async function buildCoachReadStrip(
   userId: string,
   type: MeasurementType,
+  /**
+   * The reader's locale. Line 2 arrives as a finished sentence and is printed
+   * verbatim by both clients, so it has to be written in the reader's language
+   * HERE — the strip component only translates the wrapper around it. Required
+   * on purpose: while this parameter did not exist, the route said nothing and
+   * the sentence came back in English, which is how a German weight page ended
+   * up with a German first line and an English second one.
+   */
+  locale: Locale,
 ): Promise<CoachReadStripData> {
   const profile = await loadBaselineProfile(prisma, userId);
 
@@ -78,7 +88,9 @@ export async function buildCoachReadStrip(
     computeVitalsBaseline(userId, profile, { type }),
     readLatestValue(userId, type),
     // Line 2 is best-effort: a correlation failure must never sink the band.
-    readCoachCorrelations(userId).catch(() => ({ present: false }) as const),
+    readCoachCorrelations(userId, locale).catch(
+      () => ({ present: false }) as const,
+    ),
   ]);
 
   let baseline: CoachReadBaseline | null = null;

src/lib/insights/metric-correlation-context.ts

@@ -22,6 +22,7 @@
  */
 import type { MeasurementType } from "@/generated/prisma/client";
 import { prisma } from "@/lib/db";
+import type { Locale } from "@/lib/i18n/config";
 import {
   discoverCorrelations,
   discoveryMeasurementTypes,
@@ -82,6 +83,13 @@ function channelKeyForType(type: MeasurementType): string | null {
 export async function getRelevantCorrelationsForMetric(
   userId: string,
   measurementType: MeasurementType,
+  /**
+   * The reader's locale. These interpretations are finished sentences that go
+   * into the assessment prompt as grounded context; handing a German card's
+   * prompt English prose invites the model to echo it. Required — every caller
+   * is a status builder that already knows the reader's language.
+   */
+  locale: Locale,
 ): Promise<RelevantCorrelation[]> {
   const channel = channelKeyForType(measurementType);
   if (!channel) return [];
@@ -162,7 +170,7 @@ export async function getRelevantCorrelationsForMetric(
     }
     series.push(...environmentSeries, ...customMetricSeries);
 
-    const result = discoverCorrelations(series);
+    const result = discoverCorrelations(series, { locale });
     const decisions = await syncAcceptedPatterns({
       userId,
       family: PATTERN_FAMILIES.discoveryRetrospective,

src/lib/insights/metric-status.ts

@@ -454,6 +454,7 @@ export async function generateMetricStatus(args: {
   const relations = await getRelevantCorrelationsForMetric(
     args.userId,
     meta.measurementType,
+    locale,
   );
   const contextBlock = buildAssessmentContextBlock(
     {

src/lib/insights/narrative/__tests__/period-narrative-db.test.ts

@@ -103,6 +103,7 @@ describe("buildPeriodNarrativeContext — grain (QA F3)", () => {
     const result = await buildPeriodNarrativeContext("u1", {
       period: "week",
       now,
+      locale: "en",
     });
 
     expect(result.status).toBe("ready");
@@ -122,7 +123,11 @@ describe("buildPeriodNarrativeContext — grain (QA F3)", () => {
     const now = new Date("2026-06-30T12:00:00.000Z");
     measurementFindMany.mockResolvedValue([]);
 
-    await buildPeriodNarrativeContext("u1", { period: "week", now });
+    await buildPeriodNarrativeContext("u1", {
+      period: "week",
+      now,
+      locale: "en",
+    });
 
     expect(measurementFindMany).toHaveBeenCalledWith(
       expect.objectContaining({

src/lib/insights/narrative/__tests__/period-narrative.test.ts

@@ -40,6 +40,7 @@ function monthInput(
     },
     seriesByMetric,
     discoverySeries,
+    locale: "en",
     computedAt: "2026-04-30T12:00:00.000Z",
   };
 }

src/lib/insights/narrative/period-narrative-generate.ts

@@ -329,7 +329,7 @@ export async function generatePeriodNarrative(
     }
   }
 
-  const context = await buildContext(userId, { period, now });
+  const context = await buildContext(userId, { period, now, locale });
   if (context.status === "insufficient") {
     annotate({
       action: { name: "insights.narrative.insufficient" },

src/lib/insights/narrative/period-narrative.ts

@@ -31,6 +31,7 @@
  */
 import type { MeasurementType } from "@/generated/prisma/client";
 import { prisma } from "@/lib/db";
+import type { Locale } from "@/lib/i18n/config";
 import { annotate } from "@/lib/logging/context";
 import { wallClockInTz } from "@/lib/tz/wall-clock";
 import {
@@ -286,6 +287,12 @@ export interface AssembleInput {
   seriesByMetric: Map<string, DailySeriesPoint[]>;
   /** Named series feeding the discovery matrix (same window). */
   discoverySeries: NamedSeries[];
+  /**
+   * The reader's locale for the drivers' narrated `interpretation`. Required —
+   * the assembler writes sentences, so it has to be told which language they
+   * are in rather than assuming one.
+   */
+  locale: Locale;
   /** Compute time (ISO). */
   computedAt: string;
 }
@@ -305,6 +312,7 @@ export function assemblePeriodNarrativeContext(
     window,
     seriesByMetric,
     discoverySeries,
+    locale,
     computedAt,
   } = input;
 
@@ -380,7 +388,7 @@ export function assemblePeriodNarrativeContext(
   }
 
   // ── drivers (FDR-surviving correlations, descriptive-only) ──────────────
-  const discovery = discoverCorrelations(discoverySeries);
+  const discovery = discoverCorrelations(discoverySeries, { locale });
   const drivers: NarrativeDriver[] = discovery.discovered.map((d) => ({
     behaviour: d.behaviour,
     outcome: d.outcome,
@@ -486,6 +494,14 @@ export interface BuildPeriodNarrativeContextOpts {
   period: NarrativePeriod;
   /** Injected clock for deterministic behaviour; defaults to now. */
   now?: Date;
+  /**
+   * The reader's locale, for the drivers' narrated `interpretation`. Required:
+   * those are finished sentences that reach the prompt and the deterministic
+   * fallback, and both callers already know which language the narrative is
+   * being written in. An optional locale here would mean an English driver line
+   * inside a German summary whenever a caller stayed quiet.
+   */
+  locale: Locale;
 }
 
 /**
@@ -673,6 +689,7 @@ export async function buildPeriodNarrativeContext(
     window: { from: since.toISOString(), to: now.toISOString() },
     seriesByMetric,
     discoverySeries,
+    locale: opts.locale,
     computedAt: now.toISOString(),
   });
 }

src/lib/insights/pulse-status.ts

@@ -427,7 +427,11 @@ export async function preparePulseStatusForUser(
   // v1.12.7 — diversity / anti-repetition block (see blood-pressure-status).
   const varietyLead = pickVarietyLead(userId, "pulse", todayKey);
   const steadyRun = computeSteadyRun(pulseGraded.weekly, pulseGraded.monthly);
-  const relations = await getRelevantCorrelationsForMetric(userId, "PULSE");
+  const relations = await getRelevantCorrelationsForMetric(
+    userId,
+    "PULSE",
+    locale,
+  );
   const assessmentContextBlock = buildAssessmentContextBlock(
     {
       varietyLead,

src/lib/insights/weight-status.ts

@@ -586,7 +586,11 @@ export async function prepareWeightStatusForUser(
   // v1.12.7 — diversity / anti-repetition block (see blood-pressure-status).
   const varietyLead = pickVarietyLead(userId, "weight", todayKey);
   const steadyRun = computeSteadyRun(weightGraded.weekly, weightGraded.monthly);
-  const relations = await getRelevantCorrelationsForMetric(userId, "WEIGHT");
+  const relations = await getRelevantCorrelationsForMetric(
+    userId,
+    "WEIGHT",
+    locale,
+  );
   const assessmentContextBlock = buildAssessmentContextBlock(
     {
       varietyLead,

src/lib/mcp/__tests__/rich-reads-module-gate.test.ts

@@ -32,11 +32,14 @@ vi.mock("@/lib/rollups/measurement-read-wmy", async (importOriginal) => {
     await importOriginal<typeof import("@/lib/rollups/measurement-read-wmy")>();
   return { ...actual, readBestGranularityRollups: vi.fn() };
 });
-const { labResult, measurement } = vi.hoisted(() => ({
+const { labResult, measurement, user } = vi.hoisted(() => ({
   labResult: { findMany: vi.fn() },
   measurement: { count: vi.fn(async () => 0), groupBy: vi.fn(async () => []) },
+  // The rich reads resolve the account's language before asking for a
+  // narrated sentence, which reads the stored preference.
+  user: { findUnique: vi.fn(async () => ({ locale: "de" })) },
 }));
-vi.mock("@/lib/db", () => ({ prisma: { labResult, measurement } }));
+vi.mock("@/lib/db", () => ({ prisma: { labResult, measurement, user } }));
 
 import {
   compareMetric,

src/lib/mcp/__tests__/rich-reads.test.ts

@@ -18,13 +18,16 @@ vi.mock("@/lib/rollups/measurement-read-wmy", async (importOriginal) => {
     await importOriginal<typeof import("@/lib/rollups/measurement-read-wmy")>();
   return { ...actual, readBestGranularityRollups: vi.fn() };
 });
-const { labResult, measurement } = vi.hoisted(() => ({
+const { labResult, measurement, user } = vi.hoisted(() => ({
   labResult: { findMany: vi.fn() },
   // `withHrvFallback`'s presence probe (`count`) + the discovery presence
   // query (`groupBy`). Unused by metrics that don't exercise them.
   measurement: { count: vi.fn(), groupBy: vi.fn(async () => []) },
+  // The rich reads resolve the account's language before asking for a
+  // narrated sentence, which reads the stored preference.
+  user: { findUnique: vi.fn(async () => ({ locale: "de" })) },
 }));
-vi.mock("@/lib/db", () => ({ prisma: { labResult, measurement } }));
+vi.mock("@/lib/db", () => ({ prisma: { labResult, measurement, user } }));
 
 import {
   getCorrelation,
@@ -215,6 +218,10 @@ describe("get_correlation", () => {
     expect(res.pair?.r).toBe(-0.42);
     expect(res.association).toBe("descriptive");
     expect(res.windowDays).toBe(180);
+    // The `note` on the pair is a finished sentence an MCP client shows as it
+    // stands, so the account's language has to reach the reader. Nothing else
+    // in this file would notice if the read went back to asking in English.
+    expect(readCoachCorrelations).toHaveBeenCalledWith(USER, "de");
   });
 
   it("honest-null when the pair has no surviving association", async () => {
@@ -396,7 +403,7 @@ describe("get_metric_baseline", () => {
     const res = await getMetricBaseline(USER, { metric: "hrv" });
 
     expect(res.present).toBe(true);
-    expect(buildCoachReadStrip).toHaveBeenCalledWith(USER, "HRV_RMSSD");
+    expect(buildCoachReadStrip).toHaveBeenCalledWith(USER, "HRV_RMSSD", "de");
   });
 
   it("stays on SDNN when the user has any HEART_RATE_VARIABILITY rows", async () => {
@@ -419,6 +426,7 @@ describe("get_metric_baseline", () => {
     expect(buildCoachReadStrip).toHaveBeenCalledWith(
       USER,
       "HEART_RATE_VARIABILITY",
+      "de",
     );
   });
 });

src/lib/mcp/rich-reads.ts

@@ -36,6 +36,7 @@ import { isModuleEnabled } from "@/lib/modules/gate";
 import { moduleForMeasurementType } from "@/lib/modules/measurement-scope";
 import { readCoachCorrelations } from "@/lib/ai/coach/tools/correlations-read";
 import { buildCoachReadStrip } from "@/lib/insights/derived/coach-read";
+import { resolveLocaleForUser } from "@/lib/i18n/user-locale";
 import {
   readBestGranularityRollups,
   aggregateWmyBuckets,
@@ -668,7 +669,12 @@ export async function getCorrelation(
 ): Promise<CorrelationResult> {
   const a = norm(args.metricA);
   const b = norm(args.metricB);
-  const result = await readCoachCorrelations(userId);
+  // The `note` on each driver is a finished sentence the MCP client shows as
+  // it stands, so it is written in the account's language, not English.
+  const result = await readCoachCorrelations(
+    userId,
+    await resolveLocaleForUser(userId),
+  );
 
   if (!result.present || !result.drivers || result.drivers.length === 0) {
     annotate({
@@ -973,7 +979,11 @@ export async function getMetricBaseline(
   }
   const metric = resolved.metric;
 
-  const strip = await buildCoachReadStrip(userId, metric.measurementType);
+  const strip = await buildCoachReadStrip(
+    userId,
+    metric.measurementType,
+    await resolveLocaleForUser(userId),
+  );
 
   if (!strip.baseline) {
     annotate({

src/lib/query-keys/cycle.ts

@@ -22,7 +22,14 @@ export const cycleKeys = {
    * on-ramp reads this so a non-FEMALE account can opt in before the gated
    * cycle page is reachable. */
   cyclePrefs: () => ["cycle", "prefs"] as const,
-  cycleInsights: () => ["cycle", "insights"] as const,
+  /**
+   * The FDR-guarded phase-correlation read (`/api/cycle/insights`). Keyed by
+   * locale: each surviving lagged row now carries an `interpretation` the
+   * server writes in the reader's language, so a cached payload belongs to one
+   * language only. `["cycle"]` stays the prefix, so every cycle write still
+   * evicts it through `cycleDependentKeys`.
+   */
+  cycleInsights: (locale: string) => ["cycle", "insights", locale] as const,
   cycleDayLog: (date: string) => ["cycle", "day-log", date] as const,
   /** The caller's own custom symptoms (decrypted labels) the log-day sheet
    * merges into the seeded chip grid. */

src/lib/query-keys/insights.ts

@@ -172,9 +172,16 @@ export const insightsKeys = {
    * MeasurementType. Pure compute over the baseline + correlation engines —
    * the `["insights"]` prefix keeps it in the standard invalidation fan-out
    * (a measurement write busts it so the placement refreshes).
+   *
+   * Keyed by locale as well: the strip's second line is prose the server
+   * writes in the reader's language, so a cache entry belongs to one language
+   * only. Without the segment, switching language re-used the German page's
+   * cached English sentence — the fix on the server would have been invisible
+   * to anyone who switched rather than reloaded. Same reason
+   * `insightsMetricStatus` carries it.
    */
-  insightsCoachRead: (metric: string) =>
-    ["insights", "coach-read", metric] as const,
+  insightsCoachRead: (metric: string, locale: string) =>
+    ["insights", "coach-read", metric, locale] as const,
 
   /**
    * v1.25 — baseline-drift read (`/api/insights/health-status`). Combines the