diff --git a/src/lib/ai/prompts/__tests__/output-language-adoption.test.ts b/src/lib/ai/prompts/__tests__/output-language-adoption.test.ts
new file mode 100644
index 000000000..309f71fae
--- /dev/null
+++ b/src/lib/ai/prompts/__tests__/output-language-adoption.test.ts
@@ -0,0 +1,271 @@
+import { describe, expect, it } from "vitest";
+
+import { locales, type Locale } from "@/lib/i18n/config";
+import {
+  outputLanguageDirective,
+  targetLanguageName,
+} from "@/lib/ai/prompts/output-language";
+import { getCoachSystemPrompt } from "@/lib/ai/coach/system-prompt";
+import {
+  getGeneralStatusSystemPrompt,
+  getGeneralStatusUserPrompt,
+} from "@/lib/ai/prompts/general-status";
+import { getBmiSystemPrompt, getBmiUserPrompt } from "@/lib/ai/prompts/bmi";
+import {
+  getBloodPressureSystemPrompt,
+  getBloodPressureUserPrompt,
+} from "@/lib/ai/prompts/blood-pressure";
+import {
+  getWeightSystemPrompt,
+  getWeightUserPrompt,
+} from "@/lib/ai/prompts/weight";
+import {
+  getPulseSystemPrompt,
+  getPulseUserPrompt,
+} from "@/lib/ai/prompts/pulse";
+import { getMoodSystemPrompt, getMoodUserPrompt } from "@/lib/ai/prompts/mood";
+import {
+  getMedicationComplianceSystemPrompt,
+  getMedicationComplianceUserPrompt,
+} from "@/lib/ai/prompts/medication-compliance";
+import {
+  getMetricArchetypeSystemPrompt,
+  getMetricArchetypeUserPrompt,
+} from "@/lib/ai/prompts/metric-archetypes";
+import { getStatusBatchSystemPrompt } from "@/lib/ai/prompts/status-batch";
+import { buildInterpretationBlock } from "@/lib/ai/prompts/interpretation-block";
+import {
+  buildUserPrompt,
+  buildComparisonBlock,
+} from "@/lib/ai/prompts/insight-system-prompt";
+import type { MetricStatusMeta } from "@/lib/insights/metric-status-registry";
+
+/**
+ * Adoption contract for the assessment prompt family.
+ *
+ * `output-language-contract.test.ts` pins the helper and the base system
+ * prompt. This file pins the SIBLING modules that compose on top of it — the
+ * per-metric system prompts, their user-prompt scaffolding, the batch wrapper,
+ * the interpretation block, the insights user prompt and the Coach prefs
+ * prefix.
+ *
+ * The invariant each module is held to: strip the one directive out of the
+ * four-locale prompt and what remains is the English prompt, byte for byte.
+ * That single assertion carries three guarantees. The four locales compose the
+ * ENGLISH body (no German leak); the body they compose is byte-identical to
+ * the English one, so an English prompt edit can never silently diverge from
+ * what a French reader receives; and the directive rides EXACTLY once — the
+ * double-append a module would produce by adding its own directive on top of
+ * the one `getBaseSystemPrompt` already carries shows up here as a leftover,
+ * and again in the occurrence count below.
+ *
+ * Note where the directive sits. `getBaseSystemPrompt` appends it last, and
+ * each of these modules then appends its per-metric section AFTER that — so on
+ * a composed prompt the directive sits at the base/section boundary rather
+ * than at the very end. It is present and unambiguous either way; moving it
+ * back to terminal position would mean composing the base without the
+ * directive and re-appending after the section, which is a change to the
+ * shared base prompt, not to these modules.
+ *
+ * User-prompt scaffolding carries no directive of its own (it rides the system
+ * prompt), so there the invariant is plain equality with the English text.
+ */
+
+/** Words that appear in a German instruction body and in no English one. */
+const GERMAN_SENTINEL =
+  /AUSGABEFORMAT|Antworte ausschließlich|Einschätzung|Schreibe eine|GEBÜNDELTE|EINORDNUNGS-KONTEXT|VERGLEICHSMODUS|Analysiere/;
+
+/** The four locales that ride the English body plus their own directive. */
+const RIDERS = ["fr", "es", "it", "pl"] as const;
+
+const ARGS = ["{SNAPSHOT}", "2026-07-18"] as const;
+
+const META: MetricStatusMeta = {
+  key: "STEP_COUNT",
+  displayName: "Steps",
+  unit: "steps",
+  direction: "higher-better",
+  archetype: "activity-fitness",
+  normalRange: { low: 7000, high: 12000 },
+} as unknown as MetricStatusMeta;
+
+/**
+ * System-prompt builders: every one of these composes `getBaseSystemPrompt`,
+ * which already carries the directive — so none of them may append its own.
+ */
+const SYSTEM_BUILDERS: ReadonlyArray<{
+  name: string;
+  build: (locale: Locale) => string;
+}> = [
+  { name: "general-status", build: getGeneralStatusSystemPrompt },
+  { name: "bmi", build: getBmiSystemPrompt },
+  { name: "blood-pressure", build: getBloodPressureSystemPrompt },
+  { name: "weight", build: getWeightSystemPrompt },
+  { name: "pulse", build: getPulseSystemPrompt },
+  { name: "mood", build: getMoodSystemPrompt },
+  {
+    name: "medication-compliance",
+    build: getMedicationComplianceSystemPrompt,
+  },
+  {
+    name: "metric-archetypes",
+    build: (l) => getMetricArchetypeSystemPrompt(META, l),
+  },
+  {
+    name: "status-batch",
+    build: (l) => getStatusBatchSystemPrompt(l, ["bp", "weight"]),
+  },
+];
+
+/** User-prompt scaffolding — English text for every non-German locale. */
+const USER_BUILDERS: ReadonlyArray<{
+  name: string;
+  build: (locale: Locale) => string | undefined;
+}> = [
+  {
+    name: "general-status",
+    build: (l) => getGeneralStatusUserPrompt(...ARGS, l),
+  },
+  { name: "bmi", build: (l) => getBmiUserPrompt(...ARGS, l) },
+  {
+    name: "blood-pressure",
+    build: (l) => getBloodPressureUserPrompt(...ARGS, l),
+  },
+  { name: "weight", build: (l) => getWeightUserPrompt(...ARGS, l) },
+  { name: "pulse", build: (l) => getPulseUserPrompt(...ARGS, l) },
+  { name: "mood", build: (l) => getMoodUserPrompt(...ARGS, l) },
+  {
+    name: "medication-compliance",
+    build: (l) => getMedicationComplianceUserPrompt(...ARGS, l),
+  },
+  {
+    name: "metric-archetypes",
+    build: (l) => getMetricArchetypeUserPrompt(META, ...ARGS, l),
+  },
+  {
+    name: "interpretation-block",
+    build: (l) =>
+      buildInterpretationBlock({
+        metricKey: "RESTING_HEART_RATE",
+        value: 58,
+        sex: "MALE",
+        locale: l,
+      }),
+  },
+  {
+    name: "insight-system-prompt",
+    build: (l) => buildUserPrompt("{FEATURES}", "raw", l),
+  },
+  {
+    name: "insight-comparison-block",
+    build: (l) =>
+      buildComparisonBlock(l, {
+        baseline: "lastMonth",
+        metrics: [
+          {
+            type: "weight",
+            currentAvg: 82.1,
+            baselineAvg: 83.2,
+            delta: -1.1,
+            deltaPercent: -1.3,
+            unit: "kg",
+          },
+        ],
+      }),
+  },
+  {
+    name: "insight-comparison-block (no metrics)",
+    build: (l) =>
+      buildComparisonBlock(l, { baseline: "lastYear", metrics: [] }),
+  },
+];
+
+describe.each(SYSTEM_BUILDERS)("$name system prompt", ({ build }) => {
+  it.each(RIDERS)("%s composes the English body plus its directive", (l) => {
+    const directive = outputLanguageDirective(l);
+    expect(directive).not.toBe("");
+    // Two things legitimately differ from the English prompt: the directive
+    // itself, and the language name the base body interpolates into its
+    // output clause ("the complete assessment in Polish"). Undo exactly those
+    // two and nothing else may remain.
+    const normalised = build(l)
+      .replace(`\n\n${directive}`, "")
+      .split(targetLanguageName(l))
+      .join("English");
+    expect(normalised).toBe(build("en"));
+  });
+
+  it("appends the directive exactly once — never twice", () => {
+    // A module that appended its own directive on top of the one
+    // `getBaseSystemPrompt` already carries would count two here.
+    for (const l of RIDERS) {
+      expect(occurrences(build(l), "OUTPUT LANGUAGE:"), `${l}`).toBe(1);
+    }
+    // de and en name their language inside their own body; no directive.
+    expect(occurrences(build("de"), "OUTPUT LANGUAGE:")).toBe(0);
+    expect(occurrences(build("en"), "OUTPUT LANGUAGE:")).toBe(0);
+  });
+
+  it("sends German instruction text to German readers only", () => {
+    expect(GERMAN_SENTINEL.test(build("de"))).toBe(true);
+    for (const l of locales) {
+      if (l === "de") continue;
+      expect(GERMAN_SENTINEL.test(build(l)), `${l}`).toBe(false);
+    }
+  });
+
+  it("keeps German and English distinct", () => {
+    expect(build("de")).not.toBe(build("en"));
+  });
+});
+
+describe.each(USER_BUILDERS)("$name user prompt", ({ build }) => {
+  it.each(RIDERS)("%s receives the English scaffolding verbatim", (l) => {
+    expect(build(l)).toBe(build("en"));
+  });
+
+  it("sends German scaffolding to German readers only", () => {
+    expect(GERMAN_SENTINEL.test(build("de") ?? "")).toBe(true);
+    for (const l of locales) {
+      if (l === "de") continue;
+      expect(GERMAN_SENTINEL.test(build(l) ?? ""), `${l}`).toBe(false);
+    }
+  });
+});
+
+describe("coach preference overrides", () => {
+  const prefs = { tone: "neutral", verbosity: "detailed" } as never;
+
+  it("never carries German override lines on a non-German prompt", () => {
+    for (const l of locales) {
+      const prompt = getCoachSystemPrompt(l, prefs);
+      if (l === "de") {
+        expect(prompt).toContain("TONFALL-OVERRIDE:");
+        continue;
+      }
+      // The French body is native; only the override prefix is at issue here.
+      expect(prompt, `${l} carries a German tone override`).not.toContain(
+        "TONFALL-OVERRIDE:",
+      );
+      expect(prompt).toContain("TONE OVERRIDE:");
+      expect(prompt).not.toContain("AUSFÜHRLICHKEITS-OVERRIDE:");
+      expect(prompt).toContain("VERBOSITY OVERRIDE:");
+    }
+  });
+
+  it("never carries a German v1.22 addendum on a non-German prompt", () => {
+    for (const l of locales) {
+      const prompt = getCoachSystemPrompt(l);
+      if (l === "de") {
+        expect(prompt).toContain("COACH-ZUSATZ (v1.22)");
+        continue;
+      }
+      expect(prompt, `${l}`).not.toContain("COACH-ZUSATZ (v1.22)");
+      expect(prompt).toContain("COACH ADDENDUM (v1.22)");
+    }
+  });
+});
+
+function occurrences(haystack: string, needle: string): number {
+  return haystack.split(needle).length - 1;
+}
diff --git a/src/lib/insights/biomarker-status.ts b/src/lib/insights/biomarker-status.ts
index a2f388e92..de66cff7e 100644
--- a/src/lib/insights/biomarker-status.ts
+++ b/src/lib/insights/biomarker-status.ts
@@ -32,7 +32,6 @@ import { getNoKeyGeneralStatusText } from "@/lib/insights/no-key-fallbacks";
 import { hashInsightSnapshot } from "@/lib/insights/snapshot-hash";
 import { runStatusCompletion } from "@/lib/insights/status-provider";
 import {
-  type SupportedLocale,
   normalizeLocale,
   normalizeSummaryText,
   parseSummaryFromContent,
diff --git a/src/lib/insights/derived/derived-assessment-ai.ts b/src/lib/insights/derived/derived-assessment-ai.ts
index 9ee22913f..bd6d002c4 100644
--- a/src/lib/insights/derived/derived-assessment-ai.ts
+++ b/src/lib/insights/derived/derived-assessment-ai.ts
@@ -31,7 +31,6 @@ import {
   normalizeSummaryText,
   parseSummaryFromContent,
   persistStatusInsight,
-  type SupportedLocale,
 } from "@/lib/insights/status-shared";
 import { runStatusCompletion } from "@/lib/insights/status-provider";
 import {