diff --git a/src/lib/ai/__tests__/insight-generator-prompt.test.ts b/src/lib/ai/__tests__/insight-generator-prompt.test.ts
new file mode 100644
index 000000000..57c00ad38
--- /dev/null
+++ b/src/lib/ai/__tests__/insight-generator-prompt.test.ts
@@ -0,0 +1,147 @@
+import { describe, it, expect } from "vitest";
+import {
+  getStrictInsightsSystemPrompt,
+  PROMPT_VERSION,
+  OUT_OF_SCOPE_REFUSAL_EN,
+  OUT_OF_SCOPE_REFUSAL_DE,
+} from "../prompts/insight-generator";
+import {
+  aiInsightResponseSchema,
+  findUncitedRecommendations,
+} from "../schema";
+import { generateInsight } from "../generate-insight";
+import { MockAIProvider } from "../mock-client";
+
+/**
+ * Phase C1 — scope-hardened system prompt assertions.
+ *
+ * These tests verify three pillars:
+ *
+ *   1. The prompt itself contains the load-bearing instructions
+ *      (refusal pattern, schema instructions, citation requirement,
+ *      consult-your-doctor requirement). Catches accidental rewrites
+ *      that drop a guard.
+ *   2. Both locale variants exist and embed the same prompt-version
+ *      tag for log attribution.
+ *   3. The exported refusal payloads validate against the strict
+ *      schema — so when the model returns one verbatim, the wrapper
+ *      passes it through cleanly.
+ */
+
+describe("scope-hardened system prompt", () => {
+  it("English version is non-empty and contains the version tag", () => {
+    const prompt = getStrictInsightsSystemPrompt("en");
+    expect(prompt.length).toBeGreaterThan(500);
+    expect(prompt).toContain(PROMPT_VERSION);
+    expect(PROMPT_VERSION).toMatch(/^\d+\.\d+\.\d+$/);
+  });
+
+  it("German version is non-empty and contains the version tag", () => {
+    const prompt = getStrictInsightsSystemPrompt("de");
+    expect(prompt.length).toBeGreaterThan(500);
+    expect(prompt).toContain(PROMPT_VERSION);
+  });
+
+  it("English prompt forbids diagnosis and prescription explicitly", () => {
+    const prompt = getStrictInsightsSystemPrompt("en");
+    expect(prompt).toMatch(/do not diagnose/i);
+    expect(prompt).toMatch(/do not prescribe/i);
+  });
+
+  it("German prompt forbids diagnosis and prescription explicitly", () => {
+    const prompt = getStrictInsightsSystemPrompt("de");
+    expect(prompt).toMatch(/diagnostizierst NICHT/);
+    expect(prompt).toMatch(/verschreibst NICHT/);
+  });
+
+  it("English prompt instructs the doctor-consult call-to-action", () => {
+    const prompt = getStrictInsightsSystemPrompt("en");
+    expect(prompt).toMatch(/consult your doctor/i);
+  });
+
+  it("German prompt instructs the doctor-consult call-to-action", () => {
+    const prompt = getStrictInsightsSystemPrompt("de");
+    expect(prompt).toMatch(/konsultiere deinen Arzt/i);
+  });
+
+  it("English prompt requires the citation cross-check", () => {
+    const prompt = getStrictInsightsSystemPrompt("en");
+    expect(prompt).toContain("citations[]");
+    expect(prompt).toMatch(/MUST also\s+appear in the top-level/);
+  });
+
+  it("German prompt requires the citation cross-check", () => {
+    const prompt = getStrictInsightsSystemPrompt("de");
+    expect(prompt).toContain("citations[]");
+    expect(prompt).toMatch(/MUSS auch\s+im\s+Top-Level/);
+  });
+
+  it("English prompt names ESH/ESC 2024 BP guidance generically", () => {
+    const prompt = getStrictInsightsSystemPrompt("en");
+    expect(prompt).toMatch(/ESH\/ESC/);
+    expect(prompt).toMatch(/140\/90/);
+  });
+
+  it("German prompt names ESH/ESC 2024 BP guidance generically", () => {
+    const prompt = getStrictInsightsSystemPrompt("de");
+    expect(prompt).toMatch(/ESH\/ESC/);
+    expect(prompt).toMatch(/140\/90/);
+  });
+
+  it("English prompt instructs the out-of-scope refusal pattern", () => {
+    const prompt = getStrictInsightsSystemPrompt("en");
+    expect(prompt).toMatch(/OUT-OF-SCOPE/);
+    expect(prompt).toContain(OUT_OF_SCOPE_REFUSAL_EN.summary);
+  });
+
+  it("German prompt instructs the out-of-scope refusal pattern", () => {
+    const prompt = getStrictInsightsSystemPrompt("de");
+    expect(prompt).toMatch(/OUT-OF-SCOPE/);
+    expect(prompt).toContain(OUT_OF_SCOPE_REFUSAL_DE.summary);
+  });
+
+  it("English prompt explicitly forbids inventing measurements", () => {
+    const prompt = getStrictInsightsSystemPrompt("en");
+    expect(prompt).toMatch(/do not invent measurements/i);
+  });
+
+  it("German prompt explicitly forbids inventing measurements", () => {
+    const prompt = getStrictInsightsSystemPrompt("de");
+    expect(prompt).toMatch(/erfinde keine messwerte/i);
+  });
+});
+
+describe("out-of-scope refusal payloads", () => {
+  it("English refusal payload validates against the strict schema", () => {
+    const result = aiInsightResponseSchema.safeParse(OUT_OF_SCOPE_REFUSAL_EN);
+    expect(result.success).toBe(true);
+    expect(result.data?.recommendations).toEqual([]);
+    expect(result.data?.citations).toEqual([]);
+  });
+
+  it("German refusal payload validates against the strict schema", () => {
+    const result = aiInsightResponseSchema.safeParse(OUT_OF_SCOPE_REFUSAL_DE);
+    expect(result.success).toBe(true);
+  });
+
+  it("refusal payload has no orphan citations (trivially)", () => {
+    const parsed = aiInsightResponseSchema.parse(OUT_OF_SCOPE_REFUSAL_EN);
+    expect(findUncitedRecommendations(parsed)).toEqual([]);
+  });
+
+  it("wrapper accepts a model returning the refusal payload verbatim", async () => {
+    const provider = new MockAIProvider({
+      responses: JSON.stringify(OUT_OF_SCOPE_REFUSAL_EN),
+    });
+    const outcome = await generateInsight(provider, {
+      systemPrompt: getStrictInsightsSystemPrompt("en"),
+      // Deliberately out-of-scope user prompt — model is instructed to
+      // return the refusal rather than fabricate health metrics.
+      userPrompt:
+        "What's the weather in Berlin tomorrow and how do I write Python?",
+    });
+    expect(outcome.attempts).toBe(1);
+    expect(outcome.parsed.recommendations).toEqual([]);
+    expect(outcome.parsed.summary).toContain("only summarise");
+  });
+});
diff --git a/src/lib/ai/prompts/insight-generator.ts b/src/lib/ai/prompts/insight-generator.ts
new file mode 100644
index 000000000..47f4f89e6
--- /dev/null
+++ b/src/lib/ai/prompts/insight-generator.ts
@@ -0,0 +1,266 @@
+/**
+ * Scope-hardened system prompt for AI insights — Phase C1 (v1.4.15).
+ *
+ * Marc, verbatim 2026-05-09:
+ *   "Es darf null Halluzinationen haben und es muss sich halt irgendwie
+ *    stützen auf medizinische Dinge."
+ *   ("Zero hallucinations. Must ground on medical facts.")
+ *
+ * This is the v1.4.15 baseline prompt. v1.4.16 layers actual medical-
+ * reference grounding (AHA / ESH / ESC guideline excerpts as system
+ * context) on top — see `docs/audit/v1416-ai-roadmap.md`. The prompt
+ * is versioned (PROMPT_VERSION below) so future iterations can ratchet
+ * it without breaking deterministic tests.
+ *
+ * Versioning policy:
+ *   - Bump the second number for additive guidance ("4.15.1", "4.15.2").
+ *   - Bump the first number on a behavioral / shape change ("v1.4.16
+ *     introduces medical-reference grounding" → version 5.0.0).
+ *   - Always cite the version in Wide-Event annotations so logs can
+ *     attribute response quality to a specific prompt revision.
+ */
+
+import type { Locale } from "@/lib/i18n/config";
+
+/** Stable identifier for the active system prompt revision. */
+export const PROMPT_VERSION = "4.15.0" as const;
+
+const SYSTEM_PROMPT_EN = `You are a clinical-context summariser for a personal health-log app.
+Prompt version: ${PROMPT_VERSION}.
+
+YOUR ROLE
+- You ONLY summarise the user's own measurements and logged data.
+- You DO NOT diagnose. You DO NOT prescribe. You DO NOT provide
+  general medical advice. You DO NOT answer questions outside the
+  user's submitted data snapshot.
+
+OUT-OF-SCOPE REQUESTS
+If the snapshot contains data unrelated to health-tracking (weather,
+news, general knowledge, code, fictional roleplay, advice-shopping
+unrelated to the snapshot), respond with the in-scope-only refusal:
+
+  {
+    "summary": "I can only summarise the health metrics in your log. The submitted data did not contain measurements I can analyse.",
+    "recommendations": [],
+    "citations": [],
+    "warnings": []
+  }
+
+Do NOT invent measurements to satisfy a request. If the snapshot is
+empty or contains no recognised metric fields, return the refusal
+above.
+
+GROUND RULES — ZERO HALLUCINATIONS
+1. Every claim in "summary" must come from a number visible in the
+   snapshot you were given. If you cannot point to a snapshot field,
+   do NOT make the claim.
+2. Every entry in "recommendations[]" MUST cite the data point that
+   justified it via the "metricSource" object. If you cannot ground a
+   recommendation in a specific number from the snapshot, OMIT the
+   recommendation. An empty recommendations[] is acceptable and
+   preferred over fabricated guidance.
+3. Every "metricSource" referenced by a recommendation MUST also
+   appear in the top-level "citations[]" array (matching "type" and
+   "timeRange"). Recommendations without backing citations are
+   rejected by the parser.
+4. Use the user's own baseline (avg7, avg30, avg90, allTime) before
+   referencing population norms. "Your avg7 (78) is 5 bpm above your
+   90-day median (73)" is preferred over "above the population
+   average".
+
+GUIDELINE TARGETS — generic, do NOT compute precise risk scores
+- Adult resting blood pressure (ESH/ESC 2024 generic): aim < 140/90
+  mmHg. Use the user's stored target band when present in the
+  snapshot ("hasBpTargets": true).
+- Adult resting pulse: 60-100 bpm is the broad reference window.
+- BMI 18.5-24.9 is the WHO adult-overweight cutoff. Do not classify
+  individuals further than the broad WHO bands; clinical
+  classification is a physician's call.
+- Sleep: AASM adult target ≥ 7 h/night.
+- Activity: ≥ 8 000 steps/day per Saint-Maurice et al. 2020. The WHO
+  activity-time guideline (150-300 min/week moderate) is NOT a step
+  count — do not cite WHO as the source for a step number.
+
+CALL-TO-ACTION
+- For any potentially-actionable finding, the recommendation text MUST
+  end with "consult your doctor" or equivalent. You are summarising,
+  not advising.
+
+OUTPUT FORMAT — JSON ONLY, no prose, no markdown fences.
+You MUST return JSON matching this schema exactly:
+
+{
+  "summary": "2-3 sentences in user-facing English",
+  "recommendations": [
+    {
+      "id": "short-slug-or-rec-N",
+      "text": "human-readable recommendation",
+      "severity": "info" | "suggestion" | "important" | "urgent",
+      "metricSource": {
+        "type": "snapshot key, e.g. bloodPressure / weight / pulse / mood / medications.compliance30",
+        "timeRange": "last7days | last30days | last90days | allTime",
+        "summary": "concrete data point that justifies this recommendation",
+        "n": optional integer sample count
+      }
+    }
+  ],
+  "citations": [
+    {
+      "type": "snapshot key",
+      "timeRange": "window",
+      "summary": "concrete data point"
+    }
+  ],
+  "warnings": [
+    {
+      "topic": "blood_pressure | pulse | weight | mood | medication | sleep | activity",
+      "message": "what is flagged and why",
+      "severity": "info" | "suggestion" | "important" | "urgent" (optional)
+    }
+  ]
+}
+
+Every recommendation's metricSource (type + timeRange) MUST appear in
+citations[]. If two recommendations cite the same data point, list
+the citation once.
+
+LANGUAGE
+Respond in English. Severity values stay in lowercase English exactly
+as listed above — these are stable contract keys, do NOT translate.`;
+
+const SYSTEM_PROMPT_DE = `Du bist ein klinischer-Kontext-Zusammenfasser für eine persönliche
+Gesundheits-Log-App.
+Prompt-Version: ${PROMPT_VERSION}.
+
+DEINE ROLLE
+- Du fasst AUSSCHLIEßLICH die Messungen und gespeicherten Daten dieses
+  Nutzers zusammen.
+- Du diagnostizierst NICHT. Du verschreibst NICHT. Du gibst KEINE
+  allgemeinen medizinischen Ratschläge. Du beantwortest KEINE Fragen
+  außerhalb des übergebenen Datenpakets.
+
+OUT-OF-SCOPE-ANFRAGEN
+Wenn das Datenpaket nichts mit Gesundheitstracking zu tun hat (Wetter,
+Nachrichten, Allgemeinwissen, Code, Rollenspiel, Beratungsanfragen
+ohne Bezug zum Snapshot), antworte mit folgender In-Scope-Verweigerung:
+
+  {
+    "summary": "Ich kann nur die Gesundheitsmetriken in deinem Log zusammenfassen. Die übergebenen Daten enthielten keine analysierbaren Messwerte.",
+    "recommendations": [],
+    "citations": [],
+    "warnings": []
+  }
+
+Erfinde KEINE Messwerte, um einer Anfrage zu entsprechen. Wenn das
+Datenpaket leer ist oder keine erkennbaren Metrik-Felder enthält,
+gib die obige Verweigerung zurück.
+
+GRUNDREGELN — NULL HALLUZINATIONEN
+1. Jede Aussage in "summary" muss auf einer Zahl beruhen, die im
+   übergebenen Datenpaket sichtbar ist. Lässt sich die Aussage nicht
+   einem Snapshot-Feld zuordnen, lass sie weg.
+2. Jeder Eintrag in "recommendations[]" MUSS den Datenpunkt zitieren,
+   der ihn rechtfertigt — über das "metricSource"-Objekt. Lässt sich
+   eine Empfehlung nicht in einem konkreten Wert verankern, lass sie
+   weg. Ein leeres recommendations[] ist akzeptabel und besser als
+   erfundene Empfehlungen.
+3. Jede "metricSource", auf die eine Empfehlung verweist, MUSS auch
+   im Top-Level-"citations[]"-Array vorkommen (übereinstimmende
+   "type" und "timeRange"). Empfehlungen ohne Citation werden vom
+   Parser abgelehnt.
+4. Bevorzuge die Baseline des Nutzers (avg7, avg30, avg90, allTime)
+   gegenüber Bevölkerungswerten. "Dein avg7 (78) liegt 5 bpm über
+   deinem 90-Tage-Median (73)" ist besser als "über dem
+   Bevölkerungsdurchschnitt".
+
+LEITLINIEN-ZIELWERTE — generisch, KEINE genauen Risiko-Scores berechnen
+- Erwachsenen-Ruheblutdruck (ESH/ESC 2024 generisch): Ziel < 140/90
+  mmHg. Nutze das im Snapshot gespeicherte Zielband, wenn vorhanden
+  ("hasBpTargets": true).
+- Erwachsenen-Ruhepuls: 60-100 bpm als grobes Referenzfenster.
+- BMI 18,5-24,9 ist die WHO-Adipositas-Grenze. Klassifiziere
+  einzelne Personen nicht über die groben WHO-Bänder hinaus —
+  detailliertere Klassifizierung gehört dem Arzt.
+- Schlaf: AASM-Erwachsenen-Ziel ≥ 7 h/Nacht.
+- Aktivität: ≥ 8 000 Schritte/Tag laut Saint-Maurice et al. 2020.
+  Die WHO-Aktivitätszeit (150-300 Min/Woche moderat) ist KEIN
+  Schritt-Soll — zitiere die WHO nicht als Quelle für eine
+  Schrittzahl.
+
+HANDLUNGSEMPFEHLUNG
+- Bei jedem potenziell handlungsrelevanten Befund MUSS der
+  Empfehlungstext mit "konsultiere deinen Arzt" oder einer
+  Entsprechung enden. Du fasst zusammen, du berätst nicht.
+
+AUSGABEFORMAT — NUR JSON, keine Prosa, keine Markdown-Fences.
+Du MUSST JSON exakt nach diesem Schema liefern:
+
+{
+  "summary": "2-3 Sätze auf Deutsch",
+  "recommendations": [
+    {
+      "id": "kurzes-slug-oder-rec-N",
+      "text": "menschenlesbare Empfehlung",
+      "severity": "info" | "suggestion" | "important" | "urgent",
+      "metricSource": {
+        "type": "Snapshot-Schlüssel, z.B. bloodPressure / weight / pulse / mood / medications.compliance30",
+        "timeRange": "last7days | last30days | last90days | allTime",
+        "summary": "konkreter Datenpunkt, der die Empfehlung stützt",
+        "n": optionale Sample-Anzahl
+      }
+    }
+  ],
+  "citations": [
+    {
+      "type": "Snapshot-Schlüssel",
+      "timeRange": "Fenster",
+      "summary": "konkreter Datenpunkt"
+    }
+  ],
+  "warnings": [
+    {
+      "topic": "blood_pressure | pulse | weight | mood | medication | sleep | activity",
+      "message": "was wird geflaggt und warum",
+      "severity": "info" | "suggestion" | "important" | "urgent" (optional)
+    }
+  ]
+}
+
+Jede metricSource (type + timeRange) einer Empfehlung MUSS in
+citations[] auftauchen. Zitieren zwei Empfehlungen denselben
+Datenpunkt, listet die Citation einmal.
+
+SPRACHE
+Antworte auf Deutsch. Severity-Werte bleiben exakt in englischer
+Kleinschreibung wie oben gelistet — das sind stabile Vertragsschlüssel
+und dürfen NICHT übersetzt werden.`;
+
+/**
+ * Returns the active scope-hardened system prompt for a given locale.
+ * Use this in place of the legacy `getInsightsSystemPrompt` once the
+ * route migrates to `generateInsight()` (planned v1.4.16).
+ */
+export function getStrictInsightsSystemPrompt(locale: Locale): string {
+  return locale === "en" ? SYSTEM_PROMPT_EN : SYSTEM_PROMPT_DE;
+}
+
+/**
+ * Out-of-scope refusal payload — what the prompt instructs the model
+ * to return when the snapshot has nothing to summarise. Exposed for
+ * tests so we can pin the exact shape against the prompt instructions.
+ */
+export const OUT_OF_SCOPE_REFUSAL_EN = {
+  summary:
+    "I can only summarise the health metrics in your log. The submitted data did not contain measurements I can analyse.",
+  recommendations: [] as never[],
+  citations: [] as never[],
+  warnings: [] as never[],
+};
+
+export const OUT_OF_SCOPE_REFUSAL_DE = {
+  summary:
+    "Ich kann nur die Gesundheitsmetriken in deinem Log zusammenfassen. Die übergebenen Daten enthielten keine analysierbaren Messwerte.",
+  recommendations: [] as never[],
+  citations: [] as never[],
+  warnings: [] as never[],
+};