diff --git a/src/lib/integrations/__tests__/status.test.ts b/src/lib/integrations/__tests__/status.test.ts
index f624e7ccb..f9a8dfe99 100644
--- a/src/lib/integrations/__tests__/status.test.ts
+++ b/src/lib/integrations/__tests__/status.test.ts
@@ -269,6 +269,68 @@ describe("recordSyncFailure — reauth_required", () => {
   });
 });
 
+describe("recordSyncFailure — persistent (v1.4.42 W6)", () => {
+  it("marks state=error_transient (next sync still runs) and audits with kind=persistent", async () => {
+    // Contract-mismatch failures (Withings 293 invalid params) MUST
+    // surface in the audit log with kind=persistent so operations can
+    // grep for upstream contract bugs, but they MUST NOT park the
+    // integration — the next sync should still run because the
+    // mismatch can be one-sided (e.g. Withings introducing a new
+    // required field that the client release fixes).
+    vi.mocked(prisma.integrationStatus.upsert).mockResolvedValueOnce({
+      consecutiveFailures: 1,
+    } as never);
+    await recordSyncFailure({
+      userId: "u1",
+      integration: "withings",
+      kind: "persistent",
+      message: "Withings measure error: 293 - invalid params",
+      errorCode: "293",
+    });
+    const upsertArgs = vi.mocked(prisma.integrationStatus.upsert).mock
+      .calls[0][0];
+    expect(upsertArgs.update).toMatchObject({ state: "error_transient" });
+    expect(auditLog).toHaveBeenCalledWith(
+      "integrations.sync.failed",
+      expect.objectContaining({
+        details: expect.objectContaining({
+          kind: "persistent",
+          errorCode: "293",
+        }),
+      }),
+    );
+  });
+
+  it("at threshold, the admin alert message includes the 'persistent error' label", async () => {
+    vi.mocked(prisma.integrationStatus.upsert).mockResolvedValueOnce({
+      consecutiveFailures: 3,
+      alertedAt: null,
+    } as never);
+    vi.mocked(prisma.user.findUnique).mockResolvedValueOnce({
+      email: "user@example.com",
+    } as never);
+    vi.mocked(prisma.user.findMany).mockResolvedValueOnce([
+      { id: "admin-1" },
+    ] as never);
+    vi.mocked(prisma.integrationStatus.update).mockResolvedValueOnce(
+      {} as never,
+    );
+
+    await recordSyncFailure({
+      userId: "u1",
+      integration: "withings",
+      kind: "persistent",
+      message: "Withings activity error: 293",
+      errorCode: "293",
+    });
+
+    expect(dispatchNotification).toHaveBeenCalledOnce();
+    const payload = vi.mocked(dispatchNotification).mock.calls[0][0];
+    expect(payload.message).toContain("persistent error");
+    expect(payload.message).toContain("Action: investigate the upstream contract");
+  });
+});
+
 describe("recordSyncFailure — admin Telegram skipped silently when no admins", () => {
   it("logs a wide-event warning but does NOT throw when no admin users exist", async () => {
     vi.mocked(prisma.integrationStatus.upsert).mockResolvedValueOnce({
diff --git a/src/lib/integrations/status.ts b/src/lib/integrations/status.ts
index 537c770ea..bcec36c66 100644
--- a/src/lib/integrations/status.ts
+++ b/src/lib/integrations/status.ts
@@ -162,7 +162,26 @@ export async function recordSyncSuccess(
   });
 }
 
-export type FailureKind = "transient" | "reauth_required";
+/**
+ * Failure kinds carried into `recordSyncFailure`.
+ *
+ *   - `transient`        : retry on the next sync; user not blocked.
+ *   - `reauth_required`  : permanent revoke / invalid_grant; park at
+ *                          `error_reauth` until the user reconnects.
+ *   - `persistent`       : contract mismatch (invalid params, missing
+ *                          field, unknown action). Surfaces in the
+ *                          integration-status card AND audit log so an
+ *                          operator can investigate, but does NOT skip
+ *                          future sync attempts — those may succeed once
+ *                          the upstream side resolves.
+ *
+ * v1.4.42 W6 extended this union from `transient | reauth_required` to
+ * the three-state taxonomy above; the state-mapping function turns
+ * `persistent` into `error_transient` for now (a Withings 293 still
+ * lets the next sync run), but the audit detail carries the explicit
+ * kind so operations can filter.
+ */
+export type FailureKind = "transient" | "reauth_required" | "persistent";
 
 export interface RecordSyncFailureInput {
   userId: string;
@@ -193,6 +212,12 @@ export async function recordSyncFailure(
 ): Promise<void> {
   const { userId, integration, kind, message, errorCode } = input;
   const now = new Date();
+  // State mapping:
+  //   reauth_required → error_reauth (sync entry-point short-circuits)
+  //   transient       → error_transient (next sync still runs)
+  //   persistent      → error_transient (next sync still runs, but the
+  //                     audit detail carries `kind: "persistent"` so
+  //                     operations can grep for contract-bug bursts)
   const newState: IntegrationState =
     kind === "reauth_required" ? "error_reauth" : "error_transient";
 
@@ -486,7 +511,11 @@ export function formatAdminAlertPayload(input: AlertInput): {
     input.integration === "withings" ? "Withings" : "moodLog";
   const subjectLabel = input.subjectLabel ?? input.userId;
   const reasonLabel =
-    input.kind === "reauth_required" ? "re-auth required" : "transient error";
+    input.kind === "reauth_required"
+      ? "re-auth required"
+      : input.kind === "persistent"
+        ? "persistent error"
+        : "transient error";
   const codeLabel = input.errorCode ? ` (${input.errorCode})` : "";
   const trimmed =
     input.message.length > 280
@@ -497,7 +526,13 @@ export function formatAdminAlertPayload(input: AlertInput): {
   const message =
     `${integrationLabel} sync has failed ${input.consecutiveFailures} times in a row for ${subjectLabel}.\n` +
     `Last error: ${reasonLabel}${codeLabel} — ${trimmed}\n` +
-    `Action: ${input.kind === "reauth_required" ? "ask the user to reconnect the integration." : "investigate the upstream service."}`;
+    `Action: ${
+      input.kind === "reauth_required"
+        ? "ask the user to reconnect the integration."
+        : input.kind === "persistent"
+          ? "investigate the upstream contract — params/scope/action likely mismatched."
+          : "investigate the upstream service."
+    }`;
 
   return {
     title,
diff --git a/src/lib/withings/__tests__/client.test.ts b/src/lib/withings/__tests__/client.test.ts
index 5e3723b2f..20d71b219 100644
--- a/src/lib/withings/__tests__/client.test.ts
+++ b/src/lib/withings/__tests__/client.test.ts
@@ -2,11 +2,14 @@ import { afterEach, describe, expect, it, vi } from "vitest";
 import {
   MEASURE_TYPE_MAP,
   WITHINGS_OAUTH_SCOPE,
+  exchangeCode,
   fetchMeasurements,
   hasActivityScope,
   parseWithingsScope,
+  refreshAccessToken,
   subscribeWebhook,
 } from "../client";
+import { WithingsApiError } from "../response-classifier";
 import { WITHINGS_NOTIFY_APPLIS } from "../sync";
 
 /**
@@ -376,3 +379,142 @@ describe("fetchMeasurements — pulse-wave velocity + vascular age", () => {
     expect(out[0]).toMatchObject({ type: "VASCULAR_AGE", value: 42 });
   });
 });
+
+// ── v1.4.42 W6 — off-response classifier wire-through ──
+//
+// Each client entrypoint must throw `WithingsApiError` carrying the
+// classification verdict, NOT a plain `new Error("Withings ... error:
+// <status>")`. Downstream catch-blocks read `err.classification`; the
+// regex-fallback in `classifyError` covers serialised retries but the
+// typed-throw is the contract.
+
+describe("client off-response handling — refreshAccessToken", () => {
+  const creds = { clientId: "id", clientSecret: "secret" };
+
+  it("throws WithingsApiError(classification: 'reauth_required') on Withings 101", async () => {
+    vi.stubGlobal(
+      "fetch",
+      vi.fn(async () => ({
+        status: 200,
+        json: async () => ({ status: 101, error: "Invalid token" }),
+      })),
+    );
+
+    await expect(refreshAccessToken("rt", creds)).rejects.toMatchObject({
+      name: "WithingsApiError",
+      classification: "reauth_required",
+      withingsStatus: 101,
+    });
+  });
+
+  it("throws WithingsApiError(classification: 'transient') on HTTP 503", async () => {
+    vi.stubGlobal(
+      "fetch",
+      vi.fn(async () => ({
+        status: 503,
+        json: async () => ({ status: 0 }),
+      })),
+    );
+
+    const err = await refreshAccessToken("rt", creds).catch((e) => e);
+    expect(err).toBeInstanceOf(WithingsApiError);
+    expect((err as WithingsApiError).classification).toBe("transient");
+    expect((err as WithingsApiError).reason).toBe("http_503");
+  });
+});
+
+describe("client off-response handling — exchangeCode", () => {
+  const creds = { clientId: "id", clientSecret: "secret" };
+
+  it("classifies Withings 100 (Authentication failed) as reauth_required", async () => {
+    vi.stubGlobal(
+      "fetch",
+      vi.fn(async () => ({
+        status: 200,
+        json: async () => ({ status: 100, error: "Authentication failed" }),
+      })),
+    );
+    const err = await exchangeCode("auth-code", creds).catch((e) => e);
+    expect(err).toBeInstanceOf(WithingsApiError);
+    expect((err as WithingsApiError).classification).toBe("reauth_required");
+  });
+});
+
+describe("client off-response handling — fetchMeasurements", () => {
+  it("throws WithingsApiError(classification: 'transient') on Withings 601 (rate-limit)", async () => {
+    vi.stubGlobal(
+      "fetch",
+      vi.fn(async () => ({
+        status: 200,
+        json: async () => ({ status: 601 }),
+      })),
+    );
+    const err = await fetchMeasurements("token").catch((e) => e);
+    expect(err).toBeInstanceOf(WithingsApiError);
+    expect((err as WithingsApiError).classification).toBe("transient");
+    expect((err as WithingsApiError).withingsStatus).toBe(601);
+  });
+
+  it("throws WithingsApiError(classification: 'persistent') on Withings 293 (invalid params)", async () => {
+    vi.stubGlobal(
+      "fetch",
+      vi.fn(async () => ({
+        status: 200,
+        json: async () => ({ status: 293 }),
+      })),
+    );
+    const err = await fetchMeasurements("token").catch((e) => e);
+    expect(err).toBeInstanceOf(WithingsApiError);
+    expect((err as WithingsApiError).classification).toBe("persistent");
+  });
+
+  it("returns the empty list on a healthy but empty body (status 0, no measuregrps)", async () => {
+    // Off-response edge case: a healthy connection that has no data
+    // in the requested window is NOT a failure — it's success with
+    // zero rows.
+    vi.stubGlobal(
+      "fetch",
+      vi.fn(async () => ({
+        status: 200,
+        json: async () => ({ status: 0, body: { measuregrps: [] } }),
+      })),
+    );
+    const out = await fetchMeasurements("token");
+    expect(out).toEqual([]);
+  });
+});
+
+describe("client off-response handling — subscribeWebhook 294 idempotency", () => {
+  it("does NOT throw when Withings replies status 294 (already-subscribed)", async () => {
+    // 294 at the subscribe call-site is documented idempotent success;
+    // every other endpoint sees it as `persistent`, but here we
+    // downgrade to success so reconnect flows don't fail.
+    vi.stubGlobal(
+      "fetch",
+      vi.fn(async () => ({
+        status: 200,
+        json: async () => ({ status: 294 }),
+      })),
+    );
+    await expect(
+      subscribeWebhook("token", "https://example.com/webhook", 1),
+    ).resolves.toBeUndefined();
+  });
+
+  it("throws WithingsApiError(classification: 'transient') when Withings replies status 2554 (notify-subscribe busy)", async () => {
+    vi.stubGlobal(
+      "fetch",
+      vi.fn(async () => ({
+        status: 200,
+        json: async () => ({ status: 2554 }),
+      })),
+    );
+    const err = await subscribeWebhook(
+      "token",
+      "https://example.com/webhook",
+      1,
+    ).catch((e) => e);
+    expect(err).toBeInstanceOf(WithingsApiError);
+    expect((err as WithingsApiError).classification).toBe("transient");
+  });
+});
diff --git a/src/lib/withings/__tests__/response-classifier.test.ts b/src/lib/withings/__tests__/response-classifier.test.ts
new file mode 100644
index 000000000..93ff0226d
--- /dev/null
+++ b/src/lib/withings/__tests__/response-classifier.test.ts
@@ -0,0 +1,212 @@
+/**
+ * v1.4.42 W6 — Withings off-response classifier.
+ *
+ * The classifier turns a `(httpStatus, body)` pair into a verdict of
+ * `success / transient / reauth_required / persistent`. Every other
+ * Withings code path branches on the verdict; this suite pins the
+ * three-way taxonomy plus the "off-spec response" edge cases the
+ * legacy client treated as silent no-data.
+ */
+import { describe, expect, it } from "vitest";
+import {
+  WithingsApiError,
+  classifyError,
+  classifyWithingsResponse,
+} from "../response-classifier";
+
+describe("classifyWithingsResponse — success branch", () => {
+  it("treats HTTP 200 + status 0 as success", () => {
+    const v = classifyWithingsResponse(200, { status: 0 });
+    expect(v.classification).toBe("success");
+    expect(v.withingsStatus).toBe(0);
+    expect(v.reason).toBe("ok");
+  });
+});
+
+describe("classifyWithingsResponse — transient branch", () => {
+  it("HTTP 503 → transient (upstream outage)", () => {
+    const v = classifyWithingsResponse(503, null);
+    expect(v.classification).toBe("transient");
+    expect(v.reason).toBe("http_503");
+  });
+
+  it("HTTP 502 → transient", () => {
+    expect(classifyWithingsResponse(502, null).classification).toBe(
+      "transient",
+    );
+  });
+
+  it("HTTP 429 → transient (rate-limit)", () => {
+    const v = classifyWithingsResponse(429, null);
+    expect(v.classification).toBe("transient");
+    expect(v.reason).toBe("http_429");
+  });
+
+  it("Withings status 503 (service unavailable) → transient", () => {
+    const v = classifyWithingsResponse(200, { status: 503 });
+    expect(v.classification).toBe("transient");
+    expect(v.withingsStatus).toBe(503);
+  });
+
+  it("Withings status 601 (rate-limited) → transient", () => {
+    expect(
+      classifyWithingsResponse(200, { status: 601 }).classification,
+    ).toBe("transient");
+  });
+
+  it("Withings status 2554 (notify-subscribe transient) → transient", () => {
+    expect(
+      classifyWithingsResponse(200, { status: 2554 }).classification,
+    ).toBe("transient");
+  });
+
+  it("off-spec body without a `status` field → transient (one-off CDN page)", () => {
+    const v = classifyWithingsResponse(200, {});
+    expect(v.classification).toBe("transient");
+    expect(v.reason).toBe("no_status_field");
+  });
+
+  it("unknown Withings status defaults to transient (admin-alert ladder catches recurrence)", () => {
+    const v = classifyWithingsResponse(200, { status: 9999 });
+    expect(v.classification).toBe("transient");
+    expect(v.reason).toBe("withings_9999_unknown");
+  });
+});
+
+describe("classifyWithingsResponse — reauth_required branch", () => {
+  it.each([100, 101, 102])(
+    "Withings status %i → reauth_required (permanent revoke)",
+    (status) => {
+      const v = classifyWithingsResponse(200, { status });
+      expect(v.classification).toBe("reauth_required");
+      expect(v.withingsStatus).toBe(status);
+    },
+  );
+
+  it("Withings 200 (invalid_grant boundary) → reauth_required", () => {
+    expect(
+      classifyWithingsResponse(200, { status: 200 }).classification,
+    ).toBe("reauth_required");
+  });
+
+  it("Withings 250 (mid-range invalid_grant) → reauth_required", () => {
+    expect(
+      classifyWithingsResponse(200, { status: 250 }).classification,
+    ).toBe("reauth_required");
+  });
+
+  it("Withings 299 (invalid_grant upper boundary) → reauth_required", () => {
+    expect(
+      classifyWithingsResponse(200, { status: 299 }).classification,
+    ).toBe("reauth_required");
+  });
+});
+
+describe("classifyWithingsResponse — persistent branch", () => {
+  it("Withings status 293 (invalid params) → persistent", () => {
+    const v = classifyWithingsResponse(200, { status: 293 });
+    expect(v.classification).toBe("persistent");
+    expect(v.withingsStatus).toBe(293);
+  });
+
+  it("Withings status 294 (already-subscribed) → persistent (callers downgrade for subscribe)", () => {
+    // 294 is idempotent-success at the subscribeWebhook call-site;
+    // the classifier still surfaces it as persistent so any OTHER
+    // endpoint that sees 294 is loud about the contract bug.
+    expect(
+      classifyWithingsResponse(200, { status: 294 }).classification,
+    ).toBe("persistent");
+  });
+
+  it("HTTP 400 (unexpected from Withings) → persistent", () => {
+    const v = classifyWithingsResponse(400, null);
+    expect(v.classification).toBe("persistent");
+    expect(v.reason).toBe("http_400");
+  });
+
+  it("HTTP 404 → persistent", () => {
+    expect(classifyWithingsResponse(404, null).classification).toBe(
+      "persistent",
+    );
+  });
+});
+
+describe("WithingsApiError", () => {
+  it("carries the classification + status on the typed error", () => {
+    const err = new WithingsApiError({
+      verb: "refresh",
+      classification: "reauth_required",
+      withingsStatus: 100,
+      reason: "withings_100",
+      upstreamError: "Authentication failed",
+    });
+    expect(err.classification).toBe("reauth_required");
+    expect(err.withingsStatus).toBe(100);
+    expect(err.reason).toBe("withings_100");
+    expect(err.verb).toBe("refresh");
+    expect(err.name).toBe("WithingsApiError");
+  });
+
+  it("preserves the legacy `Withings <verb> error: <status> - <error>` message shape", () => {
+    const err = new WithingsApiError({
+      verb: "measure",
+      classification: "transient",
+      withingsStatus: 503,
+      reason: "withings_503",
+      upstreamError: "service unavailable",
+    });
+    // Legacy regex consumers (extractWithingsStatus, etc.) expect
+    // exactly this shape.
+    expect(err.message).toBe(
+      "Withings measure error: 503 - service unavailable",
+    );
+    expect(/Withings\s+\w+\s+error:\s*(\d+)/.exec(err.message)?.[1]).toBe(
+      "503",
+    );
+  });
+
+  it("omits the dash-segment when upstreamError is absent", () => {
+    const err = new WithingsApiError({
+      verb: "subscribe",
+      classification: "persistent",
+      withingsStatus: 293,
+      reason: "withings_293",
+    });
+    expect(err.message).toBe("Withings subscribe error: 293");
+  });
+});
+
+describe("classifyError", () => {
+  it("reads the verdict directly off a WithingsApiError instance", () => {
+    const err = new WithingsApiError({
+      verb: "refresh",
+      classification: "reauth_required",
+      withingsStatus: 101,
+      reason: "withings_101",
+    });
+    expect(classifyError(err)).toBe("reauth_required");
+  });
+
+  it("falls back to the message regex when the prototype was lost (pg-boss round-trip)", () => {
+    // pg-boss serializes job errors to JSON and re-throws as plain Error
+    // on retry — the WithingsApiError prototype does NOT survive.
+    const rehydrated = new Error("Withings refresh error: 102 - User does not exist");
+    expect(classifyError(rehydrated)).toBe("reauth_required");
+  });
+
+  it("classifies a recovered Withings 503 transient via the regex fallback", () => {
+    const rehydrated = new Error("Withings measure error: 503");
+    expect(classifyError(rehydrated)).toBe("transient");
+  });
+
+  it("classifies a recovered Withings 293 persistent via the regex fallback", () => {
+    const rehydrated = new Error("Withings activity error: 293");
+    expect(classifyError(rehydrated)).toBe("persistent");
+  });
+
+  it("defaults to transient for unrelated errors (network drop, timeout)", () => {
+    expect(classifyError(new Error("ECONNRESET"))).toBe("transient");
+    expect(classifyError("string error")).toBe("transient");
+    expect(classifyError(null)).toBe("transient");
+  });
+});
diff --git a/src/lib/withings/client.ts b/src/lib/withings/client.ts
index 82cfde9d0..daa685442 100644
--- a/src/lib/withings/client.ts
+++ b/src/lib/withings/client.ts
@@ -3,6 +3,10 @@
  * Docs: https://developer.withings.com/api-reference
  */
 import { getEvent } from "@/lib/logging/context";
+import {
+  WithingsApiError,
+  classifyWithingsResponse,
+} from "./response-classifier";
 
 const WITHINGS_OAUTH_URL =
   "https://account.withings.com/oauth2_user/authorize2";
@@ -112,15 +116,22 @@ export async function exchangeCode(
   });
 
   const json = await res.json();
+  const verdict = classifyWithingsResponse(res.status, json);
   getEvent()?.addExternalCall({
     service: "withings",
     method: "exchangeCode",
     duration_ms: Math.round(performance.now() - start),
     status: res.status,
-    error: json.status !== 0 ? `status=${json.status}` : undefined,
+    error: verdict.classification === "success" ? undefined : verdict.reason,
   });
-  if (json.status !== 0) {
-    throw new Error(`Withings token error: ${json.status} - ${json.error}`);
+  if (verdict.classification !== "success") {
+    throw new WithingsApiError({
+      verb: "token",
+      classification: verdict.classification,
+      withingsStatus: verdict.withingsStatus,
+      reason: verdict.reason,
+      upstreamError: typeof json?.error === "string" ? json.error : undefined,
+    });
   }
   return json.body as WithingsTokenResponse;
 }
@@ -148,15 +159,22 @@ export async function refreshAccessToken(
   });
 
   const json = await res.json();
+  const verdict = classifyWithingsResponse(res.status, json);
   getEvent()?.addExternalCall({
     service: "withings",
     method: "refreshAccessToken",
     duration_ms: Math.round(performance.now() - start),
     status: res.status,
-    error: json.status !== 0 ? `status=${json.status}` : undefined,
+    error: verdict.classification === "success" ? undefined : verdict.reason,
   });
-  if (json.status !== 0) {
-    throw new Error(`Withings refresh error: ${json.status} - ${json.error}`);
+  if (verdict.classification !== "success") {
+    throw new WithingsApiError({
+      verb: "refresh",
+      classification: verdict.classification,
+      withingsStatus: verdict.withingsStatus,
+      reason: verdict.reason,
+      upstreamError: typeof json?.error === "string" ? json.error : undefined,
+    });
   }
   return json.body as WithingsTokenResponse;
 }
@@ -261,15 +279,22 @@ export async function fetchMeasurements(
     });
 
     const json = await res.json();
+    const verdict = classifyWithingsResponse(res.status, json);
     getEvent()?.addExternalCall({
       service: "withings",
       method: `fetchMeasurements(page=${pageCount})`,
       duration_ms: Math.round(performance.now() - pageStart),
       status: res.status,
-      error: json.status !== 0 ? `status=${json.status}` : undefined,
+      error: verdict.classification === "success" ? undefined : verdict.reason,
     });
-    if (json.status !== 0) {
-      throw new Error(`Withings measure error: ${json.status}`);
+    if (verdict.classification !== "success") {
+      throw new WithingsApiError({
+        verb: "measure",
+        classification: verdict.classification,
+        withingsStatus: verdict.withingsStatus,
+        reason: verdict.reason,
+        upstreamError: typeof json?.error === "string" ? json.error : undefined,
+      });
     }
 
     const body = json.body ?? {};
@@ -331,19 +356,29 @@ export async function subscribeWebhook(
   });
 
   const json = await res.json();
+  const verdict = classifyWithingsResponse(res.status, json);
+  // 294 = already-subscribed is idempotent success at the subscribe
+  // call-site (Withings preserves the existing subscription). The
+  // classifier surfaces it as `persistent` because any OTHER endpoint
+  // receiving 294 is a contract bug; here we explicitly downgrade it.
+  const isAlreadySubscribed = json?.status === 294;
+  const treatAsSuccess =
+    verdict.classification === "success" || isAlreadySubscribed;
   getEvent()?.addExternalCall({
     service: "withings",
     method: "subscribeWebhook",
     duration_ms: Math.round(performance.now() - start),
     status: res.status,
-    error:
-      json.status !== 0 && json.status !== 294
-        ? `status=${json.status}`
-        : undefined,
+    error: treatAsSuccess ? undefined : verdict.reason,
   });
-  // Status 0 = success, 294 = already subscribed (both OK)
-  if (json.status !== 0 && json.status !== 294) {
-    throw new Error(`Withings subscribe error: ${json.status}`);
+  if (!treatAsSuccess) {
+    throw new WithingsApiError({
+      verb: "subscribe",
+      classification: verdict.classification,
+      withingsStatus: verdict.withingsStatus,
+      reason: verdict.reason,
+      upstreamError: typeof json?.error === "string" ? json.error : undefined,
+    });
   }
 }
 
@@ -372,14 +407,21 @@ export async function unsubscribeWebhook(
   });
 
   const json = await res.json();
+  const verdict = classifyWithingsResponse(res.status, json);
   getEvent()?.addExternalCall({
     service: "withings",
     method: "unsubscribeWebhook",
     duration_ms: Math.round(performance.now() - start),
     status: res.status,
-    error: json.status !== 0 ? `status=${json.status}` : undefined,
+    error: verdict.classification === "success" ? undefined : verdict.reason,
   });
-  if (json.status !== 0) {
-    getEvent()?.addWarning(`Withings unsubscribe warning: ${json.status}`);
+  // Unsubscribe is best-effort — a stale subscription will eventually
+  // be garbage-collected by Withings, so we never throw here. We do
+  // bubble the verdict into a warning so the audit-log path still
+  // sees the off-response.
+  if (verdict.classification !== "success") {
+    getEvent()?.addWarning(
+      `Withings unsubscribe warning: ${verdict.reason}`,
+    );
   }
 }
diff --git a/src/lib/withings/response-classifier.ts b/src/lib/withings/response-classifier.ts
new file mode 100644
index 000000000..210885515
--- /dev/null
+++ b/src/lib/withings/response-classifier.ts
@@ -0,0 +1,281 @@
+/**
+ * Off-response classifier for Withings API replies.
+ *
+ * The Withings API has three failure shapes the legacy client treated
+ * uniformly as "throw with a message":
+ *
+ *   1. HTTP 5xx — upstream outage, retry on the next sync.
+ *   2. HTTP 200 + `status: <non-zero>` — Withings returns its semantic
+ *      error code in the JSON body, with HTTP 200. The status code
+ *      taxonomy is documented at
+ *      https://developer.withings.com/api-reference/#section/Response-status .
+ *   3. HTTP 200 + `status: 0` + empty `measuregrps` — no data, but the
+ *      connection is healthy. Not a failure at all.
+ *
+ * Pre-v1.4.42 the sync path inferred reauth-vs-transient from the error
+ * MESSAGE via a regex (`isWithingsRefreshReauthFailure` in sync.ts).
+ * That worked because the only failure shape the client threw was
+ * `Withings <verb> error: <status>`, but it tightly coupled the
+ * classifier to the literal message format and meant rate-limit
+ * responses (601) and quota responses (293) were silently bucketed as
+ * transient errors that never escalated even when they kept recurring.
+ *
+ * This module introduces an explicit classifier:
+ *
+ *   - `classifyWithingsResponse(httpStatus, body)` — pure function,
+ *     returns a `WithingsClassification`.
+ *   - `WithingsApiError` — typed Error subclass carrying the
+ *     classification so downstream catch-blocks don't need to re-parse
+ *     a string.
+ *
+ * Classification buckets:
+ *
+ *   - `success`         → HTTP 200 + body.status === 0
+ *   - `transient`       → retryable: HTTP 5xx, 429, Withings status 503
+ *                         (upstream unavailable), 601 (rate-limited),
+ *                         2554 (too many subscriptions in flight),
+ *                         3xx network/CDN glitches
+ *   - `reauth_required` → permanent revoke: Withings 100, 101, 102,
+ *                         200–299 (invalid grant family). The user has
+ *                         to redo OAuth.
+ *   - `persistent`      → operator-attention error: invalid params
+ *                         (293), missing scope (293 with body.scope_hint),
+ *                         unknown action (294 outside subscribeWebhook).
+ *                         These should NOT be retried silently — they
+ *                         indicate a contract mismatch.
+ *
+ * The classifier is deliberately conservative: anything it doesn't
+ * recognise defaults to `transient` so we don't hard-fail an integration
+ * over a Withings status we've never seen. The audit log + admin-alert
+ * ladder in `recordSyncFailure` already catches the "this keeps
+ * happening" case for unknown transients.
+ */
+
+/** Outcome buckets for a Withings response. */
+export type WithingsClassification =
+  | "success"
+  | "transient"
+  | "reauth_required"
+  | "persistent";
+
+export interface ClassifiedResponse {
+  classification: WithingsClassification;
+  /** The Withings body status code (or undefined if HTTP-only failure). */
+  withingsStatus: number | undefined;
+  /** Short human-readable label for logs / audit details. */
+  reason: string;
+}
+
+/**
+ * The minimum body shape we read. Withings adds many additional fields
+ * (`body`, `error`, `scope_hint`, `more`, `measuregrps`, …); callers
+ * keep their own typed views — we only need `status` and (optionally)
+ * `error` for the reason string.
+ */
+export interface WithingsBodyLike {
+  status?: number;
+  error?: string;
+}
+
+/**
+ * Withings status codes that map to a permanent-revoke (user MUST
+ * redo OAuth). See https://developer.withings.com/api-reference for
+ * the full table — these are the values both Withings' own SDKs and
+ * the existing HealthLog code treated as terminal.
+ */
+const REAUTH_CODES = new Set<number>([100, 101, 102]);
+
+/**
+ * Withings status codes that map to a transient / retryable error.
+ * Conservative on purpose — 503 is the documented "service unavailable"
+ * response, 601 is rate-limit, 2554/2555/2556 are notify-subscribe
+ * transients.
+ */
+const TRANSIENT_CODES = new Set<number>([503, 601, 2554, 2555, 2556]);
+
+/**
+ * Withings status codes that indicate a persistent contract mismatch —
+ * the request itself is malformed or the integration is missing a
+ * required field. Retrying without operator intervention will not
+ * succeed; the audit log surfaces these.
+ *
+ *   - 293 = "invalid params" / "missing required field"
+ *   - 294 = "already subscribed" (subscribeWebhook treats it as success;
+ *           any OTHER endpoint receiving 294 is a contract bug)
+ *
+ * Note: 294 is intentionally listed here. `subscribeWebhook` overrides
+ * the classification for its own call site (already-subscribed is
+ * idempotent there); every other client call that sees 294 surfaces it
+ * as a persistent failure so the bug doesn't hide.
+ */
+const PERSISTENT_CODES = new Set<number>([293, 294]);
+
+/**
+ * Classify a single Withings API response.
+ *
+ * @param httpStatus  The HTTP status code from fetch (200 for the
+ *                    happy path, 5xx for upstream outages).
+ * @param body        The parsed JSON body. Only `status` (and
+ *                    optionally `error`) are read.
+ */
+export function classifyWithingsResponse(
+  httpStatus: number,
+  body: WithingsBodyLike | null | undefined,
+): ClassifiedResponse {
+  // HTTP layer first — a 503 / 502 / 504 from Withings means we never
+  // got to the JSON envelope.
+  if (httpStatus >= 500 && httpStatus < 600) {
+    return {
+      classification: "transient",
+      withingsStatus: undefined,
+      reason: `http_${httpStatus}`,
+    };
+  }
+  if (httpStatus === 429) {
+    return {
+      classification: "transient",
+      withingsStatus: undefined,
+      reason: "http_429",
+    };
+  }
+  // 4xx outside 429 is unexpected from Withings — they always reply 200
+  // even on auth failures, embedding the real status in the body. We
+  // bucket as persistent so a regression to a 4xx-throwing upstream
+  // surfaces in audit logs rather than retrying forever.
+  if (httpStatus >= 400 && httpStatus < 500) {
+    return {
+      classification: "persistent",
+      withingsStatus: undefined,
+      reason: `http_${httpStatus}`,
+    };
+  }
+
+  // Body status — Withings' own taxonomy.
+  const status = body?.status;
+  if (status === 0) {
+    return {
+      classification: "success",
+      withingsStatus: 0,
+      reason: "ok",
+    };
+  }
+  if (typeof status !== "number") {
+    // Off-spec body — no `status` field at all. The legacy client
+    // would have crashed reading `json.status`; we treat it as a
+    // transient so the retry catches a one-off CDN error page.
+    return {
+      classification: "transient",
+      withingsStatus: undefined,
+      reason: "no_status_field",
+    };
+  }
+
+  // Explicit lists take precedence over ranges so a contract-mismatch
+  // code (293, 294) that happens to live inside the 200..299 OAuth
+  // range stays classified as persistent rather than reauth.
+  if (PERSISTENT_CODES.has(status)) {
+    return {
+      classification: "persistent",
+      withingsStatus: status,
+      reason: `withings_${status}`,
+    };
+  }
+  if (TRANSIENT_CODES.has(status)) {
+    return {
+      classification: "transient",
+      withingsStatus: status,
+      reason: `withings_${status}`,
+    };
+  }
+  if (REAUTH_CODES.has(status)) {
+    return {
+      classification: "reauth_required",
+      withingsStatus: status,
+      reason: `withings_${status}`,
+    };
+  }
+  // The 200..299 invalid_grant family — Withings uses this range for
+  // OAuth grant failures. Membership is by range rather than an
+  // explicit list because individual codes in there can shift between
+  // Withings firmware revisions. Explicit persistent/transient codes
+  // above this check are deliberate exemptions.
+  if (status >= 200 && status <= 299) {
+    return {
+      classification: "reauth_required",
+      withingsStatus: status,
+      reason: `withings_${status}`,
+    };
+  }
+
+  // Default: anything we don't recognise becomes transient. The
+  // 3-strike admin alert ladder in `recordSyncFailure` already catches
+  // the "keeps happening" case, and silently degrading to "no data"
+  // (the legacy behaviour) is strictly worse than a retry.
+  return {
+    classification: "transient",
+    withingsStatus: status,
+    reason: `withings_${status}_unknown`,
+  };
+}
+
+/**
+ * Typed Error subclass that carries the classification verdict so
+ * `try/catch` blocks downstream can branch without re-parsing the
+ * message.
+ *
+ * The `message` field still uses the legacy
+ * `"Withings <verb> error: <status> - <error>"` format so the existing
+ * regex-based `extractWithingsStatus` / `isWithingsRefreshReauthFailure`
+ * helpers in sync.ts continue to work for callers that haven't been
+ * migrated yet. Callers that import the new class can read
+ * `err.classification` directly.
+ */
+export class WithingsApiError extends Error {
+  readonly classification: WithingsClassification;
+  readonly withingsStatus: number | undefined;
+  readonly reason: string;
+  readonly verb: string;
+
+  constructor(opts: {
+    verb: string;
+    classification: WithingsClassification;
+    withingsStatus: number | undefined;
+    reason: string;
+    upstreamError?: string;
+  }) {
+    const statusLabel =
+      typeof opts.withingsStatus === "number" ? opts.withingsStatus : "?";
+    const errSegment = opts.upstreamError ? ` - ${opts.upstreamError}` : "";
+    super(`Withings ${opts.verb} error: ${statusLabel}${errSegment}`);
+    this.name = "WithingsApiError";
+    this.verb = opts.verb;
+    this.classification = opts.classification;
+    this.withingsStatus = opts.withingsStatus;
+    this.reason = opts.reason;
+  }
+}
+
+/**
+ * Read the classification from any caught error. Returns
+ * `transient` for non-`WithingsApiError` inputs so existing call
+ * sites that surface a plain `Error` continue to retry rather than
+ * permanently disabling the integration.
+ *
+ * Falls back to message-regex parsing so callers that wrap
+ * `WithingsApiError` in `new Error(`... ${err.message}`)` (e.g.
+ * pg-boss job retries that lose the original prototype) still
+ * classify correctly.
+ */
+export function classifyError(err: unknown): WithingsClassification {
+  if (err instanceof WithingsApiError) return err.classification;
+
+  // Best-effort regex fallback for unwrapped messages — same shape
+  // sync.ts's `extractWithingsStatus` reads.
+  const msg = err instanceof Error ? err.message : String(err);
+  const m = /Withings\s+\w+\s+error:\s*(\d+)/.exec(msg);
+  if (!m) return "transient";
+  const status = Number.parseInt(m[1]!, 10);
+  if (!Number.isFinite(status)) return "transient";
+  // Run the body classifier with a synthetic HTTP-200 envelope.
+  return classifyWithingsResponse(200, { status }).classification;
+}
diff --git a/src/lib/withings/sync-activity.ts b/src/lib/withings/sync-activity.ts
index 3ce27fcb8..e58d2c96b 100644
--- a/src/lib/withings/sync-activity.ts
+++ b/src/lib/withings/sync-activity.ts
@@ -44,6 +44,10 @@ import {
 } from "@/lib/rollups/measurement-rollups";
 
 import { hasActivityScope } from "./client";
+import {
+  WithingsApiError,
+  classifyWithingsResponse,
+} from "./response-classifier";
 import {
   extractWithingsStatus,
   isWithingsRefreshReauthFailure,
@@ -157,15 +161,22 @@ export async function fetchWithingsActivity(
       body: params.toString(),
     });
     const json = await res.json();
+    const verdict = classifyWithingsResponse(res.status, json);
     getEvent()?.addExternalCall({
       service: "withings",
       method: `fetchWithingsActivity(page=${pageCount})`,
       duration_ms: Math.round(performance.now() - pageStart),
       status: res.status,
-      error: json.status !== 0 ? `status=${json.status}` : undefined,
+      error: verdict.classification === "success" ? undefined : verdict.reason,
     });
-    if (json.status !== 0) {
-      throw new Error(`Withings activity error: ${json.status}`);
+    if (verdict.classification !== "success") {
+      throw new WithingsApiError({
+        verb: "activity",
+        classification: verdict.classification,
+        withingsStatus: verdict.withingsStatus,
+        reason: verdict.reason,
+        upstreamError: typeof json?.error === "string" ? json.error : undefined,
+      });
     }
 
     const body = json.body ?? {};
diff --git a/src/lib/withings/sync-sleep.ts b/src/lib/withings/sync-sleep.ts
index 24a2f2d3d..f9d70fc85 100644
--- a/src/lib/withings/sync-sleep.ts
+++ b/src/lib/withings/sync-sleep.ts
@@ -42,6 +42,10 @@ import {
 } from "@/lib/rollups/measurement-rollups";
 
 import { hasActivityScope } from "./client";
+import {
+  WithingsApiError,
+  classifyWithingsResponse,
+} from "./response-classifier";
 import {
   extractWithingsStatus,
   isWithingsRefreshReauthFailure,
@@ -127,15 +131,22 @@ export async function fetchWithingsSleep(
     body: params.toString(),
   });
   const json = await res.json();
+  const verdict = classifyWithingsResponse(res.status, json);
   getEvent()?.addExternalCall({
     service: "withings",
     method: "fetchWithingsSleep",
     duration_ms: Math.round(performance.now() - pageStart),
     status: res.status,
-    error: json.status !== 0 ? `status=${json.status}` : undefined,
+    error: verdict.classification === "success" ? undefined : verdict.reason,
   });
-  if (json.status !== 0) {
-    throw new Error(`Withings sleep error: ${json.status}`);
+  if (verdict.classification !== "success") {
+    throw new WithingsApiError({
+      verb: "sleep",
+      classification: verdict.classification,
+      withingsStatus: verdict.withingsStatus,
+      reason: verdict.reason,
+      upstreamError: typeof json?.error === "string" ? json.error : undefined,
+    });
   }
   const series: WithingsSleepSegment[] = json.body?.series ?? [];
   return series;
diff --git a/src/lib/withings/sync.ts b/src/lib/withings/sync.ts
index d85076385..25e4ef4cb 100644
--- a/src/lib/withings/sync.ts
+++ b/src/lib/withings/sync.ts
@@ -17,7 +17,13 @@ import {
   isReauthRequired,
   recordSyncFailure,
   recordSyncSuccess,
+  type FailureKind,
 } from "@/lib/integrations/status";
+import {
+  classifyError,
+  WithingsApiError,
+  type WithingsClassification,
+} from "./response-classifier";
 import {
   collapseToTypeDayKeys,
   recomputeBucketsForMeasurement,
@@ -109,19 +115,22 @@ export async function getValidToken(userId: string): Promise<{
       };
     } catch (err) {
       getEvent()?.addWarning(`Token refresh failed for user ${userId}: ${err}`);
-      // Withings refresh failures expose the upstream `status` code in
-      // the error message (see refreshAccessToken). We treat any of the
-      // documented permanent-revoke statuses (100, 101, 102, 200..299
-      // for invalid_grant) as reauth-required and everything else as a
-      // transient retryable failure.
+      // v1.4.42 W6 — the typed `WithingsApiError` carries the
+      // classification verdict directly; the regex fallback in
+      // `classifyError` handles plain `Error` instances (e.g. a
+      // pg-boss job retry that lost the prototype during JSON
+      // round-trip).
       const message = err instanceof Error ? err.message : String(err);
-      const isReauth = isWithingsRefreshReauthFailure(message);
+      const classification = classifyError(err);
       await recordSyncFailure({
         userId,
         integration: "withings",
-        kind: isReauth ? "reauth_required" : "transient",
+        kind: classificationToFailureKind(classification),
         message,
-        errorCode: extractWithingsStatus(message),
+        errorCode:
+          err instanceof WithingsApiError
+            ? err.withingsStatus?.toString()
+            : extractWithingsStatus(message),
       });
       return null;
     }
@@ -185,14 +194,16 @@ export async function syncUserMeasurements(
     measures = await fetchMeasurements(tokenInfo.accessToken, startDate);
   } catch (err) {
     const message = err instanceof Error ? err.message : String(err);
+    const classification = classifyError(err);
     await recordSyncFailure({
       userId,
       integration: "withings",
-      kind: isWithingsRefreshReauthFailure(message)
-        ? "reauth_required"
-        : "transient",
+      kind: classificationToFailureKind(classification),
       message,
-      errorCode: extractWithingsStatus(message),
+      errorCode:
+        err instanceof WithingsApiError
+          ? err.withingsStatus?.toString()
+          : extractWithingsStatus(message),
     });
     throw err;
   }
@@ -287,6 +298,33 @@ export async function syncUserMeasurements(
   return imported;
 }
 
+/**
+ * Map a Withings response classification onto the `FailureKind` carried
+ * into `recordSyncFailure`. Pure function — no IO, no state.
+ *
+ *   `success`         → not a failure; callers must not invoke this.
+ *   `transient`       → `transient` (retry next sync)
+ *   `reauth_required` → `reauth_required` (park at `error_reauth`)
+ *   `persistent`      → `persistent` (audited; next sync still runs)
+ */
+export function classificationToFailureKind(
+  classification: WithingsClassification,
+): FailureKind {
+  switch (classification) {
+    case "reauth_required":
+      return "reauth_required";
+    case "persistent":
+      return "persistent";
+    case "transient":
+      return "transient";
+    case "success":
+      // Defensive: a caller asking for the FailureKind of a success
+      // is a contract bug. Default to `transient` so the audit log
+      // still surfaces the bug rather than silently ignoring it.
+      return "transient";
+  }
+}
+
 /**
  * Withings status codes that indicate a permanent-revoke condition
  * (the refresh_token will not work again — the user has to redo OAuth).
@@ -302,6 +340,13 @@ export async function syncUserMeasurements(
  *
  * The error message format from `refreshAccessToken()` is
  * "Withings refresh error: <status> - <error>".
+ *
+ * v1.4.42 W6 — superseded for new code paths by
+ * `classifyError`/`classifyWithingsResponse` in `response-classifier.ts`.
+ * Kept for the W17b/c sync-activity + sync-sleep catch-blocks that
+ * haven't yet been migrated; both paths now also benefit from the
+ * typed-error throw upstream because `classifyError` falls back to the
+ * same regex.
  */
 export function isWithingsRefreshReauthFailure(message: string): boolean {
   const status = extractWithingsStatus(message);