diff --git a/scripts/diagnose-implausible-measurements.ts b/scripts/diagnose-implausible-measurements.ts
new file mode 100644
index 000000000..0be518a26
--- /dev/null
+++ b/scripts/diagnose-implausible-measurements.ts
@@ -0,0 +1,150 @@
+/**
+ * Read-only diagnostic for stored measurement rows outside the app's own
+ * declared plausibility domain.
+ *
+ * `VALUE_RANGES` in `@/lib/validations/measurement` is the band the
+ * application says a metric can occupy. Every path a person can drive has
+ * always enforced it; the provider sync writers did not, so rows the
+ * application itself calls impossible reached the table — and every mean,
+ * median and personal band built from them inherited the impossibility. The
+ * writers enforce it now, which stops new rows landing. It says nothing about
+ * the rows already there.
+ *
+ * This reports them and repairs nothing. What to do with a stored impossible
+ * row is a judgement the data cannot make: a pulse of 55 million is plainly a
+ * decode slip and a candidate for deletion, while a weight of 0.9 kg might be
+ * a neonatal record on a shared scale, or a unit mix-up, or a real reading the
+ * band is simply too narrow for. Deleting somebody's health record on a
+ * heuristic is not a thing a script gets to decide.
+ *
+ * Output is aggregate: one line per (metric, source) bucket with the row
+ * count, the number of distinct records affected, the observed extremes, and
+ * the window they fall in. Individual readings are not printed — the bucket
+ * plus the metric's own declared bound is what a decision needs, and a log
+ * file full of somebody's vitals is not.
+ *
+ * Run (inside the app container, which has DATABASE_URL):
+ *   pnpm dlx tsx scripts/diagnose-implausible-measurements.ts
+ */
+import "dotenv/config";
+
+import { prisma } from "@/lib/db";
+import { VALUE_RANGES } from "@/lib/validations/measurement";
+
+interface BucketRow {
+  type: string;
+  source: string;
+  row_count: bigint;
+  record_count: bigint;
+  min_value: number;
+  max_value: number;
+  first_at: Date;
+  last_at: Date;
+  tombstoned: bigint;
+}
+
+function ymd(date: Date): string {
+  return date.toISOString().slice(0, 10);
+}
+
+async function main(): Promise<void> {
+  const types = Object.keys(VALUE_RANGES).sort();
+  const mins = types.map((t) => VALUE_RANGES[t]!.min);
+  const maxes = types.map((t) => VALUE_RANGES[t]!.max);
+
+  // The band table rides in as three parallel bound arrays rather than a
+  // spliced VALUES list, so the query carries no interpolated text at all.
+  // `value <> value` is the NaN probe: a double-precision column can hold one,
+  // and it compares false against every bound including itself.
+  const buckets = await prisma.$queryRaw<BucketRow[]>`
+    WITH bands(type, min_value, max_value) AS (
+      SELECT * FROM unnest(
+        ${types}::text[],
+        ${mins}::double precision[],
+        ${maxes}::double precision[]
+      )
+    )
+    SELECT
+      m."type"::text                                        AS type,
+      m."source"::text                                      AS source,
+      COUNT(*)::bigint                                      AS row_count,
+      COUNT(DISTINCT m."user_id")::bigint                   AS record_count,
+      MIN(m."value")                                        AS min_value,
+      MAX(m."value")                                        AS max_value,
+      MIN(m."measured_at")                                  AS first_at,
+      MAX(m."measured_at")                                  AS last_at,
+      COUNT(*) FILTER (WHERE m."deleted_at" IS NOT NULL)::bigint AS tombstoned
+    FROM "measurements" m
+    JOIN bands b ON b."type" = m."type"::text
+    WHERE m."value" < b.min_value
+       OR m."value" > b.max_value
+       OR m."value" <> m."value"
+    GROUP BY m."type", m."source"
+    ORDER BY COUNT(*) DESC, m."type"::text ASC, m."source"::text ASC
+  `;
+
+  if (buckets.length === 0) {
+    console.log(
+      `Every stored measurement sits inside its metric's declared range ` +
+        `(${types.length} metric bands checked). Nothing to review.`,
+    );
+    return;
+  }
+
+  const totalRows = buckets.reduce((sum, b) => sum + Number(b.row_count), 0);
+  const affectedTypes = new Set(buckets.map((b) => b.type));
+  const affectedSources = new Set(buckets.map((b) => b.source));
+
+  console.log(
+    `${totalRows} stored row(s) sit outside the declared plausibility band, ` +
+      `across ${affectedTypes.size} metric(s) and ${affectedSources.size} source(s).\n` +
+      `Nothing below is modified. Each line is a (metric, source) bucket; the ` +
+      `band is the metric's own declared domain.\n`,
+  );
+
+  // Group by metric so the declared band is stated once per heading, and rank
+  // the metrics by how much of the problem each one is. The query's own
+  // ordering already ranks the sources inside a metric.
+  const byType = new Map<string, BucketRow[]>();
+  for (const bucket of buckets) {
+    const bucketsForType = byType.get(bucket.type);
+    if (bucketsForType) bucketsForType.push(bucket);
+    else byType.set(bucket.type, [bucket]);
+  }
+  const ranked = [...byType.entries()].sort(
+    (a, b) =>
+      b[1].reduce((sum, x) => sum + Number(x.row_count), 0) -
+      a[1].reduce((sum, x) => sum + Number(x.row_count), 0),
+  );
+
+  for (const [type, rows] of ranked) {
+    const band = VALUE_RANGES[type]!;
+    console.log(`${type} — declared band ${band.min}…${band.max}`);
+    for (const bucket of rows) {
+      const live = Number(bucket.row_count) - Number(bucket.tombstoned);
+      console.log(
+        `  ${bucket.source.padEnd(14)} ` +
+          `${String(bucket.row_count).padStart(7)} row(s) ` +
+          `(${live} live, ${bucket.tombstoned} tombstoned) ` +
+          `across ${bucket.record_count} record(s) — ` +
+          `observed ${bucket.min_value}…${bucket.max_value}, ` +
+          `${ymd(bucket.first_at)} → ${ymd(bucket.last_at)}`,
+      );
+    }
+  }
+
+  console.log(
+    `\nThe sync writers refuse these values now, so the population above is ` +
+      `closed unless a writer without the gate is still in play. Removing a ` +
+      `row is a maintainer decision, per bucket.`,
+  );
+}
+
+main()
+  .catch((err) => {
+    console.error(err);
+    process.exitCode = 1;
+  })
+  .finally(() => {
+    void prisma.$disconnect();
+  });
diff --git a/src/__tests__/measurement-write-plausibility-guard.test.ts b/src/__tests__/measurement-write-plausibility-guard.test.ts
new file mode 100644
index 000000000..88fbafee2
--- /dev/null
+++ b/src/__tests__/measurement-write-plausibility-guard.test.ts
@@ -0,0 +1,200 @@
+/**
+ * Structural guard on the measurement ingest boundary.
+ *
+ * `VALUE_RANGES` is the band the application declares a metric can occupy.
+ * Every path a person can drive enforced it from the start; the provider sync
+ * writers never did, and nothing noticed, because each writer is correct on
+ * its own terms — it wrote what the remote sent. The gap was only visible from
+ * above, by listing every writer at once and asking which of them had ever
+ * been told about the band.
+ *
+ * So the list lives here. Every file that writes a `Measurement` row is
+ * enumerated with the reason it is allowed to, and the guard fails when the
+ * discovered set stops matching. A provider added tomorrow lands a file that
+ * is not in the map and has to answer the question before it can ship.
+ *
+ * This is a tripwire, not a proof. It cannot show a disposition is the right
+ * one — only that nobody added a writer without writing down which gate covers
+ * it. A reviewer who waves through a bogus `derived` label defeats it, and no
+ * test substitutes for that review.
+ */
+import { describe, it, expect } from "vitest";
+import { readFileSync } from "node:fs";
+import { join } from "node:path";
+
+import { walkSourceFiles } from "./helpers/source-files";
+
+const SRC = join(process.cwd(), "src");
+
+/** The gate module every provider-fed writer has to reach. */
+const GATE_MODULE = "@/lib/measurements/plausibility-gate";
+/** The predicate every person-driven write path has always used. */
+const VALIDATOR = "validateMeasurementRange";
+
+/**
+ * How a writer satisfies the plausibility domain.
+ *
+ * - `gate` — provider-fed; must reach {@link GATE_MODULE} itself.
+ * - `validator` — person-driven; must call {@link VALIDATOR} itself.
+ * - `upstream` — validated in a named module before the row is built; the
+ *   named module is checked, so the label cannot be a shrug.
+ * - `derived` — the value is arithmetic over rows already in the table, not a
+ *   number from outside. Gating here would drop a consolidation of data that
+ *   IS present, which is a different failure from admitting one that is not.
+ * - `literal` — the written value is a constant in the file, so no external
+ *   number can reach the column.
+ * - `restore` — replays the record's own backup verbatim. Fidelity is the
+ *   contract; a restore that silently dropped rows would be the worse bug,
+ *   and the diagnostic script is how a stored impossible row gets found.
+ */
+type Disposition =
+  | { kind: "gate" }
+  | { kind: "validator" }
+  | { kind: "upstream"; validatedIn: string }
+  | { kind: "derived"; why: string }
+  | { kind: "literal"; why: string }
+  | { kind: "restore"; why: string };
+
+const WRITERS: Record<string, Disposition> = {
+  // ── Provider sync writers, gated ──────────────────────────────────────
+  "lib/withings/sync.ts": { kind: "gate" },
+  "lib/withings/sync-activity.ts": { kind: "gate" },
+  "lib/fitbit/sync-core.ts": { kind: "gate" },
+  "lib/google-health/sync-core.ts": { kind: "gate" },
+  "lib/nightscout/sync.ts": { kind: "gate" },
+  // The shared identity seam. Polar, Oura, WHOOP, the Withings sleep leg,
+  // the Apple export importer and the batch endpoint all write through it,
+  // so the gate here is what makes a new provider fail closed.
+  "lib/measurements/reconcile-external-measurement.ts": { kind: "gate" },
+
+  // ── Person-driven write paths, validated ──────────────────────────────
+  "app/api/measurements/route.ts": {
+    kind: "upstream",
+    validatedIn: "lib/validations/measurement.ts",
+  },
+  "app/api/import/route.ts": { kind: "validator" },
+  "lib/mcp/writes.ts": { kind: "validator" },
+  "lib/measurements/create-from-telegram.ts": { kind: "validator" },
+  "lib/measurements/import-apple-health-export.ts": { kind: "validator" },
+  "app/api/import/csv/route.ts": {
+    kind: "upstream",
+    validatedIn: "lib/import/csv-measurements.ts",
+  },
+
+  // ── Fixed values ──────────────────────────────────────────────────────
+  "lib/withings/sync-ecg.ts": {
+    kind: "literal",
+    why: "an ECG row carries value 1 — the file's own constant, never a provider number",
+  },
+  "app/api/mental-health/assessments/route.ts": {
+    kind: "literal",
+    why: "the questionnaire score is summed from Zod-bounded item answers",
+  },
+  "lib/insights/score-row.ts": {
+    kind: "literal",
+    why: "the health score is computed in-app on a closed 0-100 scale",
+  },
+
+  // ── Derived from rows already stored ──────────────────────────────────
+  "lib/measurements/consolidate-daily-mean.ts": {
+    kind: "derived",
+    why: "writes the mean of the day's own stored rows",
+  },
+  "lib/measurements/consolidate-legacy-steps.ts": {
+    kind: "derived",
+    why: "folds stored per-sample step rows into the day's canonical row",
+  },
+  "lib/measurements/dense-intraday-retention.ts": {
+    kind: "derived",
+    why: "rebuilds hourly buckets from stored per-sample rows",
+  },
+  "lib/measurements/drain-per-sample-cumulative.ts": {
+    kind: "derived",
+    why: "reduces stored cumulative per-sample rows to one daily total",
+  },
+
+  // ── Restore ───────────────────────────────────────────────────────────
+  "app/api/admin/backups/[id]/restore/route.ts": {
+    kind: "restore",
+    why: "replays the record's own export; dropping rows would break the restore contract",
+  },
+};
+
+/**
+ * Any Prisma client handle followed by a measurement write. Whitespace- and
+ * handle-tolerant on purpose: the earlier generation of this repo's guards
+ * demanded a literal `prisma.x.findUnique(` and went green because it matched
+ * nothing, not because nothing was there. The transaction handles in use are
+ * `tx` and `pc`; a future one is caught by the `[A-Za-z_$][\w$]*` head.
+ */
+const MEASUREMENT_WRITE =
+  /[A-Za-z_$][\w$]*\s*\.\s*measurement\s*\.\s*(create|createMany|createManyAndReturn|upsert)\s*\(/;
+
+function sourceFiles(): string[] {
+  return walkSourceFiles(SRC, { floor: 3000 })
+    .filter((p) => !p.startsWith("generated/"))
+    .filter((p) => !p.includes("__tests__"))
+    .filter((p) => !p.endsWith(".test.ts") && !p.endsWith(".test.tsx"))
+    .sort();
+}
+
+function read(rel: string): string {
+  return readFileSync(join(SRC, rel), "utf8");
+}
+
+function discoveredWriters(): string[] {
+  return sourceFiles().filter((rel) => MEASUREMENT_WRITE.test(read(rel)));
+}
+
+describe("every measurement writer declares how it meets the plausibility domain", () => {
+  it("finds the writers at all — an empty sweep is a broken guard, not a clean repo", () => {
+    // The map is the claim; this asserts the matcher can still see the
+    // population it describes. A regex that silently stops matching would
+    // otherwise turn every check below into a tautology.
+    expect(discoveredWriters().length).toBeGreaterThanOrEqual(15);
+  });
+
+  it("the discovered writer set matches the declared map exactly", () => {
+    expect(discoveredWriters()).toEqual(Object.keys(WRITERS).sort());
+  });
+
+  it("every provider-fed writer reaches the gate module", () => {
+    const gated = Object.entries(WRITERS)
+      .filter(([, d]) => d.kind === "gate")
+      .map(([rel]) => rel);
+    expect(gated.length).toBeGreaterThanOrEqual(6);
+    for (const rel of gated) {
+      expect(read(rel), `${rel} must import ${GATE_MODULE}`).toContain(
+        GATE_MODULE,
+      );
+    }
+  });
+
+  it("every person-driven writer calls the range validator", () => {
+    for (const [rel, disposition] of Object.entries(WRITERS)) {
+      if (disposition.kind === "validator") {
+        expect(read(rel), `${rel} must call ${VALIDATOR}`).toContain(VALIDATOR);
+      }
+      if (disposition.kind === "upstream") {
+        expect(
+          read(disposition.validatedIn),
+          `${disposition.validatedIn} must call ${VALIDATOR} on behalf of ${rel}`,
+        ).toContain(VALIDATOR);
+      }
+    }
+  });
+
+  it("the exempt writers carry a written reason", () => {
+    for (const [rel, disposition] of Object.entries(WRITERS)) {
+      if (
+        disposition.kind === "derived" ||
+        disposition.kind === "literal" ||
+        disposition.kind === "restore"
+      ) {
+        expect(disposition.why.length, `${rel} needs a reason`).toBeGreaterThan(
+          20,
+        );
+      }
+    }
+  });
+});
diff --git a/src/app/api/measurements/batch/route.ts b/src/app/api/measurements/batch/route.ts
index df8ffcd59..1199e2354 100644
--- a/src/app/api/measurements/batch/route.ts
+++ b/src/app/api/measurements/batch/route.ts
@@ -620,6 +620,17 @@ async function postBatch(request: NextRequest): Promise<Response> {
           };
           failedCount++;
           continue;
+        case "rejected_range":
+          // The reconciler carries the same plausibility gate this route
+          // already applied above, so reaching here means the two disagreed —
+          // report it exactly as the earlier guard would have, and let the
+          // wide-event tally record which end refused.
+          results[p.index] = {
+            index: p.index,
+            status: "skipped",
+            reason: "value_out_of_range",
+          };
+          continue;
       }
 
       if (isMergeableSource(p.row.source as string)) {
diff --git a/src/lib/fitbit/__tests__/sync-plausibility.test.ts b/src/lib/fitbit/__tests__/sync-plausibility.test.ts
new file mode 100644
index 000000000..7efe59f99
--- /dev/null
+++ b/src/lib/fitbit/__tests__/sync-plausibility.test.ts
@@ -0,0 +1,179 @@
+/**
+ * `upsertFitbitMeasurements` is the single write seam for every Fitbit
+ * resource leg — activity, sleep, daily metrics — and it holds Prisma
+ * directly, so it does not inherit the shared reconciler's gate. These cases
+ * pin that the provider's numbers clear the app's own declared plausibility
+ * band before any of them can become a row, and that a refusal is reported.
+ */
+import { beforeEach, describe, expect, it, vi } from "vitest";
+
+const { findManyMock, createManyMock, updateMock } = vi.hoisted(() => ({
+  findManyMock: vi.fn(async () => [] as unknown[]),
+  createManyMock: vi.fn<
+    () => Promise<Array<{ id: string; type: string; measuredAt: Date }>>
+  >(async () => []),
+  updateMock: vi.fn(async () => ({})),
+}));
+
+vi.mock("@/lib/db", () => ({
+  prisma: {
+    measurement: {
+      findMany: findManyMock,
+      createManyAndReturn: createManyMock,
+      update: updateMock,
+    },
+  },
+}));
+vi.mock("@/lib/crypto", () => ({ encrypt: vi.fn(), decrypt: vi.fn() }));
+vi.mock("@/lib/integrations/status", () => ({
+  isReauthRequired: vi.fn(async () => false),
+  recordSyncFailure: vi.fn(async () => {}),
+  recordSyncSuccess: vi.fn(async () => {}),
+}));
+vi.mock("@/lib/rollups/measurement-rollups", () => ({
+  collapseToTypeDayKeys: vi.fn(() => []),
+  recomputeBucketsForMeasurement: vi.fn(async () => {}),
+  recomputeUserRollups: vi.fn(async () => {}),
+}));
+vi.mock("@/lib/insights/comprehensive-generate", () => ({
+  invalidateStatusInsightsForTypes: vi.fn(async () => {}),
+}));
+vi.mock("@/lib/integrations/oauth-refresh", () => ({
+  persistRotatedToken: vi.fn(async () => {}),
+}));
+vi.mock("../credentials", () => ({
+  getUserFitbitCredentials: vi.fn(async () => null),
+}));
+vi.mock("../client", () => ({ refreshAccessToken: vi.fn() }));
+
+import { eventStorage } from "@/lib/logging/context";
+import { WideEventBuilder } from "@/lib/logging/event-builder";
+import { RANGE_REJECTED_META_KEY } from "@/lib/measurements/plausibility-gate";
+import type { RangeRejectionTally } from "@/lib/measurements/plausibility-gate";
+
+import { upsertFitbitMeasurements } from "../sync-core";
+
+/** RESTING_HEART_RATE is declared 25–220. */
+const REAL_READING = {
+  type: "RESTING_HEART_RATE",
+  value: 58,
+  unit: "bpm",
+  measuredAt: new Date("2026-07-08T00:00:00.000Z"),
+  externalId: "rhr:2026-07-08",
+};
+
+const IMPOSSIBLE_READING = {
+  type: "RESTING_HEART_RATE",
+  value: 4_915_200,
+  unit: "bpm",
+  measuredAt: new Date("2026-07-09T00:00:00.000Z"),
+  externalId: "rhr:2026-07-09",
+};
+
+let event: WideEventBuilder;
+
+function tally(): RangeRejectionTally | undefined {
+  return event.toJSON().meta?.[RANGE_REJECTED_META_KEY] as
+    RangeRejectionTally | undefined;
+}
+
+beforeEach(() => {
+  findManyMock.mockReset().mockResolvedValue([]);
+  createManyMock.mockReset().mockResolvedValue([]);
+  updateMock.mockReset().mockResolvedValue({});
+  event = new WideEventBuilder("background");
+});
+
+describe("upsertFitbitMeasurements — plausibility gate", () => {
+  it("writes a reading inside the metric's declared band", async () => {
+    createManyMock.mockResolvedValue([
+      {
+        id: "row-1",
+        type: "RESTING_HEART_RATE",
+        measuredAt: REAL_READING.measuredAt,
+      },
+    ]);
+
+    const result = await eventStorage.run(event, () =>
+      upsertFitbitMeasurements("user-1", [REAL_READING], { deferRollup: true }),
+    );
+
+    expect(result.imported).toBe(1);
+    const planned = (createManyMock.mock.calls[0] as unknown[])[0] as {
+      data: Array<{ value: number }>;
+    };
+    expect(planned.data).toHaveLength(1);
+    expect(planned.data[0]!.value).toBe(58);
+    expect(tally()).toBeUndefined();
+  });
+
+  it("refuses an out-of-band reading and annotates the drop", async () => {
+    createManyMock.mockResolvedValue([
+      {
+        id: "row-1",
+        type: "RESTING_HEART_RATE",
+        measuredAt: REAL_READING.measuredAt,
+      },
+    ]);
+
+    const result = await eventStorage.run(event, () =>
+      upsertFitbitMeasurements("user-1", [REAL_READING, IMPOSSIBLE_READING], {
+        deferRollup: true,
+      }),
+    );
+
+    expect(result.imported).toBe(1);
+    const planned = (createManyMock.mock.calls[0] as unknown[])[0] as {
+      data: Array<{ value: number; externalId: string }>;
+    };
+    expect(planned.data).toHaveLength(1);
+    expect(planned.data[0]!.externalId).toBe("rhr:2026-07-08");
+    // The refused reading never even reached the existence probe.
+    expect(
+      (
+        (findManyMock.mock.calls[0] as unknown[])[0] as {
+          where: { externalId: { in: string[] } };
+        }
+      ).where.externalId.in,
+    ).toEqual(["rhr:2026-07-08"]);
+    expect(tally()).toEqual({
+      total: 1,
+      buckets: [
+        {
+          source: "fitbit",
+          type: "RESTING_HEART_RATE",
+          direction: "above_max",
+          count: 1,
+        },
+      ],
+    });
+    expect(event.getLevel()).toBe("warn");
+  });
+
+  it("does not overwrite a stored reading with an out-of-band one", async () => {
+    // The key already exists; ungated, the update branch rewrote its value.
+    findManyMock.mockResolvedValue([
+      {
+        id: "existing",
+        type: "RESTING_HEART_RATE",
+        externalId: "rhr:2026-07-09",
+        value: 58,
+        unit: "bpm",
+        measuredAt: IMPOSSIBLE_READING.measuredAt,
+        sleepStage: null,
+        deletedAt: null,
+      },
+    ]);
+
+    const result = await eventStorage.run(event, () =>
+      upsertFitbitMeasurements("user-1", [IMPOSSIBLE_READING], {
+        deferRollup: true,
+      }),
+    );
+
+    expect(result.imported).toBe(0);
+    expect(updateMock).not.toHaveBeenCalled();
+    expect(createManyMock).not.toHaveBeenCalled();
+    expect(tally()?.total).toBe(1);
+  });
+});
diff --git a/src/lib/fitbit/sync-core.ts b/src/lib/fitbit/sync-core.ts
index 74637e01b..f67f4422b 100644
--- a/src/lib/fitbit/sync-core.ts
+++ b/src/lib/fitbit/sync-core.ts
@@ -15,6 +15,7 @@ import { persistRotatedToken } from "@/lib/integrations/oauth-refresh";
 import { recordSyncFailure } from "@/lib/integrations/status";
 import type { FailureKind } from "@/lib/integrations/status";
 import { getEvent } from "@/lib/logging/context";
+import { dropImplausibleMeasurements } from "@/lib/measurements/plausibility-gate";
 import {
   collapseToTypeDayKeys,
   recomputeBucketsForMeasurement,
@@ -530,7 +531,7 @@ export async function replaceStaleFitbitSleep(
  */
 export async function upsertFitbitMeasurements(
   userId: string,
-  readings: FitbitMeasurementUpsert[],
+  incoming: FitbitMeasurementUpsert[],
   opts: { deferRollup?: boolean } = {},
 ): Promise<{
   imported: number;
@@ -541,6 +542,15 @@ export async function upsertFitbitMeasurements(
     measuredAt: Date;
   }>;
 }> {
+  // Every resource leg funnels through here, so this is the one place the
+  // provider's numbers have to clear the app's declared plausibility band
+  // before they can become rows. A refused reading is dropped and tallied on
+  // the ambient event, never clamped and never written for a later reader to
+  // discover.
+  const readings = dropImplausibleMeasurements("fitbit", incoming, (r) => ({
+    type: r.type,
+    value: r.value,
+  }));
   if (readings.length === 0) return { imported: 0, touched: [], inserted: [] };
 
   // Probe EVERY existing row (live AND tombstoned) for the batch's externalIds
diff --git a/src/lib/google-health/__tests__/sync-plausibility.test.ts b/src/lib/google-health/__tests__/sync-plausibility.test.ts
new file mode 100644
index 000000000..2fb5b3404
--- /dev/null
+++ b/src/lib/google-health/__tests__/sync-plausibility.test.ts
@@ -0,0 +1,180 @@
+/**
+ * `upsertGoogleHealthMeasurements` is the single write seam for every Google
+ * Health resource leg and it holds Prisma directly, so it does not inherit the
+ * shared reconciler's gate. These cases pin that the provider's numbers clear
+ * the app's own declared plausibility band before any of them can become a
+ * row, and that a refusal is reported rather than swallowed.
+ */
+import { beforeEach, describe, expect, it, vi } from "vitest";
+
+const { findManyMock, createManyMock, updateMock } = vi.hoisted(() => ({
+  findManyMock: vi.fn(async () => [] as unknown[]),
+  createManyMock: vi.fn<
+    () => Promise<Array<{ id: string; type: string; measuredAt: Date }>>
+  >(async () => []),
+  updateMock: vi.fn(async () => ({})),
+}));
+
+vi.mock("@/lib/db", () => ({
+  prisma: {
+    measurement: {
+      findMany: findManyMock,
+      createManyAndReturn: createManyMock,
+      update: updateMock,
+    },
+  },
+}));
+vi.mock("@/lib/crypto", () => ({ encrypt: vi.fn(), decrypt: vi.fn() }));
+vi.mock("@/lib/integrations/status", () => ({
+  isReauthRequired: vi.fn(async () => false),
+  recordSyncFailure: vi.fn(async () => {}),
+  recordSyncSuccess: vi.fn(async () => {}),
+}));
+vi.mock("@/lib/rollups/measurement-rollups", () => ({
+  collapseToTypeDayKeys: vi.fn(() => []),
+  recomputeBucketsForMeasurement: vi.fn(async () => {}),
+  recomputeUserRollups: vi.fn(async () => {}),
+}));
+vi.mock("@/lib/insights/comprehensive-generate", () => ({
+  invalidateStatusInsightsForTypes: vi.fn(async () => {}),
+}));
+vi.mock("../credentials", () => ({
+  getUserGoogleHealthCredentials: vi.fn(async () => null),
+}));
+vi.mock("../client", async () => {
+  const actual = await vi.importActual<typeof import("../client")>("../client");
+  return {
+    ...actual,
+    refreshAccessToken: vi.fn(),
+    noteGoogleHealthMapped: vi.fn(),
+    noteGoogleHealthWritten: vi.fn(),
+    noteGoogleHealthOutcomeFailure: vi.fn(),
+  };
+});
+
+import { eventStorage } from "@/lib/logging/context";
+import { WideEventBuilder } from "@/lib/logging/event-builder";
+import { RANGE_REJECTED_META_KEY } from "@/lib/measurements/plausibility-gate";
+import type { RangeRejectionTally } from "@/lib/measurements/plausibility-gate";
+
+import { upsertGoogleHealthMeasurements } from "../sync-core";
+
+/** OXYGEN_SATURATION is declared 50–100. */
+const REAL_READING = {
+  type: "OXYGEN_SATURATION",
+  value: 97,
+  unit: "%",
+  measuredAt: new Date("2026-07-08T00:00:00.000Z"),
+  externalId: "spo2:2026-07-08",
+};
+
+const IMPOSSIBLE_READING = {
+  type: "OXYGEN_SATURATION",
+  value: 9_700,
+  unit: "%",
+  measuredAt: new Date("2026-07-09T00:00:00.000Z"),
+  externalId: "spo2:2026-07-09",
+};
+
+let event: WideEventBuilder;
+
+function tally(): RangeRejectionTally | undefined {
+  return event.toJSON().meta?.[RANGE_REJECTED_META_KEY] as
+    RangeRejectionTally | undefined;
+}
+
+beforeEach(() => {
+  findManyMock.mockReset().mockResolvedValue([]);
+  createManyMock.mockReset().mockResolvedValue([]);
+  updateMock.mockReset().mockResolvedValue({});
+  event = new WideEventBuilder("background");
+});
+
+describe("upsertGoogleHealthMeasurements — plausibility gate", () => {
+  it("writes a reading inside the metric's declared band", async () => {
+    createManyMock.mockResolvedValue([
+      {
+        id: "row-1",
+        type: "OXYGEN_SATURATION",
+        measuredAt: REAL_READING.measuredAt,
+      },
+    ]);
+
+    const result = await eventStorage.run(event, () =>
+      upsertGoogleHealthMeasurements("user-1", [REAL_READING], {
+        deferRollup: true,
+      }),
+    );
+
+    expect(result.imported).toBe(1);
+    const planned = (createManyMock.mock.calls[0] as unknown[])[0] as {
+      data: Array<{ value: number }>;
+    };
+    expect(planned.data).toHaveLength(1);
+    expect(planned.data[0]!.value).toBe(97);
+    expect(tally()).toBeUndefined();
+  });
+
+  it("refuses an out-of-band reading and annotates the drop", async () => {
+    createManyMock.mockResolvedValue([
+      {
+        id: "row-1",
+        type: "OXYGEN_SATURATION",
+        measuredAt: REAL_READING.measuredAt,
+      },
+    ]);
+
+    const result = await eventStorage.run(event, () =>
+      upsertGoogleHealthMeasurements(
+        "user-1",
+        [REAL_READING, IMPOSSIBLE_READING],
+        { deferRollup: true },
+      ),
+    );
+
+    expect(result.imported).toBe(1);
+    const planned = (createManyMock.mock.calls[0] as unknown[])[0] as {
+      data: Array<{ externalId: string }>;
+    };
+    expect(planned.data).toHaveLength(1);
+    expect(planned.data[0]!.externalId).toBe("spo2:2026-07-08");
+    expect(tally()).toEqual({
+      total: 1,
+      buckets: [
+        {
+          source: "googleHealth",
+          type: "OXYGEN_SATURATION",
+          direction: "above_max",
+          count: 1,
+        },
+      ],
+    });
+    expect(event.getLevel()).toBe("warn");
+  });
+
+  it("does not overwrite a stored reading with an out-of-band one", async () => {
+    findManyMock.mockResolvedValue([
+      {
+        id: "existing",
+        type: "OXYGEN_SATURATION",
+        externalId: "spo2:2026-07-09",
+        value: 97,
+        unit: "%",
+        measuredAt: IMPOSSIBLE_READING.measuredAt,
+        sleepStage: null,
+        deletedAt: null,
+      },
+    ]);
+
+    const result = await eventStorage.run(event, () =>
+      upsertGoogleHealthMeasurements("user-1", [IMPOSSIBLE_READING], {
+        deferRollup: true,
+      }),
+    );
+
+    expect(result.imported).toBe(0);
+    expect(updateMock).not.toHaveBeenCalled();
+    expect(createManyMock).not.toHaveBeenCalled();
+    expect(tally()?.total).toBe(1);
+  });
+});
diff --git a/src/lib/google-health/sync-core.ts b/src/lib/google-health/sync-core.ts
index 27ccaa139..d0545ee0d 100644
--- a/src/lib/google-health/sync-core.ts
+++ b/src/lib/google-health/sync-core.ts
@@ -20,6 +20,7 @@ import { prisma } from "@/lib/db";
 import type { MeasurementType } from "@/generated/prisma/client";
 import { encrypt, decrypt } from "@/lib/crypto";
 import { getEvent } from "@/lib/logging/context";
+import { dropImplausibleMeasurements } from "@/lib/measurements/plausibility-gate";
 import { emitInsertedMeasurementArrivals } from "@/lib/arrivals/measurement-emit";
 import { recordSyncFailure, type FailureKind } from "@/lib/integrations/status";
 import {
@@ -487,7 +488,7 @@ export async function replaceStaleGoogleHealthSleep(
  */
 export async function upsertGoogleHealthMeasurements(
   userId: string,
-  readings: GoogleHealthMeasurementUpsert[],
+  incoming: GoogleHealthMeasurementUpsert[],
   opts: { deferRollup?: boolean } = {},
 ): Promise<{
   imported: number;
@@ -498,7 +499,17 @@ export async function upsertGoogleHealthMeasurements(
     measuredAt: Date;
   }>;
 }> {
-  noteGoogleHealthMapped(readings.length);
+  noteGoogleHealthMapped(incoming.length);
+  // Every resource leg funnels through here, so this is the one place the
+  // provider's numbers have to clear the app's declared plausibility band
+  // before they can become rows. A refused reading is dropped and tallied on
+  // the ambient event, never clamped and never written for a later reader to
+  // discover.
+  const readings = dropImplausibleMeasurements(
+    "googleHealth",
+    incoming,
+    (r) => ({ type: r.type, value: r.value }),
+  );
   if (readings.length === 0) return { imported: 0, touched: [], inserted: [] };
 
   // Probe EVERY existing row (live AND tombstoned) for the batch's externalIds
diff --git a/src/lib/measurements/__tests__/plausibility-gate.test.ts b/src/lib/measurements/__tests__/plausibility-gate.test.ts
new file mode 100644
index 000000000..074d32600
--- /dev/null
+++ b/src/lib/measurements/__tests__/plausibility-gate.test.ts
@@ -0,0 +1,222 @@
+/**
+ * The ingest-side plausibility gate.
+ *
+ * `VALUE_RANGES` is the band the application declares a metric can occupy, and
+ * every path a person can drive has always enforced it. The provider sync
+ * writers did not, which is how a row the application itself calls impossible
+ * reached the table and poisoned every mean and personal band derived from it.
+ * These cases pin the two halves of the fix: the refusal, and the fact that a
+ * refusal is never silent.
+ */
+import { beforeEach, describe, expect, it } from "vitest";
+
+import { WideEventBuilder } from "@/lib/logging/event-builder";
+import { eventStorage } from "@/lib/logging/context";
+import {
+  RANGE_REJECTED_META_KEY,
+  classifyRangeRejection,
+  dropImplausibleMeasurements,
+  recordRangeRejections,
+  type RangeRejectionTally,
+} from "../plausibility-gate";
+
+let event: WideEventBuilder;
+
+function withEvent<T>(run: () => T): T {
+  return eventStorage.run(event, run);
+}
+
+function tally(): RangeRejectionTally | undefined {
+  return event.toJSON().meta?.[RANGE_REJECTED_META_KEY] as
+    RangeRejectionTally | undefined;
+}
+
+beforeEach(() => {
+  event = new WideEventBuilder("background");
+});
+
+describe("classifyRangeRejection", () => {
+  it("passes a value inside the metric's declared band", () => {
+    // PULSE is declared 20–300; both edges are inclusive.
+    expect(classifyRangeRejection("PULSE", 60)).toBeNull();
+    expect(classifyRangeRejection("PULSE", 20)).toBeNull();
+    expect(classifyRangeRejection("PULSE", 300)).toBeNull();
+  });
+
+  it("names the edge an out-of-band value broke", () => {
+    expect(classifyRangeRejection("PULSE", 55643821.505)).toEqual({
+      type: "PULSE",
+      direction: "above_max",
+    });
+    expect(classifyRangeRejection("PULSE", 19)).toEqual({
+      type: "PULSE",
+      direction: "below_min",
+    });
+  });
+
+  it("refuses a non-finite value whatever the metric", () => {
+    expect(classifyRangeRejection("PULSE", Number.NaN)).toEqual({
+      type: "PULSE",
+      direction: "not_finite",
+    });
+    // No declared band, so the value itself is the only thing to judge.
+    expect(
+      classifyRangeRejection("NOT_A_METRIC", Number.POSITIVE_INFINITY),
+    ).toEqual({ type: "NOT_A_METRIC", direction: "not_finite" });
+  });
+
+  it("passes a metric with no declared band — an absent range is an absent fact", () => {
+    expect(classifyRangeRejection("NOT_A_METRIC", 9_999_999)).toBeNull();
+  });
+});
+
+describe("dropImplausibleMeasurements", () => {
+  it("keeps the in-band rows and returns them untouched", () => {
+    const rows = [
+      { type: "PULSE", value: 60 },
+      { type: "WEIGHT", value: 82.4 },
+    ];
+    const kept = withEvent(() =>
+      dropImplausibleMeasurements("withings", rows, (r) => r),
+    );
+
+    expect(kept).toEqual(rows);
+    expect(kept[0]).toBe(rows[0]);
+    // Nothing was refused, so the event carries no tally at all — absence
+    // reads as absence.
+    expect(tally()).toBeUndefined();
+    expect(event.toJSON().warnings).toBeUndefined();
+  });
+
+  it("drops the out-of-band rows and tallies them on the wide event", () => {
+    const kept = withEvent(() =>
+      dropImplausibleMeasurements(
+        "withings",
+        [
+          { type: "PULSE", value: 60 },
+          { type: "PULSE", value: 55643821.505 },
+          { type: "PULSE", value: 900 },
+          { type: "WEIGHT", value: 0.2 },
+        ],
+        (r) => r,
+      ),
+    );
+
+    expect(kept).toEqual([{ type: "PULSE", value: 60 }]);
+
+    const recorded = tally();
+    expect(recorded).toEqual({
+      total: 3,
+      buckets: [
+        {
+          source: "withings",
+          type: "PULSE",
+          direction: "above_max",
+          count: 2,
+        },
+        {
+          source: "withings",
+          type: "WEIGHT",
+          direction: "below_min",
+          count: 1,
+        },
+      ],
+    });
+  });
+
+  it("keeps the refused readings out of the log line", () => {
+    withEvent(() =>
+      dropImplausibleMeasurements(
+        "withings",
+        [{ type: "PULSE", value: 55643821.505 }],
+        (r) => r,
+      ),
+    );
+
+    // The value is a health reading. The tally says which metric, which
+    // provider and which edge — never the number itself.
+    const serialised = JSON.stringify(event.toJSON());
+    expect(serialised).not.toContain("55643821");
+  });
+
+  it("lifts the event to warn once per provider", () => {
+    withEvent(() => {
+      dropImplausibleMeasurements(
+        "withings",
+        [{ type: "PULSE", value: 900 }],
+        (r) => r,
+      );
+      dropImplausibleMeasurements(
+        "withings",
+        [{ type: "PULSE", value: 901 }],
+        (r) => r,
+      );
+      dropImplausibleMeasurements(
+        "fitbit",
+        [{ type: "PULSE", value: 902 }],
+        (r) => r,
+      );
+    });
+
+    expect(event.getLevel()).toBe("warn");
+    expect(event.toJSON().warnings).toHaveLength(2);
+    expect(event.toJSON().warnings?.[0]).toContain("withings");
+    expect(event.toJSON().warnings?.[1]).toContain("fitbit");
+  });
+
+  it("accumulates across calls instead of overwriting the previous tally", () => {
+    // Fitbit gates once per resource leg, and the reconciler gates once per
+    // row: a tally that replaced rather than merged would report only the
+    // last batch, and a run that dropped hundreds would read as dropping one.
+    withEvent(() => {
+      dropImplausibleMeasurements(
+        "fitbit",
+        [{ type: "PULSE", value: 900 }],
+        (r) => r,
+      );
+      dropImplausibleMeasurements(
+        "fitbit",
+        [
+          { type: "PULSE", value: 901 },
+          { type: "OXYGEN_SATURATION", value: 4 },
+        ],
+        (r) => r,
+      );
+    });
+
+    expect(tally()).toEqual({
+      total: 3,
+      buckets: [
+        {
+          source: "fitbit",
+          type: "OXYGEN_SATURATION",
+          direction: "below_min",
+          count: 1,
+        },
+        { source: "fitbit", type: "PULSE", direction: "above_max", count: 2 },
+      ],
+    });
+  });
+
+  it("still drops outside a wide-event context", () => {
+    // A sync running without an ambient event has nowhere to report. The row
+    // is refused anyway: observability is what is lost, never the guarantee.
+    const kept = dropImplausibleMeasurements(
+      "nightscout",
+      [
+        { type: "BLOOD_GLUCOSE", value: 110 },
+        { type: "BLOOD_GLUCOSE", value: 9_000 },
+      ],
+      (r) => r,
+    );
+    expect(kept).toEqual([{ type: "BLOOD_GLUCOSE", value: 110 }]);
+  });
+});
+
+describe("recordRangeRejections", () => {
+  it("writes nothing when there is nothing to report", () => {
+    withEvent(() => recordRangeRejections("withings", []));
+    expect(tally()).toBeUndefined();
+    expect(event.getLevel()).toBe("info");
+  });
+});
diff --git a/src/lib/measurements/__tests__/reconcile-external-measurement.test.ts b/src/lib/measurements/__tests__/reconcile-external-measurement.test.ts
index 5a2d012fd..9fa5c3cf7 100644
--- a/src/lib/measurements/__tests__/reconcile-external-measurement.test.ts
+++ b/src/lib/measurements/__tests__/reconcile-external-measurement.test.ts
@@ -1,6 +1,9 @@
 import { describe, expect, it, vi } from "vitest";
 
 import type { Prisma } from "@/generated/prisma/client";
+import { eventStorage } from "@/lib/logging/context";
+import { WideEventBuilder } from "@/lib/logging/event-builder";
+import { RANGE_REJECTED_META_KEY } from "../plausibility-gate";
 import {
   reconcileExternalMeasurement,
   type ExternalMeasurementWrite,
@@ -383,3 +386,104 @@ describe("reconcileExternalMeasurement", () => {
     );
   });
 });
+
+/**
+ * The reconciler is the shared write seam for every provider that does not
+ * hold Prisma itself — Polar, Oura, WHOOP, the Withings sleep leg, the Apple
+ * export importer, the batch endpoint. Gating here rather than at each caller
+ * is what makes a provider added tomorrow inherit the refusal instead of
+ * having to remember a check it never knew about.
+ */
+describe("reconcileExternalMeasurement — plausibility gate", () => {
+  it("reconciles a value inside the metric's declared band", async () => {
+    const { tx, measurement } = transaction([]);
+    measurement.create.mockResolvedValue(row({ id: "fresh" }));
+
+    // PULSE is declared 20–300.
+    const verdict = await reconcileExternalMeasurement(
+      tx,
+      desired({ type: "PULSE", value: 112, unit: "bpm", sleepStage: null }),
+    );
+
+    expect(verdict.status).toBe("inserted");
+    expect(measurement.create).toHaveBeenCalledTimes(1);
+  });
+
+  it("refuses an out-of-band value without touching the transaction", async () => {
+    const { tx, measurement } = transaction([]);
+
+    const verdict = await reconcileExternalMeasurement(
+      tx,
+      desired({
+        type: "PULSE",
+        value: 55643821.505,
+        unit: "bpm",
+        sleepStage: null,
+      }),
+    );
+
+    expect(verdict).toEqual({
+      status: "rejected_range",
+      rejection: { type: "PULSE", direction: "above_max" },
+    });
+    // No row written, no row updated, and no savepoint opened — the refusal
+    // happens before the reconciler does any work at all.
+    expect(measurement.create).not.toHaveBeenCalled();
+    expect(measurement.update).not.toHaveBeenCalled();
+    expect(tx.$executeRawUnsafe).not.toHaveBeenCalled();
+  });
+
+  it("refuses rather than overwriting a good stored row", async () => {
+    // The dangerous shape: a live row already holds a real reading, and the
+    // provider re-sends the same identity carrying an impossible value. The
+    // update branch would have rewritten a good value with a bad one.
+    const existing = row({
+      id: "exact-row",
+      type: "PULSE",
+      value: 62,
+      unit: "bpm",
+      sleepStage: null,
+      measuredAt: new Date("2026-07-20T06:30:00.000Z"),
+    });
+    const { tx, measurement } = transaction([existing]);
+
+    const verdict = await reconcileExternalMeasurement(
+      tx,
+      desired({
+        type: "PULSE",
+        value: 55643821.505,
+        unit: "bpm",
+        sleepStage: null,
+      }),
+    );
+
+    expect(verdict.status).toBe("rejected_range");
+    expect(measurement.update).not.toHaveBeenCalled();
+  });
+
+  it("tallies the refusal on the ambient wide event under the provider", async () => {
+    const { tx } = transaction([]);
+    const event = new WideEventBuilder("background");
+
+    await eventStorage.run(event, () =>
+      reconcileExternalMeasurement(
+        tx,
+        desired({
+          type: "PULSE",
+          value: 55643821.505,
+          unit: "bpm",
+          source: "WHOOP",
+          sleepStage: null,
+        }),
+      ),
+    );
+
+    expect(event.toJSON().meta?.[RANGE_REJECTED_META_KEY]).toEqual({
+      total: 1,
+      buckets: [
+        { source: "whoop", type: "PULSE", direction: "above_max", count: 1 },
+      ],
+    });
+    expect(event.getLevel()).toBe("warn");
+  });
+});
diff --git a/src/lib/measurements/import-apple-health-export.ts b/src/lib/measurements/import-apple-health-export.ts
index fc1e9f838..31d0b7dc2 100644
--- a/src/lib/measurements/import-apple-health-export.ts
+++ b/src/lib/measurements/import-apple-health-export.ts
@@ -720,6 +720,16 @@ export async function streamParseExportXml(
           ),
         );
 
+        // The reconciler carries the same plausibility gate the block above
+        // already applied, so this arm means the two disagreed. Tally it under
+        // the same counter the earlier guard uses and count no row.
+        if (verdict.status === "rejected_range") {
+          unknown[`${type}::aggregate_out_of_range`] =
+            (unknown[`${type}::aggregate_out_of_range`] ?? 0) + 1;
+          stat.durationMs += Date.now() - rowStart;
+          continue;
+        }
+
         cumulativeEstimatedDays.add(dayKey);
         cumulativeEstimatedRows += 1;
         if (verdict.status === "inserted") {
diff --git a/src/lib/measurements/plausibility-gate.ts b/src/lib/measurements/plausibility-gate.ts
new file mode 100644
index 000000000..d079f5391
--- /dev/null
+++ b/src/lib/measurements/plausibility-gate.ts
@@ -0,0 +1,192 @@
+/**
+ * The ingest-side half of the app's plausibility domain.
+ *
+ * `VALUE_RANGES` in `@/lib/validations/measurement` is the band the
+ * application declares a metric can occupy — PULSE 20–300, WEIGHT 1–500,
+ * SLEEP_DURATION 0–1440 minutes. Every path a person can drive enforces it:
+ * the measurement routes, the batch endpoint, the CSV and Apple-export
+ * importers, MCP, Telegram. The provider sync writers did not. They took the
+ * number the remote handed over and wrote it, so a decode slip or a glitched
+ * device landed a row the application itself calls impossible, and every mean,
+ * median and personal band downstream digested it as a reading.
+ *
+ * `isPlausibleMetricValue` (in `value-domain.ts`) already answers the question
+ * for the READ side, which is where the damage surfaced first. This module is
+ * the same question asked one layer earlier, at the point of writing, so the
+ * row never lands. The two ends share the one predicate on purpose: a band the
+ * reader hides but the writer accepts is a row that exists and cannot be seen,
+ * which is the worse of the two failures.
+ *
+ * ## Disposition: drop, and say so
+ *
+ * An out-of-band incoming value is DROPPED. Not clamped (a clamp invents a
+ * reading nobody took), not stored-and-flagged (a quarantined row is still a
+ * row, and every consumer would need to learn the flag), not fatal to the
+ * batch (one glitched sample must not strand the sync's other readings behind
+ * a held watermark). Absence reads as absence: the sample is simply not part
+ * of the record.
+ *
+ * A silent drop, though, is indistinguishable from a provider that sent
+ * nothing — so every drop is tallied onto the ambient wide event under
+ * {@link RANGE_REJECTED_META_KEY}, with the metric type, the provider, and the
+ * direction it broke the band in. Counts only. The value itself is a health
+ * reading and never enters the log line; the direction plus the metric's own
+ * declared bound is what a diagnosis actually needs.
+ *
+ * ## Why meta and not `action`
+ *
+ * A wide event holds exactly ONE action, and `setAction` replaces it. A drop
+ * happens in the middle of a sync whose action is decided at the end, so an
+ * `action` set here is either clobbered a moment later (the drop vanishes) or,
+ * on the paths that annotate no terminal action, relabels an otherwise
+ * successful sync as a rejection. Meta accumulates, survives every later
+ * `setAction`, and is the shape a dashboard can sum. One warning per provider
+ * per event rides along so the drop also lifts the event's level to `warn`
+ * rather than hiding inside a green line.
+ */
+import { annotate, getEvent } from "@/lib/logging/context";
+import { isPlausibleMetricValue } from "@/lib/measurements/value-domain";
+import { VALUE_RANGES } from "@/lib/validations/measurement";
+
+/**
+ * Stable meta key the tally lands under. Pinned so dashboards can count it
+ * across providers without knowing which one wrote the event.
+ */
+export const RANGE_REJECTED_META_KEY = "measurement.range_rejected";
+
+/** Which edge of the declared band the value broke, or that it was not a number. */
+export type RangeRejectionDirection = "below_min" | "above_max" | "not_finite";
+
+/** One refused reading, reduced to the facts that carry no health value. */
+export interface MeasurementRangeRejection {
+  /** `MeasurementType` of the refused reading. */
+  type: string;
+  direction: RangeRejectionDirection;
+}
+
+/** One (provider, metric, direction) cell of the event's running tally. */
+export interface RangeRejectionBucket {
+  source: string;
+  type: string;
+  direction: RangeRejectionDirection;
+  count: number;
+}
+
+/** The pinned meta shape written under {@link RANGE_REJECTED_META_KEY}. */
+export interface RangeRejectionTally {
+  total: number;
+  buckets: RangeRejectionBucket[];
+}
+
+/**
+ * Classify a candidate write against the metric's declared band.
+ *
+ * Returns `null` when the value may be written. A type with no declared range
+ * passes — an absent range is an absent fact, never a guessed one, and the
+ * same asymmetry the read side uses (`isPlausibleMetricValue`). A non-finite
+ * value never passes, whatever the type: NaN is not an input to anything.
+ */
+export function classifyRangeRejection(
+  type: string,
+  value: number,
+): MeasurementRangeRejection | null {
+  if (isPlausibleMetricValue(type, value)) return null;
+  if (!Number.isFinite(value)) return { type, direction: "not_finite" };
+  const range = VALUE_RANGES[type];
+  // `isPlausibleMetricValue` only answers false for a finite value when the
+  // type HAS a range, so the lookup cannot miss here.
+  return {
+    type,
+    direction: value < range!.min ? "below_min" : "above_max",
+  };
+}
+
+function readTally(): RangeRejectionTally {
+  const existing = getEvent()?.toJSON().meta?.[RANGE_REJECTED_META_KEY];
+  if (
+    existing !== null &&
+    typeof existing === "object" &&
+    Array.isArray((existing as RangeRejectionTally).buckets)
+  ) {
+    return existing as RangeRejectionTally;
+  }
+  return { total: 0, buckets: [] };
+}
+
+/**
+ * Fold refusals onto the ambient wide event.
+ *
+ * Merges with whatever the event already carries, so a provider that gates
+ * several batches (or reconciles row by row) accumulates instead of
+ * overwriting. Outside a wide-event context this is a no-op — the caller still
+ * drops the row, it simply has nowhere to report.
+ */
+export function recordRangeRejections(
+  source: string,
+  rejections: readonly MeasurementRangeRejection[],
+): void {
+  if (rejections.length === 0) return;
+  const evt = getEvent();
+  if (!evt) return;
+
+  const tally = readTally();
+  const firstForSource = !tally.buckets.some((b) => b.source === source);
+
+  for (const rejection of rejections) {
+    const bucket = tally.buckets.find(
+      (b) =>
+        b.source === source &&
+        b.type === rejection.type &&
+        b.direction === rejection.direction,
+    );
+    if (bucket) bucket.count += 1;
+    else
+      tally.buckets.push({
+        source,
+        type: rejection.type,
+        direction: rejection.direction,
+        count: 1,
+      });
+    tally.total += 1;
+  }
+
+  tally.buckets.sort(
+    (a, b) =>
+      a.source.localeCompare(b.source) ||
+      a.type.localeCompare(b.type) ||
+      a.direction.localeCompare(b.direction),
+  );
+
+  annotate({ meta: { [RANGE_REJECTED_META_KEY]: tally } });
+  if (firstForSource) {
+    evt.addWarning(
+      `${source}: refused reading(s) outside the declared plausibility range — ` +
+        `dropped, see meta.${RANGE_REJECTED_META_KEY}`,
+    );
+  }
+}
+
+/**
+ * Keep the rows whose value sits inside the metric's declared band; tally and
+ * drop the rest.
+ *
+ * `select` exists because the sync writers carry their own row shapes and none
+ * of them is worth reshaping at the call site: the gate reads the two fields
+ * it needs and returns the caller's own objects untouched.
+ */
+export function dropImplausibleMeasurements<T>(
+  source: string,
+  rows: readonly T[],
+  select: (row: T) => { type: string; value: number },
+): T[] {
+  const kept: T[] = [];
+  const rejections: MeasurementRangeRejection[] = [];
+  for (const row of rows) {
+    const { type, value } = select(row);
+    const rejection = classifyRangeRejection(type, value);
+    if (rejection) rejections.push(rejection);
+    else kept.push(row);
+  }
+  recordRangeRejections(source, rejections);
+  return kept;
+}
diff --git a/src/lib/measurements/reconcile-external-measurement.ts b/src/lib/measurements/reconcile-external-measurement.ts
index 031dda441..d4fb50784 100644
--- a/src/lib/measurements/reconcile-external-measurement.ts
+++ b/src/lib/measurements/reconcile-external-measurement.ts
@@ -4,6 +4,11 @@ import {
   type MeasurementAggregationProvenance,
   type Prisma as PrismaTypes,
 } from "@/generated/prisma/client";
+import {
+  classifyRangeRejection,
+  recordRangeRejections,
+  type MeasurementRangeRejection,
+} from "@/lib/measurements/plausibility-gate";
 
 const SAVEPOINT = "measurement_identity_reconcile";
 
@@ -50,6 +55,18 @@ export type MeasurementReconciliationVerdict =
   | {
       status: "failed";
       error: { message: string; code?: string };
+    }
+  | {
+      /**
+       * The incoming value sits outside the metric's declared plausibility
+       * band, so nothing was written and no existing row was touched. Not a
+       * failure: the write was refused on purpose and the caller must carry
+       * on with the rest of its batch. Every caller narrows on this status
+       * because it carries no `row` — the compiler is what makes a future
+       * provider handle it rather than dereference a row that does not exist.
+       */
+      status: "rejected_range";
+      rejection: MeasurementRangeRejection;
     };
 
 export type FailedMeasurementReconciliation = Extract<
@@ -219,12 +236,26 @@ async function retireCollision(
  * advisory lock serializes cooperative writers sharing either identity, and a
  * savepoint turns a non-benign database error into a structured `failed`
  * verdict without poisoning the caller's surrounding transaction.
+ *
+ * This is also the plausibility gate for every provider that reconciles rather
+ * than writing Prisma directly. Gating HERE rather than at each caller is the
+ * point: a provider added tomorrow inherits the refusal by construction, and
+ * cannot land an impossible reading by forgetting a check it never knew about.
  */
 export async function reconcileExternalMeasurement(
   tx: PrismaTypes.TransactionClient,
   desired: ExternalMeasurementWrite,
   options: MeasurementReconciliationOptions = {},
 ): Promise<MeasurementReconciliationVerdict> {
+  const rejection = classifyRangeRejection(desired.type, desired.value);
+  if (rejection) {
+    recordRangeRejections(
+      desired.source ? String(desired.source).toLowerCase() : "external",
+      [rejection],
+    );
+    return { status: "rejected_range", rejection };
+  }
+
   await tx.$executeRawUnsafe(`SAVEPOINT ${SAVEPOINT}`);
 
   try {
diff --git a/src/lib/nightscout/__tests__/sync-plausibility.test.ts b/src/lib/nightscout/__tests__/sync-plausibility.test.ts
new file mode 100644
index 000000000..24c023eb7
--- /dev/null
+++ b/src/lib/nightscout/__tests__/sync-plausibility.test.ts
@@ -0,0 +1,115 @@
+/**
+ * The Nightscout pull writes BLOOD_GLUCOSE rows directly, so it does not
+ * inherit the shared reconciler's gate. It is also first-write-wins: a row
+ * that lands here is immutable, so an impossible sample never corrects itself
+ * on a later pass. These cases pin the refusal and its annotation.
+ */
+import { beforeEach, describe, expect, it, vi } from "vitest";
+
+const {
+  upsertMock,
+  findManyMock,
+  emitArrivalsMock,
+  recomputeMock,
+  invalidateMock,
+} = vi.hoisted(() => ({
+  upsertMock: vi.fn(),
+  findManyMock: vi.fn(),
+  emitArrivalsMock: vi.fn(),
+  recomputeMock: vi.fn(),
+  invalidateMock: vi.fn(),
+}));
+
+vi.mock("@/lib/db", () => ({
+  prisma: { measurement: { upsert: upsertMock, findMany: findManyMock } },
+}));
+
+vi.mock("@/lib/arrivals/measurement-emit", () => ({
+  emitInsertedMeasurementArrivals: emitArrivalsMock,
+}));
+
+vi.mock("@/lib/rollups/measurement-rollups", () => ({
+  collapseToTypeDayKeys: (rows: Array<{ type: string; measuredAt: Date }>) =>
+    rows.map((r) => ({ type: r.type, measuredAt: r.measuredAt })),
+  recomputeBucketsForMeasurement: recomputeMock,
+}));
+
+vi.mock("@/lib/insights/comprehensive-generate", () => ({
+  invalidateStatusInsightsForTypes: invalidateMock,
+}));
+
+import { eventStorage } from "@/lib/logging/context";
+import { WideEventBuilder } from "@/lib/logging/event-builder";
+import { RANGE_REJECTED_META_KEY } from "@/lib/measurements/plausibility-gate";
+import type { RangeRejectionTally } from "@/lib/measurements/plausibility-gate";
+
+import { upsertNightscoutEntries } from "../sync";
+
+/** BLOOD_GLUCOSE is declared 20–800 mg/dL. */
+const REAL_ENTRY = { id: "abc", sgv: 112, date: 1718000000000 };
+/** A sensor fault, or an mg/dL field carrying something else entirely. */
+const IMPOSSIBLE_ENTRY = { id: "def", sgv: 41_600, date: 1718000300000 };
+
+let event: WideEventBuilder;
+
+function tally(): RangeRejectionTally | undefined {
+  return event.toJSON().meta?.[RANGE_REJECTED_META_KEY] as
+    RangeRejectionTally | undefined;
+}
+
+beforeEach(() => {
+  vi.clearAllMocks();
+  findManyMock.mockResolvedValue([]);
+  upsertMock.mockResolvedValue({ id: "row-1" });
+  emitArrivalsMock.mockResolvedValue(undefined);
+  recomputeMock.mockResolvedValue(undefined);
+  invalidateMock.mockResolvedValue(undefined);
+  event = new WideEventBuilder("background");
+});
+
+describe("upsertNightscoutEntries — plausibility gate", () => {
+  it("writes a reading inside the metric's declared band", async () => {
+    const result = await eventStorage.run(event, () =>
+      upsertNightscoutEntries("user-1", [REAL_ENTRY]),
+    );
+
+    expect(result).toEqual({ inserted: 1, failedRows: 0 });
+    expect(upsertMock).toHaveBeenCalledTimes(1);
+    expect(tally()).toBeUndefined();
+  });
+
+  it("refuses an out-of-band reading and annotates the drop", async () => {
+    const result = await eventStorage.run(event, () =>
+      upsertNightscoutEntries("user-1", [REAL_ENTRY, IMPOSSIBLE_ENTRY]),
+    );
+
+    expect(result).toEqual({ inserted: 1, failedRows: 0 });
+    expect(upsertMock).toHaveBeenCalledTimes(1);
+    expect(
+      (upsertMock.mock.calls[0]![0] as { create: { value: number } }).create
+        .value,
+    ).toBe(112);
+    expect(tally()).toEqual({
+      total: 1,
+      buckets: [
+        {
+          source: "nightscout",
+          type: "BLOOD_GLUCOSE",
+          direction: "above_max",
+          count: 1,
+        },
+      ],
+    });
+    expect(event.getLevel()).toBe("warn");
+  });
+
+  it("writes nothing when the whole page is out of band", async () => {
+    const result = await eventStorage.run(event, () =>
+      upsertNightscoutEntries("user-1", [IMPOSSIBLE_ENTRY]),
+    );
+
+    expect(result).toEqual({ inserted: 0, failedRows: 0 });
+    expect(upsertMock).not.toHaveBeenCalled();
+    expect(tally()?.total).toBe(1);
+  });
+});
diff --git a/src/lib/nightscout/sync.ts b/src/lib/nightscout/sync.ts
index adef2de55..f92104550 100644
--- a/src/lib/nightscout/sync.ts
+++ b/src/lib/nightscout/sync.ts
@@ -22,6 +22,7 @@
 import { prisma } from "@/lib/db";
 import type { MeasurementType } from "@/generated/prisma/client";
 import { getEvent } from "@/lib/logging/context";
+import { dropImplausibleMeasurements } from "@/lib/measurements/plausibility-gate";
 import {
   markSyncFailureRecorded,
   recordSyncFailure,
@@ -221,7 +222,16 @@ export async function upsertNightscoutEntries(
   if (entries.length === 0) return { inserted: 0, failedRows: 0 };
 
   const type = "BLOOD_GLUCOSE" as MeasurementType;
-  const mapped = entries.map((entry) => mapSgvEntryToMeasurement(entry));
+  // A CGM that reports a glucose the app's own band calls impossible is
+  // reporting a sensor fault or a unit slip, not a reading. Drop it: the row
+  // would otherwise be immutable (first-write-wins), so an impossible sample
+  // that lands here never corrects itself on a later pass.
+  const mapped = dropImplausibleMeasurements(
+    "nightscout",
+    entries.map((entry) => mapSgvEntryToMeasurement(entry)),
+    (m) => ({ type, value: m.value }),
+  );
+  if (mapped.length === 0) return { inserted: 0, failedRows: 0 };
 
   // Probe which keys already exist so a re-confirmed reading (the upsert's
   // no-op update branch) is not miscounted as a fresh import and the arrival
diff --git a/src/lib/oura/sync.ts b/src/lib/oura/sync.ts
index 4dfefaa83..aa11c88f5 100644
--- a/src/lib/oura/sync.ts
+++ b/src/lib/oura/sync.ts
@@ -575,6 +575,10 @@ export async function upsertOuraMeasurements(
   for (let index = 0; index < readings.length; index++) {
     const reading = readings[index]!;
     const verdict = verdicts[index]!;
+    // Refused by the plausibility gate: nothing was written and nothing was
+    // touched, so the reading counts as neither imported nor a rollup dirt
+    // mark. The drop is already on the wide event.
+    if (verdict.status === "rejected_range") continue;
     for (const dirty of verdict.dirtyIdentities ?? []) {
       touched.push(dirty);
     }
diff --git a/src/lib/polar/sync.ts b/src/lib/polar/sync.ts
index 17b6505c5..8b44200fe 100644
--- a/src/lib/polar/sync.ts
+++ b/src/lib/polar/sync.ts
@@ -364,6 +364,10 @@ export async function upsertPolarMeasurements(
   for (let index = 0; index < readings.length; index++) {
     const reading = readings[index]!;
     const verdict = verdicts[index]!;
+    // Refused by the plausibility gate: nothing was written and nothing was
+    // touched, so the reading counts as neither imported nor a rollup dirt
+    // mark. The drop is already on the wide event.
+    if (verdict.status === "rejected_range") continue;
     for (const dirty of verdict.dirtyIdentities ?? []) {
       touched.push(dirty);
     }
diff --git a/src/lib/whoop/sync-core.ts b/src/lib/whoop/sync-core.ts
index 2e2e41212..464ab6718 100644
--- a/src/lib/whoop/sync-core.ts
+++ b/src/lib/whoop/sync-core.ts
@@ -480,6 +480,10 @@ export async function upsertWhoopMeasurements(
     for (let index = chunkStart; index < chunkEnd; index++) {
       const reading = readings[index]!;
       const verdict = verdicts[index - chunkStart]!;
+      // Refused by the plausibility gate: nothing was written and nothing was
+      // touched, so the reading counts as neither imported nor a rollup dirt
+      // mark. The drop is already on the wide event.
+      if (verdict.status === "rejected_range") continue;
       for (const dirty of verdict.dirtyIdentities ?? []) {
         touched.push(dirty);
         touchedTypes.add(dirty.type);
diff --git a/src/lib/withings/__tests__/sync-plausibility.test.ts b/src/lib/withings/__tests__/sync-plausibility.test.ts
new file mode 100644
index 000000000..8aa7572c7
--- /dev/null
+++ b/src/lib/withings/__tests__/sync-plausibility.test.ts
@@ -0,0 +1,261 @@
+/**
+ * Withings writes the measurement table directly — the measures leg row by
+ * row, the activity leg as a planned batch — so neither inherits the shared
+ * reconciler's gate. These cases pin that both legs now clear the app's own
+ * declared plausibility band before anything reaches Prisma, and that a
+ * refusal lands on the wide event rather than vanishing.
+ *
+ * The measures leg matters twice over: it does not only insert, it OVERWRITES
+ * the stored value of an existing slot, so an ungated impossible reading could
+ * replace a real one.
+ */
+import { afterEach, beforeEach, describe, expect, it, vi } from "vitest";
+
+vi.mock("@/lib/db", () => ({
+  prisma: {
+    measurement: {
+      findFirst: vi.fn(),
+      findMany: vi.fn(),
+      create: vi.fn(),
+      createMany: vi.fn(),
+      update: vi.fn(),
+    },
+    withingsConnection: {
+      findUnique: vi.fn(),
+      update: vi.fn(),
+    },
+    $queryRaw: vi.fn(),
+    $transaction: vi.fn((ops: unknown) =>
+      Array.isArray(ops) ? Promise.all(ops) : Promise.resolve(),
+    ),
+  },
+}));
+
+vi.mock("@/lib/crypto", () => ({
+  decrypt: vi.fn((v: string) => v),
+  encrypt: vi.fn((v: string) => v),
+}));
+
+vi.mock("../client", async () => {
+  const actual = await vi.importActual<typeof import("../client")>("../client");
+  return {
+    ...actual,
+    fetchMeasurements: vi.fn(),
+    refreshAccessToken: vi.fn(),
+    subscribeWebhook: vi.fn(),
+  };
+});
+
+vi.mock("../credentials", () => ({
+  getUserWithingsCredentials: vi.fn(async () => ({
+    clientId: "cid",
+    clientSecret: "secret",
+  })),
+}));
+
+vi.mock("@/lib/integrations/status", () => ({
+  isReauthRequired: vi.fn().mockResolvedValue(false),
+  parkIntegrationAtReauth: vi.fn(),
+  recordSyncFailure: vi.fn().mockResolvedValue(undefined),
+  recordSyncSuccess: vi.fn().mockResolvedValue(undefined),
+}));
+
+vi.mock("@/lib/rollups/measurement-rollups", async () => {
+  const actual = await vi.importActual<
+    typeof import("@/lib/rollups/measurement-rollups")
+  >("@/lib/rollups/measurement-rollups");
+  return {
+    ...actual,
+    recomputeBucketsForMeasurement: vi.fn().mockResolvedValue(undefined),
+  };
+});
+
+vi.mock("@/lib/insights/comprehensive-generate", () => ({
+  invalidateStatusInsightsForTypes: vi.fn().mockResolvedValue(undefined),
+}));
+
+vi.mock("@/lib/cache/invalidate", () => ({
+  invalidateUserDashboardSnapshot: vi.fn(),
+}));
+
+import { prisma } from "@/lib/db";
+import { eventStorage } from "@/lib/logging/context";
+import { WideEventBuilder } from "@/lib/logging/event-builder";
+import { RANGE_REJECTED_META_KEY } from "@/lib/measurements/plausibility-gate";
+import type { RangeRejectionTally } from "@/lib/measurements/plausibility-gate";
+
+import { fetchMeasurements } from "../client";
+import { syncUserActivity } from "../sync-activity";
+import { syncUserMeasurements } from "../sync";
+
+let event: WideEventBuilder;
+
+function tally(): RangeRejectionTally | undefined {
+  return event.toJSON().meta?.[RANGE_REJECTED_META_KEY] as
+    RangeRejectionTally | undefined;
+}
+
+function installActivityFetch(
+  entries: Array<{ date: string; steps?: number; distance?: number }>,
+) {
+  vi.stubGlobal(
+    "fetch",
+    vi.fn(async () => ({
+      status: 200,
+      headers: new Headers(),
+      json: async () => ({
+        status: 0,
+        body: { activities: entries, more: false, offset: 0 },
+      }),
+    })),
+  );
+}
+
+beforeEach(() => {
+  vi.clearAllMocks();
+  event = new WideEventBuilder("background");
+  vi.mocked(prisma.withingsConnection.findUnique).mockResolvedValue({
+    id: "conn-1",
+    withingsUserId: "wu-1",
+    accessToken: "enc-access",
+    refreshToken: "enc-refresh",
+    scope: "user.metrics,user.activity",
+    tokenExpiresAt: new Date(Date.now() + 60 * 60 * 1000),
+    lastSyncedAt: new Date(Date.now() - 60 * 60 * 1000),
+  } as never);
+  vi.mocked(prisma.withingsConnection.update).mockResolvedValue({} as never);
+  vi.mocked(prisma.measurement.findFirst).mockResolvedValue(null);
+  vi.mocked(prisma.measurement.findMany).mockResolvedValue([] as never);
+  vi.mocked(prisma.measurement.create).mockResolvedValue({
+    id: "m-1",
+    type: "PULSE",
+    measuredAt: new Date(),
+  } as never);
+  vi.mocked(prisma.measurement.createMany).mockResolvedValue({
+    count: 1,
+  } as never);
+  vi.mocked(prisma.measurement.update).mockResolvedValue({} as never);
+});
+
+afterEach(() => {
+  vi.unstubAllGlobals();
+});
+
+describe("syncUserMeasurements — plausibility gate", () => {
+  it("writes a reading inside the metric's declared band", async () => {
+    vi.mocked(fetchMeasurements).mockResolvedValue([
+      {
+        type: "PULSE",
+        value: 112,
+        measuredAt: new Date("2026-05-16T08:00:00Z"),
+      },
+    ] as never);
+
+    const result = await eventStorage.run(event, () =>
+      syncUserMeasurements("user-1"),
+    );
+
+    expect(result).toEqual({ imported: 1, failed: false });
+    expect(prisma.measurement.create).toHaveBeenCalledTimes(1);
+    expect(tally()).toBeUndefined();
+  });
+
+  it("refuses an out-of-band reading and annotates the drop", async () => {
+    vi.mocked(fetchMeasurements).mockResolvedValue([
+      {
+        type: "PULSE",
+        value: 112,
+        measuredAt: new Date("2026-05-16T08:00:00Z"),
+      },
+      {
+        type: "PULSE",
+        value: 55643821.505,
+        measuredAt: new Date("2026-05-16T08:05:00Z"),
+      },
+    ] as never);
+
+    const result = await eventStorage.run(event, () =>
+      syncUserMeasurements("user-1"),
+    );
+
+    // Only the real reading landed, and the run is still a clean success: one
+    // impossible sample must not strand the rest behind a held watermark.
+    expect(result).toEqual({ imported: 1, failed: false });
+    expect(prisma.measurement.create).toHaveBeenCalledTimes(1);
+    expect(tally()).toEqual({
+      total: 1,
+      buckets: [
+        { source: "withings", type: "PULSE", direction: "above_max", count: 1 },
+      ],
+    });
+    expect(event.getLevel()).toBe("warn");
+  });
+
+  it("does not overwrite a stored reading with an out-of-band one", async () => {
+    // The slot already holds a real pulse; the provider re-sends the same
+    // instant carrying a decode slip. Ungated, the update branch rewrote the
+    // good value with the bad one.
+    vi.mocked(prisma.measurement.findFirst).mockResolvedValue({
+      id: "existing-row",
+    } as never);
+    vi.mocked(fetchMeasurements).mockResolvedValue([
+      {
+        type: "PULSE",
+        value: 55643821.505,
+        measuredAt: new Date("2026-05-16T08:00:00Z"),
+      },
+    ] as never);
+
+    const result = await eventStorage.run(event, () =>
+      syncUserMeasurements("user-1"),
+    );
+
+    expect(result).toEqual({ imported: 0, failed: false });
+    expect(prisma.measurement.update).not.toHaveBeenCalled();
+    expect(tally()?.total).toBe(1);
+  });
+});
+
+describe("syncUserActivity — plausibility gate", () => {
+  it("plans a daily total inside the metric's declared band", async () => {
+    installActivityFetch([{ date: "2026-05-16", steps: 8_400 }]);
+
+    const imported = await eventStorage.run(event, () =>
+      syncUserActivity("user-1"),
+    );
+
+    expect(imported).toBe(1);
+    expect(prisma.measurement.createMany).toHaveBeenCalledTimes(1);
+    expect(tally()).toBeUndefined();
+  });
+
+  it("refuses an out-of-band daily total and annotates the drop", async () => {
+    // ACTIVITY_STEPS is declared 0–200 000.
+    installActivityFetch([
+      { date: "2026-05-16", steps: 8_400 },
+      { date: "2026-05-17", steps: 9_900_000 },
+    ]);
+
+    const imported = await eventStorage.run(event, () =>
+      syncUserActivity("user-1"),
+    );
+
+    expect(imported).toBe(1);
+    const created = vi.mocked(prisma.measurement.createMany).mock.calls[0]?.[0]
+      ?.data as Array<{ value: number }>;
+    expect(created).toHaveLength(1);
+    expect(created[0]!.value).toBe(8_400);
+    expect(tally()).toEqual({
+      total: 1,
+      buckets: [
+        {
+          source: "withings",
+          type: "ACTIVITY_STEPS",
+          direction: "above_max",
+          count: 1,
+        },
+      ],
+    });
+    expect(event.getLevel()).toBe("warn");
+  });
+});
diff --git a/src/lib/withings/sync-activity.ts b/src/lib/withings/sync-activity.ts
index 869fb1d18..a4a0098b2 100644
--- a/src/lib/withings/sync-activity.ts
+++ b/src/lib/withings/sync-activity.ts
@@ -39,6 +39,7 @@ import { prisma } from "@/lib/db";
 import { getEvent } from "@/lib/logging/context";
 import { safeFetch } from "@/lib/safe-fetch";
 import { getUnitForType } from "@/lib/validations/measurement";
+import { dropImplausibleMeasurements } from "@/lib/measurements/plausibility-gate";
 import {
   collapseToTypeDayKeys,
   recomputeBucketsForMeasurement,
@@ -342,7 +343,7 @@ export async function syncUserActivity(
     value: number;
     externalId: string;
   }
-  const planned: PlannedRow[] = [];
+  const collected: PlannedRow[] = [];
   for (const entry of entries) {
     if (!entry.date) continue;
     const measuredAt = activityMeasuredAt(entry.date);
@@ -352,7 +353,7 @@ export async function syncUserActivity(
       // valid (a day of rest still records 0 steps). Only skip
       // undefined / null / NaN.
       if (raw == null || !Number.isFinite(raw)) continue;
-      planned.push({
+      collected.push({
         type,
         measuredAt,
         value: raw,
@@ -361,6 +362,15 @@ export async function syncUserActivity(
     }
   }
 
+  // A daily-activity total the app's own band calls impossible (a step count
+  // past 200 000, a negative distance) is a provider artefact, not a day. Drop
+  // it before the plan is read against the table, so it can neither insert a
+  // fresh impossible slot nor overwrite a good stored one.
+  const planned = dropImplausibleMeasurements("withings", collected, (p) => ({
+    type: p.type,
+    value: p.value,
+  }));
+
   if (planned.length > 0) {
     const slotKey = (type: MeasurementType, measuredAt: Date) =>
       `${type}|${measuredAt.getTime()}`;
diff --git a/src/lib/withings/sync-sleep.ts b/src/lib/withings/sync-sleep.ts
index 840b5d8d3..65099ea7c 100644
--- a/src/lib/withings/sync-sleep.ts
+++ b/src/lib/withings/sync-sleep.ts
@@ -474,6 +474,9 @@ export async function syncUserSleep(
           if (verdict.status === "failed") {
             throw new MeasurementReconciliationError(verdict);
           }
+          // Refused by the plausibility gate: nothing was written and nothing
+          // was touched. The drop is already on the wide event.
+          if (verdict.status === "rejected_range") continue;
           for (const dirty of verdict.dirtyIdentities ?? []) {
             touched.push(dirty);
           }
@@ -542,6 +545,9 @@ export async function syncUserSleep(
             if (verdict.status === "failed") {
               throw new MeasurementReconciliationError(verdict);
             }
+            // Refused by the plausibility gate: nothing was written and
+            // nothing was touched. The drop is already on the wide event.
+            if (verdict.status === "rejected_range") continue;
             for (const dirty of verdict.dirtyIdentities ?? []) {
               touched.push(dirty);
             }
diff --git a/src/lib/withings/sync.ts b/src/lib/withings/sync.ts
index fcee1cc7c..d500a030c 100644
--- a/src/lib/withings/sync.ts
+++ b/src/lib/withings/sync.ts
@@ -15,6 +15,7 @@ import { getUserWithingsCredentials } from "./credentials";
 import { annotate, getEvent } from "@/lib/logging/context";
 import { isP2002 } from "@/lib/prisma-errors";
 import { emitInsertedMeasurementArrivals } from "@/lib/arrivals/measurement-emit";
+import { dropImplausibleMeasurements } from "@/lib/measurements/plausibility-gate";
 import {
   isReauthRequired,
   recordSyncFailure,
@@ -229,14 +230,24 @@ export async function syncUserMeasurements(
         )
       : new Date(Date.now() - 30 * 24 * 60 * 60 * 1000); // 30 days back
 
-  let measures: Awaited<ReturnType<typeof fetchMeasurements>>;
+  let fetched: Awaited<ReturnType<typeof fetchMeasurements>>;
   try {
-    measures = await fetchMeasurements(tokenInfo.accessToken, startDate);
+    fetched = await fetchMeasurements(tokenInfo.accessToken, startDate);
   } catch (err) {
     await recordWithingsSyncFailure(userId, err, WITHINGS_LEG_MEASURES);
     throw err;
   }
 
+  // The app's own plausibility band applies to a provider reading exactly as
+  // it applies to a typed one. A remote that reports a pulse in the millions
+  // is reporting a decode slip, not a heartbeat, and the row would poison
+  // every mean and personal band derived from it. Refuse it here, before the
+  // update branch below can overwrite a good stored value with it.
+  const measures = dropImplausibleMeasurements("withings", fetched, (m) => ({
+    type: m.type,
+    value: m.value,
+  }));
+
   let imported = 0;
   // v1.4.39.1 — track every (type, measuredAt) we touched so the
   // persistent rollup tier can be re-folded at the end of the sync.