diff --git a/docs/api/openapi.yaml b/docs/api/openapi.yaml
index 7938a5939..eb8909465 100644
--- a/docs/api/openapi.yaml
+++ b/docs/api/openapi.yaml
@@ -491,7 +491,15 @@ paths:
         "401": *a1
         "403": *a3
         "409": *a7
-        "422": *a4
+        "422":
+          description: "The whole batch was refused and nothing was written: more than 500 entries (`meta.errorCode` =
+            `cycle.bulk.too_large`, no issue list), or the body failed validation (`meta.errorCode` =
+            `cycle.bulk.invalid`, every issue under `details.issues`). Both are permanent for the batch as sent. A
+            problem with one entry never refuses the batch; it comes back as that entry's `skipped` status."
+          content:
+            application/json:
+              schema:
+                $ref: "#/components/schemas/ErrorEnvelope"
         "429": *a5
   /api/cycle/period:
     post:
@@ -16810,8 +16818,11 @@ paths:
         Upsert key is (day, nutrient): a re-post replaces the stored total and reports `updated`. Post the current +
         previous local day per sync; backfill 30 days on first enable. Max 500 entries; Idempotency-Key supported; rate
         limit 60/min. Per-entry skips (`unit_mismatch` | `value_out_of_range` | `day_invalid`) are terminal — log and
-        drop, do not retry the entry. Behind the opt-in `nutrients` module (403 `module.disabled` when off). Query
-        HealthKit with exactly the listed unit per code (`ug` = microgram; do not convert to IU):
+        drop, do not retry the entry. `upsert_failed` is not one of them: it is a server-side write failure for the
+        entry's whole write group, nothing was stored, and the entry should be sent again on a later sync; a response
+        carrying it is `Cache-Control: no-store` and is not replayed for its `Idempotency-Key`. Behind the opt-in
+        `nutrients` module (403 `module.disabled` when off). Query HealthKit with exactly the listed unit per code (`ug`
+        = microgram; do not convert to IU):
 
 
         | code | HealthKit identifier | unit |
@@ -30760,6 +30771,12 @@ components:
                   externalId:
                     type: string
                   reason:
+                    description: "On a `skipped` entry. `unstable_external_id` is a verdict on the entry and terminal: resending it
+                      unchanged is skipped again. `constraint` means the write met a database constraint (a key another
+                      write claimed at the same moment, or a reference that no longer exists). `upsert_failed` is a
+                      write failure on the server's side: nothing was stored, the entry should be sent again, and the
+                      response carrying it is `Cache-Control: no-store`, so a retry under the same `Idempotency-Key`
+                      writes again rather than replaying the failure."
                     type: string
                 required:
                   - index
@@ -64336,7 +64353,12 @@ components:
         - status
       additionalProperties: false
       description: "Per-entry ingest outcome. skipped reasons: unit_mismatch | value_out_of_range | day_invalid |
-        upsert_failed. Log and drop a skipped entry — do not retry it."
+        upsert_failed. The first three are verdicts on the entry itself and terminal: resending it unchanged is skipped
+        again, so log and drop it. `upsert_failed` is a write failure on the server's side, not a verdict on the entry:
+        nothing was stored for it, and it should be sent again on a later sync. A write failure takes out the whole
+        group it was written in (every new day total, or every existing one), so several entries can carry it at once. A
+        response carrying it is marked `Cache-Control: no-store` and is not kept for `Idempotency-Key` replay, so the
+        retry writes again rather than replaying the failure."
     NutrientOverviewEnvelope:
       type: object
       properties:
diff --git a/src/app/api/cycle/day-logs/bulk/__tests__/route.test.ts b/src/app/api/cycle/day-logs/bulk/__tests__/route.test.ts
index d75526e8e..8b59f8fec 100644
--- a/src/app/api/cycle/day-logs/bulk/__tests__/route.test.ts
+++ b/src/app/api/cycle/day-logs/bulk/__tests__/route.test.ts
@@ -203,3 +203,82 @@ describe("POST /api/cycle/day-logs/bulk — external-id stability floor", () =>
     expect(upsertCycleDayLog).toHaveBeenCalledTimes(1);
   });
 });
+
+describe("POST /api/cycle/day-logs/bulk — refusal codes", () => {
+  /**
+   * The native client files rows under a permanent-refusal register as soon
+   * as it can tell the refusal is permanent. It can only tell from the code,
+   * so both whole-batch 422s are pinned here, with the envelope they ride.
+   */
+  async function refusal(body: unknown) {
+    const res = await POST(postReq(body));
+    return {
+      status: res.status,
+      body: (await res.json()) as {
+        data: null;
+        error: string;
+        details?: { issues: unknown[] };
+        meta?: { errorCode?: string };
+      },
+    };
+  }
+
+  it("names a malformed batch cycle.bulk.invalid, with every issue listed", async () => {
+    const { status, body } = await refusal({
+      entries: [{ date: "not-a-day" }, { date: "2026-13-40" }],
+    });
+    expect(status).toBe(422);
+    expect(body.data).toBeNull();
+    expect(body.meta?.errorCode).toBe("cycle.bulk.invalid");
+    expect(body.details?.issues.length).toBeGreaterThanOrEqual(2);
+    expect(upsertCycleDayLog).not.toHaveBeenCalled();
+  });
+
+  it("names a batch with no entries array cycle.bulk.invalid", async () => {
+    const { status, body } = await refusal({});
+    expect(status).toBe(422);
+    expect(body.meta?.errorCode).toBe("cycle.bulk.invalid");
+  });
+
+  it("names an over-cap batch cycle.bulk.too_large", async () => {
+    const entries = Array.from({ length: 501 }, (_, i) => ({
+      date: "2026-01-01",
+      externalId: `e${i}`,
+    }));
+    const { status, body } = await refusal({ entries });
+    expect(status).toBe(422);
+    expect(body.meta?.errorCode).toBe("cycle.bulk.too_large");
+  });
+});
+
+describe("POST /api/cycle/day-logs/bulk — a failed write is not cached", () => {
+  it("marks a response carrying upsert_failed no-store and leaves a clean one alone", async () => {
+    vi.mocked(upsertCycleDayLog).mockRejectedValueOnce(new Error("reset"));
+    const failed = await POST(
+      postReq({
+        entries: [{ date: "2026-01-01", loggedAt: "2026-01-01T08:00:00.000Z" }],
+      }),
+    );
+    expect(failed.status).toBe(200);
+    const body = (await failed.json()) as {
+      data: { entries: Array<{ status: string; reason?: string }> };
+    };
+    expect(body.data.entries[0]).toMatchObject({
+      status: "skipped",
+      reason: "upsert_failed",
+    });
+    expect(failed.headers.get("Cache-Control")).toBe("private, no-store");
+
+    vi.mocked(upsertCycleDayLog).mockResolvedValueOnce({
+      id: "d1",
+      existed: false,
+      changed: true,
+    } as never);
+    const clean = await POST(
+      postReq({
+        entries: [{ date: "2026-01-02", loggedAt: "2026-01-02T08:00:00.000Z" }],
+      }),
+    );
+    expect(clean.headers.get("Cache-Control")).not.toBe("private, no-store");
+  });
+});
diff --git a/src/app/api/cycle/day-logs/bulk/route.ts b/src/app/api/cycle/day-logs/bulk/route.ts
index 890bbf240..93c56116b 100644
--- a/src/app/api/cycle/day-logs/bulk/route.ts
+++ b/src/app/api/cycle/day-logs/bulk/route.ts
@@ -249,7 +249,7 @@ async function postBulk(request: NextRequest): Promise<Response> {
     },
   });
 
-  return apiSuccess({
+  const response = apiSuccess({
     processed: entries.length,
     inserted,
     updated,
@@ -257,4 +257,12 @@ async function postBulk(request: NextRequest): Promise<Response> {
     skipped,
     entries: results,
   });
+  // `upsert_failed` is a write that did not happen, not a verdict on the
+  // entry, and the client sends it again. `no-store` keeps the idempotency
+  // layer from caching this answer, so a retry under the same key writes
+  // instead of replaying the failure.
+  if (results.some((r) => r.reason === "upsert_failed")) {
+    response.headers.set("Cache-Control", "private, no-store");
+  }
+  return response;
 }
diff --git a/src/app/api/nutrients/batch/__tests__/route.test.ts b/src/app/api/nutrients/batch/__tests__/route.test.ts
index 09dba1152..9544efec6 100644
--- a/src/app/api/nutrients/batch/__tests__/route.test.ts
+++ b/src/app/api/nutrients/batch/__tests__/route.test.ts
@@ -69,6 +69,7 @@ import { checkRateLimit } from "@/lib/rate-limit";
 import { requireModuleEnabled } from "@/lib/modules/gate";
 import { auditLog } from "@/lib/auth/audit";
 import { invalidateUserDashboardSnapshot } from "@/lib/cache/invalidate";
+import { nutrientEntryResultSchema } from "@/lib/validations/nutrients";
 
 const SESSION_OK = {
   session: { id: "sess-1", expiresAt: new Date(Date.now() + 3_600_000) },
@@ -449,6 +450,46 @@ describe("POST /api/nutrients/batch — write semantics", () => {
     spy.mockRestore();
   });
 
+  it("marks a response carrying upsert_failed no-store, so a retry with the same Idempotency-Key writes again", async () => {
+    const spy = vi.spyOn(console, "error").mockImplementation(() => {});
+    vi.mocked(prisma.nutrientIntakeDay.createMany).mockRejectedValue(
+      new Error("connection reset"),
+    );
+    const failed = await POST(
+      postReq({
+        entries: [
+          { day: recentDay(), nutrient: "zinc", unit: "mg", amount: 10 },
+        ],
+      }),
+    );
+    expect(failed.status).toBe(200);
+    expect(failed.headers.get("Cache-Control")).toBe("private, no-store");
+    spy.mockRestore();
+
+    vi.mocked(prisma.nutrientIntakeDay.createMany).mockResolvedValue({
+      count: 1,
+    } as never);
+    const clean = await POST(
+      postReq({
+        entries: [
+          { day: recentDay(), nutrient: "zinc", unit: "mg", amount: 10 },
+        ],
+      }),
+    );
+    expect(clean.headers.get("Cache-Control")).not.toBe("private, no-store");
+  });
+
+  it("documents upsert_failed as a transient write failure to retry, and the validation skips as terminal", () => {
+    const description = String(
+      (nutrientEntryResultSchema.meta() as { description?: string })
+        .description,
+    );
+    expect(description).toMatch(/`upsert_failed` is a write failure/);
+    expect(description).toMatch(/sent again on a later sync/);
+    expect(description).not.toMatch(/log and drop a skipped entry/i);
+    expect(description).toMatch(/unit_mismatch/);
+  });
+
   it("a race that drops one createMany row still counts consistently and is observable", async () => {
     vi.mocked(prisma.nutrientIntakeDay.createMany).mockResolvedValue({
       count: 1,
diff --git a/src/app/api/nutrients/batch/route.ts b/src/app/api/nutrients/batch/route.ts
index cdfadcc57..fb15fc1a3 100644
--- a/src/app/api/nutrients/batch/route.ts
+++ b/src/app/api/nutrients/batch/route.ts
@@ -18,7 +18,10 @@
  * per-row updates (existing pairs), the measurements-batch shape. A write
  * failure marks its WHOLE group `skipped upsert_failed` rather than
  * isolating the one entry that would have caused it (unavoidable once the
- * write is batched) — the request itself never fails.
+ * write is batched) — the request itself never fails. That reason is
+ * transient, unlike the validation skips: the client sends those entries
+ * again, and the response is marked `no-store` so an idempotent retry is not
+ * answered from the cache.
  *
  * Module gate FIRST: the opt-in `nutrients` module (default off)
  * refuses ingest with the 403 `module.disabled` envelope, so a phone
@@ -360,11 +363,19 @@ async function postBatch(request: NextRequest): Promise<Response> {
     },
   });
 
-  return apiSuccess({
+  const response = apiSuccess({
     processed: entries.length,
     inserted,
     updated,
     skipped,
     entries: results,
   });
+  // `upsert_failed` is transient: nothing was stored for those entries and the
+  // client sends them again. `no-store` keeps the idempotency layer from
+  // caching this answer, so a retry under the same key writes instead of
+  // replaying the failure. The measurement batch does the same for `failed`.
+  if (skippedByReason.upsert_failed) {
+    response.headers.set("Cache-Control", "private, no-store");
+  }
+  return response;
 }
diff --git a/src/lib/openapi/routes/cycle.ts b/src/lib/openapi/routes/cycle.ts
index 86306fa4d..a5ebfe264 100644
--- a/src/lib/openapi/routes/cycle.ts
+++ b/src/lib/openapi/routes/cycle.ts
@@ -420,7 +420,12 @@ const cycleBulkEntryResult = z.object({
   status: z.enum(["inserted", "duplicate", "updated", "skipped"]),
   id: z.string().optional(),
   externalId: z.string().optional(),
-  reason: z.string().optional(),
+  reason: z
+    .string()
+    .optional()
+    .describe(
+      "On a `skipped` entry. `unstable_external_id` is a verdict on the entry and terminal: resending it unchanged is skipped again. `constraint` means the write met a database constraint (a key another write claimed at the same moment, or a reference that no longer exists). `upsert_failed` is a write failure on the server's side: nothing was stored, the entry should be sent again, and the response carrying it is `Cache-Control: no-store`, so a retry under the same `Idempotency-Key` writes again rather than replaying the failure.",
+    ),
 });
 
 const cycleBulkResponse = z.object({
@@ -791,6 +796,11 @@ export const cyclePaths: NonNullable<ZodOpenApiObject["paths"]> = {
         },
         ...cycleDisabledResponse,
         ...stdResponses,
+        "422": {
+          description:
+            "The whole batch was refused and nothing was written: more than 500 entries (`meta.errorCode` = `cycle.bulk.too_large`, no issue list), or the body failed validation (`meta.errorCode` = `cycle.bulk.invalid`, every issue under `details.issues`). Both are permanent for the batch as sent. A problem with one entry never refuses the batch; it comes back as that entry's `skipped` status.",
+          content: { "application/json": { schema: errorEnvelope } },
+        },
       },
     },
   },
diff --git a/src/lib/openapi/routes/nutrients.ts b/src/lib/openapi/routes/nutrients.ts
index 99f170029..a1256ad3e 100644
--- a/src/lib/openapi/routes/nutrients.ts
+++ b/src/lib/openapi/routes/nutrients.ts
@@ -56,7 +56,7 @@ export const nutrientPaths: NonNullable<ZodOpenApiObject["paths"]> = {
       tags: ["Nutrients"],
       summary: "Ingest micronutrient day totals (v1.28)",
       description:
-        "Bulk upsert of supplement-style daily totals synced from HealthKit — vitamins, minerals, water, caffeine. Day totals only: compute HKStatisticsCollectionQuery cumulative sums day-anchored in the user's current IANA timezone; `day` is yyyy-MM-dd in that timezone (the same rule as the `stats:` day keys). Never post raw samples. Upsert key is (day, nutrient): a re-post replaces the stored total and reports `updated`. Post the current + previous local day per sync; backfill 30 days on first enable. Max 500 entries; Idempotency-Key supported; rate limit 60/min. Per-entry skips (`unit_mismatch` | `value_out_of_range` | `day_invalid`) are terminal — log and drop, do not retry the entry. Behind the opt-in `nutrients` module (403 `module.disabled` when off). Query HealthKit with exactly the listed unit per code (`ug` = microgram; do not convert to IU):\n\n| code | HealthKit identifier | unit |\n| --- | --- | --- |\n" +
+        "Bulk upsert of supplement-style daily totals synced from HealthKit — vitamins, minerals, water, caffeine. Day totals only: compute HKStatisticsCollectionQuery cumulative sums day-anchored in the user's current IANA timezone; `day` is yyyy-MM-dd in that timezone (the same rule as the `stats:` day keys). Never post raw samples. Upsert key is (day, nutrient): a re-post replaces the stored total and reports `updated`. Post the current + previous local day per sync; backfill 30 days on first enable. Max 500 entries; Idempotency-Key supported; rate limit 60/min. Per-entry skips (`unit_mismatch` | `value_out_of_range` | `day_invalid`) are terminal — log and drop, do not retry the entry. `upsert_failed` is not one of them: it is a server-side write failure for the entry's whole write group, nothing was stored, and the entry should be sent again on a later sync; a response carrying it is `Cache-Control: no-store` and is not replayed for its `Idempotency-Key`. Behind the opt-in `nutrients` module (403 `module.disabled` when off). Query HealthKit with exactly the listed unit per code (`ug` = microgram; do not convert to IU):\n\n| code | HealthKit identifier | unit |\n| --- | --- | --- |\n" +
         catalogTable,
       requestBody: {
         required: true,
diff --git a/src/lib/validations/nutrients.ts b/src/lib/validations/nutrients.ts
index a48fc76af..e65014ed6 100644
--- a/src/lib/validations/nutrients.ts
+++ b/src/lib/validations/nutrients.ts
@@ -64,7 +64,7 @@ export const nutrientEntryResultSchema = z
   .meta({
     id: "NutrientEntryResult",
     description:
-      "Per-entry ingest outcome. skipped reasons: unit_mismatch | value_out_of_range | day_invalid | upsert_failed. Log and drop a skipped entry — do not retry it.",
+      "Per-entry ingest outcome. skipped reasons: unit_mismatch | value_out_of_range | day_invalid | upsert_failed. The first three are verdicts on the entry itself and terminal: resending it unchanged is skipped again, so log and drop it. `upsert_failed` is a write failure on the server's side, not a verdict on the entry: nothing was stored for it, and it should be sent again on a later sync. A write failure takes out the whole group it was written in (every new day total, or every existing one), so several entries can carry it at once. A response carrying it is marked `Cache-Control: no-store` and is not kept for `Idempotency-Key` replay, so the retry writes again rather than replaying the failure.",
   });
 
 export const nutrientBatchResponseSchema = z