diff --git a/docs/api/openapi.yaml b/docs/api/openapi.yaml
index ecc92d47d..808c289bc 100644
--- a/docs/api/openapi.yaml
+++ b/docs/api/openapi.yaml
@@ -1274,7 +1274,9 @@ paths:
         - in: query
           name: _offset
           schema:
+            description: At most 100000; a larger offset is a 400 OperationOutcome.
             type: number
+          description: At most 100000; a larger offset is a 400 OperationOutcome.
       responses:
         "200":
           description: searchset Bundle (application/fhir+json).
@@ -1310,7 +1312,9 @@ paths:
         - in: query
           name: _offset
           schema:
+            description: At most 100000; a larger offset is a 400 OperationOutcome.
             type: number
+          description: At most 100000; a larger offset is a 400 OperationOutcome.
         - in: query
           name: date
           schema:
@@ -1345,7 +1349,9 @@ paths:
         - in: query
           name: _offset
           schema:
+            description: At most 100000; a larger offset is a 400 OperationOutcome.
             type: number
+          description: At most 100000; a larger offset is a 400 OperationOutcome.
       responses:
         "200":
           description: searchset Bundle (application/fhir+json).
@@ -1372,7 +1378,9 @@ paths:
         - in: query
           name: _offset
           schema:
+            description: At most 100000; a larger offset is a 400 OperationOutcome.
             type: number
+          description: At most 100000; a larger offset is a 400 OperationOutcome.
       responses:
         "200":
           description: searchset Bundle (application/fhir+json).
@@ -1400,7 +1408,9 @@ paths:
         - in: query
           name: _offset
           schema:
+            description: At most 100000; a larger offset is a 400 OperationOutcome.
             type: number
+          description: At most 100000; a larger offset is a 400 OperationOutcome.
       responses:
         "200":
           description: searchset Bundle (application/fhir+json).
@@ -1595,7 +1605,9 @@ paths:
         - in: query
           name: _offset
           schema:
+            description: At most 100000; a larger offset is a 400 OperationOutcome.
             type: number
+          description: At most 100000; a larger offset is a 400 OperationOutcome.
       responses:
         "200":
           description: searchset Bundle (application/fhir+json).
diff --git a/src/app/api/fhir/Coverage/route.ts b/src/app/api/fhir/Coverage/route.ts
index ce45cbc0f..de38c6414 100644
--- a/src/app/api/fhir/Coverage/route.ts
+++ b/src/app/api/fhir/Coverage/route.ts
@@ -43,7 +43,11 @@ export const GET = apiHandler(async (request: NextRequest) => {
   if (!rl.allowed) {
     return operationOutcome(429, "throttled", "Rate limit exceeded");
   }
-  const { count, offset } = parsePaging(request.nextUrl.searchParams);
+  const paging = parsePaging(request.nextUrl.searchParams);
+  if ("error" in paging) {
+    return operationOutcome(400, "invalid", paging.error);
+  }
+  const { count, offset } = paging;
   const context = await loadFhirContext(user.id);
 
   const all = [coverageResource(context.data, context.identity)].filter(
diff --git a/src/app/api/fhir/MedicationAdministration/route.ts b/src/app/api/fhir/MedicationAdministration/route.ts
index 361d1eed6..639472eba 100644
--- a/src/app/api/fhir/MedicationAdministration/route.ts
+++ b/src/app/api/fhir/MedicationAdministration/route.ts
@@ -41,7 +41,11 @@ export const GET = apiHandler(async (request: NextRequest) => {
   if (!rl.allowed) {
     return operationOutcome(429, "throttled", "Rate limit exceeded");
   }
-  const { count, offset } = parsePaging(request.nextUrl.searchParams);
+  const paging = parsePaging(request.nextUrl.searchParams);
+  if ("error" in paging) {
+    return operationOutcome(400, "invalid", paging.error);
+  }
+  const { count, offset } = paging;
   const context = await loadFhirContext(user.id);
 
   const all = medicationAdministrationsFromReportData(context.data, {
diff --git a/src/app/api/fhir/MedicationStatement/route.ts b/src/app/api/fhir/MedicationStatement/route.ts
index bd205854c..848f6562d 100644
--- a/src/app/api/fhir/MedicationStatement/route.ts
+++ b/src/app/api/fhir/MedicationStatement/route.ts
@@ -41,7 +41,11 @@ export const GET = apiHandler(async (request: NextRequest) => {
   if (!rl.allowed) {
     return operationOutcome(429, "throttled", "Rate limit exceeded");
   }
-  const { count, offset } = parsePaging(request.nextUrl.searchParams);
+  const paging = parsePaging(request.nextUrl.searchParams);
+  if ("error" in paging) {
+    return operationOutcome(400, "invalid", paging.error);
+  }
+  const { count, offset } = paging;
   const context = await loadFhirContext(user.id);
 
   const all = medicationStatementsFromReportData(context.data, {
diff --git a/src/app/api/fhir/Observation/route.ts b/src/app/api/fhir/Observation/route.ts
index 1988f78c4..8267b28d3 100644
--- a/src/app/api/fhir/Observation/route.ts
+++ b/src/app/api/fhir/Observation/route.ts
@@ -57,7 +57,11 @@ export const GET = apiHandler(async (request: NextRequest) => {
     return operationOutcome(429, "throttled", "Rate limit exceeded");
   }
   const params = request.nextUrl.searchParams;
-  const { count, offset } = parsePaging(params);
+  const paging = parsePaging(params);
+  if ("error" in paging) {
+    return operationOutcome(400, "invalid", paging.error);
+  }
+  const { count, offset } = paging;
   const codeValues = params.getAll("code");
   const range = parseDateParams(params.getAll("date"));
   if (range && "error" in range) {
diff --git a/src/app/api/fhir/Patient/$everything/route.ts b/src/app/api/fhir/Patient/$everything/route.ts
index 68faf6304..bebc9d12c 100644
--- a/src/app/api/fhir/Patient/$everything/route.ts
+++ b/src/app/api/fhir/Patient/$everything/route.ts
@@ -69,7 +69,11 @@ export const GET = apiHandler(async (request: NextRequest) => {
     return operationOutcome(429, "throttled", "Rate limit exceeded");
   }
 
-  const { count, offset } = parsePaging(request.nextUrl.searchParams);
+  const paging = parsePaging(request.nextUrl.searchParams);
+  if ("error" in paging) {
+    return operationOutcome(400, "invalid", paging.error);
+  }
+  const { count, offset } = paging;
   const { data, identity, germanAtc, records } = await loadFhirContext(user.id);
 
   const document = buildFhirDocumentBundle(
diff --git a/src/app/api/fhir/Patient/route.ts b/src/app/api/fhir/Patient/route.ts
index dcf03b802..27a8b90f0 100644
--- a/src/app/api/fhir/Patient/route.ts
+++ b/src/app/api/fhir/Patient/route.ts
@@ -43,7 +43,11 @@ export const GET = apiHandler(async (request: NextRequest) => {
   if (!rl.allowed) {
     return operationOutcome(429, "throttled", "Rate limit exceeded");
   }
-  const { count, offset } = parsePaging(request.nextUrl.searchParams);
+  const paging = parsePaging(request.nextUrl.searchParams);
+  if ("error" in paging) {
+    return operationOutcome(400, "invalid", paging.error);
+  }
+  const { count, offset } = paging;
   const context = await loadFhirContext(user.id);
 
   const all = [patientResource(context.data, context.identity)];
diff --git a/src/app/api/fhir/__tests__/route.test.ts b/src/app/api/fhir/__tests__/route.test.ts
index af4bdfe81..eaf538a41 100644
--- a/src/app/api/fhir/__tests__/route.test.ts
+++ b/src/app/api/fhir/__tests__/route.test.ts
@@ -147,6 +147,23 @@ describe("GET /api/fhir/Observation — searchset", () => {
     expect(bundle.entry[0].resource.id).toBe("obs-3");
   });
 
+  it("refuses an offset past the ceiling with a 400 OperationOutcome", async () => {
+    vi.mocked(observationsFromReportData).mockReturnValue([obs(1)] as never);
+    for (const offset of ["100001", "1e20", "Infinity"]) {
+      const res = await observationGet(req(`?_offset=${offset}`));
+      expect(res.status).toBe(400);
+      const outcome = (await res.json()) as {
+        resourceType: string;
+        issue: Array<{ code: string }>;
+      };
+      expect(outcome.resourceType).toBe("OperationOutcome");
+      expect(outcome.issue[0].code).toBe("invalid");
+    }
+    // The ceiling itself still pages (an empty page past the end).
+    const atCeiling = await observationGet(req("?_offset=100000"));
+    expect(atCeiling.status).toBe(200);
+  });
+
   it("enforces the fhir:read scope (Bearer narrow token without it 403s)", async () => {
     // No session → Bearer path. requireAuth(FHIR_READ_SCOPE) must reject a
     // token lacking the scope; here no auth at all yields 401, proving the
diff --git a/src/lib/fhir/rest.ts b/src/lib/fhir/rest.ts
index 75e87ecf2..ebd1c54e3 100644
--- a/src/lib/fhir/rest.ts
+++ b/src/lib/fhir/rest.ts
@@ -91,21 +91,33 @@ export const DEFAULT_COUNT = 50;
 /** Hard ceiling on `_count` so a single search can never page the whole store. */
 export const MAX_COUNT = 200;
 
+/**
+ * Ceiling on `_offset`. No record pages that far (the largest search, the
+ * Observation time series, is bounded by the readings in a date range), and
+ * an unbounded offset reached the database as a skip it cannot represent and
+ * answered 500.
+ */
+export const MAX_OFFSET = 100_000;
+
 /**
  * Parsed + clamped paging parameters. `count` is bounded to `[1, MAX_COUNT]`;
  * `offset` floors at 0. A non-numeric / negative input collapses to the
  * default rather than erroring — FHIR search is forgiving on paging params.
+ * An offset above {@link MAX_OFFSET} is the one refusal: the caller answers
+ * it with a 400 `invalid` OperationOutcome.
  */
-export function parsePaging(searchParams: URLSearchParams): {
-  count: number;
-  offset: number;
-} {
+export function parsePaging(
+  searchParams: URLSearchParams,
+): { count: number; offset: number } | { error: string } {
   const rawCount = Number(searchParams.get("_count"));
   const count =
     Number.isFinite(rawCount) && rawCount > 0
       ? Math.min(Math.floor(rawCount), MAX_COUNT)
       : DEFAULT_COUNT;
   const rawOffset = Number(searchParams.get("_offset"));
+  if (rawOffset > MAX_OFFSET) {
+    return { error: `_offset must be at most ${MAX_OFFSET}` };
+  }
   const offset =
     Number.isFinite(rawOffset) && rawOffset > 0 ? Math.floor(rawOffset) : 0;
   return { count, offset };
diff --git a/src/lib/openapi/routes/health-record.ts b/src/lib/openapi/routes/health-record.ts
index 9673e3aa3..20e0e4042 100644
--- a/src/lib/openapi/routes/health-record.ts
+++ b/src/lib/openapi/routes/health-record.ts
@@ -376,7 +376,12 @@ export const healthRecordPaths: NonNullable<ZodOpenApiObject["paths"]> = {
       requestParams: {
         query: z.object({
           _count: z.coerce.number().optional(),
-          _offset: z.coerce.number().optional(),
+          _offset: z.coerce
+            .number()
+            .optional()
+            .describe(
+              "At most 100000; a larger offset is a 400 OperationOutcome.",
+            ),
         }),
       },
       responses: {
@@ -401,7 +406,12 @@ export const healthRecordPaths: NonNullable<ZodOpenApiObject["paths"]> = {
       requestParams: {
         query: z.object({
           _count: z.coerce.number().optional(),
-          _offset: z.coerce.number().optional(),
+          _offset: z.coerce
+            .number()
+            .optional()
+            .describe(
+              "At most 100000; a larger offset is a 400 OperationOutcome.",
+            ),
           date: z.string().optional(),
           code: z.string().optional(),
         }),
@@ -428,7 +438,12 @@ export const healthRecordPaths: NonNullable<ZodOpenApiObject["paths"]> = {
       requestParams: {
         query: z.object({
           _count: z.coerce.number().optional(),
-          _offset: z.coerce.number().optional(),
+          _offset: z.coerce
+            .number()
+            .optional()
+            .describe(
+              "At most 100000; a larger offset is a 400 OperationOutcome.",
+            ),
         }),
       },
       responses: {
@@ -453,7 +468,12 @@ export const healthRecordPaths: NonNullable<ZodOpenApiObject["paths"]> = {
       requestParams: {
         query: z.object({
           _count: z.coerce.number().optional(),
-          _offset: z.coerce.number().optional(),
+          _offset: z.coerce
+            .number()
+            .optional()
+            .describe(
+              "At most 100000; a larger offset is a 400 OperationOutcome.",
+            ),
         }),
       },
       responses: {
@@ -478,7 +498,12 @@ export const healthRecordPaths: NonNullable<ZodOpenApiObject["paths"]> = {
       requestParams: {
         query: z.object({
           _count: z.coerce.number().optional(),
-          _offset: z.coerce.number().optional(),
+          _offset: z.coerce
+            .number()
+            .optional()
+            .describe(
+              "At most 100000; a larger offset is a 400 OperationOutcome.",
+            ),
         }),
       },
       responses: {
@@ -653,7 +678,12 @@ export const healthRecordPaths: NonNullable<ZodOpenApiObject["paths"]> = {
       requestParams: {
         query: z.object({
           _count: z.coerce.number().optional(),
-          _offset: z.coerce.number().optional(),
+          _offset: z.coerce
+            .number()
+            .optional()
+            .describe(
+              "At most 100000; a larger offset is a 400 OperationOutcome.",
+            ),
         }),
       },
       responses: {