diff --git a/.changeset/strict-byte-ranges.md b/.changeset/strict-byte-ranges.md
new file mode 100644
index 0000000000..4a883a01ac
--- /dev/null
+++ b/.changeset/strict-byte-ranges.md
@@ -0,0 +1,5 @@
+---
+"@moonshot-ai/kimi-code": patch
+---
+
+Serve malformed byte-range requests as full downloads instead of partial responses.
diff --git a/packages/kap-server/src/lib/httpRange.ts b/packages/kap-server/src/lib/httpRange.ts
index dbc7b872eb..c8f577833c 100644
--- a/packages/kap-server/src/lib/httpRange.ts
+++ b/packages/kap-server/src/lib/httpRange.ts
@@ -22,13 +22,10 @@ export function parseRangeHeader(
   size: number,
 ): { start: number; end: number; length: number } | null {
   if (raw === undefined) return null;
-  if (!raw.startsWith('bytes=')) return null;
-  const spec = raw.slice('bytes='.length);
-  if (spec.includes(',')) return null;
-  const dash = spec.indexOf('-');
-  if (dash < 0) return null;
-  const leftRaw = spec.slice(0, dash);
-  const rightRaw = spec.slice(dash + 1);
+  const match = /^bytes=(\d*)-(\d*)$/i.exec(raw.trim());
+  if (match === null) return null;
+  const leftRaw = match[1] ?? '';
+  const rightRaw = match[2] ?? '';
   if (leftRaw === '' && rightRaw === '') return null;
   let start: number;
   let end: number;
diff --git a/packages/kap-server/src/routes/files.ts b/packages/kap-server/src/routes/files.ts
index ff74e99ffc..bb835a3473 100644
--- a/packages/kap-server/src/routes/files.ts
+++ b/packages/kap-server/src/routes/files.ts
@@ -22,6 +22,7 @@ import {
 } from '@moonshot-ai/agent-core-v2';
 import { z } from 'zod';
 
+import { parseRangeHeader, pickHeader } from '../lib/httpRange';
 import { requestLog } from '../lib/requestLog';
 import { defineRoute } from '../middleware/defineRoute';
 import { ErrorCode } from '../protocol/error-codes';
@@ -180,13 +181,10 @@ export function registerFilesRoutes(app: FilesRouteHost, core: Scope): void {
         // Browsers load <video>/<audio> via byte-range requests (Range: bytes=…).
         // Without 206 Partial Content + Content-Range the media stalls at 0:00
         // and refuses to play or seek, so honor Range when the client sends one.
-        const range = parseRange(
-          readRangeHeader((req as unknown as FastifyRequestLike).headers['range']),
-          size,
-        );
-        if (range) {
+        const range = parseRangeHeader(pickHeader(req.headers, 'range'), size);
+        if (range !== null) {
           r.header('content-range', `bytes ${range.start}-${range.end}/${size}`)
-            .header('content-length', range.end - range.start + 1)
+            .header('content-length', range.length)
             .code(206);
           return r.send(file.stream(range)) as unknown as void;
         }
@@ -285,10 +283,6 @@ function readFieldNumber(field: unknown): number | undefined {
   return undefined;
 }
 
-function readRangeHeader(value: string | string[] | undefined): string | undefined {
-  return Array.isArray(value) ? value[0] : value;
-}
-
 function buildContentDisposition(name: string, mediaType?: string): string {
   const disposition = /^(image|video|audio)\//.test(mediaType ?? '') ? 'inline' : 'attachment';
   if (/^[\w. ()+[\]-]+$/.test(name)) {
@@ -296,37 +290,3 @@ function buildContentDisposition(name: string, mediaType?: string): string {
   }
   return disposition;
 }
-
-interface ByteRange {
-  start: number;
-  end: number;
-}
-
-/** Parse a `Range: bytes=start-end` header against the file size. Returns
- *  undefined for a missing / malformed / unsatisfiable range, in which case the
- *  caller serves the whole file with 200 (browsers accept that response). */
-function parseRange(header: string | undefined, size: number): ByteRange | undefined {
-  if (!header || size <= 0) return undefined;
-  const m = /^bytes=(\d*)-(\d*)$/i.exec(header.trim());
-  if (!m) return undefined;
-  const startStr = m[1]!;
-  const endStr = m[2]!;
-  if (startStr === '' && endStr === '') return undefined;
-
-  let start: number;
-  let end: number;
-  if (startStr === '') {
-    // Suffix range: `bytes=-N` -> the last N bytes.
-    const suffix = Number(endStr);
-    if (!Number.isFinite(suffix) || suffix <= 0) return undefined;
-    start = Math.max(size - suffix, 0);
-    end = size - 1;
-  } else {
-    start = Number(startStr);
-    if (!Number.isFinite(start) || start < 0 || start >= size) return undefined;
-    end = endStr === '' ? size - 1 : Number(endStr);
-    if (!Number.isFinite(end) || end < 0) return undefined;
-  }
-  if (start > end) return undefined;
-  return { start, end: Math.min(end, size - 1) };
-}
diff --git a/packages/kap-server/test/files.test.ts b/packages/kap-server/test/files.test.ts
index 6a9060d1d2..e0cbc39fd3 100644
--- a/packages/kap-server/test/files.test.ts
+++ b/packages/kap-server/test/files.test.ts
@@ -14,6 +14,7 @@ import { join } from 'node:path';
 
 import { afterEach, beforeEach, describe, expect, it } from 'vitest';
 
+import { parseRangeHeader } from '../src/lib/httpRange';
 import { type RunningServer, startServer } from '../src/start';
 
 let home: string;
@@ -80,6 +81,14 @@ interface Envelope<T = unknown> {
   details?: unknown;
 }
 
+describe('parseRangeHeader', () => {
+  it('rejects malformed numeric ranges instead of parsing a prefix', () => {
+    expect(parseRangeHeader('bytes=1abc-3', 10)).toBeNull();
+    expect(parseRangeHeader('bytes=1-3xyz', 10)).toBeNull();
+    expect(parseRangeHeader('bytes=1-3', 10)).toEqual({ start: 1, end: 3, length: 3 });
+  });
+});
+
 function buildMultipart(parts: {
   file: { fieldName: string; filename: string; contentType: string; data: Buffer };
   fields?: Array<{ name: string; value: string }>;
@@ -301,6 +310,15 @@ describe('POST /api/v1/files (server-v2)', () => {
     expect(tail.statusCode).toBe(206);
     expect(tail.headers['content-range']).toBe(`bytes 30-${data.length - 1}/${data.length}`);
     expect(tail.rawPayload).toEqual(data.subarray(30));
+
+    const malformed = await appOf(r).inject({
+      method: 'GET',
+      url: `/api/v1/files/${meta.id}`,
+      headers: { range: 'bytes=4abc-9' },
+    });
+    expect(malformed.statusCode).toBe(200);
+    expect(malformed.headers['content-range']).toBeUndefined();
+    expect(malformed.rawPayload).toEqual(data);
   });
 
   it('missing file part → 40001 validation error', async () => {