diff --git a/src/media/content-length.ts b/src/media/content-length.ts
new file mode 100644
index 000000000000..75aee95973ae
--- /dev/null
+++ b/src/media/content-length.ts
@@ -0,0 +1,14 @@
+export function parseMediaContentLength(raw: string | null): number | null {
+  if (raw === null) {
+    return null;
+  }
+  const trimmed = raw.trim();
+  if (!/^\d+$/.test(trimmed)) {
+    throw new Error(`invalid content-length header: ${raw}`);
+  }
+  const size = Number(trimmed);
+  if (!Number.isSafeInteger(size)) {
+    throw new Error(`invalid content-length header: ${raw}`);
+  }
+  return size;
+}
diff --git a/src/media/fetch.test.ts b/src/media/fetch.test.ts
index bf74e81f5cbb..1541f831e7d5 100644
--- a/src/media/fetch.test.ts
+++ b/src/media/fetch.test.ts
@@ -286,6 +286,28 @@ describe("readRemoteMediaBuffer", () => {
     expect(body.wasCanceled()).toBe(true);
   });
 
+  it("rejects malformed content-length before remote buffer reads", async () => {
+    const body = makeCancelableStream([new Uint8Array([1, 2, 3, 4, 5])]);
+    const fetchImpl = vi.fn(
+      async () =>
+        new Response(body.stream, {
+          status: 200,
+          headers: { "content-length": "1e9" },
+        }),
+    );
+
+    await expect(
+      readRemoteMediaBuffer({
+        url: "https://example.com/file.bin",
+        fetchImpl,
+        maxBytes: 4,
+        lookupFn: makeLookupFn(),
+      }),
+    ).rejects.toThrow("invalid content-length header: 1e9");
+
+    expect(body.wasCanceled()).toBe(true);
+  });
+
   it("applies a default stream limit when maxBytes is omitted", async () => {
     const fetchImpl = vi.fn(
       async () =>
@@ -632,6 +654,25 @@ describe("readRemoteMediaBuffer", () => {
     expect(body.wasCanceled()).toBe(true);
   });
 
+  it("rejects malformed content-length before saving responses", async () => {
+    const body = makeCancelableStream([new Uint8Array([1, 2, 3, 4, 5])]);
+
+    await expect(
+      saveResponseMedia(
+        new Response(body.stream, {
+          status: 200,
+          headers: { "content-length": "1e9" },
+        }),
+        {
+          maxBytes: 4,
+          sourceUrl: "https://example.com/file.bin",
+        },
+      ),
+    ).rejects.toThrow("invalid content-length header: 1e9");
+
+    expect(body.wasCanceled()).toBe(true);
+  });
+
   it("decodes URL path basenames when deriving remote media filenames", async () => {
     const fetchImpl = vi.fn(
       async () =>
diff --git a/src/media/fetch.ts b/src/media/fetch.ts
index cc850f377ad9..935002df8ba2 100644
--- a/src/media/fetch.ts
+++ b/src/media/fetch.ts
@@ -9,6 +9,7 @@ import { retryAsync, type RetryOptions } from "../infra/retry.js";
 import { isAbortError, isTransientNetworkError } from "../infra/unhandled-rejections.js";
 import { redactSensitiveText } from "../logging/redact.js";
 import { MAX_DOCUMENT_BYTES } from "./constants.js";
+import { parseMediaContentLength } from "./content-length.js";
 import { basenameFromAnyPath, extnameFromAnyPath } from "./file-name.js";
 import { detectMime, extensionForMime } from "./mime.js";
 import { readResponseTextSnippet, readResponseWithLimit } from "./read-response-with-limit.js";
@@ -286,12 +287,21 @@ async function assertMediaContentLength(params: {
   sourceUrl: string;
   maxBytes: number;
 }): Promise<void> {
-  const contentLength = params.res.headers.get("content-length");
-  if (!contentLength) {
+  let length: number | null;
+  try {
+    length = parseMediaContentLength(params.res.headers.get("content-length"));
+  } catch (err) {
+    await discardIgnoredResponseBody(params.res);
+    throw new MediaFetchError(
+      "http_error",
+      `Failed to fetch media from ${params.sourceUrl}: ${formatErrorMessage(err)}`,
+      { cause: err },
+    );
+  }
+  if (length === null) {
     return;
   }
-  const length = Number(contentLength);
-  if (Number.isFinite(length) && length > params.maxBytes) {
+  if (length > params.maxBytes) {
     await discardIgnoredResponseBody(params.res);
     throw new MediaFetchError(
       "max_bytes",
diff --git a/src/media/input-files.fetch-guard.test.ts b/src/media/input-files.fetch-guard.test.ts
index 68eb20996b14..f215e544a987 100644
--- a/src/media/input-files.fetch-guard.test.ts
+++ b/src/media/input-files.fetch-guard.test.ts
@@ -299,6 +299,39 @@ describe("fetchWithGuard", () => {
     expect(release).toHaveBeenCalledTimes(1);
   });
 
+  it("rejects malformed content-length before reading input files", async () => {
+    let canceled = false;
+    const stream = new ReadableStream<Uint8Array>({
+      start(controller) {
+        controller.enqueue(new Uint8Array([1, 2, 3, 4]));
+      },
+      cancel() {
+        canceled = true;
+      },
+    });
+    const release = vi.fn(async () => {});
+    fetchWithSsrFGuardMock.mockResolvedValueOnce({
+      response: new Response(stream, {
+        status: 200,
+        headers: { "content-length": "1e9", "content-type": "application/octet-stream" },
+      }),
+      release,
+      finalUrl: "https://example.com/file.bin",
+    });
+
+    await expect(
+      fetchWithGuard({
+        url: "https://example.com/file.bin",
+        maxBytes: 1024,
+        timeoutMs: 1000,
+        maxRedirects: 0,
+      }),
+    ).rejects.toThrow("invalid content-length header: 1e9");
+
+    expect(canceled).toBe(true);
+    expect(release).toHaveBeenCalledTimes(1);
+  });
+
   it("rejects oversized streamed payloads and cancels the stream", async () => {
     let canceled = false;
     let pulls = 0;
diff --git a/src/media/input-files.ts b/src/media/input-files.ts
index a89a8557c3d5..ad84fa934064 100644
--- a/src/media/input-files.ts
+++ b/src/media/input-files.ts
@@ -7,6 +7,7 @@ import {
   normalizeOptionalString,
 } from "../shared/string-coerce.js";
 import { canonicalizeBase64, estimateBase64DecodedBytes } from "./base64.js";
+import { parseMediaContentLength } from "./content-length.js";
 import { convertHeicToJpeg } from "./media-services.js";
 import { detectMime } from "./mime.js";
 import { extractPdfContent, type PdfExtractedImage } from "./pdf-extract.js";
@@ -196,13 +197,18 @@ export async function fetchWithGuard(params: {
       throw new Error(`Failed to fetch: ${response.status} ${response.statusText}`);
     }
 
-    const contentLength = response.headers.get("content-length");
-    if (contentLength) {
-      const size = Number(contentLength);
-      if (Number.isFinite(size) && size > params.maxBytes) {
-        await discardIgnoredResponseBody(response);
-        throw new Error(`Content too large: ${size} bytes (limit: ${params.maxBytes} bytes)`);
-      }
+    let contentLength: number | null;
+    try {
+      contentLength = parseMediaContentLength(response.headers.get("content-length"));
+    } catch (err) {
+      await discardIgnoredResponseBody(response);
+      throw err;
+    }
+    if (contentLength !== null && contentLength > params.maxBytes) {
+      await discardIgnoredResponseBody(response);
+      throw new Error(
+        `Content too large: ${contentLength} bytes (limit: ${params.maxBytes} bytes)`,
+      );
     }
 
     const buffer = await readResponseWithLimit(response, params.maxBytes);