diff --git a/packages/agents-client/src/client.ts b/packages/agents-client/src/client.ts
index a25b4743c..ac5a9e755 100644
--- a/packages/agents-client/src/client.ts
+++ b/packages/agents-client/src/client.ts
@@ -2144,6 +2144,7 @@ export class OpenAIAgentsClient implements AgentCore {
       cacheControl !== "no-store" ||
       nosniff?.toLowerCase() !== "nosniff"
     ) {
+      await response.body?.cancel().catch(() => undefined);
       return invalidSourceFileContent("OpenAgentCore returned invalid Source File content headers.");
     }
     const data = await readExactSourceFileBody(response, Number(contentLength));
diff --git a/packages/agents-client/src/source-file-download.test.ts b/packages/agents-client/src/source-file-download.test.ts
new file mode 100644
index 000000000..8e01f25ef
--- /dev/null
+++ b/packages/agents-client/src/source-file-download.test.ts
@@ -0,0 +1,103 @@
+import { describe, expect, it, vi } from "vitest";
+
+import { AgentCoreError, OpenAIAgentsClient } from "./client";
+
+const fileId = "file-16e1f26e-8cf6-4272-9c31-d470b08d31af";
+
+function contentHeaders(bytes = 3): Headers {
+  return new Headers({
+    "Content-Type": "application/octet-stream",
+    "Content-Disposition": 'attachment; filename="notes.txt"',
+    "Content-Length": String(bytes),
+    "Cache-Control": "no-store",
+    "X-Content-Type-Options": "nosniff",
+  });
+}
+
+function clientFor(response: Response) {
+  const fetch = vi.fn<typeof globalThis.fetch>().mockResolvedValue(response);
+  return { client: new OpenAIAgentsClient({ fetch }), fetch };
+}
+
+describe("Source File download body cleanup", () => {
+  it.each([
+    { header: "Content-Type", value: null },
+    { header: "Content-Type", value: "application/json" },
+    { header: "Content-Disposition", value: null },
+    { header: "Content-Disposition", value: "inline" },
+    { header: "Content-Length", value: null },
+    { header: "Content-Length", value: "" },
+    { header: "Content-Length", value: "03" },
+    { header: "Content-Length", value: "-1" },
+    { header: "Content-Length", value: "1.5" },
+    { header: "Content-Length", value: "9007199254740992" },
+    { header: "Content-Length", value: String(512 * 1024 * 1024 + 1) },
+    { header: "Cache-Control", value: null },
+    { header: "Cache-Control", value: "public" },
+    { header: "X-Content-Type-Options", value: null },
+    { header: "X-Content-Type-Options", value: "sniff" },
+  ])("cancels rejected content with $header=$value", async ({ header, value }) => {
+    const headers = contentHeaders();
+    if (value === null) headers.delete(header);
+    else headers.set(header, value);
+    const cancel = vi.fn();
+    const response = new Response(new ReadableStream<Uint8Array>({ cancel }, { highWaterMark: 0 }), { headers });
+    const { client, fetch } = clientFor(response);
+
+    await expect(client.downloadSourceFile(fileId)).rejects.toMatchObject({
+      status: 502,
+      code: "invalid_source_file_content",
+      message: "OpenAgentCore returned invalid Source File content headers.",
+    });
+    expect(cancel).toHaveBeenCalledOnce();
+    expect(fetch).toHaveBeenCalledOnce();
+  });
+
+  it("preserves the typed header error when cancelling the body fails", async () => {
+    const headers = contentHeaders();
+    headers.delete("Content-Length");
+    const cancel = vi.fn().mockRejectedValue(new Error("body cancellation failed"));
+    const response = new Response(new ReadableStream<Uint8Array>({ cancel }, { highWaterMark: 0 }), { headers });
+    const { client, fetch } = clientFor(response);
+
+    const download = client.downloadSourceFile(fileId);
+    await expect(download).rejects.toBeInstanceOf(AgentCoreError);
+    await expect(download).rejects.toMatchObject({
+      status: 502,
+      code: "invalid_source_file_content",
+      message: "OpenAgentCore returned invalid Source File content headers.",
+    });
+    expect(cancel).toHaveBeenCalledOnce();
+    expect(fetch).toHaveBeenCalledOnce();
+  });
+
+  it("preserves the header error when there is no response body", async () => {
+    const headers = contentHeaders();
+    headers.delete("Content-Length");
+    const { client } = clientFor(new Response(null, { headers }));
+
+    await expect(client.downloadSourceFile(fileId)).rejects.toMatchObject({
+      status: 502,
+      code: "invalid_source_file_content",
+      message: "OpenAgentCore returned invalid Source File content headers.",
+    });
+  });
+
+  it.each([{ bytes: [] }, { bytes: [1, 2, 3] }])("reads valid content $bytes without cancelling it", async ({ bytes }) => {
+    const cancel = vi.fn();
+    const response = new Response(new ReadableStream<Uint8Array>({
+      pull(controller) {
+        controller.enqueue(new Uint8Array(bytes));
+        controller.close();
+      },
+      cancel,
+    }, { highWaterMark: 0 }), { headers: contentHeaders(bytes.length) });
+    const { client, fetch } = clientFor(response);
+
+    const content = await client.downloadSourceFile(fileId);
+    expect(Array.from(content.data)).toEqual(bytes);
+    expect(content.bytes).toBe(bytes.length);
+    expect(cancel).not.toHaveBeenCalled();
+    expect(fetch).toHaveBeenCalledOnce();
+  });
+});