diff --git a/extensions/slack/src/monitor/media.test.ts b/extensions/slack/src/monitor/media.test.ts
index b8dcae3c59c3..2aa74800162b 100644
--- a/extensions/slack/src/monitor/media.test.ts
+++ b/extensions/slack/src/monitor/media.test.ts
@@ -291,10 +291,11 @@ describe("fetchWithSlackAuth", () => {
 
   it("strips Authorization header on cross-origin redirects", async () => {
     // First call: redirect response from Slack
-    const redirectResponse = new Response(null, {
+    const redirectResponse = new Response("redirect body", {
       status: 302,
       headers: { location: "https://cdn.slack-edge.com/presigned-url?sig=abc123" },
     });
+    const cancel = vi.spyOn(redirectResponse.body!, "cancel").mockResolvedValue(undefined);
 
     // Second call: actual file content from CDN
     const fileResponse = new Response(Buffer.from("actual image data"), {
@@ -321,13 +322,15 @@ describe("fetchWithSlackAuth", () => {
       "https://cdn.slack-edge.com/presigned-url?sig=abc123",
       { redirect: "follow" },
     );
+    expect(cancel).toHaveBeenCalledOnce();
   });
 
   it("preserves Authorization header on same-origin redirects", async () => {
-    const redirectResponse = new Response(null, {
+    const redirectResponse = new Response("redirect body", {
       status: 302,
       headers: { location: "/files/redirect-target" },
     });
+    const cancel = vi.spyOn(redirectResponse.body!, "cancel").mockResolvedValue(undefined);
 
     const fileResponse = new Response(Buffer.from("image data"), {
       status: 200,
@@ -342,6 +345,7 @@ describe("fetchWithSlackAuth", () => {
       headers: { Authorization: "Bearer xoxb-test-token" },
       redirect: "follow",
     });
+    expect(cancel).toHaveBeenCalledOnce();
   });
 
   it("returns redirect response when no location header is provided", async () => {
diff --git a/extensions/slack/src/monitor/media.ts b/extensions/slack/src/monitor/media.ts
index 9cdd0e7435a0..93d87d5f5993 100644
--- a/extensions/slack/src/monitor/media.ts
+++ b/extensions/slack/src/monitor/media.ts
@@ -104,6 +104,12 @@ function resolveSlackFetchForRuntime(): typeof fetch {
   return isMockedFetch(globalThis.fetch) ? globalThis.fetch : fetchWithRuntimeDispatcher;
 }
 
+async function cancelUnreadResponseBody(response: Response): Promise<void> {
+  if (!response.bodyUsed) {
+    await response.body?.cancel().catch(() => undefined);
+  }
+}
+
 /**
  * Fetches a URL with Authorization header while keeping same-origin redirects
  * authenticated and dropping auth once the redirect crosses origins.
@@ -136,6 +142,7 @@ export async function fetchWithSlackAuth(url: string, token: string): Promise<Re
   if (resolvedUrl.protocol !== "https:") {
     return initialRes;
   }
+  await cancelUnreadResponseBody(initialRes);
   if (resolvedUrl.origin === parsed.origin) {
     return fetchImpl(resolvedUrl.toString(), {
       headers: authHeaders,