diff --git a/src/agents/tools/slack-actions.test.ts b/src/agents/tools/slack-actions.test.ts
index f4ba99488df..8a57602f58e 100644
--- a/src/agents/tools/slack-actions.test.ts
+++ b/src/agents/tools/slack-actions.test.ts
@@ -216,6 +216,33 @@ describe("handleSlackAction", () => {
     );
   });
 
+  it("passes download scope (channel/thread) to downloadSlackFile", async () => {
+    downloadSlackFile.mockResolvedValueOnce(null);
+
+    const result = await handleSlackAction(
+      {
+        action: "downloadFile",
+        fileId: "F123",
+        to: "channel:C1",
+        replyTo: "123.456",
+      },
+      slackConfig(),
+    );
+
+    expect(downloadSlackFile).toHaveBeenCalledWith(
+      "F123",
+      expect.objectContaining({
+        channelId: "C1",
+        threadId: "123.456",
+      }),
+    );
+    expect(result).toEqual(
+      expect.objectContaining({
+        details: expect.objectContaining({ ok: false }),
+      }),
+    );
+  });
+
   it.each([
     {
       name: "JSON blocks",
diff --git a/src/agents/tools/slack-actions.ts b/src/agents/tools/slack-actions.ts
index a56eb2b3686..20a491c350d 100644
--- a/src/agents/tools/slack-actions.ts
+++ b/src/agents/tools/slack-actions.ts
@@ -290,12 +290,17 @@ export async function handleSlackAction(
       }
       case "downloadFile": {
         const fileId = readStringParam(params, "fileId", { required: true });
+        const channelTarget = readStringParam(params, "channelId") ?? readStringParam(params, "to");
+        const channelId = channelTarget ? resolveSlackChannelId(channelTarget) : undefined;
+        const threadId = readStringParam(params, "threadId") ?? readStringParam(params, "replyTo");
         const maxBytes = account.config?.mediaMaxMb
           ? account.config.mediaMaxMb * 1024 * 1024
           : 20 * 1024 * 1024;
         const downloaded = await downloadSlackFile(fileId, {
           ...readOpts,
           maxBytes,
+          channelId,
+          threadId: threadId ?? undefined,
         });
         if (!downloaded) {
           return jsonResult({
diff --git a/src/plugin-sdk/slack-message-actions.test.ts b/src/plugin-sdk/slack-message-actions.test.ts
index 14f584f9ca2..109b825fab9 100644
--- a/src/plugin-sdk/slack-message-actions.test.ts
+++ b/src/plugin-sdk/slack-message-actions.test.ts
@@ -16,6 +16,7 @@ describe("handleSlackMessageAction", () => {
         params: {
           channelId: "C1",
           fileId: "F123",
+          threadId: "111.222",
         },
       } as never,
       invoke: invoke as never,
@@ -25,6 +26,39 @@ describe("handleSlackMessageAction", () => {
       expect.objectContaining({
         action: "downloadFile",
         fileId: "F123",
+        channelId: "C1",
+        threadId: "111.222",
+      }),
+      expect.any(Object),
+    );
+  });
+
+  it("maps download-file target aliases to scope fields", async () => {
+    const invoke = vi.fn(async (action: Record<string, unknown>) => ({
+      ok: true,
+      content: action,
+    }));
+
+    await handleSlackMessageAction({
+      providerId: "slack",
+      ctx: {
+        action: "download-file",
+        cfg: {},
+        params: {
+          to: "channel:C2",
+          fileId: "F999",
+          replyTo: "333.444",
+        },
+      } as never,
+      invoke: invoke as never,
+    });
+
+    expect(invoke).toHaveBeenCalledWith(
+      expect.objectContaining({
+        action: "downloadFile",
+        fileId: "F999",
+        channelId: "channel:C2",
+        threadId: "333.444",
       }),
       expect.any(Object),
     );
diff --git a/src/plugin-sdk/slack-message-actions.ts b/src/plugin-sdk/slack-message-actions.ts
index 16abb4f9b4f..d9e0fa333a5 100644
--- a/src/plugin-sdk/slack-message-actions.ts
+++ b/src/plugin-sdk/slack-message-actions.ts
@@ -178,7 +178,20 @@ export async function handleSlackMessageAction(params: {
 
   if (action === "download-file") {
     const fileId = readStringParam(actionParams, "fileId", { required: true });
-    return await invoke({ action: "downloadFile", fileId, accountId }, cfg);
+    const channelId =
+      readStringParam(actionParams, "channelId") ?? readStringParam(actionParams, "to");
+    const threadId =
+      readStringParam(actionParams, "threadId") ?? readStringParam(actionParams, "replyTo");
+    return await invoke(
+      {
+        action: "downloadFile",
+        fileId,
+        channelId: channelId ?? undefined,
+        threadId: threadId ?? undefined,
+        accountId,
+      },
+      cfg,
+    );
   }
 
   throw new Error(`Action ${action} is not supported for provider ${providerId}.`);
diff --git a/src/slack/actions.download-file.test.ts b/src/slack/actions.download-file.test.ts
index 3bc7ae76cab..b7afe84b149 100644
--- a/src/slack/actions.download-file.test.ts
+++ b/src/slack/actions.download-file.test.ts
@@ -1,5 +1,5 @@
 import type { WebClient } from "@slack/web-api";
-import { describe, expect, it, vi } from "vitest";
+import { beforeEach, describe, expect, it, vi } from "vitest";
 
 const resolveSlackMedia = vi.fn();
 
@@ -22,6 +22,10 @@ function createClient() {
 }
 
 describe("downloadSlackFile", () => {
+  beforeEach(() => {
+    resolveSlackMedia.mockReset();
+  });
+
   it("returns null when files.info has no private download URL", async () => {
     const client = createClient();
     client.files.info.mockResolvedValueOnce({
@@ -85,4 +89,89 @@ describe("downloadSlackFile", () => {
       placeholder: "[Slack file: image.png]",
     });
   });
+
+  it("returns null when channel scope definitely mismatches file shares", async () => {
+    const client = createClient();
+    client.files.info.mockResolvedValueOnce({
+      file: {
+        id: "F123",
+        name: "image.png",
+        mimetype: "image/png",
+        url_private_download: "https://files.slack.com/files-pri/T1-F123/image.png",
+        channels: ["C999"],
+      },
+    });
+
+    const result = await downloadSlackFile("F123", {
+      client,
+      token: "xoxb-test",
+      maxBytes: 1024,
+      channelId: "C123",
+    });
+
+    expect(result).toBeNull();
+    expect(resolveSlackMedia).not.toHaveBeenCalled();
+  });
+
+  it("returns null when thread scope definitely mismatches file share thread", async () => {
+    const client = createClient();
+    client.files.info.mockResolvedValueOnce({
+      file: {
+        id: "F123",
+        name: "image.png",
+        mimetype: "image/png",
+        url_private_download: "https://files.slack.com/files-pri/T1-F123/image.png",
+        shares: {
+          private: {
+            C123: [{ ts: "111.111", thread_ts: "111.111" }],
+          },
+        },
+      },
+    });
+
+    const result = await downloadSlackFile("F123", {
+      client,
+      token: "xoxb-test",
+      maxBytes: 1024,
+      channelId: "C123",
+      threadId: "222.222",
+    });
+
+    expect(result).toBeNull();
+    expect(resolveSlackMedia).not.toHaveBeenCalled();
+  });
+
+  it("keeps legacy behavior when file metadata does not expose channel/thread shares", async () => {
+    const client = createClient();
+    client.files.info.mockResolvedValueOnce({
+      file: {
+        id: "F123",
+        name: "image.png",
+        mimetype: "image/png",
+        url_private_download: "https://files.slack.com/files-pri/T1-F123/image.png",
+      },
+    });
+    resolveSlackMedia.mockResolvedValueOnce([
+      {
+        path: "/tmp/image.png",
+        contentType: "image/png",
+        placeholder: "[Slack file: image.png]",
+      },
+    ]);
+
+    const result = await downloadSlackFile("F123", {
+      client,
+      token: "xoxb-test",
+      maxBytes: 1024,
+      channelId: "C123",
+      threadId: "222.222",
+    });
+
+    expect(result).toEqual({
+      path: "/tmp/image.png",
+      contentType: "image/png",
+      placeholder: "[Slack file: image.png]",
+    });
+    expect(resolveSlackMedia).toHaveBeenCalledTimes(1);
+  });
 });
diff --git a/src/slack/actions.ts b/src/slack/actions.ts
index 3feedcc5314..d2e57959b0e 100644
--- a/src/slack/actions.ts
+++ b/src/slack/actions.ts
@@ -280,6 +280,129 @@ export async function listSlackPins(
   return (result.items ?? []) as SlackPin[];
 }
 
+type SlackFileInfoSummary = {
+  id?: string;
+  name?: string;
+  mimetype?: string;
+  url_private?: string;
+  url_private_download?: string;
+  channels?: unknown;
+  groups?: unknown;
+  ims?: unknown;
+  shares?: unknown;
+};
+
+type SlackFileThreadShare = {
+  channelId: string;
+  ts?: string;
+  threadTs?: string;
+};
+
+function normalizeSlackScopeValue(value: string | undefined): string | undefined {
+  const trimmed = value?.trim();
+  return trimmed ? trimmed : undefined;
+}
+
+function collectSlackDirectShareChannelIds(file: SlackFileInfoSummary): Set<string> {
+  const ids = new Set<string>();
+  for (const group of [file.channels, file.groups, file.ims]) {
+    if (!Array.isArray(group)) {
+      continue;
+    }
+    for (const entry of group) {
+      if (typeof entry !== "string") {
+        continue;
+      }
+      const normalized = normalizeSlackScopeValue(entry);
+      if (normalized) {
+        ids.add(normalized);
+      }
+    }
+  }
+  return ids;
+}
+
+function collectSlackShareMaps(file: SlackFileInfoSummary): Array<Record<string, unknown>> {
+  if (!file.shares || typeof file.shares !== "object" || Array.isArray(file.shares)) {
+    return [];
+  }
+  const shares = file.shares as Record<string, unknown>;
+  return [shares.public, shares.private].filter(
+    (value): value is Record<string, unknown> =>
+      Boolean(value) && typeof value === "object" && !Array.isArray(value),
+  );
+}
+
+function collectSlackSharedChannelIds(file: SlackFileInfoSummary): Set<string> {
+  const ids = new Set<string>();
+  for (const shareMap of collectSlackShareMaps(file)) {
+    for (const channelId of Object.keys(shareMap)) {
+      const normalized = normalizeSlackScopeValue(channelId);
+      if (normalized) {
+        ids.add(normalized);
+      }
+    }
+  }
+  return ids;
+}
+
+function collectSlackThreadShares(
+  file: SlackFileInfoSummary,
+  channelId: string,
+): SlackFileThreadShare[] {
+  const matches: SlackFileThreadShare[] = [];
+  for (const shareMap of collectSlackShareMaps(file)) {
+    const rawEntries = shareMap[channelId];
+    if (!Array.isArray(rawEntries)) {
+      continue;
+    }
+    for (const rawEntry of rawEntries) {
+      if (!rawEntry || typeof rawEntry !== "object" || Array.isArray(rawEntry)) {
+        continue;
+      }
+      const entry = rawEntry as Record<string, unknown>;
+      const ts = typeof entry.ts === "string" ? normalizeSlackScopeValue(entry.ts) : undefined;
+      const threadTs =
+        typeof entry.thread_ts === "string" ? normalizeSlackScopeValue(entry.thread_ts) : undefined;
+      matches.push({ channelId, ts, threadTs });
+    }
+  }
+  return matches;
+}
+
+function hasSlackScopeMismatch(params: {
+  file: SlackFileInfoSummary;
+  channelId?: string;
+  threadId?: string;
+}): boolean {
+  const channelId = normalizeSlackScopeValue(params.channelId);
+  if (!channelId) {
+    return false;
+  }
+  const threadId = normalizeSlackScopeValue(params.threadId);
+
+  const directIds = collectSlackDirectShareChannelIds(params.file);
+  const sharedIds = collectSlackSharedChannelIds(params.file);
+  const hasChannelEvidence = directIds.size > 0 || sharedIds.size > 0;
+  const inChannel = directIds.has(channelId) || sharedIds.has(channelId);
+  if (hasChannelEvidence && !inChannel) {
+    return true;
+  }
+
+  if (!threadId) {
+    return false;
+  }
+  const threadShares = collectSlackThreadShares(params.file, channelId);
+  if (threadShares.length === 0) {
+    return false;
+  }
+  const threadEvidence = threadShares.filter((entry) => entry.threadTs || entry.ts);
+  if (threadEvidence.length === 0) {
+    return false;
+  }
+  return !threadEvidence.some((entry) => entry.threadTs === threadId || entry.ts === threadId);
+}
+
 /**
  * Downloads a Slack file by ID and saves it to the local media store.
  * Fetches a fresh download URL via files.info to avoid using stale private URLs.
@@ -287,26 +410,21 @@ export async function listSlackPins(
  */
 export async function downloadSlackFile(
   fileId: string,
-  opts: SlackActionClientOpts & { maxBytes: number },
+  opts: SlackActionClientOpts & { maxBytes: number; channelId?: string; threadId?: string },
 ): Promise<SlackMediaResult | null> {
   const token = resolveToken(opts.token, opts.accountId);
   const client = await getClient(opts);
 
   // Fetch fresh file metadata (includes a current url_private_download).
   const info = await client.files.info({ file: fileId });
-  const file = info.file as
-    | {
-        id?: string;
-        name?: string;
-        mimetype?: string;
-        url_private?: string;
-        url_private_download?: string;
-      }
-    | undefined;
+  const file = info.file as SlackFileInfoSummary | undefined;
 
   if (!file?.url_private_download && !file?.url_private) {
     return null;
   }
+  if (hasSlackScopeMismatch({ file, channelId: opts.channelId, threadId: opts.threadId })) {
+    return null;
+  }
 
   const results = await resolveSlackMedia({
     files: [