diff --git a/extensions/slack/src/monitor/slash.test.ts b/extensions/slack/src/monitor/slash.test.ts
index 9200b2e119fc..be8a752ae200 100644
--- a/extensions/slack/src/monitor/slash.test.ts
+++ b/extensions/slack/src/monitor/slash.test.ts
@@ -502,6 +502,8 @@ async function runArgMenuAction(
     userName?: string;
     channelId?: string;
     channelName?: string;
+    message?: { ts: string; thread_ts?: string };
+    container?: { message_ts: string; thread_ts?: string };
     respond?: ReturnType<typeof vi.fn>;
     includeRespond?: boolean;
   },
@@ -515,6 +517,8 @@ async function runArgMenuAction(
       user: { id: params.userId ?? "U1", name: params.userName ?? "Ada" },
       channel: { id: params.channelId ?? "C1", name: params.channelName ?? "directmessage" },
       trigger_id: "t1",
+      ...(params.message ? { message: params.message } : {}),
+      ...(params.container ? { container: params.container } : {}),
     },
   };
   if (includeRespond) {
@@ -1223,17 +1227,28 @@ describe("Slack native command argument menus", () => {
     expect(firstDispatchArg().ctx?.OriginatingTo).toBe("team:TGRID1:user:U1");
   });
 
-  it("does not apply the response_url call cap to Web API action replies", async () => {
+  it.each([
+    { name: "top-level", message: undefined, threadTs: undefined },
+    {
+      name: "threaded",
+      message: { ts: "171.222", thread_ts: "170.111" },
+      threadTs: "170.111",
+    },
+  ])("does not cap $name Web API action replies", async ({ message, threadTs }) => {
     mockSixDispatchedReplies();
 
     await runArgMenuAction(argMenuHandler, {
       action: {
         value: encodeValue({ command: "usage", arg: "mode", value: "tokens", userId: "U1" }),
       },
+      message,
       includeRespond: false,
     });
 
     expect(harness.postEphemeral).toHaveBeenCalledTimes(6);
+    for (const [payload] of harness.postEphemeral.mock.calls) {
+      expect(payload.thread_ts).toBe(threadTs);
+    }
   });
 
   it("keeps table fallback tokens literal in Web API action replies", async () => {
@@ -1280,6 +1295,7 @@ describe("Slack native command argument menus", () => {
       action: {
         value: encodeValue({ command: "usage", arg: "mode", value: "tokens", userId: "U1" }),
       },
+      message: { ts: "171.222", thread_ts: "170.111" },
     });
 
     expect(respond).toHaveBeenCalledTimes(5);
@@ -1488,17 +1504,37 @@ describe("Slack native command argument menus", () => {
     expect(trackEvent).toHaveBeenCalledTimes(1);
   });
 
-  it("falls back to postEphemeral with token when respond is unavailable", async () => {
-    await runArgMenuAction(argMenuHandler, {
-      action: { value: "garbage" },
-      includeRespond: false,
-    });
+  it.each([
+    { name: "a top-level action", message: undefined, container: undefined, threadTs: undefined },
+    {
+      name: "the action container's parent thread",
+      message: { ts: "171.222", thread_ts: "169.999" },
+      container: { message_ts: "171.222", thread_ts: "170.111" },
+      threadTs: "170.111",
+    },
+    {
+      name: "the action message's parent thread",
+      message: { ts: "171.222", thread_ts: "170.111" },
+      container: { message_ts: "171.222" },
+      threadTs: "170.111",
+    },
+  ])(
+    "keeps $name when respond falls back to postEphemeral",
+    async ({ message, container, threadTs }) => {
+      await runArgMenuAction(argMenuHandler, {
+        action: { value: "garbage" },
+        message,
+        container,
+        includeRespond: false,
+      });
 
-    const payload = firstCallPayload(harness.postEphemeral, "postEphemeral");
-    expect(payload.token).toBe("bot-token");
-    expect(payload.channel).toBe("C1");
-    expect(payload.user).toBe("U1");
-  });
+      const payload = firstCallPayload(harness.postEphemeral, "postEphemeral");
+      expect(payload.token).toBe("bot-token");
+      expect(payload.channel).toBe("C1");
+      expect(payload.user).toBe("U1");
+      expect(payload.thread_ts).toBe(threadTs);
+    },
+  );
 
   it("treats malformed percent-encoding as an invalid button", async () => {
     await runArgMenuAction(argMenuHandler, {
diff --git a/extensions/slack/src/monitor/slash.ts b/extensions/slack/src/monitor/slash.ts
index 9f3fda3fa040..fc715e288d39 100644
--- a/extensions/slack/src/monitor/slash.ts
+++ b/extensions/slack/src/monitor/slash.ts
@@ -1,6 +1,7 @@
 // Slack plugin module implements slash behavior.
 import type {
   AllMiddlewareArgs,
+  BlockAction,
   SlackActionMiddlewareArgs,
   SlackCommandMiddlewareArgs,
   SlackOptionsMiddlewareArgs,
@@ -91,7 +92,7 @@ const SLACK_COMMAND_ARG_ACTION_BLOCKS_MAX = SLACK_MAX_BLOCKS - SLACK_COMMAND_ARG
 
 type SlackCommandHandlerArgs = SlackCommandMiddlewareArgs &
   Pick<AllMiddlewareArgs, "context" | "client">;
-type SlackArgActionHandlerArgs = SlackActionMiddlewareArgs &
+type SlackArgActionHandlerArgs = SlackActionMiddlewareArgs<BlockAction> &
   Pick<AllMiddlewareArgs, "context" | "client">;
 type SlackArgOptionsHandlerArgs = SlackOptionsMiddlewareArgs<"block_suggestion"> &
   Pick<AllMiddlewareArgs, "context" | "client">;
@@ -1177,11 +1178,13 @@ export async function registerSlackMonitorSlashCommands(params: {
                   blocks?: (Block | KnownBlock)[];
                   mrkdwn?: boolean;
                 });
+          const threadTs = body.container?.thread_ts ?? body.message?.thread_ts;
           await args.client.chat.postEphemeral({
             token: ctx.botToken,
             channel: body.channel.id,
             user: body.user.id,
             text: payload.text ?? "",
+            ...(threadTs ? { thread_ts: threadTs } : {}),
             ...(payload.blocks ? { blocks: payload.blocks } : {}),
             ...(typeof payload.mrkdwn === "boolean" ? { mrkdwn: payload.mrkdwn } : {}),
           });