diff --git a/agent/prompt.py b/agent/prompt.py
index 6726595b..575ef83d 100644
--- a/agent/prompt.py
+++ b/agent/prompt.py
@@ -150,7 +150,7 @@ Do not use this tool for GitHub API calls. Use `GH_TOKEN=dummy gh` in the sandbo
 Posts a comment to a Linear ticket given a `ticket_id`. Call this after opening/updating the pull request to notify stakeholders and include the PR link. You can tag Linear users with `@username` (their Linear display name).
 
 #### `slack_thread_reply`
-Posts a message to the active Slack thread. Use this for clarifying questions, status updates, and final summaries when the task was triggered from Slack.
+Posts a message to the active Slack thread. Use this for clarifying questions, mid-run progress updates, and final summaries when the task was triggered from Slack. You can call it multiple times during a run — if you're about to do something long-running (cloning a large repo, big refactors, running heavy test suites), post a short status update first so the user knows what's happening. Always end the run with a final reply that summarizes what you did or answers the question. Do not post a status reply before quick, single-tool answers — only when the user would otherwise be left waiting.
 Format messages using Slack's mrkdwn format, NOT standard Markdown.
     Key differences: *bold*, _italic_, ~strikethrough~, <url|link text>,
     bullet lists with "• ", ```code blocks```, > blockquotes.
diff --git a/agent/tools/slack_thread_reply.py b/agent/tools/slack_thread_reply.py
index 9210d14e..0dfef4b9 100644
--- a/agent/tools/slack_thread_reply.py
+++ b/agent/tools/slack_thread_reply.py
@@ -9,6 +9,12 @@ from ..utils.slack import convert_mentions_to_slack_format, post_slack_thread_re
 def slack_thread_reply(message: str) -> dict[str, Any]:
     """Post a message to the current Slack thread.
 
+    Use this for clarifying questions, mid-run progress updates, and the final
+    summary. You can call this multiple times during a run — if you're about to
+    do long-running work (cloning, large refactors, big test runs) consider
+    posting a brief status update first so the user knows what's happening.
+    Always end the run with a final reply summarizing what you did.
+
     Format messages using Slack's mrkdwn format, NOT standard Markdown.
     Key differences: *bold*, _italic_, ~strikethrough~, <url|link text>,
     bullet lists with "• ", ```code blocks```, > blockquotes.
diff --git a/agent/utils/slack.py b/agent/utils/slack.py
index fefb3c61..c03beb57 100644
--- a/agent/utils/slack.py
+++ b/agent/utils/slack.py
@@ -7,6 +7,7 @@ import hashlib
 import hmac
 import logging
 import os
+import random
 import re
 import time
 from dataclasses import dataclass
@@ -596,10 +597,25 @@ async def resolve_slack_links_in_context(
     return resolved_links_section, image_urls
 
 
+TRACE_REPLY_PHRASES: tuple[str, ...] = (
+    "Working on it!",
+    "On it!",
+    "Diving in!",
+    "Powering up!",
+    "Heads down.",
+    "Cracking knuckles...",
+    "Spinning up...",
+    "Looking...",
+    "Time to cook. 🧑‍🍳",
+)
+
+
 async def post_slack_trace_reply(
-    channel_id: str, thread_ts: str, thread_id: str, message: str = "Working on it!"
+    channel_id: str, thread_ts: str, thread_id: str, message: str | None = None
 ) -> None:
     """Post a trace URL reply in a Slack thread."""
+    if message is None:
+        message = random.choice(TRACE_REPLY_PHRASES)
     trace_url = get_langsmith_trace_url(thread_id)
     if trace_url:
         await post_slack_thread_reply(channel_id, thread_ts, f"{message} <{trace_url}|View trace>")
diff --git a/agent/webapp.py b/agent/webapp.py
index d89abffa..6a36ab50 100644
--- a/agent/webapp.py
+++ b/agent/webapp.py
@@ -863,6 +863,7 @@ async def process_slack_mention(event_data: dict[str, Any], repo_config: dict[st
     }
 
     langgraph_client = get_client(url=LANGGRAPH_URL)
+    is_first_mention = not await _thread_exists(thread_id)
     await _upsert_slack_thread_repo_metadata(thread_id, repo_config, langgraph_client)
 
     thread_active = await is_thread_active(thread_id)
@@ -895,7 +896,13 @@ async def process_slack_mention(event_data: dict[str, Any], repo_config: dict[st
         _run_id_for_logging(run),
         thread_id,
     )
-    await post_slack_trace_reply(channel_id, thread_ts, thread_id)
+    if is_first_mention:
+        await post_slack_trace_reply(channel_id, thread_ts, thread_id)
+    else:
+        logger.info(
+            "Skipping Slack trace reply for thread %s — agent will reply when run completes",
+            thread_id,
+        )
 
 
 async def process_slack_pr_review_request(
diff --git a/tests/test_slack_context.py b/tests/test_slack_context.py
index ba2e7bcb..c0322b93 100644
--- a/tests/test_slack_context.py
+++ b/tests/test_slack_context.py
@@ -3,12 +3,15 @@ import asyncio
 import pytest
 
 from agent import webapp
+from agent.utils import slack as slack_utils
 from agent.utils.slack import (
+    TRACE_REPLY_PHRASES,
     convert_mentions_to_slack_format,
     format_slack_messages_for_prompt,
     looks_like_slack_pr_review_command,
     parse_github_pr_url,
     parse_slack_review_command,
+    post_slack_trace_reply,
     replace_bot_mention_with_username,
     select_slack_context_messages,
     strip_bot_mention,
@@ -218,6 +221,39 @@ def test_format_slack_messages_for_prompt_replaces_bot_id_mention_in_text() -> N
     assert formatted == "@alice(U123): @open-swe status update?"
 
 
+def test_post_slack_trace_reply_picks_random_phrase_when_no_message(
+    monkeypatch: pytest.MonkeyPatch,
+) -> None:
+    posted: list[str] = []
+
+    async def fake_post_slack_thread_reply(channel_id: str, thread_ts: str, text: str) -> None:
+        posted.append(text)
+
+    monkeypatch.setattr(slack_utils, "post_slack_thread_reply", fake_post_slack_thread_reply)
+    monkeypatch.setattr(slack_utils, "get_langsmith_trace_url", lambda thread_id: None)
+
+    asyncio.run(post_slack_trace_reply("C123", "1.0", "thread-id"))
+
+    assert len(posted) == 1
+    assert posted[0] in TRACE_REPLY_PHRASES
+
+
+def test_post_slack_trace_reply_uses_explicit_message_when_provided(
+    monkeypatch: pytest.MonkeyPatch,
+) -> None:
+    posted: list[str] = []
+
+    async def fake_post_slack_thread_reply(channel_id: str, thread_ts: str, text: str) -> None:
+        posted.append(text)
+
+    monkeypatch.setattr(slack_utils, "post_slack_thread_reply", fake_post_slack_thread_reply)
+    monkeypatch.setattr(slack_utils, "get_langsmith_trace_url", lambda thread_id: None)
+
+    asyncio.run(post_slack_trace_reply("C123", "1.0", "thread-id", message="Taking a look..."))
+
+    assert posted == ["Taking a look..."]
+
+
 def test_select_slack_context_messages_detects_username_mention() -> None:
     selected, mode = select_slack_context_messages(
         [
@@ -416,11 +452,9 @@ def test_get_slack_repo_config_repo_name_only_space_syntax(
     assert repo == {"owner": "langchain-ai", "name": "open-swe"}
 
 
-def test_process_slack_mention_creates_thread_followup_run(
-    monkeypatch: pytest.MonkeyPatch,
+def _setup_slack_mention_fakes(
+    monkeypatch: pytest.MonkeyPatch, captured: dict[str, object]
 ) -> None:
-    captured: dict[str, object] = {}
-
     async def fake_add_slack_reaction(channel_id: str, message_ts: str, emoji: str) -> bool:
         captured["reaction"] = {
             "channel_id": channel_id,
@@ -465,7 +499,7 @@ def test_process_slack_mention_creates_thread_followup_run(
         return False
 
     async def fake_post_slack_trace_reply(
-        channel_id: str, thread_ts: str, thread_id: str, message: str = "Working on it!"
+        channel_id: str, thread_ts: str, thread_id: str, message: str | None = None
     ) -> None:
         captured["trace_reply"] = {
             "channel_id": channel_id,
@@ -503,6 +537,19 @@ def test_process_slack_mention_creates_thread_followup_run(
     monkeypatch.setattr(webapp, "post_slack_trace_reply", fake_post_slack_trace_reply)
     monkeypatch.setattr(webapp, "get_client", lambda url: _FakeLangGraphClientForProcess())
 
+
+def test_process_slack_mention_creates_thread_first_run_with_trace_reply(
+    monkeypatch: pytest.MonkeyPatch,
+) -> None:
+    captured: dict[str, object] = {}
+    _setup_slack_mention_fakes(monkeypatch, captured)
+
+    async def fake_thread_exists(thread_id: str) -> bool:
+        captured["thread_exists_check"] = thread_id
+        return False
+
+    monkeypatch.setattr(webapp, "_thread_exists", fake_thread_exists)
+
     thread_ts = "1700000000.000100"
     event_ts = "1700000000.000200"
     expected_thread_id = generate_thread_id_from_slack_thread("C123", thread_ts)
@@ -521,6 +568,7 @@ def test_process_slack_mention_creates_thread_followup_run(
         )
     )
 
+    assert captured["thread_exists_check"] == expected_thread_id
     assert captured["fetch_thread"] == {"channel_id": "C123", "thread_ts": thread_ts}
     assert captured["active_thread_id"] == expected_thread_id
     assert captured["metadata_update"] == {
@@ -531,7 +579,7 @@ def test_process_slack_mention_creates_thread_followup_run(
         "channel_id": "C123",
         "thread_ts": thread_ts,
         "thread_id": expected_thread_id,
-        "message": "Working on it!",
+        "message": None,
     }
 
     run_create = captured["run_create"]
@@ -548,6 +596,44 @@ def test_process_slack_mention_creates_thread_followup_run(
     assert "## Latest Mention Request\ncontinue on the branch" in prompt_block["text"]
 
 
+def test_process_slack_mention_skips_trace_reply_on_followup_mention(
+    monkeypatch: pytest.MonkeyPatch,
+) -> None:
+    """Subsequent mentions in a Slack thread should not post 'Working on it!'."""
+    captured: dict[str, object] = {}
+    _setup_slack_mention_fakes(monkeypatch, captured)
+
+    async def fake_thread_exists(thread_id: str) -> bool:
+        captured["thread_exists_check"] = thread_id
+        return True
+
+    monkeypatch.setattr(webapp, "_thread_exists", fake_thread_exists)
+
+    thread_ts = "1700000000.000100"
+    event_ts = "1700000000.000300"
+    expected_thread_id = generate_thread_id_from_slack_thread("C123", thread_ts)
+
+    asyncio.run(
+        webapp.process_slack_mention(
+            {
+                "channel_id": "C123",
+                "thread_ts": thread_ts,
+                "event_ts": event_ts,
+                "user_id": "U123",
+                "text": "<@UBOT> follow up question",
+                "bot_user_id": "UBOT",
+            },
+            {"owner": "langchain-ai", "name": "open-swe"},
+        )
+    )
+
+    assert captured["thread_exists_check"] == expected_thread_id
+    assert "trace_reply" not in captured
+    run_create = captured["run_create"]
+    assert isinstance(run_create, dict)
+    assert run_create["thread_id"] == expected_thread_id
+
+
 def test_process_slack_mention_queues_active_thread_message(
     monkeypatch: pytest.MonkeyPatch,
 ) -> None:
@@ -604,6 +690,9 @@ def test_process_slack_mention_queues_active_thread_message(
     async def fake_post_slack_trace_reply(*args, **kwargs) -> None:
         raise AssertionError("trace reply should not be posted for queued mid-run Slack messages")
 
+    async def fake_thread_exists(thread_id: str) -> bool:
+        return True
+
     class _FakeRunsClient:
         async def create(self, *args, **kwargs) -> None:
             raise AssertionError("run should not be created for active Slack threads")
@@ -628,6 +717,7 @@ def test_process_slack_mention_queues_active_thread_message(
     monkeypatch.setattr(webapp, "is_thread_active", fake_is_thread_active)
     monkeypatch.setattr(webapp, "queue_message_for_thread", fake_queue_message_for_thread)
     monkeypatch.setattr(webapp, "post_slack_trace_reply", fake_post_slack_trace_reply)
+    monkeypatch.setattr(webapp, "_thread_exists", fake_thread_exists)
     monkeypatch.setattr(webapp, "get_client", lambda url: _FakeLangGraphClientForProcess())
 
     thread_ts = "1700000000.000100"