diff --git a/agent/prompt.py b/agent/prompt.py
index c415e1ec..b6192e2c 100644
--- a/agent/prompt.py
+++ b/agent/prompt.py
@@ -83,6 +83,7 @@ OPEN_SWE_SHARED_BASE = """You are **Open SWE**, an open-source agent built on La
 ### Communication
 
 - Focus on the substance and keep summaries brief. Use light markdown (`###`/`####` headings, bold, code) — avoid `#`/`##` titles.
+- In Slack, when a user asks to "break out," "split out," or "start a separate thread" for part of the work, summarize the requested aspect and relevant context into self-contained instructions, then call `slack_start_new_thread` instead of only replying in the current thread.
 - When you post to Slack with `slack_thread_reply`, do not repeat that text in a later assistant message; the user can already see the Slack message.
 - When delegated work to a subagent: the calling agent only sees your final message, so make it the complete answer.
 
diff --git a/agent/server.py b/agent/server.py
index 12a6fa80..c16d36ab 100644
--- a/agent/server.py
+++ b/agent/server.py
@@ -86,6 +86,7 @@ from .tools import (
     save_plan,
     schedule_thread_wakeup,
     slack_read_thread_messages,
+    slack_start_new_thread,
     slack_thread_reply,
     web_search,
 )
@@ -624,6 +625,7 @@ PLAN_MODE_EXCLUDED_TOOLS: frozenset[str] = frozenset(
         "http_request",
         "open_pull_request",
         "request_pr_review",
+        "slack_start_new_thread",
         "linear_create_issue",
         "linear_update_issue",
         "linear_delete_issue",
@@ -958,6 +960,7 @@ async def get_agent(config: RunnableConfig) -> Pregel:
             request_pr_review,
             schedule_thread_wakeup,
             slack_read_thread_messages,
+            slack_start_new_thread,
             slack_thread_reply,
             *corridor_tools,
             *observability_tools,
diff --git a/agent/tools/__init__.py b/agent/tools/__init__.py
index ef411c17..532324d2 100644
--- a/agent/tools/__init__.py
+++ b/agent/tools/__init__.py
@@ -21,6 +21,7 @@ from .save_plan import save_plan
 from .schedule_thread_wakeup import schedule_thread_wakeup
 from .search_repo_code import search_repo_code
 from .slack_read_thread_messages import slack_read_thread_messages
+from .slack_start_new_thread import slack_start_new_thread
 from .slack_thread_reply import slack_thread_reply
 from .update_finding import update_finding
 from .web_search import web_search
@@ -49,6 +50,7 @@ __all__ = [
     "schedule_thread_wakeup",
     "search_repo_code",
     "slack_read_thread_messages",
+    "slack_start_new_thread",
     "slack_thread_reply",
     "update_finding",
     "web_search",
diff --git a/agent/tools/slack_start_new_thread.py b/agent/tools/slack_start_new_thread.py
new file mode 100644
index 00000000..cf9de0ce
--- /dev/null
+++ b/agent/tools/slack_start_new_thread.py
@@ -0,0 +1,260 @@
+import os
+import re
+from typing import Any
+
+from langgraph.config import get_config
+from langgraph_sdk import get_client
+
+from ..dispatch import dispatch_agent_run
+from ..utils.dashboard_links import dashboard_thread_url
+from ..utils.slack import (
+    post_slack_top_level_message_with_ts,
+    post_slack_trace_reply,
+    store_slack_run_mapping,
+)
+from ..utils.thread_ids import generate_thread_id_from_slack_thread
+
+LANGGRAPH_URL = os.environ.get("LANGGRAPH_URL") or os.environ.get(
+    "LANGGRAPH_URL_PROD", "http://localhost:2024"
+)
+
+_TITLE_MAX_CHARS = 160
+_INSTRUCTIONS_MAX_CHARS = 12000
+_VISIBLE_INSTRUCTIONS_MAX_CHARS = 2800
+_REPO_RE = re.compile(r"^[A-Za-z0-9_.-]+/[A-Za-z0-9_.-]+$")
+
+
+def _failure_hint(slack_error: str | None) -> str:
+    if slack_error == "msg_too_long":
+        return "Slack rejected the message as too long; retry with shorter title or instructions."
+    if slack_error in {"channel_not_found", "not_in_channel"}:
+        return "Slack rejected the channel; do not retry with another channel."
+    if slack_error and slack_error.startswith("rate_limited"):
+        retry_after = slack_error.partition(":")[2].strip()
+        if retry_after:
+            return f"Slack rate limited the request; wait at least {retry_after}s before retrying."
+        return "Slack rate limited the request; wait before retrying."
+    if slack_error == "missing_slack_bot_token":
+        return "Slack bot token is missing; do not retry."
+    if slack_error and slack_error.startswith("http_error:"):
+        return "Slack posting hit an HTTP error; retry once."
+    return "Slack post failed; retry once with concise instructions."
+
+
+def _validate_text(value: str, *, field: str, max_chars: int) -> str | dict[str, Any]:
+    text = value.strip() if isinstance(value, str) else ""
+    if not text:
+        return {"success": False, "error": f"{field} is required"}
+    if len(text) > max_chars:
+        return {
+            "success": False,
+            "error": f"{field} is too long",
+            "max_chars": max_chars,
+            "actual_chars": len(text),
+        }
+    return text
+
+
+def _resolve_repo(configurable: dict[str, Any], default_repo: str | None) -> dict[str, str] | None:
+    if default_repo and default_repo.strip():
+        candidate = default_repo.strip()
+        if not _REPO_RE.fullmatch(candidate):
+            return None
+        owner, name = candidate.split("/", 1)
+        return {"owner": owner, "name": name}
+
+    repo = configurable.get("repo")
+    if isinstance(repo, dict):
+        owner = repo.get("owner")
+        name = repo.get("name")
+        if isinstance(owner, str) and owner.strip() and isinstance(name, str) and name.strip():
+            return {"owner": owner.strip(), "name": name.strip()}
+    return None
+
+
+def _truncate_for_slack(text: str) -> str:
+    if len(text) <= _VISIBLE_INSTRUCTIONS_MAX_CHARS:
+        return text
+    omitted = len(text) - _VISIBLE_INSTRUCTIONS_MAX_CHARS
+    return f"{text[:_VISIBLE_INSTRUCTIONS_MAX_CHARS].rstrip()}\n\n…truncated {omitted} chars; the new Open SWE thread received the full instructions."
+
+
+def _visible_message(title: str, instructions: str, repo: dict[str, str] | None) -> str:
+    repo_line = f"\n*Repository:* `{repo['owner']}/{repo['name']}`" if repo else ""
+    return (
+        f"*Open SWE breakout thread:* {title}{repo_line}\n\n"
+        f"*Instructions for the new thread:*\n{_truncate_for_slack(instructions)}"
+    )
+
+
+def _run_prompt(
+    title: str,
+    instructions: str,
+    repo: dict[str, str] | None,
+    original_slack_thread: dict[str, Any],
+) -> str:
+    repo_text = f"{repo['owner']}/{repo['name']}" if repo else "(no repository specified)"
+    channel_id = original_slack_thread.get("channel_id", "")
+    thread_ts = original_slack_thread.get("thread_ts", "")
+    return (
+        "You were started from another Open SWE Slack thread as a breakout task.\n\n"
+        f"## Breakout Title\n{title}\n\n"
+        f"## Default Repository Hint\n{repo_text}\n"
+        "Use this repository unless the instructions below clearly identify a different repository.\n\n"
+        "## Source Slack Thread\n"
+        f"- Channel: {channel_id}\n"
+        f"- Thread TS: {thread_ts}\n\n"
+        "## Breakout Instructions\n"
+        f"{instructions}\n\n"
+        "Use `slack_thread_reply` to communicate in this new Slack thread for clarifications, "
+        "status updates, and final summaries."
+    )
+
+
+def _new_slack_thread_context(
+    original: dict[str, Any],
+    *,
+    channel_id: str,
+    thread_ts: str,
+) -> dict[str, Any]:
+    return {
+        "channel_id": channel_id,
+        "thread_ts": thread_ts,
+        "triggering_user_id": original.get("triggering_user_id", ""),
+        "triggering_user_name": original.get("triggering_user_name", ""),
+        "triggering_user_email": original.get("triggering_user_email", ""),
+        "triggering_event_ts": thread_ts,
+    }
+
+
+async def slack_start_new_thread(
+    title: str,
+    instructions: str,
+    default_repo: str | None = None,
+) -> dict[str, Any]:
+    """Start a new Open SWE thread in a top-level Slack message in the current channel."""
+    config = get_config()
+    configurable = config.get("configurable", {})
+    current_slack_thread = configurable.get("slack_thread")
+    if not isinstance(current_slack_thread, dict):
+        return {"success": False, "error": "Missing slack_thread config"}
+
+    channel_id = current_slack_thread.get("channel_id")
+    current_thread_ts = current_slack_thread.get("thread_ts")
+    if not isinstance(channel_id, str) or not channel_id.strip():
+        return {"success": False, "error": "Missing slack_thread.channel_id in config"}
+
+    clean_title = _validate_text(title, field="title", max_chars=_TITLE_MAX_CHARS)
+    if isinstance(clean_title, dict):
+        return clean_title
+    clean_instructions = _validate_text(
+        instructions, field="instructions", max_chars=_INSTRUCTIONS_MAX_CHARS
+    )
+    if isinstance(clean_instructions, dict):
+        return clean_instructions
+
+    repo = _resolve_repo(configurable, default_repo)
+    if default_repo and default_repo.strip() and repo is None:
+        return {
+            "success": False,
+            "error": "default_repo must be a simple owner/name repository string",
+        }
+
+    message_ts, slack_error = await post_slack_top_level_message_with_ts(
+        channel_id.strip(),
+        _visible_message(clean_title, clean_instructions, repo),
+        unfurl_links=False,
+        unfurl_media=False,
+    )
+    if message_ts is None:
+        return {
+            "success": False,
+            "error": slack_error or "post failed",
+            "slack_error": slack_error,
+            "hint": _failure_hint(slack_error),
+        }
+
+    thread_id = generate_thread_id_from_slack_thread(channel_id.strip(), message_ts)
+    new_slack_thread = _new_slack_thread_context(
+        current_slack_thread,
+        channel_id=channel_id.strip(),
+        thread_ts=message_ts,
+    )
+    breakout_from = {
+        "channel_id": channel_id.strip(),
+        "thread_ts": current_thread_ts or "",
+        "message_ts": current_slack_thread.get("triggering_event_ts", ""),
+    }
+
+    metadata: dict[str, Any] = {
+        "source": "slack",
+        "title": clean_title[:80],
+        "source_context": {
+            "slack_thread": new_slack_thread,
+            "breakout_from": breakout_from,
+        },
+    }
+    if repo:
+        metadata.update(
+            {
+                "repo": repo,
+                "repo_owner": repo["owner"],
+                "repo_name": repo["name"],
+            }
+        )
+    github_login = configurable.get("github_login")
+    if isinstance(github_login, str) and github_login:
+        metadata["github_login"] = github_login
+    user_email = configurable.get("user_email")
+    if isinstance(user_email, str) and user_email:
+        metadata["triggering_user_email"] = user_email.strip().lower()
+
+    new_configurable: dict[str, Any] = {
+        "slack_thread": new_slack_thread,
+        "source": "slack",
+    }
+    if repo:
+        new_configurable["repo"] = repo
+    for key in ("user_email", "github_login", "agent_model_id", "agent_effort"):
+        value = configurable.get(key)
+        if value:
+            new_configurable[key] = value
+
+    client = get_client(url=LANGGRAPH_URL)
+    await client.threads.create(thread_id=thread_id, if_exists="do_nothing", metadata=metadata)
+    await client.threads.update(thread_id=thread_id, metadata=metadata)
+
+    run = await dispatch_agent_run(
+        thread_id,
+        _run_prompt(clean_title, clean_instructions, repo, current_slack_thread),
+        new_configurable,
+        source="slack",
+        client=client,
+    )
+    run_id = run.get("run_id") if isinstance(run, dict) else None
+    trace_message_ts = await post_slack_trace_reply(channel_id.strip(), message_ts, thread_id)
+    if isinstance(run_id, str) and run_id:
+        await store_slack_run_mapping(
+            client,
+            channel_id.strip(),
+            message_ts,
+            run_id,
+            message_ts=message_ts,
+            triggering_user_id=new_slack_thread.get("triggering_user_id") or None,
+        )
+        if trace_message_ts:
+            await store_slack_run_mapping(
+                client,
+                channel_id.strip(),
+                message_ts,
+                run_id,
+                message_ts=trace_message_ts,
+                triggering_user_id=new_slack_thread.get("triggering_user_id") or None,
+            )
+
+    return {
+        "success": True,
+        "thread_id": thread_id,
+        "thread_ts": message_ts,
+        "dashboard_url": dashboard_thread_url(thread_id),
+    }
diff --git a/tests/test_plan_mode.py b/tests/test_plan_mode.py
index cc3ea0ae..cbf8ca59 100644
--- a/tests/test_plan_mode.py
+++ b/tests/test_plan_mode.py
@@ -29,6 +29,7 @@ def test_plan_mode_excluded_tools_cover_mutating_tools() -> None:
         "task",
         "open_pull_request",
         "request_pr_review",
+        "slack_start_new_thread",
         "linear_create_issue",
         "linear_update_issue",
         "linear_delete_issue",
diff --git a/tests/test_slack_start_new_thread_tool.py b/tests/test_slack_start_new_thread_tool.py
new file mode 100644
index 00000000..b7edf311
--- /dev/null
+++ b/tests/test_slack_start_new_thread_tool.py
@@ -0,0 +1,259 @@
+from __future__ import annotations
+
+import importlib
+from typing import Any
+
+import pytest
+
+from agent.utils.thread_ids import generate_thread_id_from_slack_thread
+
+slack_breakout_tool = importlib.import_module("agent.tools.slack_start_new_thread")
+
+
+def _config() -> dict[str, Any]:
+    return {
+        "configurable": {
+            "repo": {"owner": "langchain-ai", "name": "open-swe"},
+            "github_login": "alice",
+            "user_email": "alice@example.com",
+            "agent_model_id": "bedrock_converse:us.anthropic.claude-opus-4-8",
+            "agent_effort": "high",
+            "slack_thread": {
+                "channel_id": "C1",
+                "thread_ts": "1700000000.000001",
+                "triggering_user_id": "U1",
+                "triggering_user_name": "Alice",
+                "triggering_user_email": "alice@example.com",
+                "triggering_event_ts": "1700000000.000002",
+            },
+        }
+    }
+
+
+class _FakeThreadsClient:
+    def __init__(self, captured: dict[str, Any]) -> None:
+        self.captured = captured
+
+    async def create(self, *, thread_id: str, if_exists: str, metadata: dict[str, Any]) -> None:
+        self.captured["thread_create"] = {
+            "thread_id": thread_id,
+            "if_exists": if_exists,
+            "metadata": metadata,
+        }
+
+    async def update(self, *, thread_id: str, metadata: dict[str, Any]) -> None:
+        self.captured["thread_update"] = {"thread_id": thread_id, "metadata": metadata}
+
+
+class _FakeClient:
+    def __init__(self, captured: dict[str, Any]) -> None:
+        self.threads = _FakeThreadsClient(captured)
+
+
+async def test_slack_start_new_thread_success(monkeypatch: pytest.MonkeyPatch) -> None:
+    captured: dict[str, Any] = {"stored_mappings": []}
+    new_ts = "1700000000.111111"
+    trace_ts = "1700000000.222222"
+
+    async def fake_post_top_level(
+        channel_id: str,
+        text: str,
+        *,
+        unfurl_links: bool = True,
+        unfurl_media: bool = True,
+        blocks: list[dict[str, Any]] | None = None,
+    ) -> tuple[str | None, str | None]:
+        captured["top_level_post"] = {
+            "channel_id": channel_id,
+            "text": text,
+            "unfurl_links": unfurl_links,
+            "unfurl_media": unfurl_media,
+            "blocks": blocks,
+        }
+        return new_ts, None
+
+    async def fake_dispatch_agent_run(
+        thread_id: str,
+        content: str,
+        configurable: dict[str, Any],
+        *,
+        source: str,
+        client: Any,
+        **kwargs: Any,
+    ) -> dict[str, str]:
+        captured["dispatch"] = {
+            "thread_id": thread_id,
+            "content": content,
+            "configurable": configurable,
+            "source": source,
+            "client": client,
+            "kwargs": kwargs,
+        }
+        return {"run_id": "run-123"}
+
+    async def fake_post_trace(channel_id: str, thread_ts: str, thread_id: str) -> str:
+        captured["trace"] = {
+            "channel_id": channel_id,
+            "thread_ts": thread_ts,
+            "thread_id": thread_id,
+        }
+        return trace_ts
+
+    async def fake_store_mapping(
+        client: Any,
+        channel_id: str,
+        thread_ts: str,
+        run_id: str,
+        *,
+        message_ts: str | None = None,
+        triggering_user_id: str | None = None,
+    ) -> None:
+        captured["stored_mappings"].append(
+            {
+                "client": client,
+                "channel_id": channel_id,
+                "thread_ts": thread_ts,
+                "run_id": run_id,
+                "message_ts": message_ts,
+                "triggering_user_id": triggering_user_id,
+            }
+        )
+
+    fake_client = _FakeClient(captured)
+    monkeypatch.setattr(slack_breakout_tool, "get_config", _config)
+    monkeypatch.setattr(slack_breakout_tool, "get_client", lambda url: fake_client)
+    monkeypatch.setattr(
+        slack_breakout_tool, "post_slack_top_level_message_with_ts", fake_post_top_level
+    )
+    monkeypatch.setattr(slack_breakout_tool, "dispatch_agent_run", fake_dispatch_agent_run)
+    monkeypatch.setattr(slack_breakout_tool, "post_slack_trace_reply", fake_post_trace)
+    monkeypatch.setattr(slack_breakout_tool, "store_slack_run_mapping", fake_store_mapping)
+    monkeypatch.setattr(
+        slack_breakout_tool,
+        "dashboard_thread_url",
+        lambda thread_id: f"https://dashboard.example/agents/{thread_id}",
+    )
+
+    result = await slack_breakout_tool.slack_start_new_thread(
+        "Investigate follow-up",
+        "Use the same repo and investigate the follow-up aspect in detail.",
+    )
+
+    expected_thread_id = generate_thread_id_from_slack_thread("C1", new_ts)
+    assert result == {
+        "success": True,
+        "thread_id": expected_thread_id,
+        "thread_ts": new_ts,
+        "dashboard_url": f"https://dashboard.example/agents/{expected_thread_id}",
+    }
+    assert captured["top_level_post"]["channel_id"] == "C1"
+    assert "Investigate follow-up" in captured["top_level_post"]["text"]
+    assert "langchain-ai/open-swe" in captured["top_level_post"]["text"]
+    assert captured["top_level_post"]["unfurl_links"] is False
+    assert captured["thread_create"]["if_exists"] == "do_nothing"
+    assert captured["thread_create"]["thread_id"] == expected_thread_id
+    metadata = captured["thread_update"]["metadata"]
+    assert metadata["source"] == "slack"
+    assert metadata["repo"] == {"owner": "langchain-ai", "name": "open-swe"}
+    assert metadata["github_login"] == "alice"
+    assert metadata["triggering_user_email"] == "alice@example.com"
+    assert metadata["source_context"]["slack_thread"]["thread_ts"] == new_ts
+    assert metadata["source_context"]["slack_thread"]["triggering_user_id"] == "U1"
+    assert metadata["source_context"]["breakout_from"] == {
+        "channel_id": "C1",
+        "thread_ts": "1700000000.000001",
+        "message_ts": "1700000000.000002",
+    }
+    dispatch = captured["dispatch"]
+    assert dispatch["thread_id"] == expected_thread_id
+    assert dispatch["source"] == "slack"
+    assert dispatch["configurable"]["slack_thread"]["thread_ts"] == new_ts
+    assert dispatch["configurable"]["repo"] == {"owner": "langchain-ai", "name": "open-swe"}
+    assert dispatch["configurable"]["github_login"] == "alice"
+    assert (
+        dispatch["configurable"]["agent_model_id"]
+        == "bedrock_converse:us.anthropic.claude-opus-4-8"
+    )
+    assert "Breakout Instructions" in dispatch["content"]
+    assert captured["trace"] == {
+        "channel_id": "C1",
+        "thread_ts": new_ts,
+        "thread_id": expected_thread_id,
+    }
+    assert [item["message_ts"] for item in captured["stored_mappings"]] == [new_ts, trace_ts]
+    assert all(item["triggering_user_id"] == "U1" for item in captured["stored_mappings"])
+
+
+async def test_slack_start_new_thread_requires_slack_config(
+    monkeypatch: pytest.MonkeyPatch,
+) -> None:
+    monkeypatch.setattr(slack_breakout_tool, "get_config", lambda: {"configurable": {}})
+
+    result = await slack_breakout_tool.slack_start_new_thread("Title", "Instructions")
+
+    assert result == {"success": False, "error": "Missing slack_thread config"}
+
+
+@pytest.mark.parametrize(
+    ("title", "instructions", "error"),
+    [
+        ("", "Instructions", "title is required"),
+        ("Title", "", "instructions is required"),
+        ("x" * 161, "Instructions", "title is too long"),
+        ("Title", "x" * 12001, "instructions is too long"),
+    ],
+)
+async def test_slack_start_new_thread_validates_text(
+    title: str,
+    instructions: str,
+    error: str,
+    monkeypatch: pytest.MonkeyPatch,
+) -> None:
+    monkeypatch.setattr(slack_breakout_tool, "get_config", _config)
+
+    result = await slack_breakout_tool.slack_start_new_thread(title, instructions)
+
+    assert result["success"] is False
+    assert result["error"] == error
+
+
+async def test_slack_start_new_thread_rejects_invalid_repo_override(
+    monkeypatch: pytest.MonkeyPatch,
+) -> None:
+    monkeypatch.setattr(slack_breakout_tool, "get_config", _config)
+
+    result = await slack_breakout_tool.slack_start_new_thread(
+        "Title", "Instructions", default_repo="https://github.com/langchain-ai/open-swe"
+    )
+
+    assert result == {
+        "success": False,
+        "error": "default_repo must be a simple owner/name repository string",
+    }
+
+
+async def test_slack_start_new_thread_returns_slack_failure_without_dispatch(
+    monkeypatch: pytest.MonkeyPatch,
+) -> None:
+    captured: dict[str, bool] = {"dispatched": False}
+
+    async def fake_post_top_level(*args: Any, **kwargs: Any) -> tuple[str | None, str | None]:
+        return None, "msg_too_long"
+
+    async def fake_dispatch_agent_run(*args: Any, **kwargs: Any) -> dict[str, str]:
+        captured["dispatched"] = True
+        return {"run_id": "run-123"}
+
+    monkeypatch.setattr(slack_breakout_tool, "get_config", _config)
+    monkeypatch.setattr(
+        slack_breakout_tool, "post_slack_top_level_message_with_ts", fake_post_top_level
+    )
+    monkeypatch.setattr(slack_breakout_tool, "dispatch_agent_run", fake_dispatch_agent_run)
+
+    result = await slack_breakout_tool.slack_start_new_thread("Title", "Instructions")
+
+    assert result["success"] is False
+    assert result["error"] == "msg_too_long"
+    assert result["slack_error"] == "msg_too_long"
+    assert "shorter" in result["hint"]
+    assert captured["dispatched"] is False