fix: propagate Slack reply errors (#1358)

* fix: propagate Slack reply errors

Co-authored-by: Johannes du Plessis <51395795+johannes117@users.noreply.github.com>

* fix: classify Slack 429 as rate_limited with retry-after

Slack chat.postMessage rate limiting returns HTTP 429, which raise_for_status
turned into a generic http_error and told the agent to retry immediately,
ignoring Slack's retry window. Special-case 429 (threading Retry-After) and
normalize the ratelimited body code before the generic HTTP path so the
existing rate_limited hint actually fires.

---------

Co-authored-by: open-swe[bot] <open-swe@users.noreply.github.com>
Co-authored-by: Johannes du Plessis <51395795+johannes117@users.noreply.github.com>
Co-authored-by: Johannes du Plessis <johannes@langchain.dev>

agent/prompt.py

@@ -166,6 +166,7 @@ Posts a comment to a Linear ticket given a `ticket_id`. Call this after opening/
 
 #### `slack_thread_reply`
 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.
+If `slack_thread_reply` returns `success: False`, treat it like any other tool failure. Read the `slack_error` and `hint` fields. Never emit a final response message as if the user received it when the Slack post failed.
 Format messages using Slack's mrkdwn format, NOT standard Markdown.
     Key differences: *bold*, _italic_, ~strikethrough~, <url|link text>,
     bullet lists with "• ", ```code blocks```, > blockquotes.

agent/tools/slack_thread_reply.py

@@ -49,13 +49,40 @@ def slack_thread_reply(message: str) -> dict[str, Any]:
         return {"success": False, "error": "Message cannot be empty"}
 
     message = convert_mentions_to_slack_format(message)
-    message_ts = asyncio.run(_post_and_store_mapping(channel_id, thread_ts, message))
+    message_ts, slack_error = asyncio.run(_post_and_store_mapping(channel_id, thread_ts, message))
-    return {"success": message_ts is not None}
+    if message_ts is None:
+        return {
+            "success": False,
+            "error": slack_error or "post failed",
+            "slack_error": slack_error,
+            "message_chars": len(message),
+            "hint": _slack_reply_failure_hint(slack_error),
+        }
+    return {"success": True}
 
 
-async def _post_and_store_mapping(channel_id: str, thread_ts: str, message: str) -> str | None:
+def _slack_reply_failure_hint(slack_error: str | None) -> str:
-    message_ts = await post_slack_thread_reply_with_ts(channel_id, thread_ts, message)
+    if slack_error == "msg_too_long":
+        return "Slack rejected the message as too long; retry with a shorter message."
+    if slack_error in {"channel_not_found", "not_in_channel"}:
+        return "Slack rejected the channel; do not retry. Surface the failure to the user via the trace output instead."
+    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, or surface the failure to the user via the trace output."
+        return "Slack rate limited the request; wait before retrying, or surface the failure to the user via the trace output."
+    if slack_error == "missing_slack_bot_token":
+        return "Slack bot token is missing; do not retry. Surface the failure to the user via the trace output instead."
+    if slack_error and slack_error.startswith("http_error:"):
+        return "Slack posting hit an HTTP error; retry once, then surface the failure to the user via the trace output."
+    return "Slack post failed; retry once with a concise message or surface the failure to the user via the trace output."
+
+
+async def _post_and_store_mapping(
+    channel_id: str, thread_ts: str, message: str
+) -> tuple[str | None, str | None]:
+    message_ts, slack_error = await post_slack_thread_reply_with_ts(channel_id, thread_ts, message)
     if message_ts:
         langgraph_client = get_client(url=LANGGRAPH_URL)
         await store_slack_message_run_mapping(langgraph_client, channel_id, thread_ts, message_ts)
-    return message_ts
+    return message_ts, slack_error

agent/utils/slack.py

@@ -330,10 +330,10 @@ async def post_slack_thread_reply_with_ts(
     *,
     unfurl_links: bool = True,
     unfurl_media: bool = True,
-) -> str | None:
+) -> tuple[str | None, str | None]:
-    """Post a reply in a Slack thread and return its Slack timestamp."""
+    """Post a reply in a Slack thread and return its Slack timestamp and error."""
     if not SLACK_BOT_TOKEN:
-        return None
+        return None, "missing_slack_bot_token"
 
     payload: dict[str, Any] = {
         "channel": channel_id,
@@ -350,21 +350,33 @@ async def post_slack_thread_reply_with_ts(
                 headers=_slack_headers(),
                 json=payload,
             )
+            if response.status_code == 429:
+                retry_after = response.headers.get("Retry-After")
+                logger.warning("Slack chat.postMessage rate limited (retry-after=%s)", retry_after)
+                if retry_after:
+                    return None, f"rate_limited: {retry_after}"
+                return None, "rate_limited"
             response.raise_for_status()
             data = response.json()
             if not data.get("ok"):
-                logger.warning("Slack chat.postMessage failed: %s", data.get("error"))
+                error = data.get("error")
-                return None
+                logger.warning("Slack chat.postMessage failed: %s", error)
+                if error == "ratelimited":
+                    return None, "rate_limited"
+                return None, error
             message_ts = data.get("ts")
-            return message_ts if isinstance(message_ts, str) and message_ts else None
+            if isinstance(message_ts, str) and message_ts:
-        except httpx.HTTPError:
+                return message_ts, None
+            return None, None
+        except httpx.HTTPError as exc:
             logger.exception("Slack chat.postMessage request failed")
-            return None
+            return None, f"http_error: {type(exc).__name__}"
 
 
 async def post_slack_thread_reply(channel_id: str, thread_ts: str, text: str) -> bool:
     """Post a reply in a Slack thread."""
-    return await post_slack_thread_reply_with_ts(channel_id, thread_ts, text) is not None
+    message_ts, _ = await post_slack_thread_reply_with_ts(channel_id, thread_ts, text)
+    return message_ts is not None
 
 
 async def post_slack_ephemeral_message(
@@ -713,13 +725,14 @@ def _format_trace_reply(trace_url: str | None) -> str:
 async def post_slack_trace_reply(channel_id: str, thread_ts: str, thread_id: str) -> str | None:
     """Post a trace URL reply in a Slack thread and return its Slack timestamp."""
     trace_url = get_langsmith_trace_url(thread_id)
-    return await post_slack_thread_reply_with_ts(
+    message_ts, _ = await post_slack_thread_reply_with_ts(
         channel_id,
         thread_ts,
         _format_trace_reply(trace_url),
         unfurl_links=False,
         unfurl_media=False,
     )
+    return message_ts
 
 
 _SLACK_RUN_MAP_NAMESPACE = "slack_run_map"

tests/test_slack_assistants_status.py

@@ -23,6 +23,13 @@ def _err_response(error: str = "channel_not_found") -> MagicMock:
     return response
 
 
+def _rate_limited_response(retry_after: str | None = None) -> MagicMock:
+    response = MagicMock()
+    response.status_code = 429
+    response.headers = {"Retry-After": retry_after} if retry_after else {}
+    return response
+
+
 def _async_client_cm(post_response: MagicMock) -> AsyncMock:
     client_cm = AsyncMock()
     client_cm.__aenter__.return_value = client_cm
@@ -140,3 +147,96 @@ async def test_post_slack_thread_reply_does_not_call_set_status(
     assert ok is True
     assert client_cm.post.await_count == 1
     assert client_cm.post.call_args.args[0].endswith("/chat.postMessage")
+
+
+@pytest.mark.asyncio
+async def test_post_slack_thread_reply_with_ts_returns_missing_token_error(
+    monkeypatch: pytest.MonkeyPatch,
+) -> None:
+    monkeypatch.setattr(slack_utils, "SLACK_BOT_TOKEN", "")
+
+    client_cm = _async_client_cm(_ok_response())
+    with patch.object(slack_utils.httpx, "AsyncClient", return_value=client_cm):
+        result = await slack_utils.post_slack_thread_reply_with_ts("C1", "1.0", "hello")
+
+    assert result == (None, "missing_slack_bot_token")
+    client_cm.post.assert_not_called()
+
+
+@pytest.mark.asyncio
+async def test_post_slack_thread_reply_with_ts_returns_slack_error(
+    monkeypatch: pytest.MonkeyPatch,
+) -> None:
+    monkeypatch.setattr(slack_utils, "SLACK_BOT_TOKEN", "xoxb-test")
+
+    client_cm = _async_client_cm(_err_response("msg_too_long"))
+    with patch.object(slack_utils.httpx, "AsyncClient", return_value=client_cm):
+        result = await slack_utils.post_slack_thread_reply_with_ts("C1", "1.0", "hello")
+
+    assert result == (None, "msg_too_long")
+
+
+@pytest.mark.asyncio
+async def test_post_slack_thread_reply_with_ts_returns_rate_limited_with_retry_after(
+    monkeypatch: pytest.MonkeyPatch,
+) -> None:
+    monkeypatch.setattr(slack_utils, "SLACK_BOT_TOKEN", "xoxb-test")
+
+    client_cm = _async_client_cm(_rate_limited_response(retry_after="30"))
+    with patch.object(slack_utils.httpx, "AsyncClient", return_value=client_cm):
+        result = await slack_utils.post_slack_thread_reply_with_ts("C1", "1.0", "hello")
+
+    assert result == (None, "rate_limited: 30")
+
+
+@pytest.mark.asyncio
+async def test_post_slack_thread_reply_with_ts_returns_rate_limited_without_retry_after(
+    monkeypatch: pytest.MonkeyPatch,
+) -> None:
+    monkeypatch.setattr(slack_utils, "SLACK_BOT_TOKEN", "xoxb-test")
+
+    client_cm = _async_client_cm(_rate_limited_response())
+    with patch.object(slack_utils.httpx, "AsyncClient", return_value=client_cm):
+        result = await slack_utils.post_slack_thread_reply_with_ts("C1", "1.0", "hello")
+
+    assert result == (None, "rate_limited")
+
+
+@pytest.mark.asyncio
+async def test_post_slack_thread_reply_with_ts_normalizes_ratelimited_body_error(
+    monkeypatch: pytest.MonkeyPatch,
+) -> None:
+    monkeypatch.setattr(slack_utils, "SLACK_BOT_TOKEN", "xoxb-test")
+
+    client_cm = _async_client_cm(_err_response("ratelimited"))
+    with patch.object(slack_utils.httpx, "AsyncClient", return_value=client_cm):
+        result = await slack_utils.post_slack_thread_reply_with_ts("C1", "1.0", "hello")
+
+    assert result == (None, "rate_limited")
+
+
+@pytest.mark.asyncio
+async def test_post_slack_thread_reply_with_ts_returns_http_error(
+    monkeypatch: pytest.MonkeyPatch,
+) -> None:
+    monkeypatch.setattr(slack_utils, "SLACK_BOT_TOKEN", "xoxb-test")
+
+    client_cm = _async_client_cm(_ok_response())
+    client_cm.post = AsyncMock(side_effect=slack_utils.httpx.ConnectError("boom"))
+    with patch.object(slack_utils.httpx, "AsyncClient", return_value=client_cm):
+        result = await slack_utils.post_slack_thread_reply_with_ts("C1", "1.0", "hello")
+
+    assert result == (None, "http_error: ConnectError")
+
+
+@pytest.mark.asyncio
+async def test_post_slack_thread_reply_preserves_bool_return_on_error(
+    monkeypatch: pytest.MonkeyPatch,
+) -> None:
+    monkeypatch.setattr(slack_utils, "SLACK_BOT_TOKEN", "xoxb-test")
+
+    client_cm = _async_client_cm(_err_response("channel_not_found"))
+    with patch.object(slack_utils.httpx, "AsyncClient", return_value=client_cm):
+        ok = await slack_utils.post_slack_thread_reply("C1", "1.0", "hello")
+
+    assert ok is False

tests/test_slack_context.py

@@ -233,9 +233,9 @@ def test_post_slack_trace_reply_emits_tip_only_when_no_trace_url(
         *,
         unfurl_links: bool = True,
         unfurl_media: bool = True,
-    ) -> str | None:
+    ) -> tuple[str | None, str | None]:
         posted.append({"text": text, "unfurl_links": unfurl_links, "unfurl_media": unfurl_media})
-        return "1.1"
+        return "1.1", None
 
     monkeypatch.setattr(
         slack_utils, "post_slack_thread_reply_with_ts", fake_post_slack_thread_reply_with_ts
@@ -264,9 +264,9 @@ def test_post_slack_trace_reply_includes_trace_link_and_tip(
         *,
         unfurl_links: bool = True,
         unfurl_media: bool = True,
-    ) -> str | None:
+    ) -> tuple[str | None, str | None]:
         posted.append({"text": text, "unfurl_links": unfurl_links, "unfurl_media": unfurl_media})
-        return "1.1"
+        return "1.1", None
 
     monkeypatch.setattr(
         slack_utils, "post_slack_thread_reply_with_ts", fake_post_slack_thread_reply_with_ts

tests/test_slack_thread_reply_tool.py

@@ -0,0 +1,121 @@
+from __future__ import annotations
+
+import importlib
+from typing import Any
+
+import pytest
+
+slack_reply_tool = importlib.import_module("agent.tools.slack_thread_reply")
+
+
+def _config() -> dict[str, Any]:
+    return {
+        "configurable": {
+            "slack_thread": {
+                "channel_id": "C1",
+                "thread_ts": "1.0",
+            }
+        }
+    }
+
+
+def test_slack_thread_reply_returns_structured_error_for_msg_too_long(
+    monkeypatch: pytest.MonkeyPatch,
+) -> None:
+    async def fake_post_and_store_mapping(
+        channel_id: str, thread_ts: str, message: str
+    ) -> tuple[str | None, str | None]:
+        return None, "msg_too_long"
+
+    monkeypatch.setattr(slack_reply_tool, "get_config", _config)
+    monkeypatch.setattr(slack_reply_tool, "_post_and_store_mapping", fake_post_and_store_mapping)
+
+    result = slack_reply_tool.slack_thread_reply("hello")
+
+    assert result == {
+        "success": False,
+        "error": "msg_too_long",
+        "slack_error": "msg_too_long",
+        "message_chars": 5,
+        "hint": "Slack rejected the message as too long; retry with a shorter message.",
+    }
+
+
+@pytest.mark.parametrize("slack_error", ["channel_not_found", "not_in_channel"])
+def test_slack_thread_reply_hints_not_to_retry_channel_errors(
+    slack_error: str,
+    monkeypatch: pytest.MonkeyPatch,
+) -> None:
+    async def fake_post_and_store_mapping(
+        channel_id: str, thread_ts: str, message: str
+    ) -> tuple[str | None, str | None]:
+        return None, slack_error
+
+    monkeypatch.setattr(slack_reply_tool, "get_config", _config)
+    monkeypatch.setattr(slack_reply_tool, "_post_and_store_mapping", fake_post_and_store_mapping)
+
+    result = slack_reply_tool.slack_thread_reply("hello")
+
+    assert result["success"] is False
+    assert result["error"] == slack_error
+    assert result["slack_error"] == slack_error
+    assert result["message_chars"] == 5
+    assert "do not retry" in result["hint"]
+    assert "trace output" in result["hint"]
+
+
+def test_slack_thread_reply_rate_limited_hint_includes_retry_after(
+    monkeypatch: pytest.MonkeyPatch,
+) -> None:
+    async def fake_post_and_store_mapping(
+        channel_id: str, thread_ts: str, message: str
+    ) -> tuple[str | None, str | None]:
+        return None, "rate_limited: 30"
+
+    monkeypatch.setattr(slack_reply_tool, "get_config", _config)
+    monkeypatch.setattr(slack_reply_tool, "_post_and_store_mapping", fake_post_and_store_mapping)
+
+    result = slack_reply_tool.slack_thread_reply("hello")
+
+    assert result["success"] is False
+    assert result["error"] == "rate_limited: 30"
+    assert result["slack_error"] == "rate_limited: 30"
+    assert "30s" in result["hint"]
+    assert "wait" in result["hint"]
+
+
+def test_slack_thread_reply_rate_limited_hint_without_retry_after(
+    monkeypatch: pytest.MonkeyPatch,
+) -> None:
+    async def fake_post_and_store_mapping(
+        channel_id: str, thread_ts: str, message: str
+    ) -> tuple[str | None, str | None]:
+        return None, "rate_limited"
+
+    monkeypatch.setattr(slack_reply_tool, "get_config", _config)
+    monkeypatch.setattr(slack_reply_tool, "_post_and_store_mapping", fake_post_and_store_mapping)
+
+    result = slack_reply_tool.slack_thread_reply("hello")
+
+    assert result["success"] is False
+    assert result["slack_error"] == "rate_limited"
+    assert "wait" in result["hint"]
+
+
+def test_slack_thread_reply_uses_post_failed_without_slack_error(
+    monkeypatch: pytest.MonkeyPatch,
+) -> None:
+    async def fake_post_and_store_mapping(
+        channel_id: str, thread_ts: str, message: str
+    ) -> tuple[str | None, str | None]:
+        return None, None
+
+    monkeypatch.setattr(slack_reply_tool, "get_config", _config)
+    monkeypatch.setattr(slack_reply_tool, "_post_and_store_mapping", fake_post_and_store_mapping)
+
+    result = slack_reply_tool.slack_thread_reply("hello")
+
+    assert result["success"] is False
+    assert result["error"] == "post failed"
+    assert result["slack_error"] is None
+    assert result["message_chars"] == 5