fix: convert @Name(USER_ID) mentions to Slack's <@USER_ID> format in thread replies (#1126)

The agent sees users formatted as @Name(USER_ID) in conversation context and
reproduces that pattern in replies, but Slack requires <@USER_ID> for real
mentions. This adds automatic conversion and updates prompt instructions.

Co-authored-by: open-swe[bot] <open-swe@users.noreply.github.com>

agent/prompt.py

@@ -89,6 +89,7 @@ Format messages using Slack's mrkdwn format, NOT standard Markdown.
     Key differences: *bold*, _italic_, ~strikethrough~, <url|link text>,
     bullet lists with "• ", ```code blocks```, > blockquotes.
     Do NOT use **bold**, [link](url), or other standard Markdown syntax.
+    To mention/tag a user, use `<@USER_ID>` (e.g. `<@U06KD8BFY95>`). You can find user IDs in the conversation context next to display names (e.g. `@Name(U06KD8BFY95)`).
 
 #### `github_comment`
 Posts a comment to a GitHub issue or pull request. Provide the `issue_number` explicitly. Use this when the task was triggered from GitHub — to reply with updates, answers, or a summary after completing work."""

agent/tools/slack_thread_reply.py

@@ -3,7 +3,7 @@ from typing import Any
 
 from langgraph.config import get_config
 
-from ..utils.slack import post_slack_thread_reply
+from ..utils.slack import convert_mentions_to_slack_format, post_slack_thread_reply
 
 
 def slack_thread_reply(message: str) -> dict[str, Any]:
@@ -12,7 +12,11 @@ def slack_thread_reply(message: str) -> dict[str, Any]:
     Format messages using Slack's mrkdwn format, NOT standard Markdown.
     Key differences: *bold*, _italic_, ~strikethrough~, <url|link text>,
     bullet lists with "• ", ```code blocks```, > blockquotes.
-    Do NOT use **bold**, [link](url), or other standard Markdown syntax."""
+    Do NOT use **bold**, [link](url), or other standard Markdown syntax.
+
+    To mention/tag a user, use Slack's mention format: <@USER_ID>.
+    You can find user IDs in the conversation context (e.g. @Name(U06KD8BFY95)).
+    Example: <@U06KD8BFY95> will tag that user in the message."""
     config = get_config()
     configurable = config.get("configurable", {})
     slack_thread = configurable.get("slack_thread", {})
@@ -28,5 +32,6 @@ def slack_thread_reply(message: str) -> dict[str, Any]:
     if not message.strip():
         return {"success": False, "error": "Message cannot be empty"}
 
+    message = convert_mentions_to_slack_format(message)
     success = asyncio.run(post_slack_thread_reply(channel_id, thread_ts, message))
     return {"success": success}

agent/utils/slack.py

@@ -7,6 +7,7 @@ import hashlib
 import hmac
 import logging
 import os
+import re
 import time
 from typing import Any
 
@@ -66,6 +67,11 @@ def replace_bot_mention_with_username(text: str, bot_user_id: str, bot_username:
     return text
 
 
+def convert_mentions_to_slack_format(text: str) -> str:
+    """Convert @Name(USER_ID) patterns to Slack's <@USER_ID> mention format."""
+    return re.sub(r"@[^()]+\(([A-Z0-9]+)\)", r"<@\1>", text)
+
+
 def verify_slack_signature(
     body: bytes,
     timestamp: str,

tests/test_slack_context.py

@@ -4,6 +4,7 @@ import pytest
 
 from agent import webapp
 from agent.utils.slack import (
+    convert_mentions_to_slack_format,
     format_slack_messages_for_prompt,
     replace_bot_mention_with_username,
     select_slack_context_messages,
@@ -107,6 +108,28 @@ def test_replace_bot_mention_with_username() -> None:
     )
 
 
+def test_convert_mentions_to_slack_format_basic() -> None:
+    assert (
+        convert_mentions_to_slack_format("Hey @Brace Sproul(U06KD8BFY95), check this")
+        == "Hey <@U06KD8BFY95>, check this"
+    )
+
+
+def test_convert_mentions_to_slack_format_multiple() -> None:
+    text = "@Alice(U111) and @Bob(U222) please review"
+    assert convert_mentions_to_slack_format(text) == "<@U111> and <@U222> please review"
+
+
+def test_convert_mentions_to_slack_format_no_match() -> None:
+    text = "No mentions here, just @plain text"
+    assert convert_mentions_to_slack_format(text) == text
+
+
+def test_convert_mentions_to_slack_format_preserves_existing_slack_mentions() -> None:
+    text = "Already tagged <@U06KD8BFY95> correctly"
+    assert convert_mentions_to_slack_format(text) == text
+
+
 def test_format_slack_messages_for_prompt_uses_name_and_id() -> None:
     formatted = format_slack_messages_for_prompt(
         [{"ts": "1.0", "text": "hello", "user": "U123"}],