What does this PR do?

Treats an exact NO_REPLY response from the Slack adapter as intentional silence instead of user-visible message text.

Slack channels and threads are shared human surfaces, so a gateway-backed agent may intentionally decide that a message should not receive a visible response. This adds a small final-send guard so the Slack adapter suppresses the explicit sentinel before chat_postMessage, while still allowing ordinary messages that merely contain the text NO_REPLY.

Reference PRs/context from this repo:

• fix: surface self-improvement review summaries across CLI, TUI, and gateway #18073 surfaced self-improvement review summaries across gateway surfaces; this PR follows the same gateway-boundary discipline by handling a Slack-specific final-send concern at the adapter boundary.
• feat(gateway): deliver background review notifications to user chat #3293 added gateway delivery for background review notifications; this PR is another narrow gateway delivery behaviour fix, but for explicit final-response suppression.
• Empty-response retry loop on claude-opus-4-7 in Slack group threads (non-@mention discussion messages) #13248 is adjacent issue context around Slack group-thread non-reply handling.

Related Issue

Fixes #30644

Type of Change

• 🐛 Bug fix (non-breaking change that fixes an issue)
• ✨ New feature (non-breaking change that adds functionality)
• 🔒 Security fix
• 📝 Documentation update
• ✅ Tests (adding or improving test coverage)
• ♻️ Refactor (no behavior change)
• 🎯 New skill (bundled or hub)

Changes Made

• gateway/platforms/slack.py
  • Adds _is_no_reply_sentinel() for exact trimmed NO_REPLY detection.
  • Suppresses exact sentinel sends before Slack chat_postMessage is called.
  • Returns a successful SendResult with suppression metadata so callers do not treat the turn as a send failure.
• tests/gateway/test_slack_no_reply_sentinel.py
  • Adds regression coverage for exact sentinel matching.
  • Verifies whitespace-trimmed NO_REPLY does not call chat_postMessage.
  • Verifies longer messages containing NO_REPLY still post normally.

How to Test

1. Run the targeted regression test:

   python -m pytest tests/gateway/test_slack_no_reply_sentinel.py -q -o 'addopts='

2. Run the relevant Slack gateway tests:

   python -m pytest tests/gateway/test_slack.py tests/gateway/test_slack_approval_buttons.py tests/gateway/test_slack_no_reply_sentinel.py -q -o 'addopts='

3. Optional broader gateway sweep:

   python -m pytest tests/gateway -q -o 'addopts='

Checklist

Code

• I've read the Contributing Guide
• My commit messages follow Conventional Commits (fix(scope):, feat(scope):, etc.)
• I searched for existing PRs to make sure this isn't a duplicate
• My PR contains only changes related to this fix/feature (no unrelated commits)
• I've run pytest tests/ -q and all tests pass
• I've added tests for my changes (required for bug fixes, strongly encouraged for features)
• I've tested on my platform: Ubuntu/Linux container

Documentation & Housekeeping

• I've updated relevant documentation (README, docs/, docstrings) — or N/A
• I've updated cli-config.yaml.example if I added/changed config keys — or N/A
• I've updated CONTRIBUTING.md or AGENTS.md if I changed architecture or workflows — or N/A
• I've considered cross-platform impact (Windows, macOS) per the compatibility guide — or N/A
• I've updated tool descriptions/schemas if I changed tool behavior — or N/A

For New Skills

N/A

Screenshots / Logs

Targeted regression:

$ python -m pytest tests/gateway/test_slack_no_reply_sentinel.py -q -o 'addopts='
...                                                                      [100%]
3 passed in 0.29s

Relevant Slack gateway tests:

$ python -m pytest tests/gateway/test_slack.py tests/gateway/test_slack_approval_buttons.py tests/gateway/test_slack_no_reply_sentinel.py -q -o 'addopts='
212 passed, 37 warnings in 2.02s

Broader gateway sweep was also run. It reached 5599 passed, 74 skipped; 6 Telegram tests failed on existing ParseMode repr expectations ('MarkdownV2' vs MARKDOWN_V2) and are unrelated to this Slack-only change.