The MCP serve event bridge polls two files to decide whether there is new
conversation activity to surface to MCP clients: the gateway sessions.json
index and state.db. Its skip-when-unchanged guard was self-defeating — it
refreshed self._sessions_json_mtime with the current value *before*
comparing against it, so the sessions.json term was always true and the
guard collapsed to a state.db-only check.
The impact is silent message loss on the event stream. The gateway commonly
persists a message to state.db on one tick and registers the owning
conversation in sessions.json a moment later. On that later tick only
sessions.json has changed, so the broken guard takes the early return and
never processes the freshly-registered chat. Its messages are withheld from
every connected MCP client (events_poll / events_wait) until state.db
happens to change again — which, for an otherwise-idle conversation, may be
never. A polling bridge that quietly swallows new conversations is exactly
the failure mode this watcher exists to prevent.
The fix is minimal and low-risk: capture the previously-seen sessions.json
mtime before the cache refresh and compare against that, so the guard skips
only when NEITHER file changed since the last poll. The hot-path mtime
optimization is fully preserved (a genuinely idle tick still short-circuits),
and all existing EventBridge polling tests continue to pass unchanged.
## What does this PR do?
Fixes a logic error in `EventBridge._poll_once` (`mcp_serve.py`) where the
"nothing changed, skip this poll" guard compared `sj_mtime` against
`self._sessions_json_mtime` *after* that attribute had already been
overwritten with `sj_mtime`. The comparison was therefore always true,
reducing the intended "skip only if both files are unchanged" check to a
state.db-only check and discarding any tick in which only sessions.json
changed. The guard now compares against the mtime observed on the previous
poll, restoring the intended behavior.
## Related Issue
N/A
## Type of Change
- [x] 🐛 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
- `mcp_serve.py`: in `EventBridge._poll_once`, snapshot
`prev_sessions_json_mtime = self._sessions_json_mtime` before refreshing the
cached index, and use it in the skip guard
(`sj_mtime == prev_sessions_json_mtime`) so a sessions.json-only change no
longer triggers the early return. Added a comment explaining the seam.
- `tests/test_mcp_serve.py`: added
`TestEventBridgePollE2E::test_poll_picks_up_new_conversation_when_only_sessions_json_changed`,
a regression test that reproduces the boundary state (state.db unchanged,
sessions.json newly updated) and asserts the new conversation's message is
emitted.
## How to Test
1. Reproduce the failure on the old code: with the guard comparing against
`self._sessions_json_mtime`, the new test fails — the freshly-registered
conversation yields `0` events instead of `1`.
2. Apply the fix and run `pytest tests/test_mcp_serve.py -q` — all 46 tests
pass (40 skipped require the optional `mcp` SDK), including the three
pre-existing `TestEventBridgePollE2E` polling tests and the new regression
guard.
3. `ruff check mcp_serve.py tests/test_mcp_serve.py` and
`python scripts/check-windows-footguns.py mcp_serve.py` both report clean.
## Checklist
### Code
- [x] I've read the [Contributing Guide](https://github.com/NousResearch/hermes-agent/blob/main/CONTRIBUTING.md)
- [x] My commit messages follow [Conventional Commits](https://www.conventionalcommits.org/) (`fix(scope):`, `feat(scope):`, etc.)
- [x] I searched for [existing PRs](https://github.com/NousResearch/hermes-agent/pulls) to make sure this isn't a duplicate
- [x] My PR contains **only** changes related to this fix/feature (no unrelated commits)
- [x] I've run `pytest tests/test_mcp_serve.py -q` and all tests pass
- [x] I've added tests for my changes (required for bug fixes, strongly encouraged for features)
- [x] I've tested on my platform: macOS 15 (Darwin)
### Documentation & Housekeeping
- [x] I've updated relevant documentation (README, `docs/`, docstrings) — or N/A
- [x] I've updated `cli-config.yaml.example` if I added/changed config keys — or N/A
- [x] I've updated `CONTRIBUTING.md` or `AGENTS.md` if I changed architecture or workflows — or N/A
- [x] I've considered cross-platform impact (Windows, macOS) per the [compatibility guide](https://github.com/NousResearch/hermes-agent/blob/main/CONTRIBUTING.md#cross-platform-compatibility) — or N/A
- [x] I've updated tool descriptions/schemas if I changed tool behavior — or N/A
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