diff --git a/openviking/server/routers/sessions.py b/openviking/server/routers/sessions.py
index bc173b98a2..9e4bef711c 100644
--- a/openviking/server/routers/sessions.py
+++ b/openviking/server/routers/sessions.py
@@ -222,6 +222,27 @@ async def list_sessions(
     return Response(status="ok", result=result)
 
 
+async def _count_on_disk_archives(session) -> int:
+    """Count archive_NNN/ directories on disk for a session.
+
+    Returns 0 on any error (missing history dir, transient FS issue) so the
+    fallback never makes the read path fail; the persisted counter remains
+    the response in that case.
+    """
+    viking_fs = getattr(session, "_viking_fs", None)
+    if viking_fs is None:
+        return 0
+    try:
+        items = await viking_fs.ls(f"{session.uri}/history", ctx=session.ctx)
+    except Exception:
+        return 0
+    return sum(
+        1
+        for item in items
+        if isinstance(item, dict) and str(item.get("name", "")).startswith("archive_")
+    )
+
+
 @router.get("/{session_id}")
 async def get_session(
     session_id: str = Path(..., description="Session ID"),
@@ -240,6 +261,19 @@ async def get_session(
     result["uri"] = session.uri
     result["user"] = session.user.to_dict()
     result["pending_tokens"] = int(session.meta.pending_tokens or 0)
+
+    # Observability fallback for sessions written through async / out-of-band
+    # paths (e.g. the Hermes provider) that materialize archives on disk but
+    # leave the persisted counter at zero. The persisted value remains
+    # authoritative for the synchronous write path; we only override when it
+    # is stale relative to disk.
+    archive_count = await _count_on_disk_archives(session)
+    result["archive_count"] = archive_count
+    if archive_count > 0:
+        if int(result.get("commit_count") or 0) == 0:
+            result["commit_count"] = archive_count
+        if int(result.get("message_count") or 0) == 0:
+            result["message_count"] = archive_count
     return Response(status="ok", result=result)
 
 
diff --git a/tests/server/test_session_counters.py b/tests/server/test_session_counters.py
new file mode 100644
index 0000000000..a4afae5847
--- /dev/null
+++ b/tests/server/test_session_counters.py
@@ -0,0 +1,171 @@
+# Copyright (c) 2026 Beijing Volcano Engine Technology Co., Ltd.
+# SPDX-License-Identifier: AGPL-3.0
+
+"""Tests for the GET /sessions/{id} archive-derived counter fallback (#1550).
+
+Sessions written through async / out-of-band paths (e.g. the Hermes
+provider) materialize archive_NNN/ directories on disk but leave the
+persisted .meta.json counter at zero. The read endpoint must surface the
+on-disk truth so the CLI does not report "nothing happened".
+
+These tests directly exercise the router handler with a fake service so
+they do not require the native vectordb backend.
+"""
+
+from types import SimpleNamespace
+
+import pytest
+
+from openviking.server.routers import sessions as sessions_router
+from openviking.session.session import SessionMeta
+from openviking_cli.exceptions import NotFoundError
+from openviking_cli.session.user_id import UserIdentifier
+
+
+def _fake_session(
+    *,
+    session_id: str = "sess-1",
+    message_count: int = 0,
+    commit_count: int = 0,
+    archive_dir_names=(),
+    raise_on_ls: bool = False,
+):
+    user = UserIdentifier.the_default_user("u1")
+    meta = SessionMeta(
+        session_id=session_id,
+        message_count=message_count,
+        commit_count=commit_count,
+    )
+
+    async def _ls(uri, ctx=None):
+        if raise_on_ls:
+            raise FileNotFoundError(uri)
+        return [{"name": name, "isDir": True} for name in archive_dir_names]
+
+    viking_fs = SimpleNamespace(ls=_ls)
+    return SimpleNamespace(
+        session_id=session_id,
+        uri=f"viking://session/{session_id}",
+        meta=meta,
+        user=user,
+        ctx=SimpleNamespace(),
+        _viking_fs=viking_fs,
+    )
+
+
+def _patch_service(monkeypatch, *, session=None, exc=None):
+    async def _get(session_id, ctx, auto_create=False):
+        if exc is not None:
+            raise exc
+        return session
+
+    fake_service = SimpleNamespace(sessions=SimpleNamespace(get=_get))
+    monkeypatch.setattr(sessions_router, "get_service", lambda: fake_service)
+
+
+@pytest.mark.asyncio
+async def test_count_on_disk_archives_counts_archive_dirs():
+    session = _fake_session(
+        archive_dir_names=("archive_001", "archive_002", "summary.md", "extras")
+    )
+    assert await sessions_router._count_on_disk_archives(session) == 2
+
+
+@pytest.mark.asyncio
+async def test_count_on_disk_archives_returns_zero_on_missing_history():
+    session = _fake_session(raise_on_ls=True)
+    assert await sessions_router._count_on_disk_archives(session) == 0
+
+
+@pytest.mark.asyncio
+async def test_get_session_overrides_zero_counters_when_archives_exist(monkeypatch):
+    """The Hermes-style stale-counter case."""
+    session = _fake_session(
+        message_count=0,
+        commit_count=0,
+        archive_dir_names=("archive_001", "archive_002"),
+    )
+    _patch_service(monkeypatch, session=session)
+
+    response = await sessions_router.get_session(
+        session_id="sess-1", auto_create=False, _ctx=SimpleNamespace()
+    )
+    result = response.result
+    assert result["archive_count"] == 2
+    assert result["commit_count"] == 2
+    assert result["message_count"] == 2
+
+
+@pytest.mark.asyncio
+async def test_get_session_does_not_downgrade_persisted_counters(monkeypatch):
+    """Persisted counters > derived value must remain authoritative."""
+    session = _fake_session(
+        message_count=42,
+        commit_count=5,
+        archive_dir_names=("archive_001", "archive_002"),
+    )
+    _patch_service(monkeypatch, session=session)
+
+    response = await sessions_router.get_session(
+        session_id="sess-1", auto_create=False, _ctx=SimpleNamespace()
+    )
+    result = response.result
+    assert result["archive_count"] == 2
+    assert result["message_count"] == 42
+    assert result["commit_count"] == 5
+
+
+@pytest.mark.asyncio
+async def test_get_session_no_archives_keeps_zero(monkeypatch):
+    """No archives on disk → archive_count is 0, no override."""
+    session = _fake_session(message_count=0, commit_count=0, archive_dir_names=())
+    _patch_service(monkeypatch, session=session)
+
+    response = await sessions_router.get_session(
+        session_id="sess-1", auto_create=False, _ctx=SimpleNamespace()
+    )
+    result = response.result
+    assert result["archive_count"] == 0
+    assert result["message_count"] == 0
+    assert result["commit_count"] == 0
+
+
+@pytest.mark.asyncio
+async def test_get_session_response_shape_unchanged(monkeypatch):
+    """Existing fields must remain present; only archive_count is added."""
+    session = _fake_session(message_count=0, commit_count=0, archive_dir_names=())
+    _patch_service(monkeypatch, session=session)
+
+    response = await sessions_router.get_session(
+        session_id="sess-1", auto_create=False, _ctx=SimpleNamespace()
+    )
+    result = response.result
+    for required_key in (
+        "session_id",
+        "uri",
+        "user",
+        "message_count",
+        "commit_count",
+        "memories_extracted",
+        "llm_token_usage",
+        "embedding_token_usage",
+        "pending_tokens",
+        "archive_count",
+    ):
+        assert required_key in result, f"missing key {required_key!r}"
+
+
+@pytest.mark.asyncio
+async def test_get_session_not_found_passthrough(monkeypatch):
+    """Missing session still returns NOT_FOUND error response."""
+    import json as _json
+
+    _patch_service(monkeypatch, exc=NotFoundError("missing"))
+
+    response = await sessions_router.get_session(
+        session_id="ghost", auto_create=False, _ctx=SimpleNamespace()
+    )
+    # error_response returns a starlette JSONResponse with the standard payload.
+    body = _json.loads(response.body)
+    assert body["status"] == "error"
+    assert body["error"]["code"] == "NOT_FOUND"