diff --git a/plugins/memory/byterover/__init__.py b/plugins/memory/byterover/__init__.py
index eafd9b2cfe5f..7824ca9ecc9e 100644
--- a/plugins/memory/byterover/__init__.py
+++ b/plugins/memory/byterover/__init__.py
@@ -185,6 +185,49 @@ def is_available(self) -> bool:
         """Check if brv CLI is installed. No network calls."""
         return _resolve_brv_path() is not None
 
+    def health_check(self) -> tuple[bool, str]:
+        """Probe ByteRover by running ``brv status`` — verifies the CLI
+        is BOTH installed AND logged in. ``is_available()`` only
+        confirmed the binary was on PATH; an unauthenticated user
+        would still see the green check from doctor.
+
+        Returns (RFC #42 conventions):
+          (True, "")                     — `brv status` exits 0.
+          (False, "sdk_missing")         — brv CLI not on PATH.
+          (False, "auth: ...")           — non-zero exit + stderr
+                                            mentions auth / login.
+          (False, "unreachable: timeout")— probe timed out.
+          (False, "unreachable: ...")    — anything else (config
+                                            issue, malformed install).
+
+        MUST NOT raise — _run_brv already catches subprocess errors
+        and returns a dict; we just translate.
+        """
+        if _resolve_brv_path() is None:
+            return (False, "sdk_missing")
+        try:
+            result = _run_brv(["status"], timeout=3)
+        except Exception as exc:  # noqa: BLE001
+            # Defense in depth — _run_brv is already wrapped, but
+            # health_check must never propagate.
+            return (False, f"unreachable: {exc}")
+        if result.get("success"):
+            return (True, "")
+        err = (result.get("error") or "").strip()
+        err_lc = err.lower()
+        if "timeout" in err_lc or "timed out" in err_lc:
+            return (False, "unreachable: timeout")
+        is_auth = (
+            "auth" in err_lc
+            or "login" in err_lc
+            or "unauthorized" in err_lc
+            or "not signed in" in err_lc
+            or "401" in err
+            or "403" in err
+        )
+        prefix = "auth" if is_auth else "unreachable"
+        return (False, f"{prefix}: {err[:200]}")
+
     def get_config_schema(self):
         return [
             {
diff --git a/tests/plugins/memory/test_byterover_health_check.py b/tests/plugins/memory/test_byterover_health_check.py
new file mode 100644
index 000000000000..c77b1e374541
--- /dev/null
+++ b/tests/plugins/memory/test_byterover_health_check.py
@@ -0,0 +1,101 @@
+"""Tests for ``ByteRoverMemoryProvider.health_check`` (#42 step 2c).
+
+The override runs ``brv status`` to verify the CLI is both
+installed AND logged in — ``is_available()`` only confirmed the
+binary was on PATH, so an unauthenticated user previously saw a
+false-positive green check from doctor. Reasons follow the RFC #42
+prefix taxonomy (auth: / unreachable: / sdk_missing).
+"""
+from __future__ import annotations
+
+from typing import Optional
+
+import pytest
+
+from plugins.memory.byterover import ByteRoverMemoryProvider
+import plugins.memory.byterover as bv
+
+
+# ── Helpers ────────────────────────────────────────────────────
+
+def _stub_brv(monkeypatch, *, brv_path: Optional[str] = "/usr/bin/brv",
+              run_result: Optional[dict] = None,
+              run_raises: Optional[Exception] = None):
+    """Patch the brv-resolve + _run_brv helpers."""
+    monkeypatch.setattr(bv, "_resolve_brv_path", lambda: brv_path)
+
+    def _run(args, timeout=10, cwd=None):
+        if run_raises is not None:
+            raise run_raises
+        return run_result if run_result is not None else {"success": True, "output": ""}
+
+    monkeypatch.setattr(bv, "_run_brv", _run)
+
+
+# ── Tests ──────────────────────────────────────────────────────
+
+
+def test_returns_true_when_brv_status_succeeds(monkeypatch):
+    _stub_brv(monkeypatch, run_result={"success": True, "output": "logged in as alice"})
+    healthy, reason = ByteRoverMemoryProvider().health_check()
+    assert healthy is True
+    assert reason == ""
+
+
+def test_sdk_missing_when_brv_absent(monkeypatch):
+    _stub_brv(monkeypatch, brv_path=None)
+    healthy, reason = ByteRoverMemoryProvider().health_check()
+    assert healthy is False
+    assert reason == "sdk_missing"
+
+
+@pytest.mark.parametrize("err", [
+    "not signed in. run: brv auth login",
+    "Unauthorized: token expired",
+    "auth required",
+    "401: missing token",
+    "login required",
+])
+def test_auth_classified_correctly(monkeypatch, err):
+    _stub_brv(monkeypatch, run_result={"success": False, "error": err})
+    healthy, reason = ByteRoverMemoryProvider().health_check()
+    assert healthy is False
+    assert reason.startswith("auth:"), reason
+    assert err[:30] in reason
+
+
+def test_timeout_classified_as_unreachable(monkeypatch):
+    _stub_brv(monkeypatch, run_result={"success": False, "error": "command timed out"})
+    healthy, reason = ByteRoverMemoryProvider().health_check()
+    assert healthy is False
+    assert reason == "unreachable: timeout"
+
+
+def test_generic_failure_classified_as_unreachable(monkeypatch):
+    _stub_brv(monkeypatch,
+              run_result={"success": False, "error": "unknown subcommand 'status'"})
+    healthy, reason = ByteRoverMemoryProvider().health_check()
+    assert healthy is False
+    assert reason.startswith("unreachable:")
+    assert "unknown subcommand" in reason
+
+
+def test_never_raises_even_when_run_brv_throws(monkeypatch):
+    """RFC #42 contract: health_check MUST NOT raise. Defense in
+    depth — _run_brv already catches subprocess errors and returns
+    a dict, but if for some reason it propagates, the override
+    must still return a tuple."""
+    _stub_brv(monkeypatch, run_raises=RuntimeError("brv exploded"))
+    healthy, reason = ByteRoverMemoryProvider().health_check()
+    assert healthy is False
+    assert reason.startswith("unreachable:")
+    assert "brv exploded" in reason
+
+
+def test_truncates_long_error_messages(monkeypatch):
+    long_err = "x" * 500
+    _stub_brv(monkeypatch, run_result={"success": False, "error": long_err})
+    healthy, reason = ByteRoverMemoryProvider().health_check()
+    assert healthy is False
+    # prefix ("unreachable: ") + 200 = 213
+    assert len(reason) <= 220