diff --git a/api/config.py b/api/config.py
index 8230f2f79..8efafbb4d 100644
--- a/api/config.py
+++ b/api/config.py
@@ -7603,6 +7603,25 @@ def _evict_session_agent(session_id: str) -> None:
             agent = entry[0] if isinstance(entry, tuple) else None
     if agent is None:
         return
+    # A live run for this session may still hold this agent's _session_db (the
+    # worker assigns agent._session_db at run start). Never close it out from
+    # under an in-flight turn — ACTIVE_RUNS is the authoritative liveness signal
+    # (mirrors the worker's own LRU-eviction guard in streaming.py). When a run
+    # is live we still drop the cache handle above (harmless — the worker holds
+    # a local ref), but skip the lifecycle commit + _session_db.close() so the
+    # running turn can finish persisting. Hardens /clear + model-switch eviction
+    # too, not just truncate (#5096 Bug D).
+    _run_active = False
+    try:
+        with ACTIVE_RUNS_LOCK:
+            for _entry in (ACTIVE_RUNS or {}).values():
+                if (_entry or {}).get("session_id") == session_id:
+                    _run_active = True
+                    break
+    except Exception:
+        _run_active = False
+    if _run_active:
+        return
     should_close = True
     try:
         from api.session_lifecycle import commit_session_memory, discard_session, has_uncommitted_work, unregister_agent
diff --git a/tests/test_evict_session_agent_active_run_guard.py b/tests/test_evict_session_agent_active_run_guard.py
new file mode 100644
index 000000000..c1033fb0b
--- /dev/null
+++ b/tests/test_evict_session_agent_active_run_guard.py
@@ -0,0 +1,80 @@
+"""Regression: _evict_session_agent must not close a live session's _session_db.
+
+Bug-D follow-up (#5096): truncate/clear/model-switch all call
+api.config._evict_session_agent(). The worker assigns agent._session_db at run
+start, so eviction must consult ACTIVE_RUNS (the authoritative liveness signal,
+same as the worker's own LRU-eviction guard) and skip the lifecycle commit +
+_session_db.close() while a run is in flight on that session. Otherwise a
+truncate racing an in-flight turn on the same session (reachable via a second
+client / direct API; the UI gates it behind S.busy) closes the SessionDB the
+running worker is still persisting through.
+"""
+
+import api.config as config
+
+
+class _FakeSessionDB:
+    def __init__(self):
+        self.closed = False
+
+    def close(self):
+        self.closed = True
+
+
+class _FakeAgent:
+    def __init__(self):
+        self._session_db = _FakeSessionDB()
+
+
+def _seed_cache(session_id, agent):
+    with config.SESSION_AGENT_CACHE_LOCK:
+        config.SESSION_AGENT_CACHE[session_id] = (agent, "sig")
+
+
+def _clear_active_runs():
+    with config.ACTIVE_RUNS_LOCK:
+        config.ACTIVE_RUNS.clear()
+
+
+def test_evict_skips_session_db_close_when_run_active(monkeypatch):
+    """A live run on the session => the cache handle drops but the DB stays open."""
+    sid = "live-session-evict-guard"
+    agent = _FakeAgent()
+    _seed_cache(sid, agent)
+    _clear_active_runs()
+    config.register_active_run("stream-xyz", session_id=sid)
+    try:
+        config._evict_session_agent(sid)
+        # The agent handle is removed from the cache (harmless — the worker
+        # holds its own local ref) ...
+        with config.SESSION_AGENT_CACHE_LOCK:
+            assert sid not in config.SESSION_AGENT_CACHE
+        # ... but the live worker's SessionDB must NOT be closed.
+        assert agent._session_db.closed is False
+    finally:
+        _clear_active_runs()
+        with config.SESSION_AGENT_CACHE_LOCK:
+            config.SESSION_AGENT_CACHE.pop(sid, None)
+
+
+def test_evict_closes_session_db_when_no_run_active(monkeypatch):
+    """No live run => normal eviction closes the SessionDB (idle path unchanged)."""
+    sid = "idle-session-evict-guard"
+    agent = _FakeAgent()
+    _seed_cache(sid, agent)
+    _clear_active_runs()
+
+    # Neutralize the lifecycle commit machinery so the test isolates the
+    # ACTIVE_RUNS guard + close decision (no uncommitted work => should_close).
+    monkeypatch.setattr("api.session_lifecycle.has_uncommitted_work", lambda *_a, **_k: False)
+    monkeypatch.setattr("api.session_lifecycle.unregister_agent", lambda *_a, **_k: None)
+    monkeypatch.setattr("api.session_lifecycle.discard_session", lambda *_a, **_k: None)
+    try:
+        config._evict_session_agent(sid)
+        with config.SESSION_AGENT_CACHE_LOCK:
+            assert sid not in config.SESSION_AGENT_CACHE
+        assert agent._session_db.closed is True
+    finally:
+        _clear_active_runs()
+        with config.SESSION_AGENT_CACHE_LOCK:
+            config.SESSION_AGENT_CACHE.pop(sid, None)