diff --git a/backend/CLAUDE.md b/backend/CLAUDE.md
index a7a22e7af..6ab5b69e7 100644
--- a/backend/CLAUDE.md
+++ b/backend/CLAUDE.md
@@ -122,10 +122,12 @@ Blocking-IO runtime gate (`tests/blocking_io/`):
   `tests/support/detectors/blocking_io_runtime.py`). Any sync blocking IO
   call whose stack passes through DeerFlow business code while running on
   the asyncio event loop raises `BlockingError` and fails the test.
-- Two regression anchors live there: `test_skills_load.py` (locks the
+- Regression anchors live there: `test_skills_load.py` (locks the
   `asyncio.to_thread` offload around `LocalSkillStorage.load_skills`, fix
-  for #1917) and `test_sqlite_lifespan.py` (locks the offload around
-  SQLite path resolution plus `ensure_sqlite_parent_dir`, fix for #1912).
+  for #1917); `test_sqlite_lifespan.py` (locks the offload around
+  SQLite path resolution plus `ensure_sqlite_parent_dir`, fix for #1912);
+  and `test_jsonl_run_event_store.py` (locks `JsonlRunEventStore`'s async
+  API offloading its file IO via `asyncio.to_thread`, fix #3084).
 - `test_gate_smoke.py` is a meta-test asserting the gate actually catches
   unoffloaded blocking IO and that the `@pytest.mark.allow_blocking_io`
   opt-out works.
diff --git a/backend/docs/BLOCKING_IO_DETECTION.md b/backend/docs/BLOCKING_IO_DETECTION.md
index 8cfc49ada..b874020fa 100644
--- a/backend/docs/BLOCKING_IO_DETECTION.md
+++ b/backend/docs/BLOCKING_IO_DETECTION.md
@@ -134,11 +134,16 @@ that production actually executes.
 
 ## Current runtime coverage
 
-The initial runtime anchors protect confirmed blocking-IO bug shapes:
+The runtime anchors protect confirmed blocking-IO bug shapes:
 
 - SQLite checkpointer setup, including path resolution and parent-directory
   creation.
 - Subagent skill metadata loading through `SubagentExecutor._load_skills()`.
+- `JsonlRunEventStore` async API (`put` / `list_*` / `delete_*`): the JSONL
+  run-event backend offloads its synchronous file IO via `asyncio.to_thread`
+  (fix #3084); this anchor drives the real async API under the gate so any
+  blocking IO reintroduced on the loop fails, not only removal of one
+  `to_thread` call.
 - Gate health checks: Blockbuster catches unoffloaded calls, opt-out works, and
   patches are restored after exceptions.
 
diff --git a/backend/tests/blocking_io/test_jsonl_run_event_store.py b/backend/tests/blocking_io/test_jsonl_run_event_store.py
new file mode 100644
index 000000000..a7590de1a
--- /dev/null
+++ b/backend/tests/blocking_io/test_jsonl_run_event_store.py
@@ -0,0 +1,62 @@
+"""Regression anchor: JsonlRunEventStore async API must not block the loop.
+
+``JsonlRunEventStore`` is the ``run_events.backend == "jsonl"`` implementation.
+Its ``async def`` methods perform synchronous filesystem IO (``Path.glob``,
+``read_text``, ``open``, ``unlink``) that must be offloaded with
+``asyncio.to_thread`` (fixed in #3084). ``put`` runs on every emitted run event,
+so any blocking IO here stalls the event loop on the hot path.
+
+#3084 added a mock-based offload assertion in
+``tests/test_jsonl_event_store_async_io.py`` that covers ``put`` only. This
+anchor complements it by driving the **full** async surface (``put``,
+``put_batch``, ``list_messages``, ``list_events``, ``list_messages_by_run``,
+``count_messages``, ``delete_by_run``, ``delete_by_thread``) under the strict
+Blockbuster runtime gate, so any blocking IO reintroduced on the event loop in
+any of these methods — not just removal of a specific ``to_thread`` call —
+fails CI.
+"""
+
+from __future__ import annotations
+
+from pathlib import Path
+
+import pytest
+
+pytestmark = pytest.mark.asyncio
+
+
+async def test_jsonl_run_event_store_async_api_does_not_block_event_loop(tmp_path: Path) -> None:
+    from deerflow.runtime.events.store.jsonl import JsonlRunEventStore
+
+    store = JsonlRunEventStore(base_dir=str(tmp_path))
+
+    # Seed an existing run file so put()'s seq-load globs + reads, and the
+    # read/delete paths have files to scan. Test-side IO is invisible to the
+    # gate (this module is not in scanned_modules).
+    thread_dir = tmp_path / "threads" / "t1" / "runs"
+    thread_dir.mkdir(parents=True, exist_ok=True)
+    (thread_dir / "r0.jsonl").write_text('{"seq": 1, "category": "message", "run_id": "r0"}\n', encoding="utf-8")
+
+    # writes: put + put_batch
+    record = await store.put(thread_id="t1", run_id="r1", event_type="message", category="message", content="hi")
+    assert record["seq"] >= 2
+    batch = await store.put_batch(
+        [
+            {"thread_id": "t1", "run_id": "r2", "event_type": "message", "category": "message", "content": "a"},
+            {"thread_id": "t1", "run_id": "r2", "event_type": "trace", "category": "trace", "content": "b"},
+        ]
+    )
+    assert len(batch) == 2
+
+    # reads: list_messages / list_events / list_messages_by_run / count_messages.
+    # list_events is exercised both without and with the event_types filter so
+    # the filter branch runs after _read_run_events' filesystem IO.
+    assert isinstance(await store.list_messages("t1"), list)
+    assert isinstance(await store.list_events("t1", "r1"), list)
+    assert isinstance(await store.list_events("t1", "r1", event_types=["message"]), list)
+    assert isinstance(await store.list_messages_by_run("t1", "r2"), list)
+    assert await store.count_messages("t1") >= 1
+
+    # deletes: delete_by_run (single file) then delete_by_thread (remaining)
+    assert await store.delete_by_run("t1", "r2") >= 1
+    assert await store.delete_by_thread("t1") >= 1