diff --git a/src/agents/sandbox/session/sinks.py b/src/agents/sandbox/session/sinks.py
index 77d90cc086..d9fdfee609 100644
--- a/src/agents/sandbox/session/sinks.py
+++ b/src/agents/sandbox/session/sinks.py
@@ -8,7 +8,6 @@
 from pathlib import Path
 from types import ModuleType
 from typing import Literal, Protocol, runtime_checkable
-from urllib.error import HTTPError, URLError
 from urllib.request import Request, urlopen
 
 from ..errors import WorkspaceReadNotFoundError
@@ -181,7 +180,6 @@ def __init__(
         self._seen = 0
         self._lock = asyncio.Lock()
         self._flush_every = max(1, int(flush_every))
-        self._existing_outbox_loaded = False
 
     def _resolve_relpath(self) -> Path:
         rel = self.workspace_relpath
@@ -234,23 +232,23 @@ async def _can_flush_to_workspace(self) -> bool:
             return False
 
     async def _flush_buffer(self) -> None:
-        if self._session is None:
+        if self._session is None or not self._buf:
             return
 
-        await self._ensure_existing_outbox_loaded()
         relpath = self._resolved_workspace_relpath or self.workspace_relpath
-        await self._session.write(relpath, io.BytesIO(bytes(self._buf)))
+        existing = await self._read_existing_outbox(relpath)
+        pending = bytes(self._buf)
+        await self._session.write(relpath, io.BytesIO(existing + pending))
+        self._buf.clear()
 
-    async def _ensure_existing_outbox_loaded(self) -> None:
-        if self._session is None or self._existing_outbox_loaded:
-            return
+    async def _read_existing_outbox(self, relpath: Path) -> bytes:
+        if self._session is None:
+            return b""
 
-        relpath = self._resolved_workspace_relpath or self.workspace_relpath
         try:
             existing = await self._session.read(relpath)
         except (FileNotFoundError, WorkspaceReadNotFoundError):
-            self._existing_outbox_loaded = True
-            return
+            return b""
 
         try:
             payload = existing.read()
@@ -258,10 +256,8 @@ async def _ensure_existing_outbox_loaded(self) -> None:
             existing.close()
 
         if isinstance(payload, str):
-            payload = payload.encode("utf-8")
-        if payload:
-            self._buf = bytearray(payload) + self._buf
-        self._existing_outbox_loaded = True
+            return payload.encode("utf-8")
+        return bytes(payload)
 
     async def handle(self, event: SandboxSessionEvent) -> None:
         # If unbound (e.g., audit event emission used without a SandboxSession wrapper),
@@ -317,7 +313,7 @@ def _post(self, body: bytes, spool_line: str | None) -> None:
         try:
             with urlopen(req, timeout=self.timeout_s) as resp:
                 _ = resp.read(1)  # ensure request completes
-        except (HTTPError, URLError) as e:
+        except OSError as e:
             if spool_line is not None and self.spool_path is not None:
                 try:
                     self.spool_path.parent.mkdir(parents=True, exist_ok=True)
diff --git a/tests/sandbox/test_session_sinks.py b/tests/sandbox/test_session_sinks.py
index 420f142f3e..09efbd83ca 100644
--- a/tests/sandbox/test_session_sinks.py
+++ b/tests/sandbox/test_session_sinks.py
@@ -6,6 +6,7 @@
 import tarfile
 import uuid
 from pathlib import Path
+from unittest.mock import patch
 
 import pytest
 from inline_snapshot import snapshot
@@ -21,6 +22,7 @@
     CallbackSink,
     ChainedSink,
     EventPayloadPolicy,
+    HttpProxySink,
     Instrumentation,
     JsonlOutboxSink,
     SandboxSession,
@@ -277,6 +279,32 @@ async def test_workspace_jsonl_sink_does_not_duplicate_lines_across_flushes(
     assert [json.loads(line)["seq"] for line in lines] == [1, 2, 3]
 
 
+@pytest.mark.asyncio
+async def test_workspace_jsonl_sink_clears_flushed_buffer(tmp_path: Path) -> None:
+    inner = _build_unix_local_session(tmp_path)
+    relpath = Path(f"logs/events-{inner.state.session_id}.jsonl")
+
+    async with inner:
+        sink = WorkspaceJsonlSink(mode="sync", on_error="raise", ephemeral=False, flush_every=1)
+        sink.bind(inner)
+
+        for seq in (1, 2):
+            await sink.handle(
+                SandboxSessionStartEvent(
+                    session_id=inner.state.session_id,
+                    seq=seq,
+                    op="write",
+                    span_id=str(uuid.uuid4()),
+                )
+            )
+            assert sink._buf == bytearray()
+
+        outbox_stream = await inner.read(relpath)
+        lines = outbox_stream.read().decode("utf-8").splitlines()
+
+    assert [json.loads(line)["seq"] for line in lines] == [1, 2]
+
+
 @pytest.mark.asyncio
 async def test_workspace_jsonl_sink_ephemeral_excludes_runtime_outbox_with_existing_parent(
     tmp_path: Path,
@@ -363,6 +391,31 @@ def _callback(event: SandboxSessionEvent, session: BaseSandboxSession) -> None:
     assert all(session is inner for _op, session in seen)
 
 
+@pytest.mark.asyncio
+async def test_http_proxy_sink_spools_direct_timeout(tmp_path: Path) -> None:
+    spool_path = tmp_path / "events.jsonl"
+    sink = HttpProxySink(
+        "http://127.0.0.1:9/events",
+        mode="sync",
+        on_error="raise",
+        spool_path=spool_path,
+    )
+    event = SandboxSessionStartEvent(
+        session_id=uuid.uuid4(),
+        seq=1,
+        op="write",
+        span_id=str(uuid.uuid4()),
+    )
+
+    with patch("agents.sandbox.session.sinks.urlopen", side_effect=TimeoutError("timed out")):
+        with pytest.raises(RuntimeError, match="http proxy sink POST failed"):
+            await sink.handle(event)
+
+    lines = spool_path.read_text(encoding="utf-8").splitlines()
+    assert len(lines) == 1
+    assert json.loads(lines[0])["seq"] == 1
+
+
 @pytest.mark.asyncio
 async def test_sandbox_session_error_events_and_traces_include_retryability(
     tmp_path: Path,