diff --git a/headroom/proxy/handlers/streaming.py b/headroom/proxy/handlers/streaming.py
index 060c18aaa..0b875d923 100644
--- a/headroom/proxy/handlers/streaming.py
+++ b/headroom/proxy/handlers/streaming.py
@@ -588,6 +588,38 @@ class StreamingMixin:
         from headroom.proxy.cost import _summarize_transforms
 
         total_latency = (time.time() - start_time) * 1000
+
+        # Per-chunk SSE parsing only flushes events terminated by ``\n\n``.
+        # When upstream truncates mid-event (client disconnect, network
+        # drop, connection reset), the message_start (cache_read /
+        # cache_creation) or message_delta (output_tokens) usage events
+        # can sit in the residual buffer and never be parsed — surfacing
+        # as cache_read=cache_write=0 in PERF logs and poisoning the
+        # downstream freeze heuristic for the next request. Append the
+        # terminator so the buffer parser drains whatever's there. The
+        # per-event try/except in the parser swallows incomplete JSON,
+        # so this is safe even when the truncation cut mid-payload.
+        sse_buffer = stream_state.get("sse_buffer")
+        if isinstance(sse_buffer, bytearray) and len(sse_buffer) > 0:
+            sse_buffer.extend(b"\n\n")
+            late_usage = self._parse_sse_usage_from_buffer(stream_state, provider) or {}
+            for key in (
+                "input_tokens",
+                "output_tokens",
+                "cache_read_input_tokens",
+                "cache_creation_input_tokens",
+                "cache_creation_ephemeral_5m_input_tokens",
+                "cache_creation_ephemeral_1h_input_tokens",
+            ):
+                if key not in late_usage:
+                    continue
+                current = stream_state.get(key)
+                # Only fill in unset (None) or default-zero slots so a
+                # real cache_read=0 from earlier in the stream isn't
+                # clobbered by a later partial event.
+                if current is None or current == 0:
+                    stream_state[key] = late_usage[key]
+
         output_tokens = stream_state["output_tokens"]
         if output_tokens is None:
             output_tokens = stream_state["total_bytes"] // 40
diff --git a/tests/test_proxy_streaming_request_logger.py b/tests/test_proxy_streaming_request_logger.py
index aad6d50c0..035d9570c 100644
--- a/tests/test_proxy_streaming_request_logger.py
+++ b/tests/test_proxy_streaming_request_logger.py
@@ -228,6 +228,57 @@ async def test_finalize_openai_responses_stream_uses_provider_usage_for_dashboar
     assert cost_kwargs["uncached_tokens"] == 186_600
 
 
+@pytest.mark.asyncio
+async def test_finalize_stream_response_recovers_usage_from_truncated_buffer() -> None:
+    """When upstream truncates mid-event (no trailing \\n\\n), the per-chunk
+    parser leaves the message_start usage event sitting in sse_buffer and
+    PERF logs cache_read=cache_write=0 — which then poisons the freeze
+    heuristic on the next request. The finalizer must flush the residual
+    buffer so the real cache_read / cache_creation tokens still land in
+    the log even on aborted streams.
+    """
+    proxy = _build_proxy_with_real_logger(log_full_messages=False)
+
+    partial_message_start = (
+        b"event: message_start\n"
+        b'data: {"type":"message_start","message":{"id":"msg_x",'
+        b'"type":"message","role":"assistant","model":"claude-sonnet-4-6",'
+        b'"content":[],"stop_reason":null,"usage":{'
+        b'"input_tokens":1234,"cache_read_input_tokens":50000,'
+        b'"cache_creation_input_tokens":2500,"output_tokens":1}}}'
+    )
+
+    state = {
+        "output_tokens": None,
+        "total_bytes": len(partial_message_start),
+        "ttfb_ms": 35.0,
+        "input_tokens": None,
+        "cache_read_input_tokens": 0,
+        "cache_creation_input_tokens": 0,
+        "cache_creation_ephemeral_5m_input_tokens": 0,
+        "cache_creation_ephemeral_1h_input_tokens": 0,
+        "sse_buffer": bytearray(partial_message_start),
+    }
+
+    await proxy._finalize_stream_response(
+        body={"messages": [{"role": "user", "content": "hi"}]},
+        provider="anthropic",
+        model="claude-sonnet-4-6",
+        request_id="req-stream-truncated",
+        original_tokens=2000,
+        optimized_tokens=1800,
+        tokens_saved=200,
+        transforms_applied=[],
+        optimization_latency=5.0,
+        stream_state=state,
+        start_time=0.0,
+    )
+
+    assert state["input_tokens"] == 1234
+    assert state["cache_read_input_tokens"] == 50000
+    assert state["cache_creation_input_tokens"] == 2500
+
+
 @pytest.mark.asyncio
 async def test_finalize_stream_response_no_op_when_logger_disabled():
     proxy = _build_proxy_with_real_logger(log_full_messages=False)