diff --git a/src/plyngent/lmproto/openai_compatible/client.py b/src/plyngent/lmproto/openai_compatible/client.py
index 0004774..fce7bd1 100644
--- a/src/plyngent/lmproto/openai_compatible/client.py
+++ b/src/plyngent/lmproto/openai_compatible/client.py
@@ -25,6 +25,10 @@ if TYPE_CHECKING:
     from niquests.models import AsyncResponse
 
 
+_HTTP_ERROR = 400
+_ERROR_BODY_PREVIEW = 500
+
+
 async def _close_async_response(resp: AsyncResponse) -> None:
     """Close a streaming response; support sync or async close()."""
     for name in ("aclose", "close"):
@@ -37,6 +41,43 @@ async def _close_async_response(resp: AsyncResponse) -> None:
         return
 
 
+def sse_data_payload(line: bytes) -> bytes | None | Literal[False]:
+    """Parse one SSE line.
+
+    Returns:
+        ``bytes`` payload after ``data: ``,
+        ``None`` to skip (comment/empty/non-data),
+        ``False`` when the stream is finished (``[DONE]``).
+    """
+    stripped = line.strip()
+    if not stripped:
+        return None
+    if stripped in {b"data: [DONE]", b"[DONE]"}:
+        return False
+    if not stripped.startswith(b"data: "):
+        return None
+    payload = stripped[6:].strip()
+    if payload == b"[DONE]":
+        return False
+    if not payload:
+        return None
+    return payload
+
+
+def http_error_message(status_code: int, body: bytes | str | None = None) -> str | None:
+    """Return an error string for HTTP ``status_code`` >= 400, else ``None``."""
+    if status_code < _HTTP_ERROR:
+        return None
+    msg = f"chat completions HTTP {status_code}"
+    if body is None:
+        return msg
+    if isinstance(body, (bytes, bytearray)):
+        text = bytes(body[:_ERROR_BODY_PREVIEW]).decode(errors="replace")
+    else:
+        text = str(body)[:_ERROR_BODY_PREVIEW]
+    return f"{msg}: {text}" if text else msg
+
+
 class BaseOpenAIClient:
     session: AsyncSession
     encoder: msgspec.json.Encoder
@@ -54,22 +95,20 @@ class BaseOpenAIClient:
 
     async def _ensure_ok(self, resp: object) -> None:
         """Raise if the HTTP response is an error (stream or non-stream)."""
-        http_error = 400
         status = getattr(resp, "status_code", None)
-        if not isinstance(status, int) or status < http_error:
+        if not isinstance(status, int):
             return
-        body = ""
         content = getattr(resp, "content", None)
-        if content is not None:
-            if isinstance(content, (bytes, bytearray)):
-                body = bytes(content[:500]).decode(errors="replace")
-            else:
-                body = str(content)[:500]
+        body: bytes | str | None = None
+        if isinstance(content, bytes | bytearray):
+            body = bytes(content)
+        elif isinstance(content, str):
+            body = content
+        msg = http_error_message(status, body)
+        if msg is None:
+            return
         if hasattr(resp, "close") or hasattr(resp, "aclose"):
             await _close_async_response(cast("AsyncResponse", resp))
-        msg = f"chat completions HTTP {status}"
-        if body:
-            msg = f"{msg}: {body}"
         raise RuntimeError(msg)
 
     async def _parse_sse(self, resp: AsyncResponse) -> AsyncIterator[ChatCompletionChunk]:
@@ -79,17 +118,12 @@ class BaseOpenAIClient:
             async for raw in resp.iter_lines():
                 if not raw:
                     continue
-                line = bytes(raw).strip()
-                if line in {b"data: [DONE]", b"[DONE]"}:
-                    break
-                if not line.startswith(b"data: "):
+                parsed = sse_data_payload(bytes(raw))
+                if parsed is None:
                     continue
-                payload = line[6:].strip()
-                if payload == b"[DONE]":
+                if parsed is False:
                     break
-                if not payload:
-                    continue
-                yield self.chunk_decoder.decode(payload)
+                yield self.chunk_decoder.decode(parsed)
         finally:
             await _close_async_response(resp)
 
diff --git a/tests/test_lmproto/test_sse_parse.py b/tests/test_lmproto/test_sse_parse.py
index c6ecb17..878d014 100644
--- a/tests/test_lmproto/test_sse_parse.py
+++ b/tests/test_lmproto/test_sse_parse.py
@@ -1,57 +1,71 @@
 from __future__ import annotations
 
-from typing import TYPE_CHECKING, cast
-
-from plyngent.lmproto.openai_compatible.client import BaseOpenAIClient
-from plyngent.lmproto.openai_compatible.config import OpenAIConfig
-
-if TYPE_CHECKING:
-    from collections.abc import AsyncIterator
-
-    from niquests.models import AsyncResponse
+from plyngent.lmproto.openai_compatible.client import http_error_message, sse_data_payload
+from plyngent.lmproto.openai_compatible.model import ChatCompletionChunk
 
 
-class _FakeResp:
-    status_code: int
-    closed: bool
-    content: bytes
-    _lines: list[bytes]
-
-    def __init__(self, lines: list[bytes], *, status_code: int = 200) -> None:
-        self.status_code = status_code
-        self.closed = False
-        self._lines = lines
-        self.content = b""
-
-    async def iter_lines(self) -> AsyncIterator[bytes]:
-        for line in self._lines:
-            yield line
-
-    def close(self) -> None:
-        self.closed = True
+def test_sse_data_payload_done_variants() -> None:
+    assert sse_data_payload(b"data: [DONE]") is False
+    assert sse_data_payload(b"[DONE]") is False
+    assert sse_data_payload(b"data: [DONE]\n") is False
+    assert sse_data_payload(b"data:  [DONE]  ") is False
 
 
-async def test_parse_sse_stops_at_done() -> None:
-    client = BaseOpenAIClient(OpenAIConfig(access_key_or_token="t", base_url="http://x"))
-    # Minimal OpenAI-style chunk payload
-    chunk = (
+def test_sse_data_payload_skip() -> None:
+    assert sse_data_payload(b"") is None
+    assert sse_data_payload(b": comment") is None
+    assert sse_data_payload(b"event: message") is None
+    assert sse_data_payload(b"data: ") is None
+
+
+def test_sse_data_payload_json() -> None:
+    line = (
         b'data: {"id":"1","object":"chat.completion.chunk","created":0,'
         b'"model":"m","choices":[{"index":0,"delta":{"content":"hi"},'
         b'"finish_reason":null}]}'
     )
-    resp = _FakeResp([chunk, b"", b"data: [DONE]", b"data: should-not-parse"])
-    out = [c async for c in client._parse_sse(cast("AsyncResponse", cast("object", resp)))]
-    assert len(out) == 1
-    assert out[0].choices[0].delta.content == "hi"
-    assert resp.closed is True
+    payload = sse_data_payload(line)
+    assert isinstance(payload, bytes)
+    chunk = msgspec_decode_chunk(payload)
+    assert chunk.choices[0].delta.content == "hi"
 
 
-async def test_parse_sse_http_error() -> None:
-    import pytest
+def msgspec_decode_chunk(payload: bytes) -> ChatCompletionChunk:
+    import msgspec
 
-    client = BaseOpenAIClient(OpenAIConfig(access_key_or_token="t", base_url="http://x"))
-    resp = _FakeResp([], status_code=500)
-    resp.content = b'{"error":"boom"}'
-    with pytest.raises(RuntimeError, match="500"):
-        _ = [c async for c in client._parse_sse(cast("AsyncResponse", cast("object", resp)))]
-    assert resp.closed is True
+    return msgspec.json.decode(payload, type=ChatCompletionChunk)
+
+
+def test_sse_stream_stops_at_done() -> None:
+    """Simulate a line iterator: only pre-DONE data lines are decoded."""
+    lines = [
+        b'data: {"id":"1","object":"chat.completion.chunk","created":0,'
+        b'"model":"m","choices":[{"index":0,"delta":{"content":"a"},'
+        b'"finish_reason":null}]}',
+        b"",
+        b"data: [DONE]",
+        b'data: {"id":"1","choices":[{"delta":{"content":"never"}}]}',
+    ]
+    payloads: list[bytes] = []
+    for line in lines:
+        parsed = sse_data_payload(line)
+        if parsed is None:
+            continue
+        if parsed is False:
+            break
+        payloads.append(parsed)
+    assert len(payloads) == 1
+    assert msgspec_decode_chunk(payloads[0]).choices[0].delta.content == "a"
+
+
+def test_http_error_message() -> None:
+    assert http_error_message(200) is None
+    assert http_error_message(399) is None
+    err = http_error_message(500, b'{"error":"boom"}')
+    assert err is not None
+    assert "500" in err
+    assert "boom" in err
+    err503 = http_error_message(503, "unavailable")
+    assert err503 is not None
+    assert "503" in err503
+    assert "unavailable" in err503