diff --git a/src-new/astrbot_sdk/protocol/wire_codecs.py b/src-new/astrbot_sdk/protocol/wire_codecs.py
index db326553b..494df4435 100644
--- a/src-new/astrbot_sdk/protocol/wire_codecs.py
+++ b/src-new/astrbot_sdk/protocol/wire_codecs.py
@@ -18,7 +18,7 @@ class ProtocolCodec(ABC):
     websocket_frame_type: WebSocketFrameType
 
     @abstractmethod
-    def encode_message(self, message: ProtocolMessage) -> bytes:
+    def encode_message(self, message: ProtocolMessage) -> bytes | str:
         raise NotImplementedError
 
     @abstractmethod
@@ -31,8 +31,8 @@ class JsonProtocolCodec(ProtocolCodec):
     stdio_framing: StdioFraming = "line"
     websocket_frame_type: WebSocketFrameType = "text"
 
-    def encode_message(self, message: ProtocolMessage) -> bytes:
-        return message.model_dump_json(exclude_none=True).encode("utf-8")
+    def encode_message(self, message: ProtocolMessage) -> str:
+        return message.model_dump_json(exclude_none=True)
 
     def decode_message(self, payload: bytes | str) -> ProtocolMessage:
         if isinstance(payload, bytes):
diff --git a/src-new/astrbot_sdk/runtime/bootstrap.py b/src-new/astrbot_sdk/runtime/bootstrap.py
index e0f26b709..14fd0138b 100644
--- a/src-new/astrbot_sdk/runtime/bootstrap.py
+++ b/src-new/astrbot_sdk/runtime/bootstrap.py
@@ -55,22 +55,16 @@ async def run_supervisor(
     env_manager: PluginEnvironmentManager | None = None,
     wire_codec: str = "json",
 ) -> None:
-    codec = make_protocol_codec(wire_codec)
     transport_stdin, transport_stdout, original_stdout = _prepare_stdio_transport(
         stdin,
         stdout,
-        binary=codec.stdio_framing == "length_prefixed",
-    )
-    transport = StdioTransport(
-        stdin=transport_stdin,
-        stdout=transport_stdout,
-        framing=codec.stdio_framing,
     )
+    transport = StdioTransport(stdin=transport_stdin, stdout=transport_stdout)
     runtime = SupervisorRuntime(
         transport=transport,
         plugins_dir=plugins_dir,
         env_manager=env_manager,
-        codec=codec,
+        worker_wire_codec_name=wire_codec,
     )
 
     try:
diff --git a/src-new/astrbot_sdk/runtime/supervisor.py b/src-new/astrbot_sdk/runtime/supervisor.py
index 14ad9b109..80211fc88 100644
--- a/src-new/astrbot_sdk/runtime/supervisor.py
+++ b/src-new/astrbot_sdk/runtime/supervisor.py
@@ -395,14 +395,14 @@ class SupervisorRuntime:
         plugins_dir: Path,
         env_manager: PluginEnvironmentManager | None = None,
         codec: ProtocolCodec | None = None,
-        wire_codec_name: str = "json",
+        worker_wire_codec_name: str = "json",
     ) -> None:
         self.transport = transport
         self.plugins_dir = plugins_dir.resolve()
         self.repo_root = Path(__file__).resolve().parents[3]
         self.env_manager = env_manager or PluginEnvironmentManager(self.repo_root)
-        self.codec = codec or make_protocol_codec(wire_codec_name)
-        self.wire_codec_name = self.codec.name
+        self.codec = codec or make_protocol_codec("json")
+        self.worker_wire_codec_name = worker_wire_codec_name
         self.capability_router = CapabilityRouter()
         self.peer = Peer(
             transport=self.transport,
@@ -646,7 +646,7 @@ class SupervisorRuntime:
                             repo_root=self.repo_root,
                             env_manager=self.env_manager,
                             capability_router=self.capability_router,
-                            wire_codec_name=self.wire_codec_name,
+                            wire_codec_name=self.worker_wire_codec_name,
                             on_closed=lambda group_id=group.id: (
                                 self._handle_worker_closed(group_id)
                             ),
@@ -660,7 +660,7 @@ class SupervisorRuntime:
                             repo_root=self.repo_root,
                             env_manager=self.env_manager,
                             capability_router=self.capability_router,
-                            wire_codec_name=self.wire_codec_name,
+                            wire_codec_name=self.worker_wire_codec_name,
                             on_closed=lambda plugin_name=plugin.name: (
                                 self._handle_worker_closed(plugin_name)
                             ),
diff --git a/tests_v4/test_peer.py b/tests_v4/test_peer.py
index 0801317aa..7a8aab17b 100644
--- a/tests_v4/test_peer.py
+++ b/tests_v4/test_peer.py
@@ -17,6 +17,7 @@ from astrbot_sdk.protocol.wire_codecs import MsgpackProtocolCodec
 from astrbot_sdk.runtime.capability_router import CapabilityRouter, StreamExecution
 from astrbot_sdk.runtime.peer import Peer
 from astrbot_sdk.runtime.transport import (
+    Transport,
     WebSocketClientTransport,
     WebSocketServerTransport,
 )
@@ -41,6 +42,21 @@ def make_linked_transport_pair() -> tuple[LinkedMemoryTransport, LinkedMemoryTra
     return left, right
 
 
+class RecordingTextTransport(Transport):
+    def __init__(self) -> None:
+        super().__init__()
+        self.sent_payloads: list[bytes | str] = []
+
+    async def start(self) -> None:
+        self._closed.clear()
+
+    async def stop(self) -> None:
+        self._closed.set()
+
+    async def send(self, payload):
+        self.sent_payloads.append(payload)
+
+
 class PeerRuntimeTest(unittest.IsolatedAsyncioTestCase):
     async def asyncSetUp(self) -> None:
         self.left, self.right = make_transport_pair()
@@ -90,6 +106,20 @@ class PeerRuntimeTest(unittest.IsolatedAsyncioTestCase):
         await plugin.stop()
         await core.stop()
 
+    async def test_default_json_codec_preserves_text_transport_payloads(self) -> None:
+        transport = RecordingTextTransport()
+        peer = Peer(
+            transport=transport,
+            peer_info=PeerInfo(name="plugin", role="plugin", version="v4"),
+        )
+
+        await peer.start()
+        await peer.cancel("request-1")
+        await peer.stop()
+
+        self.assertEqual(len(transport.sent_payloads), 1)
+        self.assertIsInstance(transport.sent_payloads[0], str)
+
     async def test_initialize_carries_remote_provided_capabilities(self) -> None:
         provided = CapabilityDescriptor(
             name="demo.echo",
diff --git a/tests_v4/test_wire_codecs.py b/tests_v4/test_wire_codecs.py
index 92f48e41f..cacbac042 100644
--- a/tests_v4/test_wire_codecs.py
+++ b/tests_v4/test_wire_codecs.py
@@ -34,7 +34,7 @@ class TestJsonProtocolCodec:
         encoded = codec.encode_message(message)
         decoded = codec.decode_message(encoded)
 
-        assert isinstance(encoded, bytes)
+        assert isinstance(encoded, str)
         assert decoded == message