asyncio StreamReader LimitOverrunError causes ACP server crash on large JSON-RPC messages

[ZENOTME]

What version of Kimi Code CLI is running?

main

Which open platform/subscription were you using?

kimi code

Which model were you using?

No response

What platform is your computer?

No response

What issue are you seeing?

Problem Summary

When using ACP (Agent Communication Protocol) over stdio, the server process crashes with LimitOver
runError when the client sends a single-line JSON-RPC message exceeding 64KB (the default asyncio.S
treamReader buffer limit). E..g the client is responible to read big file.

Error Log

  asyncio.exceptions.LimitOverrunError: Separator is found, but chunk is longer than limit
  ValueError: Separator is found, but chunk is longer than limit

Stack trace points to:

  File "acp/connection.py", line 151, in _receive_loop
      line = await self._reader.readline()

Root Cause Analysis

1. Protocol Constraint: JSON-RPC over stdio requires each message to be a single line terminated by
2. asyncio Implementation: StreamReader.readline() uses a fixed-size buffer (default 64KB) and rais
   LimitOverrunError when the buffer is full but no separator is found
3. No Backpressure: Unlike TCP, stdio/pipes lack flow control mechanisms - the reader cannot signal
   e writer to pause
4. Fatal Error: ACP library doesn't handle LimitOverrunError, causing the connection loop to crash
   the process to exit with code 1

Current Workaround

Increase the buffer limit to 100MB when creating stdio streams:

reader, writer = await acp.stdio_streams(limit=100 * 1024 * 1024)

What steps can reproduce the bug?

Config ACP client config to have file read capability so that the agent will request the ACP client to read the file.

What is the expected behavior?

No response

Additional information

No response

[xiaoju111a]

#758

[cjnchk]

+1, my error log

2026-02-04 23:02:52.948 | ERROR    | threading:run:992 - ERROR:root:Connection main loop failed
2026-02-04 23:02:52.949 | ERROR    | threading:run:992 - Traceback (most recent call last):
2026-02-04 23:02:52.950 | ERROR    | threading:run:992 -   File "/Users/cjnchk/.pyenv/versions/3.13.3/lib/python3.13/asyncio/streams.py", line 562, in readline
2026-02-04 23:02:52.951 | ERROR    | threading:run:992 -     line = await self.readuntil(sep)
2026-02-04 23:02:52.951 | ERROR    | threading:run:992 -            ^^^^^^^^^^^^^^^^^^^^^^^^^
2026-02-04 23:02:52.952 | ERROR    | threading:run:992 -   File "/Users/cjnchk/.pyenv/versions/3.13.3/lib/python3.13/asyncio/streams.py", line 663, in readuntil
2026-02-04 23:02:52.952 | ERROR    | threading:run:992 -     raise exceptions.LimitOverrunError(
2026-02-04 23:02:52.952 | ERROR    | threading:run:992 -         'Separator is not found, and chunk exceed the limit',
2026-02-04 23:02:52.952 | ERROR    | threading:run:992 -         offset)
2026-02-04 23:02:52.953 | ERROR    | threading:run:992 - asyncio.exceptions.LimitOverrunError: Separator is not found, and chunk exceed the limit
2026-02-04 23:02:52.953 | ERROR    | threading:run:992 - During handling of the above exception, another exception occurred:
2026-02-04 23:02:52.953 | ERROR    | threading:run:992 - Traceback (most recent call last):
2026-02-04 23:02:52.953 | ERROR    | threading:run:992 -   File "/Users/cjnchk/.local/share/uv/tools/kimi-cli/lib/python3.13/site-packages/acp/connection.py", line 118, in main_loop
2026-02-04 23:02:52.954 | ERROR    | threading:run:992 -     await self._receive_loop()
2026-02-04 23:02:52.954 | ERROR    | threading:run:992 -   File "/Users/cjnchk/.local/share/uv/tools/kimi-cli/lib/python3.13/site-packages/acp/connection.py", line 151, in _receive_loop
2026-02-04 23:02:52.954 | ERROR    | threading:run:992 -     line = await self._reader.readline()
2026-02-04 23:02:52.954 | ERROR    | threading:run:992 -            ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
2026-02-04 23:02:52.955 | ERROR    | threading:run:992 -   File "/Users/cjnchk/.pyenv/versions/3.13.3/lib/python3.13/asyncio/streams.py", line 571, in readline
2026-02-04 23:02:52.955 | ERROR    | threading:run:992 -     raise ValueError(e.args[0])
2026-02-04 23:02:52.955 | ERROR    | threading:run:992 - ValueError: Separator is not found, and chunk exceed the limit
2026-02-04 23:02:52.955 | ERROR    | threading:run:992 - ERROR:root:Receive loop failed
2026-02-04 23:02:52.956 | ERROR    | threading:run:992 - Traceback (most recent call last):
2026-02-04 23:02:52.956 | ERROR    | threading:run:992 -   File "/Users/cjnchk/.pyenv/versions/3.13.3/lib/python3.13/asyncio/streams.py", line 562, in readline
2026-02-04 23:02:52.956 | ERROR    | threading:run:992 -     line = await self.readuntil(sep)
2026-02-04 23:02:52.957 | ERROR    | threading:run:992 -            ^^^^^^^^^^^^^^^^^^^^^^^^^
2026-02-04 23:02:52.957 | ERROR    | threading:run:992 -   File "/Users/cjnchk/.pyenv/versions/3.13.3/lib/python3.13/asyncio/streams.py", line 663, in readuntil
2026-02-04 23:02:52.957 | ERROR    | threading:run:992 -     raise exceptions.LimitOverrunError(
2026-02-04 23:02:52.958 | ERROR    | threading:run:992 -         'Separator is not found, and chunk exceed the limit',
2026-02-04 23:02:52.959 | ERROR    | threading:run:992 -         offset)
2026-02-04 23:02:52.959 | ERROR    | threading:run:992 - asyncio.exceptions.LimitOverrunError: Separator is not found, and chunk exceed the limit
2026-02-04 23:02:52.961 | ERROR    | threading:run:992 - During handling of the above exception, another exception occurred:
2026-02-04 23:02:52.961 | ERROR    | threading:run:992 - Traceback (most recent call last):
2026-02-04 23:02:52.961 | ERROR    | threading:run:992 -   File "/Users/cjnchk/.local/share/uv/tools/kimi-cli/lib/python3.13/site-packages/acp/connection.py", line 118, in main_loop
2026-02-04 23:02:52.961 | ERROR    | threading:run:992 -     await self._receive_loop()
2026-02-04 23:02:52.961 | ERROR    | threading:run:992 -   File "/Users/cjnchk/.local/share/uv/tools/kimi-cli/lib/python3.13/site-packages/acp/connection.py", line 151, in _receive_loop
2026-02-04 23:02:52.961 | ERROR    | threading:run:992 -     line = await self._reader.readline()
2026-02-04 23:02:52.962 | ERROR    | threading:run:992 -            ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
2026-02-04 23:02:52.962 | ERROR    | threading:run:992 -   File "/Users/cjnchk/.pyenv/versions/3.13.3/lib/python3.13/asyncio/streams.py", line 571, in readline
2026-02-04 23:02:52.962 | ERROR    | threading:run:992 -     raise ValueError(e.args[0])
2026-02-04 23:02:52.962 | ERROR    | threading:run:992 - ValueError: Separator is not found, and chunk exceed the limit

[cjnchk]

@ZENOTME As you said, we should increase the buffer limit to 100MB in acp when creating stdio streams like in wire. I've created a PR to fix this issue. Before the PR is merged and released, you can manually modify the "~/.local/share/uv/tools/kimi-cli/lib/python3.13/site-packages/kimi_cli/acp/__init__.py" file by referring to the implementation in the PR, and then we’ll be able to use it happily and smoothly 😊.

[ZENOTME]

@ZENOTME As you said, we should increase the buffer limit to 100MB in acp when creating stdio streams like in wire. I've created a PR to fix this issue. Before the PR is merged and released, you can manually modify the "~/.local/share/uv/tools/kimi-cli/lib/python3.13/site-packages/kimi_cli/acp/init.py" file by referring to the implementation in the PR, and then we’ll be able to use it happily and smoothly 😊.

Thanks @cjnchk ! For now I use another way to bypass this limit: disable file capability in acp client so that the agent will read file in agent rather than depends on acp client.

Increase the buffer limit to 100MB when creating stdio streams:

I think this solution can work for most of case but may also not be suitable for reading big file cases (e.g. file bigger than 100MB). One solution could be to allow response in a stream way using multiple messages. I'm not sure where it's compliant with the specification: https://agentclientprotocol.com/protocol/file-system