fix(issue #27): added an optional limit parameter

README.md

@@ -122,6 +122,8 @@ if __name__ == "__main__":
 
 Full example with streaming and lifecycle hooks lives in [examples/echo_agent.py](examples/echo_agent.py).
 
+Python's asyncio applies a 64KB default line buffer for stdio. If your agent streams larger JSON lines, pass `limit=10 * 1024 * 1024` (or similar) to `stdio_streams()` and `spawn_stdio_transport()` to raise the cap.
+
 ## Examples
 
 - `examples/echo_agent.py`: the canonical streaming agent with lifecycle hooks

src/acp/stdio.py

@@ -96,8 +96,11 @@ def get_extra_info(self, name: str, default=None):  # type: ignore[override]
         return default
 
 
-async def _windows_stdio_streams(loop: asyncio.AbstractEventLoop) -> tuple[asyncio.StreamReader, asyncio.StreamWriter]:
-    reader = asyncio.StreamReader()
+async def _windows_stdio_streams(
+    loop: asyncio.AbstractEventLoop,
+    limit: int | None = None,
+) -> tuple[asyncio.StreamReader, asyncio.StreamWriter]:
+    reader = asyncio.StreamReader(limit=limit) if limit is not None else asyncio.StreamReader()
     _ = asyncio.StreamReaderProtocol(reader)
 
     _start_stdin_feeder(loop, reader)
@@ -108,9 +111,12 @@ async def _windows_stdio_streams(loop: asyncio.AbstractEventLoop) -> tuple[async
     return reader, writer
 
 
-async def _posix_stdio_streams(loop: asyncio.AbstractEventLoop) -> tuple[asyncio.StreamReader, asyncio.StreamWriter]:
+async def _posix_stdio_streams(
+    loop: asyncio.AbstractEventLoop,
+    limit: int | None = None,
+) -> tuple[asyncio.StreamReader, asyncio.StreamWriter]:
     # Reader from stdin
-    reader = asyncio.StreamReader()
+    reader = asyncio.StreamReader(limit=limit) if limit is not None else asyncio.StreamReader()
     reader_protocol = asyncio.StreamReaderProtocol(reader)
     await loop.connect_read_pipe(lambda: reader_protocol, sys.stdin)
 
@@ -121,12 +127,16 @@ async def _posix_stdio_streams(loop: asyncio.AbstractEventLoop) -> tuple[asyncio
     return reader, writer
 
 
-async def stdio_streams() -> tuple[asyncio.StreamReader, asyncio.StreamWriter]:
-    """Create stdio asyncio streams; on Windows use a thread feeder + custom stdout transport."""
+async def stdio_streams(limit: int | None = None) -> tuple[asyncio.StreamReader, asyncio.StreamWriter]:
+    """Create stdio asyncio streams; on Windows use a thread feeder + custom stdout transport.
+
+    Args:
+        limit: Optional buffer limit for the stdin reader.
+    """
     loop = asyncio.get_running_loop()
     if platform.system() == "Windows":
-        return await _windows_stdio_streams(loop)
-    return await _posix_stdio_streams(loop)
+        return await _windows_stdio_streams(loop, limit=limit)
+    return await _posix_stdio_streams(loop, limit=limit)
 
 
 @asynccontextmanager

src/acp/transports.py

@@ -51,6 +51,7 @@ async def spawn_stdio_transport(
     env: Mapping[str, str] | None = None,
     cwd: str | Path | None = None,
     stderr: int | None = aio_subprocess.PIPE,
+    limit: int | None = None,
     shutdown_timeout: float = 2.0,
 ) -> AsyncIterator[tuple[asyncio.StreamReader, asyncio.StreamWriter, aio_subprocess.Process]]:
     """Launch a subprocess and expose its stdio streams as asyncio transports.
@@ -62,15 +63,27 @@ async def spawn_stdio_transport(
     if env:
         merged_env.update(env)
 
-    process = await asyncio.create_subprocess_exec(
-        command,
-        *args,
-        stdin=aio_subprocess.PIPE,
-        stdout=aio_subprocess.PIPE,
-        stderr=stderr,
-        env=merged_env,
-        cwd=str(cwd) if cwd is not None else None,
-    )
+    if limit is None:
+        process = await asyncio.create_subprocess_exec(
+            command,
+            *args,
+            stdin=aio_subprocess.PIPE,
+            stdout=aio_subprocess.PIPE,
+            stderr=stderr,
+            env=merged_env,
+            cwd=str(cwd) if cwd is not None else None,
+        )
+    else:
+        process = await asyncio.create_subprocess_exec(
+            command,
+            *args,
+            stdin=aio_subprocess.PIPE,
+            stdout=aio_subprocess.PIPE,
+            stderr=stderr,
+            env=merged_env,
+            cwd=str(cwd) if cwd is not None else None,
+            limit=limit,
+        )
 
     if process.stdout is None or process.stdin is None:
         process.kill()

tests/real_user/test_stdio_limits.py

@@ -0,0 +1,41 @@
+import sys
+import textwrap
+
+import pytest
+
+from acp.transports import spawn_stdio_transport
+
+LARGE_LINE_SIZE = 70 * 1024
+
+
+def _large_line_script(size: int = LARGE_LINE_SIZE) -> str:
+    return textwrap.dedent(
+        f"""
+        import sys
+        sys.stdout.write("X" * {size})
+        sys.stdout.write("\\n")
+        sys.stdout.flush()
+        """
+    ).strip()
+
+
+@pytest.mark.asyncio
+async def test_spawn_stdio_transport_hits_default_limit() -> None:
+    script = _large_line_script()
+    async with spawn_stdio_transport(sys.executable, "-c", script) as (reader, writer, _process):
+        # readline() re-raises LimitOverrunError as ValueError on CPython 3.12+.
+        with pytest.raises(ValueError):
+            await reader.readline()
+
+
+@pytest.mark.asyncio
+async def test_spawn_stdio_transport_custom_limit_handles_large_line() -> None:
+    script = _large_line_script()
+    async with spawn_stdio_transport(
+        sys.executable,
+        "-c",
+        script,
+        limit=LARGE_LINE_SIZE * 2,
+    ) as (reader, writer, _process):
+        line = await reader.readline()
+        assert len(line) == LARGE_LINE_SIZE + 1