Bug fix: Handle multiline json

src/claude_code_sdk/_internal/transport/subprocess_cli.py

@@ -182,23 +182,54 @@ async def read_stderr() -> None:
         async with anyio.create_task_group() as tg:
             tg.start_soon(read_stderr)
 
+            buffer = ""
+            is_multiline_json = False
+
             try:
                 async for line in self._stdout_stream:
                     line_str = line.strip()
                     if not line_str:
                         continue
 
-                    try:
-                        data = json.loads(line_str)
+                    if is_multiline_json:
+                        # We're collecting lines for multiline JSON
+                        buffer += line_str
+                        try:
+                            data = json.loads(buffer)
+                            # Success! Yield and reset
+                            try:
+                                yield data
+                            except GeneratorExit:
+                                return
+                            buffer = ""
+                            is_multiline_json = False
+                        except json.JSONDecodeError:
+                            # Still not valid, keep collecting
+                            continue
+                    else:
+                        # Try to parse line as complete JSON first
                         try:
-                            yield data
-                        except GeneratorExit:
-                            # Handle generator cleanup gracefully
-                            return
+                            data = json.loads(line_str)
+                            try:
+                                yield data
+                            except GeneratorExit:
+                                return
+                        except json.JSONDecodeError as e:
+                            # Failed to parse - check if it looks like start of JSON
+                            if line_str.startswith("{") or line_str.startswith("["):
+                                # Start buffering for potential multiline JSON
+                                buffer = line_str
+                                is_multiline_json = True
+                            # If it doesn't look like JSON, just skip the line
+                            continue
+
+                # Handle any remaining buffer after stream ends
+                if buffer and is_multiline_json:
+                    try:
+                        data = json.loads(buffer)
+                        yield data
                     except json.JSONDecodeError as e:
-                        if line_str.startswith("{") or line_str.startswith("["):
-                            raise SDKJSONDecodeError(line_str, e) from e
-                        continue
+                        raise SDKJSONDecodeError(buffer, e) from e
 
             except anyio.ClosedResourceError:
                 pass

tests/test_transport.py

@@ -1,11 +1,13 @@
 """Tests for Claude SDK transport layer."""
 
+import json
 from unittest.mock import AsyncMock, MagicMock, patch
 
 import anyio
 import pytest
 
 from claude_code_sdk._internal.transport.subprocess_cli import SubprocessCLITransport
+from claude_code_sdk._errors import CLIJSONDecodeError as SDKJSONDecodeError
 from claude_code_sdk.types import ClaudeCodeOptions
 
 
@@ -132,3 +134,124 @@ def test_receive_messages(self):
         # So we just verify the transport can be created and basic structure is correct
         assert transport._prompt == "test"
         assert transport._cli_path == "/usr/bin/claude"
+
+    def test_multiline_json_parsing(self):
+        """Test parsing JSON that works both single-line and with buffering logic."""
+
+        async def _test():
+            # Mock process and streams
+            mock_process = MagicMock()
+            mock_process.returncode = 0
+            mock_process.wait = AsyncMock(return_value=0)
+
+            # Test data: valid single-line JSON only
+            test_lines = [
+                '{"type": "single", "data": "complete"}',  # Valid single line JSON
+                '{"type": "valid_long", "data": {"nested": "value"}, "complete": true}',  # Another valid single line
+            ]
+
+            # Create async iterator from test lines
+            class MockTextReceiveStream:
+                def __init__(self, lines):
+                    self.lines = iter(lines)
+
+                def __aiter__(self):
+                    return self
+
+                async def __anext__(self):
+                    try:
+                        return next(self.lines)
+                    except StopIteration:
+                        raise StopAsyncIteration
+
+            mock_stdout_stream = MockTextReceiveStream(test_lines)
+            mock_stderr_stream = MockTextReceiveStream([])
+
+            with patch("anyio.open_process") as mock_open_process:
+                mock_open_process.return_value = mock_process
+
+                transport = SubprocessCLITransport(
+                    prompt="test",
+                    options=ClaudeCodeOptions(),
+                    cli_path="/usr/bin/claude",
+                )
+
+                # Manually set up the streams for testing
+                transport._process = mock_process
+                transport._stdout_stream = mock_stdout_stream  # type: ignore
+                transport._stderr_stream = mock_stderr_stream  # type: ignore
+
+                # Collect all yielded messages
+                messages = []
+                async for message in transport.receive_messages():
+                    messages.append(message)
+
+                # Verify we got the expected valid JSON messages
+                assert len(messages) == 2
+
+                # Check first single line JSON
+                assert messages[0] == {"type": "single", "data": "complete"}
+
+                # Check second single line JSON 
+                assert messages[1] == {
+                    "type": "valid_long",
+                    "data": {"nested": "value"},
+                    "complete": True
+                }
+
+        anyio.run(_test)
+
+    def test_multiline_json_no_error_on_valid_completion(self):
+        """Test that valid multiline JSON doesn't raise error."""
+
+        async def _test():
+            mock_process = MagicMock()
+            mock_process.returncode = 0
+            mock_process.wait = AsyncMock(return_value=0)
+
+            # Test multiline JSON that completes properly
+            test_lines = [
+                '{"type": "multiline",',
+                '"data": "test",',
+                '"complete": true}',
+            ]
+
+            class MockTextReceiveStream:
+                def __init__(self, lines):
+                    self.lines = iter(lines)
+
+                def __aiter__(self):
+                    return self
+
+                async def __anext__(self):
+                    try:
+                        return next(self.lines)
+                    except StopIteration:
+                        raise StopAsyncIteration
+
+            mock_stdout_stream = MockTextReceiveStream(test_lines)
+            mock_stderr_stream = MockTextReceiveStream([])
+
+            with patch("anyio.open_process") as mock_open_process:
+                mock_open_process.return_value = mock_process
+
+                transport = SubprocessCLITransport(
+                    prompt="test",
+                    options=ClaudeCodeOptions(),
+                    cli_path="/usr/bin/claude",
+                )
+
+                transport._process = mock_process
+                transport._stdout_stream = mock_stdout_stream  # type: ignore
+                transport._stderr_stream = mock_stderr_stream  # type: ignore
+
+                messages = []
+                async for message in transport.receive_messages():
+                    messages.append(message)
+
+                # Should get exactly one properly parsed message
+                assert len(messages) == 1
+                expected = {"type": "multiline", "data": "test", "complete": True}
+                assert messages[0] == expected
+
+        anyio.run(_test)