Fix FastAPI SSE streaming compatibility (fixes #4)

Remove anyio.create_task_group() from receive_messages() to fix the
RuntimeError "Attempted to exit cancel scope in a different task than
it was entered in" when using the SDK with FastAPI's SSE streaming.

The issue occurred because FastAPI can move async generators between
different asyncio tasks during the streaming lifecycle, which conflicts
with anyio's cancel scope tracking.

Changes:
- Remove task group usage from receive_messages()
- Read stderr sequentially after stdout completes
- Add test to ensure no task groups are used
- Fix existing test expectation for buffer overflow

This is a minimal fix that maintains compatibility while solving the
core issue. The trade-off is that stderr is read after stdout instead
of concurrently, but this is unlikely to cause issues in practice.

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <noreply@anthropic.com>

src/claude_code_sdk/_internal/transport/subprocess_cli.py

@@ -170,57 +170,53 @@ class SubprocessCLITransport(Transport):
         if not self._process or not self._stdout_stream:
             raise CLIConnectionError("Not connected")
 
-        stderr_lines = []
+        json_buffer = ""
         
-        async def read_stderr() -> None:
-            """Read stderr in background."""
-            if self._stderr_stream:
-                try:
-                    async for line in self._stderr_stream:
-                        stderr_lines.append(line.strip())
-                except anyio.ClosedResourceError:
-                    pass
+        # Process stdout messages first
+        try:
+            async for line in self._stdout_stream:
+                line_str = line.strip()
+                if not line_str:
+                    continue
 
-        async with anyio.create_task_group() as tg:
-            tg.start_soon(read_stderr)
+                json_lines = line_str.split("\n")
 
-            json_buffer = ""
-
-            try:
-                async for line in self._stdout_stream:
-                    line_str = line.strip()
-                    if not line_str:
+                for json_line in json_lines:
+                    json_line = json_line.strip()
+                    if not json_line:
                         continue
 
-                    json_lines = line_str.split("\n")
+                    # Keep accumulating partial JSON until we can parse it
+                    json_buffer += json_line
 
-                    for json_line in json_lines:
-                        json_line = json_line.strip()
-                        if not json_line:
-                            continue
-
-                        # Keep accumulating partial JSON until we can parse it
-                        json_buffer += json_line
-
-                        if len(json_buffer) > _MAX_BUFFER_SIZE:
-                            json_buffer = ""
-                            raise SDKJSONDecodeError(
-                                f"JSON message exceeded maximum buffer size of {_MAX_BUFFER_SIZE} bytes",
-                                ValueError(
-                                    f"Buffer size {len(json_buffer)} exceeds limit {_MAX_BUFFER_SIZE}"
-                                ),
-                            )
+                    if len(json_buffer) > _MAX_BUFFER_SIZE:
+                        json_buffer = ""
+                        raise SDKJSONDecodeError(
+                            f"JSON message exceeded maximum buffer size of {_MAX_BUFFER_SIZE} bytes",
+                            ValueError(
+                                f"Buffer size {len(json_buffer)} exceeds limit {_MAX_BUFFER_SIZE}"
+                            ),
+                        )
 
+                    try:
+                        data = json.loads(json_buffer)
+                        json_buffer = ""
                         try:
-                            data = json.loads(json_buffer)
-                            json_buffer = ""
-                            try:
-                                yield data
-                            except GeneratorExit:
-                                return
-                        except json.JSONDecodeError:
-                            continue
+                            yield data
+                        except GeneratorExit:
+                            return
+                    except json.JSONDecodeError:
+                        continue
 
+        except anyio.ClosedResourceError:
+            pass
+
+        # Read stderr after stdout completes (no concurrent task group)
+        stderr_lines = []
+        if self._stderr_stream:
+            try:
+                async for line in self._stderr_stream:
+                    stderr_lines.append(line.strip())
             except anyio.ClosedResourceError:
                 pass
 

tests/test_fastapi_streaming_compatibility.py

@@ -0,0 +1,25 @@
+"""Test FastAPI streaming compatibility (issue #4 fix)."""
+
+import inspect
+
+from claude_code_sdk._internal.transport.subprocess_cli import SubprocessCLITransport
+
+
+def test_no_task_groups_in_receive_messages():
+    """Verify receive_messages doesn't use task groups (fixes FastAPI issue #4)."""
+    # Get the source code of receive_messages
+    source = inspect.getsource(SubprocessCLITransport.receive_messages)
+    
+    # The fix: ensure no task group or task creation
+    assert "create_task_group" not in source, (
+        "receive_messages must not use create_task_group to avoid "
+        "RuntimeError with FastAPI streaming"
+    )
+    assert "asyncio.create_task" not in source, (
+        "receive_messages must not create tasks to maintain "
+        "compatibility with FastAPI's generator handling"
+    )
+    
+    # Verify stderr is still being read (sequential approach)
+    assert "_stderr_stream" in source, "Should still read stderr"
+    assert "stderr_lines" in source, "Should collect stderr output"

tests/test_subprocess_buffering.py

@@ -255,9 +255,8 @@ class TestSubprocessBuffering:
                 async for msg in transport.receive_messages():
                     messages.append(msg)
 
-            assert len(exc_info.value.exceptions) == 1
-            assert isinstance(exc_info.value.exceptions[0], CLIJSONDecodeError)
-            assert "exceeded maximum buffer size" in str(exc_info.value.exceptions[0])
+            assert isinstance(exc_info.value, CLIJSONDecodeError)
+            assert "exceeded maximum buffer size" in str(exc_info.value)
 
         anyio.run(_test)