fix: wait for first result before closing stdin if SDK MCP present (#380)

Port SDK MCP fix from TypeScript to Python.

Now, when SDK MCP servers or hooks are present, stream_input() waits for
the first result message before closing stdin, allowing bidirectional
control protocol communication to complete.

Fixes repro in
https://github.com/anthropics/claude-agent-sdk-python/issues/266.

The `query()` design requires input streams to be held open by the user
for SDK MCP bidirectional communication to work. This has confused a lot
of folks, so we're moving towards a more explicit lifecycle design. In
the meantime, this is the way we've addressed it with V1 APIs in
https://github.com/anthropics/claude-agent-sdk-typescript.

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

Co-authored-by: Claude <noreply@anthropic.com>

src/claude_agent_sdk/_internal/query.py

@@ -107,6 +107,12 @@ class Query:
         self._closed = False
         self._initialization_result: dict[str, Any] | None = None
 
+        # Track first result for proper stream closure with SDK MCP servers
+        self._first_result_event = anyio.Event()
+        self._stream_close_timeout = (
+            float(os.environ.get("CLAUDE_CODE_STREAM_CLOSE_TIMEOUT", "60000")) / 1000.0
+        )  # Convert ms to seconds
+
     async def initialize(self) -> dict[str, Any] | None:
         """Initialize control protocol if in streaming mode.
 
@@ -195,6 +201,10 @@ class Query:
                     # TODO: Implement cancellation support
                     continue
 
+                # Track results for proper stream closure
+                if msg_type == "result":
+                    self._first_result_event.set()
+
                 # Regular SDK messages go to the stream
                 await self._message_send.send(message)
 
@@ -525,13 +535,35 @@ class Query:
         )
 
     async def stream_input(self, stream: AsyncIterable[dict[str, Any]]) -> None:
-        """Stream input messages to transport."""
+        """Stream input messages to transport.
+
+        If SDK MCP servers or hooks are present, waits for the first result
+        before closing stdin to allow bidirectional control protocol communication.
+        """
         try:
             async for message in stream:
                 if self._closed:
                     break
                 await self.transport.write(json.dumps(message) + "\n")
-            # After all messages sent, end input
+
+            # If we have SDK MCP servers or hooks that need bidirectional communication,
+            # wait for first result before closing the channel
+            has_hooks = bool(self.hooks)
+            if self.sdk_mcp_servers or has_hooks:
+                logger.debug(
+                    f"Waiting for first result before closing stdin "
+                    f"(sdk_mcp_servers={len(self.sdk_mcp_servers)}, has_hooks={has_hooks})"
+                )
+                try:
+                    with anyio.move_on_after(self._stream_close_timeout):
+                        await self._first_result_event.wait()
+                        logger.debug("Received first result, closing input stream")
+                except Exception:
+                    logger.debug(
+                        "Timed out waiting for first result, closing input stream"
+                    )
+
+            # After all messages sent (and result received if needed), end input
             await self.transport.end_input()
         except Exception as e:
             logger.debug(f"Error streaming input: {e}")