BerriAI/litellm · error · RuntimeError

Cannot use sync iteration on async iterator

Error message

Cannot use sync iteration on async iterator

What it means

Guard in MCPStreamingIterator.__next__: the underlying base_iterator is async while the consumer is iterating synchronously, a usage/programming error — the stream must be consumed with anext/async for.

Source

Thrown at litellm/responses/mcp/mcp_streaming_iterator.py:844

        return self

    def __next__(self) -> ResponsesAPIStreamingResponse:
        # First, emit any queued MCP events
        if self.mcp_events:
            return self.mcp_events.pop(0)

        # Then delegate to the base iterator
        if not self.is_async:
            try:
                if self.base_iterator and hasattr(self.base_iterator, "__next__"):
                    return next(cast(Any, self.base_iterator))
                else:
                    raise StopIteration
            except StopIteration:
                self.finished = True
                raise
        else:
            raise RuntimeError("Cannot use sync iteration on async iterator")

View on GitHub (pinned to 77b7c6c40c)

Solutions

1. Iterate with 'async for' instead of a synchronous for loop over the async iterator.

When it happens

Trigger: Thrown at litellm/responses/mcp/mcp_streaming_iterator.py:844 when the library encounters an invalid state.

Common situations: See trigger scenarios.

AI-assisted analysis of BerriAI/litellm@77b7c6c40c (2026-08-18). Data as JSON: /api/errors/580eeb5d387a0054. Report an issue: GitHub.