Python Agent SDK: MessageParseError on rate_limit_event from CLI 2.1.45+

Bug Description

The Python Agent SDK (0.1.38+) raises MessageParseError: Unknown message type: rate_limit_event when iterating async responses. This is because CLI 2.1.45 introduced SDKRateLimitEvent as an informational lifecycle message, but the SDK's message_parser.py was not updated to recognize it.

Reproduction

from claude_agent_sdk import query, ClaudeAgentOptions

options = ClaudeAgentOptions(model="claude-sonnet-4-6", allowed_tools=[], max_turns=1)
async for msg in query(prompt="Hello", options=options):
    print(type(msg).__name__)  # crashes with MessageParseError

Versions

• claude-agent-sdk 0.1.39 (latest on PyPI)
• Bundled CLI: 2.1.49
• First appeared: SDK 0.1.38 / CLI 2.1.45

Root Cause

message_parser.py has a fixed set of recognized types (UserMessage, AssistantMessage, SystemMessage, ResultMessage, StreamEvent). The CLI now emits rate_limit_event JSON messages that the parser cannot handle.

SDK 0.1.37 (bundled CLI 2.1.44) works fine — the event didn't exist yet.

Impact

• ask_json_native with structured output survives because ToolUseBlock arrives before the crash
• Plain text queries fail because the iterator crashes before all TextBlock content is collected
• Streaming crashes mid-stream

Suggested Fix

Either:

1. Add SDKRateLimitEvent as a recognized message type with a corresponding dataclass (preferred — consumers can use the rate limit info)
2. Skip unknown message types with a warning log instead of raising MessageParseError

Option 2 is more forward-compatible — future CLI message types won't break existing SDK versions.

Workaround

Wrap the async iterator to catch and skip unknown types:

async def _safe_stream(async_iter):
    while True:
        try:
            msg = await async_iter.__anext__()
            yield msg
        except StopAsyncIteration:
            break
        except Exception as e:
            if "Unknown message type" in str(e):
                continue
            raise