Mid-stream SSE errors get status_code=200 instead of the actual error code

Bug

When the API returns HTTP 200 (streaming started fine) but then sends an SSE error event like overloaded_error, the SDK creates an APIStatusError with status_code=200 instead of 529. The error body has the right type, but the status code is wrong.

This happens because _streaming.py passes self.response (the original HTTP response object) to _make_status_error, and _make_status_error switches on response.status_code to pick the error subclass.

Code path

In _streaming.py (line ~229):

if sse.event == "error":
    body = sse.json()
    err_msg = f"{body}"
    raise self._client._make_status_error(
        err_msg,
        body=body,
        response=self.response,  # <-- this is the original HTTP 200 response
    )

In _client.py, _make_status_error dispatches on response.status_code:

def _make_status_error(self, err_msg, *, body, response):
    if response.status_code == 429:
        return RateLimitError(...)
    if response.status_code >= 500:
        return InternalServerError(...)
    return APIStatusError(...)  # <-- falls through here because status is 200

Since the HTTP status was 200, none of the specific branches match. You get a bare APIStatusError with status_code=200.

Why this matters

Any caller checking status_code for retry/fallback decisions gets the wrong answer. An overloaded_error should look like a 529, not a 200. The body has the correct error type ({'error': {'type': 'overloaded_error'}}), but most callers don't expect to need to parse that — they check status codes.

We hit this in production with pydantic-ai's FallbackModel, which checks status_code >= 500 to decide whether to try the next model. The overloaded error had status_code=200, so fallback never fired.

Expected behavior

Mid-stream SSE errors should produce the same error subclass as if the error came back as an HTTP response. An overloaded_error → OverloadedError(status_code=529), an api_error → InternalServerError(status_code=500), etc.

Suggested fix

Map the SSE error type to the corresponding status code before calling _make_status_error, or construct the right error subclass directly:

_SSE_ERROR_TYPE_TO_STATUS = {
    "overloaded_error": 529,
    "rate_limit_error": 429,
    "api_error": 500,
    "internal_server_error": 500,
    # client errors (shouldn't appear mid-stream, but for completeness):
    "authentication_error": 401,
    "invalid_request_error": 400,
    "not_found_error": 404,
}

Then use the mapped status code (falling back to the original response.status_code) when building the error.

Versions

anthropic SDK: 0.52.0
Observed with claude-sonnet-4-20250514 via the streaming API

Confirmed the bug by reading the current source. Here's a concise breakdown:

Root cause: In _streaming.py (lines 114-117 for sync, 234-237 for async), the SSE error handler passes self.response — the original HTTP 200 response — to _client._make_status_error(). Since _make_status_error in _client.py dispatches entirely on response.status_code, every mid-stream SSE error falls through to the generic APIStatusError with status_code=200, regardless of the error type in the body.

The gap: SSE error events carry the error type (e.g. overloaded_error) in the JSON body but no HTTP status code. There is currently no translation layer between these two representations.

Fix: Add a mapping from SSE error types to their canonical HTTP status codes in _streaming.py, and use it to construct the correct error subclass. Something like:

_SSE_ERROR_TO_STATUS = {
    "overloaded_error": 529,
    "rate_limit_error": 429,
    "api_error": 500,
    "authentication_error": 401,
    "invalid_request_error": 400,
    "not_found_error": 404,
}

Then in the sse.event == "error" block, extract the error type from the parsed body, look up the status code, and either (a) build a synthetic response with the mapped status code before calling _make_status_error, or (b) construct the right error subclass directly, bypassing _make_status_error for SSE errors.

Option (b) is cleaner since it avoids mutating or faking the httpx response object.

Happy to submit a PR for this if it would be welcome.

fix: extract correct status code from SSE error events

@sarth6

Thanks @sarth6 for the detailed report! We're writing out a better approach to this that we hope to have ready for release shortly! I'll update this issue once that goes out.

fix(streaming): derive correct status code from SSE error type

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Mid-stream SSE errors get `status_code=200` instead of the actual error code #1258

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Mid-stream SSE errors get status_code=200 instead of the actual error code #1258

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weiguangli-io commented on Mar 19, 2026

dtmeadows commented on Mar 19, 2026

khalidsaidi commented on Mar 24, 2026

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Mid-stream SSE errors get `status_code=200` instead of the actual error code #1258