Summary

The Bedrock stream decoder (anthropic/lib/bedrock/_stream_decoder.py) hardcodes ServerSentEvent(event="completion") for every chunk, discarding the real event type. Downstream, the "completion" branch in _streaming.py::__stream__ never backfills data["type"] (unlike the direct-SSE branch). Chunks without a type field — notably Bedrock's amazon-bedrock-invocationMetrics trailer — then hit non-validating construct_type against the stream-event union, which falls back to the first union member and yields RawMessageStartEvent(message=None) — an object that violates the SDK's own type contract (message is typed as non-optional).

Reproduction (anthropic 0.105.2, offline)

from anthropic._models import construct_type
from anthropic.types.beta import BetaRawMessageStreamEvent

event = construct_type(
    value={"amazon-bedrock-invocationMetrics": {"inputTokenCount": 1}},
    type_=BetaRawMessageStreamEvent,
)
print(type(event).__name__, event.message)
# BetaRawMessageStartEvent None

Any consumer that trusts the type annotations (e.g. event.message.usage) crashes with AttributeError: 'NoneType' object has no attribute 'usage'. We hit this in production via pydantic-ai's streaming path on AsyncAnthropicBedrock — intermittent 500s whenever a stream contained the invocationMetrics trailer.

Expected

Either:

• the Bedrock decoder preserves/backfills the event type from the chunk payload's type field (the payloads do carry one for real events), and drops/ignores non-Messages chunks like amazon-bedrock-invocationMetrics, or
• stream-event construction validates and raises a clear error instead of silently producing a contract-violating first-union-member object.

Notes

• Related but distinct from Bedrock streaming SSE events errors are not handled gracefully #1477 (decoder raises ValueError on error SSE events) — same file, different gap: this one is about successful streams containing type-less chunks.
• Direct (non-Bedrock) SSE is unaffected: that branch carries the event type and backfills data["type"].