fix(ai): preserve providerMetadata on all smoothStream word-boundary chunks#14988
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…chunks The while loop that emits word/line boundary chunks was not forwarding providerMetadata, so Anthropic thinking signatures and any other provider-specific metadata were silently dropped for every intermediate chunk. Only the final remainder (flushed via flushBuffer) retained the metadata. Apply the same spread pattern already used in flushBuffer so all emitted chunks carry the original providerMetadata. Fixes vercel#14373 Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
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Background
`smoothStream` buffers text and emits multiple chunks from a single input chunk when a word boundary is detected (the `while` loop). The `providerMetadata` from the input chunk (e.g. Anthropic thinking signatures) was only forwarded on the first chunk — subsequent chunks emitted from the same burst lost the metadata entirely.
Summary
Spread `providerMetadata` onto every chunk emitted inside the `while` loop, not just the first. Chunks without metadata are unaffected (the spread is conditional on `providerMetadata != null`).
Manual Verification
Verified via unit test: all three word-boundary chunks emitted from a single input carry the original `providerMetadata`.
Checklist