What
The SDK lets other programs run Claude Code and read its output as a stream of messages. The program reading that stream is called the host. Messages in the stream now have an optional tag field.
The tag marks a line that a host may want to treat specially. Ordinary lines do not have it. A host is expected to ignore any tag value it does not recognise.
Why
If you build on the SDK and check messages strictly against a fixed set of fields, a new field may now appear in the stream. Make sure your code accepts it, and ignores values it does not know.
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The writer flagged doubt
It is not known which tag values Claude Code actually sends.