The spec's prompt-message ContentBlock union is
`text | image | audio | resource_link | resource`, but
PromptMessageContent omitted `Audio`. Because the enum is
`#[serde(tag = "type")]` with no catch-all, a spec-conformant
`{"type":"audio",...}` content block failed to deserialize with
"unknown variant `audio`", breaking prompts/get for any server that
returns audio prompt content (the audio analogue of #842 / #843).
The supporting AudioContent type already existed, and Audio was
already a variant of the general RawContent enum (tool results,
sampling) -- only PromptMessageContent lacked it.
Add the flattened Audio variant (mirroring Image), a
PromptMessage::new_audio constructor (mirroring new_image), and
serialization + deserialization regression tests.
Fixes#864.
Co-authored-by: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
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