A streaming, incremental update to a [Message].

MessageDelta accumulates partial outputs (text chunks, tool-call fragments, IDs, signatures, etc.) until they can be materialized as a full [Message]. It implements [Delta] to support accumulation.

Accumulation Rules

• role: merging two distinct roles fails.
• thinking: concatenated in arrival order.
• contents/tool_calls: last element is accumulated with the incoming delta when both are compatible (e.g., Text+Text, Function+Function with matching ID policy), otherwise appended as a new fragment.
• id/signature: last-writer-wins.

Finalization

• finish() converts the accumulated deltas into a fully-formed [Message]. Fails if required fields (e.g., role) are missing or inner deltas cannot be finalized.

Examples

let d1 = MessageDelta::new().with_role(Role::Assistant).with_contents([PartDelta::Text { text: \"Hel\".into() }]);
let d2 = MessageDelta::new().with_contents([PartDelta::Text { text: \"lo\".into() }]);

let merged = d1.accumulate(d2).unwrap();
let msg = merged.finish().unwrap();
assert_eq!(msg.contents[0].as_text().unwrap(), \"Hello\");
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