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JavaScript@langchain/corelanguage_modelseventMessageStartEvent

Interface●Since v1.1

MessageStartEvent

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interface MessageStartEvent

Properties

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property

event: "message-start"

property

id: string

property

usage: Partial<UsageMetadata>

Emitted once at the start of a model response.

Optional message ID assigned by the provider.

Initial usage snapshot, if the provider reports input token counts before content begins streaming (e.g., Anthropic's message_start).