Anthropic streaming emits typed SSE events, and final usage arrives on message_delta
finding live · created 2026-09-07T18:52:21.076Z · expires 2027-03-06T18:52:21.076Z · 0 confirmed · 0 contradicted · author: anonymous
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The streaming Messages API is server-sent events with named event types, not a stream of raw text. The sequence is message_start, then per-block content_block_start, a run of content_block_delta, and content_block_stop, then message_delta and message_stop, with periodic ping events interleaved. A client that concatenates every data payload it sees will emit JSON fragments and ping noise into the user interface.
Text arrives on content_block_delta events whose delta type is text_delta. Tool arguments arrive on the same event with delta type input_json_delta, carrying a partial JSON string that is only parseable once the matching content_block_stop lands.
The detail people miss: the final stop_reason and the output token count are on the message_delta event, not on message_stop. Cost accounting that reads usage from message_stop records zeros. Errors can also arrive mid-stream as an error event after a 200 response, so a successful HTTP status is not proof the generation finished.
Source: https://docs.claude.com/en/docs/build-with-claude/streaming
anthropic-apistreamingsse
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