Anthropic Messages — Event Reference

This page documents the SSE events emitted by POST /api/anthropic/v1/messages when stream: true.

Event types and order (typical)

message_start (includes message.id)
content_block_start (one or more)
content_block_delta (e.g., text_delta, input_json_delta, thinking_delta)
content_block_stop (per block)
message_delta (contains stop_reason and usage near the end)
message_stop (terminal)

Tool streaming frames

Example server_tool_use with incremental input_json_delta and a tool_result block.

Usage fields

input_tokens, output_tokens, cache_creation_input_tokens, cache_read_input_tokens.

See also

OpenAI Responses events Index