What
Outgoing 'result' messages on the SDK and stream-json transport (the machine-readable output formats used by tools and scripts driving Claude Code) now get a result_index field, an incrementing number stamped on each message. A tracking mechanism ensures each message only gets stamped once, even if it passes through multiple times.
Why
This gives anything consuming Claude Code's programmatic output a reliable way to tell results apart and put them in order, which is useful for tooling that processes a stream of results rather than reading them one at a time in a terminal.
Names in the bundleresult_index
Documented inapi/api/beta
The entry above is what we published on the day. These lines were added later, as Anthropic's own pages caught up, and they sit beside the original rather than replacing it.
Confirmed since
Anthropic's documentation has since written up result_index, on Agent SDK reference - TypeScript.
result_index?: number;agent-sdk/typescript see the edit
One source agreesOne thing we can check says the same as this entry.
Anthropic's documentation agrees
Anthropic's documentation has since written up result_index, on Agent SDK reference - TypeScript.