What
When Claude Code finishes a run with an error, it sends a result message saying how the run ended. Programs that drive Claude Code through the SDK read this message. The SDK is the toolkit for using Claude Code from your own code.
That error result used to carry only a subtype and the error text. It can now also carry an error_code field. The field appears only when the error matches one of the codes Claude Code knows about.
Why
If you build on the SDK, your program can check a fixed code and decide what to do next. It no longer has to match the wording of the error text.
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Which error codes are on the known list, and so when the field appears, is not stated.