What
Claude Code includes a copy of Anthropic's SDK, the library programs use to talk to Claude. Its tool runner, the part that runs a loop of tool calls, can now change its tool list while it is running:
addToolsadds tools.removeToolsremoves tools.
Previously the tool list was fixed when the run started. Each change is now reported to the model as a system message of type tool_addition or tool_removal. When a conversation is compacted (shortened to save space), the record of tool changes is kept. Messages are also copied with a different method.
Why
This mostly matters to developers who build their own programs on the SDK. For them, tools can change partway through a run without starting over.
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The writer flagged doubt
It is not clear whether Claude Code itself uses the new add and remove methods.