Use it now
A new flag lets you supply a subagent system-prompt append from a file instead of typing it inline.
A new --append-subagent-system-prompt-file <file> flag lets a subagent system-prompt append be supplied from a file instead of inline. It's mutually exclusive with the existing --append-subagent-system-prompt flag.
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Anthropic's release notes agree
Added --append-subagent-system-prompt-file to read the subagent system prompt from a file, for prompts too large to pass on the command line