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DisclaimerUnofficial, and not affiliated with Anthropic. Nearly all of this is read straight out of what ships: npm bundles, captured prompts, published docs. Anthropic's own notes go in verbatim, marked as theirs. The rest is my reading, and every entry carries the strings behind it. If one looks wrong, vote it down and say why.

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Claude Code v2.0.59 4 Dec 2025

Main Thread Agent Definition Support

What: New architecture allowing main thread agents to provide custom system prompts.

Details:

New mainThreadAgentDefinition parameter added to main UI component at line 480951
HJ9() helper function at line 480920 handles system prompt resolution
Conditionally calls agent's getSystemPrompt() method with tool context
Enables more flexible agent customization
Evidence: 7 occurrences in v2.0.59, zero in v2.0.58

See this entry in the whole of v2.0.59 →