feat: add tool-scoping for fork-mode skills#3260
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…rtable A readonly agent could previously gain mutating tools via a fork skill's assistive toolset; forkSkillToolSets now applies toolset.ReadOnly || a.ReadOnly. Each toolset is also wrapped in tools.NewStartable for lazy single-flight start semantics, since skillSubSessionTools calls Start() on every run-loop iteration. Assisted-By: Claude
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Jun 26, 2026
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Fork-mode skills run in an isolated sub-session, but until now they inherited every tool available to the parent agent with no way to restrict or augment that set. This made it impossible to write a skill that only needs a small, safe subset of tools, or one that requires a dedicated toolset not part of the main agent's config.
Two new fields address this.
allowed_tools(frontmatter keyallowed-toolsinSKILL.mdfiles) is an allow-list of glob or exact patterns applied to the parent agent's inherited tools — the Claude Code-compatible field was previously parsed but never enforced; it is now enforced in the fork sub-session.toolsetsis a list of top-leveltoolsetsnames that are injected into the sub-session in addition to the filtered inherited tools, bypassing the allow-list. Both fields are validated at load time: toolset references must exist in the top-level config, and both fields are rejected on non-fork skills. The agent-levelreadonlyflag is honored for assistive toolsets, and assistive toolsets are wrapped with lazy single-flight start semantics viaNewStartable.Both fields work for inline skills defined in YAML and for file-based skills loaded from
SKILL.mdfrontmatter. An example demonstrating the feature is inexamples/skills_fork_toolsets.yaml, and the skills documentation indocs/features/skills/index.mdhas been updated accordingly.