Skill tool aborts with "Sibling tool call errored" when a parallel tool call fails

Summary

When the Skill tool is invoked in parallel with other tool calls (i.e. as a sibling in the same response turn), and one of those other sibling tool calls fails, the Skill invocation is aborted with the error message \"Sibling tool call errored\" — even though the skill itself and the error are completely unrelated.

Retrying the skill invocation in isolation (no other parallel tool calls) succeeds immediately.

Steps to Reproduce

1. Configure a CLAUDE.md instruction that automatically triggers a skill load at the start of a task (e.g. \"When working with Swift or SwiftUI, always load swiftui-expert:swiftui-expert-skill\").
2. Send a message that triggers both the skill load and other tool calls in the same response turn (e.g. a file read, glob, or bash command that fails or times out).
3. Observe that the Skill tool invocation reports Error: Sibling tool call errored and the skill content is not injected into the context.

Observed Behaviour

⏺ Skill(swiftui-expert:swiftui-expert-skill)
  ⎿  Initializing…
  ⎿  Error: Sibling tool call errored

The skill does not load. Its context/content is not available for the rest of the turn.

Expected Behaviour

A failing sibling tool call should not abort unrelated parallel tool calls. The Skill invocation should either:

• Complete independently of sibling failures, or
• Surface its own error (if it genuinely failed), not a cascaded one from an unrelated call.

Additional Context

• The skill (swiftui-expert:swiftui-expert-skill) is correctly installed and functional — retrying it alone succeeds every time.
• The skill's SKILL.md is a static text document with no network calls or file reads, so it cannot itself be the source of failure.
• The error appears to be a cascade: one sibling tool call fails → the failure propagates to and aborts other in-flight sibling tool calls, including the Skill invocation.
• This is confusing because the error message on the Skill output gives no indication that the root cause is elsewhere.

Workaround

Invoke the skill in a standalone message before starting the task (no other tool calls in the same turn). This prevents any siblings from failing and taking the skill load down with them.

Environment

• Platform: macOS (darwin 25.3.0)
• Shell: zsh