Summary

Five SKILL.md files declare argument-hint using the flow-sequence form:

argument-hint: ["[path] [--full] [--language <lang>] [--chunk <N>]"]

In YAML, a bare [ opens a flow sequence, so this parses to a one-element array of strings, not a string. That worked as long as skill loaders were permissive.

GitHub Copilot CLI 1.0.65 tightened its skill loader to require argument-hint to be a plain string. Under 1.0.65+ these five sprang skills silently fail to load — the manifest is dropped without a user-visible error.

The fix strips the outer brackets so the value is a plain double-quoted scalar. The semantics inside the string (Copilot / VS Code Agent Skills argument-hint syntax) are unchanged.

-argument-hint: ["[path] [--full] [--language <lang>] [--chunk <N>]"]
+argument-hint: "[path] [--full] [--language <lang>] [--chunk <N>]"

Files changed

I checked the other six SKILL.md files in each tree (sprang, sprang-why, sprang-team, sprang-onboard, sprang-health, sprang-diff, sprang-domain) — none declare argument-hint, so they are unaffected. .claude-plugin/ and .copilot-plugin/ do not vendor per-skill SKILL.md files in this repo (they carry plugin.json / marketplace manifests only), so no other trees are impacted.

Grep verification after the fix:

$ grep -rEn 'argument-hint\s*:\s*\[' --include=SKILL.md .
(no matches)

Why this shape

Per the VS Code Agent Skills docs and the copilot-cli-for-beginners 05-skills tutorial, the canonical frontmatter shape is:

---
name: my-skill
description: ...
argument-hint: "<free-form hint shown to the user>"
---

argument-hint is a single string. It's not a list of alternatives, and it's not structured — it's the hint text that Copilot renders next to the argument prompt.

Test plan

• pnpm install --frozen-lockfile && pnpm lint && pnpm typecheck && pnpm test — nothing in this diff touches code paths those exercise, but running them confirms the CI matrix in .github/workflows/ci.yml still passes.
• Sanity: grep -rEn 'argument-hint\s*:\s*\[' --include=SKILL.md . returns no results.
• Sanity: for each of the five changed files, head -6 shows argument-hint: "..." (plain scalar, no [).
• Manual on Copilot CLI 1.0.65+: run copilot in a project with sprang skills installed and confirm sprang-analyze, sprang-chat, sprang-explain, sprang-knowledge now show up in the skill list. (Before this fix, they are silently missing on 1.0.65+.)

I did not run the local pnpm build here (fork-side, no side effects on skill definitions), but the CI job on this PR will exercise pnpm build, pnpm typecheck, pnpm lint, pnpm test, and the Validate plugin manifests step from .github/workflows/ci.yml.

Refs

• GitHub Copilot CLI 1.0.65 release notes — skill loader tightening (argument-hint must be a string)
• VS Code Agent Skills docs — argument-hint field spec
• microsoft/copilot-cli-for-beginners/05-skills — canonical SKILL.md shape
• Same latent bug also affects Claude Code: anthropics/claude-code#22161

YAML rule (for reviewers)

A bare [ at the start of a value in YAML opens a flow sequence — see the YAML 1.2 spec §7.4.1. So:

key: ["value"]        # → key is an array: ["value"]
key: "value"          # → key is a string: "value"

Strict consumers reject the array form when the schema expects a string. That's what Copilot CLI 1.0.65 now does.