SKILL.md files with description > 1024 chars are silently dropped from skills.list() — no warning, no error

[K8Rayner]

Environment

• Copilot CLI: 1.0.36-0
• Python SDK: github_copilot_sdk==0.3.0
• OS: Windows 11
• Discovery method: skill_directories=[...] on Session(...)

Problem

The Agent Skills specification states:

description: Max 1024 characters. Non-empty.

The CLI enforces this cap correctly during skill load, but silently:

• Over-cap files are materialised on disk by the SDK loader.
• They are then dropped during YAML/frontmatter parse.
• No warning is emitted (stderr, log, or otherwise).
• No error is raised.
• The skill simply does not appear in session.skills.list().

This is particularly painful for skills generated by tooling (we generate description from a keyword pool that grows as the codebase grows). The skill silently disappears between releases and there's no signal to debug.

Reproducer

Save this as bad-skill/SKILL.md (description is 1,200 chars, deliberately over cap):

---
name: bad-skill
description: AAAAAAAAAA[…repeat letter 'A' 1200 times…]
---

# Bad Skill

Body.

(Padding character + exact length is the only thing that matters; the body is irrelevant.)

Place the bad-skill/ folder in the current directory, then:

import asyncio
from copilot.session import Session

async def main():
    async with Session(skill_directories=["."]) as s:
        result = await s.rpc.skills.list()
        print([sk.name for sk in result.skills])

asyncio.run(main())
# Expected: ["bad-skill"]
# Actual:   []  (silent drop, no log)

Trim the description to ≤ 1,024 chars and the same call returns ["bad-skill"].

Suggested fix

Emit a warning at load time the way the loader does for unknown frontmatter fields:

[warn] skill 'bad-skill' (./bad-skill/SKILL.md):
       description exceeds 1024-char cap (got 1200); skill not loaded

Documenting the 1,024-char cap in docs.github.com/copilot/concepts/agents/about-agent-skills would also help — currently only the spec page mentions it.

Workaround

Trim description under the cap before shipping. We've added a lint step that fails CI if any SKILL.md description ≥ 1024 chars.