cc-slash-command-argument-hint-yaml

name	cc-slash-command-argument-hint-yaml
description	Fix for Claude Code slash command frontmatter that fails to parse when `argument-hint:` is given square-bracketed placeholder values. Use when: (1) authoring `commands/<name>.md` files in a CC plugin and the frontmatter shows `argument-hint: [working-dir] [team-name]` or similar multi-token bracketed values; (2) a frontmatter-validation test (e.g. `yaml.safe_load(block)` over the SKILL/command frontmatter) raises `ScannerError` or `ConstructorError` complaining about flow sequences or "expected a mapping"; (3) the slash command silently doesn't appear in `/help` after install, or a plugin-marketplace test asserting the `description` field is present trips because the whole frontmatter parse blew up. The fix is to QUOTE the value as a single string. Covers the same trap for `commands/<name>.md` directly authored in CC format AND APM-managed marketplaces shipping commands via APM.
author	Claude Code
version	1.0.0
date	2026-05-08T00:00:00.000Z

CC slash command argument-hint YAML parse trap

Problem

When authoring a Claude Code slash command (plugins/<plugin>/commands/<name>.md), the convention shown in many examples is:

---
description: Run a multi-team competition.
argument-hint: [working-dir] [team-name]
allowed-tools: Bash
---

This does not parse as YAML. [working-dir] is a single-element flow sequence. [working-dir] [team-name] is two flow sequences with whitespace between them — invalid syntax for a scalar value. yaml.safe_load raises ScannerError or ConstructorError.

Claude Code's command loader may swallow the parse error silently and the command never appears in /help. Worse: in a plugin marketplace context, a downstream test that asserts every command's frontmatter has a description field will fail because the whole frontmatter block failed to parse, not because description was actually missing — leading to ~30 minutes of looking at the wrong field.

Trigger conditions

Any of these:

• yaml.safe_load(frontmatter) raises yaml.scanner.ScannerError or yaml.constructor.ConstructorError, often pointing at the argument-hint: line.

• A plugin-marketplace test like:

  fm = yaml.safe_load(frontmatter_block)
  assert "description" in fm
  

  fails with a YAML error instead of an assert failure.

• The slash command was added to plugins/<name>/commands/<cmd>.md but doesn't show up in Claude Code's /help after installing the plugin.

• The frontmatter on disk shows a value with multiple bracketed tokens:

  argument-hint: [working-dir] [team-name]
  argument-hint: [path]
  argument-hint: [up|down]
  

Solution

Quote the value as a single string. The angle-bracket form is the idiomatic placeholder convention for shell argument hints:

argument-hint: "<working-dir> <team-name>"
argument-hint: "<path>"
argument-hint: "[up|down]"          # square brackets = optional; OK INSIDE the quoted string

Quoting turns the value into a YAML scalar; the brackets are now just characters in a string and don't trigger flow-sequence parsing.

If you genuinely want a YAML list of hint tokens (rare; the loader typically wants a single string), use the explicit list form:

argument-hint:
  - working-dir
  - team-name

But this is almost never what argument-hint consumers expect — they treat it as display text shown next to the slash command in /help. Stick with the quoted-string form.

Verification

After quoting:

import yaml
fm = yaml.safe_load(frontmatter_block)   # no longer raises
assert fm["argument-hint"] == "<working-dir> <team-name>"

In Claude Code:

/help                                    # the command now appears

In the plugin tests (e.g., tests/test_plugin_marketplace.py):

uv run pytest tests/test_plugin_marketplace.py -v
# every command's `description` assertion runs against a parsed dict,
# not a parse exception

Example — full command file (corrected)

---
description: Scaffold a new team folder inside an existing competition working directory.
argument-hint: "<working-dir> <team-name>"
allowed-tools: Bash
---

You are running `/nautilus-competition:init-team`. Invoke:

    compete init-team {{1}} {{2}}

If the binary isn't on PATH, surface the install hint from
`plugins/nautilus-competition/README.md` (Install section).

Notes

• The trap fires specifically on multi-token bracketed values. A single bare bracketed token like argument-hint: [path] parses as a one-element list, which the command loader may also reject (depending on how strictly it expects a string). Quote it for safety.
• Prettier and other YAML formatters generally leave bracketed flow sequences alone; they don't auto-quote, so the fix has to be by hand.
• This is unrelated to the [tool.ruff.lint.per-file-ignores] style square-bracket usage in pyproject.toml (TOML, different parser).
• Origin: discovered while shipping slash commands for the cajias/nautilus-competition APM plugin (May 2026). The agent's _parse_frontmatter() test helper caught it on first run; quoting all four argument-hint values made the test pass.

References

• YAML 1.2 flow-sequence productions: https://yaml.org/spec/1.2.2/#742-flow-sequence
• Claude Code command authoring: https://docs.claude.com/en/docs/claude-code/plugins (commands section)
• Plugin development skill (plugin-dev:command-development) — the authoritative reference for command frontmatter fields. This skill documents the YAML-parse pitfall the command-development skill doesn't cover head-on.