Instructions when creating or editing skills.

What is a Skill?

A skill (.claude/skills/<name>/SKILL.md) extends what Claude can do in the current conversation. Skills come in two flavors:

Reference skills — conventions, patterns, domain knowledge. Claude auto-loads them when relevant and applies them inline alongside conversation context.
Task skills — step-by-step instructions for a specific action (deploy, commit, generate). Typically invoked explicitly via /name. Can run inline or fork to a subagent with context: fork.

Skills follow the open Agent Skills standard; Claude Code adds invocation control, subagent execution, and dynamic context injection. Custom commands have been merged into skills — .claude/commands/foo.md and .claude/skills/foo/SKILL.md both produce /foo; if both exist, the skill wins.

For agent .md files, see the doc-agents skill.

SKILL.md YAML Frontmatter

Sources:

https://code.claude.com/docs/en/skills (Claude Code skills reference)
https://platform.claude.com/docs/en/agents-and-tools/agent-skills/best-practices (authoring guide)
Use a double-quoted string for the description field (not > folded scalar) — VS Code's YAML parser chokes on the folded syntax.
The /slash-command name comes from the skill's directory name, not the frontmatter name field. name is only a display label. (Exception: in a plugin-root SKILL.md, name does set the command, because there's no containing skill directory.)

All SKILL.md frontmatter fields

All fields are optional. Only description is recommended.

Field	Required	Description
`name`	No	Display name in skill listings. Defaults to the directory name. Max 64 chars, lowercase letters/numbers/hyphens only, no XML tags, cannot contain "anthropic" or "claude". Does NOT set the slash-command name except in a plugin-root `SKILL.md`.
`description`	Recommended	What the skill does and when to use it. Claude uses this to decide when to auto-load. Max 1024 chars, no XML tags, third person ("Processes PDFs", not "I help with PDFs"). The combined `description` + `when_to_use` text is truncated at 1,536 chars in the skill listing — put the key use case first. If omitted, the first paragraph of markdown content is used as fallback.
`when_to_use`	No	Additional trigger context appended to `description` in the skill listing — trigger phrases, example requests. Counts toward the 1,536-char listing cap.
`argument-hint`	No	Hint shown during autocomplete. Example: `[issue-number]` or `[filename] [format]`.
`arguments`	No	Named positional arguments for `$name` substitution in skill content. Accepts a space-separated string or a YAML list. Names map to positions in order (e.g. `arguments: [issue, branch]` makes `$issue` = arg 0, `$branch` = arg 1).
`disable-model-invocation`	No	`true` = explicit-only; prevents Claude from auto-loading. Only you can invoke via `/name`. Description is removed from context entirely. Also prevents preloading into subagents. Use for side-effect workflows (`/deploy`, `/commit`). Default: `false`.
`user-invocable`	No	`false` = hidden from the `/` menu; only Claude can invoke automatically. Description stays in context. Use for background knowledge that isn't actionable as a command. Default: `true`.
`allowed-tools`	No	Tools Claude can use without asking permission while this skill is active. Accepts a space- or comma-separated string, or a YAML list. Example: `Read Grep Glob` or `Bash(git *)`. Use `[]` to grant no tools (useful for delegate-only skills). Does not restrict the tool pool — only pre-approves. Takes effect after workspace trust is accepted for project skills.
`disallowed-tools`	No	Tools removed from Claude's pool while this skill is active. Useful for autonomous skills that should never call certain tools (e.g. `AskUserQuestion` in a background loop). Same format as `allowed-tools`. Restriction clears on next user message.
`model`	No	Model to use while this skill is active. Same values as `/model`, plus `inherit` to keep the session model. Override applies for the current turn only; the session model resumes on the next prompt.
`effort`	No	Effort level while this skill is active. Options: `low`, `medium`, `high`, `xhigh`, `max` (levels available depend on the model). Overrides the session effort level.
`context`	No	Set to `fork` to run in a forked subagent context. The skill content becomes the subagent's task prompt; it won't have access to conversation history. Only makes sense for task skills, not reference content.
`agent`	No	Which subagent type to use when `context: fork` is set. Options: built-in (`Explore`, `Plan`, `general-purpose`) or any custom subagent from `.claude/agents/`. Default: `general-purpose`.
`paths`	No	Glob patterns that limit when Claude auto-loads this skill. Comma-separated string or YAML list. When set, Claude considers the skill only when working with matching files.
`shell`	No	Shell for dynamic-context-injection commands (see "Dynamic context injection" below). `bash` (default) or `powershell`. PowerShell requires `CLAUDE_CODE_USE_POWERSHELL_TOOL=1`.
`hooks`	No	Lifecycle hooks scoped to this skill (`PreToolUse`, `PostToolUse`, `Stop`). See hooks in skills and agents.

Skill content lifecycle

When invoked, the rendered SKILL.md enters the conversation as a single message and stays there for the rest of the session — Claude Code does not re-read the file on later turns. Write guidance as standing instructions, not one-time steps.

On auto-compaction, the most recent invocation of each skill is re-attached after the summary, capped at 5,000 tokens per skill and 25,000 tokens combined (filled most-recent-first; older skills can drop entirely). If a large skill seems to stop influencing behavior after compaction, re-invoke it.

Skill authoring guidelines

SKILL.md body under 500 lines. Move detailed reference material to separate files in the skill directory and link from SKILL.md.
References one level deep. SKILL.md links directly to reference.md, never reference.md → details.md. Claude may partially read deeply nested files (e.g. head -100) and miss content.
Table of contents for long reference files. Files over 100 lines should open with a TOC so partial reads still reveal full scope.
Naming convention: gerund form when practical (processing-pdfs, analyzing-spreadsheets). Noun-phrase (pdf-processing) is acceptable. Avoid vague names (helper, utils, tools).
Write descriptions in third person. "Processes PDF files", not "I help you process PDFs" or "You can use this to…". Include both what and when. Skill selection is pure LLM reasoning — no embeddings, no keyword match — so the description must contain terms a user would naturally say.
Don't over-explain. Claude already knows common concepts. Only add what it doesn't have (domain schemas, project conventions, tool usage). Every token in a loaded skill competes with conversation history.
Match degrees of freedom to fragility. Low freedom (specific commands, no params) for fragile/critical ops; medium (scripts with knobs) for preferred-pattern tasks; high (textual guidance) for open-ended work.
Only grant tools the skill actually needs. Prefer allowed-tools: Read Grep over a kitchen-sink list. Use Bash(git *) prefix-matching to scope shells.
Consistent terminology. Pick one term and use it throughout (always "field", never mixing "field"/"box"/"element").
Avoid time-sensitive phrasing. Don't write "before August 2025, use the old API" — put deprecated guidance in an "Old patterns" section instead.
Forward slashes in all paths, even when documenting Windows usage.
MCP tool references use ServerName:tool_name (e.g. BigQuery:bigquery_schema) to avoid "tool not found" when multiple MCP servers are loaded.
Skill descriptions share a character budget (~2% of context window, fallback 16,000 chars). If skills get excluded, run /context to check. Override with SLASH_COMMAND_TOOL_CHAR_BUDGET env var.

Authoring expert / reference skills (external-knowledge domains)

For a skill that makes Claude an "expert" on an external product, tool, or service whose facts live online and change over time (e.g. riverside), encode the retrieval and verification method, not a snapshot of the facts:

Encode the map, not the corpus. The skill should make Claude expert by knowing where to look and how to confirm — a routing map of authoritative sources plus a verification protocol — not by memorizing facts that go stale. You can't fit the whole product's docs in a skill anyway.
Never answer from memory; retrieve then verify. Model memory of a specific app is usually wrong or stale, and a confident wrong answer is worse than an honest "couldn't confirm." State that rule and the reason rather than relying on bare all-caps MUST/NEVER — Claude generalizes better from the why.
Tier the trusted sources (official → first-party-adjacent → a small vetted third-party allowlist) and require citing the source URL and labeling official vs third-party. Explicitly exclude SEO listicles, affiliate roundups, and forums; a fact found only on an untrusted source is reported as unverified.
Search beats blind fetch for bot-blocked docs. Many help centers (Zendesk, Intercom) return HTTP 403 to WebFetch but are fully search-indexed. Prefer WebSearch with allowed_domains scoped to the official domain to find articles, then fetch only pages that allow it; corroborate against a second source when full text can't be fetched. Never guess or construct a doc URL — find the real one.
Verify every URL before it goes in the skill (same discipline as the research-purchase skill's link-verification rule), and keep the verified link catalog in a one-level-deep bundled sources.md (with a TOC) so SKILL.md stays a short routing table — progressive disclosure.
Pre-approve research tools via allowed-tools: WebSearch WebFetch Read so answering doesn't trigger permission prompts.

String substitutions in skill content

The Variable column below writes a space after $ so the loader doesn't substitute these examples while loading this skill — the real variables have no space. (Same self-documentation trap as dynamic injection, below.)

Variable	Description
`$ ARGUMENTS`	All arguments passed when invoking the skill. If not present in content, arguments are appended as `ARGUMENTS: <value>`.
`$ ARGUMENTS[N]`	Access a specific argument by 0-based index (shell-style quoting — wrap multi-word values in quotes to keep them as one argument).
`$ N`	Shorthand for `$ ARGUMENTS[N]` (`$ 0` = first argument).
`$ name`	Named argument declared in the `arguments` frontmatter list. With `arguments: [issue, branch]`, `$ issue` expands to arg 0 and `$ branch` to arg 1.
`$ {CLAUDE_SESSION_ID}`	Current session ID. Useful for logging or session-specific files.
`$ {CLAUDE_EFFORT}`	Current effort level (`low`, `medium`, `high`, `xhigh`, `max`). Use to adapt instructions to the active effort.
`$ {CLAUDE_SKILL_DIR}`	Directory containing the skill's `SKILL.md`. For plugin skills, the skill's subdirectory, not the plugin root. Use in dynamic-injection commands to reference bundled scripts.

Dynamic context injection

A line consisting of an exclamation mark immediately followed by a backtick-quoted command — ! + `git diff HEAD` with no space between them — is executed before Claude sees the skill content; the command output is inlined in its place. A fenced code block whose opening fence is three backticks immediately followed by ! works the same way for multi-line scripts. Use this to ground a skill in live state (current diff, current branch, file listing) without making Claude run a tool first. The shell frontmatter field selects bash (default) or powershell.

(The literal syntax is deliberately not written out in this file — the skill loader would execute it on load. That's also the trap to avoid in any skill that documents injection rather than using it.)

Other skill features (not frontmatter)

Extended thinking: Include "ultrathink" anywhere in skill content to enable thinking mode.
Supporting files: Additional .md, scripts, templates in the skill's directory, referenced from SKILL.md. Bundled files cost no tokens until read; scripts can be executed via bash without loading their contents.
Live change detection: edits to SKILL.md files under watched skill dirs take effect mid-session. Creating a brand-new top-level skills directory requires a restart.