Slash Commands in Claude Agent SDK Sessions

## Overview

Slash commands control a Claude Agent SDK session with special `/` commands. Only
commands that work without an interactive terminal are dispatchable through the
SDK; the `system` `init` message lists the ones available in your session.

## Discover available commands

```typescript
for await (const message of query({ prompt: "Hello", options: { maxTurns: 1 } })) {
  if (message.type === "system" && message.subtype === "init") {
    console.log("Available slash commands:", message.slash_commands);
    // e.g. ["clear", "compact", "context", "usage"]
  }
}
```

## Send a command

Send a slash command by putting it in the prompt string:

```typescript
for await (const message of query({ prompt: "/compact", options: { maxTurns: 1 } })) {
  if (message.type === "result" && message.subtype === "success") {
    console.log("Command executed:", message.result);
  }
}
```

Common built-ins include `/compact` (summarize history; emits a
`compact_boundary` system message with `pre_tokens`) and `/clear` (reset context;
useful in streaming-input mode, requires v2.1.117+). For one-shot `query()` calls
`/clear` has no effect since each call starts empty.

## Define custom commands

Custom commands are markdown files; the filename becomes the command name. The
recommended format is `.claude/skills/<name>/SKILL.md` (which also supports
autonomous invocation); `.claude/commands/` is the legacy format and still works.

```markdown
---
allowed-tools: Read, Grep, Glob
description: Run security vulnerability scan
---

Analyze the codebase for security vulnerabilities including SQL injection,
XSS, exposed credentials, and insecure configurations.
```

Custom commands support arguments (`$0`, `$1`, `$ARGUMENTS`), embedded bash output
(`` !`git status` ``), and file references (`@package.json`). They appear in the
`slash_commands` list and are invoked like built-ins:

```typescript
for await (const message of query({ prompt: "/security-check", options: { maxTurns: 3 } })) {
  /* ... */
}
```

## Organization

Group commands in subdirectories (for example `.claude/commands/frontend/`); the
subdirectory shows in the description but not the command name.

## Source

- Slash Commands in the SDK: https://code.claude.com/docs/en/agent-sdk/slash-commands