[FEATURE] Feature request: Session start hook or --init-command CLI flag for automatic setup on launch

Preflight Checklist

• [x] I have searched existing requests and this feature hasn't been requested yet
• [x] This is a single feature request (not multiple features)

Problem Statement

Summary

I'd like the ability to automatically run a slash command (or shell command) when a Claude Code session starts,
optionally conditioned on the working directory.

## Current Limitation

• Available hooks (PreToolUse, PostToolUse, Notification) only fire during tool execution, not on session

start.

• There's no CLI flag to inject an initial command into an interactive session.
• Shell wrappers (<<<, expect) are fragile and break interactive stdin.

Proposed Solution

Proposed Solutions (any one would work)

1. SessionStart hook type — fires once when a session begins, with access to cwd for conditional logic:

```json
{
"hooks": {
"SessionStart": [
{
"matcher": { "cwd_pattern": "/my/sandbox/dir/abc/.*" },
"commands": ["/sandbox"]
}
]
}
}

1. CLI flag like --init-command "/sandbox" that executes a slash command after interactive startup completes.
2. startup_commands field in .claude/settings.json that lists slash commands to run on launch:

{
"startup_commands": ["/sandbox"]
}

Alternative Solutions

_No response_

Priority

Medium - Would be very helpful

Feature Category

CLI commands and flags

Use Case Example

Use Case

I work across multiple project trees. When launching Claude from directories under a specific path (e.g.,
/my/sandbox/dir/abc/), I need /sandbox to run automatically to configure the session environment.
Currently I have to remember to type it manually every time.

Additional Context

_No response_