Claude Code

Claude Code is Anthropic's official CLI for Claude. It supports both interactive and headless operation.

Installation

npm install -g @anthropic-ai/claude-code

The binary is named claude.

Headless mode

Claude Code's headless mode is invoked with the -p (print) flag:

claude -p "explain this codebase" --output-format stream-json --verbose

How harness maps to Claude Code

harness	Claude Code
`PermissionMode::FullAccess`	`--dangerously-skip-permissions`
`PermissionMode::ReadOnly`	`--permission-mode plan`
`config.prompt`	`-p <prompt>`
`config.model`	`--model <model>`
`config.max_turns`	`--max-turns <n>`
`config.max_budget_usd`	`--max-budget-usd <n>`
`config.system_prompt`	`--system-prompt <text>`
`config.append_system_prompt`	`--append-system-prompt <text>`
`config.resume_session`	`--resume <session-id>`
Output format	`--output-format stream-json --verbose`

Key flags

Flag	Description
`-p <prompt>`	Required for headless. Run non-interactively with this prompt.
`--output-format <fmt>`	Output format: `text`, `json`, `stream-json`
`--verbose`	Include detailed events in stream output
`--model <model>`	Model to use (e.g. `claude-opus-4-6`)
`--max-turns <n>`	Maximum agentic turns before stopping
`--max-budget-usd <n>`	Maximum spend in USD
`--system-prompt <text>`	Replace the default system prompt
`--append-system-prompt <text>`	Append to the default system prompt
`--resume <session-id>`	Resume a previous session

Permission modes

Flag	Effect
(none)	Default — may prompt for tool approvals
`--permission-mode acceptEdits`	Auto-approve file edits
`--permission-mode plan`	Read-only / plan mode
`--dangerously-skip-permissions`	Auto-approve everything

Stream-JSON output format

With --output-format stream-json, Claude Code emits NDJSON. Event types:

System init

{
  "type": "system",
  "subtype": "init",
  "session_id": "abc-123",
  "model": "claude-opus-4-6",
  "cwd": "/path/to/project"
}

Assistant message

{
  "type": "assistant",
  "message": {
    "role": "assistant",
    "content": [
      { "type": "text", "text": "I found the bug in line 42." }
    ]
  }
}

Stream event (partial delta)

{
  "type": "stream_event",
  "event": {
    "delta": {
      "type": "text_delta",
      "text": "Hi"
    }
  }
}

Result

{
  "type": "result",
  "subtype": "success",
  "result": "Task completed successfully.",
  "session_id": "abc-123",
  "duration_ms": 12345,
  "total_cost_usd": 0.05
}

Environment variables

Variable	Purpose
`ANTHROPIC_API_KEY`	API key for Claude models

Claude Code

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