The Act II pivot from driving one Claude Code session by hand to calling it from a script: same agent, same loop, but you pre-decide what's allowed in code before the run ever starts.
The tutorial. Print mode (claude -p) as a Unix citizen — piping stdin (and the 10MB cap), the --bare flag for deterministic CI runs, and structured output via --output-format json (the result, session_id, total_cost_usd, and subtype fields), stream-json with the init and api_retry events, and --json-schema for typed data instead of prose. The run-bounding flags — --max-turns, --max-budget-usd, --model/--fallback-model, --allowedTools/--permission-mode — and chaining turns with --resume/--session-id/--fork-session. Why a model refusal can't be caught from the exit code. Copyable patterns: a commit-message generator (and the space-before-* permission footgun), a stdin-fed typo linter that needs no Bash permission, and a locked-down CI run. Then the Claude Agent SDK (renamed from the Claude Code SDK in September 2025): query() and the options that mirror the CLI flags, custom in-process tools, the Python ClaudeSDKClient, hooks and subagents in code, and the can_use_tool permission callback. Full reference in the headless docs and the migration guide.
The pitfalls. --dangerously-skip-permissions in an unattended run — how to recognize the silent-success failure, and the least-privilege allowlist that replaces it — and the June 15, 2026 billing change that moves Agent SDK and claude -p usage to a separate metered credit pool, plus how to watch total_cost_usd and bound it.
News. Claude Code 2.1.166 (June 6): a fallbackModel setting (up to three), thinking-off controls, a "*" deny-all glob, and a cross-session permission-escalation fix; latest is 2.1.167 (changelog). 2.1.163 added additionalContext from Stop hooks, /plugin list, and version-pinning settings. And Claude Opus 4.1 is deprecated, retiring on the API August 5, 2026 (release notes).
Earlier episodes referenced: CLAUDE.md and --resume, permissions and plan mode, custom slash commands and hooks, skills, subagents and the orchestrator pattern, MCP servers, cost and rate-limit engineering and evals, ultraplan/ultrareview, and parallel sessions with git worktrees.