Claude Code Complete Guide

Claude Code is a terminal-based AI coding agent. This handbook is for advanced users who want to run it as an operating system for software work: multiple sessions, explicit plans, automated checks, project memory, subagents, hooks, MCP tools, and repeatable team workflows.

Latest Update Baseline

• August 3 command re-check: there is no newer stable release and no command addition, removal, or rename after v2.1.220. The August 1 guidance for /fork and /subtask, manual review/verify, Opus 5, subagent limits, sandboxing, and headless operation remains current.

• Treat v2.1.206~v2.1.220 as a model, session, and containment refresh: Opus 5 becomes the default Opus path; /fork creates a separate background session while /subtask owns the inherited-context subagent path; session and concurrency caps bound subagents; and new controls cover strict network allowlisting, optional filesystem isolation, DirectoryAdded, MCP init errors, and nested subagent forwarding in headless output.

• Treat v2.1.202~v2.1.205 as a workflow, background-session, and diagnostics refresh: dynamic workflow sizing and OTel attributes, a fast single-pass /review, MCP root updates, extensive Agent View/background reliability fixes, headless SessionStart hook streaming, a full /doctor setup checkup, transcript-tampering protection in Auto mode, and corrected --json-schema validation.

• Treat v2.1.197 as the Sonnet 5 baseline: Sonnet 5 is now the default Sonnet path in Claude Code, maps from the Anthropic API sonnet alias, has native 1M context, and supports xhigh effort.

• Treat v2.1.181~v2.1.201 as an operations and safety refresh: sandbox.credentials, destructive-command sandbox auto-approval, Windows PowerShell sandbox fixes, MCP login/logout, MCP idle timeout, Korean paste handling, non-interactive fallback stability, background/Remote recovery, plugin validation fixes, and app/schema bug fixes. v2.1.198 makes subagents run in the background by default (plus Claude in Chrome GA and the new /dataviz skill), v2.1.200 changes the default permission mode to "Manual", and v2.1.201 removes the mid-conversation system role on Sonnet 5 sessions.

• Track model restrictions and warnings: main model restrictions now center on Opus, Sonnet, Haiku, and Fable families; Haiku is not available as the main model but can be used for subagents, and deprecated model warnings are stronger.

• Keep subagent work bounded: v2.1.219 defaults to three nested layers below the main conversation, while v2.1.212+ caps a session at 200 spawned subagents and v2.1.217+ caps concurrent subagents at 20 unless configured otherwise.

• Use Fable 5 for the hardest and longest-running autonomous tasks when your organization has access. It requires Claude Code v2.1.170 or later, is selected with /model fable, and is not the default model.

• Treat Opus 5 as the default high-capability Opus path for complex architecture, multi-step refactors, and long-running agentic work when Fable is unavailable or not appropriate.

• Check model aliases by provider. best uses Fable 5 where available and otherwise falls back to the latest Opus model; opus maps to Opus 5 on the Anthropic API, Claude Platform on AWS, Bedrock, and Google Cloud Agent Platform. Microsoft Foundry remains on Opus 4.6, so pin a full model name or ANTHROPIC_DEFAULT_*_MODEL when reproducibility matters.

• Plan for Fable-specific fallback. Claude Code automatically reroutes Fable requests flagged by safety classifiers to the default Opus path; use --safe-mode when repository context itself may be triggering a classifier.

• Note the Fable 5 redeployment. Fable 5 and Mythos 5 were suspended 2026-06-12 under US export controls (~19 days) and Fable 5 was restored globally on 2026-07-01 with an improved safety classifier targeting the reported vulnerability-finding behavior — the automatic fallback follows it. Pro/Max/Team/select Enterprise include Fable 5 up to 50% of weekly usage limits through 2026-07-07, then via usage credits. Mythos 5 (no classifiers) is Project Glasswing invitation-only and cannot be selected through /model.

• Use Agent View and background sessions for parallel work: claude agents, /bg, /background, claude --bg, and claude --bg --exec.

• Configure fallbackModel chains for availability failures before long autonomous sessions so overloads fail over deliberately instead of stopping the turn.

• Use /goal for completion-condition driven work and /workflows for dynamic workflows that orchestrate many background agents.

• Prefer /usage-credits over legacy /extra-usage, and use /reload-skills when commands or skills change during a session.

• Use native installation paths first; treat npm installation as legacy where official tooling does.

• Keep every agent session scoped to a concrete working directory, branch, and verification target.

• Prefer worktrees for parallel implementation, review, and experiment tracks.

• Put durable project rules in CLAUDE.md, but keep them short enough that the agent can obey them.

• Use subagents for bounded parallel work, not for urgent blocking decisions.

• Use hooks and MCP only after the manual workflow is clear enough to automate.

• Make verification part of the prompt contract: tests, lint, typecheck, browser checks, or review notes.

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