Sessions & resumption

A session in Claude Code is a persistent conversation thread that records every prompt, agent reasoning step, tool call, tool result, and file interaction to disk. The SDK writes this transcript automatically so you can pause work and return to it later without losing context. Each session is tied to a specific project directory and stored locally under ~/.claude/projects/ as a transcript file (JSONL format). Resuming a session means the agent has full context from before: files it already read, analysis it already performed, decisions it already made. You can ask follow-up questions, recover from interruptions, or branch off to try a different approach without re-explaining your codebase or repeating prior work. The SDK exposes three distinct patterns: 'continue' (automatically picks up the most recent session in the current directory), 'resume' (targets a specific session by ID, required for multi-user or multi-session workflows), and 'fork' (creates a new independent session that starts with a copy of an existing session's history, leaving the original unchanged). These patterns cover the full spectrum from simple day-to-day continuity to advanced parallel experimentation.