The one extension point that edits the system prompt itself. Five built-in styles, the single frontmatter field that decides whether Claude still behaves like an engineer, and how output styles differ from the four mechanisms they get confused with.

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A session is a JSONL file you can resume, branch and rewind. What each of those restores, what a resumed session quietly forgets, and the four categories of change that rewind cannot undo.

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The budget every other chapter spends. What is already in the window before you type, what each file read costs, what compaction keeps and drops, and why the prompt cache makes a mid-task model switch more expensive than it looks.

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Two ways instructions persist without you retyping them: rules you scope to file paths so they cost nothing until they are relevant, and the notes Claude writes for itself between sessions.

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The file that stops you re-explaining your project every session. Where it can live, how several of them combine, why it is context rather than configuration, and the test for whether an instruction belongs in it at all.

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Four settings files, one precedence stack, and the handful of keys that break the stack's rules. Where each file reaches, why some keys merge instead of overriding, and how to find out which file is beating yours.

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Rules decide whether a tool call happens; the sandbox decides what it can touch once it does. Rule syntax and the wildcard placement that widens a rule further than intended, path anchoring, working directories, and where the OS boundary goes.

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Six permission modes decide what runs without asking. The evaluation order they sit inside, what each one actually approves, how auto mode's classifier makes its call, and the paths no mode will ever wave through.

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CLI flags, slash commands and in-prompt sigils are three separate input channels, distinguished by when they are parsed and whether they reach the model at all. The mental model, the handful you actually use, and where the full reference lives.

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A language model has no filesystem, no shell and no network. Claude Code is the process that supplies them and drives the model in a loop. The execution model, the tools, and the one rule the permission system grows from.

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