Chapter 9 already gave a brief introduction to plan mode. Now that the recipe planner has several interconnected parts (backend route, frontend form, validation, tests), it's the right time to show it in detail on a genuinely larger change: extending recipes with categories (e.g. "starter", "main course", "dessert"), including filtering by them.

Why this change needs planning

Unlike the ingredient search from chapter 12, this change touches almost every layer of the project at once: the data structure in data/recipes.json, the validation in src/recipes.js, several routes in server.js, the form and the display in public/, and new tests. With that many places affected, it's worth agreeing on a plan first, rather than diving straight into changes.

Turning on plan mode

In plan mode, Claude Code reads the project as usual but makes no changes. A fitting starting prompt:

> I want to add a category to recipes (starter, main course, dessert) and make the list filterable by it, similar to the existing ingredient search. Create a plan for this before anything gets changed.

What a good plan looks like

A sensible plan for this task lists the affected files and the respective change, for example:

1. data/recipes.json: add a category field to existing recipes (migrating existing data).
2. src/recipes.js: extend validateRecipe() with a check that category is one of the allowed values.
3. server.js: extend the existing route with a further query parameter category, similar to zutat.
4. public/index.html and public/app.js: a category selector in the form and in the filter bar.
5. tests/recipes.test.js: new tests for the category validation.

Exactly this plan gives you the chance to step in before implementation: maybe you'd prefer fixed, predefined categories instead of free text input, or the migration of existing recipes should work differently. All of that can be clarified on the plan itself, before a single file gets touched.

Adjusting and approving the plan

> I like the plan, but please only allow these three fixed categories: starter, main course, dessert - no free text input. Everything else is fine, go ahead.

Only after this approval does Claude Code switch from planning to actual implementation - using the tools from block 2, just like with any other change.

When plan mode is especially worth it

• For changes that touch several files and layers of the project at once.
• When there are several reasonable approaches and you want to decide which one yourself.
• In unfamiliar or particularly important code, where an unexpected change would be costly.