Build a Remix App

Use this skill for end-to-end Remix app work. It should help the agent choose the right layer first, reach for the right package, and avoid the most common Remix-specific mistakes.

What Remix Is

Remix 3 is a server-first web framework built on Web APIs such as Request, Response, URL, and FormData. All packages ship from a single npm package, remix, and are imported via subpath. There is no top-level remix import.

A Remix app has four main pieces:

• Routes in app/routes.ts define the typed URL contract and power href() generation.
• Controllers and actions implement that contract and return Response objects.
• Middleware composes request lifecycle behavior and populates typed context via context.set(Key, value).
• Components render UI with remix/ui. This is not React. A component receives a handle, reads current props from handle.props, and returns a render function.

When To Use This Skill

Use this skill for:

• new features or refactors that touch routing, controllers, middleware, data, auth, sessions, UI, or tests
• reviewing Remix app code for correctness, architecture, or framework usage
• answering "how should this be structured in Remix?" questions
• finding the right package, reference doc, or default pattern for a task

Load Only The References You Need

Classify the task first, then load the smallest useful reference set. Each reference file starts with a "What This Covers" section that lists the topics inside it — read that first to confirm the file is relevant before reading the rest.

Use the table below to find candidates. Loading more than two or three files at once is usually a sign that the task hasn't been narrowed enough yet.

Common bundles:

• Form or CRUD feature -> routing, data and validation, testing; add auth if user-specific
• Protected area -> auth and sessions, routing, testing
• Interactive widget -> component model, mixins and styling; add hydration only if it runs in the browser
• Browser asset pipeline -> assets and browser modules, hydration, middleware and server
• File upload -> middleware and server, data and validation, testing
• Navigation or frames -> hydration, frames, navigation

Default Workflow

1. Classify the change. Decide whether it changes the route contract, request lifecycle, data model, auth or session behavior, or only UI.
2. Start from the server contract. Add or update app/routes.ts before wiring handlers or UI.
3. Put code in the narrowest owner. Favor route-local code first, then promote only when reuse is real.
4. Make the server path correct before adding browser behavior. A route should return the right Response via router.fetch(...) before you add clientEntry(...), animations, or DOM effects.
5. Add middleware deliberately. Keep fast-exit middleware early and request-enriching middleware later. Export a typed AppContext from the root middleware stack and use it in controllers.
6. Validate input at the boundary. Parse and validate Request, FormData, params, cookies, and external payloads before they reach rendering or persistence logic.
7. Hydrate only when necessary. Prefer server-rendered UI. Use clientEntry(...) and run(...) only for real browser interactivity or browser-only APIs.
8. Test the narrowest meaningful layer. Prefer router tests for route behavior. Use component tests when the behavior is truly interactive or DOM-specific.
9. Finish with verification. Re-read the route flow, confirm auth and authorization boundaries, and run the smallest relevant test and typecheck loop.

Project Layout

Use these root directories consistently:

• app/ for runtime application code
• db/ for migrations and local database files
• public/ for static assets served as-is
• test/ for shared helpers, fixtures, and integration coverage
• tmp/ for uploads, caches, local session files, and other scratch data

Inside app/, organize by responsibility:

• assets/ for client entrypoints and client-owned browser behavior
• controllers/ for route-owned handlers and route-local UI
• data/ for schema, queries, persistence setup, migrations, and runtime data initialization
• middleware/ for request lifecycle concerns such as auth, sessions, uploads, and database injection
• ui/ for shared cross-route UI primitives
• utils/ only for genuinely cross-layer helpers that do not clearly belong elsewhere
• routes.ts for the route contract
• router.ts for router setup and wiring

Placement Precedence

When code could live in multiple places:

1. Put it in the narrowest owner first.
2. If it belongs to one route, keep it with that route.
3. If it is shared UI across route areas, move it to app/ui/.
4. If it is request lifecycle setup, keep it in app/middleware/.
5. If it is schema, query, persistence, or startup data logic, keep it in app/data/.
6. Use app/utils/ only as a last resort for truly cross-layer helpers.

Route Ownership

• Use a flat file in app/controllers/ for a simple leaf action, such as app/controllers/home.tsx
• Use a folder with controller.tsx when a route owns nested routes or multiple actions, such as app/controllers/account/controller.tsx
• Mirror nested route structure on disk, such as app/controllers/auth/login/controller.tsx
• Keep route-local UI next to its owner, such as app/controllers/contact/page.tsx
• Move shared UI to app/ui/
• If a flat leaf grows child routes or multiple actions, promote it to a controller folder

Layout Anti-Patterns

• Do not create app/lib/ as a generic dumping ground
• Do not create app/components/ as a second shared UI bucket when app/ui/ already owns that role
• Do not put shared cross-route UI in app/controllers/
• Do not put middleware or persistence helpers in app/utils/ when they have a clearer home
• Do not create folders for simple leaf actions unless they are real controllers

Core Remix Rules

• Import from remix/<subpath>, never import { ... } from 'remix'
• Treat app/routes.ts as the source of truth for URLs. Use routes.<name>.href(...) for redirects, links, tests, and internal URL construction
• Controllers and actions should return explicit Response objects, including redirects, 404s, and validation failures. At the route boundary, prefer returning a Response for expected outcomes (validation errors, conflicts, not found) over throwing for control flow
• Model HTTP behavior explicitly. Status codes, headers, redirects, cache rules, and content types are part of the route contract
• Make the server route correct first. A POST should already return the right HTML, redirect, or error response on its own before clientEntry(...) layers interactivity on top
• Validate input at the boundary using remix/data-schema (and remix/data-schema/form-data for forms). parseSafe makes the failure path a return value instead of an exception
• Derive AppContext from the root middleware stack so get(Database), get(Session), get(Auth), and similar keys stay typed. If the controller never reads from context, it doesn't need the harness
• Outside actions and controllers, only use getContext() when asyncContext() is in the middleware stack
• Remix Component is not React: read props from handle.props, keep state in setup-scope variables, call handle.update() explicitly, and do DOM-sensitive work in event handlers or queueTask(...), not in render
• Prefer host-element mixins via mix={mixin(...)} for behavior and styling instead of inventing custom host prop conventions. Use mix={[...]} only when composing multiple mixins
• Hydrated clientEntry(...) props must be serializable. Do not pass functions, class instances, or opaque runtime objects

Security And Session Defaults

• Never ship demo secrets. In non-test environments, require session and provider secrets from the environment and fail fast if they are missing
• Use hardened cookies: httpOnly always, sameSite by default, and secure when serving over HTTPS
• Regenerate session IDs on login, logout, and privilege changes
• Use requireAuth() to protect authenticated route areas, but still authorize resource ownership inside handlers and data writes
• Add CSRF protection when browser forms mutate state using cookie-backed sessions
• Add CORS only for endpoints that must be called cross-origin. Prefer same-origin by default
• Prefer JSX or remix/html-template for HTML generation so escaping stays correct
• Validate uploads for size, type, and destination. Treat filenames and content as untrusted input

Testing Defaults

• Prefer server and router tests first. Drive the app with router.fetch(new Request(...)) and assert on the returned Response
• Build a fresh router per test or per suite so sessions, in-memory storage, and database state stay isolated
• Use routes.<name>.href(...) in tests so URLs stay coupled to the route contract
• For auth or session scenarios, use a test cookie and createMemorySessionStorage() instead of production storage
• Use component tests only for interactive or DOM-specific behavior. Render with createRoot(...), interact with the real DOM, and call root.flush() between steps
• Prefer one representative behavior test over many repetitive assertion variants

Common Mistakes To Avoid

• Treating Remix Component like React and reaching for hooks or implicit rerendering
• Importing from a top-level remix entry instead of a subpath
• Adding clientEntry(...) before the server-rendered route behavior is correct
• Passing non-serializable props into clientEntry(...)
• Calling getContext() without asyncContext() in the middleware stack
• Getting middleware order wrong; fast exits like static files belong early, request enrichment later
• Skipping boundary validation and trusting raw FormData, params, cookies, or external payloads
• Letting route-local domain errors leak out of the controller. Translate expected outcomes (validation, conflicts, not-found) into the HTTP Response the route means to return rather than throwing a custom Error subclass and catching it elsewhere
• Reaching for createCookie when a tamper-sensitive or server-managed per-browser fact really wants remix/session. If editing the value would be a bug, use a session
• Building a JSON-only RPC layer when a normal form POST, redirect, or resource route would be simpler. Fetch-from-the-client is a layer on top of sound route behavior, not a replacement for it
• Treating JSON state endpoints and <Frame> reloads as mutually exclusive patterns. Pick the lightest sync mechanism that fits the UX; small widgets may reasonably poll a JSON endpoint
• Assuming authentication is enough without per-resource authorization checks
• Dropping shared code into vague buckets like utils.ts, helpers.ts, or common.ts when ownership is known
• Writing only component tests for a feature whose main behavior is really an HTTP route concern

Package Map

Use this map to find the right package quickly. Each entry says what the package is for, not just what it exports. Open the linked reference file when you need full examples.

Routing, Server, and Responses

• remix/router — the router itself. Use for createRouter, controller and middleware types, and registering routes. A URL that matches a route pattern but not the request method gets 405 Method Not Allowed with an Allow header (it does not reach defaultHandler); register an ANY route for a per-URL catch-all. GET routes also serve HEAD with the same status and headers and an empty body — do not add explicit HEAD routes just for that
• remix/routes — declarative route builders. Use for route, get, post, put, del, form, resources when defining app/routes.ts
• remix/node-fetch-server — adapter from Node's http module to a Fetch-style router. Use for createRequestListener in server.ts
• remix/assets — browser asset server. Use for createAssetServer when serving compiled scripts and styles. Scripts resolve imports through import maps: read getScriptEntry() and render <ImportMap value={importMap} /> (from remix/ui/server) before the preloads and module script; use fingerprint: true (content hashes) plus files.cacheKey for persistent caches, and directory mounts (default { app, npm }). Shared compiler options such as target, sourceMaps, sourceMapSourcePaths, and minify live at the top level. Kody's origin website does not use this: hydration URLs come from Pitlane ?assets=
• remix/multiple-import-maps-polyfill — importModule(), detectMultipleImportMapSupport(), preloadShim() for browsers without native multiple-import-map support. Only relevant with remix/assets
• remix/middleware/render — pairs createAssetServer with frame rendering for Node apps. Do not use it on the origin Worker; Kody owns renderToStream plus Workers Assets
• remix/spa — client-only run(router) for apps with no server document. Kody keeps a custom client router on top of server-rendered HTML: it runs the destination's loader before committing the URL, and route components read the payload through createRouteData (#client/route-data.tsx) so the previous page stays until the next one is ready — see docs/contributing/no-flash-navigation.md before adding a client route
• remix/headers — typed header parsers and builders. Use when reading Accept, Cookie, or setting CacheControl, Vary, etc., instead of hand-formatting strings
• remix/response/redirect — redirect(href, status?). Use for the canonical "POST then redirect" pattern and other location changes
• remix/response/html — createHtmlResponse. Use when you need an HTML Response from a string or stream without rendering through remix/ui
• remix/response/compress — compressResponse. Use when compressing one-off responses outside the global compression() middleware
• remix/response/file — file-download responses. Use for Content-Disposition: attachment responses
• remix/route-pattern — low-level URL matching and generation. Use when working with raw patterns outside the router (custom matchers, scripts)
• remix/fetch-proxy — Fetch-based HTTP proxying. Use to forward a request to another origin; pass xForwardedHeaders when the upstream needs forwarded proto, host, and port

Data, Validation, and Persistence

• remix/data-schema — schema builders for runtime validation. Use for parse and parseSafe to validate any input that crosses a trust boundary, and .transform(...) when validated output should map to a different value or type
• remix/data-schema/checks — common check helpers (email, minLength, maxLength, etc.). Use to compose into a schema
• remix/data-schema/coerce — coercion helpers for strings, numbers, booleans, dates, and ids. Use when input arrives as a string but should be a typed value
• remix/data-schema/form-data — f.object and f.field for parsing FormData directly. Use in actions that read browser forms
• remix/data-table — typed tables and a Database interface. Use for table, column, and new Database(driver) when modeling persisted data
• remix/data-table/sqlite, remix/data-table/postgres, remix/data-table/mysql — official drivers. Use createSqliteDatabase(...) for Node, Bun, and compatible synchronous SQLite clients. Remix does not ship a D1 factory; Kody owns D1DatabaseDriver and passes it to new Database
• remix/data-table/migrations — migration authoring and runners. Use for createMigration, createMigrationRunner
• remix/data-table/migrations/node — loadMigrations from disk. Use in startup scripts that apply migrations
• remix/data-table/operators — query operators such as inList(...). Use when where clauses need set or comparison logic

Auth, Sessions, and Cookies

• remix/session — the Session object: get, set, flash, unset, regenerateId. Use for any per-browser state where tampering would be a bug (login, "I submitted this form already", cart, flash messages)
• remix/middleware/session — session(cookie, storage). Use to wire a session cookie and storage backend into the root middleware stack
• remix/session-storage/fs, remix/session-storage/memory, remix/session-storage/cookie — storage backends. Use fs-storage for single-process apps, memory-storage for tests, cookie-storage for stateless deployments where data fits in a cookie
• remix/session-storage/redis — Redis-backed storage. Use for multi-process or multi-host deployments
• remix/session-storage/memcache — Memcache-backed storage. Same multi-host use case as Redis
• remix/cookie — createCookie for plain signed/unsigned cookies. Use for non-sensitive preferences where the client is allowed to control the value (theme, locale, dismissed banner). For state where tampering matters, prefer remix/session
• remix/auth — credentials, OAuth, and OIDC providers. Use to define how identity is verified, start/finish external login, and refresh stored OAuth/OIDC token bundles with refreshExternalAuth(...). Use OAuthTokens. There is no built-in Atmosphere/atproto provider.
• remix/middleware/auth — auth({ schemes }), requireAuth, the Auth context key. Use to resolve identity into the request context and to gate routes

UI, Hydration, and Browser Behavior

• remix/ui — the component runtime: components, core mixins, clientEntry, run, <Frame>, navigation helpers, and createRoot. Use for app UI behavior. Framework attributes are data-rmx-* (data-rmx-target, data-rmx-document, data-rmx-history, …). Listen with native target.addEventListener(type, listener, { signal }); there is no addEventListeners() helper. Server-rendered <script> elements must have a single string child (or stay empty with src); non-string children render empty and error. handle.update() during setup warns and is skipped — move it to an event handler or handle.queueTask(). Optional run({ resolveFrame }) defaults to fetching the frame source as HTML and rendering 3xx/4xx HTML responses in the frame; Kody keeps a custom resolver for the frame registry, prefetch cache, and retries but mirrors that status acceptance. run() falls back to document navigation when the Navigation API is missing, so no window.navigation stub is needed.
• remix/ui/server — server rendering: renderToStream, renderToString. Use in the render(...) helper that returns HTML responses
• remix/ui/animation — animation APIs: animateEntrance, animateExit, animateLayout, spring, tween, and easings
• remix/ui/<primitive> — UI primitives, mixins, glyphs, and theme helpers. Import from remix/ui/accordion, remix/ui/button, remix/ui/select, etc.
• remix/ui/test — component test rendering helpers such as render
• remix/ui/jsx-runtime — JSX transform target. Configured in tsconfig.json, rarely imported directly
• remix/html-template — escaped HTML template literals. Use when generating HTML outside the component system (RSS feeds, email bodies, error pages)
• remix/file-storage — backend-agnostic File storage interface. Use as the type bound for upload destinations
• remix/file-storage/fs, remix/file-storage/memory, remix/file-storage-s3 — storage backends. Use to implement an upload destination

Middleware

• remix/middleware/static — staticFiles(dir). Use to serve files from public/ exactly as they exist on disk
• remix/middleware/form-data — formData(). Use to parse FormData once and expose it via get(FormData) instead of calling await request.formData() in each action
• remix/form-data-parser — lower-level parseFormData, FileUpload. Use when implementing custom upload handlers. Upload handler errors propagate directly
• remix/multipart-parser and remix/multipart-parser/node — low-level multipart stream parsing. MultipartPart.headers is a plain object keyed by lower-case header name; read values with bracket notation such as part.headers['content-type']
• remix/middleware/compression — compression(). Use globally for text-like responses
• remix/middleware/logger — logger(). Use in development for request logs; pass colors to force terminal color output on or off
• remix/middleware/method-override — methodOverride(). Use when HTML forms need PUT, PATCH, or DELETE
• remix/middleware/async-context — asyncContext(), getContext(). Use when helpers outside actions need request context without threading it through every call
• remix/middleware/cors — cors(opts?). Use for endpoints called cross-origin
• remix/middleware/csrf — csrf(opts?). Use when session-backed forms mutate state and need synchronizer-token CSRF protection
• remix/middleware/cop — cross-origin protection. Use to reject unsafe cross-origin browser requests

Test

• remix/test — describe, it, and lifecycle hooks. Use as the test framework
• remix/test/cli — programmatic test runner APIs such as runRemixTest
• remix/cli — programmatic Remix CLI API. Use the remix executable for project commands such as remix test, remix routes, and remix doctor
• remix/assert — assertion helpers. Use in place of node:assert so messages render cleanly in the runner
• remix/terminal — ANSI styles, color detection, style factories, and testable terminal streams. Use for CLIs and terminal output instead of hand-rolled escape sequences

Canonical Patterns

Define routes first

import { form, get, post, resources, route } from 'remix/routes'

export const routes = route({
	home: '/',
	contact: form('contact'),
	books: {
		index: '/books',
		show: '/books/:slug',
	},
	auth: route('auth', {
		login: form('login'),
		logout: post('logout'),
	}),
	admin: route('admin', {
		index: get('/'),
		books: resources('books', { param: 'bookId' }),
	}),
})

Type controllers against the route contract

import type { Controller } from 'remix/router'

import type { AppContext } from '../router.ts'
import { routes } from '../routes.ts'

export default {
  actions: {
    async index({ get }) {
      let db = get(Database)
      let allBooks = await db.findMany(books, { orderBy: ['id', 'asc'] })
      return render(<BooksIndexPage allBooks={allBooks} />)
    },
    async show({ get, params }) {
      let db = get(Database)
      let book = await db.findOne(books, { where: { slug: params.slug } })
      if (!book) return new Response('Not Found', { status: 404 })
      return render(<BookShowPage book={book} />)
    },
  },
} satisfies Controller<typeof routes.books, AppContext>

Compose middleware deliberately

import {
	createRouter,
	type AnyParams,
	type MiddlewareContext,
	type WithParams,
} from 'remix/router'

export type RootMiddleware = [
	ReturnType<typeof formData>,
	ReturnType<typeof session>,
	ReturnType<typeof loadDatabase>,
	ReturnType<typeof loadAuth>,
]

export type AppContext<params extends AnyParams = AnyParams> = WithParams<
	MiddlewareContext<RootMiddleware>,
	params
>

let middleware = []

if (process.env.NODE_ENV === 'development') {
	middleware.push(logger())
}

middleware.push(compression())
middleware.push(staticFiles('./public'))
middleware.push(formData())
middleware.push(methodOverride())
middleware.push(session(cookie, storage))
middleware.push(asyncContext())
middleware.push(loadDatabase())
middleware.push(loadAuth())

let router = createRouter({ middleware })

Mutate, validate, and respond

import { redirect } from 'remix/response/redirect'
import * as s from 'remix/data-schema'
import * as f from 'remix/data-schema/form-data'
import { Session } from 'remix/session'
import { Database } from 'remix/data-table'

let bookSchema = f.object({
  slug: f.field(s.string()),
  title: f.field(s.string()),
})

export default {
  actions: {
    async create({ get }) {
      let parsed = s.parseSafe(bookSchema, get(FormData))
      if (!parsed.success) {
        return render(<NewBookPage errors={parsed.issues} />, { status: 400 })
      }

      let db = get(Database)
      let book = await db.create(books, parsed.value)

      let session = get(Session)
      session.flash('message', `Added ${book.title}.`)

      return redirect(routes.books.show.href({ slug: book.slug }))
    },
  },
} satisfies Controller<typeof routes.books, AppContext>

This shape works without JavaScript, returns a Response for every outcome, and is ready for clientEntry(...) interactivity when the UI needs it.

Build UI from handle props plus render

import { on, type Handle } from 'remix/ui'

function Counter(handle: Handle<{ initialCount?: number; label: string }>) {
	let count = handle.props.initialCount ?? 0

	return () => (
		<button
			mix={on('click', () => {
				count++
				handle.update()
			})}
		>
			{handle.props.label}: {count}
		</button>
	)
}

Only add clientEntry(...) and run(...) when the component needs browser interactivity or browser-only APIs.