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Sim Development Guidelines

You are a professional software engineer. All code must follow best practices: accurate, readable, clean, and efficient.

Global Standards

  • Linting / Audit: bun run check:api-validation must pass on PRs. Do not introduce route-local boundary Zod schemas, direct route Zod imports, or ad-hoc client wire types — see "API Contracts" and "API Route Pattern" below
  • Logging: Import createLogger from @sim/logger. Use logger.info, logger.warn, logger.error instead of console.log. Inside API routes wrapped with withRouteHandler, loggers automatically include the request ID — no manual withMetadata({ requestId }) needed
  • API Route Handlers: All API route handlers (GET, POST, PUT, DELETE, PATCH) must be wrapped with withRouteHandler from @/lib/core/utils/with-route-handler. This provides request ID tracking, automatic error logging for 4xx/5xx responses, and unhandled error catching. See "API Route Pattern" section below
  • Comments: Use TSDoc for documentation. No ==== separators. No non-TSDoc comments
  • Styling: Never update global styles. Keep all styling local to components
  • ID Generation: Never use crypto.randomUUID(), nanoid, or uuid package. Use generateId() (UUID v4) or generateShortId() (compact) from @sim/utils/id
  • Common Utilities: Use shared helpers from @sim/utils instead of inline implementations:
    • sleep(ms) from @sim/utils/helpers — never new Promise(resolve => setTimeout(resolve, ms))
    • toError(e) from @sim/utils/errors — normalize caught values to Error
    • getErrorMessage(e, fallback?) from @sim/utils/errors — extract message string from unknown caught value; never write e instanceof Error ? e.message : 'fallback'
    • structuredClone(value) — built-in deep clone; never JSON.parse(JSON.stringify(...))
    • omit(obj, keys) / filterUndefined(obj) from @sim/utils/object — object trimming; never Object.fromEntries(Object.entries(...).filter(...))
    • truncate(str, maxLength, suffix?) from @sim/utils/string — never inline slice + ellipsis
    • backoffWithJitter(attempt, retryAfterMs, options?) / parseRetryAfter(header) from @sim/utils/retry — shared retry pacing; never reimplement exponential backoff inline
  • Package Manager: Use bun and bunx, not npm and npx

Architecture

Core Principles

  1. Single Responsibility: Each component, hook, store has one clear purpose
  2. Composition Over Complexity: Break down complex logic into smaller pieces
  3. Type Safety First: TypeScript interfaces for all props, state, return types
  4. Predictable State: Zustand for global state, useState for UI-only concerns

Root Structure

apps/
├── sim/                    # Next.js app (UI + API routes + workflow editor)
│   ├── app/                # Next.js app router (pages, API routes)
│   ├── blocks/             # Block definitions and registry
│   ├── components/         # Shared UI (emcn/, ui/)
│   ├── executor/           # Workflow execution engine
│   ├── hooks/              # Shared hooks (queries/, selectors/)
│   ├── lib/                # App-wide utilities
│   ├── providers/          # LLM provider integrations
│   ├── stores/             # Zustand stores
│   ├── tools/              # Tool definitions
│   └── triggers/           # Trigger definitions
└── realtime/               # Bun Socket.IO server (collaborative canvas)

packages/
├── audit/                  # @sim/audit
├── auth/                   # @sim/auth — shared Better Auth verifier
├── db/                     # @sim/db — drizzle schema + client
├── logger/                 # @sim/logger
├── realtime-protocol/      # @sim/realtime-protocol — socket op constants + zod schemas
├── security/               # @sim/security — safeCompare
├── tsconfig/               # shared tsconfig presets
├── utils/                  # @sim/utils
├── workflow-authz/         # @sim/workflow-authz
├── workflow-persistence/   # @sim/workflow-persistence
└── workflow-types/         # @sim/workflow-types — pure BlockState/Loop/Parallel types

Package boundaries

  • apps/* → packages/* only. Packages never import from apps/*.
  • apps/realtime intentionally avoids Next.js, React, the block/tool registry, provider SDKs, and the executor. Do not add imports from @/lib/webhooks/providers/*, @/executor/*, @/blocks/*, or @/tools/* to any package consumed by apps/realtime. CI enforces this via scripts/check-monorepo-boundaries.ts and scripts/check-realtime-prune-graph.ts.
  • Auth is shared across both apps via the Better Auth "Shared Database Session" pattern (same BETTER_AUTH_SECRET, same DB via @sim/db).

Naming Conventions

  • Components: PascalCase (WorkflowList)
  • Hooks: use prefix (useWorkflowOperations)
  • Files: kebab-case (workflow-list.tsx)
  • Stores: stores/feature/store.ts
  • Constants: SCREAMING_SNAKE_CASE
  • Interfaces: PascalCase with suffix (WorkflowListProps)

Imports

Always use absolute imports. Never use relative imports.

// ✓ Good
import { useWorkflowStore } from '@/stores/workflows/store'

// ✗ Bad
import { useWorkflowStore } from '../../../stores/workflows/store'

Use barrel exports (index.ts) when a folder has 3+ exports. Do not re-export from non-barrel files; import directly from the source.

Import Order

  1. React/core libraries
  2. External libraries
  3. UI components (@/components/emcn, @/components/ui)
  4. Utilities (@/lib/...)
  5. Stores (@/stores/...)
  6. Feature imports
  7. CSS imports

Use import type { X } for type-only imports.

TypeScript

  1. No any - Use proper types or unknown with type guards
  2. Always define props interface for components
  3. as const for constant objects/arrays
  4. Explicit ref types: useRef<HTMLDivElement>(null)

Components

'use client' // Only if using hooks

const CONFIG = { SPACING: 8 } as const

interface ComponentProps {
  requiredProp: string
  optionalProp?: boolean
}

export function Component({ requiredProp, optionalProp = false }: ComponentProps) {
  // Order: refs → external hooks → store hooks → custom hooks → state → useMemo → useCallback → useEffect → return
}

Extract when: 50+ lines, used in 2+ files, or has own state/logic. Keep inline when: < 10 lines, single use, purely presentational.

API Contracts

Boundary HTTP request and response shapes for all routes under apps/sim/app/api/** live in apps/sim/lib/api/contracts/** (one file per resource family — folders.ts, chats.ts, knowledge.ts, etc.). Routes never define route-local boundary Zod schemas, and clients never define ad-hoc wire types — both sides consume the same contract.

  • Each contract is built with defineRouteContract({ method, path, params?, query?, body?, headers?, response: { mode: 'json', schema } }) from @/lib/api/contracts
  • Contracts export named schemas (e.g., createFolderBodySchema) AND named TypeScript type aliases (e.g., export type CreateFolderBody = z.input<typeof createFolderBodySchema>)
  • Clients (hooks, utilities, components) import the named type aliases from the contract file. They must never write z.input<...> / z.output<...> themselves
  • Shared identifier schemas live in apps/sim/lib/api/contracts/primitives.ts (e.g., workspaceIdSchema, workflowIdSchema). Reuse these instead of redefining string-based ID schemas
  • Audit script: bun run check:api-validation enforces boundary policy and prints ratchet metrics for route Zod imports, route-local schema constructors, route ZodError references, client hook Zod imports, and related counters. It must pass on PRs. bun run check:api-validation:strict is the strict CI gate and additionally fails on annotations with empty reasons

Domain validators that are not HTTP boundaries — tools, blocks, triggers, connectors, realtime handlers, and internal helpers — may still use Zod directly. The contract rule is boundary-only.

Boundary annotations

A small number of legitimate exceptions to the boundary rules are tolerated when annotated. The audit script recognizes four annotation forms:

  • // boundary-raw-fetch: <reason> — placed on the line directly above a raw fetch( call in client hooks (apps/sim/hooks/queries/**, apps/sim/hooks/selectors/**) AND any same-origin /api/... fetch elsewhere under apps/sim/** outside an API route handler. Use only for documented exceptions: streaming responses, binary downloads, multipart uploads, signed-URL flows, OAuth redirects, and external-origin requests
  • // double-cast-allowed: <reason> — placed on the line directly above an as unknown as X cast outside test files
  • // boundary-raw-json: <reason> — placed on the line directly above a raw await request.json() / await req.json() read in a route handler. Use only when the body is a JSON-RPC envelope, a tolerant .catch(() => ({})) parse, or otherwise cannot go through parseRequest
  • // untyped-response: <reason> — placed on the line directly above a schema: z.unknown() response declaration in a contract file. Use only when the response body is genuinely opaque (user-supplied data, third-party passthrough)

Placement rule: the annotation must immediately precede the call or cast. Up to three non-empty preceding comment lines are tolerated, so additional context comments above the annotation are fine. The reason must be non-empty after trimming — annotations with empty reasons fail strict mode (annotationsMissingReason).

Whole-file allowlists for routes (legitimate non-boundary or auth-handled routes that legitimately import Zod for non-boundary reasons) go through INDIRECT_ZOD_ROUTES in scripts/check-api-validation-contracts.ts, not per-line annotations.

Examples:

// boundary-raw-fetch: streaming SSE chunks must be processed as they arrive
const response = await fetch(`/api/copilot/chat/stream?chatId=${chatId}`, { signal })
// double-cast-allowed: legacy provider type lacks the discriminator field we need
const provider = config as unknown as LegacyProvider

API Route Pattern

Every API route handler must be wrapped with withRouteHandler. This sets up AsyncLocalStorage-based request context so all loggers in the request lifecycle automatically include the request ID.

Routes never import { z } from 'zod' and never define route-local boundary schemas. They consume the contract from @/lib/api/contracts/** and validate with canonical helpers from @/lib/api/server:

  • parseRequest(contract, request, context, options?) — fully contract-bound routes; parses params, query, body, and headers in one call. Pass {} for context on routes without route params, or the route's context argument when route params exist. Returns a discriminated union; check parsed.success and return parsed.response on failure
  • validationErrorResponse(error) and getValidationErrorMessage(error, fallback) — produce 400 responses from a ZodError
  • validationErrorResponseFromError(error) — when handling unknown caught errors that may or may not be a ZodError
  • isZodError(error) — type guard. Routes never use instanceof z.ZodError

Fully contract-bound route (parseRequest)

import { createLogger } from '@sim/logger'
import type { NextRequest } from 'next/server'
import { NextResponse } from 'next/server'
import { createFolderContract } from '@/lib/api/contracts/folders'
import { parseRequest } from '@/lib/api/server'
import { withRouteHandler } from '@/lib/core/utils/with-route-handler'

const logger = createLogger('FoldersAPI')

export const POST = withRouteHandler(async (request: NextRequest) => {
 const parsed = await parseRequest(createFolderContract, request, {})
 if (!parsed.success) return parsed.response
 const { body } = parsed.data
 logger.info('Creating folder', { workspaceId: body.workspaceId })
 return NextResponse.json({ ok: true })
})

Composing with other middleware

export const POST = withRouteHandler(withAdminAuth(async (request) => {
  return NextResponse.json({ ok: true })
}))

Routes under apps/sim/app/api/v1/** use the shared middleware in apps/sim/app/api/v1/middleware.ts for auth, rate-limit, and workspace access. Compose contract validation inside that middleware — never reimplement auth/rate-limit per-route.

Never export a bare async function GET/POST/... — always use export const METHOD = withRouteHandler(...).

Adding a new boundary feature end-to-end

When adding a new route + client surface, follow this order. Each step has one place it lives.

  1. Author the contract first in apps/sim/lib/api/contracts/<domain>.ts (or a subdirectory for large domains: knowledge/, selectors/, tools/). Define one schema per request slice (params, query, body, headers) and one for the response, then wrap with defineRouteContract. Export named type aliases (z.input for inputs, z.output for outputs).
  2. Implement the route in apps/sim/app/api/<path>/route.ts. Auth always runs before parseRequest — never validate untrusted input before authenticating the caller. The route returns exactly the shape declared in contract.response.schema.
  3. Add the React Query hook in apps/sim/hooks/queries/<domain>.ts. Use requestJson(contract, input) for the call. Build a hierarchical query-key factory (all → lists() → list(workspaceId) → details() → detail(id)) so invalidations can target prefixes.
  4. Use the hook in the component. The mutation's data and error are fully typed from the contract; surface error.message (already extracted from the response body's error or message field by requestJson).

Schema review checklist (read the contract diff like a DB migration)

LLMs will write contracts that compile but are sloppy. The human reviewer should optimize attention on:

  • required vs optional vs nullable is correct. optional() allows omission; nullable() allows null; chaining both creates a tri-state that's almost never what you want.
  • Response schema matches the route's actual JSON output. The most common drift bug — route emits a field the schema doesn't declare, or omits a required field. Walk every NextResponse.json(...) callsite against the schema.
  • Error messages are descriptive. 'fileName cannot be empty' beats 'Required'. Use the second arg of min(1, '...'), nonempty('...'), etc. For cross-field refines, use superRefine with a path and a message that names the failing field.
  • Bounds are set on arrays (.min(1), .max(N)), strings (.min(1).max(N) for IDs/names), and numbers (.min().max() for limits/sizes).
  • z.unknown() is a smell unless the data is genuinely arbitrary (provider passthrough, user-defined tool result, JSON-RPC envelope). When kept, must be annotated // untyped-response: <specific reason> in a schema: slot.
  • Discriminated unions over plain unions when the wire has a discriminant field — gives clients exhaustive narrowing.

CI (bun run check:api-validation:strict) catches structural violations (Zod imports in routes, raw request.json(), double casts, missing annotations). It does not catch these schema-quality judgments — that's the human's job in PR review.

Hooks

interface UseFeatureProps { id: string }

export function useFeature({ id }: UseFeatureProps) {
  const idRef = useRef(id)
  const [data, setData] = useState<Data | null>(null)
  
  useEffect(() => { idRef.current = id }, [id])
  
  const fetchData = useCallback(async () => { ... }, []) // Empty deps when using refs
  
  return { data, fetchData }
}

Zustand Stores

Stores live in stores/. Complex stores split into store.ts + types.ts.

import { create } from 'zustand'
import { devtools } from 'zustand/middleware'

const initialState = { items: [] as Item[] }

export const useFeatureStore = create<FeatureState>()(
  devtools(
    (set, get) => ({
      ...initialState,
      setItems: (items) => set({ items }),
      reset: () => set(initialState),
    }),
    { name: 'feature-store' }
  )
)

Use devtools middleware. Use persist only when data should survive reload with partialize to persist only necessary state.

React Query

All React Query hooks live in hooks/queries/. All server state must go through React Query — never use useState + fetch in components for data fetching or mutations.

Client Boundary

Hooks consume contracts the same way routes do. Every same-origin JSON call must go through requestJson(contract, ...) from @/lib/api/client/request instead of raw fetch:

  • Hooks import named type aliases from @/lib/api/contracts/**. Never write z.input<...> / z.output<...> in hooks, and never import { z } from 'zod' in client code
  • requestJson parses params, query, body, and headers against the contract on the way out and validates the JSON response on the way back. Hooks always forward signal for cancellation
  • Documented exceptions for raw fetch: streaming responses, binary downloads, multipart uploads, signed-URL flows, OAuth redirects, and external-origin requests. Mark each raw fetch with a TSDoc comment explaining which exception applies. The // boundary-raw-fetch annotation is required not only in client hooks but for any same-origin /api/... fetch anywhere under apps/sim/** outside an API route handler — strict CI flags these regardless of location
import { keepPreviousData, useQuery } from '@tanstack/react-query'
import { requestJson } from '@/lib/api/client/request'
import { listEntitiesContract, type EntityList } from '@/lib/api/contracts/entities'

async function fetchEntities(workspaceId: string, signal?: AbortSignal): Promise<EntityList> {
  const data = await requestJson(listEntitiesContract, {
    query: { workspaceId },
    signal,
  })
  return data.entities
}

export function useEntityList(workspaceId?: string) {
  return useQuery({
    queryKey: entityKeys.list(workspaceId),
    queryFn: ({ signal }) => fetchEntities(workspaceId as string, signal),
    enabled: Boolean(workspaceId),
    staleTime: 60 * 1000,
    placeholderData: keepPreviousData,
  })
}

Query Key Factory

Every file must have a hierarchical key factory with an all root key and intermediate plural keys for prefix invalidation:

export const entityKeys = {
  all: ['entity'] as const,
  lists: () => [...entityKeys.all, 'list'] as const,
  list: (workspaceId?: string) => [...entityKeys.lists(), workspaceId ?? ''] as const,
  details: () => [...entityKeys.all, 'detail'] as const,
  detail: (id?: string) => [...entityKeys.details(), id ?? ''] as const,
}

Query Hooks

  • Every queryFn must forward signal for request cancellation
  • Every query must have an explicit staleTime
  • Use keepPreviousData only on variable-key queries (where params change), never on static keys
export function useEntityList(workspaceId?: string) {
  return useQuery({
    queryKey: entityKeys.list(workspaceId),
    queryFn: ({ signal }) => fetchEntities(workspaceId as string, signal),
    enabled: Boolean(workspaceId),
    staleTime: 60 * 1000,
    placeholderData: keepPreviousData, // OK: workspaceId varies
  })
}

Mutation Hooks

  • Use targeted invalidation (entityKeys.lists()) not broad (entityKeys.all) when possible
  • For optimistic updates: use onSettled (not onSuccess) for cache reconciliation — onSettled fires on both success and error
  • Don't include mutation objects in useCallback deps — .mutate() is stable in TanStack Query v5
export function useUpdateEntity() {
  const queryClient = useQueryClient()
  return useMutation({
    mutationFn: async (variables) => { /* ... */ },
    onMutate: async (variables) => {
      await queryClient.cancelQueries({ queryKey: entityKeys.detail(variables.id) })
      const previous = queryClient.getQueryData(entityKeys.detail(variables.id))
      queryClient.setQueryData(entityKeys.detail(variables.id), /* optimistic */)
      return { previous }
    },
    onError: (_err, variables, context) => {
      queryClient.setQueryData(entityKeys.detail(variables.id), context?.previous)
    },
    onSettled: (_data, _error, variables) => {
      queryClient.invalidateQueries({ queryKey: entityKeys.lists() })
      queryClient.invalidateQueries({ queryKey: entityKeys.detail(variables.id) })
    },
  })
}

Styling

Use Tailwind only, no inline styles. Use cn() from @/lib/core/utils/cn for conditional classes.

<div className={cn('base-classes', isActive && 'active-classes')} />

For equal height and width, use the size-* shorthand — never h-[Npx] w-[Npx] or h-N w-N. Default icon size is size-[14px].

<Icon className='size-[14px] text-[var(--text-icon)]' />

On chip components (see "EMCN Components"), drive chrome through PROPS, not className: error for the error state, icon/endAdornment for adornments, inputClassName for the inner field. className carries ONLY layout/sizing — never re-specify canonical chrome (border, fill, radius, height, text/icon color) or add focus rings. Full consumer rules in .claude/rules/sim-styling.md.

EMCN Components

Import from @/components/emcn, never from subpaths (except CSS files). Use CVA only when 2+ genuine variants exist; otherwise plain cn().

The chip family is the canonical UI chrome and is progressively replacing the legacy EMCN primitives — always reach for the chip equivalent: ChipInput over Input, ChipTextarea over Textarea, ChipModal/ChipModalField over Modal, ChipSelect/ChipCombobox (searchable) or ChipDropdown (simple menu-select) over Select/Combobox, ChipSwitch over Switch, ChipDatePicker over a raw date field, Chip/ChipLink for pill buttons/links, ChipTag for inline tags/badges. For context/action menus the canonical control is DropdownMenu (not a chip, but the standard menu — not a hand-rolled popover). Components OWN their chrome (single source of truth) — consumers pass props, not class overrides. Authoring rules in .claude/rules/emcn-components.md; consumer rules in .claude/rules/sim-styling.md.

Inside a ChipModalBody, EVERY labeled field MUST be a ChipModalField — never hand-roll a field row (a raw <div> + a hand-rolled <p>/<label> title + a bare ChipInput/ChipTextarea). ChipModalBody applies px-2 + gap-4; ChipModalField adds ANOTHER px-2, so each field lands at effective px-4, exactly matching ChipModalHeader/ChipModalFooter (px-4). Hand-rolled rows skip the field's gutter and sit at px-2, visibly misaligned with the header/footer. For controls ChipModalField does not cover (ChipCombobox, ChipSelect, DatePicker, TimePicker, ButtonGroup, arbitrary JSX), use ChipModalField type='custom' with a title — it still applies the px-2 gutter and renders the canonical Label. Drive intent via props (title/value/onChange/error/hint/required/flush); never pass variant/className/id to the inner control, and never add a body-level wrapper <div> with a custom gap-* that fights ChipModalBody's gap-4.

Design-System Consolidation

Principles when building or migrating shared UI:

  • One canonical source of truth for shared chrome — compose it, never re-derive it per consumer.
  • Props-driven API over className overrides — reaching for className to change chrome is a smell; expose a prop instead.
  • Discriminated-union props for modes (e.g. ChipDropdown multiple) over near-duplicate components.
  • Delete legacy variants/components after migration — no parallel paths left behind.
  • Plain cn() for a single error/state toggle; CVA only for genuinely multiple variants.
  • Align consumers to the canonical defaults — normal weight, --text-body text, --text-icon icons.
  • Verify referenced CSS vars exist — an undefined var silently falls back to currentColor (black-bug).

Testing

Use Vitest. Test files: feature.ts → feature.test.ts. See .cursor/rules/sim-testing.mdc for full details.

Global Mocks (vitest.setup.ts)

@sim/db, @sim/db/schema, drizzle-orm, @sim/logger, @sim/workflow-authz, @/blocks/registry, @/lib/auth, @/lib/auth/hybrid, @/lib/core/utils/request, @trigger.dev/sdk, and store mocks are provided globally. Do NOT re-mock them unless overriding behavior. (The vi.mock('@/lib/auth', ...) in the example below is an override of the global mock so getSession can be controlled per-test.)

Standard Test Pattern

/**
 * @vitest-environment node
 */
import { createMockRequest } from '@sim/testing'
import { beforeEach, describe, expect, it, vi } from 'vitest'

const { mockGetSession } = vi.hoisted(() => ({
  mockGetSession: vi.fn(),
}))

vi.mock('@/lib/auth', () => ({
  auth: { api: { getSession: vi.fn() } },
  getSession: mockGetSession,
}))

import { GET } from '@/app/api/my-route/route'

describe('my route', () => {
  beforeEach(() => {
    vi.clearAllMocks()
    mockGetSession.mockResolvedValue({ user: { id: 'user-1' } })
  })
  it('returns data', async () => { ... })
})

Performance Rules

  • NEVER use vi.resetModules() + vi.doMock() + await import() — use vi.hoisted() + vi.mock() + static imports
  • NEVER use vi.importActual() — mock everything explicitly
  • NEVER use mockAuth(), mockConsoleLogger(), setupCommonApiMocks() from @sim/testing — they use vi.doMock() internally
  • Mock heavy deps (@/blocks, @/tools/registry, @/triggers) in tests that don't need them
  • Use @vitest-environment node unless DOM APIs are needed (window, document, FormData)
  • Avoid real timers — use 1ms delays or vi.useFakeTimers()

Use @sim/testing mocks/factories over local test data.

Utils Rules

  • Never create utils.ts for single consumer - inline it
  • Create utils.ts when 2+ files need the same helper
  • Check existing sources in lib/ before duplicating

Adding Integrations

New integrations are built in order: Tools → Block → Icon → (optional) Trigger. Always look up the service's API docs first.

Two hard rules that the skills assume:

  • Tool IDs are snake_case (service_action) and must be registered in tools/registry.ts; blocks register in blocks/registry.ts (alphabetically).
  • tools.config.tool runs during serialization (before variable resolution) — never do Number() or other type coercions there, or dynamic references like <Block.output> are destroyed. Put all type coercions in tools.config.params, which runs during execution after variables resolve.

For the full authoring instructions — SubBlock property tables, condition/dependsOn/required/mode/canonicalParamId syntax, required block metadata (integrationType, tags, authMode, docsLink, {Service}BlockMeta), file-input/normalizeFileInput patterns, and checklists — use the skills: /add-integration (end-to-end), /add-tools, /add-block, /add-trigger.