Convex + TanStack Start

Overview

This skill provides guidance for building reactive, real-time full-stack applications using Convex (reactive backend-as-a-service) with TanStack Start (full-stack React meta-framework). The stack provides live-updating queries, type-safe end-to-end development, SSR support, and automatic cache invalidation.

When to Use This Skill

• Implementing Convex queries, mutations, or actions
• Setting up or troubleshooting Better Auth authentication
• Configuring TanStack Router routes and loaders
• Writing schema definitions and indexes
• Implementing data fetching patterns (useQuery, useSuspenseQuery)
• Working with file storage, scheduling, or cron jobs
• Building AI agents with @convex-dev/agent
• Debugging SSR or hydration issues

Quick Reference

Essential Imports

// Data fetching (always use cached version)
import { useQuery } from 'convex-helpers/react/cache'
import { useMutation, useAction } from 'convex/react'

// SSR with React Query
import { useSuspenseQuery } from '@tanstack/react-query'
import { convexQuery } from '@convex-dev/react-query'

// API and types
import { api } from '~/convex/_generated/api'
import type { Id, Doc } from '~/convex/_generated/dataModel'

// Backend functions
import { query, mutation, action } from "./_generated/server"
import { v } from "convex/values"

The Skip Pattern

Never call hooks conditionally. Use "skip" instead:

const user = useQuery(api.users.get, userId ? { userId } : "skip")
const org = useQuery(api.orgs.get, user?.orgId ? { orgId: user.orgId } : "skip")

Three-State Query Handling

if (data === undefined) return <Skeleton />  // Loading
if (data === null) return <NotFound />       // Not found
return <Content data={data} />               // Success

Function Syntax (Always Include Returns Validator)

export const getUser = query({
  args: { userId: v.id("users") },
  returns: v.union(
    v.object({ _id: v.id("users"), name: v.string() }),
    v.null()
  ),
  handler: async (ctx, args) => {
    return await ctx.db.get(args.userId)
  },
})

Index Best Practices

// Schema - name includes all fields
.index("by_organizationId_status", ["organizationId", "status"])

// Query - fields in same order as index
.withIndex("by_organizationId_status", (q) =>
  q.eq("organizationId", orgId).eq("status", "published")
)

Auth Check (Backend)

import { authComponent } from "./auth"

const user = await authComponent.getAuthUser(ctx)
if (!user) throw new Error("Not authenticated")

Core Principles

1. Use queries for reads - Queries are reactive, cacheable, and consistent
2. Keep functions fast - Finish in < 100ms, work with < a few hundred records
3. Prefer queries/mutations over actions - Actions are for external API calls only
4. Always use indexes - Never do table scans with .filter()
5. Minimize client state - Rely on Convex's real-time sync

Common Anti-Patterns

Reference Files

Load the appropriate reference file based on the task:

When to Load References

• Starting a new project: Load 01-setup.md
• Adding authentication: Load 03-auth.md
• Writing backend functions: Load 05-backend.md
• Implementing data fetching: Load 04-data-fetching.md
• Building UI components: Load 10-frontend.md
• Need quick syntax: Load 13-quick-reference.md