/convex-backend

name: convex-backend
description: Convex backend development guidelines. Use when writing Convex functions, schemas, queries, mutations, actions, or any backend code in a Convex project. Triggers on tasks involving Convex database operations, real-time subscriptions, file storage, or serverless functions.

Convex Backend Guidelines

When to Load

• **Trigger**: Convex-specific development, writing Convex functions, schemas, queries, mutations, actions, or real-time subscriptions
• **Skip**: Project does not use Convex as its backend

Comprehensive guide for building Convex backends with TypeScript. Covers function syntax, validators, schemas, queries, mutations, actions, scheduling, and file storage.

When to Apply

Reference these guidelines when:

• Writing new Convex functions (queries, mutations, actions)
• Defining database schemas and validators
• Implementing real-time data fetching
• Setting up cron jobs or scheduled functions
• Working with file storage
• Designing API structure

Rule Categories

| Category | Impact | Description | | ----------------- | -------- | --------------------------------------------- | | Function Syntax | CRITICAL | New function syntax with args/returns/handler | | Validators | CRITICAL | Type-safe argument and return validation | | Schema Design | HIGH | Table definitions, indexes, system fields | | Query Patterns | HIGH | Efficient data fetching with indexes | | Mutation Patterns | MEDIUM | Database writes, patch vs replace | | Action Patterns | MEDIUM | External API calls, Node.js runtime | | Scheduling | MEDIUM | Crons and delayed function execution | | File Storage | LOW | Blob storage and metadata |

Quick Reference

Function Registration

// Public functions (exposed to clients)
import { query, mutation, action } from "./_generated/server";

// Internal functions (only callable from other Convex functions)
import {
  internalQuery,
  internalMutation,
  internalAction,
} from "./_generated/server";

Function Syntax (Always Use This)

export const myFunction = query({
  args: { name: v.string() },
  returns: v.string(),
  handler: async (ctx, args) => {
    return "Hello " + args.name;
  },
});

Common Validators

| Type | Validator | Example | | -------- | --------------------------------- | ------------- | | String | `v.string()` | `"hello"` | | Number | `v.number()` | `3.14` | | Boolean | `v.boolean()` | `true` | | ID | `v.id("tableName")` | `doc._id` | | Array | `v.array(v.string())` | `["a", "b"]` | | Object | `v.object({...})` | `{name: "x"}` | | Optional | `v.optional(v.string())` | `undefined` | | Union | `v.union(v.string(), v.number())` | `"x"` or `1` | | Literal | `v.literal("status")` | `"status"` | | Null | `v.null()` | `null` |

Function References

// Public functions
import { api } from "./_generated/api";
api.example.myQuery; // convex/example.ts → myQuery

// Internal functions
import { internal } from "./_generated/api";
internal.example.myInternalMutation;

Query with Index

// Schema
messages: defineTable({...}).index("by_channel", ["channelId"])

// Query
await ctx.db
  .query("messages")
  .withIndex("by_channel", (q) => q.eq("channelId", channelId))
  .order("desc")
  .take(10);

Key Rules

1. **Always include `args` and `returns` validators** on all functions 2. **Use `v.null()` for void returns** - never omit return validator 3. **Use `withIndex()` not `filter()`** - define indexes in schema 4. **Use `internalQuery/Mutation/Action`** for private functions 5. **Actions cannot access `ctx.db`** - use runQuery/runMutation instead 6. **Include type annotations** when calling functions in same file

Full Compiled Document

For the complete guide with all rules and detailed code examples, see [AGENTS.md](AGENTS.md).