Convex Backend Guidelines Skill

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

Always include args and returns validators on all functions
Use v.null() for void returns - never omit return validator
Use withIndex() not filter() - define indexes in schema
Use internalQuery/Mutation/Action for private functions
Actions cannot access ctx.db - use runQuery/runMutation instead
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

Agent Skills: Convex Backend Guidelines

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