LWC1503: @wire decorator's parameter must be a function

LWC's compile-time errors are clearer than Aura's, and LWC1503 is one of the friendliest: the wire's first argument has to be a recognised wire adapter at compile time, or the build refuses to ship.

What @wire actually expects

import { LightningElement, wire } from 'lwc';
import getAccounts from '@salesforce/apex/AccountController.getAccounts';

export default class MyComponent extends LightningElement {
  @wire(getAccounts) accounts;          // ✅ direct method reference
  @wire(getAccounts, { name: 'Acme' }) filtered;  // ✅ method + reactive params
}

The first arg has to be a value the compiler recognises as a wire adapter — either an @salesforce/apex/... import or one of the built-in adapters (getRecord, getObjectInfo, etc.).

Common causes of LWC1503

1. The import path doesn't resolve

import getAccounts from '@salesforce/apex/AccountController.getAccount';  // typo
@wire(getAccounts) accounts;

If the Apex method name doesn't match exactly (case-sensitive, full method name), the import becomes undefined and the @wire errors out. Cross-check the Apex class:

public with sharing class AccountController {
    @AuraEnabled(cacheable=true)
    public static List<Account> getAccounts() { ... }
}

The method must be @AuraEnabled and cacheable=true for @wire. Without those, the import won't work.

2. Import is a default-vs-named confusion

The Apex bridge uses default imports:

import getAccounts from '@salesforce/apex/AccountController.getAccounts';  // ✅
import { getAccounts } from '@salesforce/apex/AccountController.getAccounts'; // ❌ undefined

LDS adapters are named imports:

import { getRecord } from 'lightning/uiRecordApi';                          // ✅
import getRecord from 'lightning/uiRecordApi';                              // ❌ undefined

If you have the wrong shape, the symbol resolves to undefined and @wire(undefined) fails.

3. Conditionally-built wire reference

const adapter = condition ? getAccounts : null;
@wire(adapter) accounts;   // ❌ LWC1503

The compiler resolves @wire decorators at build time. The argument can't depend on runtime branching. If you need conditional wiring, use the imperative form:

import getAccounts from '@salesforce/apex/AccountController.getAccounts';

handleClick() {
  getAccounts({ name: 'Acme' })
    .then(rows => { this.accounts = rows; })
    .catch(err => { this.error = err; });
}

4. Apex method isn't cacheable=true

For @wire (not imperative), Apex must declare cacheable=true:

@AuraEnabled(cacheable=true)
public static List<Account> getAccounts() { ... }

Without it, the @wire adapter throws at compile time. cacheable=true means no DML — the method must be a pure read. If you need to write, use imperative invocation instead.

Reading the LWC compiler output

The error includes a file path and line. Open the file at that line; the offending decorator is right there. If your editor has the LWC extension installed (VS Code recommended), most LWC1xxx errors appear as red squigglies before you even build.