If two 'blocks' fields reference the same block definition, only one of the fields will reference the interfaceName of the block in the generated types.

[galenwarren]

Describe the Bug

If you add two 'blocks' fields to the same collection that reference the same blocks, only one of them will properly reference the interface name in the generated payload-types.ts.

For example, with this definition for Users:

import type { Block, CollectionConfig } from 'payload'

export const SampleBlock: Block = {
  slug: 'sample-block',
  interfaceName: 'SampleBlock',
  fields: [
    {
      name: 'field',
      type: 'text',
    },
  ],
}

export const Users: CollectionConfig = {
  slug: 'users',
  admin: {
    useAsTitle: 'email',
  },
  auth: true,
  fields: [
    // Email added by default
    // Add more fields as needed
    {
      name: 'field1',
      type: 'blocks',
      blocks: [SampleBlock],
    },
    {
      name: 'field2',
      type: 'blocks',
      blocks: [SampleBlock],
    },    
  ],
}

... the generated type is:

export interface User {
  id: number;
  field1?:
    | {
        field?: string | null;
        id?: string | null;
        blockName?: string | null;
        blockType: 'sample-block';
      }[]
    | null;
  field2?: SampleBlock[] | null;
  [snip]

... but it should be:

export interface User {
  id: number;
  field1?: SampleBlock[] | null;
  field2?: SampleBlock[] | null;
  [snip]

It looks to me that only the last field gets assigned the proper interface type?

Link to the code that reproduces this issue

https://github.com/galenwarren/payloadcms-bug-demo

Reproduction Steps

Run npm run generate:types and see what is generated for User.

Which area(s) are affected?

area: core

Environment Info

Binaries:                                                                                                                                                                                     Node: 24.6.0                                                                                                                                                                                npm: 11.5.1                                                                                                                                                                                 Yarn: N/A                                                                                                                                                                                   pnpm: N/A                                                                                                                                                                                 Relevant Packages:                                                                                                                                                                            payload: 3.79.0                                                                                                                                                                             next: 15.4.11                                                                                                                                                                               @payloadcms/db-sqlite: 3.79.0                                                                                                                                                               @payloadcms/drizzle: 3.79.0                                                                                                                                                                 @payloadcms/graphql: 3.79.0                                                                                                                                                                 @payloadcms/next/utilities: 3.79.0                                                                                                                                                          @payloadcms/richtext-lexical: 3.79.0                                                                                                                                                        @payloadcms/translations: 3.79.0                                                                                                                                                            @payloadcms/ui/shared: 3.79.0                                                                                                                                                               react: 19.2.1                                                                                                                                                                               react-dom: 19.2.1                                                                                                                                                                         Operating System:                                                                                                                                                                             Platform: linux                                                                                                                                                                             Arch: x64                                                                                                                                                                                   Version: #1 SMP PREEMPT_DYNAMIC PMX 6.17.4-2 (2025-12-19T07:49Z)                                                                                                                            Available memory (MB): 16384                                                                                                                                                                Available CPU cores: 8

[galenwarren]

It looks like what triggers it is if the same interfaceName is used in multiple blocks fields. If you change it so the interfaceName is different for the two fields, it uses the types properly.

[codeflorist]

I'm seeing an increase of payload-types.ts size from 120KB to 3MB, with interfaceName not getting used correctly anymore after upgrading from v3.77 to v3.79.

Could this be related to this issue? Was this issue introduced with v3.78 or v3.79?

[galenwarren]

@codeflorist I can't be sure, but in my case I was running an older version (3.3x if I remember properly) when I noticed the issue. I upgraded to the most recent version to see if it fixed it before submitting the issue. So at least my issue seems to have been there longer than just in 3.78.