Problem with extends configuration from repository

[piotar]

How are you running Renovate?

Self-hosted Renovate

Which platform you running Renovate on?

GitLab (.com or self-hosted)

Which version of Renovate are you using?

42.27.5

Please tell us more about your question or problem

For over two months, the migration of static configuration from the repository has been failing.

The extends property from the repository-provided config file is not migrated correctly when its type is a simple string.

When running in debug mode, I get the following output:

DEBUG: Found renovate.json config file (repository=frontend/abc)
DEBUG: Repository config (repository=frontend/abc)
       "fileName": "renovate.json",
       "config": {"extends": "goo/renovate-bot//presets/microfrontend"}
DEBUG: migrateAndValidate() (repository=frontend/abc)
DEBUG: No config migration necessary (repository=frontend/abc)

After that, Renovate attempts to find a preset named renovate-config-g, which suggests that the string was incorrectly transformed into an array and only the first character (g) was taken as an entry.

It seems related to this issue:
https://github.com/renovatebot/renovate/pull/37115/files

Based on the code, the extends field is destructured before migrations run, which might cause this behavior:

• https://github.com/renovatebot/renovate/blob/main/lib/workers/repository/init/merge.ts#L214
• https://github.com/renovatebot/renovate/blob/main/lib/workers/repository/init/merge.ts#L219

Actual behavior

• The extends value is not migrated at all (DEBUG: No config migration necessary).
• Renovate appears to destructure the extends string before migration, treating it as an array and incorrectly extracting only its first character.

Expected behavior

• The extends field provided in the repository's renovate.json (even when defined as a simple string) should be correctly migrated during the config migration phase.
• Renovate should recognize and load the preset referenced in the extends string without altering or splitting it.

Logs (if relevant)

Logs

Replace this text with your logs, between the starting and ending triple backticks

[RahulGautamSingh]

Is the problem with extends within renovate config of your repo, or within the config in the static repo?

[piotar]

The migration issue only occurs with the configuration file provided by the repository.

[github-actions[bot]]

Hi there,

Please help this Discussion progress by creating a minimal reproduction. This means a repository dedicated to reproducing this issue with the minimal dependencies and config possible.

Before we start working on your issue we need to know exactly what's causing the current behavior. A minimal reproduction helps us with this. Discussions without reproductions are less likely to be converted to Issues.

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[piotar]

Minimal reproduce: https://github.com/piotar/renovate-reproductions-39659

[piotar]

This does not work correctly because I put extends as a string rather than an array of string, the name of the preset does not matter for my case (I put it as example).

The documentation provides the correct configuration (https://docs.renovatebot.com/configuration-options/#extends), but in the old implementation, extends allowed the use of a simple string rather than an array.
In the repository, you can find a migrator that migrates the string case to a string array: https://github.com/renovatebot/renovate/blob/main/lib/config/migrations/custom/extends-migration.ts

I suppose that the changes in these places caused the extends migration not to be performed

• https://github.com/renovatebot/renovate/blob/main/lib/workers/repository/init/merge.ts#L214
• https://github.com/renovatebot/renovate/blob/main/lib/workers/repository/init/merge.ts#L219

In my case, I just migrate all my files with extends from a single string to an array string.

logs:
piotar_renovate-reproductions-39659_2025-12-08_07-58_019afb59-7cc1-7ec6-ae44-0c11a64d49a2.log

[jamietanna]

To confirm, can you please make sure that the reproduction i.e. uses "extends": "config:recommended"? That'll confirm that it's a string value that gets migrated (but the extended preset is valid and doesn't need a migration)

[piotar]

I modified the repository using config:recommended.

https://github.com/piotar/renovate-reproductions-39659/blob/main/renovate.json#L2

log:

DEBUG: Repository config
{
  "fileName": "renovate.json",
  "config": {
    "extends": "config:recommended"
  }
}

DEBUG: migrateAndValidate()
DEBUG: No config migration necessary
WARN: Using npm packages for Renovate presets is now deprecated. Please migrate to repository-based presets instead.
{
  "registryUrl": "https://registry.npmjs.org",
  "pkg": "renovate-config-c"
}

piotar_renovate-reproductions-39659_2025-12-10_14-00_019b007f-cd93-7af7-9b85-5e6d63cb7974.log

[jamietanna]

Awesome thank you for the confirmation! I've raised #39898 to cover this - we can move any additional conversation into there (and close this Discussion)