# Incompatible with the v2 merge mechanism

**URL:** https://discourse.nixos.org/t/incompatible-with-the-v2-merge-mechanism/80073
**Category:** Help
**Created:** [September 13, 2026, 11:02am UTC](https://discourse.nixos.org/t/incompatible-with-the-v2-merge-mechanism/80073 "2026-09-13T11:02:00Z")
**Posts on this page:** 6
**Page:** 1

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### Author: ![Geobomatic](https://discourse.nixos.org/user_avatar/discourse.nixos.org/geobomatic/32/35770_2.png) [@Geobomatic](https://discourse.nixos.org/u/Geobomatic)
#### Post date: [September 13, 2026, 11:02am UTC](https://discourse.nixos.org/t/incompatible-with-the-v2-merge-mechanism/80073/1 "2026-09-13T11:02:00Z")

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Hi,

I did not change my config and just did an update (on unstable) and had this:

```auto
error: The option `home-manager.users.geob.programs.fish.binds.ctrl-backspace.command' has a type `string…
┃ an ad-hoc `type // { check = ...; }' override, which is incompatible with
┃ the v2 merge mechanism.
┃
┃ Please use `lib.types.addCheck` instead of `type // { check }' to add
┃ custom validation. For example:
┃
┃ lib.types.addCheck baseType (value: /* your check */)
┃
┃ instead of:
┃
┃ baseType // { check = value: /* your check */; }
┃
┃ Alternatively, this message may also occur as false positive when mixing Nixpkgs
┃ versions, if one Nixpkgs is between 83fed2e6..58696117 (Aug 28 - Oct 28 2025)

```

What’s happening?

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<div class="post-metadata">

### Author: ![Infinisil](https://discourse.nixos.org/user_avatar/discourse.nixos.org/infinisil/32/26523_2.png) [@Infinisil](https://discourse.nixos.org/u/Infinisil)
#### Post date: [September 13, 2026, 11:12am UTC](https://discourse.nixos.org/t/incompatible-with-the-v2-merge-mechanism/80073/2 "2026-09-13T11:12:33Z")

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As the error says, home-manager needs patching here: [home-manager/modules/programs/fish.nix at fdc36b12804b0179b24a5ffd282e4c95c37072e2 · nix-community/home-manager · GitHub](https://github.com/nix-community/home-manager/blob/fdc36b12804b0179b24a5ffd282e4c95c37072e2/modules/programs/fish.nix#L277)

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<div class="post-metadata">

### Author: ![Geobomatic](https://discourse.nixos.org/user_avatar/discourse.nixos.org/geobomatic/32/35770_2.png) [@Geobomatic](https://discourse.nixos.org/u/Geobomatic)
#### Post date: [September 13, 2026, 11:16am UTC](https://discourse.nixos.org/t/incompatible-with-the-v2-merge-mechanism/80073/3 "2026-09-13T11:16:08Z")

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Thanks for your answer!

So just need to wait for HM to update then?

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<div class="post-metadata">

### Author: ![waffle8946](https://discourse.nixos.org/user_avatar/discourse.nixos.org/waffle8946/32/27221_2.png) [@waffle8946](https://discourse.nixos.org/u/waffle8946)
#### Post date: [September 13, 2026, 1:07pm UTC](https://discourse.nixos.org/t/incompatible-with-the-v2-merge-mechanism/80073/4 "2026-09-13T13:07:40Z")

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Specifically, someone needs to fix it on their end.

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<div class="post-metadata">

### Author: ![Geobomatic](https://discourse.nixos.org/user_avatar/discourse.nixos.org/geobomatic/32/35770_2.png) [@Geobomatic](https://discourse.nixos.org/u/Geobomatic)
#### Post date: [September 13, 2026, 1:21pm UTC](https://discourse.nixos.org/t/incompatible-with-the-v2-merge-mechanism/80073/5 "2026-09-13T13:21:44Z")

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Someone? I though the link above was upstream

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<div class="post-metadata">

### Author: ![waffle8946](https://discourse.nixos.org/user_avatar/discourse.nixos.org/waffle8946/32/27221_2.png) [@waffle8946](https://discourse.nixos.org/u/waffle8946)
#### Post date: [September 13, 2026, 1:39pm UTC](https://discourse.nixos.org/t/incompatible-with-the-v2-merge-mechanism/80073/6 "2026-09-13T13:39:29Z")

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Anyone can make a pull request to h-m, even you if you wanted to. Someone in h-m will then have to review and merge it. ~~No one has done any of that yet.~~

~~Ideally you can report the error to h-m if you want to speed up that process.~~

After all that, _then_ you can update h-m to fix the error.

edit: seems there is a PR already open after all, so no issue needed - [Use addCheck in fish module by s9gf4ult · Pull Request #9939 · nix-community/home-manager · GitHub](https://github.com/nix-community/home-manager/pull/9939)