diff --git a/CHANGELOG.md b/CHANGELOG.md
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -4,6 +4,14 @@
 
 The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.0.0/)
 
+## [0.1.3.0] - 2021-06-22
+
+### Added
+
+- Utilities for servant's `Generic` feature
+  - `Substitute`
+  - `useApiE`, `enterApi`, `enterApiE`
+
 ## [0.1.2.0] - 2020-03-07
 
 - Use `uuid` type in swagger
@@ -17,6 +25,8 @@
 
 - Initial release, split out of hercules-ci-api
 
+[0.1.3.0]: https://github.com/hercules-ci/hercules-ci-agent/releases/tag/hercules-ci-api-core-0.1.3.0
+[0.1.2.0]: https://github.com/hercules-ci/hercules-ci-agent/releases/tag/hercules-ci-api-core-0.1.2.0
 [0.1.1.0]: https://github.com/hercules-ci/hercules-ci-agent/releases/tag/hercules-ci-api-core-0.1.1.0
 [0.1.0.0]: https://github.com/hercules-ci/hercules-ci-agent/releases/tag/hercules-ci-api-core-0.1.0.0
 [Unreleased]: https://github.com/hercules-ci/hercules-ci-agent/compare/stable...master
diff --git a/hercules-ci-api-core.cabal b/hercules-ci-api-core.cabal
--- a/hercules-ci-api-core.cabal
+++ b/hercules-ci-api-core.cabal
@@ -1,7 +1,7 @@
 cabal-version: 1.12
 
 name:           hercules-ci-api-core
-version:        0.1.2.0
+version:        0.1.3.0
 synopsis:       Types and convenience modules use across Hercules CI API packages
 category:       API, CI, Testing, DevOps, Nix
 homepage:       https://github.com/hercules-ci/hercules-ci-agent#readme
diff --git a/src/Hercules/API/Prelude.hs b/src/Hercules/API/Prelude.hs
--- a/src/Hercules/API/Prelude.hs
+++ b/src/Hercules/API/Prelude.hs
@@ -19,6 +19,11 @@
     FromJSON,
     ToSchema,
     schemaCompatibleOptions,
+
+    -- * Servant
+    Hercules.API.Servant.Substitute,
+    Hercules.API.Servant.Placeholder,
+    Hercules.API.Servant.enterApiE,
   )
 where
 
@@ -37,6 +42,7 @@
 import GHC.Generics (Generic)
 import Hercules.API.Id (Id)
 import Hercules.API.Name (Name)
+import qualified Hercules.API.Servant
 import Prelude
 
 schemaCompatibleOptions :: Aeson.Options
diff --git a/src/Hercules/API/Servant.hs b/src/Hercules/API/Servant.hs
--- a/src/Hercules/API/Servant.hs
+++ b/src/Hercules/API/Servant.hs
@@ -1,6 +1,22 @@
+{-# LANGUAGE PolyKinds #-}
+{-# LANGUAGE TypeFamilies #-}
+
 -- | Extras for working with servant
-module Hercules.API.Servant where
+module Hercules.API.Servant
+  ( noContent,
 
+    -- * 'Generic'
+    useApi,
+    useApiE,
+    enterApi,
+    enterApiE,
+
+    -- * Substitution
+    Substitute,
+    Placeholder,
+  )
+where
+
 import Control.Monad (void)
 import Servant.API
 import Servant.API.Generic
@@ -12,15 +28,56 @@
 --
 -- Ideally, this functionality would be built into a new combinator.
 useApi ::
-  (GenericServant f mode, GenericServant g mode) =>
-  (f mode -> ToServant g mode) ->
-  f mode ->
-  g mode
+  forall subapi api mode.
+  (GenericServant api mode, GenericServant subapi mode) =>
+  (api mode -> ToServant subapi mode) ->
+  api mode ->
+  subapi mode
 useApi = (Servant.API.Generic.fromServant .)
 
+-- | Like 'useApi' but constrains the @auth@ type variable that's passed to
+-- subapis.
+useApiE ::
+  forall subapi api mode a.
+  (GenericServant (api a) mode, GenericServant (subapi a) mode) =>
+  (api a mode -> ToServant (subapi a) mode) ->
+  api a mode ->
+  subapi a mode
+useApiE = useApi
+
+-- | @flip 'useApi'
+enterApi ::
+  forall subapi api mode.
+  (GenericServant api mode, GenericServant subapi mode) =>
+  api mode ->
+  (api mode -> ToServant subapi mode) ->
+  subapi mode
+enterApi = flip useApi
+
+-- | @flip 'useApiE'
+enterApiE ::
+  forall subapi api mode a.
+  (GenericServant (api a) mode, GenericServant (subapi a) mode) =>
+  api a mode ->
+  (api a mode -> ToServant (subapi a) mode) ->
+  subapi a mode
+enterApiE = flip useApi
+
 -- | 'Control.Monad.void' specialised to 'NoContent' to soothe the
 -- compiler that rightfully warns about throwing away a do notation
 -- result. By specialising, we make sure that we still get warnings
 -- if the result type changes in the future. (We'll get an error)
 noContent :: Functor m => m Servant.API.NoContent -> m ()
 noContent = void
+
+-- | A reference to the @subapi@ parameter in 'Substitute'
+data Placeholder
+
+-- | Replaces 'Placeholder' by @subapi@ in the API @target@.
+type family Substitute (target :: k) subapi :: *
+
+type instance Substitute Placeholder subapi = subapi
+
+type instance Substitute (a :> b) subapi = a :> Substitute b subapi
+
+type instance Substitute (a :<|> b) subapi = Substitute a subapi :<|> Substitute b subapi