diff --git a/core/cli/src/install.ts b/core/cli/src/install.ts
index 8a77afb59..537001c57 100644
--- a/core/cli/src/install.ts
+++ b/core/cli/src/install.ts
@@ -31,7 +31,7 @@ async function asyncFilter<T>(items: T[], predicate: (item: T) => Promise<boolea
   return results.filter(({ keep }) => keep).map(({ item }) => item)
 }
 
-const loadHookEntrypoints = async (
+export const loadHookEntrypoints = async (
   logger: Logger,
   config: ValidConfig
 ): Promise<Validated<Record<string, HookModule>>> => {
@@ -54,13 +54,9 @@ export const loadHookInstallations = async (
   config: ValidConfig
 ): Promise<Validated<Hook[]>> => {
   const hookClassResults = await loadHookEntrypoints(logger, config)
-  const installationResults = (
-    await hookClassResults
-      .map((hookClasses) =>
-        reducePluginHookInstallations(logger, config, hookClasses, config.plugins['app root'])
-      )
-      .awaitValue()
-  ).flatMap((installation) => installation)
+  const installationResults = hookClassResults.flatMap((hookClasses) =>
+    reducePluginHookInstallations(logger, config, hookClasses, config.plugins['app root'])
+  )
 
   const installationsWithoutConflicts = installationResults.flatMap((installations) => {
     const conflicts = findConflicts(installations)
diff --git a/core/cli/src/plugin/reduce-installations.ts b/core/cli/src/plugin/reduce-installations.ts
index aed78ff50..fcf46ef64 100644
--- a/core/cli/src/plugin/reduce-installations.ts
+++ b/core/cli/src/plugin/reduce-installations.ts
@@ -46,23 +46,19 @@ const extractForHook = (installation: HookInstallation | Conflict<HookInstallati
 //   - call `Hook.overrideChildInstallations` with the appropriate concrete Hook class, and
 
 //     the resulting installations and/or conflicts from `Hook.mergeChildInstallations` and `p`
-export async function reducePluginHookInstallations(
+export function reducePluginHookInstallations(
   logger: Logger,
   config: ValidConfig,
   hookModules: Record<string, HookModule>,
   plugin: Plugin
-): Promise<Validated<(HookInstallation | Conflict<HookInstallation>)[]>> {
+): Validated<(HookInstallation | Conflict<HookInstallation>)[]> {
   if (!plugin.rcFile || config.resolutionTrackers.reducedInstallationPlugins.has(plugin.id)) {
     return valid([])
   }
   config.resolutionTrackers.reducedInstallationPlugins.add(plugin.id)
 
   const rawChildInstallations = reduceValidated(
-    await Promise.all(
-      (plugin.children ?? []).map((child) =>
-        reducePluginHookInstallations(logger, config, hookModules, child)
-      )
-    )
+    (plugin.children ?? []).map((child) => reducePluginHookInstallations(logger, config, hookModules, child))
   ).map((installations) => installations.flat())
 
   if (!rawChildInstallations.valid) {
diff --git a/core/cli/test/plugin/files/single-install/merge-children/.toolkitrc.yml b/core/cli/test/plugin/files/single-install/merge-children/.toolkitrc.yml
new file mode 100644
index 000000000..41bb74e56
--- /dev/null
+++ b/core/cli/test/plugin/files/single-install/merge-children/.toolkitrc.yml
@@ -0,0 +1,3 @@
+plugins:
+  - './a'
+  - './b'
diff --git a/core/cli/test/plugin/files/single-install/merge-children/a/.toolkitrc.yml b/core/cli/test/plugin/files/single-install/merge-children/a/.toolkitrc.yml
new file mode 100644
index 000000000..c004d8f62
--- /dev/null
+++ b/core/cli/test/plugin/files/single-install/merge-children/a/.toolkitrc.yml
@@ -0,0 +1,6 @@
+version: 2
+
+options:
+  hooks:
+    - TestHook:
+        a: true
diff --git a/core/cli/test/plugin/files/single-install/merge-children/b/.toolkitrc.yml b/core/cli/test/plugin/files/single-install/merge-children/b/.toolkitrc.yml
new file mode 100644
index 000000000..7259da3d2
--- /dev/null
+++ b/core/cli/test/plugin/files/single-install/merge-children/b/.toolkitrc.yml
@@ -0,0 +1,6 @@
+version: 2
+
+options:
+  hooks:
+    - TestHook:
+        b: true
diff --git a/core/cli/test/plugin/files/single-install/nested-override/.toolkitrc.yml b/core/cli/test/plugin/files/single-install/nested-override/.toolkitrc.yml
new file mode 100644
index 000000000..43c53afe7
--- /dev/null
+++ b/core/cli/test/plugin/files/single-install/nested-override/.toolkitrc.yml
@@ -0,0 +1,2 @@
+plugins:
+  - './b'
diff --git a/core/cli/test/plugin/files/single-install/nested-override/a/.toolkitrc.yml b/core/cli/test/plugin/files/single-install/nested-override/a/.toolkitrc.yml
new file mode 100644
index 000000000..c004d8f62
--- /dev/null
+++ b/core/cli/test/plugin/files/single-install/nested-override/a/.toolkitrc.yml
@@ -0,0 +1,6 @@
+version: 2
+
+options:
+  hooks:
+    - TestHook:
+        a: true
diff --git a/core/cli/test/plugin/files/single-install/nested-override/b/.toolkitrc.yml b/core/cli/test/plugin/files/single-install/nested-override/b/.toolkitrc.yml
new file mode 100644
index 000000000..dff0a3a4c
--- /dev/null
+++ b/core/cli/test/plugin/files/single-install/nested-override/b/.toolkitrc.yml
@@ -0,0 +1,9 @@
+version: 2
+
+plugins:
+  - '../a'
+
+options:
+  hooks:
+    - TestHook:
+        b: true
diff --git a/core/cli/test/plugin/files/single-install/override-children/.toolkitrc.yml b/core/cli/test/plugin/files/single-install/override-children/.toolkitrc.yml
new file mode 100644
index 000000000..47d439f34
--- /dev/null
+++ b/core/cli/test/plugin/files/single-install/override-children/.toolkitrc.yml
@@ -0,0 +1,8 @@
+plugins:
+  - './a'
+  - './b'
+
+options:
+  hooks:
+    - TestHook:
+        c: true
diff --git a/core/cli/test/plugin/files/single-install/override-children/a/.toolkitrc.yml b/core/cli/test/plugin/files/single-install/override-children/a/.toolkitrc.yml
new file mode 100644
index 000000000..c004d8f62
--- /dev/null
+++ b/core/cli/test/plugin/files/single-install/override-children/a/.toolkitrc.yml
@@ -0,0 +1,6 @@
+version: 2
+
+options:
+  hooks:
+    - TestHook:
+        a: true
diff --git a/core/cli/test/plugin/files/single-install/override-children/b/.toolkitrc.yml b/core/cli/test/plugin/files/single-install/override-children/b/.toolkitrc.yml
new file mode 100644
index 000000000..7259da3d2
--- /dev/null
+++ b/core/cli/test/plugin/files/single-install/override-children/b/.toolkitrc.yml
@@ -0,0 +1,6 @@
+version: 2
+
+options:
+  hooks:
+    - TestHook:
+        b: true
diff --git a/core/cli/test/plugin/reduce-installations.test.ts b/core/cli/test/plugin/reduce-installations.test.ts
new file mode 100644
index 000000000..48f12ad7c
--- /dev/null
+++ b/core/cli/test/plugin/reduce-installations.test.ts
@@ -0,0 +1,308 @@
+import path from 'path'
+import { Hook, HookClass } from '@dotcom-tool-kit/base'
+import { isConflict } from '@dotcom-tool-kit/conflict'
+import { z } from 'zod'
+import winston, { Logger } from 'winston'
+import { loadConfig } from '../../src/config'
+import { HookModule, reducePluginHookInstallations } from '../../src/plugin/reduce-installations'
+
+const logger = winston as unknown as Logger
+
+const testHookSchema = z.any()
+
+class TestHook extends Hook<
+  {
+    hook: typeof testHookSchema
+  },
+  void
+> {
+  static schema = z.any()
+
+  async isInstalled() {
+    return true
+  }
+
+  // eslint-disable-next-line @typescript-eslint/no-empty-function
+  async install() {}
+}
+
+const hookClasses = {
+  TestHook: {
+    hookClass: TestHook as HookClass,
+    schema: testHookSchema
+  }
+} satisfies Record<string, HookModule>
+
+describe('reducePluginHookInstallations', () => {
+  afterEach(() => {
+    jest.restoreAllMocks()
+  })
+
+  describe('single installation per plugin', () => {
+    it("should return conflict if mergeChildInstallations returns conflict that's not resolved by overrideChildInstallations", async () => {
+      const config = await loadConfig(logger, {
+        root: path.resolve(__dirname, './files/single-install/merge-children')
+      })
+
+      jest.spyOn(TestHook, 'mergeChildInstallations').mockImplementation(
+        // NOTE: this is the default implementation from the Hook base class,
+        // repeating it here in the mock for clarity and consistency with other tests
+        (plugin, childInstallations) => [
+          {
+            plugin,
+            conflicting: childInstallations.flatMap((installation) =>
+              isConflict(installation) ? installation.conflicting : installation
+            )
+          }
+        ]
+      )
+
+      const mergedInstallations = reducePluginHookInstallations(
+        logger,
+        config,
+        hookClasses,
+        config.plugins['app root']
+      ).unwrap('hook installations were invalid')
+
+      expect(mergedInstallations).toMatchObject([
+        {
+          plugin: config.plugins['app root'],
+          conflicting: [
+            {
+              plugin: config.plugins['./a'],
+              options: {
+                a: true
+              }
+            },
+            {
+              plugin: config.plugins['./b'],
+              options: {
+                b: true
+              }
+            }
+          ]
+        }
+      ])
+    })
+
+    it('should return resolved installation returned by mergeChildInstallations', async () => {
+      const config = await loadConfig(logger, {
+        root: path.resolve(__dirname, './files/single-install/merge-children')
+      })
+
+      jest.spyOn(TestHook, 'mergeChildInstallations').mockImplementation((plugin, childInstallations) => [
+        {
+          plugin,
+          forHook: 'TestHook',
+          hookConstructor: TestHook,
+          options: Object.fromEntries(
+            childInstallations.flatMap((child) => (isConflict(child) ? [] : Object.entries(child.options)))
+          )
+        }
+      ])
+
+      const mergedInstallations = reducePluginHookInstallations(
+        logger,
+        config,
+        hookClasses,
+        config.plugins['app root']
+      ).unwrap('hook installations were invalid')
+
+      expect(mergedInstallations).toMatchObject([
+        {
+          plugin: config.plugins['app root'],
+          options: {
+            a: true,
+            b: true
+          }
+        }
+      ])
+    })
+
+    it('should return resolved installation returned by overrideChildInstallations', async () => {
+      const config = await loadConfig(logger, {
+        root: path.resolve(__dirname, './files/single-install/override-children')
+      })
+
+      jest.spyOn(TestHook, 'overrideChildInstallations').mockImplementation(
+        // NOTE: this is the default implementation from the Hook base class,
+        // repeating it here in the mock for clarity and consistency with other tests
+        (plugin, parentInstallation, _childInstallations) => [parentInstallation]
+      )
+
+      const mergedInstallations = reducePluginHookInstallations(
+        logger,
+        config,
+        hookClasses,
+        config.plugins['app root']
+      ).unwrap('hook installations were invalid')
+
+      expect(mergedInstallations).toMatchObject([
+        {
+          plugin: config.plugins['app root'],
+          options: {
+            c: true
+          }
+        }
+      ])
+    })
+
+    it('should return resolved installation merged by overrideChildInstallations', async () => {
+      const config = await loadConfig(logger, {
+        root: path.resolve(__dirname, './files/single-install/override-children')
+      })
+
+      jest.spyOn(TestHook, 'overrideChildInstallations').mockImplementation(
+        // NOTE: this implementation naively merges installations between parents and children.
+        // that's not a usual use case, but CircleCi and PackageJson both do this in limited cases.
+        (plugin, parentInstallation, childInstallations) => [
+          {
+            ...parentInstallation,
+            options: Object.fromEntries(
+              [parentInstallation, ...childInstallations].flatMap((installation) =>
+                isConflict(installation)
+                  ? installation.conflicting.flatMap((child) => Object.entries(child.options))
+                  : Object.entries(installation.options)
+              )
+            ),
+            plugin
+          }
+        ]
+      )
+
+      const mergedInstallations = reducePluginHookInstallations(
+        logger,
+        config,
+        hookClasses,
+        config.plugins['app root']
+      ).unwrap('hook installations were invalid')
+
+      expect(mergedInstallations).toMatchObject([
+        {
+          plugin: config.plugins['app root'],
+          options: {
+            a: true,
+            b: true
+          }
+        }
+      ])
+    })
+
+    it('should return conflict returned by overrideChildInstallations', async () => {
+      const config = await loadConfig(logger, {
+        root: path.resolve(__dirname, './files/single-install/override-children')
+      })
+
+      jest.spyOn(TestHook, 'overrideChildInstallations').mockImplementation(
+        // NOTE: check which plugin is overriding; only return a conflict at the app root stage
+        (plugin, parentInstallation, childInstallations) =>
+          plugin.id === 'app root'
+            ? [
+                {
+                  plugin,
+                  conflicting: childInstallations.flatMap((installation) =>
+                    isConflict(installation) ? installation.conflicting : installation
+                  )
+                }
+              ]
+            : Hook.overrideChildInstallations(plugin, parentInstallation, childInstallations)
+      )
+
+      const mergedInstallations = reducePluginHookInstallations(
+        logger,
+        config,
+        hookClasses,
+        config.plugins['app root']
+      ).unwrap('hook installations were invalid')
+
+      expect(mergedInstallations).toMatchObject([
+        {
+          plugin: config.plugins['app root'],
+          conflicting: [
+            {
+              plugin: config.plugins['./a'],
+              options: {
+                a: true
+              }
+            },
+            {
+              plugin: config.plugins['./b'],
+              options: {
+                b: true
+              }
+            }
+          ]
+        }
+      ])
+    })
+
+    it('should return resolved installation returned by overrideChildInstallations at multiple layers of nesting', async () => {
+      const config = await loadConfig(logger, {
+        root: path.resolve(__dirname, './files/single-install/nested-override')
+      })
+
+      jest.spyOn(TestHook, 'overrideChildInstallations').mockImplementation(
+        // NOTE: this is the default implementation from the Hook base class,
+        // repeating it here in the mock for clarity and consistency with other tests
+        (plugin, parentInstallation, _childInstallations) => [parentInstallation]
+      )
+
+      const mergedInstallations = reducePluginHookInstallations(
+        logger,
+        config,
+        hookClasses,
+        config.plugins['app root']
+      ).unwrap('hook installations were invalid')
+
+      expect(mergedInstallations).toMatchObject([
+        {
+          plugin: config.plugins['./b'],
+          options: {
+            b: true
+          }
+        }
+      ])
+    })
+
+    it('should return resolved installation merged by overrideChildInstallations at multiple layers of nesting', async () => {
+      const config = await loadConfig(logger, {
+        root: path.resolve(__dirname, './files/single-install/nested-override')
+      })
+
+      jest.spyOn(TestHook, 'overrideChildInstallations').mockImplementation(
+        // NOTE: this implementation naively merges installations between parents and children.
+        // that's not a usual use case, but CircleCi and PackageJson both do this in limited cases.
+        (plugin, parentInstallation, childInstallations) => [
+          {
+            ...parentInstallation,
+            options: Object.fromEntries(
+              [parentInstallation, ...childInstallations].flatMap((installation) =>
+                isConflict(installation)
+                  ? installation.conflicting.flatMap((child) => Object.entries(child.options))
+                  : Object.entries(installation.options)
+              )
+            ),
+            plugin
+          }
+        ]
+      )
+
+      const mergedInstallations = reducePluginHookInstallations(
+        logger,
+        config,
+        hookClasses,
+        config.plugins['app root']
+      ).unwrap('hook installations were invalid')
+
+      expect(mergedInstallations).toMatchObject([
+        {
+          plugin: config.plugins['./b'],
+          options: {
+            a: true,
+            b: true
+          }
+        }
+      ])
+    })
+  })
+})
diff --git a/lib/base/src/hook.ts b/lib/base/src/hook.ts
index 2f568e70e..f8bf0d3a1 100644
--- a/lib/base/src/hook.ts
+++ b/lib/base/src/hook.ts
@@ -39,6 +39,10 @@ export abstract class Hook<
     plugin: Plugin,
     childInstallations: (HookInstallation | Conflict<HookInstallation>)[]
   ): (HookInstallation | Conflict<HookInstallation>)[] {
+    if (childInstallations.length === 1) {
+      return childInstallations
+    }
+
     return [
       {
         plugin,