diff --git a/config/scripts/ci-unit-files.mjs b/config/scripts/ci-unit-files.mjs
index e1c265dbcd6..4187cfde1a1 100644
--- a/config/scripts/ci-unit-files.mjs
+++ b/config/scripts/ci-unit-files.mjs
@@ -13,6 +13,14 @@ export const UNIT_INCLUDE = [
   'tests/e2e/**/*.unit.test.ts'
 ]
 
+export const UNIT_SETUP_FILES = [
+  'config/scripts/vitest-real-agent-home-write-guard.ts',
+  'config/scripts/vitest-bun-node-builtins.ts',
+  'config/scripts/vitest-dom-setup.ts',
+  'config/scripts/vitest-host-ports-setup.ts',
+  'config/scripts/vitest-caller-identity-env-setup.ts'
+]
+
 export const UNIT_EXCLUDE = [
   ...defaultExclude,
   'src/main/daemon/repro-13767-shell-ready-marker-lost-to-exec.test.ts',
diff --git a/config/scripts/ci-unit-selection.mjs b/config/scripts/ci-unit-selection.mjs
index 5a71c5f87fe..2e06f1d3e38 100644
--- a/config/scripts/ci-unit-selection.mjs
+++ b/config/scripts/ci-unit-selection.mjs
@@ -1,5 +1,19 @@
 import { unitConsumers } from './ci-unit-dependency-graph.mjs'
 import { balanceFiles } from './ci-shard-assignment.mjs'
+import { UNIT_SETUP_FILES } from './ci-unit-files.mjs'
+
+const RUNTIME_ENTRY_POINTS = new Set([
+  'config/vitest.config.ts',
+  'config/scripts/run-vitest.mjs',
+  ...UNIT_SETUP_FILES
+])
+
+function isUnitRuntimeEntryPoint(file) {
+  return (
+    RUNTIME_ENTRY_POINTS.has(file) ||
+    (/^config\/scripts\/vitest[^/]*\.(?:ts|mjs)$/.test(file) && !file.includes('.test.'))
+  )
+}
 
 export function selectUnitFiles(files, changed, graphInput) {
   const full = (reason) => ({ files, reason, full: true })
@@ -15,6 +29,9 @@ export function selectUnitFiles(files, changed, graphInput) {
   }
   for (const file of changed) {
     const consumers = unitConsumers([file], graph.reverse)
+    if ([...consumers].some(isUnitRuntimeEntryPoint)) {
+      return full('Shared unit runtime input')
+    }
     if (!files.some((test) => consumers.has(test))) {
       return full('No proven test coverage for changed inputs')
     }
diff --git a/config/scripts/ci-unit-selection.test.mjs b/config/scripts/ci-unit-selection.test.mjs
index f3764a54c1b..601e3c269b2 100644
--- a/config/scripts/ci-unit-selection.test.mjs
+++ b/config/scripts/ci-unit-selection.test.mjs
@@ -1,5 +1,6 @@
 import { describe, expect, it } from 'vitest'
 import { buildUnitDependencyGraph } from './ci-unit-dependency-graph.mjs'
+import { UNIT_SETUP_FILES } from './ci-unit-files.mjs'
 import {
   FULL_SHARD_COUNT,
   auditUnitSelection,
@@ -26,6 +27,52 @@ const graph = { ...buildUnitDependencyGraph(sources), files: new Set(sources.key
 const files = [...sources.keys()].filter((file) => file.endsWith('.test.ts')).sort()
 
 describe('conservative unit selection', () => {
+  it.each([
+    'config/vitest.config.ts',
+    'config/scripts/run-vitest.mjs',
+    ...UNIT_SETUP_FILES,
+    'config/scripts/vitest-node-runtime-worker.mjs'
+  ])('retains every test for a transitive dependency of %s', (runtime) => {
+    const sources = new Map([
+      ['src/input.ts', 'export const value = 1'],
+      ['config/scripts/setup-forward.ts', "export * from '../../src/input'"],
+      [
+        runtime,
+        `import '${runtime === 'config/vitest.config.ts' ? './scripts' : '.'}/setup-forward'`
+      ],
+      ['src/direct.test.ts', "import './input'"],
+      ['src/unrelated.test.ts', 'export const value = 1']
+    ])
+    const files = ['src/direct.test.ts', 'src/unrelated.test.ts']
+    const graph = { ...buildUnitDependencyGraph(sources), files: new Set(sources.keys()) }
+    expect(selectUnitFiles(files, ['src/input.ts'], graph)).toMatchObject({ full: true, files })
+    expect(
+      planUnitSelection({
+        files,
+        changed: ['src/input.ts'],
+        graph,
+        timings: {},
+        mode: 'selected',
+        event: { pull_request: { draft: true } }
+      })
+    ).toMatchObject({ mode: 'shadow', executionFiles: files, selectionAvailable: false })
+  })
+
+  it('does not mistake a Vitest harness test for a shared runtime entry point', () => {
+    const sources = new Map([
+      ['src/input.ts', 'export const value = 1'],
+      ['config/scripts/vitest-harness.test.mjs', "import '../../src/input'"],
+      ['src/direct.test.ts', "import './input'"],
+      ['src/unrelated.test.ts', 'export const value = 1']
+    ])
+    expect(
+      selectUnitFiles(['src/direct.test.ts', 'src/unrelated.test.ts'], ['src/input.ts'], {
+        ...buildUnitDependencyGraph(sources),
+        files: new Set(sources.keys())
+      })
+    ).toMatchObject({ full: false, files: ['src/direct.test.ts'] })
+  })
+
   it('follows re-exports, literal dynamic imports and requires, retaining indirect readers', () => {
     expect(selectUnitFiles(files, ['src/leaf.ts'], graph).files).toEqual([
       'src/consumer.test.ts',
diff --git a/config/vitest.config.ts b/config/vitest.config.ts
index aac0a83051f..b080ce434b1 100644
--- a/config/vitest.config.ts
+++ b/config/vitest.config.ts
@@ -2,7 +2,7 @@ import { readFileSync } from 'node:fs'
 import { createRequire } from 'node:module'
 import { resolve } from 'node:path'
 import { defaultExclude, defineConfig } from 'vitest/config'
-import { UNIT_INCLUDE, UNIT_EXCLUDE } from './scripts/ci-unit-files.mjs'
+import { UNIT_INCLUDE, UNIT_EXCLUDE, UNIT_SETUP_FILES } from './scripts/ci-unit-files.mjs'
 import TimingSequencer from './scripts/ci-unit-sequencer.mjs'
 import RuntimeSequencer from './scripts/vitest-runtime-sequencer.mjs'
 import { getNodeRuntimeInclude } from './scripts/vitest-node-runtime-files.mjs'
@@ -64,13 +64,7 @@ const testOptions = {
   server: { deps: { inline: ['zod'] } },
   // Node's storage globals and V8 retention checks require the existing child flags.
   execArgv: ['--no-experimental-webstorage', '--expose-gc'],
-  setupFiles: [
-    resolve('config/scripts/vitest-real-agent-home-write-guard.ts'),
-    resolve('config/scripts/vitest-bun-node-builtins.ts'),
-    resolve('config/scripts/vitest-dom-setup.ts'),
-    resolve('config/scripts/vitest-host-ports-setup.ts'),
-    resolve('config/scripts/vitest-caller-identity-env-setup.ts')
-  ],
+  setupFiles: UNIT_SETUP_FILES.map((file) => resolve(file)),
   include: UNIT_INCLUDE,
   exclude: balancedShards ? UNIT_EXCLUDE : defaultExclude,
   hookTimeout: 60_000,