@@ -16,6 +16,7 @@ dist
 .vite-node
 ltex*
 .DS_Store
+.zed
 bench/test/*/*/
 **/bench.json
 **/browser/browser.json
@@ -35,4 +36,4 @@ test/browser/html/
 test/core/html/
 .vitest-attachments
 explainFiles.txt
-.vitest-dump
+.vitest-dump
@@ -1,4 +1,5 @@
 import type { Awaitable } from '@vitest/utils'
+import type { ContextTestEnvironment } from '../types/worker'
 import type { Vitest } from './core'
 import type { PoolTask } from './pools/types'
 import type { TestProject } from './project'
@@ -87,7 +88,7 @@ export function createPool(ctx: Vitest): ProcessPool {
 
     const sorted = await sequencer.sort(specs)
     const environments = await getSpecificationsEnvironments(specs)
-    const groups = groupSpecs(sorted)
+    const groups = groupSpecs(sorted, environments)
 
     const projectEnvs = new WeakMap<TestProject, Partial<NodeJS.ProcessEnv>>()
     const projectExecArgvs = new WeakMap<TestProject, string[]>()
@@ -330,9 +331,8 @@ function getMemoryLimit(config: ResolvedConfig, pool: string) {
   return null
 }
 
-function groupSpecs(specs: TestSpecification[]) {
-  // Test files are passed to test runner one at a time, except Typechecker.
-  // TODO: Should non-isolated test files be passed to test runner all at once?
+function groupSpecs(specs: TestSpecification[], environments: Awaited<ReturnType<typeof getSpecificationsEnvironments>>) {
+  // Test files are passed to test runner one at a time, except for Typechecker or when "--maxWorker=1 --no-isolate"
   type SpecsForRunner = TestSpecification[]
 
   // Tests in a single group are executed with `maxWorkers` parallelism.
@@ -346,6 +346,43 @@ function groupSpecs(specs: TestSpecification[]) {
   // Type tests are run in a single group, per project
   const typechecks: Record<string, TestSpecification[]> = {}
 
+  const serializedEnvironmentOptions = new Map<ContextTestEnvironment, string>()
+
+  function getSerializedOptions(env: ContextTestEnvironment) {
+    const options = serializedEnvironmentOptions.get(env)
+
+    if (options) {
+      return options
+    }
+
+    const serialized = JSON.stringify(env.options)
+    serializedEnvironmentOptions.set(env, serialized)
+    return serialized
+  }
+
+  function isEqualEnvironments(a: TestSpecification, b: TestSpecification) {
+    const aEnv = environments.get(a)
+    const bEnv = environments.get(b)
+
+    if (!aEnv && !bEnv) {
+      return true
+    }
+
+    if (!aEnv || !bEnv || aEnv.name !== bEnv.name) {
+      return false
+    }
+
+    if (!aEnv.options && !bEnv.options) {
+      return true
+    }
+
+    if (!aEnv.options || !bEnv.options) {
+      return false
+    }
+
+    return getSerializedOptions(aEnv) === getSerializedOptions(bEnv)
+  }
+
   specs.forEach((spec) => {
     if (spec.pool === 'typescript') {
       typechecks[spec.project.name] ||= []
@@ -361,6 +398,7 @@ function groupSpecs(specs: TestSpecification[]) {
     }
 
     const maxWorkers = resolveMaxWorkers(spec.project)
+    const isolate = spec.project.config.isolate
     groups[order] ||= { specs: [], maxWorkers }
 
     // Multiple projects with different maxWorkers but same groupId
@@ -370,6 +408,15 @@ function groupSpecs(specs: TestSpecification[]) {
       throw new Error(`Projects "${last}" and "${spec.project.name}" have different 'maxWorkers' but same 'sequence.groupId'.\nProvide unique 'sequence.groupId' for them.`)
     }
 
+    // Non-isolated single worker can receive all files at once
+    if (isolate === false && maxWorkers === 1) {
+      const previous = groups[order].specs[0]?.[0]
+
+      if (previous && previous.project.name === spec.project.name && isEqualEnvironments(spec, previous)) {
+        return groups[order].specs[0].push(spec)
+      }
+    }
+
     groups[order].specs.push([spec])
   })
 
 
@@ -0,0 +1,3 @@
+import { test } from "vitest"
+
+test("a", () => { })
@@ -0,0 +1,3 @@
+import { test } from "vitest"
+
+test("b", () => { })
@@ -0,0 +1,3 @@
+import { test } from "vitest"
+
+test("c", () => { })
@@ -0,0 +1,6 @@
+import { test } from 'vitest'
+
+test('print config', () => {
+  // @ts-expect-error -- internal
+  console.log(JSON.stringify(globalThis.__vitest_worker__.ctx.files.map(file => file.filepath)))
+})
@@ -1,7 +1,8 @@
 import type { SerializedConfig } from 'vitest'
 import type { TestUserConfig } from 'vitest/node'
+import { normalize } from 'pathe'
 import { assert, describe, expect, test } from 'vitest'
-import { runVitest } from '../../test-utils'
+import { runVitest, StableTestFileOrderSorter } from '../../test-utils'
 
 describe.each(['forks', 'threads', 'vmThreads', 'vmForks'])('%s', async (pool) => {
   test('resolves top-level pool', async () => {
@@ -51,8 +52,39 @@ test('project level pool options overwrites top-level', async () => {
   expect(config.fileParallelism).toBe(false)
 })
 
-async function getConfig(options: Partial<TestUserConfig>, cliOptions: Partial<TestUserConfig> = {}) {
-  let config: SerializedConfig | undefined
+test('isolated single worker pool receives single testfile at once', async () => {
+  const files = await getConfig<string[]>({
+    maxWorkers: 1,
+    isolate: true,
+    sequence: { sequencer: StableTestFileOrderSorter },
+  }, { include: ['print-testfiles.test.ts', 'a.test.ts', 'b.test.ts', 'c.test.ts'] })
+
+  expect(files.map(normalizeFilename)).toMatchInlineSnapshot(`
+    [
+      "<process-cwd>/fixtures/pool/print-testfiles.test.ts",
+    ]
+  `)
+})
+
+test('non-isolated single worker pool receives all testfiles at once', async () => {
+  const files = await getConfig<string[]>({
+    maxWorkers: 1,
+    isolate: false,
+    sequence: { sequencer: StableTestFileOrderSorter },
+  }, { include: ['print-testfiles.test.ts', 'a.test.ts', 'b.test.ts', 'c.test.ts'] })
+
+  expect(files.map(normalizeFilename)).toMatchInlineSnapshot(`
+    [
+      "<process-cwd>/fixtures/pool/a.test.ts",
+      "<process-cwd>/fixtures/pool/b.test.ts",
+      "<process-cwd>/fixtures/pool/c.test.ts",
+      "<process-cwd>/fixtures/pool/print-testfiles.test.ts",
+    ]
+  `)
+})
+
+async function getConfig<T = SerializedConfig>(options: Partial<TestUserConfig>, cliOptions: Partial<TestUserConfig> = {}): Promise<T> {
+  let config: T | undefined
 
   await runVitest({
     root: './fixtures/pool',
@@ -66,3 +98,8 @@ async function getConfig(options: Partial<TestUserConfig>, cliOptions: Partial<T
   assert(config)
   return config
 }
+
+function normalizeFilename(filename: string) {
+  return normalize(filename)
+    .replace(normalize(process.cwd()), '<process-cwd>')
+}
Original file line number	Diff line number	Diff line change
`@@ -0,0 +1,3 @@`
	`1`	`+import { test } from "vitest"`
	`2`	`+`
	`3`	`+test("a", () => { })`
Original file line number	Diff line number	Diff line change
`@@ -0,0 +1,3 @@`
	`1`	`+import { test } from "vitest"`
	`2`	`+`
	`3`	`+test("b", () => { })`
Original file line number	Diff line number	Diff line change
`@@ -0,0 +1,3 @@`
	`1`	`+import { test } from "vitest"`
	`2`	`+`
	`3`	`+test("c", () => { })`