diff --git a/packages/coverage-istanbul/src/constants.ts b/packages/coverage-istanbul/src/constants.ts
index 1f81d8ad9..a8ff606d2 100644
--- a/packages/coverage-istanbul/src/constants.ts
+++ b/packages/coverage-istanbul/src/constants.ts
@@ -1 +1 @@
-export const COVERAGE_STORE_KEY = '__VITEST_COVERAGE__'
+export const COVERAGE_STORE_KEY: string = '__VITEST_COVERAGE__'
diff --git a/packages/coverage-istanbul/src/index.ts b/packages/coverage-istanbul/src/index.ts
index f20c95582..b02518138 100644
--- a/packages/coverage-istanbul/src/index.ts
+++ b/packages/coverage-istanbul/src/index.ts
@@ -1,6 +1,7 @@
 import { COVERAGE_STORE_KEY } from './constants'
+import type { IstanbulCoverageProvider } from './provider'
 
-export async function getProvider() {
+export async function getProvider(): Promise<IstanbulCoverageProvider> {
   // to not bundle the provider
   const providerPath = './provider.js'
   const { IstanbulCoverageProvider } = (await import(
@@ -10,7 +11,7 @@ export async function getProvider() {
   return new IstanbulCoverageProvider()
 }
 
-export function takeCoverage() {
+export function takeCoverage(): any {
   // @ts-expect-error -- untyped global
   const coverage = globalThis[COVERAGE_STORE_KEY]
 
@@ -21,7 +22,12 @@ export function takeCoverage() {
   return coverage
 }
 
-export default {
+const _default: {
+  getProvider: () => Promise<IstanbulCoverageProvider>
+  takeCoverage: () => any
+} = {
   getProvider,
   takeCoverage,
 }
+
+export default _default
diff --git a/packages/coverage-istanbul/src/provider.ts b/packages/coverage-istanbul/src/provider.ts
index ae909f519..8597ef200 100644
--- a/packages/coverage-istanbul/src/provider.ts
+++ b/packages/coverage-istanbul/src/provider.ts
@@ -57,13 +57,11 @@ interface TestExclude {
   }
 }
 
-const DEFAULT_PROJECT = Symbol.for('default-project')
+const DEFAULT_PROJECT: unique symbol = Symbol.for('default-project')
 const debug = createDebug('vitest:coverage')
 let uniqueId = 0
 
-export class IstanbulCoverageProvider
-  extends BaseCoverageProvider
-  implements CoverageProvider {
+export class IstanbulCoverageProvider extends BaseCoverageProvider implements CoverageProvider {
   name = 'istanbul'
 
   ctx!: Vitest
@@ -71,11 +69,11 @@ export class IstanbulCoverageProvider
   instrumenter!: Instrumenter
   testExclude!: InstanceType<TestExclude>
 
-  coverageFiles = new Map<ProjectName, CoverageFilesByTransformMode>()
+  coverageFiles: Map<ProjectName, CoverageFilesByTransformMode> = new Map()
   coverageFilesDirectory!: string
   pendingPromises: Promise<void>[] = []
 
-  initialize(ctx: Vitest) {
+  initialize(ctx: Vitest): void {
     const config: CoverageIstanbulOptions = ctx.config.coverage
 
     this.ctx = ctx
@@ -145,11 +143,14 @@ export class IstanbulCoverageProvider
     )
   }
 
-  resolveOptions() {
+  resolveOptions(): Options {
     return this.options
   }
 
-  onFileTransform(sourceCode: string, id: string, pluginCtx: any) {
+  onFileTransform(sourceCode: string, id: string, pluginCtx: any): {
+    code: string
+    map: any
+  } | undefined {
     if (!this.testExclude.shouldInstrument(id)) {
       return
     }
@@ -178,7 +179,7 @@ export class IstanbulCoverageProvider
    * Note that adding new entries here and requiring on those without
    * backwards compatibility is a breaking change.
    */
-  onAfterSuiteRun({ coverage, transformMode, projectName }: AfterSuiteRunMeta) {
+  onAfterSuiteRun({ coverage, transformMode, projectName }: AfterSuiteRunMeta): void {
     if (!coverage) {
       return
     }
@@ -204,7 +205,7 @@ export class IstanbulCoverageProvider
     this.pendingPromises.push(promise)
   }
 
-  async clean(clean = true) {
+  async clean(clean = true): Promise<void> {
     if (clean && existsSync(this.options.reportsDirectory)) {
       await fs.rm(this.options.reportsDirectory, {
         recursive: true,
@@ -227,7 +228,7 @@ export class IstanbulCoverageProvider
     this.pendingPromises = []
   }
 
-  async generateCoverage({ allTestsRun }: ReportContext) {
+  async generateCoverage({ allTestsRun }: ReportContext): Promise<CoverageMap> {
     const coverageMap = libCoverage.createCoverageMap({})
     let index = 0
     const total = this.pendingPromises.length
@@ -282,7 +283,7 @@ export class IstanbulCoverageProvider
     return coverageMap
   }
 
-  async reportCoverage(coverageMap: unknown, { allTestsRun }: ReportContext) {
+  async reportCoverage(coverageMap: unknown, { allTestsRun }: ReportContext): Promise<void> {
     await this.generateReports(
       (coverageMap as CoverageMap) || libCoverage.createCoverageMap({}),
       allTestsRun,
@@ -305,7 +306,7 @@ export class IstanbulCoverageProvider
   async generateReports(
     coverageMap: CoverageMap,
     allTestsRun: boolean | undefined,
-  ) {
+  ): Promise<void> {
     const context = libReport.createContext({
       dir: this.options.reportsDirectory,
       coverageMap,
@@ -370,7 +371,7 @@ export class IstanbulCoverageProvider
     }
   }
 
-  async mergeReports(coverageMaps: unknown[]) {
+  async mergeReports(coverageMaps: unknown[]): Promise<void> {
     const coverageMap = libCoverage.createCoverageMap({})
 
     for (const coverage of coverageMaps) {
diff --git a/packages/coverage-istanbul/tsconfig.json b/packages/coverage-istanbul/tsconfig.json
index 8bcb003f8..73cb13106 100644
--- a/packages/coverage-istanbul/tsconfig.json
+++ b/packages/coverage-istanbul/tsconfig.json
@@ -1,5 +1,8 @@
 {
   "extends": "../../tsconfig.base.json",
+  "compilerOptions": {
+    "moduleResolution": "Bundler"
+  },
   "include": ["./src/**/*.ts"],
   "exclude": ["./dist"]
 }
diff --git a/packages/coverage-v8/src/index.ts b/packages/coverage-v8/src/index.ts
index c7a0ee47a..fb03b3b8b 100644
--- a/packages/coverage-v8/src/index.ts
+++ b/packages/coverage-v8/src/index.ts
@@ -1,8 +1,18 @@
+import type { Profiler } from 'node:inspector'
 import * as coverage from './takeCoverage'
+import type { V8CoverageProvider } from './provider'
 
 export default {
-  ...coverage,
-  async getProvider() {
+  startCoverage(): void {
+    return coverage.startCoverage()
+  },
+  takeCoverage(): Promise<{ result: Profiler.ScriptCoverage[] }> {
+    return coverage.takeCoverage()
+  },
+  stopCoverage(): void {
+    return coverage.stopCoverage()
+  },
+  async getProvider(): Promise<V8CoverageProvider> {
     // to not bundle the provider
     const name = './provider.js'
     const { V8CoverageProvider } = (await import(
diff --git a/packages/coverage-v8/src/provider.ts b/packages/coverage-v8/src/provider.ts
index aa01dced6..8b032e72a 100644
--- a/packages/coverage-v8/src/provider.ts
+++ b/packages/coverage-v8/src/provider.ts
@@ -73,25 +73,23 @@ const VITE_EXPORTS_LINE_PATTERN
   = /Object\.defineProperty\(__vite_ssr_exports__.*\n/g
 const DECORATOR_METADATA_PATTERN
   = /_ts_metadata\("design:paramtypes", \[[^\]]*\]\),*/g
-const DEFAULT_PROJECT = Symbol.for('default-project')
+const DEFAULT_PROJECT: unique symbol = Symbol.for('default-project')
 
 const debug = createDebug('vitest:coverage')
 let uniqueId = 0
 
-export class V8CoverageProvider
-  extends BaseCoverageProvider
-  implements CoverageProvider {
+export class V8CoverageProvider extends BaseCoverageProvider implements CoverageProvider {
   name = 'v8'
 
   ctx!: Vitest
   options!: Options
   testExclude!: InstanceType<TestExclude>
 
-  coverageFiles = new Map<ProjectName, CoverageFilesByTransformMode>()
+  coverageFiles: Map<ProjectName, CoverageFilesByTransformMode> = new Map()
   coverageFilesDirectory!: string
   pendingPromises: Promise<void>[] = []
 
-  initialize(ctx: Vitest) {
+  initialize(ctx: Vitest): void {
     const config: CoverageV8Options = ctx.config.coverage
 
     this.ctx = ctx
@@ -142,11 +140,11 @@ export class V8CoverageProvider
     )
   }
 
-  resolveOptions() {
+  resolveOptions(): Options {
     return this.options
   }
 
-  async clean(clean = true) {
+  async clean(clean = true): Promise<void> {
     if (clean && existsSync(this.options.reportsDirectory)) {
       await fs.rm(this.options.reportsDirectory, {
         recursive: true,
@@ -174,7 +172,7 @@ export class V8CoverageProvider
    * Note that adding new entries here and requiring on those without
    * backwards compatibility is a breaking change.
    */
-  onAfterSuiteRun({ coverage, transformMode, projectName }: AfterSuiteRunMeta) {
+  onAfterSuiteRun({ coverage, transformMode, projectName }: AfterSuiteRunMeta): void {
     if (transformMode !== 'web' && transformMode !== 'ssr') {
       throw new Error(`Invalid transform mode: ${transformMode}`)
     }
@@ -196,7 +194,7 @@ export class V8CoverageProvider
     this.pendingPromises.push(promise)
   }
 
-  async generateCoverage({ allTestsRun }: ReportContext) {
+  async generateCoverage({ allTestsRun }: ReportContext): Promise<CoverageMap> {
     const coverageMap = libCoverage.createCoverageMap({})
     let index = 0
     const total = this.pendingPromises.length
@@ -255,7 +253,7 @@ export class V8CoverageProvider
     return coverageMap
   }
 
-  async reportCoverage(coverageMap: unknown, { allTestsRun }: ReportContext) {
+  async reportCoverage(coverageMap: unknown, { allTestsRun }: ReportContext): Promise<void> {
     if (provider === 'stackblitz') {
       this.ctx.logger.log(
         c.blue(' % ')
@@ -284,7 +282,7 @@ export class V8CoverageProvider
     }
   }
 
-  async generateReports(coverageMap: CoverageMap, allTestsRun?: boolean) {
+  async generateReports(coverageMap: CoverageMap, allTestsRun?: boolean): Promise<void> {
     const context = libReport.createContext({
       dir: this.options.reportsDirectory,
       coverageMap,
@@ -349,7 +347,7 @@ export class V8CoverageProvider
     }
   }
 
-  async mergeReports(coverageMaps: unknown[]) {
+  async mergeReports(coverageMaps: unknown[]): Promise<void> {
     const coverageMap = libCoverage.createCoverageMap({})
 
     for (const coverage of coverageMaps) {
diff --git a/packages/coverage-v8/src/takeCoverage.ts b/packages/coverage-v8/src/takeCoverage.ts
index 0172ff0d8..4ffabbd93 100644
--- a/packages/coverage-v8/src/takeCoverage.ts
+++ b/packages/coverage-v8/src/takeCoverage.ts
@@ -8,7 +8,7 @@ import { provider } from 'std-env'
 
 const session = new inspector.Session()
 
-export function startCoverage() {
+export function startCoverage(): void {
   session.connect()
   session.post('Profiler.enable')
   session.post('Profiler.startPreciseCoverage', {
@@ -17,7 +17,7 @@ export function startCoverage() {
   })
 }
 
-export async function takeCoverage() {
+export async function takeCoverage(): Promise<{ result: Profiler.ScriptCoverage[] }> {
   return new Promise((resolve, reject) => {
     session.post('Profiler.takePreciseCoverage', async (error, coverage) => {
       if (error) {
@@ -36,7 +36,7 @@ export async function takeCoverage() {
   })
 }
 
-export function stopCoverage() {
+export function stopCoverage(): void {
   session.post('Profiler.stopPreciseCoverage')
   session.post('Profiler.disable')
   session.disconnect()
diff --git a/packages/coverage-v8/tsconfig.json b/packages/coverage-v8/tsconfig.json
index 8bcb003f8..73cb13106 100644
--- a/packages/coverage-v8/tsconfig.json
+++ b/packages/coverage-v8/tsconfig.json
@@ -1,5 +1,8 @@
 {
   "extends": "../../tsconfig.base.json",
+  "compilerOptions": {
+    "moduleResolution": "Bundler"
+  },
   "include": ["./src/**/*.ts"],
   "exclude": ["./dist"]
 }
diff --git a/packages/snapshot/src/client.ts b/packages/snapshot/src/client.ts
index dcbdff37b..4faf6802a 100644
--- a/packages/snapshot/src/client.ts
+++ b/packages/snapshot/src/client.ts
@@ -1,6 +1,6 @@
 import { deepMergeSnapshot } from './port/utils'
 import SnapshotState from './port/state'
-import type { SnapshotStateOptions } from './types'
+import type { SnapshotResult, SnapshotStateOptions } from './types'
 import type { RawSnapshotInfo } from './port/rawSnapshot'
 
 function createMismatchError(
@@ -53,7 +53,7 @@ export class SnapshotClient {
   filepath?: string
   name?: string
   snapshotState: SnapshotState | undefined
-  snapshotStateMap = new Map<string, SnapshotState>()
+  snapshotStateMap: Map<string, SnapshotState> = new Map()
 
   constructor(private options: SnapshotClientOptions = {}) {}
 
@@ -61,7 +61,7 @@ export class SnapshotClient {
     filepath: string,
     name: string,
     options: SnapshotStateOptions,
-  ) {
+  ): Promise<void> {
     this.filepath = filepath
     this.name = name
 
@@ -78,16 +78,16 @@ export class SnapshotClient {
     }
   }
 
-  getSnapshotState(filepath: string) {
+  getSnapshotState(filepath: string): SnapshotState {
     return this.snapshotStateMap.get(filepath)!
   }
 
-  clearTest() {
+  clearTest(): void {
     this.filepath = undefined
     this.name = undefined
   }
 
-  skipTestSnapshots(name: string) {
+  skipTestSnapshots(name: string): void {
     this.snapshotState?.markSnapshotsAsCheckedForTest(name)
   }
 
@@ -189,7 +189,7 @@ export class SnapshotClient {
     return this.assert(options)
   }
 
-  async finishCurrentRun() {
+  async finishCurrentRun(): Promise<SnapshotResult | null> {
     if (!this.snapshotState) {
       return null
     }
@@ -199,7 +199,7 @@ export class SnapshotClient {
     return result
   }
 
-  clear() {
+  clear(): void {
     this.snapshotStateMap.clear()
   }
 }
diff --git a/packages/snapshot/src/env/node.ts b/packages/snapshot/src/env/node.ts
index 32a61795d..4463e23fb 100644
--- a/packages/snapshot/src/env/node.ts
+++ b/packages/snapshot/src/env/node.ts
@@ -13,7 +13,7 @@ export class NodeSnapshotEnvironment implements SnapshotEnvironment {
     return `// Snapshot v${this.getVersion()}`
   }
 
-  async resolveRawPath(testPath: string, rawPath: string) {
+  async resolveRawPath(testPath: string, rawPath: string): Promise<string> {
     return isAbsolute(rawPath) ? rawPath : resolve(dirname(testPath), rawPath)
   }
 
diff --git a/packages/snapshot/src/manager.ts b/packages/snapshot/src/manager.ts
index 16c1f0fe9..44c88aaca 100644
--- a/packages/snapshot/src/manager.ts
+++ b/packages/snapshot/src/manager.ts
@@ -15,18 +15,17 @@ export class SnapshotManager {
     this.clear()
   }
 
-  clear() {
+  clear(): void {
     this.summary = emptySummary(this.options)
   }
 
-  add(result: SnapshotResult) {
+  add(result: SnapshotResult): void {
     addSnapshotResult(this.summary, result)
   }
 
-  resolvePath(testPath: string) {
+  resolvePath(testPath: string): string {
     const resolver
-      = this.options.resolveSnapshotPath
-      || (() => {
+      = this.options.resolveSnapshotPath || (() => {
         return join(
           join(dirname(testPath), '__snapshots__'),
           `${basename(testPath)}${this.extension}`,
@@ -37,7 +36,7 @@ export class SnapshotManager {
     return path
   }
 
-  resolveRawPath(testPath: string, rawPath: string) {
+  resolveRawPath(testPath: string, rawPath: string): string {
     return isAbsolute(rawPath) ? rawPath : resolve(dirname(testPath), rawPath)
   }
 }
diff --git a/packages/snapshot/src/port/inlineSnapshot.ts b/packages/snapshot/src/port/inlineSnapshot.ts
index af69121b5..11487738e 100644
--- a/packages/snapshot/src/port/inlineSnapshot.ts
+++ b/packages/snapshot/src/port/inlineSnapshot.ts
@@ -17,7 +17,7 @@ export interface InlineSnapshot {
 export async function saveInlineSnapshots(
   environment: SnapshotEnvironment,
   snapshots: Array<InlineSnapshot>,
-) {
+): Promise<void> {
   const MagicString = (await import('magic-string')).default
   const files = new Set(snapshots.map(i => i.file))
   await Promise.all(
@@ -148,7 +148,7 @@ export function replaceInlineSnap(
   s: MagicString,
   currentIndex: number,
   newSnap: string,
-) {
+): boolean {
   const { code: codeStartingAtIndex, index } = getCodeStartingAtIndex(code, currentIndex)
 
   const startMatch = startRegex.exec(codeStartingAtIndex)
@@ -182,7 +182,7 @@ export function replaceInlineSnap(
 }
 
 const INDENTATION_REGEX = /^([^\S\n]*)\S/m
-export function stripSnapshotIndentation(inlineSnapshot: string) {
+export function stripSnapshotIndentation(inlineSnapshot: string): string {
   // Find indentation if exists.
   const match = inlineSnapshot.match(INDENTATION_REGEX)
   if (!match || !match[1]) {
diff --git a/packages/snapshot/src/port/rawSnapshot.ts b/packages/snapshot/src/port/rawSnapshot.ts
index f544c5532..c7d445bcc 100644
--- a/packages/snapshot/src/port/rawSnapshot.ts
+++ b/packages/snapshot/src/port/rawSnapshot.ts
@@ -14,7 +14,7 @@ export interface RawSnapshot extends RawSnapshotInfo {
 export async function saveRawSnapshots(
   environment: SnapshotEnvironment,
   snapshots: Array<RawSnapshot>,
-) {
+): Promise<void> {
   await Promise.all(
     snapshots.map(async (snap) => {
       if (!snap.readonly) {
diff --git a/packages/snapshot/src/port/state.ts b/packages/snapshot/src/port/state.ts
index 0f47b4e5a..eab28e81e 100644
--- a/packages/snapshot/src/port/state.ts
+++ b/packages/snapshot/src/port/state.ts
@@ -94,7 +94,7 @@ export default class SnapshotState {
     this._environment = options.snapshotEnvironment
   }
 
-  static async create(testFilePath: string, options: SnapshotStateOptions) {
+  static async create(testFilePath: string, options: SnapshotStateOptions): Promise<SnapshotState> {
     const snapshotPath = await options.snapshotEnvironment.resolvePath(
       testFilePath,
     )
@@ -104,7 +104,7 @@ export default class SnapshotState {
     return new SnapshotState(testFilePath, snapshotPath, content, options)
   }
 
-  get environment() {
+  get environment(): SnapshotEnvironment {
     return this._environment
   }
 
@@ -116,7 +116,7 @@ export default class SnapshotState {
     })
   }
 
-  protected _inferInlineSnapshotStack(stacks: ParsedStack[]) {
+  protected _inferInlineSnapshotStack(stacks: ParsedStack[]): ParsedStack | null {
     // if called inside resolves/rejects, stacktrace is different
     const promiseIndex = stacks.findIndex(i =>
       i.method.match(/__VITEST_(RESOLVES|REJECTS)__/),
diff --git a/packages/snapshot/src/port/utils.ts b/packages/snapshot/src/port/utils.ts
index d247616f2..35b31dfd1 100644
--- a/packages/snapshot/src/port/utils.ts
+++ b/packages/snapshot/src/port/utils.ts
@@ -131,7 +131,7 @@ function printBacktickString(str: string): string {
   return `\`${escapeBacktickString(str)}\``
 }
 
-export function normalizeNewlines(string: string) {
+export function normalizeNewlines(string: string): string {
   return string.replace(/\r\n|\r/g, '\n')
 }
 
@@ -139,7 +139,7 @@ export async function saveSnapshotFile(
   environment: SnapshotEnvironment,
   snapshotData: SnapshotData,
   snapshotPath: string,
-) {
+): Promise<void> {
   const snapshots = Object.keys(snapshotData)
     .sort(naturalCompare)
     .map(
@@ -164,7 +164,7 @@ export async function saveSnapshotFileRaw(
   environment: SnapshotEnvironment,
   content: string,
   snapshotPath: string,
-) {
+): Promise<void> {
   const oldContent = await environment.readSnapshotFile(snapshotPath)
   const skipWriting = oldContent != null && oldContent === content
 
@@ -175,7 +175,7 @@ export async function saveSnapshotFileRaw(
   await environment.saveSnapshotFile(snapshotPath, content)
 }
 
-export function prepareExpected(expected?: string) {
+export function prepareExpected(expected?: string): string | undefined {
   function findStartIndent() {
     // Attempts to find indentation for objects.
     // Matches the ending tag of the object.
diff --git a/packages/snapshot/tsconfig.json b/packages/snapshot/tsconfig.json
index 8bcb003f8..93e30d6aa 100644
--- a/packages/snapshot/tsconfig.json
+++ b/packages/snapshot/tsconfig.json
@@ -1,5 +1,9 @@
 {
   "extends": "../../tsconfig.base.json",
+  "compilerOptions": {
+    "moduleResolution": "Bundler",
+    "isolatedDeclarations": true
+  },
   "include": ["./src/**/*.ts"],
   "exclude": ["./dist"]
 }
diff --git a/packages/spy/src/index.ts b/packages/spy/src/index.ts
index 9e698adf8..6ce989bf6 100644
--- a/packages/spy/src/index.ts
+++ b/packages/spy/src/index.ts
@@ -380,7 +380,7 @@ export type Mocked<T> = {
       : T[P];
 } & T
 
-export const mocks = new Set<MockInstance>()
+export const mocks: Set<MockInstance> = new Set()
 
 export function isMockFunction(fn: any): fn is MockInstance {
   return (
diff --git a/packages/spy/tsconfig.json b/packages/spy/tsconfig.json
index 8bcb003f8..93e30d6aa 100644
--- a/packages/spy/tsconfig.json
+++ b/packages/spy/tsconfig.json
@@ -1,5 +1,9 @@
 {
   "extends": "../../tsconfig.base.json",
+  "compilerOptions": {
+    "moduleResolution": "Bundler",
+    "isolatedDeclarations": true
+  },
   "include": ["./src/**/*.ts"],
   "exclude": ["./dist"]
 }