diff --git a/packages/cli/src/services/check-parser/__tests__/check-parser-fixtures/import-js-from-ts/dep1.js b/packages/cli/src/services/check-parser/__tests__/check-parser-fixtures/import-js-from-ts/dep1.js
new file mode 100644
index 00000000..d99d91de
--- /dev/null
+++ b/packages/cli/src/services/check-parser/__tests__/check-parser-fixtures/import-js-from-ts/dep1.js
@@ -0,0 +1 @@
+export const value = 'hello'
diff --git a/packages/cli/src/services/check-parser/__tests__/check-parser-fixtures/import-js-from-ts/dep2.js b/packages/cli/src/services/check-parser/__tests__/check-parser-fixtures/import-js-from-ts/dep2.js
new file mode 100644
index 00000000..51a1e4c5
--- /dev/null
+++ b/packages/cli/src/services/check-parser/__tests__/check-parser-fixtures/import-js-from-ts/dep2.js
@@ -0,0 +1 @@
+export const value = 'world'
diff --git a/packages/cli/src/services/check-parser/__tests__/check-parser-fixtures/import-js-from-ts/dep3.ts b/packages/cli/src/services/check-parser/__tests__/check-parser-fixtures/import-js-from-ts/dep3.ts
new file mode 100644
index 00000000..e2befb13
--- /dev/null
+++ b/packages/cli/src/services/check-parser/__tests__/check-parser-fixtures/import-js-from-ts/dep3.ts
@@ -0,0 +1 @@
+export const value = 3
diff --git a/packages/cli/src/services/check-parser/__tests__/check-parser-fixtures/import-js-from-ts/entrypoint.ts b/packages/cli/src/services/check-parser/__tests__/check-parser-fixtures/import-js-from-ts/entrypoint.ts
new file mode 100644
index 00000000..f80d396f
--- /dev/null
+++ b/packages/cli/src/services/check-parser/__tests__/check-parser-fixtures/import-js-from-ts/entrypoint.ts
@@ -0,0 +1,3 @@
+import { } from './dep1'
+import { } from './dep2.js'
+import { } from './dep3.js'
diff --git a/packages/cli/src/services/check-parser/__tests__/check-parser-fixtures/no-import-ts-from-js/dep1.ts b/packages/cli/src/services/check-parser/__tests__/check-parser-fixtures/no-import-ts-from-js/dep1.ts
new file mode 100644
index 00000000..d99d91de
--- /dev/null
+++ b/packages/cli/src/services/check-parser/__tests__/check-parser-fixtures/no-import-ts-from-js/dep1.ts
@@ -0,0 +1 @@
+export const value = 'hello'
diff --git a/packages/cli/src/services/check-parser/__tests__/check-parser-fixtures/no-import-ts-from-js/dep2.js b/packages/cli/src/services/check-parser/__tests__/check-parser-fixtures/no-import-ts-from-js/dep2.js
new file mode 100644
index 00000000..51a1e4c5
--- /dev/null
+++ b/packages/cli/src/services/check-parser/__tests__/check-parser-fixtures/no-import-ts-from-js/dep2.js
@@ -0,0 +1 @@
+export const value = 'world'
diff --git a/packages/cli/src/services/check-parser/__tests__/check-parser-fixtures/no-import-ts-from-js/entrypoint.js b/packages/cli/src/services/check-parser/__tests__/check-parser-fixtures/no-import-ts-from-js/entrypoint.js
new file mode 100644
index 00000000..7bee0690
--- /dev/null
+++ b/packages/cli/src/services/check-parser/__tests__/check-parser-fixtures/no-import-ts-from-js/entrypoint.js
@@ -0,0 +1,4 @@
+import { } from './dep1'
+import { } from './dep1.ts'
+import { } from './dep1.js'
+import { } from './dep2'
diff --git a/packages/cli/src/services/check-parser/__tests__/check-parser-fixtures/tsconfig-allow-importing-ts-extensions/src/dep1.ts b/packages/cli/src/services/check-parser/__tests__/check-parser-fixtures/tsconfig-allow-importing-ts-extensions/src/dep1.ts
new file mode 100644
index 00000000..062a3151
--- /dev/null
+++ b/packages/cli/src/services/check-parser/__tests__/check-parser-fixtures/tsconfig-allow-importing-ts-extensions/src/dep1.ts
@@ -0,0 +1 @@
+export const value1 = 'value1'
diff --git a/packages/cli/src/services/check-parser/__tests__/check-parser-fixtures/tsconfig-allow-importing-ts-extensions/src/dep2.ts b/packages/cli/src/services/check-parser/__tests__/check-parser-fixtures/tsconfig-allow-importing-ts-extensions/src/dep2.ts
new file mode 100644
index 00000000..2407fa53
--- /dev/null
+++ b/packages/cli/src/services/check-parser/__tests__/check-parser-fixtures/tsconfig-allow-importing-ts-extensions/src/dep2.ts
@@ -0,0 +1 @@
+export const value2 = 'value2'
diff --git a/packages/cli/src/services/check-parser/__tests__/check-parser-fixtures/tsconfig-allow-importing-ts-extensions/src/dep3.ts b/packages/cli/src/services/check-parser/__tests__/check-parser-fixtures/tsconfig-allow-importing-ts-extensions/src/dep3.ts
new file mode 100644
index 00000000..28b05968
--- /dev/null
+++ b/packages/cli/src/services/check-parser/__tests__/check-parser-fixtures/tsconfig-allow-importing-ts-extensions/src/dep3.ts
@@ -0,0 +1 @@
+export const value3 = 'value3'
diff --git a/packages/cli/src/services/check-parser/__tests__/check-parser-fixtures/tsconfig-allow-importing-ts-extensions/src/entrypoint.ts b/packages/cli/src/services/check-parser/__tests__/check-parser-fixtures/tsconfig-allow-importing-ts-extensions/src/entrypoint.ts
new file mode 100644
index 00000000..5ab303a3
--- /dev/null
+++ b/packages/cli/src/services/check-parser/__tests__/check-parser-fixtures/tsconfig-allow-importing-ts-extensions/src/entrypoint.ts
@@ -0,0 +1,3 @@
+export { value1 } from './dep1.ts'
+export { value2 } from './dep2.js'
+export { value3 } from './dep3'
diff --git a/packages/cli/src/services/check-parser/__tests__/check-parser-fixtures/tsconfig-allow-importing-ts-extensions/tsconfig.json b/packages/cli/src/services/check-parser/__tests__/check-parser-fixtures/tsconfig-allow-importing-ts-extensions/tsconfig.json
new file mode 100644
index 00000000..ba221115
--- /dev/null
+++ b/packages/cli/src/services/check-parser/__tests__/check-parser-fixtures/tsconfig-allow-importing-ts-extensions/tsconfig.json
@@ -0,0 +1,11 @@
+{
+  "compilerOptions": {
+    "target": "ES2022",
+    "module": "NodeNext",
+    "moduleResolution": "nodenext",
+    "esModuleInterop": true,
+    "baseUrl": ".",
+    "noEmit": true,
+    "allowImportingTsExtensions": true
+  }
+}
diff --git a/packages/cli/src/services/check-parser/__tests__/check-parser.spec.ts b/packages/cli/src/services/check-parser/__tests__/check-parser.spec.ts
index 098894aa..65c0ec4c 100644
--- a/packages/cli/src/services/check-parser/__tests__/check-parser.spec.ts
+++ b/packages/cli/src/services/check-parser/__tests__/check-parser.spec.ts
@@ -156,6 +156,51 @@ describe('dependency-parser - parser()', () => {
     expect(dependencies.map(d => d.filePath).sort()).toEqual([])
   })
 
+  it('should support importing ts extensions if allowed', () => {
+    const toAbsolutePath = (...filepath: string[]) => path.join(__dirname, 'check-parser-fixtures', 'tsconfig-allow-importing-ts-extensions', ...filepath)
+    const parser = new Parser({
+      supportedNpmModules: defaultNpmModules,
+    })
+    const { dependencies } = parser.parse(toAbsolutePath('src', 'entrypoint.ts'))
+    expect(dependencies.map(d => d.filePath).sort()).toEqual([
+      toAbsolutePath('src', 'dep1.ts'),
+      toAbsolutePath('src', 'dep2.ts'),
+      toAbsolutePath('src', 'dep3.ts'),
+    ])
+  })
+
+  it('should not import TS files from a JS file', () => {
+    const toAbsolutePath = (...filepath: string[]) => path.join(__dirname, 'check-parser-fixtures', 'no-import-ts-from-js', ...filepath)
+    const parser = new Parser({
+      supportedNpmModules: defaultNpmModules,
+    })
+    try {
+      // eslint-disable-next-line @typescript-eslint/no-unused-vars
+      const { dependencies } = parser.parse(toAbsolutePath('entrypoint.js'))
+    } catch (err) {
+      expect(err).toMatchObject({
+        missingFiles: [
+          toAbsolutePath('dep1'),
+          toAbsolutePath('dep1.ts'),
+          toAbsolutePath('dep1.js'),
+        ],
+      })
+    }
+  })
+
+  it('should import JS files from a TS file', () => {
+    const toAbsolutePath = (...filepath: string[]) => path.join(__dirname, 'check-parser-fixtures', 'import-js-from-ts', ...filepath)
+    const parser = new Parser({
+      supportedNpmModules: defaultNpmModules,
+    })
+    const { dependencies } = parser.parse(toAbsolutePath('entrypoint.ts'))
+    expect(dependencies.map(d => d.filePath).sort()).toEqual([
+      toAbsolutePath('dep1.js'),
+      toAbsolutePath('dep2.js'),
+      toAbsolutePath('dep3.ts'),
+    ])
+  })
+
   it('should handle ES Modules', () => {
     const toAbsolutePath = (...filepath: string[]) => path.join(__dirname, 'check-parser-fixtures', 'esmodules-example', ...filepath)
     const parser = new Parser({
diff --git a/packages/cli/src/services/check-parser/package-files/extension.ts b/packages/cli/src/services/check-parser/package-files/extension.ts
new file mode 100644
index 00000000..d1510f5e
--- /dev/null
+++ b/packages/cli/src/services/check-parser/package-files/extension.ts
@@ -0,0 +1,76 @@
+import path from 'node:path'
+
+export type CoreExtension = '.js' | '.mjs' | '.cjs' | '.json'
+
+export const CoreExtensions: CoreExtension[] = ['.js', '.mjs', '.cjs', '.json']
+
+type CoreExtensionMapping = {
+  [key in CoreExtension]: string[]
+}
+
+/**
+ * @see https://www.typescriptlang.org/docs/handbook/modules/reference.html#file-extension-substitution
+ */
+export const tsCoreExtensionLookupOrder: CoreExtensionMapping = {
+  '.js': ['.ts', '.tsx', '.js', '.jsx'],
+  '.mjs': ['.mts', '.mjs'],
+  '.cjs': ['.cts', '.cjs'],
+  '.json': ['.json'],
+}
+
+export type TSExtension = '.ts' | '.mts' | '.tsx'
+
+export const TSExtensions: TSExtension[] = ['.ts', '.mts', '.tsx']
+
+type TSExtensionMapping = {
+  [key in TSExtension]: string[]
+}
+
+export const tsExtensionLookupOrder: TSExtensionMapping = {
+  '.ts': ['.ts'],
+  '.mts': ['.mts'],
+  '.tsx': ['.tsx'],
+}
+
+function stripKnownSuffix (value: string, suffix: string): string {
+  return value.substring(0, value.length - suffix.length)
+}
+
+export class FileExtPath {
+  filePath: string
+  ext: string
+
+  private constructor (filePath: string, ext: string) {
+    this.filePath = filePath
+    this.ext = ext
+  }
+
+  static fromFilePath (filePath: string) {
+    const ext = path.extname(filePath)
+    return new FileExtPath(filePath, ext)
+  }
+
+  hasCoreExtension () {
+    return this.ext in tsCoreExtensionLookupOrder
+  }
+
+  hasTypeScriptExtension () {
+    return this.ext in tsExtensionLookupOrder
+  }
+
+  replaceExt (ext: string) {
+    return stripKnownSuffix(this.filePath, this.ext) + ext
+  }
+
+  appendExt (ext: string) {
+    return this.filePath + ext
+  }
+
+  resolve (...paths: string[]) {
+    return path.resolve(this.filePath, ...paths)
+  }
+
+  self () {
+    return this.filePath
+  }
+}
diff --git a/packages/cli/src/services/check-parser/package-files/lookup.ts b/packages/cli/src/services/check-parser/package-files/lookup.ts
new file mode 100644
index 00000000..64a4d8dd
--- /dev/null
+++ b/packages/cli/src/services/check-parser/package-files/lookup.ts
@@ -0,0 +1,78 @@
+import { PackageJsonFile } from './package-json-file'
+import { CoreExtension, FileExtPath, tsCoreExtensionLookupOrder, TSExtension, tsExtensionLookupOrder } from './extension'
+
+type Options = {
+  plainJs?: boolean
+  allowImportingTsExtensions?: boolean
+}
+
+export class LookupContext {
+  #plainJs: boolean
+  #allowImportingTsExtensions: boolean
+
+  constructor (options: Options) {
+    this.#plainJs = options.plainJs ?? false
+    this.#allowImportingTsExtensions = options.allowImportingTsExtensions ?? false
+  }
+
+  static forFilePath (filePath: string, options?: Options) {
+    return new LookupContext({
+      plainJs: FileExtPath.fromFilePath(filePath).hasCoreExtension(),
+      allowImportingTsExtensions: options?.allowImportingTsExtensions,
+    })
+  }
+
+  switch (options: Options) {
+    return new LookupContext({
+      plainJs: options.plainJs ?? this.#plainJs,
+      allowImportingTsExtensions: options.allowImportingTsExtensions ?? this.#allowImportingTsExtensions,
+    })
+  }
+
+  collectLookupPaths (filePath: string): string[] {
+    const extPath = FileExtPath.fromFilePath(filePath)
+
+    if (this.#plainJs) {
+      if (extPath.hasCoreExtension()) {
+        return [extPath.self()]
+      }
+
+      return this.extlessCoreLookupPaths(extPath)
+    }
+
+    if (extPath.hasCoreExtension()) {
+      const extensions = tsCoreExtensionLookupOrder[extPath.ext as CoreExtension]
+      return extensions.map(ext => extPath.replaceExt(ext))
+    }
+
+    if (this.#allowImportingTsExtensions) {
+      if (extPath.hasTypeScriptExtension()) {
+        const extensions = tsExtensionLookupOrder[extPath.ext as TSExtension]
+        return extensions.map(ext => extPath.replaceExt(ext))
+      }
+    }
+
+    return this.extlessTSLookupPaths(extPath)
+  }
+
+  private extlessCoreLookupPaths (extPath: FileExtPath): string[] {
+    return [
+      extPath.appendExt('.js'),
+      extPath.appendExt('.mjs'),
+      extPath.appendExt('.cjs'),
+      extPath.appendExt('.json'),
+      extPath.resolve(PackageJsonFile.FILENAME),
+      extPath.resolve('index.js'),
+      extPath.resolve('index.mjs'),
+      extPath.resolve('index.cjs'),
+    ]
+  }
+
+  private extlessTSLookupPaths (extPath: FileExtPath): string[] {
+    return this.extlessCoreLookupPaths(extPath).flatMap(filePath => {
+      const extPath = FileExtPath.fromFilePath(filePath)
+      const extensions = tsCoreExtensionLookupOrder[extPath.ext as CoreExtension]
+      return extensions.map(ext => extPath.replaceExt(ext))
+    })
+  }
+}
diff --git a/packages/cli/src/services/check-parser/package-files/resolver.ts b/packages/cli/src/services/check-parser/package-files/resolver.ts
index 418a7d39..891c0e70 100644
--- a/packages/cli/src/services/check-parser/package-files/resolver.ts
+++ b/packages/cli/src/services/check-parser/package-files/resolver.ts
@@ -9,6 +9,7 @@ import { JSConfigFile } from './jsconfig-json-file'
 import { isLocalPath, PathResult } from './paths'
 import { FileLoader, LoadFile } from './loader'
 import { JsonSourceFile } from './json-source-file'
+import { LookupContext } from './lookup'
 
 class PackageFilesCache {
   #sourceFileCache = new FileLoader(SourceFile.loadFromFilePath)
@@ -33,11 +34,9 @@ class PackageFilesCache {
   #tsconfigJsonCache = new FileLoader(this.#jsonFileLoader(TSConfigFile.loadFromJsonSourceFile))
   #jsconfigJsonCache = new FileLoader(this.#jsonFileLoader(JSConfigFile.loadFromJsonSourceFile))
 
-  sourceFile (filePath: string, suffixes?: string[]) {
-    for (const suffix of ['', ...suffixes ?? []]) {
-      const suffixFilePath = filePath + suffix
-
-      const sourceFile = this.#sourceFileCache.load(suffixFilePath)
+  sourceFile (filePath: string, context: LookupContext) {
+    for (const lookupPath of context.collectLookupPaths(filePath)) {
+      const sourceFile = this.#sourceFileCache.load(lookupPath)
       if (sourceFile === undefined) {
         continue
       }
@@ -185,7 +184,7 @@ export class PackageFilesResolver {
     return files
   }
 
-  private resolveSourceFile (sourceFile: SourceFile): SourceFile[] {
+  private resolveSourceFile (sourceFile: SourceFile, context: LookupContext): SourceFile[] {
     if (sourceFile.meta.basename === PackageJsonFile.FILENAME) {
       const packageJson = this.cache.packageJson(sourceFile.meta.filePath)
       if (packageJson === undefined) {
@@ -208,9 +207,13 @@ export class PackageFilesResolver {
             continue
           }
 
-          const candidatePaths = configJson.collectLookupPaths(mainPath)
+          const candidatePaths = configJson.collectLookupPaths(mainPath).flatMap(filePath => {
+            return context.collectLookupPaths(filePath)
+          })
           for (const candidatePath of candidatePaths) {
-            const mainSourceFile = this.cache.sourceFile(candidatePath)
+            const mainSourceFile = this.cache.sourceFile(candidatePath, context.switch({
+              allowImportingTsExtensions: configJson.allowImportingTsExtensions,
+            }))
             if (mainSourceFile === undefined) {
               continue
             }
@@ -221,7 +224,7 @@ export class PackageFilesResolver {
           }
         }
 
-        const mainSourceFile = this.cache.sourceFile(mainPath)
+        const mainSourceFile = this.cache.sourceFile(mainPath, context)
         if (mainSourceFile === undefined) {
           continue
         }
@@ -239,7 +242,6 @@ export class PackageFilesResolver {
   resolveDependenciesForFilePath (
     filePath: string,
     dependencies: string[],
-    suffixes: string[],
   ): Dependencies {
     const resolved: Dependencies = {
       external: [],
@@ -250,14 +252,19 @@ export class PackageFilesResolver {
     const dirname = path.dirname(filePath)
 
     const { packageJson, tsconfigJson, jsconfigJson } = this.loadPackageFiles(filePath)
+    const mainConfigJson = tsconfigJson ?? jsconfigJson
+
+    const context = LookupContext.forFilePath(filePath, {
+      allowImportingTsExtensions: mainConfigJson?.allowImportingTsExtensions,
+    })
 
     resolve:
     for (const importPath of dependencies) {
       if (isLocalPath(importPath)) {
         const relativeDepPath = path.resolve(dirname, importPath)
-        const sourceFile = this.cache.sourceFile(relativeDepPath, suffixes)
+        const sourceFile = this.cache.sourceFile(relativeDepPath, context)
         if (sourceFile !== undefined) {
-          const resolvedFiles = this.resolveSourceFile(sourceFile)
+          const resolvedFiles = this.resolveSourceFile(sourceFile, context)
           let found = false
           for (const resolvedFile of resolvedFiles) {
             resolved.local.push({
@@ -278,20 +285,23 @@ export class PackageFilesResolver {
         continue resolve
       }
 
-      // TODO: Prefer jsconfig.json when dealing with a JavaScript file.
       for (const configJson of [tsconfigJson, jsconfigJson]) {
         if (configJson === undefined) {
           continue
         }
 
+        const configContext = context.switch({
+          allowImportingTsExtensions: configJson.allowImportingTsExtensions,
+        })
+
         const resolvedPaths = configJson.resolvePath(importPath)
         if (resolvedPaths.length > 0) {
           let found = false
           for (const { source, target } of resolvedPaths) {
             const relativePath = path.resolve(configJson.basePath, target.path)
-            const sourceFile = this.cache.sourceFile(relativePath, suffixes)
+            const sourceFile = this.cache.sourceFile(relativePath, configContext)
             if (sourceFile !== undefined) {
-              const resolvedFiles = this.resolveSourceFile(sourceFile)
+              const resolvedFiles = this.resolveSourceFile(sourceFile, configContext)
               for (const resolvedFile of resolvedFiles) {
                 configJson.registerRelatedSourceFile(resolvedFile)
                 resolved.local.push({
@@ -326,9 +336,9 @@ export class PackageFilesResolver {
 
         if (configJson.baseUrl !== undefined) {
           const relativePath = path.resolve(configJson.basePath, configJson.baseUrl, importPath)
-          const sourceFile = this.cache.sourceFile(relativePath, suffixes)
+          const sourceFile = this.cache.sourceFile(relativePath, configContext)
           if (sourceFile !== undefined) {
-            const resolvedFiles = this.resolveSourceFile(sourceFile)
+            const resolvedFiles = this.resolveSourceFile(sourceFile, configContext)
             let found = false
             for (const resolvedFile of resolvedFiles) {
               configJson.registerRelatedSourceFile(resolvedFile)
@@ -356,9 +366,9 @@ export class PackageFilesResolver {
       if (packageJson !== undefined) {
         if (packageJson.supportsPackageRelativePaths()) {
           const relativePath = path.resolve(packageJson.basePath, importPath)
-          const sourceFile = this.cache.sourceFile(relativePath, suffixes)
+          const sourceFile = this.cache.sourceFile(relativePath, context)
           if (sourceFile !== undefined) {
-            const resolvedFiles = this.resolveSourceFile(sourceFile)
+            const resolvedFiles = this.resolveSourceFile(sourceFile, context)
             let found = false
             for (const resolvedFile of resolvedFiles) {
               resolved.local.push({
diff --git a/packages/cli/src/services/check-parser/package-files/tsconfig-json-file.ts b/packages/cli/src/services/check-parser/package-files/tsconfig-json-file.ts
index d8f9ec71..50ae15d4 100644
--- a/packages/cli/src/services/check-parser/package-files/tsconfig-json-file.ts
+++ b/packages/cli/src/services/check-parser/package-files/tsconfig-json-file.ts
@@ -54,29 +54,18 @@ interface CompilerOptions {
    * @see https://www.typescriptlang.org/tsconfig/#composite
    */
   composite?: boolean
+
+  /**
+   * If true, TypeScript-specific extensions can be imported. Requires
+   * either `noEmit: true` or `emitDeclarationOnly: true`.
+   */
+  allowImportingTsExtensions?: boolean
 }
 
 export interface Schema {
   compilerOptions?: CompilerOptions
 }
 
-type JSExtension = '.js' | '.mjs' | '.cjs'
-
-const JSExtensions: JSExtension[] = ['.js', '.mjs', '.cjs']
-
-type JSExtensionMappings = {
-  [key in JSExtension]: string[]
-}
-
-/**
- * @see https://www.typescriptlang.org/docs/handbook/modules/reference.html#file-extension-substitution
- */
-const extensionMappings: JSExtensionMappings = {
-  '.js': ['.ts', '.tsx', '.js', '.jsx'],
-  '.mjs': ['.mts', '.mjs'],
-  '.cjs': ['.cts', '.cjs'],
-}
-
 export class TSConfigFile {
   static FILENAME = 'tsconfig.json'
 
@@ -88,6 +77,7 @@ export class TSConfigFile {
   moduleResolution: string
   baseUrl?: string
   pathResolver: PathResolver
+  allowImportingTsExtensions: boolean
 
   relatedSourceFiles: SourceFile[] = []
 
@@ -104,6 +94,8 @@ export class TSConfigFile {
     }
 
     this.pathResolver = PathResolver.createFromPaths(this.baseUrl ?? '.', jsonFile.data.compilerOptions?.paths ?? {})
+
+    this.allowImportingTsExtensions = jsonFile.data.compilerOptions?.allowImportingTsExtensions ?? false
   }
 
   public get meta () {
@@ -133,22 +125,6 @@ export class TSConfigFile {
     return this.pathResolver.resolve(importPath)
   }
 
-  private extifyLookupPaths (filePaths: string[]): string[] {
-    return filePaths.flatMap(filePath => {
-      let extensions = extensionMappings['.js']
-      let extlessPath = filePath
-
-      for (const ext of JSExtensions) {
-        if (filePath.endsWith(ext)) {
-          extensions = extensionMappings[ext]
-          extlessPath = filePath.substring(0, filePath.length - ext.length)
-        }
-      }
-
-      return extensions.map(ext => path.resolve(this.basePath, extlessPath + ext))
-    })
-  }
-
   collectLookupPaths (filePath: string): string[] {
     let {
       outDir,
@@ -160,8 +136,8 @@ export class TSConfigFile {
     const candidates = []
 
     if (outDir === undefined) {
-      candidates.push(filePath)
-      return this.extifyLookupPaths(candidates) // Nothing more we can do.
+      candidates.push(path.resolve(this.basePath, filePath))
+      return candidates // Nothing more we can do.
     }
 
     if (composite === undefined) {
@@ -187,8 +163,8 @@ export class TSConfigFile {
     // something that wasn't compiled using this tsconfig (or at all), and
     // stop looking.
     if (relativePath.startsWith('..')) {
-      candidates.push(filePath)
-      return this.extifyLookupPaths(candidates)
+      candidates.push(path.resolve(this.basePath, filePath))
+      return candidates
     }
 
     candidates.push(path.resolve(this.basePath, rootDir, relativePath))
@@ -200,7 +176,7 @@ export class TSConfigFile {
       candidates.push(path.resolve(this.basePath, multiRootDir, relativePath))
     }
 
-    return this.extifyLookupPaths(candidates)
+    return candidates
   }
 
   registerRelatedSourceFile (file: SourceFile) {
diff --git a/packages/cli/src/services/check-parser/parser.ts b/packages/cli/src/services/check-parser/parser.ts
index 1ecc2abc..10c807f1 100644
--- a/packages/cli/src/services/check-parser/parser.ts
+++ b/packages/cli/src/services/check-parser/parser.ts
@@ -20,26 +20,6 @@ type SupportedFileExtension = '.js' | '.mjs' | '.ts'
 
 const PACKAGE_EXTENSION = `${path.sep}package.json`
 
-const JS_RESOLVE_ORDER = [
-  '.js',
-  '.mjs',
-  PACKAGE_EXTENSION,
-  `${path.sep}index.js`,
-  // TODO: Check the module type in package.json to figure out the esm and common js file extensions
-  `${path.sep}index.mjs`,
-]
-
-const TS_RESOLVE_ORDER = [
-  '.ts',
-  '.js',
-  '.mjs',
-  PACKAGE_EXTENSION,
-  `${path.sep}index.ts`,
-  `${path.sep}index.js`,
-  // TODO: Check the module type in package.json to figure out the esm and common js file extensions
-  `${path.sep}index.mjs`,
-]
-
 const supportedBuiltinModules = [
   'assert', 'buffer', 'crypto', 'dns', 'fs', 'path', 'querystring', 'readline ', 'stream', 'string_decoder',
   'timers', 'tls', 'url', 'util', 'zlib',
@@ -131,9 +111,7 @@ export class Parser {
         continue
       }
 
-      const suffixes = extension === '.js' ? JS_RESOLVE_ORDER : TS_RESOLVE_ORDER
-
-      const resolved = resolver.resolveDependenciesForFilePath(item.filePath, module.dependencies, suffixes)
+      const resolved = resolver.resolveDependenciesForFilePath(item.filePath, module.dependencies)
 
       if (this.checkUnsupportedModules) {
         const unsupportedDependencies = resolved.external.flatMap(dep => {