diff --git a/src/compiler/builder.ts b/src/compiler/builder.ts
index d4f2dc7f31f23..550baef382984 100644
--- a/src/compiler/builder.ts
+++ b/src/compiler/builder.ts
@@ -1161,6 +1161,7 @@ function getBuildInfo(state: BuilderProgramState, bundle: BundleBuildInfo | unde
 
 function convertToReusableCompilerOptionValue(option: CommandLineOption | undefined, value: CompilerOptionsValue, relativeToBuildInfo: (path: string) => string) {
     if (option) {
+        Debug.assert(option.type !== "listOrElement");
         if (option.type === "list") {
             const values = value as readonly (string | number)[];
             if (option.element.isFilePath && values.length) {
diff --git a/src/compiler/commandLineParser.ts b/src/compiler/commandLineParser.ts
index cd354820ad3f7..97a09f716345e 100644
--- a/src/compiler/commandLineParser.ts
+++ b/src/compiler/commandLineParser.ts
@@ -103,6 +103,7 @@ import {
     returnTrue,
     ScriptTarget,
     startsWith,
+    stringContains,
     StringLiteral,
     SyntaxKind,
     sys,
@@ -1658,11 +1659,14 @@ export function parseCustomTypeOption(opt: CommandLineOptionOfCustomType, value:
 }
 
 /** @internal */
-export function parseListTypeOption(opt: CommandLineOptionOfListType, value = "", errors: Push<Diagnostic>): (string | number)[] | undefined {
+export function parseListTypeOption(opt: CommandLineOptionOfListType, value = "", errors: Push<Diagnostic>): string | (string | number)[] | undefined {
     value = trimString(value);
     if (startsWith(value, "-")) {
         return undefined;
     }
+    if (opt.type === "listOrElement" && !stringContains(value, ",")) {
+        return validateJsonOptionValue(opt, value, errors);
+    }
     if (value === "") {
         return [];
     }
@@ -1672,6 +1676,9 @@ export function parseListTypeOption(opt: CommandLineOptionOfListType, value = ""
             return mapDefined(values, v => validateJsonOptionValue(opt.element, parseInt(v), errors));
         case "string":
             return mapDefined(values, v => validateJsonOptionValue(opt.element, v || "", errors));
+        case "boolean":
+        case "object":
+            return Debug.fail(`List of ${opt.element.type} is not yet supported.`);
         default:
             return mapDefined(values, v => parseCustomTypeOption(opt.element as CommandLineOptionOfCustomType, v, errors));
     }
@@ -1850,6 +1857,9 @@ function parseOptionValue(
                         i++;
                     }
                     break;
+                case "listOrElement":
+                    Debug.fail("listOrElement not supported here");
+                    break;
                 // If not a primitive, the possible types are specified in what is effectively a map of options.
                 default:
                     options[opt.name] = parseCustomTypeOption(opt as CommandLineOptionOfCustomType, args[i], errors);
@@ -2090,6 +2100,15 @@ function getCommandLineTypeAcquisitionMap() {
     return commandLineTypeAcquisitionMapCache || (commandLineTypeAcquisitionMapCache = commandLineOptionsToMap(typeAcquisitionDeclarations));
 }
 
+const extendsOptionDeclaration: CommandLineOptionOfListType = {
+    name: "extends",
+    type: "listOrElement",
+    element: {
+        name: "extends",
+        type: "string"
+    },
+    category: Diagnostics.File_Management,
+};
 let _tsconfigRootOptions: TsConfigOnlyOption;
 function getTsconfigRootOptionsMap() {
     if (_tsconfigRootOptions === undefined) {
@@ -2121,11 +2140,7 @@ function getTsconfigRootOptionsMap() {
                     elementOptions: getCommandLineTypeAcquisitionMap(),
                     extraKeyDiagnostics: typeAcquisitionDidYouMeanDiagnostics
                 },
-                {
-                    name: "extends",
-                    type: "string",
-                    category: Diagnostics.File_Management,
-                },
+                extendsOptionDeclaration,
                 {
                     name: "references",
                     type: "list",
@@ -2339,11 +2354,11 @@ export function convertToObjectWorker(
         let invalidReported: boolean | undefined;
         switch (valueExpression.kind) {
             case SyntaxKind.TrueKeyword:
-                reportInvalidOptionValue(option && option.type !== "boolean");
+                reportInvalidOptionValue(option && option.type !== "boolean" && (option.type !== "listOrElement" || option.element.type !== "boolean"));
                 return validateValue(/*value*/ true);
 
             case SyntaxKind.FalseKeyword:
-                reportInvalidOptionValue(option && option.type !== "boolean");
+                reportInvalidOptionValue(option && option.type !== "boolean"&& (option.type !== "listOrElement" || option.element.type !== "boolean"));
                 return validateValue(/*value*/ false);
 
             case SyntaxKind.NullKeyword:
@@ -2354,8 +2369,11 @@ export function convertToObjectWorker(
                 if (!isDoubleQuotedString(valueExpression)) {
                     errors.push(createDiagnosticForNodeInSourceFile(sourceFile, valueExpression, Diagnostics.String_literal_with_double_quotes_expected));
                 }
-                reportInvalidOptionValue(option && (isString(option.type) && option.type !== "string"));
+                reportInvalidOptionValue(option && isString(option.type) && option.type !== "string" && (option.type !== "listOrElement" || (isString(option.element.type) && option.element.type !== "string")));
                 const text = (valueExpression as StringLiteral).text;
+                if (option) {
+                    Debug.assert(option.type !== "listOrElement" || option.element.type === "string", "Only string or array of string is handled for now");
+                }
                 if (option && !isString(option.type)) {
                     const customOption = option as CommandLineOptionOfCustomType;
                     // Validate custom option type
@@ -2372,18 +2390,18 @@ export function convertToObjectWorker(
                 return validateValue(text);
 
             case SyntaxKind.NumericLiteral:
-                reportInvalidOptionValue(option && option.type !== "number");
+                reportInvalidOptionValue(option && option.type !== "number" && (option.type !== "listOrElement" || option.element.type !== "number"));
                 return validateValue(Number((valueExpression as NumericLiteral).text));
 
             case SyntaxKind.PrefixUnaryExpression:
                 if ((valueExpression as PrefixUnaryExpression).operator !== SyntaxKind.MinusToken || (valueExpression as PrefixUnaryExpression).operand.kind !== SyntaxKind.NumericLiteral) {
                     break; // not valid JSON syntax
                 }
-                reportInvalidOptionValue(option && option.type !== "number");
+                reportInvalidOptionValue(option && option.type !== "number" && (option.type !== "listOrElement" || option.element.type !== "number"));
                 return validateValue(-Number(((valueExpression as PrefixUnaryExpression).operand as NumericLiteral).text));
 
             case SyntaxKind.ObjectLiteralExpression:
-                reportInvalidOptionValue(option && option.type !== "object");
+                reportInvalidOptionValue(option && option.type !== "object" && (option.type !== "listOrElement" || option.element.type !== "object"));
                 const objectLiteralExpression = valueExpression as ObjectLiteralExpression;
 
                 // Currently having element option declaration in the tsconfig with type "object"
@@ -2404,7 +2422,7 @@ export function convertToObjectWorker(
                 }
 
             case SyntaxKind.ArrayLiteralExpression:
-                reportInvalidOptionValue(option && option.type !== "list");
+                reportInvalidOptionValue(option && option.type !== "list" && option.type !== "listOrElement");
                 return validateValue(convertArrayLiteralExpressionToJson(
                     (valueExpression as ArrayLiteralExpression).elements,
                     option && (option as CommandLineOptionOfListType).element));
@@ -2444,8 +2462,10 @@ export function convertToObjectWorker(
     }
 }
 
-function getCompilerOptionValueTypeString(option: CommandLineOption) {
-    return option.type === "list" ?
+function getCompilerOptionValueTypeString(option: CommandLineOption): string {
+    return (option.type === "listOrElement") ?
+            `${getCompilerOptionValueTypeString(option.element)} or Array`:
+            option.type === "list" ?
         "Array" :
         isString(option.type) ? option.type : "string";
 }
@@ -2456,6 +2476,9 @@ function isCompilerOptionsValue(option: CommandLineOption | undefined, value: an
         if (option.type === "list") {
             return isArray(value);
         }
+        if (option.type === "listOrElement") {
+            return isArray(value) || isCompilerOptionsValue(option.element, value);
+        }
         const expectedType = isString(option.type) ? option.type : "string";
         return typeof value === expectedType;
     }
@@ -2560,15 +2583,18 @@ function matchesSpecs(path: string, includeSpecs: readonly string[] | undefined,
 }
 
 function getCustomTypeMapOfCommandLineOption(optionDefinition: CommandLineOption): Map<string, string | number> | undefined {
-    if (optionDefinition.type === "string" || optionDefinition.type === "number" || optionDefinition.type === "boolean" || optionDefinition.type === "object") {
-        // this is of a type CommandLineOptionOfPrimitiveType
-        return undefined;
-    }
-    else if (optionDefinition.type === "list") {
-        return getCustomTypeMapOfCommandLineOption(optionDefinition.element);
-    }
-    else {
-        return optionDefinition.type;
+    switch (optionDefinition.type) {
+        case "string":
+        case "number":
+        case "boolean":
+        case "object":
+            // this is of a type CommandLineOptionOfPrimitiveType
+            return undefined;
+        case "list":
+        case "listOrElement":
+            return getCustomTypeMapOfCommandLineOption(optionDefinition.element);
+        default:
+            return optionDefinition.type;
     }
 }
 
@@ -2612,6 +2638,7 @@ function serializeOptionBaseObject(
             const value = options[name] as CompilerOptionsValue;
             const optionDefinition = optionsNameMap.get(name.toLowerCase());
             if (optionDefinition) {
+                Debug.assert(optionDefinition.type !== "listOrElement");
                 const customTypeMap = getCustomTypeMapOfCommandLineOption(optionDefinition);
                 if (!customTypeMap) {
                     // There is no map associated with this compiler option then use the value as-is
@@ -2805,6 +2832,7 @@ function convertToOptionValueWithAbsolutePaths(option: CommandLineOption | undef
         else if (option.isFilePath) {
             return toAbsolutePath(value as string);
         }
+        Debug.assert(option.type !== "listOrElement");
     }
     return value;
 }
@@ -3076,13 +3104,22 @@ export interface ParsedTsconfig {
     /**
      * Note that the case of the config path has not yet been normalized, as no files have been imported into the project yet
      */
-    extendedConfigPath?: string;
+    extendedConfigPath?: string | string[];
 }
 
 function isSuccessfulParsedTsconfig(value: ParsedTsconfig) {
     return !!value.options;
 }
 
+interface ExtendsResult {
+    options: CompilerOptions;
+    watchOptions?: WatchOptions;
+    include?: string[];
+    exclude?: string[];
+    files?: string[];
+    compileOnSave?: boolean;
+    extendedSourceFiles?: Set<string>
+}
 /**
  * This *just* extracts options/include/exclude/files out of a config file.
  * It does *not* resolve the included files.
@@ -3119,34 +3156,52 @@ function parseConfig(
     if (ownConfig.extendedConfigPath) {
         // copy the resolution stack so it is never reused between branches in potential diamond-problem scenarios.
         resolutionStack = resolutionStack.concat([resolvedPath]);
-        const extendedConfig = getExtendedConfig(sourceFile, ownConfig.extendedConfigPath, host, resolutionStack, errors, extendedConfigCache);
+        const result: ExtendsResult = { options:{} };
+        if (isString(ownConfig.extendedConfigPath)) {
+            applyExtendedConfig(result, ownConfig.extendedConfigPath);
+        }
+        else {
+            ownConfig.extendedConfigPath.forEach(extendedConfigPath => applyExtendedConfig(result, extendedConfigPath));
+        }
+        if (!ownConfig.raw.include && result.include) ownConfig.raw.include = result.include;
+        if (!ownConfig.raw.exclude && result.exclude) ownConfig.raw.exclude = result.exclude;
+        if (!ownConfig.raw.files && result.files) ownConfig.raw.files = result.files;
+        if (ownConfig.raw.compileOnSave === undefined && result.compileOnSave) ownConfig.raw.compileOnSave = result.compileOnSave;
+        if (sourceFile && result.extendedSourceFiles) sourceFile.extendedSourceFiles = arrayFrom(result.extendedSourceFiles.keys());
+
+        ownConfig.options = assign(result.options, ownConfig.options);
+        ownConfig.watchOptions = ownConfig.watchOptions && result.watchOptions ?
+            assign(result.watchOptions, ownConfig.watchOptions) :
+            ownConfig.watchOptions || result.watchOptions;
+        }
+    return ownConfig;
+
+    function applyExtendedConfig(result: ExtendsResult, extendedConfigPath: string){
+        const extendedConfig = getExtendedConfig(sourceFile, extendedConfigPath, host, resolutionStack, errors, extendedConfigCache, result);
         if (extendedConfig && isSuccessfulParsedTsconfig(extendedConfig)) {
-            const baseRaw = extendedConfig.raw;
-            const raw = ownConfig.raw;
-            let relativeDifference: string | undefined;
-            const setPropertyInRawIfNotUndefined = (propertyName: string) => {
-                if (!raw[propertyName] && baseRaw[propertyName]) {
-                    raw[propertyName] = map(baseRaw[propertyName], (path: string) => isRootedDiskPath(path) ? path : combinePaths(
-                        relativeDifference ||= convertToRelativePath(getDirectoryPath(ownConfig.extendedConfigPath!), basePath, createGetCanonicalFileName(host.useCaseSensitiveFileNames)),
+            const extendsRaw = extendedConfig.raw;
+            let relativeDifference: string | undefined ;
+            const setPropertyInResultIfNotUndefined = (propertyName: "include" | "exclude" | "files") => {
+                if (extendsRaw[propertyName]) {
+                    result[propertyName] = map(extendsRaw[propertyName], (path: string) => isRootedDiskPath(path) ? path : combinePaths(
+                        relativeDifference ||= convertToRelativePath(getDirectoryPath(extendedConfigPath), basePath, createGetCanonicalFileName(host.useCaseSensitiveFileNames)),
                         path
                     ));
                 }
             };
-            setPropertyInRawIfNotUndefined("include");
-            setPropertyInRawIfNotUndefined("exclude");
-            setPropertyInRawIfNotUndefined("files");
-            if (raw.compileOnSave === undefined) {
-                raw.compileOnSave = baseRaw.compileOnSave;
+            setPropertyInResultIfNotUndefined("include");
+            setPropertyInResultIfNotUndefined("exclude");
+            setPropertyInResultIfNotUndefined("files");
+            if (extendsRaw.compileOnSave !== undefined) {
+                result.compileOnSave = extendsRaw.compileOnSave;
             }
-            ownConfig.options = assign({}, extendedConfig.options, ownConfig.options);
-            ownConfig.watchOptions = ownConfig.watchOptions && extendedConfig.watchOptions ?
-                assign({}, extendedConfig.watchOptions, ownConfig.watchOptions) :
-                ownConfig.watchOptions || extendedConfig.watchOptions;
+            assign(result.options, extendedConfig.options);
+            result.watchOptions = result.watchOptions && extendedConfig.watchOptions ?
+                assign({}, result.watchOptions, extendedConfig.watchOptions) :
+                result.watchOptions || extendedConfig.watchOptions;
             // TODO extend type typeAcquisition
         }
     }
-
-    return ownConfig;
 }
 
 function parseOwnConfigOfJson(
@@ -3166,15 +3221,28 @@ function parseOwnConfigOfJson(
     const typeAcquisition = convertTypeAcquisitionFromJsonWorker(json.typeAcquisition || json.typingOptions, basePath, errors, configFileName);
     const watchOptions = convertWatchOptionsFromJsonWorker(json.watchOptions, basePath, errors);
     json.compileOnSave = convertCompileOnSaveOptionFromJson(json, basePath, errors);
-    let extendedConfigPath: string | undefined;
+    let extendedConfigPath: string | string[] | undefined;
 
     if (json.extends) {
-        if (!isString(json.extends)) {
-            errors.push(createCompilerDiagnostic(Diagnostics.Compiler_option_0_requires_a_value_of_type_1, "extends", "string"));
+        if (!isCompilerOptionsValue(extendsOptionDeclaration, json.extends)) {
+            errors.push(createCompilerDiagnostic(Diagnostics.Compiler_option_0_requires_a_value_of_type_1, "extends", getCompilerOptionValueTypeString(extendsOptionDeclaration)));
         }
         else {
             const newBase = configFileName ? directoryOfCombinedPath(configFileName, basePath) : basePath;
-            extendedConfigPath = getExtendsConfigPath(json.extends, host, newBase, errors, createCompilerDiagnostic);
+            if (isString(json.extends)) {
+                extendedConfigPath = getExtendsConfigPath(json.extends, host, newBase, errors, createCompilerDiagnostic);
+            }
+            else {
+                extendedConfigPath = [];
+                for (const fileName of json.extends as unknown[]) {
+                    if (isString(fileName)) {
+                        extendedConfigPath = append(extendedConfigPath, getExtendsConfigPath(fileName, host, newBase, errors, createCompilerDiagnostic));
+                }
+                    else {
+                        errors.push(createCompilerDiagnostic(Diagnostics.Compiler_option_0_requires_a_value_of_type_1, "extends", getCompilerOptionValueTypeString(extendsOptionDeclaration.element)));
+                    }
+                }
+            }
         }
     }
     return { raw: json, options, watchOptions, typeAcquisition, extendedConfigPath };
@@ -3190,7 +3258,7 @@ function parseOwnConfigOfJsonSourceFile(
     const options = getDefaultCompilerOptions(configFileName);
     let typeAcquisition: TypeAcquisition | undefined, typingOptionstypeAcquisition: TypeAcquisition | undefined;
     let watchOptions: WatchOptions | undefined;
-    let extendedConfigPath: string | undefined;
+    let extendedConfigPath: string | string[] | undefined;
     let rootCompilerOptions: PropertyName[] | undefined;
 
     const optionsIterator: JsonConversionNotifier = {
@@ -3219,15 +3287,33 @@ function parseOwnConfigOfJsonSourceFile(
             switch (key) {
                 case "extends":
                     const newBase = configFileName ? directoryOfCombinedPath(configFileName, basePath) : basePath;
+                    if (isString(value)) {
                     extendedConfigPath = getExtendsConfigPath(
-                        value as string,
+                        value,
                         host,
                         newBase,
                         errors,
                         (message, arg0) =>
                             createDiagnosticForNodeInSourceFile(sourceFile, valueNode, message, arg0)
                     );
-                    return;
+                    }
+                    else {
+                        extendedConfigPath = [];
+                        for (let index = 0; index < (value as unknown[]).length; index++) {
+                            const fileName = (value as unknown[])[index];
+                            if (isString(fileName)) {
+                                extendedConfigPath = append(extendedConfigPath, getExtendsConfigPath(
+                                    fileName,
+                                    host,
+                                    newBase,
+                                    errors,
+                                    (message, arg0) =>
+                                        createDiagnosticForNodeInSourceFile(sourceFile, (valueNode as ArrayLiteralExpression).elements[index], message, arg0)
+                                ));
+                            }
+                        }
+                    }
+                return;
             }
         },
         onSetUnknownOptionKeyValueInRoot(key: string, keyNode: PropertyName, _value: CompilerOptionsValue, _valueNode: Expression) {
@@ -3301,7 +3387,8 @@ function getExtendedConfig(
     host: ParseConfigHost,
     resolutionStack: string[],
     errors: Push<Diagnostic>,
-    extendedConfigCache?: Map<string, ExtendedConfigCacheEntry>
+    extendedConfigCache: Map<string, ExtendedConfigCacheEntry> | undefined,
+    result: ExtendsResult
 ): ParsedTsconfig | undefined {
     const path = host.useCaseSensitiveFileNames ? extendedConfigPath : toFileNameLowerCase(extendedConfigPath);
     let value: ExtendedConfigCacheEntry | undefined;
@@ -3321,9 +3408,11 @@ function getExtendedConfig(
         }
     }
     if (sourceFile) {
-        sourceFile.extendedSourceFiles = [extendedResult.fileName];
+        (result.extendedSourceFiles ??= new Set()).add(extendedResult.fileName);
         if (extendedResult.extendedSourceFiles) {
-            sourceFile.extendedSourceFiles.push(...extendedResult.extendedSourceFiles);
+            for (const extenedSourceFile of extendedResult.extendedSourceFiles) {
+                result.extendedSourceFiles.add(extenedSourceFile);
+            }
         }
     }
     if (extendedResult.parseDiagnostics.length) {
@@ -3416,10 +3505,15 @@ function convertOptionsFromJson(optionsNameMap: Map<string, CommandLineOption>,
 export function convertJsonOption(opt: CommandLineOption, value: any, basePath: string, errors: Push<Diagnostic>): CompilerOptionsValue {
     if (isCompilerOptionsValue(opt, value)) {
         const optType = opt.type;
-        if (optType === "list" && isArray(value)) {
+        if ((optType === "list") && isArray(value)) {
             return convertJsonOptionOfListType(opt, value, basePath, errors);
         }
-        else if (!isString(optType)) {
+        else if (optType === "listOrElement") {
+            return isArray(value) ?
+                convertJsonOptionOfListType(opt, value, basePath, errors) :
+                convertJsonOption(opt.element, value, basePath, errors);
+        }
+        else if (!isString(opt.type)) {
             return convertJsonOptionOfCustomType(opt as CommandLineOptionOfCustomType, value as string, errors);
         }
         const validatedValue = validateJsonOptionValue(opt, value, errors);
@@ -3432,7 +3526,8 @@ export function convertJsonOption(opt: CommandLineOption, value: any, basePath:
 
 function normalizeOptionValue(option: CommandLineOption, basePath: string, value: any): CompilerOptionsValue {
     if (isNullOrUndefined(value)) return undefined;
-    if (option.type === "list") {
+    if (option.type === "listOrElement" && !isArray(value)) return normalizeOptionValue(option.element, basePath, value);
+    else if (option.type === "list" || option.type === "listOrElement") {
         const listOption = option;
         if (listOption.element.isFilePath || !isString(listOption.element.type)) {
             return filter(map(value, v => normalizeOptionValue(listOption.element, basePath, v)), v => listOption.listPreserveFalsyValues ? true : !!v) as CompilerOptionsValue;
@@ -3860,6 +3955,9 @@ function getOptionValueWithEmptyStrings(value: any, option: CommandLineOption):
             return typeof value === "number" ? value : "";
         case "boolean":
             return typeof value === "boolean" ? value : "";
+        case "listOrElement":
+            if (!isArray(value)) return getOptionValueWithEmptyStrings(value, option.element);
+            // fall through to list
         case "list":
             const elementType = option.element;
             return isArray(value) ? value.map(v => getOptionValueWithEmptyStrings(v, elementType)) : "";
@@ -3873,7 +3971,7 @@ function getOptionValueWithEmptyStrings(value: any, option: CommandLineOption):
 }
 
 
-function getDefaultValueForOption(option: CommandLineOption) {
+function getDefaultValueForOption(option: CommandLineOption): {} {
     switch (option.type) {
         case "number":
             return 1;
@@ -3884,6 +3982,8 @@ function getDefaultValueForOption(option: CommandLineOption) {
             return option.isFilePath ? `./${defaultValue && typeof defaultValue === "string" ? defaultValue : ""}` : "";
         case "list":
             return [];
+        case "listOrElement":
+            return getDefaultValueForOption(option.element);
         case "object":
             return {};
         default:
diff --git a/src/compiler/types.ts b/src/compiler/types.ts
index fcfb940152086..12bf22b8af668 100644
--- a/src/compiler/types.ts
+++ b/src/compiler/types.ts
@@ -6977,7 +6977,7 @@ export interface CreateProgramOptions {
 /** @internal */
 export interface CommandLineOptionBase {
     name: string;
-    type: "string" | "number" | "boolean" | "object" | "list" | Map<string, number | string>;    // a value of a primitive type, or an object literal mapping named values to actual values
+    type: "string" | "number" | "boolean" | "object" | "list" | "listOrElement" | Map<string, number | string>;    // a value of a primitive type, or an object literal mapping named values to actual values
     isFilePath?: boolean;                                   // True if option value is a path or fileName
     shortName?: string;                                     // A short mnemonic for convenience - for instance, 'h' can be used in place of 'help'
     description?: DiagnosticMessage;                        // The message describing what the command line switch does.
@@ -7047,7 +7047,7 @@ export interface TsConfigOnlyOption extends CommandLineOptionBase {
 
 /** @internal */
 export interface CommandLineOptionOfListType extends CommandLineOptionBase {
-    type: "list";
+    type: "list" | "listOrElement";
     element: CommandLineOptionOfCustomType | CommandLineOptionOfStringType | CommandLineOptionOfNumberType | CommandLineOptionOfBooleanType | TsConfigOnlyOption;
     listPreserveFalsyValues?: boolean;
 }
diff --git a/src/executeCommandLine/executeCommandLine.ts b/src/executeCommandLine/executeCommandLine.ts
index 58bd58cb02150..0853f9c6916f4 100644
--- a/src/executeCommandLine/executeCommandLine.ts
+++ b/src/executeCommandLine/executeCommandLine.ts
@@ -253,9 +253,9 @@ function generateOptionOutput(sys: System, option: CommandLineOption, rightAlign
         typeof option.defaultValueDescription === "object"
             ? getDiagnosticText(option.defaultValueDescription)
             : formatDefaultValue(
-                  option.defaultValueDescription,
-                  option.type === "list" ? option.element.type : option.type
-              );
+                option.defaultValueDescription,
+                option.type === "list" || option.type === "listOrElement" ? option.element.type : option.type
+            );
     const terminalWidth = sys.getWidthOfTerminal?.() ?? 0;
 
     // Note: child_process might return `terminalWidth` as undefined.
@@ -365,6 +365,7 @@ function generateOptionOutput(sys: System, option: CommandLineOption, rightAlign
         };
 
         function getValueType(option: CommandLineOption) {
+            Debug.assert(option.type !== "listOrElement");
             switch (option.type) {
                 case "string":
                 case "number":
@@ -386,7 +387,7 @@ function generateOptionOutput(sys: System, option: CommandLineOption, rightAlign
                     possibleValues = option.type;
                     break;
                 case "list":
-                    // TODO: check infinite loop
+                case "listOrElement":
                     possibleValues = getPossibleValues(option.element);
                     break;
                 case "object":
diff --git a/src/harness/harnessIO.ts b/src/harness/harnessIO.ts
index 642e4990385b3..4dcfcbca99faa 100644
--- a/src/harness/harnessIO.ts
+++ b/src/harness/harnessIO.ts
@@ -380,6 +380,7 @@ export namespace Compiler {
             }
             // If not a primitive, the possible types are specified in what is effectively a map of options.
             case "list":
+            case "listOrElement":
                 return ts.parseListTypeOption(option, value, errors);
             default:
                 return ts.parseCustomTypeOption(option as ts.CommandLineOptionOfCustomType, value, errors);
diff --git a/src/services/getEditsForFileRename.ts b/src/services/getEditsForFileRename.ts
index 4d87384d5a4e4..ef3a1107fca0b 100644
--- a/src/services/getEditsForFileRename.ts
+++ b/src/services/getEditsForFileRename.ts
@@ -120,7 +120,7 @@ function updateTsconfigFiles(program: Program, changeTracker: textChanges.Change
                 if (foundExactMatch || propertyName !== "include" || !isArrayLiteralExpression(property.initializer)) return;
                 const includes = mapDefined(property.initializer.elements, e => isStringLiteral(e) ? e.text : undefined);
                 if (includes.length === 0) return;
-                const matchers = getFileMatcherPatterns(configDir, /*excludes*/ [], includes, useCaseSensitiveFileNames, currentDirectory);
+                const matchers = getFileMatcherPatterns(configDir, /*excludes*/[], includes, useCaseSensitiveFileNames, currentDirectory);
                 // If there isn't some include for this, add a new one.
                 if (getRegexFromPattern(Debug.checkDefined(matchers.includeFilePattern), useCaseSensitiveFileNames).test(oldFileOrDirPath) &&
                     !getRegexFromPattern(Debug.checkDefined(matchers.includeFilePattern), useCaseSensitiveFileNames).test(newFileOrDirPath)) {
@@ -131,6 +131,7 @@ function updateTsconfigFiles(program: Program, changeTracker: textChanges.Change
             case "compilerOptions":
                 forEachProperty(property.initializer, (property, propertyName) => {
                     const option = getOptionFromName(propertyName);
+                    Debug.assert(option?.type !== "listOrElement");
                     if (option && (option.isFilePath || option.type === "list" && option.element.isFilePath)) {
                         updatePaths(property);
                     }
diff --git a/src/testRunner/unittests/config/configurationExtension.ts b/src/testRunner/unittests/config/configurationExtension.ts
index 8434eb350cf60..8677621a6e514 100644
--- a/src/testRunner/unittests/config/configurationExtension.ts
+++ b/src/testRunner/unittests/config/configurationExtension.ts
@@ -186,7 +186,47 @@ function createFileSystem(ignoreCase: boolean, cwd: string, root: string) {
                 "dev/tests/unit/spec.ts": "",
                 "dev/tests/utils.ts": "",
                 "dev/tests/scenarios/first.json": "",
-                "dev/tests/baselines/first/output.ts": ""
+                "dev/tests/baselines/first/output.ts": "",
+                    "dev/configs/extendsArrayFirst.json": JSON.stringify({
+                        compilerOptions: {
+                            allowJs: true,
+                            noImplicitAny: true,
+                            strictNullChecks: true
+                        }
+                    }),
+                    "dev/configs/extendsArraySecond.json": JSON.stringify({
+                        compilerOptions: {
+                            module: "amd"
+                        },
+                        include: ["../supplemental.*"]
+                    }),
+                    "dev/configs/extendsArrayThird.json": JSON.stringify({
+                        compilerOptions: {
+                            module: null, // eslint-disable-line no-null/no-null
+                            noImplicitAny: false
+                        },
+                        extends: "./extendsArrayFirst",
+                        include: ["../supplemental.*"]
+                    }),
+                    "dev/configs/extendsArrayFourth.json": JSON.stringify({
+                        compilerOptions: {
+                            module: "system",
+                            strictNullChecks: false
+                        },
+                        include: null, // eslint-disable-line no-null/no-null
+                        files: ["../main.ts"]
+                    }),
+                    "dev/configs/extendsArrayFifth.json": JSON.stringify({
+                        extends: ["./extendsArrayFirst", "./extendsArraySecond", "./extendsArrayThird", "./extendsArrayFourth"],
+                        files: [],
+                    }),
+                    "dev/extendsArrayFails.json": JSON.stringify({
+                        extends: ["./missingFile"],
+                        compilerOptions: {
+                            types: []
+                        }
+                    }),
+                    "dev/extendsArrayFails2.json": JSON.stringify({ extends: [42] }),
             }
         }
     });
@@ -295,9 +335,9 @@ describe("unittests:: config:: configurationExtension", () => {
                 messageText: `Unknown option 'excludes'. Did you mean 'exclude'?`
             }]);
 
-            testFailure("can error when 'extends' is not a string", "extends.json", [{
+            testFailure("can error when 'extends' is not a string or Array", "extends.json", [{
                 code: 5024,
-                messageText: `Compiler option 'extends' requires a value of type string.`
+                messageText: `Compiler option 'extends' requires a value of type string or Array.`
             }]);
 
             testSuccess("can overwrite compiler options using extended 'null'", "configs/third.json", {
@@ -352,5 +392,40 @@ describe("unittests:: config:: configurationExtension", () => {
                 assert.deepEqual(sourceFile.extendedSourceFiles, expected);
             });
         });
+
+        describe(testName, () => {
+            it("adds extendedSourceFiles from an array only once", () => {
+                const sourceFile = ts.readJsonConfigFile("configs/extendsArrayFifth.json", (path) => host.readFile(path));
+                const dir = ts.combinePaths(basePath, "configs");
+                const expected = [
+                    ts.combinePaths(dir, "extendsArrayFirst.json"),
+                    ts.combinePaths(dir, "extendsArraySecond.json"),
+                    ts.combinePaths(dir, "extendsArrayThird.json"),
+                    ts.combinePaths(dir, "extendsArrayFourth.json"),
+                ];
+                ts.parseJsonSourceFileConfigFileContent(sourceFile, host, dir, {}, "extendsArrayFifth.json");
+                assert.deepEqual(sourceFile.extendedSourceFiles, expected);
+                ts.parseJsonSourceFileConfigFileContent(sourceFile, host, dir, {}, "extendsArrayFifth.json");
+                assert.deepEqual(sourceFile.extendedSourceFiles, expected);
+            });
+
+            testSuccess("can overwrite top-level compilerOptions", "configs/extendsArrayFifth.json", {
+                allowJs: true,
+                noImplicitAny: false,
+                strictNullChecks: false,
+                module: ts.ModuleKind.System
+            }, []);
+
+            testFailure("can report missing configurations", "extendsArrayFails.json", [{
+                code: 6053,
+                messageText: `File './missingFile' not found.`
+            }]);
+
+            testFailure("can error when 'extends' is not a string or Array2", "extendsArrayFails2.json", [{
+                code: 5024,
+                messageText: `Compiler option 'extends' requires a value of type string.`
+            }]);
+        });
     });
 });
+
diff --git a/src/testRunner/unittests/config/showConfig.ts b/src/testRunner/unittests/config/showConfig.ts
index 438bee2f111ee..a815ec7d7750c 100644
--- a/src/testRunner/unittests/config/showConfig.ts
+++ b/src/testRunner/unittests/config/showConfig.ts
@@ -147,6 +147,10 @@ describe("unittests:: config:: showConfig", () => {
                 }
                 break;
             }
+            case "listOrElement": {
+                ts.Debug.fail();
+                break;
+            }
             case "string": {
                 if (option.isTSConfigOnly) {
                     args = ["-p", "tsconfig.json"];
diff --git a/src/testRunner/unittests/tsbuildWatch/programUpdates.ts b/src/testRunner/unittests/tsbuildWatch/programUpdates.ts
index aa385da701dfa..b4498d1f7e5f9 100644
--- a/src/testRunner/unittests/tsbuildWatch/programUpdates.ts
+++ b/src/testRunner/unittests/tsbuildWatch/programUpdates.ts
@@ -597,7 +597,7 @@ export function someFn() { }`),
     verifyTscWatch({
         scenario: "programUpdates",
         subScenario: "works with extended source files",
-        commandLineArgs: ["-b", "-w", "-v", "project1.tsconfig.json", "project2.tsconfig.json"],
+        commandLineArgs: ["-b", "-w", "-v", "project1.tsconfig.json", "project2.tsconfig.json", "project3.tsconfig.json"],
         sys: () => {
             const alphaExtendedConfigFile: File = {
                 path: "/a/b/alpha.tsconfig.json",
@@ -633,10 +633,49 @@ export function someFn() { }`),
                     files: [otherFile.path]
                 })
             };
+            const otherFile2: File = {
+                path: "/a/b/other2.ts",
+                content: "let k = 0;",
+            };
+            const extendsConfigFile1: File = {
+                path: "/a/b/extendsConfig1.tsconfig.json",
+                content: JSON.stringify({
+                    compilerOptions: {
+                        composite: true,
+                    }
+                })
+            };
+            const extendsConfigFile2: File = {
+                path: "/a/b/extendsConfig2.tsconfig.json",
+                content: JSON.stringify({
+                    compilerOptions: {
+                        strictNullChecks: false,
+                    }
+                })
+            };
+            const extendsConfigFile3: File = {
+                path: "/a/b/extendsConfig3.tsconfig.json",
+                content: JSON.stringify({
+                    compilerOptions: {
+                        noImplicitAny: true,
+                    }
+                })
+            };
+            const project3Config: File = {
+                path: "/a/b/project3.tsconfig.json",
+                content: JSON.stringify({
+                    extends: ["./extendsConfig1.tsconfig.json", "./extendsConfig2.tsconfig.json", "./extendsConfig3.tsconfig.json"],
+                    compilerOptions: {
+                        composite: false,
+                    },
+                    files: [otherFile2.path]
+                })
+            };
             return createWatchedSystem([
                 libFile,
                 alphaExtendedConfigFile, project1Config, commonFile1, commonFile2,
-                bravoExtendedConfigFile, project2Config, otherFile
+                bravoExtendedConfigFile, project2Config, otherFile, otherFile2,
+                    extendsConfigFile1, extendsConfigFile2, extendsConfigFile3, project3Config
             ], { currentDirectory: "/a/b" });
         },
         edits: [
@@ -689,7 +728,37 @@ export function someFn() { }`),
                     sys.checkTimeoutQueueLength(0);
                 },
             },
-        ]
+            {
+                caption: "Modify extendsConfigFile2",
+                edit: sys => sys.writeFile("/a/b/extendsConfig2.tsconfig.json", JSON.stringify({
+                    compilerOptions: { strictNullChecks: true }
+                })),
+                timeouts: sys => { // Build project3
+                    sys.checkTimeoutQueueLengthAndRun(1);
+                    sys.checkTimeoutQueueLength(0);
+                },
+            },
+            {
+                caption: "Modify project 3",
+                edit: sys => sys.writeFile("/a/b/project3.tsconfig.json", JSON.stringify({
+                    extends: ["./extendsConfig1.tsconfig.json", "./extendsConfig2.tsconfig.json"],
+                    compilerOptions: { composite: false },
+                    files: ["/a/b/other2.ts"]
+                })),
+                timeouts: sys => { // Build project3
+                    sys.checkTimeoutQueueLengthAndRun(1);
+                    sys.checkTimeoutQueueLength(0);
+                },
+            },
+            {
+                caption: "Delete extendedConfigFile2 and report error",
+                edit: sys => sys.deleteFile("./extendsConfig2.tsconfig.json"),
+                timeouts: sys => { // Build project3
+                    sys.checkTimeoutQueueLengthAndRun(1);
+                    sys.checkTimeoutQueueLength(0);
+                },
+            }
+        ],
     });
 
     verifyTscWatch({
diff --git a/tests/baselines/reference/api/tsserverlibrary.d.ts b/tests/baselines/reference/api/tsserverlibrary.d.ts
index be4bf4cb7a4d7..ed1562bcdeac1 100644
--- a/tests/baselines/reference/api/tsserverlibrary.d.ts
+++ b/tests/baselines/reference/api/tsserverlibrary.d.ts
@@ -9170,7 +9170,7 @@ declare namespace ts {
         /**
          * Note that the case of the config path has not yet been normalized, as no files have been imported into the project yet
          */
-        extendedConfigPath?: string;
+        extendedConfigPath?: string | string[];
     }
     interface ExtendedConfigCacheEntry {
         extendedResult: TsConfigSourceFile;
diff --git a/tests/baselines/reference/api/typescript.d.ts b/tests/baselines/reference/api/typescript.d.ts
index 8ddf62571c5a9..853c3294bead9 100644
--- a/tests/baselines/reference/api/typescript.d.ts
+++ b/tests/baselines/reference/api/typescript.d.ts
@@ -5236,7 +5236,7 @@ declare namespace ts {
         /**
          * Note that the case of the config path has not yet been normalized, as no files have been imported into the project yet
          */
-        extendedConfigPath?: string;
+        extendedConfigPath?: string | string[];
     }
     interface ExtendedConfigCacheEntry {
         extendedResult: TsConfigSourceFile;
diff --git a/tests/baselines/reference/configFileExtendsAsList.errors.txt b/tests/baselines/reference/configFileExtendsAsList.errors.txt
new file mode 100644
index 0000000000000..1accde5ea2632
--- /dev/null
+++ b/tests/baselines/reference/configFileExtendsAsList.errors.txt
@@ -0,0 +1,31 @@
+/index.ts(1,12): error TS7006: Parameter 'x' implicitly has an 'any' type.
+/index.ts(3,1): error TS2454: Variable 'y' is used before being assigned.
+
+
+==== /tsconfig.json (0 errors) ====
+    {
+        "extends": ["./tsconfig1.json", "./tsconfig2.json"]
+    }
+    
+==== /tsconfig1.json (0 errors) ====
+    {
+        "compilerOptions": {
+            "strictNullChecks": true
+        }
+    }
+    
+==== /tsconfig2.json (0 errors) ====
+    {
+        "compilerOptions": {
+            "noImplicitAny": true
+        }
+    }
+    
+==== /index.ts (2 errors) ====
+    function f(x) { } // noImplicitAny error
+               ~
+!!! error TS7006: Parameter 'x' implicitly has an 'any' type.
+    let y: string;
+    y.toLowerCase(); // strictNullChecks error
+    ~
+!!! error TS2454: Variable 'y' is used before being assigned.
\ No newline at end of file
diff --git a/tests/baselines/reference/configFileExtendsAsList.js b/tests/baselines/reference/configFileExtendsAsList.js
new file mode 100644
index 0000000000000..e805b7a293656
--- /dev/null
+++ b/tests/baselines/reference/configFileExtendsAsList.js
@@ -0,0 +1,25 @@
+//// [tests/cases/compiler/configFileExtendsAsList.ts] ////
+
+//// [tsconfig1.json]
+{
+    "compilerOptions": {
+        "strictNullChecks": true
+    }
+}
+
+//// [tsconfig2.json]
+{
+    "compilerOptions": {
+        "noImplicitAny": true
+    }
+}
+
+//// [index.ts]
+function f(x) { } // noImplicitAny error
+let y: string;
+y.toLowerCase(); // strictNullChecks error
+
+//// [index.js]
+function f(x) { } // noImplicitAny error
+var y;
+y.toLowerCase(); // strictNullChecks error
diff --git a/tests/baselines/reference/configFileExtendsAsList.symbols b/tests/baselines/reference/configFileExtendsAsList.symbols
new file mode 100644
index 0000000000000..97b24a733694e
--- /dev/null
+++ b/tests/baselines/reference/configFileExtendsAsList.symbols
@@ -0,0 +1,13 @@
+=== /index.ts ===
+function f(x) { } // noImplicitAny error
+>f : Symbol(f, Decl(index.ts, 0, 0))
+>x : Symbol(x, Decl(index.ts, 0, 11))
+
+let y: string;
+>y : Symbol(y, Decl(index.ts, 1, 3))
+
+y.toLowerCase(); // strictNullChecks error
+>y.toLowerCase : Symbol(String.toLowerCase, Decl(lib.es5.d.ts, --, --))
+>y : Symbol(y, Decl(index.ts, 1, 3))
+>toLowerCase : Symbol(String.toLowerCase, Decl(lib.es5.d.ts, --, --))
+
diff --git a/tests/baselines/reference/configFileExtendsAsList.types b/tests/baselines/reference/configFileExtendsAsList.types
new file mode 100644
index 0000000000000..dfa1eadf113be
--- /dev/null
+++ b/tests/baselines/reference/configFileExtendsAsList.types
@@ -0,0 +1,14 @@
+=== /index.ts ===
+function f(x) { } // noImplicitAny error
+>f : (x: any) => void
+>x : any
+
+let y: string;
+>y : string
+
+y.toLowerCase(); // strictNullChecks error
+>y.toLowerCase() : string
+>y.toLowerCase : () => string
+>y : string
+>toLowerCase : () => string
+
diff --git a/tests/baselines/reference/tsbuildWatch/programUpdates/works-with-extended-source-files.js b/tests/baselines/reference/tsbuildWatch/programUpdates/works-with-extended-source-files.js
index 14f5b0f8e6420..38aa01cf7af96 100644
--- a/tests/baselines/reference/tsbuildWatch/programUpdates/works-with-extended-source-files.js
+++ b/tests/baselines/reference/tsbuildWatch/programUpdates/works-with-extended-source-files.js
@@ -33,25 +33,45 @@ let y = 1
 //// [/a/b/other.ts]
 let z = 0;
 
+//// [/a/b/other2.ts]
+let k = 0;
 
-/a/lib/tsc.js -b -w -v project1.tsconfig.json project2.tsconfig.json
+//// [/a/b/extendsConfig1.tsconfig.json]
+{"compilerOptions":{"composite":true}}
+
+//// [/a/b/extendsConfig2.tsconfig.json]
+{"compilerOptions":{"strictNullChecks":false}}
+
+//// [/a/b/extendsConfig3.tsconfig.json]
+{"compilerOptions":{"noImplicitAny":true}}
+
+//// [/a/b/project3.tsconfig.json]
+{"extends":["./extendsConfig1.tsconfig.json","./extendsConfig2.tsconfig.json","./extendsConfig3.tsconfig.json"],"compilerOptions":{"composite":false},"files":["/a/b/other2.ts"]}
+
+
+/a/lib/tsc.js -b -w -v project1.tsconfig.json project2.tsconfig.json project3.tsconfig.json
 Output::
 >> Screen clear
-[12:00:25 AM] Starting compilation in watch mode...
+[12:00:35 AM] Starting compilation in watch mode...
 
-[12:00:26 AM] Projects in this build: 
+[12:00:36 AM] Projects in this build: 
     * project1.tsconfig.json
-    * project2.tsconfig.json
+    * project2.tsconfig.json
+    * project3.tsconfig.json
+
+[12:00:37 AM] Project 'project1.tsconfig.json' is out of date because output file 'project1.tsconfig.tsbuildinfo' does not exist
+
+[12:00:38 AM] Building project '/a/b/project1.tsconfig.json'...
 
-[12:00:27 AM] Project 'project1.tsconfig.json' is out of date because output file 'project1.tsconfig.tsbuildinfo' does not exist
+[12:00:52 AM] Project 'project2.tsconfig.json' is out of date because output file 'project2.tsconfig.tsbuildinfo' does not exist
 
-[12:00:28 AM] Building project '/a/b/project1.tsconfig.json'...
+[12:00:53 AM] Building project '/a/b/project2.tsconfig.json'...
 
-[12:00:42 AM] Project 'project2.tsconfig.json' is out of date because output file 'project2.tsconfig.tsbuildinfo' does not exist
+[12:01:03 AM] Project 'project3.tsconfig.json' is out of date because output file 'other2.js' does not exist
 
-[12:00:43 AM] Building project '/a/b/project2.tsconfig.json'...
+[12:01:04 AM] Building project '/a/b/project3.tsconfig.json'...
 
-[12:00:53 AM] Found 0 errors. Watching for file changes.
+[12:01:08 AM] Found 0 errors. Watching for file changes.
 
 
 
@@ -88,6 +108,21 @@ Shape signatures in builder refreshed for::
 /a/lib/lib.d.ts (used version)
 /a/b/other.ts (computed .d.ts during emit)
 
+Program root files: ["/a/b/other2.ts"]
+Program options: {"composite":false,"strictNullChecks":false,"noImplicitAny":true,"watch":true,"configFilePath":"/a/b/project3.tsconfig.json"}
+Program structureReused: Not
+Program files::
+/a/lib/lib.d.ts
+/a/b/other2.ts
+
+Semantic diagnostics in builder refreshed for::
+/a/lib/lib.d.ts
+/a/b/other2.ts
+
+Shape signatures in builder refreshed for::
+/a/lib/lib.d.ts (used version)
+/a/b/other2.ts (used version)
+
 PolledWatches::
 
 FsWatches::
@@ -105,6 +140,16 @@ FsWatches::
   {}
 /a/b/other.ts:
   {}
+/a/b/project3.tsconfig.json:
+  {}
+/a/b/extendsconfig1.tsconfig.json:
+  {}
+/a/b/extendsconfig2.tsconfig.json:
+  {}
+/a/b/extendsconfig3.tsconfig.json:
+  {}
+/a/b/other2.ts:
+  {}
 
 FsWatchesRecursive::
 
@@ -238,6 +283,10 @@ declare let z: number;
   "size": 755
 }
 
+//// [/a/b/other2.js]
+var k = 0;
+
+
 
 Change:: Modify alpha config
 
@@ -248,11 +297,11 @@ Input::
 
 Output::
 >> Screen clear
-[12:00:57 AM] File change detected. Starting incremental compilation...
+[12:01:12 AM] File change detected. Starting incremental compilation...
 
-[12:00:58 AM] Project 'project1.tsconfig.json' is out of date because output 'project1.tsconfig.tsbuildinfo' is older than input 'alpha.tsconfig.json'
+[12:01:13 AM] Project 'project1.tsconfig.json' is out of date because output 'project1.tsconfig.tsbuildinfo' is older than input 'alpha.tsconfig.json'
 
-[12:00:59 AM] Building project '/a/b/project1.tsconfig.json'...
+[12:01:14 AM] Building project '/a/b/project1.tsconfig.json'...
 
 
 
@@ -288,6 +337,16 @@ FsWatches::
   {}
 /a/b/other.ts:
   {}
+/a/b/project3.tsconfig.json:
+  {}
+/a/b/extendsconfig1.tsconfig.json:
+  {}
+/a/b/extendsconfig2.tsconfig.json:
+  {}
+/a/b/extendsconfig3.tsconfig.json:
+  {}
+/a/b/other2.ts:
+  {}
 
 FsWatchesRecursive::
 
@@ -368,11 +427,11 @@ Change:: Build project 2
 Input::
 
 Output::
-[12:01:13 AM] Project 'project2.tsconfig.json' is out of date because output 'project2.tsconfig.tsbuildinfo' is older than input 'alpha.tsconfig.json'
+[12:01:28 AM] Project 'project2.tsconfig.json' is out of date because output 'project2.tsconfig.tsbuildinfo' is older than input 'alpha.tsconfig.json'
 
-[12:01:14 AM] Building project '/a/b/project2.tsconfig.json'...
+[12:01:29 AM] Building project '/a/b/project2.tsconfig.json'...
 
-[12:01:25 AM] Found 0 errors. Watching for file changes.
+[12:01:40 AM] Found 0 errors. Watching for file changes.
 
 
 
@@ -406,6 +465,16 @@ FsWatches::
   {}
 /a/b/other.ts:
   {}
+/a/b/project3.tsconfig.json:
+  {}
+/a/b/extendsconfig1.tsconfig.json:
+  {}
+/a/b/extendsconfig2.tsconfig.json:
+  {}
+/a/b/extendsconfig3.tsconfig.json:
+  {}
+/a/b/other2.ts:
+  {}
 
 FsWatchesRecursive::
 
@@ -473,13 +542,13 @@ Input::
 
 Output::
 >> Screen clear
-[12:01:29 AM] File change detected. Starting incremental compilation...
+[12:01:44 AM] File change detected. Starting incremental compilation...
 
-[12:01:30 AM] Project 'project2.tsconfig.json' is out of date because output 'project2.tsconfig.tsbuildinfo' is older than input 'bravo.tsconfig.json'
+[12:01:45 AM] Project 'project2.tsconfig.json' is out of date because output 'project2.tsconfig.tsbuildinfo' is older than input 'bravo.tsconfig.json'
 
-[12:01:31 AM] Building project '/a/b/project2.tsconfig.json'...
+[12:01:46 AM] Building project '/a/b/project2.tsconfig.json'...
 
-[12:01:42 AM] Found 0 errors. Watching for file changes.
+[12:01:57 AM] Found 0 errors. Watching for file changes.
 
 
 
@@ -513,6 +582,16 @@ FsWatches::
   {}
 /a/b/other.ts:
   {}
+/a/b/project3.tsconfig.json:
+  {}
+/a/b/extendsconfig1.tsconfig.json:
+  {}
+/a/b/extendsconfig2.tsconfig.json:
+  {}
+/a/b/extendsconfig3.tsconfig.json:
+  {}
+/a/b/other2.ts:
+  {}
 
 FsWatchesRecursive::
 
@@ -579,17 +658,17 @@ Input::
 
 Output::
 >> Screen clear
-[12:01:46 AM] File change detected. Starting incremental compilation...
+[12:02:01 AM] File change detected. Starting incremental compilation...
 
-[12:01:47 AM] Project 'project2.tsconfig.json' is out of date because output 'commonFile1.js' is older than input 'project2.tsconfig.json'
+[12:02:02 AM] Project 'project2.tsconfig.json' is out of date because output 'other2.js' is older than input 'project2.tsconfig.json'
 
-[12:01:48 AM] Building project '/a/b/project2.tsconfig.json'...
+[12:02:03 AM] Building project '/a/b/project2.tsconfig.json'...
 
-[12:01:59 AM] Found 0 errors. Watching for file changes.
+[12:02:17 AM] Found 0 errors. Watching for file changes.
 
 
 
-Program root files: ["/a/b/commonFile1.ts","/a/b/commonFile2.ts","/a/b/other.ts"]
+Program root files: ["/a/b/commonFile1.ts","/a/b/commonFile2.ts","/a/b/other.ts","/a/b/other2.ts"]
 Program options: {"strict":true,"watch":true,"configFilePath":"/a/b/project2.tsconfig.json"}
 Program structureReused: Not
 Program files::
@@ -597,17 +676,20 @@ Program files::
 /a/b/commonFile1.ts
 /a/b/commonFile2.ts
 /a/b/other.ts
+/a/b/other2.ts
 
 Semantic diagnostics in builder refreshed for::
 /a/lib/lib.d.ts
 /a/b/commonFile1.ts
 /a/b/commonFile2.ts
 /a/b/other.ts
+/a/b/other2.ts
 
 Shape signatures in builder refreshed for::
 /a/b/commonfile1.ts (computed .d.ts)
 /a/b/commonfile2.ts (computed .d.ts)
 /a/b/other.ts (computed .d.ts)
+/a/b/other2.ts (computed .d.ts)
 
 PolledWatches::
 
@@ -624,6 +706,16 @@ FsWatches::
   {}
 /a/b/other.ts:
   {}
+/a/b/project3.tsconfig.json:
+  {}
+/a/b/extendsconfig1.tsconfig.json:
+  {}
+/a/b/extendsconfig2.tsconfig.json:
+  {}
+/a/b/extendsconfig3.tsconfig.json:
+  {}
+/a/b/other2.ts:
+  {}
 
 FsWatchesRecursive::
 /a/b:
@@ -638,6 +730,11 @@ exitCode:: ExitStatus.undefined
 var z = 0;
 
 
+//// [/a/b/other2.js]
+"use strict";
+var k = 0;
+
+
 
 Change:: update aplha config
 
@@ -648,11 +745,11 @@ Input::
 
 Output::
 >> Screen clear
-[12:02:04 AM] File change detected. Starting incremental compilation...
+[12:02:22 AM] File change detected. Starting incremental compilation...
 
-[12:02:05 AM] Project 'project1.tsconfig.json' is out of date because output 'project1.tsconfig.tsbuildinfo' is older than input 'alpha.tsconfig.json'
+[12:02:23 AM] Project 'project1.tsconfig.json' is out of date because output 'project1.tsconfig.tsbuildinfo' is older than input 'alpha.tsconfig.json'
 
-[12:02:06 AM] Building project '/a/b/project1.tsconfig.json'...
+[12:02:24 AM] Building project '/a/b/project1.tsconfig.json'...
 
 
 
@@ -686,6 +783,16 @@ FsWatches::
   {}
 /a/b/other.ts:
   {}
+/a/b/project3.tsconfig.json:
+  {}
+/a/b/extendsconfig1.tsconfig.json:
+  {}
+/a/b/extendsconfig2.tsconfig.json:
+  {}
+/a/b/extendsconfig3.tsconfig.json:
+  {}
+/a/b/other2.ts:
+  {}
 
 FsWatchesRecursive::
 /a/b:
@@ -765,15 +872,15 @@ Change:: Build project 2
 Input::
 
 Output::
-[12:02:20 AM] Project 'project2.tsconfig.json' is out of date because output 'commonFile1.js' is older than input 'alpha.tsconfig.json'
+[12:02:38 AM] Project 'project2.tsconfig.json' is out of date because output 'commonFile1.js' is older than input 'alpha.tsconfig.json'
 
-[12:02:21 AM] Building project '/a/b/project2.tsconfig.json'...
+[12:02:39 AM] Building project '/a/b/project2.tsconfig.json'...
 
-[12:02:32 AM] Found 0 errors. Watching for file changes.
+[12:02:53 AM] Found 0 errors. Watching for file changes.
 
 
 
-Program root files: ["/a/b/commonFile1.ts","/a/b/commonFile2.ts","/a/b/other.ts"]
+Program root files: ["/a/b/commonFile1.ts","/a/b/commonFile2.ts","/a/b/other.ts","/a/b/other2.ts"]
 Program options: {"watch":true,"configFilePath":"/a/b/project2.tsconfig.json"}
 Program structureReused: Not
 Program files::
@@ -781,12 +888,14 @@ Program files::
 /a/b/commonFile1.ts
 /a/b/commonFile2.ts
 /a/b/other.ts
+/a/b/other2.ts
 
 Semantic diagnostics in builder refreshed for::
 /a/lib/lib.d.ts
 /a/b/commonFile1.ts
 /a/b/commonFile2.ts
 /a/b/other.ts
+/a/b/other2.ts
 
 No shapes updated in the builder::
 
@@ -805,6 +914,16 @@ FsWatches::
   {}
 /a/b/other.ts:
   {}
+/a/b/project3.tsconfig.json:
+  {}
+/a/b/extendsconfig1.tsconfig.json:
+  {}
+/a/b/extendsconfig2.tsconfig.json:
+  {}
+/a/b/extendsconfig3.tsconfig.json:
+  {}
+/a/b/other2.ts:
+  {}
 
 FsWatchesRecursive::
 /a/b:
@@ -818,3 +937,189 @@ exitCode:: ExitStatus.undefined
 var z = 0;
 
 
+//// [/a/b/other2.js]
+var k = 0;
+
+
+
+Change:: Modify extendsConfigFile2
+
+Input::
+//// [/a/b/extendsConfig2.tsconfig.json]
+{"compilerOptions":{"strictNullChecks":true}}
+
+
+Output::
+>> Screen clear
+[12:02:57 AM] File change detected. Starting incremental compilation...
+
+[12:02:58 AM] Project 'project3.tsconfig.json' is out of date because output 'other2.js' is older than input 'extendsConfig2.tsconfig.json'
+
+[12:02:59 AM] Building project '/a/b/project3.tsconfig.json'...
+
+[12:03:00 AM] Updating unchanged output timestamps of project '/a/b/project3.tsconfig.json'...
+
+[12:03:02 AM] Found 0 errors. Watching for file changes.
+
+
+
+Program root files: ["/a/b/other2.ts"]
+Program options: {"composite":false,"strictNullChecks":true,"noImplicitAny":true,"watch":true,"configFilePath":"/a/b/project3.tsconfig.json"}
+Program structureReused: Not
+Program files::
+/a/lib/lib.d.ts
+/a/b/other2.ts
+
+Semantic diagnostics in builder refreshed for::
+/a/lib/lib.d.ts
+/a/b/other2.ts
+
+No shapes updated in the builder::
+
+PolledWatches::
+
+FsWatches::
+/a/b/project1.tsconfig.json:
+  {}
+/a/b/alpha.tsconfig.json:
+  {}
+/a/b/commonfile1.ts:
+  {}
+/a/b/commonfile2.ts:
+  {}
+/a/b/project2.tsconfig.json:
+  {}
+/a/b/other.ts:
+  {}
+/a/b/project3.tsconfig.json:
+  {}
+/a/b/extendsconfig1.tsconfig.json:
+  {}
+/a/b/extendsconfig2.tsconfig.json:
+  {}
+/a/b/extendsconfig3.tsconfig.json:
+  {}
+/a/b/other2.ts:
+  {}
+
+FsWatchesRecursive::
+/a/b:
+  {}
+
+exitCode:: ExitStatus.undefined
+
+//// [/a/b/other2.js] file changed its modified time
+
+Change:: Modify project 3
+
+Input::
+//// [/a/b/project3.tsconfig.json]
+{"extends":["./extendsConfig1.tsconfig.json","./extendsConfig2.tsconfig.json"],"compilerOptions":{"composite":false},"files":["/a/b/other2.ts"]}
+
+
+Output::
+>> Screen clear
+[12:03:06 AM] File change detected. Starting incremental compilation...
+
+[12:03:07 AM] Project 'project3.tsconfig.json' is out of date because output 'other2.js' is older than input 'project3.tsconfig.json'
+
+[12:03:08 AM] Building project '/a/b/project3.tsconfig.json'...
+
+[12:03:09 AM] Updating unchanged output timestamps of project '/a/b/project3.tsconfig.json'...
+
+[12:03:11 AM] Found 0 errors. Watching for file changes.
+
+
+
+Program root files: ["/a/b/other2.ts"]
+Program options: {"composite":false,"strictNullChecks":true,"watch":true,"configFilePath":"/a/b/project3.tsconfig.json"}
+Program structureReused: Not
+Program files::
+/a/lib/lib.d.ts
+/a/b/other2.ts
+
+Semantic diagnostics in builder refreshed for::
+/a/lib/lib.d.ts
+/a/b/other2.ts
+
+No shapes updated in the builder::
+
+PolledWatches::
+
+FsWatches::
+/a/b/project1.tsconfig.json:
+  {}
+/a/b/alpha.tsconfig.json:
+  {}
+/a/b/commonfile1.ts:
+  {}
+/a/b/commonfile2.ts:
+  {}
+/a/b/project2.tsconfig.json:
+  {}
+/a/b/other.ts:
+  {}
+/a/b/project3.tsconfig.json:
+  {}
+/a/b/extendsconfig1.tsconfig.json:
+  {}
+/a/b/extendsconfig2.tsconfig.json:
+  {}
+/a/b/other2.ts:
+  {}
+
+FsWatchesRecursive::
+/a/b:
+  {}
+
+exitCode:: ExitStatus.undefined
+
+//// [/a/b/other2.js] file changed its modified time
+
+Change:: Delete extendedConfigFile2 and report error
+
+Input::
+//// [/a/b/extendsConfig2.tsconfig.json] deleted
+
+Output::
+>> Screen clear
+[12:03:13 AM] File change detected. Starting incremental compilation...
+
+[12:03:14 AM] Project 'project3.tsconfig.json' is up to date because newest input 'other2.ts' is older than output 'other2.js'
+
+error TS5083: Cannot read file '/a/b/extendsConfig2.tsconfig.json'.
+
+[12:03:15 AM] Found 1 error. Watching for file changes.
+
+
+
+PolledWatches::
+
+FsWatches::
+/a/b/project1.tsconfig.json:
+  {}
+/a/b/alpha.tsconfig.json:
+  {}
+/a/b/commonfile1.ts:
+  {}
+/a/b/commonfile2.ts:
+  {}
+/a/b/project2.tsconfig.json:
+  {}
+/a/b/other.ts:
+  {}
+/a/b/project3.tsconfig.json:
+  {}
+/a/b/extendsconfig1.tsconfig.json:
+  {}
+/a/b/extendsconfig2.tsconfig.json:
+  {}
+/a/b/other2.ts:
+  {}
+
+FsWatchesRecursive::
+/a/b:
+  {}
+
+exitCode:: ExitStatus.undefined
+
diff --git a/tests/cases/compiler/configFileExtendsAsList.ts b/tests/cases/compiler/configFileExtendsAsList.ts
new file mode 100644
index 0000000000000..8b6e1cf971544
--- /dev/null
+++ b/tests/cases/compiler/configFileExtendsAsList.ts
@@ -0,0 +1,23 @@
+// @Filename: /tsconfig1.json
+{
+    "compilerOptions": {
+        "strictNullChecks": true
+    }
+}
+
+// @Filename: /tsconfig2.json
+{
+    "compilerOptions": {
+        "noImplicitAny": true
+    }
+}
+
+// @Filename: /tsconfig.json
+{
+    "extends": ["./tsconfig1.json", "./tsconfig2.json"]
+}
+
+// @Filename: /index.ts
+function f(x) { } // noImplicitAny error
+let y: string;
+y.toLowerCase(); // strictNullChecks error
\ No newline at end of file