diff --git a/src/compiler/types.ts b/src/compiler/types.ts
index 4b2460e2630..1850501e546 100644
--- a/src/compiler/types.ts
+++ b/src/compiler/types.ts
@@ -1708,7 +1708,7 @@ export interface AutoGenerateInfo {
 
 /** @internal */
 export interface GeneratedIdentifier extends Identifier {
-    readonly emitNode: EmitNode & { autoGenerate: AutoGenerateInfo; };
+    emitNode: EmitNode & { autoGenerate: AutoGenerateInfo; };
 }
 
 export interface QualifiedName extends Node, FlowContainer {
@@ -1787,7 +1787,7 @@ export interface PrivateIdentifier extends PrimaryExpression {
 
 /** @internal */
 export interface GeneratedPrivateIdentifier extends PrivateIdentifier {
-    readonly emitNode: EmitNode & { autoGenerate: AutoGenerateInfo; };
+    emitNode: EmitNode & { autoGenerate: AutoGenerateInfo; };
 }
 
 /** @internal */
diff --git a/src/server/project.ts b/src/server/project.ts
index db1e5307f74..aa3d64e14e9 100644
--- a/src/server/project.ts
+++ b/src/server/project.ts
@@ -346,8 +346,8 @@ export abstract class Project implements LanguageServiceHost, ModuleResolutionHo
 
     public languageServiceEnabled: boolean;
 
-    readonly trace?: (s: string) => void;
-    readonly realpath?: (path: string) => string;
+    trace?(s: string): void;
+    realpath?(path: string): string;
 
     /** @internal */
     hasInvalidatedResolutions?: HasInvalidatedResolutions | undefined;
@@ -513,7 +513,7 @@ export abstract class Project implements LanguageServiceHost, ModuleResolutionHo
     /** @internal */
     createHash = maybeBind(this.projectService.host, this.projectService.host.createHash);
 
-    readonly jsDocParsingMode: JSDocParsingMode | undefined;
+    jsDocParsingMode: JSDocParsingMode | undefined;
 
     /** @internal */
     constructor(
diff --git a/src/server/protocol.ts b/src/server/protocol.ts
index b27fb4d5c3d..46f52d532c4 100644
--- a/src/server/protocol.ts
+++ b/src/server/protocol.ts
@@ -900,15 +900,15 @@ export interface DefinitionRequest extends FileLocationRequest {
 }
 
 export interface DefinitionAndBoundSpanRequest extends FileLocationRequest {
-    readonly command: CommandTypes.DefinitionAndBoundSpan;
+    command: CommandTypes.DefinitionAndBoundSpan;
 }
 
 export interface FindSourceDefinitionRequest extends FileLocationRequest {
-    readonly command: CommandTypes.FindSourceDefinition;
+    command: CommandTypes.FindSourceDefinition;
 }
 
 export interface DefinitionAndBoundSpanResponse extends Response {
-    readonly body: DefinitionInfoAndBoundSpan;
+    body: DefinitionInfoAndBoundSpan;
 }
 
 /** @internal */
@@ -924,7 +924,7 @@ export interface EmitOutputRequestArgs extends FileRequestArgs {
 }
 /** @internal */
 export interface EmitOutputResponse extends Response {
-    readonly body: EmitOutput | ts.EmitOutput;
+    body: EmitOutput | ts.EmitOutput;
 }
 /** @internal */
 export interface EmitOutput {
@@ -1061,18 +1061,18 @@ export interface BraceCompletionRequestArgs extends FileLocationRequestArgs {
 }
 
 export interface JsxClosingTagRequest extends FileLocationRequest {
-    readonly command: CommandTypes.JsxClosingTag;
-    readonly arguments: JsxClosingTagRequestArgs;
+    command: CommandTypes.JsxClosingTag;
+    arguments: JsxClosingTagRequestArgs;
 }
 
 export interface JsxClosingTagRequestArgs extends FileLocationRequestArgs {}
 
 export interface JsxClosingTagResponse extends Response {
-    readonly body: TextInsertion;
+    body: TextInsertion;
 }
 
 export interface LinkedEditingRangeRequest extends FileLocationRequest {
-    readonly command: CommandTypes.LinkedEditingRange;
+    command: CommandTypes.LinkedEditingRange;
 }
 
 export interface LinkedEditingRangesBody {
@@ -1081,7 +1081,7 @@ export interface LinkedEditingRangesBody {
 }
 
 export interface LinkedEditingRangeResponse extends Response {
-    readonly body: LinkedEditingRangesBody;
+    body: LinkedEditingRangesBody;
 }
 
 /**
@@ -1234,13 +1234,13 @@ export interface RenameRequest extends FileLocationRequest {
 
 /** @internal */
 export interface RenameFullRequest extends FileLocationRequest {
-    readonly command: CommandTypes.RenameLocationsFull;
-    readonly arguments: RenameRequestArgs;
+    command: CommandTypes.RenameLocationsFull;
+    arguments: RenameRequestArgs;
 }
 
 /** @internal */
 export interface RenameFullResponse extends Response {
-    readonly body: readonly RenameLocation[];
+    body: readonly RenameLocation[];
 }
 
 /**
@@ -2637,8 +2637,8 @@ export interface LargeFileReferencedEventBody {
 
 export type CreateFileWatcherEventName = "createFileWatcher";
 export interface CreateFileWatcherEvent extends Event {
-    readonly event: CreateFileWatcherEventName;
-    readonly body: CreateFileWatcherEventBody;
+    event: CreateFileWatcherEventName;
+    body: CreateFileWatcherEventBody;
 }
 
 export interface CreateFileWatcherEventBody {
@@ -2648,8 +2648,8 @@ export interface CreateFileWatcherEventBody {
 
 export type CreateDirectoryWatcherEventName = "createDirectoryWatcher";
 export interface CreateDirectoryWatcherEvent extends Event {
-    readonly event: CreateDirectoryWatcherEventName;
-    readonly body: CreateDirectoryWatcherEventBody;
+    event: CreateDirectoryWatcherEventName;
+    body: CreateDirectoryWatcherEventBody;
 }
 
 export interface CreateDirectoryWatcherEventBody {
@@ -2661,8 +2661,8 @@ export interface CreateDirectoryWatcherEventBody {
 
 export type CloseFileWatcherEventName = "closeFileWatcher";
 export interface CloseFileWatcherEvent extends Event {
-    readonly event: CloseFileWatcherEventName;
-    readonly body: CloseFileWatcherEventBody;
+    event: CloseFileWatcherEventName;
+    body: CloseFileWatcherEventBody;
 }
 
 export interface CloseFileWatcherEventBody {
@@ -3050,7 +3050,7 @@ export interface PrepareCallHierarchyRequest extends FileLocationRequest {
 }
 
 export interface PrepareCallHierarchyResponse extends Response {
-    readonly body: CallHierarchyItem | CallHierarchyItem[];
+    body: CallHierarchyItem | CallHierarchyItem[];
 }
 
 export interface ProvideCallHierarchyIncomingCallsRequest extends FileLocationRequest {
@@ -3058,7 +3058,7 @@ export interface ProvideCallHierarchyIncomingCallsRequest extends FileLocationRe
 }
 
 export interface ProvideCallHierarchyIncomingCallsResponse extends Response {
-    readonly body: CallHierarchyIncomingCall[];
+    body: CallHierarchyIncomingCall[];
 }
 
 export interface ProvideCallHierarchyOutgoingCallsRequest extends FileLocationRequest {
@@ -3066,7 +3066,7 @@ export interface ProvideCallHierarchyOutgoingCallsRequest extends FileLocationRe
 }
 
 export interface ProvideCallHierarchyOutgoingCallsResponse extends Response {
-    readonly body: CallHierarchyOutgoingCall[];
+    body: CallHierarchyOutgoingCall[];
 }
 
 export const enum IndentStyle {
diff --git a/tests/cases/compiler/APILibCheck.ts b/tests/cases/compiler/APILibCheck.ts
index 7abf10e9821..d60fba58fa6 100644
--- a/tests/cases/compiler/APILibCheck.ts
+++ b/tests/cases/compiler/APILibCheck.ts
@@ -3,6 +3,7 @@
 // @strictNullChecks: true
 // @lib: es2018
 // @exactOptionalPropertyTypes: true
+// @enforceReadonly: true
 // @noTypesAndSymbols: true
 
 // @filename: node_modules/typescript/package.json