diff --git a/tests/cases/fourslash/insertReturnStatementInDuplicateIdentifierFunction.ts b/tests/cases/fourslash/insertReturnStatementInDuplicateIdentifierFunction.ts
index 76e2db2fa00..78d06459f27 100644
--- a/tests/cases/fourslash/insertReturnStatementInDuplicateIdentifierFunction.ts
+++ b/tests/cases/fourslash/insertReturnStatementInDuplicateIdentifierFunction.ts
@@ -1,5 +1,6 @@
 /// <reference path="fourslash.ts" />
 
+// @strict: true
 //// class foo { };
 //// function foo() { /**/ }
 
diff --git a/tests/cases/fourslash/jsDocFunctionSignatures5.ts b/tests/cases/fourslash/jsDocFunctionSignatures5.ts
index 61b7b381be0..0fa54b5c771 100644
--- a/tests/cases/fourslash/jsDocFunctionSignatures5.ts
+++ b/tests/cases/fourslash/jsDocFunctionSignatures5.ts
@@ -1,4 +1,5 @@
 ///<reference path="fourslash.ts" />
+// @strict: true
 // @allowJs: true
 // @Filename: Foo.js
 
diff --git a/tests/cases/fourslash/jsDocTypeTagQuickInfo1.ts b/tests/cases/fourslash/jsDocTypeTagQuickInfo1.ts
index 612e8d3ade0..08715f00091 100644
--- a/tests/cases/fourslash/jsDocTypeTagQuickInfo1.ts
+++ b/tests/cases/fourslash/jsDocTypeTagQuickInfo1.ts
@@ -1,5 +1,6 @@
 /// <reference path="fourslash.ts" />
 // @lib: es5
+// @strict: true
 // @allowJs: true
 // @Filename: jsDocTypeTag1.js
 //// /** @type {String} */
diff --git a/tests/cases/fourslash/jsDocTypeTagQuickInfo2.ts b/tests/cases/fourslash/jsDocTypeTagQuickInfo2.ts
index 67a7dcb2682..d576ef95f4e 100644
--- a/tests/cases/fourslash/jsDocTypeTagQuickInfo2.ts
+++ b/tests/cases/fourslash/jsDocTypeTagQuickInfo2.ts
@@ -1,5 +1,6 @@
 /// <reference path="fourslash.ts" />
 // @lib: es5
+// @strict: true
 // @allowJs: true
 // @Filename: jsDocTypeTag2.js
 //// /** @type {string} */