diff --git a/.github/workflows/build-and-test.yml b/.github/workflows/build-and-test.yml
index 9a597938..b5880a2c 100644
--- a/.github/workflows/build-and-test.yml
+++ b/.github/workflows/build-and-test.yml
@@ -11,12 +11,11 @@ jobs:
       # Run tests on all OS's and HHVM versions, even if one fails
       fail-fast: false
       matrix:
-        os: [ ubuntu ]
+        os: [ ubuntu-20.04 ]
         hhvm:
-          - '4.128'
-          - latest
-          - nightly
-    runs-on: ${{matrix.os}}-latest
+          - '4.153'
+          - '4.168'
+    runs-on: ${{matrix.os}}
     steps:
       - uses: actions/checkout@v2
       - uses: hhvm/actions/hack-lint-test@master
diff --git a/composer.json b/composer.json
index 0f577a62..4ec9fe48 100644
--- a/composer.json
+++ b/composer.json
@@ -3,7 +3,7 @@
     "description": "Hack Codegen is a library for programmatically generating Hack code",
     "keywords": ["code generation", "Hack"],
     "require": {
-        "hhvm": "^4.128"
+        "hhvm": "^4.153"
     },
     "bin": [ "bin/hh-codegen-verify-signatures", "bin/hh-codegen-verify-signatures.hack" ],
     "authors": [
diff --git a/src/CodegenFile.hack b/src/CodegenFile.hack
index ddc7d0c8..7c77727e 100644
--- a/src/CodegenFile.hack
+++ b/src/CodegenFile.hack
@@ -32,8 +32,9 @@ enum CodegenFileType: int {
  * depending on whether there are manual sections.
  */
 final class CodegenFile {
+  const vec<string> LEGACY_FILE_SUFFIX_TRIGGERS = vec['.php', '.hh', '.hhi'];
 
-  private CodegenFileType $fileType = CodegenFileType::HACK_STRICT;
+  private CodegenFileType $fileType;
   private ?string $docBlock;
   private string $fileName;
   private string $relativeFileName;
@@ -63,6 +64,13 @@ final class CodegenFile {
     private IHackCodegenConfig $config,
     string $file_name,
   ) {
+    $this->fileType = C\any(
+      static::LEGACY_FILE_SUFFIX_TRIGGERS,
+      $t ==> Str\ends_with($file_name, $t),
+    )
+      ? CodegenFileType::HACK_STRICT
+      : CodegenFileType::DOT_HACK;
+
     $root = $config->getRootDir();
     if (!Str\starts_with($file_name, '/')) {
       $this->relativeFileName = $file_name;
@@ -186,6 +194,10 @@ final class CodegenFile {
     return $this->fileName;
   }
 
+  public function getFileType()[]: CodegenFileType {
+    return $this->fileType;
+  }
+
   public function getRelativeFileName(): string {
     return $this->relativeFileName;
   }
diff --git a/tests/CodegenFileTest.codegen b/tests/CodegenFileTest.codegen
index ba62abba..e2e62620 100644
--- a/tests/CodegenFileTest.codegen
+++ b/tests/CodegenFileTest.codegen
@@ -1,13 +1,12 @@
 @generated
 !@#$%codegentest:testAutogenerated
-<?hh // strict
 // Codegen Tests
 /**
  * This file is generated. Do not modify it manually!
  *
  * Completely autogenerated!
  *
- * @-generated SignedSource<<e071d07d9cb3a77e37f24ecfc972a120>>
+ * @-generated SignedSource<<f8c58e9c529a492418b26efdeb95cd60>>
  */
 
 class AllAutogenerated {
@@ -18,12 +17,11 @@ class AllAutogenerated {
 }
 
 !@#$%codegentest:testConstants
-<?hh // strict
 // Codegen Tests
 /**
  * This file is generated. Do not modify it manually!
  *
- * @-generated SignedSource<<3081d5c20598b6bd1aa52a8035ffd207>>
+ * @-generated SignedSource<<30f984475edc103094e561bce9e9fdf0>>
  */
 namespace Foo\Bar;
 use namespace Herp\Derp;
@@ -67,11 +65,10 @@ function main(): void {
 main();
 
 !@#$%codegentest:testFormattingFullyGeneratedFile
-<?hh // strict
 /**
  * This file is generated. Do not modify it manually!
  *
- * @-generated SignedSource<<8873291d72dbb45b8a9e70890f7d2f0a>>
+ * @-generated SignedSource<<f78cdf7e9caaaaf7852fdf8330cf01e3>>
  */
 
 function my_func(
@@ -85,11 +82,10 @@ function my_func(
 }
 
 !@#$%codegentest:testFormattingFullyGeneratedFileWithOptions
-<?hh // strict
 /**
  * This file is generated. Do not modify it manually!
  *
- * @-generated SignedSource<<edb2ec6abbf7356621b98a5eac852996>>
+ * @-generated SignedSource<<f452b14a6164d435250f4c0088df8a86>>
  */
 
 function my_func(
@@ -103,11 +99,10 @@ function my_func(
 }
 
 !@#$%codegentest:testFormattingFullyGeneratedFileWithTabs
-<?hh // strict
 /**
  * This file is generated. Do not modify it manually!
  *
- * @-generated SignedSource<<763ad93f9a1a4caf70638a8de8786e49>>
+ * @-generated SignedSource<<f272aec490e6b17c15f61974e37838aa>>
  */
 
 function my_func(
@@ -118,12 +113,11 @@ function my_func(
 }
 
 !@#$%codegentest:testFormattingPartiallyGeneratedFile
-<?hh // strict
 /**
  * This file is partially generated. Only make modifications between BEGIN
  * MANUAL SECTION and END MANUAL SECTION designators.
  *
- * @-partially-generated SignedSource<<a6295bf3f813f9420063cf2da7478571>>
+ * @-partially-generated SignedSource<<3a7ef07d733ea4f3aa8c9d947d64c28e>>
  */
 
 function my_func(
@@ -139,7 +133,6 @@ function my_func(
 }
 
 !@#$%codegentest:testFormattingUnsignedFile
-<?hh // strict
 
 function my_func(
   string $aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa,
@@ -152,12 +145,11 @@ function my_func(
 }
 
 !@#$%codegentest:testGenerateEnums
-<?hh // strict
 // Codegen Tests
 /**
  * This file is generated. Do not modify it manually!
  *
- * @-generated SignedSource<<9627a8d7e6cf7a65a9bf2ad85d657e03>>
+ * @-generated SignedSource<<4c9d48e406a74cd17064ea2ebcd0492f>>
  */
 
 
@@ -167,12 +159,11 @@ enum TestEnum : int {
 }
 
 !@#$%codegentest:testGenerateTopLevelFunctions
-<?hh // strict
 // Codegen Tests
 /**
  * This file is generated. Do not modify it manually!
  *
- * @-generated SignedSource<<36d9f7267f98197421aa97d0c84ca8f5>>
+ * @-generated SignedSource<<e081255cf9acdc95f2d7546affae7685>>
  */
 
 function fun(): int {
@@ -180,12 +171,11 @@ function fun(): int {
 }
 
 !@#$%codegentest:testNamespace
-<?hh // strict
 // Codegen Tests
 /**
  * This file is generated. Do not modify it manually!
  *
- * @-generated SignedSource<<962f609e88de287eef0d9c8eb42c6fce>>
+ * @-generated SignedSource<<1e82f55cc7df937c367b4936f57d25b3>>
  */
 namespace MyNamespace;
 use namespace Another\Space;
@@ -197,7 +187,6 @@ class Foo {
 }
 
 !@#$%codegentest:testNoSignature
-<?hh // strict
 // Codegen Tests
 /**
  * Completely autogenerated!
@@ -211,13 +200,12 @@ class NoSignature {
 }
 
 !@#$%codegentest:testPartiallyGenerated
-<?hh // strict
 // Codegen Tests
 /**
  * This file is partially generated. Only make modifications between BEGIN
  * MANUAL SECTION and END MANUAL SECTION designators.
  *
- * @-partially-generated SignedSource<<f45c8764e5eddd1d8ce7cb55633a7810>>
+ * @-partially-generated SignedSource<<4ac3ec3e124eaa6cf732ab42f6ecbc49>>
  */
 
 class PartiallyGenerated {
@@ -248,14 +236,13 @@ class Foo {
 }
 
 !@#$%codegentest:testSaveAutogenerated
-<?hh // strict
 // Codegen Tests
 /**
  * This file is generated. Do not modify it manually!
  *
  * Testing CodegenFile with autogenerated files
  *
- * @-generated SignedSource<<6a105a3097cde1632d78096f693751c6>>
+ * @-generated SignedSource<<624100da9e208d6abeaa301bce015bfd>>
  */
 
 class Demo {
@@ -266,7 +253,6 @@ class Demo {
 }
 
 !@#$%codegentest:testSavePartiallyGenerated
-<?hh // strict
 // Codegen Tests
 /**
  * This file is partially generated. Only make modifications between BEGIN
@@ -274,7 +260,7 @@ class Demo {
  *
  * Testing CodegenFile with partially generated files
  *
- * @-partially-generated SignedSource<<500fa4945d39860935bb0c92a72477a0>>
+ * @-partially-generated SignedSource<<e0a4ec17c8e71c803d34407363ca8af6>>
  */
 
 class Demo {
@@ -287,12 +273,11 @@ class Demo {
 }
 
 !@#$%codegentest:testStrictFile
-<?hh // strict
 // Codegen Tests
 /**
  * This file is generated. Do not modify it manually!
  *
- * @-generated SignedSource<<5c0182dcd097a22f08d4dfbcc555c52c>>
+ * @-generated SignedSource<<d1d6954e25b8c7fb2bf045bede5f356b>>
  */
 
 class Foo {
diff --git a/tests/CodegenFileTest.hack b/tests/CodegenFileTest.hack
index d6246898..de13e732 100644
--- a/tests/CodegenFileTest.hack
+++ b/tests/CodegenFileTest.hack
@@ -11,6 +11,8 @@ namespace Facebook\HackCodegen;
 
 use type Facebook\HackCodegen\_Private\Filesystem;
 use namespace HH\Lib\Str;
+use type Facebook\HackTest\DataProvider;
+use function Facebook\FBExpect\expect;
 
 final class CodegenFileTest extends CodegenBaseTest {
   public function testAutogenerated(): void {
@@ -47,10 +49,12 @@ final class CodegenFileTest extends CodegenBaseTest {
     $enum = $cgf->codegenEnum('TestEnum', 'int')
       ->addMember(
         $cgf->codegenEnumMember('FIRST')
-          ->setValue(0, HackBuilderValues::export()))
+          ->setValue(0, HackBuilderValues::export()),
+      )
       ->addMember(
         $cgf->codegenEnumMember('SECOND')
-          ->setValue(1, HackBuilderValues::export()));
+          ->setValue(1, HackBuilderValues::export()),
+      );
     $code = $cgf->codegenFile('no_file')->addEnum($enum)->render();
 
     expect_with_context(static::class, $code)->toBeUnchanged();
@@ -144,7 +148,8 @@ final class CodegenFileTest extends CodegenBaseTest {
 
   public function testSaveAutogenerated(): void {
     $fname = $this->saveAutogeneratedFile();
-    expect_with_context(static::class, Filesystem::readFile($fname))->toBeUnchanged();
+    expect_with_context(static::class, Filesystem::readFile($fname))
+      ->toBeUnchanged();
   }
 
   public function testClobberManuallyWrittenCode(): void {
@@ -175,9 +180,8 @@ final class CodegenFileTest extends CodegenBaseTest {
     $fname = $this->savePartiallyGeneratedFile();
     $content = Filesystem::readFile($fname);
     expect_with_context(static::class, $content)->toBeUnchanged();
-    expect(
-      PartiallyGeneratedSignedSource::hasValidSignature($content),
-    )->toBeTrue();
+    expect(PartiallyGeneratedSignedSource::hasValidSignature($content))
+      ->toBeTrue();
   }
 
   public function testReSavePartiallyGenerated(): void {
@@ -317,7 +321,7 @@ final class CodegenFileTest extends CodegenBaseTest {
         $cgf->codegenFunction('main')
           ->setReturnType('noreturn')
           ->addEmptyUserAttribute('__EntryPoint')
-          ->setBody('exit(0);')
+          ->setBody('exit(0);'),
       )
       ->render();
     expect_with_context(static::class, $code)->toBeUnchanged();
@@ -372,20 +376,18 @@ final class CodegenFileTest extends CodegenBaseTest {
       ->render();
 
     expect_with_context(static::class, $code)->toBeUnchanged();
-    expect(
-      SignedSourceBase::hasValidSignatureFromAnySigner($code)
-    )->toBeTrue('bad signed source');
-    expect(
-      Str\ends_with($code, "\n")
-    )->toBeTrue('Should end with newline');
-    expect(
-      Str\ends_with($code, "\n\n")
-    )->toBeFalse('Should end with one newline, not multiple');
+    expect(SignedSourceBase::hasValidSignatureFromAnySigner($code))->toBeTrue(
+      'bad signed source',
+    );
+    expect(Str\ends_with($code, "\n"))->toBeTrue('Should end with newline');
+    expect(Str\ends_with($code, "\n\n"))->toBeFalse(
+      'Should end with one newline, not multiple',
+    );
 
     $lines = Str\split($code, "\n");
-    expect(
-      Str\starts_with($lines[8], ' ')
-    )->toBeTrue('use spaces instead of tabs');
+    expect(Str\starts_with($lines[8], ' '))->toBeTrue(
+      'use spaces instead of tabs',
+    );
   }
 
   public function testFormattingFullyGeneratedFileWithTabs(): void {
@@ -410,14 +412,14 @@ final class CodegenFileTest extends CodegenBaseTest {
       )
       ->render();
     expect_with_context(static::class, $code)->toBeUnchanged();
-    expect(
-      SignedSourceBase::hasValidSignatureFromAnySigner($code)
-    )->toBeTrue('bad signed source');
+    expect(SignedSourceBase::hasValidSignatureFromAnySigner($code))->toBeTrue(
+      'bad signed source',
+    );
 
     $lines = Str\split($code, "\n");
-    expect(
-      Str\starts_with($lines[9], "\t")
-    )->toBeTrue('use tabs instead of spaces');
+    expect(Str\starts_with($lines[8], "\t"))->toBeTrue(
+      'use tabs instead of spaces',
+    );
   }
 
   public function testFormattingFullyGeneratedFileWithOptions(): void {
@@ -442,14 +444,14 @@ final class CodegenFileTest extends CodegenBaseTest {
       )
       ->render();
     expect_with_context(static::class, $code)->toBeUnchanged();
-    expect(
-      SignedSourceBase::hasValidSignatureFromAnySigner($code)
-    )->toBeTrue('bad signed source');
+    expect(SignedSourceBase::hasValidSignatureFromAnySigner($code))->toBeTrue(
+      'bad signed source',
+    );
 
     $lines = Str\split($code, "\n");
-    expect(
-      Str\starts_with($lines[9], "\t")
-    )->toBeTrue('use tabs instead of spaces');
+    expect(Str\starts_with($lines[8], "\t"))->toBeTrue(
+      'use tabs instead of spaces',
+    );
   }
 
   public function testFormattingUnsignedFile(): void {
@@ -481,9 +483,9 @@ final class CodegenFileTest extends CodegenBaseTest {
       )
       ->render();
     expect_with_context(static::class, $code)->toBeUnchanged();
-    expect(
-      SignedSourceBase::hasValidSignatureFromAnySigner($code)
-    )->toBeFalse('file should be unsigned, but has valid signature');
+    expect(SignedSourceBase::hasValidSignatureFromAnySigner($code))->toBeFalse(
+      'file should be unsigned, but has valid signature',
+    );
   }
 
   public function testFormattingPartiallyGeneratedFile(): void {
@@ -516,9 +518,9 @@ final class CodegenFileTest extends CodegenBaseTest {
       )
       ->render();
     expect_with_context(static::class, $code)->toBeUnchanged();
-    expect(
-      SignedSourceBase::hasValidSignatureFromAnySigner($code)
-    )->toBeTrue('bad signed source');
+    expect(SignedSourceBase::hasValidSignatureFromAnySigner($code))->toBeTrue(
+      'bad signed source',
+    );
   }
 
   public function testConstants(): void {
@@ -541,6 +543,33 @@ final class CodegenFileTest extends CodegenBaseTest {
       ->render();
     expect_with_context(static::class, $code)->toBeUnchanged();
   }
+
+  public function provideFileNamesWithExpectedModes(
+  )[]: dict<string, (string, CodegenFileType)> {
+    return dict[
+      'Legacy extensions, such as `.php` trigger // strict mode.' =>
+        tuple('filename.php', CodegenFileType::HACK_STRICT),
+      'Legacy extensions, such as .hh trigger // strict mode.' =>
+        tuple('filename.hh', CodegenFileType::HACK_STRICT),
+      'Just guessing, but .hhi files need the <?hh sigil???' =>
+        tuple('filename.hhi', CodegenFileType::HACK_STRICT),
+      'Dot hack implies there is no need for a <?hh sigil.' =>
+        tuple('filename.hack', CodegenFileType::DOT_HACK),
+      // https://hhvm.com/blog/2021/10/26/hhvm-4.133.html
+      'As of hhvm 4.133 all files without a recognized extensions are treated as .hack' =>
+        tuple('filename', CodegenFileType::DOT_HACK),
+    ];
+  }
+
+  <<DataProvider('provideFileNamesWithExpectedModes')>>
+  public function testFileModeInference(
+    string $filename,
+    CodegenFileType $mode,
+  ): void {
+    $cfg = $this->getCodegenFactory();
+    $file = $cfg->codegenFile($filename);
+    expect($file->getFileType())->toEqual($mode);
+  }
 }
 
 final class TestTabbedCodegenConfig implements IHackCodegenConfig {
diff --git a/tests/RefactorCodegenTest.codegen b/tests/RefactorCodegenTest.codegen
index 1f29b94a..607b402b 100644
--- a/tests/RefactorCodegenTest.codegen
+++ b/tests/RefactorCodegenTest.codegen
@@ -1,12 +1,11 @@
 @generated
 !@#$%codegentest:testClassRename
-<?hh // strict
 // Codegen Tests
 /**
  * This file is partially generated. Only make modifications between BEGIN
  * MANUAL SECTION and END MANUAL SECTION designators.
  *
- * @-partially-generated SignedSource<<119dd0f6ee85b090bd68ccc9d8537b7a>>
+ * @-partially-generated SignedSource<<4b7ebf80bdd2f28f2b76ca99439f57dd>>
  */
 
 class NewClass {
@@ -21,13 +20,12 @@ class NewClass {
 }
 
 !@#$%codegentest:testManualSectionMerge
-<?hh // strict
 // Codegen Tests
 /**
  * This file is partially generated. Only make modifications between BEGIN
  * MANUAL SECTION and END MANUAL SECTION designators.
  *
- * @-partially-generated SignedSource<<4493f1e5892280993f2b53b065553d20>>
+ * @-partially-generated SignedSource<<11e6ceb684e1300fab7f316d289dadc1>>
  */
 
 class NewClass {