diff --git a/eden/fs/config/CheckoutConfig.cpp b/eden/fs/config/CheckoutConfig.cpp
index fa1c4f4653..e71ea1ebf7 100644
--- a/eden/fs/config/CheckoutConfig.cpp
+++ b/eden/fs/config/CheckoutConfig.cpp
@@ -85,16 +85,6 @@ Hash CheckoutConfig::getParentCommit() const {
   auto snapshotFileContents = readFile(snapshotFile).value();
 
   StringPiece contents{snapshotFileContents};
-  if (!contents.startsWith(kSnapshotFileMagic)) {
-    // Try reading an old-style SNAPSHOT file that just contains a single
-    // commit ID, as an ASCII hexadecimal string.
-    //
-    // TODO: In the not-to-distant future we can remove support for this old
-    // format, and simply throw an exception here if the snapshot file does not
-    // start with the correct identifier bytes.
-    auto snapshotID = folly::trimWhitespace(contents);
-    return Hash{snapshotID};
-  }
 
   if (contents.size() < kSnapshotHeaderSize) {
     throw std::runtime_error(folly::sformat(
@@ -103,6 +93,11 @@ Hash CheckoutConfig::getParentCommit() const {
         snapshotFile));
   }
 
+  if (!contents.startsWith(kSnapshotFileMagic)) {
+    throw std::runtime_error(
+        folly::sformat("unsupported legacy SNAPSHOT file"));
+  }
+
   IOBuf buf(IOBuf::WRAP_BUFFER, ByteRange{contents});
   folly::io::Cursor cursor(&buf);
   cursor += kSnapshotFileMagic.size();
diff --git a/eden/fs/config/test/CheckoutConfigTest.cpp b/eden/fs/config/test/CheckoutConfigTest.cpp
index e86027ff51..5f6947ca36 100644
--- a/eden/fs/config/test/CheckoutConfigTest.cpp
+++ b/eden/fs/config/test/CheckoutConfigTest.cpp
@@ -149,6 +149,7 @@ void CheckoutConfigTest::testBadSnapshot(
 }
 
 TEST_F(CheckoutConfigTest, testBadSnapshot) {
+  testBadSnapshot("ede", "SNAPSHOT file is too short");
   testBadSnapshot("eden", "SNAPSHOT file is too short");
   testBadSnapshot(StringPiece{"eden\0\0\0", 7}, "SNAPSHOT file is too short");
   testBadSnapshot(
@@ -179,8 +180,7 @@ TEST_F(CheckoutConfigTest, testBadSnapshot) {
   // The error type and message for this will probably change in the future
   // when we drop support for the legacy SNAPSHOT file format (of a 40-byte
   // ASCII string containing the snapshot hash).
-  testBadSnapshot<std::invalid_argument>("ede", "incorrect data size for Hash");
-  testBadSnapshot<std::invalid_argument>(
+  testBadSnapshot(
       StringPiece{
           "xden\00\00\00\01"
           "\x99\x88\x77\x66\x55\x44\x33\x22\x11\x00"
@@ -188,5 +188,5 @@ TEST_F(CheckoutConfigTest, testBadSnapshot) {
           "\xab\xcd\xef\x98\x76\x54\x32\x10\x01\x23"
           "\x45\x67\x89\xab\xcd\xef\x00\x11\x22\x33",
           48},
-      "incorrect data size for Hash");
+      "unsupported legacy SNAPSHOT file");
 }
diff --git a/eden/fs/testharness/TestMount.cpp b/eden/fs/testharness/TestMount.cpp
index a0d9bbc4e9..a0a1ad2cd7 100644
--- a/eden/fs/testharness/TestMount.cpp
+++ b/eden/fs/testharness/TestMount.cpp
@@ -424,10 +424,7 @@ size_t TestMount::drainServerExecutor() {
 
 void TestMount::setInitialCommit(Hash commitHash) {
   // Write the commit hash to the snapshot file
-  auto snapshotPath = config_->getSnapshotPath();
-  writeFileAtomic(
-      snapshotPath, folly::StringPiece(commitHash.toString() + "\n"))
-      .value();
+  config_->setParentCommit(commitHash);
 }
 
 void TestMount::setInitialCommit(Hash commitHash, Hash rootTreeHash) {