diff --git a/libc/src/stdio/scanf_core/int_converter.cpp b/libc/src/stdio/scanf_core/int_converter.cpp
index 136db2a3773e1..ecdac52e84bbd 100644
--- a/libc/src/stdio/scanf_core/int_converter.cpp
+++ b/libc/src/stdio/scanf_core/int_converter.cpp
@@ -124,13 +124,24 @@ int convert_int(Reader *reader, const FormatSection &to_conv) {
 
       if (to_lower(cur_char) == 'x') {
         // This is a valid hex prefix.
+
+        is_number = false;
+        // A valid hex prefix is not necessarily a valid number. For the
+        // conversion to be valid it needs to use all of the characters it
+        // consumes. From the standard:
+        // 7.23.6.2 paragraph 9: "An input item is defined as the longest
+        // sequence of input characters which does not exceed any specified
+        // field width and which is, or is a prefix of, a matching input
+        // sequence."
+        // 7.23.6.2 paragraph 10: "If the input item is not a matching sequence,
+        // the execution of the directive fails: this condition is a matching
+        // failure"
         base = 16;
         if (max_width > 1) {
           --max_width;
           cur_char = reader->getc();
         } else {
-          write_int_with_length(0, to_conv);
-          return READ_OK;
+          return MATCHING_FAILURE;
         }
 
       } else {
@@ -198,6 +209,9 @@ int convert_int(Reader *reader, const FormatSection &to_conv) {
   // last one back.
   reader->ungetc(cur_char);
 
+  if (!is_number)
+    return MATCHING_FAILURE;
+
   if (has_overflow) {
     write_int_with_length(MAX, to_conv);
   } else {
@@ -207,8 +221,6 @@ int convert_int(Reader *reader, const FormatSection &to_conv) {
     write_int_with_length(result, to_conv);
   }
 
-  if (!is_number)
-    return MATCHING_FAILURE;
   return READ_OK;
 }
 
diff --git a/libc/test/src/stdio/sscanf_test.cpp b/libc/test/src/stdio/sscanf_test.cpp
index 33bb0acba3e66..18addb632067c 100644
--- a/libc/test/src/stdio/sscanf_test.cpp
+++ b/libc/test/src/stdio/sscanf_test.cpp
@@ -177,13 +177,25 @@ TEST(LlvmLibcSScanfTest, IntConvMaxLengthTests) {
   EXPECT_EQ(ret_val, 1);
   EXPECT_EQ(result, 0);
 
+  result = -999;
+
+  // 0x is a valid prefix, but not a valid number. This should be a matching
+  // failure and should not modify the values.
   ret_val = LIBC_NAMESPACE::sscanf("0x1", "%2i", &result);
-  EXPECT_EQ(ret_val, 1);
-  EXPECT_EQ(result, 0);
+  EXPECT_EQ(ret_val, 0);
+  EXPECT_EQ(result, -999);
 
   ret_val = LIBC_NAMESPACE::sscanf("-0x1", "%3i", &result);
+  EXPECT_EQ(ret_val, 0);
+  EXPECT_EQ(result, -999);
+
+  ret_val = LIBC_NAMESPACE::sscanf("0x1", "%3i", &result);
   EXPECT_EQ(ret_val, 1);
-  EXPECT_EQ(result, 0);
+  EXPECT_EQ(result, 1);
+
+  ret_val = LIBC_NAMESPACE::sscanf("-0x1", "%4i", &result);
+  EXPECT_EQ(ret_val, 1);
+  EXPECT_EQ(result, -1);
 
   ret_val = LIBC_NAMESPACE::sscanf("-0x123", "%4i", &result);
   EXPECT_EQ(ret_val, 1);
@@ -212,7 +224,7 @@ TEST(LlvmLibcSScanfTest, IntConvNoWriteTests) {
   EXPECT_EQ(result, 0);
 
   ret_val = LIBC_NAMESPACE::sscanf("0x1", "%*2i", &result);
-  EXPECT_EQ(ret_val, 1);
+  EXPECT_EQ(ret_val, 0);
   EXPECT_EQ(result, 0);
 
   ret_val = LIBC_NAMESPACE::sscanf("a", "%*i", &result);
@@ -679,13 +691,17 @@ TEST(LlvmLibcSScanfTest, CombinedConv) {
   EXPECT_EQ(result, 123);
   ASSERT_STREQ(buffer, "abc");
 
+  result = -1;
+
+  // 0x is a valid prefix, but not a valid number. This should be a matching
+  // failure and should not modify the values.
   ret_val = LIBC_NAMESPACE::sscanf("0xZZZ", "%i%s", &result, buffer);
-  EXPECT_EQ(ret_val, 2);
-  EXPECT_EQ(result, 0);
-  ASSERT_STREQ(buffer, "ZZZ");
+  EXPECT_EQ(ret_val, 0);
+  EXPECT_EQ(result, -1);
+  ASSERT_STREQ(buffer, "abc");
 
   ret_val = LIBC_NAMESPACE::sscanf("0xZZZ", "%X%s", &result, buffer);
-  EXPECT_EQ(ret_val, 2);
-  EXPECT_EQ(result, 0);
-  ASSERT_STREQ(buffer, "ZZZ");
+  EXPECT_EQ(ret_val, 0);
+  EXPECT_EQ(result, -1);
+  ASSERT_STREQ(buffer, "abc");
 }