diff --git a/clang/docs/ReleaseNotes.rst b/clang/docs/ReleaseNotes.rst
index ed668ca6f207c..aecdd95f1e8fc 100644
--- a/clang/docs/ReleaseNotes.rst
+++ b/clang/docs/ReleaseNotes.rst
@@ -531,6 +531,12 @@ clang-format
 
 libclang
 --------
+- The ``Length`` field of ``CXUnsavedFile`` (in ``clang-c/Index.h``) is now a
+  fixed-width ``uint64_t`` instead of ``unsigned long``. This makes the struct
+  layout identical across data models; previously the field was 4 bytes under
+  LLP64 (Windows) and 8 bytes under LP64, which could corrupt the length or
+  crash FFI consumers. This is an intentional ABI change to the C API.
+  (#GH160729)
 
 Code Completion
 ---------------
diff --git a/clang/include/clang-c/Index.h b/clang/include/clang-c/Index.h
index be038d9165fc6..1b04cc0303e48 100644
--- a/clang/include/clang-c/Index.h
+++ b/clang/include/clang-c/Index.h
@@ -16,6 +16,8 @@
 #ifndef LLVM_CLANG_C_INDEX_H
 #define LLVM_CLANG_C_INDEX_H
 
+#include <stdint.h>
+
 #include "clang-c/BuildSystem.h"
 #include "clang-c/CXDiagnostic.h"
 #include "clang-c/CXErrorCode.h"
@@ -118,8 +120,12 @@ struct CXUnsavedFile {
 
   /**
    * The length of the unsaved contents of this buffer.
+   *
+   * A fixed-width type is used so the struct layout is identical across data
+   * models (e.g. LLP64 vs LP64); `unsigned long` previously made the field 4
+   * bytes on Windows and 8 bytes elsewhere, breaking FFI consumers.
    */
-  unsigned long Length;
+  uint64_t Length;
 };
 
 /**
diff --git a/clang/tools/libclang/CIndex.cpp b/clang/tools/libclang/CIndex.cpp
index 5aab74348967d..778648ced755f 100644
--- a/clang/tools/libclang/CIndex.cpp
+++ b/clang/tools/libclang/CIndex.cpp
@@ -4445,8 +4445,8 @@ enum CXErrorCode clang_parseTranslationUnit2FullArgv(
     for (unsigned i = 0; i != num_unsaved_files; ++i) {
       if (i)
         fprintf(stderr, ", ");
-      fprintf(stderr, "('%s', '...', %ld)", unsaved_files[i].Filename,
-              unsaved_files[i].Length);
+      fprintf(stderr, "('%s', '...', %llu)", unsaved_files[i].Filename,
+              (unsigned long long)unsaved_files[i].Length);
     }
     fprintf(stderr, "],\n");
     fprintf(stderr, "  'options' : %d,\n", options);
diff --git a/clang/tools/libclang/Indexing.cpp b/clang/tools/libclang/Indexing.cpp
index c142f142d5071..9d893e4e21cd5 100644
--- a/clang/tools/libclang/Indexing.cpp
+++ b/clang/tools/libclang/Indexing.cpp
@@ -924,8 +924,8 @@ int clang_indexSourceFileFullArgv(
     for (unsigned i = 0; i != num_unsaved_files; ++i) {
       if (i)
         fprintf(stderr, ", ");
-      fprintf(stderr, "('%s', '...', %ld)", unsaved_files[i].Filename,
-              unsaved_files[i].Length);
+      fprintf(stderr, "('%s', '...', %llu)", unsaved_files[i].Filename,
+              (unsigned long long)unsaved_files[i].Length);
     }
     fprintf(stderr, "],\n");
     fprintf(stderr, "  'options' : %d,\n", TU_options);