diff --git a/Makefile b/Makefile
index a9fa8f8208fbdb..58b870dc2c6693 100644
--- a/Makefile
+++ b/Makefile
@@ -875,6 +875,7 @@ TEST_BUILTINS_OBJS += test-submodule-config.o
 TEST_BUILTINS_OBJS += test-submodule-nested-repo-config.o
 TEST_BUILTINS_OBJS += test-submodule.o
 TEST_BUILTINS_OBJS += test-subprocess.o
+TEST_BUILTINS_OBJS += test-synthesize.o
 TEST_BUILTINS_OBJS += test-trace2.o
 TEST_BUILTINS_OBJS += test-truncate.o
 TEST_BUILTINS_OBJS += test-userdiff.o
diff --git a/builtin/index-pack.c b/builtin/index-pack.c
index b67fb0256cc831..1ea64089055887 100644
--- a/builtin/index-pack.c
+++ b/builtin/index-pack.c
@@ -37,7 +37,7 @@ static const char index_pack_usage[] =
 
 struct object_entry {
 	struct pack_idx_entry idx;
-	unsigned long size;
+	size_t size;
 	unsigned char hdr_size;
 	signed char type;
 	signed char real_type;
@@ -469,7 +469,7 @@ static int is_delta_type(enum object_type type)
 	return (type == OBJ_REF_DELTA || type == OBJ_OFS_DELTA);
 }
 
-static void *unpack_entry_data(off_t offset, unsigned long size,
+static void *unpack_entry_data(off_t offset, size_t size,
 			       enum object_type type, struct object_id *oid)
 {
 	static char fixed_buf[8192];
@@ -524,7 +524,8 @@ static void *unpack_raw_entry(struct object_entry *obj,
 			      struct object_id *oid)
 {
 	unsigned char *p;
-	unsigned long size, c;
+	size_t size;
+	unsigned long c;
 	off_t base_offset;
 	unsigned shift;
 	void *data;
@@ -542,7 +543,7 @@ static void *unpack_raw_entry(struct object_entry *obj,
 		p = fill(1);
 		c = *p;
 		use(1);
-		size += (c & 0x7f) << shift;
+		size += ((size_t)c & 0x7f) << shift;
 		shift += 7;
 	}
 	obj->size = size;
diff --git a/builtin/unpack-objects.c b/builtin/unpack-objects.c
index 6fc64e9e4b8d5a..883440ccafef19 100644
--- a/builtin/unpack-objects.c
+++ b/builtin/unpack-objects.c
@@ -533,7 +533,8 @@ static void unpack_one(unsigned nr)
 {
 	unsigned shift;
 	unsigned char *pack;
-	unsigned long size, c;
+	size_t size;
+	unsigned long c;
 	enum object_type type;
 
 	obj_list[nr].offset = consumed_bytes;
@@ -548,7 +549,7 @@ static void unpack_one(unsigned nr)
 		pack = fill(1);
 		c = *pack;
 		use(1);
-		size += (c & 0x7f) << shift;
+		size += ((size_t)c & 0x7f) << shift;
 		shift += 7;
 	}
 
diff --git a/compat/zlib-compat.h b/compat/zlib-compat.h
index ac0827662298af..5078c5ef6ce0e8 100644
--- a/compat/zlib-compat.h
+++ b/compat/zlib-compat.h
@@ -7,6 +7,8 @@
 # define z_stream_s zng_stream_s
 # define gz_header_s zng_gz_header_s
 
+# define adler32(adler, buf, len) zng_adler32(adler, buf, len)
+
 # define crc32(crc, buf, len) zng_crc32(crc, buf, len)
 
 # define inflate(strm, bits) zng_inflate(strm, bits)
diff --git a/delta.h b/delta.h
index 8a56ec07992c75..fad68cfc45f6f4 100644
--- a/delta.h
+++ b/delta.h
@@ -86,8 +86,11 @@ void *patch_delta(const void *src_buf, unsigned long src_size,
  * This must be called twice on the delta data buffer, first to get the
  * expected source buffer size, and again to get the target buffer size.
  */
-static inline unsigned long get_delta_hdr_size(const unsigned char **datap,
-					       const unsigned char *top)
+/*
+ * Size_t variant that doesn't truncate - use for >4GB objects on Windows.
+ */
+static inline size_t get_delta_hdr_size_sz(const unsigned char **datap,
+					   const unsigned char *top)
 {
 	const unsigned char *data = *datap;
 	size_t cmd, size = 0;
@@ -98,6 +101,13 @@ static inline unsigned long get_delta_hdr_size(const unsigned char **datap,
 		i += 7;
 	} while (cmd & 0x80 && data < top);
 	*datap = data;
+	return size;
+}
+
+static inline unsigned long get_delta_hdr_size(const unsigned char **datap,
+					       const unsigned char *top)
+{
+	size_t size = get_delta_hdr_size_sz(datap, top);
 	return cast_size_t_to_ulong(size);
 }
 
diff --git a/git-zlib.c b/git-zlib.c
index df9604910e3fdf..b91cb323aee916 100644
--- a/git-zlib.c
+++ b/git-zlib.c
@@ -30,6 +30,9 @@ static const char *zerr_to_string(int status)
  */
 /* #define ZLIB_BUF_MAX ((uInt)-1) */
 #define ZLIB_BUF_MAX ((uInt) 1024 * 1024 * 1024) /* 1GB */
+
+/* uLong is 32-bit on Windows, even on 64-bit systems */
+#define ULONG_MAX_VALUE maximum_unsigned_value_of_type(uLong)
 static inline uInt zlib_buf_cap(unsigned long len)
 {
 	return (ZLIB_BUF_MAX < len) ? ZLIB_BUF_MAX : len;
@@ -39,31 +42,37 @@ static void zlib_pre_call(git_zstream *s)
 {
 	s->z.next_in = s->next_in;
 	s->z.next_out = s->next_out;
-	s->z.total_in = s->total_in;
-	s->z.total_out = s->total_out;
+	s->z.total_in = (uLong)(s->total_in & ULONG_MAX_VALUE);
+	s->z.total_out = (uLong)(s->total_out & ULONG_MAX_VALUE);
 	s->z.avail_in = zlib_buf_cap(s->avail_in);
 	s->z.avail_out = zlib_buf_cap(s->avail_out);
 }
 
 static void zlib_post_call(git_zstream *s, int status)
 {
-	unsigned long bytes_consumed;
-	unsigned long bytes_produced;
+	size_t bytes_consumed;
+	size_t bytes_produced;
 
 	bytes_consumed = s->z.next_in - s->next_in;
 	bytes_produced = s->z.next_out - s->next_out;
-	if (s->z.total_out != s->total_out + bytes_produced)
+	/*
+	 * zlib's total_out/total_in are uLong which may wrap for >4GB.
+	 * We track our own totals and verify only the low bits match.
+	 */
+	if ((s->z.total_out & ULONG_MAX_VALUE) !=
+	    ((s->total_out + bytes_produced) & ULONG_MAX_VALUE))
 		BUG("total_out mismatch");
 	/*
 	 * zlib does not update total_in when it returns Z_NEED_DICT,
 	 * causing a mismatch here. Skip the sanity check in that case.
 	 */
 	if (status != Z_NEED_DICT &&
-	    s->z.total_in != s->total_in + bytes_consumed)
+	    (s->z.total_in & ULONG_MAX_VALUE) !=
+	    ((s->total_in + bytes_consumed) & ULONG_MAX_VALUE))
 		BUG("total_in mismatch");
 
-	s->total_out = s->z.total_out;
-	s->total_in = s->z.total_in;
+	s->total_out += bytes_produced;
+	s->total_in += bytes_consumed;
 	/* zlib-ng marks `next_in` as `const`, so we have to cast it away. */
 	s->next_in = (unsigned char *) s->z.next_in;
 	s->next_out = s->z.next_out;
diff --git a/git-zlib.h b/git-zlib.h
index 0e66fefa8c9f05..44380e8ad38305 100644
--- a/git-zlib.h
+++ b/git-zlib.h
@@ -7,8 +7,8 @@ typedef struct git_zstream {
 	struct z_stream_s z;
 	unsigned long avail_in;
 	unsigned long avail_out;
-	unsigned long total_in;
-	unsigned long total_out;
+	size_t total_in;
+	size_t total_out;
 	unsigned char *next_in;
 	unsigned char *next_out;
 } git_zstream;
diff --git a/object-file.c b/object-file.c
index e55bf1bfff670f..6baa36526ef9f6 100644
--- a/object-file.c
+++ b/object-file.c
@@ -1086,7 +1086,7 @@ int odb_source_loose_write_stream(struct odb_source *source,
 	} while (ret == Z_OK || ret == Z_BUF_ERROR);
 
 	if (stream.total_in != len + hdrlen)
-		die(_("write stream object %ld != %"PRIuMAX), stream.total_in,
+		die(_("write stream object %"PRIuMAX" != %"PRIuMAX), (uintmax_t)stream.total_in,
 		    (uintmax_t)len + hdrlen);
 
 	/*
@@ -2108,6 +2108,7 @@ int odb_source_loose_read_object_stream(struct odb_read_stream **out,
 	struct object_info oi = OBJECT_INFO_INIT;
 	struct odb_loose_read_stream *st;
 	unsigned long mapsize;
+	unsigned long size_ul;
 	void *mapped;
 
 	mapped = odb_source_loose_map_object(source, oid, &mapsize);
@@ -2131,11 +2132,18 @@ int odb_source_loose_read_object_stream(struct odb_read_stream **out,
 		goto error;
 	}
 
-	oi.sizep = &st->base.size;
+	/*
+	 * object_info.sizep is unsigned long* (32-bit on Windows), but
+	 * st->base.size is size_t (64-bit). Use temporary variable.
+	 * Note: loose objects >4GB would still truncate here, but such
+	 * large loose objects are uncommon (they'd normally be packed).
+	 */
+	oi.sizep = &size_ul;
 	oi.typep = &st->base.type;
 
 	if (parse_loose_header(st->hdr, &oi) < 0 || st->base.type < 0)
 		goto error;
+	st->base.size = size_ul;
 
 	st->mapped = mapped;
 	st->mapsize = mapsize;
diff --git a/odb/streaming.c b/odb/streaming.c
index 4a4474f891a07f..bd460f9adc2cb2 100644
--- a/odb/streaming.c
+++ b/odb/streaming.c
@@ -158,15 +158,26 @@ static int open_istream_incore(struct odb_read_stream **out,
 		.base.read = read_istream_incore,
 	};
 	struct odb_incore_read_stream *st;
+	unsigned long size_ul;
 	int ret;
 
 	oi.typep = &stream.base.type;
-	oi.sizep = &stream.base.size;
+	/*
+	 * object_info.sizep is unsigned long* (32-bit on Windows), but
+	 * stream.base.size is size_t (64-bit). We use a temporary variable
+	 * because the types are incompatible. Note: this path still truncates
+	 * for >4GB objects, but large objects should use pack streaming
+	 * (packfile_store_read_object_stream) which handles size_t properly.
+	 * This incore fallback is only used for small objects or when pack
+	 * streaming is unavailable.
+	 */
+	oi.sizep = &size_ul;
 	oi.contentp = (void **)&stream.buf;
 	ret = odb_read_object_info_extended(odb, oid, &oi,
 					    OBJECT_INFO_DIE_IF_CORRUPT);
 	if (ret)
 		return ret;
+	stream.base.size = size_ul;
 
 	CALLOC_ARRAY(st, 1);
 	*st = stream;
diff --git a/odb/streaming.h b/odb/streaming.h
index c7861f7e13c606..517e2ea2d3f5c3 100644
--- a/odb/streaming.h
+++ b/odb/streaming.h
@@ -21,7 +21,7 @@ struct odb_read_stream {
 	odb_read_stream_close_fn close;
 	odb_read_stream_read_fn read;
 	enum object_type type;
-	unsigned long size; /* inflated size of full object */
+	size_t size; /* inflated size of full object */
 };
 
 /*
diff --git a/packfile.c b/packfile.c
index 402c3b5dc73131..e198c9793f089f 100644
--- a/packfile.c
+++ b/packfile.c
@@ -1130,8 +1130,8 @@ unsigned long repo_approximate_object_count(struct repository *r)
 	return r->objects->approximate_object_count;
 }
 
-unsigned long unpack_object_header_buffer(const unsigned char *buf,
-		unsigned long len, enum object_type *type, unsigned long *sizep)
+static unsigned long unpack_object_header_buffer_internal(const unsigned char *buf,
+		unsigned long len, enum object_type *type, size_t *sizep)
 {
 	unsigned shift;
 	size_t size, c;
@@ -1142,7 +1142,11 @@ unsigned long unpack_object_header_buffer(const unsigned char *buf,
 	size = c & 15;
 	shift = 4;
 	while (c & 0x80) {
-		if (len <= used || (bitsizeof(long) - 7) < shift) {
+		/*
+		 * Each continuation byte adds 7 bits. Ensure shift won't
+		 * overflow size_t (use size_t not long for 64-bit on Windows).
+		 */
+		if (len <= used || (bitsizeof(size_t) - 7) < shift) {
 			error("bad object header");
 			size = used = 0;
 			break;
@@ -1151,13 +1155,25 @@ unsigned long unpack_object_header_buffer(const unsigned char *buf,
 		size = st_add(size, st_left_shift(c & 0x7f, shift));
 		shift += 7;
 	}
+	*sizep = size;
+	return used;
+}
+
+unsigned long unpack_object_header_buffer(const unsigned char *buf,
+		unsigned long len, enum object_type *type, unsigned long *sizep)
+{
+	size_t size;
+	unsigned long used = unpack_object_header_buffer_internal(buf, len, type, &size);
 	*sizep = cast_size_t_to_ulong(size);
 	return used;
 }
 
-unsigned long get_size_from_delta(struct packed_git *p,
-				  struct pack_window **w_curs,
-				  off_t curpos)
+/*
+ * Size_t variant for >4GB delta results on Windows.
+ */
+static size_t get_size_from_delta_sz(struct packed_git *p,
+				     struct pack_window **w_curs,
+				     off_t curpos)
 {
 	const unsigned char *data;
 	unsigned char delta_head[20], *in;
@@ -1204,10 +1220,44 @@ unsigned long get_size_from_delta(struct packed_git *p,
 	data = delta_head;
 
 	/* ignore base size */
-	get_delta_hdr_size(&data, delta_head+sizeof(delta_head));
+	get_delta_hdr_size_sz(&data, delta_head+sizeof(delta_head));
 
 	/* Read the result size */
-	return get_delta_hdr_size(&data, delta_head+sizeof(delta_head));
+	return get_delta_hdr_size_sz(&data, delta_head+sizeof(delta_head));
+}
+
+unsigned long get_size_from_delta(struct packed_git *p,
+				  struct pack_window **w_curs,
+				  off_t curpos)
+{
+	size_t size = get_size_from_delta_sz(p, w_curs, curpos);
+	return cast_size_t_to_ulong(size);
+}
+
+/*
+ * Like unpack_object_header(), but returns size via size_t* instead of
+ * unsigned long*. This is needed for >4GB objects on Windows where
+ * unsigned long is 32-bit but size_t is 64-bit. Used by streaming code
+ * to get the correct untruncated object size.
+ */
+static int unpack_object_header_sz(struct packed_git *p,
+				   struct pack_window **w_curs,
+				   off_t *curpos,
+				   size_t *sizep)
+{
+	unsigned char *base;
+	unsigned long left;
+	unsigned long used;
+	enum object_type type;
+
+	base = use_pack(p, w_curs, *curpos, &left);
+	used = unpack_object_header_buffer_internal(base, left, &type, sizep);
+	if (!used) {
+		type = OBJ_BAD;
+	} else
+		*curpos += used;
+
+	return type;
 }
 
 int unpack_object_header(struct packed_git *p,
@@ -1582,7 +1632,7 @@ int packed_object_info(struct packed_git *p,
 		       off_t obj_offset, struct object_info *oi)
 {
 	struct pack_window *w_curs = NULL;
-	unsigned long size;
+	size_t size;
 	off_t curpos = obj_offset;
 	enum object_type type = OBJ_NONE;
 	int ret;
@@ -1597,7 +1647,8 @@ int packed_object_info(struct packed_git *p,
 		if (!*oi->contentp)
 			type = OBJ_BAD;
 	} else if (oi->sizep || oi->typep || oi->delta_base_oid) {
-		type = unpack_object_header(p, &w_curs, &curpos, &size);
+		/* Use size_t variant to handle >4GB objects on Windows */
+		type = unpack_object_header_sz(p, &w_curs, &curpos, &size);
 	}
 
 	if (!oi->contentp && oi->sizep) {
@@ -1609,14 +1660,18 @@ int packed_object_info(struct packed_git *p,
 				ret = -1;
 				goto out;
 			}
-			*oi->sizep = get_size_from_delta(p, &w_curs, tmp_pos);
-			if (*oi->sizep == 0) {
+			/*
+			 * Use size_t variant to avoid die() on >4GB deltas.
+			 * oi->sizep is unsigned long, so truncation may occur,
+			 * but streaming code uses its own size_t tracking.
+			 */
+			size = get_size_from_delta_sz(p, &w_curs, tmp_pos);
+			if (size == 0) {
 				ret = -1;
 				goto out;
 			}
-		} else {
-			*oi->sizep = size;
 		}
+		*oi->sizep = (unsigned long)size;
 	}
 
 	if (oi->disk_sizep) {
@@ -2561,21 +2616,29 @@ int packfile_store_read_object_stream(struct odb_read_stream **out,
 	struct pack_window *window = NULL;
 	struct object_info oi = OBJECT_INFO_INIT;
 	enum object_type in_pack_type;
-	unsigned long size;
-
-	oi.sizep = &size;
+	size_t size;
 
+	/*
+	 * We need to check if this is a delta or if the object is smaller
+	 * than the big file threshold. For the initial check, we don't need
+	 * the exact size, just whether it qualifies for streaming.
+	 */
 	if (packfile_store_read_object_info(store, oid, &oi, 0) ||
 	    oi.u.packed.type == PACKED_OBJECT_TYPE_REF_DELTA ||
-	    oi.u.packed.type == PACKED_OBJECT_TYPE_OFS_DELTA ||
-	    repo_settings_get_big_file_threshold(store->source->odb->repo) >= size)
+	    oi.u.packed.type == PACKED_OBJECT_TYPE_OFS_DELTA)
 		return -1;
 
-	in_pack_type = unpack_object_header(oi.u.packed.pack,
-					    &window,
-					    &oi.u.packed.offset,
-					    &size);
+	/* Read the actual size using size_t to handle >4GB objects on Windows */
+	in_pack_type = unpack_object_header_sz(oi.u.packed.pack,
+					       &window,
+					       &oi.u.packed.offset,
+					       &size);
 	unuse_pack(&window);
+
+	/* Now check the big file threshold with the correct size */
+	if (repo_settings_get_big_file_threshold(store->source->odb->repo) >= size)
+		return -1;
+
 	switch (in_pack_type) {
 	default:
 		return -1; /* we do not do deltas for now */
diff --git a/t/helper/meson.build b/t/helper/meson.build
index cba4a9bf4f1434..d4499d26a9af1f 100644
--- a/t/helper/meson.build
+++ b/t/helper/meson.build
@@ -70,6 +70,7 @@ test_tool_sources = [
   'test-submodule-nested-repo-config.c',
   'test-submodule.c',
   'test-subprocess.c',
+  'test-synthesize.c',
   'test-tool.c',
   'test-trace2.c',
   'test-truncate.c',
diff --git a/t/helper/test-synthesize.c b/t/helper/test-synthesize.c
new file mode 100644
index 00000000000000..d4135b115bc9a2
--- /dev/null
+++ b/t/helper/test-synthesize.c
@@ -0,0 +1,250 @@
+#define USE_THE_REPOSITORY_VARIABLE
+
+#include "test-tool.h"
+#include "git-compat-util.h"
+#include "git-zlib.h"
+#include "hash.h"
+#include "hex.h"
+#include "object.h"
+#include "parse-options.h"
+#include "repository.h"
+#include "setup.h"
+#include "strbuf.h"
+
+#define BLOCK_SIZE 0xffff
+static const unsigned char zeros[BLOCK_SIZE];
+
+/*
+ * Write data as an uncompressed zlib stream.
+ * For data larger than 64KB, writes multiple uncompressed blocks.
+ * If data is NULL, writes zeros.
+ * Updates the pack checksum context.
+ */
+static void write_uncompressed_zlib(FILE *f, struct git_hash_ctx *pack_ctx,
+				    const void *data, size_t len,
+				    const struct git_hash_algo *algo)
+{
+	unsigned char zlib_header[2] = { 0x78, 0x01 }; /* CMF, FLG */
+	unsigned char block_header[5];
+	const unsigned char *p = data;
+	size_t remaining = len;
+	uint32_t adler = 1L; /* adler32 initial value */
+	unsigned char adler_buf[4];
+
+	/* Write zlib header */
+	fwrite(zlib_header, 1, 2, f);
+	algo->update_fn(pack_ctx, zlib_header, 2);
+
+	/* Write uncompressed blocks (max 64KB each) */
+	do {
+		size_t block_len = remaining > BLOCK_SIZE ? BLOCK_SIZE : remaining;
+		int is_final = (block_len == remaining);
+		const unsigned char *block_data = data ? p : zeros;
+
+		block_header[0] = is_final ? 0x01 : 0x00;
+		block_header[1] = block_len & 0xff;
+		block_header[2] = (block_len >> 8) & 0xff;
+		block_header[3] = block_header[1] ^ 0xff;
+		block_header[4] = block_header[2] ^ 0xff;
+
+		fwrite(block_header, 1, 5, f);
+		algo->update_fn(pack_ctx, block_header, 5);
+
+		if (block_len) {
+			fwrite(block_data, 1, block_len, f);
+			algo->update_fn(pack_ctx, block_data, block_len);
+			adler = adler32(adler, block_data, block_len);
+		}
+
+		if (data)
+			p += block_len;
+		remaining -= block_len;
+	} while (remaining > 0);
+
+	/* Write adler32 checksum */
+	put_be32(adler_buf, adler);
+	fwrite(adler_buf, 1, 4, f);
+	algo->update_fn(pack_ctx, adler_buf, 4);
+}
+
+/*
+ * Write a pack object header for the given type and size.
+ * Returns the number of bytes written to the buffer.
+ */
+static size_t object_header(char *buf, enum object_type type, size_t size)
+{
+	unsigned char *p = (unsigned char *)buf;
+	*p = (type << 4) | (size & 0xf);
+	size >>= 4;
+	while (size) {
+		*p++ |= 0x80;
+		*p = size & 0x7f;
+		size >>= 7;
+	}
+	p++;
+	return p - (unsigned char *)buf;
+}
+
+/*
+ * Write an uncompressed object to the pack file.
+ * If `data == NULL`, it is treated like a buffer to NUL bytes.
+ * Updates the pack checksum context.
+ */
+static void write_pack_object(FILE *f, struct git_hash_ctx *pack_ctx,
+			      enum object_type type,
+			      const void *data, size_t len,
+			      struct object_id *oid,
+			      const struct git_hash_algo *algo)
+{
+	char header[32];
+	size_t header_len;
+	struct git_hash_ctx ctx;
+
+	/* Write pack object header */
+	header_len = object_header(header, type, len);
+	fwrite(header, 1, header_len, f);
+	algo->update_fn(pack_ctx, header, header_len);
+
+	/* Write the data as uncompressed zlib */
+	write_uncompressed_zlib(f, pack_ctx, data, len, algo);
+
+	algo->init_fn(&ctx);
+	header_len = xsnprintf(header, sizeof(header), "%s %"PRIuMAX,
+			       type_name(type), (uintmax_t)len) + 1;
+	algo->update_fn(&ctx, header, header_len);
+	if (data)
+		algo->update_fn(&ctx, data, len);
+	else {
+		for (size_t i = len / BLOCK_SIZE; i; i--)
+			algo->update_fn(&ctx, zeros, BLOCK_SIZE);
+		algo->update_fn(&ctx, zeros, len % BLOCK_SIZE);
+	}
+	algo->final_oid_fn(oid, &ctx);
+}
+
+/*
+ * Generate a pack file with a single large (>4GB) reachable object.
+ *
+ * Creates:
+ *   1. A large blob (all NUL bytes)
+ *   2. A tree containing that blob as "file"
+ *   3. A commit using that tree
+ *   4. The empty tree
+ *   5. A child commit using the empty tree
+ *
+ * This is useful for testing that Git can handle objects larger than 4GB.
+ */
+static int generate_pack_with_large_object(const char *path, size_t blob_size,
+					   const struct git_hash_algo *algo)
+{
+	FILE *f = fopen_for_writing(path);
+	struct git_hash_ctx pack_ctx;
+	char header[1024];
+	struct object_id blob_oid, tree_oid, commit_oid, empty_tree_oid, final_commit_oid;
+	struct strbuf buf = STRBUF_INIT;
+	size_t object_count = 5; /* large blob, tree, commit, empty tree, final commit */
+
+	algo->init_fn(&pack_ctx);
+
+	/* Write pack header */
+	memcpy(header, "PACK", 4);
+	put_be32(header + 4, 2);
+	put_be32(header + 8, object_count);
+	fwrite(header, 1, 12, f);
+	algo->update_fn(&pack_ctx, header, 12);
+
+	/* 1. Write the large blob */
+	write_pack_object(f, &pack_ctx, OBJ_BLOB, NULL, blob_size, &blob_oid, algo);
+
+	/* 2. Write tree containing the blob as "file" */
+	strbuf_addf(&buf, "100644 file%c", '\0');
+	strbuf_add(&buf, blob_oid.hash, algo->rawsz);
+	write_pack_object(f, &pack_ctx, OBJ_TREE, buf.buf, buf.len, &tree_oid, algo);
+
+	/* 3. Write commit using that tree */
+	strbuf_reset(&buf);
+	strbuf_addf(&buf,
+		    "tree %s\n"
+		    "author A U Thor <author@example.com> 1234567890 +0000\n"
+		    "committer C O Mitter <committer@example.com> 1234567890 +0000\n"
+		    "\n"
+		    "Large blob commit\n",
+		    oid_to_hex(&tree_oid));
+	write_pack_object(f, &pack_ctx, OBJ_COMMIT, buf.buf, buf.len, &commit_oid, algo);
+
+	/* 4. Write the empty tree */
+	write_pack_object(f, &pack_ctx, OBJ_TREE, "", 0, &empty_tree_oid, algo);
+
+	/* 5. Write final commit using empty tree, with previous commit as parent */
+	strbuf_reset(&buf);
+	strbuf_addf(&buf,
+		    "tree %s\n"
+		    "parent %s\n"
+		    "author A U Thor <author@example.com> 1234567890 +0000\n"
+		    "committer C O Mitter <committer@example.com> 1234567890 +0000\n"
+		    "\n"
+		    "Empty tree commit\n",
+		    oid_to_hex(&empty_tree_oid),
+		    oid_to_hex(&commit_oid));
+	write_pack_object(f, &pack_ctx, OBJ_COMMIT, buf.buf, buf.len, &final_commit_oid, algo);
+
+	/* Write pack trailer (checksum) */
+	algo->final_fn((unsigned char *)header, &pack_ctx);
+	fwrite(header, 1, algo->rawsz, f);
+
+	fclose(f);
+
+	strbuf_release(&buf);
+
+	/* Print the final commit OID so caller can set up refs */
+	printf("%s\n", oid_to_hex(&final_commit_oid));
+
+	return 0;
+}
+
+static int cmd__synthesize__pack(int argc, const char **argv,
+				 const char *prefix UNUSED,
+				 struct repository *repo)
+{
+	int non_git;
+	const struct git_hash_algo *algo;
+	size_t count;
+	const char *path;
+	const char * const usage[] = {
+		"test-tool synthesize pack <count> <filename>",
+		NULL
+	};
+	struct option options[] = {
+		OPT_END()
+	};
+
+	setup_git_directory_gently(&non_git);
+	repo = the_repository;
+	algo = repo->hash_algo;
+
+	argc = parse_options(argc, argv, NULL, options, usage,
+			     PARSE_OPT_KEEP_ARGV0);
+	if (argc != 3)
+		usage_with_options(usage, options);
+
+	count = strtoumax(argv[1], NULL, 10);
+	path = argv[2];
+
+	return !!generate_pack_with_large_object(path, count, algo);
+}
+
+int cmd__synthesize(int argc, const char **argv)
+{
+	const char *prefix = NULL;
+	char const * const synthesize_usage[] = {
+		"test-tool synthesize pack <options>",
+		NULL,
+	};
+	parse_opt_subcommand_fn *fn = NULL;
+	struct option options[] = {
+		OPT_SUBCOMMAND("pack", &fn, cmd__synthesize__pack),
+		OPT_END()
+	};
+	argc = parse_options(argc, argv, prefix, options, synthesize_usage, 0);
+	return !!fn(argc, argv, prefix, NULL);
+}
diff --git a/t/helper/test-tool.c b/t/helper/test-tool.c
index 9d1b41c8e39b89..ee16b2cb23719e 100644
--- a/t/helper/test-tool.c
+++ b/t/helper/test-tool.c
@@ -83,6 +83,7 @@ static struct test_cmd cmds[] = {
 	{ "submodule-config", cmd__submodule_config },
 	{ "submodule-nested-repo-config", cmd__submodule_nested_repo_config },
 	{ "subprocess", cmd__subprocess },
+	{ "synthesize", cmd__synthesize },
 	{ "trace2", cmd__trace2 },
 	{ "truncate", cmd__truncate },
 	{ "userdiff", cmd__userdiff },
diff --git a/t/helper/test-tool.h b/t/helper/test-tool.h
index e18e5a9ed9de81..4cf9f935a4cdfa 100644
--- a/t/helper/test-tool.h
+++ b/t/helper/test-tool.h
@@ -76,6 +76,7 @@ int cmd__submodule(int argc, const char **argv);
 int cmd__submodule_config(int argc, const char **argv);
 int cmd__submodule_nested_repo_config(int argc, const char **argv);
 int cmd__subprocess(int argc, const char **argv);
+int cmd__synthesize(int argc, const char **argv);
 int cmd__trace2(int argc, const char **argv);
 int cmd__truncate(int argc, const char **argv);
 int cmd__userdiff(int argc, const char **argv);
diff --git a/t/t5608-clone-2gb.sh b/t/t5608-clone-2gb.sh
index 87a8cd9f98381a..67b611353de8c4 100755
--- a/t/t5608-clone-2gb.sh
+++ b/t/t5608-clone-2gb.sh
@@ -49,4 +49,15 @@ test_expect_success 'clone - with worktree, file:// protocol' '
 
 '
 
+test_expect_success 'clone with >4GB object' '
+	# Generate a pack with a single >4GB blob to test the shift overflow fix.
+	# The bug causes size calculation to overflow when shift > 32 bits.
+	git init --bare 4gb-repo &&
+	head_oid=$(test-tool synthesize pack $((4*1024*1024*1024+1)) 4gb-repo/objects/pack/test.pack) &&
+	git -C 4gb-repo index-pack objects/pack/test.pack &&
+	git -C 4gb-repo update-ref refs/heads/main $head_oid &&
+
+	git clone --no-checkout --bare 4gb-repo 4gb-clone
+'
+
 test_done