diff --git a/tests/integration/test_features.py b/tests/integration/test_features.py
index 422c21d..6474c6b 100644
--- a/tests/integration/test_features.py
+++ b/tests/integration/test_features.py
@@ -550,6 +550,147 @@ class TestIncremental:
         assert not mismatches, f"Mismatch: {mismatches}"
 
 
+class TestChecksumChoice:
+    """--checksum-choice/--cc and --checksum-seed: the whole-file digest used by
+    the --incremental/--checksum handshake is selectable and seedable.  The
+    receiver hashes the on-disk old file with the SAME algorithm+seed, so an
+    unchanged file is skipped and a changed file (even with identical size and
+    mtime) is transferred -- and the transfer always lands byte-exact.
+    FastSync accepts xxh64 (default, seed-aware) and md5; names it does not
+    implement are rejected, never silently ignored."""
+
+    def test_unsupported_algorithm_is_rejected(self, shared_server):
+        result, _ = run_client(
+            SOURCE_DIR, DEST_DIR,
+            flags=["--checksum", "--checksum-choice=sha256"],
+            port=shared_server.port,
+        )
+        assert result.returncode != 0, "sha256 must be rejected, not silently ignored"
+
+    @pytest.mark.parametrize("algo", ["xxh64", "md5"])
+    @pytest.mark.parametrize("mt", [False, True])
+    def test_unchanged_skipped_and_bytes_preserved(self, shared_server, algo, mt):
+        clean_dir(DEST_DIR)
+        result, _ = run_client(SOURCE_DIR, DEST_DIR, flags=["-M"], port=shared_server.port)
+        assert result.returncode == 0, f"seed sync failed: {result.stderr[:200]}"
+
+        flags = (["-M", "--incremental", "--checksum", f"--checksum-choice={algo}"] +
+                 (["-m"] if mt else []))
+        result, _ = run_client(SOURCE_DIR, DEST_DIR, flags=flags, port=shared_server.port)
+        assert result.returncode == 0, f"checksum {algo} run failed: {result.stderr[:200]}"
+
+        received = get_dest_received_dir(DEST_DIR, SOURCE_DIR)
+        mismatches, missing = verify_transfer(SOURCE_DIR, received)
+        assert not missing, f"Missing: {missing}"
+        assert not mismatches, f"Mismatch: {mismatches}"
+
+    # A changed source file with the SAME size and mtime must still be
+    # detected (and re-transferred byte-exactly) because the whole-file digest
+    # differs -- the explicit reason --checksum exists.  This exercises the
+    # sender/receiver digest agreement for a non-default algorithm.
+    @pytest.mark.parametrize("algo", ["xxh64", "md5"])
+    @pytest.mark.parametrize("mt", [False, True])
+    def test_changed_same_size_mtime_redetected(self, shared_server, algo, mt):
+        clean_dir(DEST_DIR)
+        result, _ = run_client(SOURCE_DIR, DEST_DIR, flags=["-M"], port=shared_server.port)
+        assert result.returncode == 0
+
+        received = get_dest_received_dir(DEST_DIR, SOURCE_DIR)
+        source_file = os.path.join(SOURCE_DIR, "small.txt")  # "hello world\n" (12 bytes)
+        received_file = os.path.join(received, "small.txt")
+        source_stat = os.stat(source_file)
+        with open(received_file, "wb") as f:
+            f.write(b"DDDDDDDDDDDD")  # same size, different content
+        os.utime(received_file, (source_stat.st_atime, source_stat.st_mtime))
+
+        flags = (["-M", "--incremental", "--checksum", f"--checksum-choice={algo}"] +
+                 (["-m"] if mt else []))
+        result, _ = run_client(SOURCE_DIR, DEST_DIR, flags=flags, port=shared_server.port)
+        assert result.returncode == 0, f"checksum {algo} redetect failed: {result.stderr[:200]}"
+        with open(received_file, "rb") as f:
+            assert f.read() == b"hello world\n"
+
+    @pytest.mark.parametrize("algo", ["xxh64", "md5"])
+    def test_unchanged_run_transfers_almost_no_data(self, shared_server, algo):
+        # A fully-unchanged --checksum run skips every file: only the config + a
+        # small handshake travels, not the payloads.  Proxy byte counts are not
+        # available for -m (multithreaded connections), so single-thread only.
+        clean_dir(DEST_DIR)
+        result, _ = run_client(SOURCE_DIR, DEST_DIR, flags=["-M"], port=shared_server.port)
+        assert result.returncode == 0
+
+        flags = ["-M", "--incremental", "--checksum", f"--checksum-choice={algo}"]
+        proxy = CountingProxy(shared_server.port)
+        cmd = (CLIENT_CMD + ["--source-dir", SOURCE_DIR, "--dest-dir", DEST_DIR,
+                             "--save-to-disk", "--server-port", str(proxy.port)] + flags)
+        result = proxy.run(cmd)
+        assert result.returncode == 0, f"checksum {algo} skip run failed: {result.stderr[:200]}"
+        assert proxy.client_to_server < 100000, \
+            f"unchanged --checksum run sent {proxy.client_to_server} bytes; expected a skip"
+
+    @pytest.mark.parametrize("mt", [False, True])
+    def test_seed_is_deterministic_and_preserves_content(self, shared_server, mt):
+        clean_dir(DEST_DIR)
+        flags = ["-M", "--incremental", "--checksum",
+                 "--checksum-choice=xxh64", "--checksum-seed=987654"] + (["-m"] if mt else [])
+        first, _ = run_client(SOURCE_DIR, DEST_DIR, flags=flags, port=shared_server.port)
+        assert first.returncode == 0, f"seeded run failed: {first.stderr[:200]}"
+
+        # A second run with the SAME seed and unchanged content skips everything
+        # deterministically (same digests both sides).
+        second, _ = run_client(SOURCE_DIR, DEST_DIR, flags=flags, port=shared_server.port)
+        assert second.returncode == 0, f"deterministic rerun failed: {second.stderr[:200]}"
+
+        received = get_dest_received_dir(DEST_DIR, SOURCE_DIR)
+        mismatches, missing = verify_transfer(SOURCE_DIR, received)
+        assert not missing and not mismatches, f"missing={missing} mismatches={mismatches}"
+
+        # A changed file with the same size and mtime is still caught and fixed
+        # (a non-zero seed does not weaken the comparison).
+        source_file = os.path.join(SOURCE_DIR, "medium.txt")
+        received_file = os.path.join(received, "medium.txt")
+        source_stat = os.stat(source_file)
+        with open(received_file, "wb") as f:
+            f.write(b"z" * os.path.getsize(source_file))
+        os.utime(received_file, (source_stat.st_atime, source_stat.st_mtime))
+        third, _ = run_client(SOURCE_DIR, DEST_DIR, flags=flags, port=shared_server.port)
+        assert third.returncode == 0, f"seeded redetect failed: {third.stderr[:200]}"
+        with open(received_file, "rb") as f:
+            assert f.read() == open(source_file, "rb").read()
+
+    # --checksum-seed also feeds the delta path's per-block strong checksum on
+    # both ends (receiver signature and sender window hash use the same seed),
+    # so a seeded delta transfer still lands byte-exact.
+    @pytest.mark.parametrize("mt", [False, True])
+    def test_seed_delta_block_hash_transfers_byte_exact(self, shared_server, mt):
+        source = os.path.join(TEST_DATA_DIR, f"ccseed_{'m' if mt else 's'}_src")
+        dest = os.path.join(TEST_DATA_DIR, f"ccseed_{'m' if mt else 's'}_dst")
+        clean_dir(source)
+        clean_dir(dest)
+        big = os.path.join(source, "big.bin")
+        with open(big, "wb") as f:
+            f.write(bytes(range(256)) * 200)  # 51200 bytes > delta 16K floor
+        result, _ = run_client(source, dest, flags=["-M"], port=shared_server.port)
+        assert result.returncode == 0, f"seed delta seed failed: {result.stderr[:200]}"
+
+        # Edit a region so the receiver must match a changed block with the seed.
+        with open(big, "r+b") as f:
+            f.seek(1000)
+            f.write(b"\x00" * 64)
+        # Force an mtime mismatch: the incremental quick-check skips files whose
+        # stored mtime second equals the source's, which can collide when the
+        # edit and the prior sync share a second.  Setting an old dest mtime
+        # guarantees the delta path is exercised deterministically.
+        os.utime(os.path.join(get_dest_received_dir(dest, source), "big.bin"), (0, 0))
+        flags = (["-M", "--incremental", "--delta", "--checksum-seed=314159"] +
+                 (["-m"] if mt else []))
+        result, _ = run_client(source, dest, flags=flags, port=shared_server.port)
+        assert result.returncode == 0, f"seed delta run failed: {result.stderr[:200]}"
+        received = get_dest_received_dir(dest, source)
+        assert _read_file(os.path.join(received, "big.bin")) == _read_file(big), \
+            "seeded delta transfer is not byte-exact"
+
+
 class TestUpdate:
     def test_update_skips_older_destination_and_allows_equal_or_newer_source(self, shared_server):
         clean_dir(DEST_DIR)
diff --git a/tests/runner.c b/tests/runner.c
index c74c6de..7232426 100644
--- a/tests/runner.c
+++ b/tests/runner.c
@@ -1,6 +1,7 @@
 #include "test_array_list.h"
 #include "test_chunk.h"
 #include "test_change_list.h"
+#include "test_checksum.h"
 #include "test_client_cli.h"
 #include "test_compression.h"
 #include "test_config.h"
@@ -46,6 +47,7 @@ int main() {
   RUN_TEST(test_config);
   RUN_TEST(test_compression);
   RUN_TEST(test_scanner);
+  RUN_TEST(test_checksum);
   RUN_TEST(test_delta);
   RUN_TEST(test_data);
   RUN_TEST(test_protocol);
diff --git a/tests/test_checksum.c b/tests/test_checksum.c
new file mode 100644
index 0000000..05a0a95
--- /dev/null
+++ b/tests/test_checksum.c
@@ -0,0 +1,146 @@
+#include "test_checksum.h"
+#include "checksum.h"
+#include "test_utils.h"
+#include <string.h>
+
+/* Known xxHash64 vector (seed 0) for the empty string and a literal.
+ * The md5 vectors are the standard NIST/RFC1321 test strings.  These pin the
+ * digest selection to genuinely distinct algorithm outputs so a --checksum-
+ * choice change is observable, not a silent no-op. */
+
+static void test_checksum_xxh64_seed0() {
+  uint8_t out[CHECKSUM_MAX_DIGEST_LEN];
+  size_t len = 0;
+  EXPECT_TRUE(checksum_digest(CHECKSUM_ALGO_XXH64, 0, "hello", 5, out, sizeof(out), &len));
+  EXPECT_TRUE(len == (size_t)8);
+  /* Hard-coded: XXH64("hello", 5, 0). */
+  uint8_t expect[8] = {0xa3, 0x6d, 0x9f, 0x88, 0x7d, 0x82, 0xc7, 0x26};
+  for (int i = 0; i < 8; i++)
+    EXPECT_EQ_INT(out[i], expect[i]);
+}
+
+static void test_checksum_xxh64_empty() {
+  uint8_t out[CHECKSUM_MAX_DIGEST_LEN];
+  size_t len = 0;
+  EXPECT_TRUE(checksum_digest(CHECKSUM_ALGO_XXH64, 0, "", 0, out, sizeof(out), &len));
+  EXPECT_TRUE(len == (size_t)8);
+  /* XXH64("", 0, 0). */
+  uint8_t expect[8] = {0x99, 0xe9, 0xd8, 0x51, 0x37, 0xdb, 0x46, 0xef};
+  for (int i = 0; i < 8; i++)
+    EXPECT_EQ_INT(out[i], expect[i]);
+}
+
+/* A nonzero seed must change the xxh64 digest: the algorithm is genuinely
+ * seed-aware, deterministic, and distinct from seed 0. */
+static void test_checksum_xxh64_seed_changes_digest() {
+  uint8_t a[CHECKSUM_MAX_DIGEST_LEN], b[CHECKSUM_MAX_DIGEST_LEN];
+  size_t alen = 0, blen = 0;
+  EXPECT_TRUE(checksum_digest(CHECKSUM_ALGO_XXH64, 7, "payload", 7, a, sizeof(a), &alen));
+  EXPECT_TRUE(checksum_digest(CHECKSUM_ALGO_XXH64, 0, "payload", 7, b, sizeof(b), &blen));
+  EXPECT_TRUE(alen == blen);
+  EXPECT_TRUE(memcmp(a, b, alen) != 0);
+}
+
+static void test_checksum_xxh64_seed_deterministic() {
+  uint8_t a[CHECKSUM_MAX_DIGEST_LEN], b[CHECKSUM_MAX_DIGEST_LEN];
+  size_t alen = 0, blen = 0;
+  EXPECT_TRUE(checksum_digest(CHECKSUM_ALGO_XXH64, 12345, "same", 4, a, sizeof(a), &alen));
+  EXPECT_TRUE(checksum_digest(CHECKSUM_ALGO_XXH64, 12345, "same", 4, b, sizeof(b), &blen));
+  EXPECT_TRUE(alen == blen);
+  EXPECT_TRUE(memcmp(a, b, alen) == 0);
+}
+
+static void test_checksum_md5_vectors() {
+  uint8_t out[CHECKSUM_MAX_DIGEST_LEN];
+  size_t len = 0;
+  EXPECT_TRUE(checksum_digest(CHECKSUM_ALGO_MD5, 0, "", 0, out, sizeof(out), &len));
+  EXPECT_TRUE(len == (size_t)16);
+  uint8_t expect_empty[16] = {0xd4, 0x1d, 0x8c, 0xd9, 0x8f, 0x00, 0xb2, 0x04,
+                              0xe9, 0x80, 0x09, 0x98, 0xec, 0xf8, 0x42, 0x7e};
+  EXPECT_TRUE(memcmp(out, expect_empty, 16) == 0);
+
+  /* MD5("abc") */
+  EXPECT_TRUE(checksum_digest(CHECKSUM_ALGO_MD5, 0, "abc", 3, out, sizeof(out), &len));
+  uint8_t expect_abc[16] = {0x90, 0x01, 0x50, 0x98, 0x3c, 0xd2, 0x4f, 0xb0,
+                            0xd6, 0x96, 0x3f, 0x7d, 0x28, 0xe1, 0x7f, 0x72};
+  EXPECT_TRUE(memcmp(out, expect_abc, 16) == 0);
+}
+
+/* md5 is 16 bytes and differs from the 8-byte xxh64 for the same input, so the
+ * choice is observably different both in length and in content. */
+static void test_checksum_algo_lengths_distinct() {
+  EXPECT_EQ_INT((int)checksum_digest_len(CHECKSUM_ALGO_XXH64), 8);
+  EXPECT_EQ_INT((int)checksum_digest_len(CHECKSUM_ALGO_MD5), 16);
+
+  uint8_t x[CHECKSUM_MAX_DIGEST_LEN], m[CHECKSUM_MAX_DIGEST_LEN];
+  size_t xl = 0, ml = 0;
+  EXPECT_TRUE(checksum_digest(CHECKSUM_ALGO_XXH64, 0, "same content", 12, x, sizeof(x), &xl));
+  EXPECT_TRUE(checksum_digest(CHECKSUM_ALGO_MD5, 0, "same content", 12, m, sizeof(m), &ml));
+  EXPECT_TRUE(xl == (size_t)8);
+  EXPECT_TRUE(ml == (size_t)16);
+  EXPECT_TRUE(memcmp(x, m, 8) != 0);
+}
+
+/* md5 has no seed: two distinct seeds give the same md5 digest (documented);
+ * the seed is only honored by xxh64 and the delta block hash (low 32 bits). */
+static void test_checksum_md5_seed_ignored() {
+  uint8_t a[CHECKSUM_MAX_DIGEST_LEN], b[CHECKSUM_MAX_DIGEST_LEN];
+  size_t alen = 0, blen = 0;
+  EXPECT_TRUE(checksum_digest(CHECKSUM_ALGO_MD5, 0, "data", 4, a, sizeof(a), &alen));
+  EXPECT_TRUE(checksum_digest(CHECKSUM_ALGO_MD5, 99, "data", 4, b, sizeof(b), &blen));
+  EXPECT_TRUE(memcmp(a, b, alen) == 0);
+}
+
+static void test_checksum_algo_name_mapping() {
+  EXPECT_EQ_INT(checksum_algo_from_name("xxh64"), (int)CHECKSUM_ALGO_XXH64);
+  EXPECT_EQ_INT(checksum_algo_from_name("XXH64"), (int)CHECKSUM_ALGO_XXH64);
+  EXPECT_EQ_INT(checksum_algo_from_name("xxhash"), (int)CHECKSUM_ALGO_XXH64);
+  EXPECT_EQ_INT(checksum_algo_from_name("XXHASH"), (int)CHECKSUM_ALGO_XXH64);
+  EXPECT_EQ_INT(checksum_algo_from_name("md5"), (int)CHECKSUM_ALGO_MD5);
+  EXPECT_EQ_INT(checksum_algo_from_name("MD5"), (int)CHECKSUM_ALGO_MD5);
+  EXPECT_TRUE(checksum_algo_from_name("sha256") < 0);
+  EXPECT_TRUE(checksum_algo_from_name("crc32") < 0);
+  EXPECT_TRUE(checksum_algo_from_name("none") < 0);
+  EXPECT_TRUE(checksum_algo_from_name("") < 0);
+  EXPECT_TRUE(checksum_algo_from_name(NULL) < 0);
+
+  EXPECT_TRUE(checksum_algo_valid((int)CHECKSUM_ALGO_XXH64));
+  EXPECT_TRUE(checksum_algo_valid((int)CHECKSUM_ALGO_MD5));
+  EXPECT_FALSE(checksum_algo_valid(99));
+  EXPECT_EQ_STR(checksum_algo_name(CHECKSUM_ALGO_XXH64), "xxh64");
+  EXPECT_EQ_STR(checksum_algo_name(CHECKSUM_ALGO_MD5), "md5");
+}
+
+static void test_checksum_truncated_buffer_rejected() {
+  uint8_t small[4];
+  size_t len = 0;
+  /* The digest cannot fit in a 4-byte buffer. */
+  EXPECT_FALSE(checksum_digest(CHECKSUM_ALGO_XXH64, 0, "x", 1, small, sizeof(small), &len));
+  EXPECT_FALSE(checksum_digest(CHECKSUM_ALGO_MD5, 0, "x", 1, small, sizeof(small), &len));
+  EXPECT_FALSE(checksum_digest(CHECKSUM_ALGO_XXH64, 0, NULL, 5, small, sizeof(small), &len));
+  EXPECT_FALSE(checksum_digest(CHECKSUM_ALGO_XXH64, 0, "x", 1, NULL, 0, &len));
+  EXPECT_FALSE(checksum_digest(CHECKSUM_ALGO_XXH64, 0, "x", 1, small, sizeof(small), NULL));
+}
+
+/* A NULL data pointer with size 0 is the empty input, not an error. */
+static void test_checksum_null_empty_digest() {
+  uint8_t a[CHECKSUM_MAX_DIGEST_LEN], b[CHECKSUM_MAX_DIGEST_LEN];
+  size_t alen = 0, blen = 0;
+  EXPECT_TRUE(checksum_digest(CHECKSUM_ALGO_XXH64, 0, NULL, 0, a, sizeof(a), &alen));
+  EXPECT_TRUE(checksum_digest(CHECKSUM_ALGO_XXH64, 0, "", 0, b, sizeof(b), &blen));
+  EXPECT_TRUE(alen == blen);
+  EXPECT_TRUE(memcmp(a, b, alen) == 0);
+}
+
+void test_checksum(void) {
+  test_checksum_xxh64_seed0();
+  test_checksum_xxh64_empty();
+  test_checksum_xxh64_seed_changes_digest();
+  test_checksum_xxh64_seed_deterministic();
+  test_checksum_md5_vectors();
+  test_checksum_algo_lengths_distinct();
+  test_checksum_md5_seed_ignored();
+  test_checksum_algo_name_mapping();
+  test_checksum_truncated_buffer_rejected();
+  test_checksum_null_empty_digest();
+}
\ No newline at end of file
diff --git a/tests/test_checksum.h b/tests/test_checksum.h
new file mode 100644
index 0000000..8849100
--- /dev/null
+++ b/tests/test_checksum.h
@@ -0,0 +1,6 @@
+#ifndef TEST_CHECKSUM_H
+#define TEST_CHECKSUM_H
+
+void test_checksum(void);
+
+#endif
\ No newline at end of file
diff --git a/tests/test_client_cli.c b/tests/test_client_cli.c
index 5882bae..f0b70ae 100644
--- a/tests/test_client_cli.c
+++ b/tests/test_client_cli.c
@@ -1,4 +1,5 @@
 #include "test_client_cli.h"
+#include "checksum.h"
 #include "client_validation.h"
 #include "chmod.h"
 #include "config.h"
@@ -787,8 +788,7 @@ static void test_parse_args_rejects_unimplemented_options() {
                                         "--ipv4",
                                         "--daemon",
                                         "--config",
-                                        "--server",
-                                        "--checksum-choice"};
+                                        "--server"};
 
   for (size_t i = 0; i < sizeof(options) / sizeof(options[0]); i++) {
     Config* cfg = config_create();
@@ -953,7 +953,8 @@ static void test_parse_args_no_preserve_blocks_implicit_metadata() {
   }
 }
 
-/* Checksum-choice spellings are recognized and rejected until algorithms are implemented. */
+/* --checksum-choice and its --cc alias select the whole-file digest algorithm
+   (default xxh64; both "xxh64" and the rsync "xxhash" spelling accepted). */
 static void test_parse_args_checksum_choice_aliases() {
   static const char* const options[] = {"--checksum-choice", "--cc"};
 
@@ -962,12 +963,88 @@ static void test_parse_args_checksum_choice_aliases() {
     char* argv[] = {"fastsync", (char*)options[i], "xxh64", "/src", "/dst"};
     int positional_args[2];
     int positional_count = 0;
+    EXPECT_EQ_INT(parse_args(cfg, 5, argv, positional_args, &positional_count), 0);
+    EXPECT_EQ_INT(cfg->checksum_algo, (int)CHECKSUM_ALGO_XXH64);
+    config_delete(cfg);
+  }
+}
 
+/* Both the "--checksum-choice=ALG" and "--cc=ALG" inline forms parse. */
+static void test_parse_args_checksum_choice_equals_forms() {
+  Config* cfg = config_create();
+  char* argv[] = {"fastsync", "--checksum-choice=md5", "/src", "/dst"};
+  int positional_args[2];
+  int positional_count = 0;
+  EXPECT_EQ_INT(parse_args(cfg, 4, argv, positional_args, &positional_count), 0);
+  EXPECT_EQ_INT(cfg->checksum_algo, (int)CHECKSUM_ALGO_MD5);
+  config_delete(cfg);
+
+  cfg = config_create();
+  char* argv2[] = {"fastsync", "--cc=xxhash", "/src", "/dst"};
+  positional_count = 0;
+  EXPECT_EQ_INT(parse_args(cfg, 4, argv2, positional_args, &positional_count), 0);
+  EXPECT_EQ_INT(cfg->checksum_algo, (int)CHECKSUM_ALGO_XXH64);
+  config_delete(cfg);
+}
+
+/* An algorithm FastSync does not support must be rejected, never a silent
+   no-op. */
+static void test_parse_args_checksum_choice_rejects_unsupported() {
+  static const char* const bad[] = {"md4", "sha256", "crc32", "none", "bogus"};
+  for (size_t i = 0; i < sizeof(bad) / sizeof(bad[0]); i++) {
+    Config* cfg = config_create();
+    char* argv[] = {"fastsync", "--checksum-choice", (char*)bad[i], "/src", "/dst"};
+    int positional_args[2];
+    int positional_count = 0;
     EXPECT_EQ_INT(parse_args(cfg, 5, argv, positional_args, &positional_count), -1);
     config_delete(cfg);
   }
 }
 
+/* --checksum-seed parses as a 64-bit non-negative integer (space and = forms);
+   invalid values are rejected. */
+static void test_parse_args_checksum_seed() {
+  Config* cfg = config_create();
+  char* argv[] = {"fastsync", "--checksum-seed=42", "/src", "/dst"};
+  int positional_args[2];
+  int positional_count = 0;
+  EXPECT_EQ_INT(parse_args(cfg, 4, argv, positional_args, &positional_count), 0);
+  EXPECT_TRUE(cfg->checksum_seed == 42ULL);
+  config_delete(cfg);
+
+  cfg = config_create();
+  char* argv2[] = {"fastsync", "--checksum-seed", "12345", "/src", "/dst"};
+  positional_count = 0;
+  EXPECT_EQ_INT(parse_args(cfg, 5, argv2, positional_args, &positional_count), 0);
+  EXPECT_TRUE(cfg->checksum_seed == 12345ULL);
+  config_delete(cfg);
+
+  /* 0 is a valid (and default) seed. */
+  cfg = config_create();
+  char* argv3[] = {"fastsync", "--checksum-seed=0", "/src", "/dst"};
+  positional_count = 0;
+  EXPECT_EQ_INT(parse_args(cfg, 4, argv3, positional_args, &positional_count), 0);
+  EXPECT_TRUE(cfg->checksum_seed == 0ULL);
+  config_delete(cfg);
+
+  /* Non-numeric and negative seeds are rejected. */
+  static const char* const bad[] = {"abc", "-5", "1.5", ""};
+  for (size_t i = 0; i < sizeof(bad) / sizeof(bad[0]); i++) {
+    cfg = config_create();
+    char* argv4[] = {"fastsync", "--checksum-seed", (char*)bad[i], "/src", "/dst"};
+    positional_count = 0;
+    EXPECT_EQ_INT(parse_args(cfg, 5, argv4, positional_args, &positional_count), -1);
+    config_delete(cfg);
+  }
+
+  /* Missing value is rejected. */
+  cfg = config_create();
+  char* argv5[] = {"fastsync", "--checksum-seed"};
+  positional_count = 0;
+  EXPECT_EQ_INT(parse_args(cfg, 2, argv5, positional_args, &positional_count), -1);
+  config_delete(cfg);
+}
+
 static void test_parse_args_rejects_unsafe_negation() {
   static const char* const options[] = {"--no-archive", "--no-timeout", "--no-unknown"};
   for (size_t i = 0; i < sizeof(options) / sizeof(options[0]); i++) {
@@ -1943,6 +2020,9 @@ void test_client_cli() {
   test_parse_args_log_file_format();
   test_parse_args_checksum_choice_aliases();
   test_parse_args_checksum_choice_requires_value();
+  test_parse_args_checksum_choice_equals_forms();
+  test_parse_args_checksum_choice_rejects_unsupported();
+  test_parse_args_checksum_seed();
   test_parse_args_temp_dir();
   test_parse_args_delay_updates();
   test_validate_config_delay_updates_rejects_inplace();
diff --git a/tests/test_config.c b/tests/test_config.c
index ced2318..10b6211 100644
--- a/tests/test_config.c
+++ b/tests/test_config.c
@@ -754,6 +754,68 @@ static void test_config_is_remote_dest() {
   EXPECT_TRUE(config_is_remote_dest("user@host:"));
 }
 
+/* --checksum-choice/--cc and --checksum-seed cross the wire intact so the
+   receiver hashes the on-disk old file with the same algorithm and seed. */
+static void test_config_checksum_options_wire_roundtrip() {
+  if (is_running_under_valgrind())
+    return;
+
+  struct {
+    int algo;
+    unsigned long long seed;
+  } cases[] = {
+      {CHECKSUM_ALGO_XXH64, 0},
+      {CHECKSUM_ALGO_XXH64, 42},
+      {CHECKSUM_ALGO_MD5, 7},
+      {CHECKSUM_ALGO_MD5, 0},
+  };
+  for (size_t i = 0; i < sizeof(cases) / sizeof(cases[0]); i++) {
+    int p[2];
+    EXPECT_EQ_INT(socketpair(AF_UNIX, SOCK_STREAM, 0, p), 0);
+    pid_t pid = fork();
+    if (pid == 0) {
+      close(p[1]);
+      io_set_fds(p[0], p[0]);
+      Config* recv = config_receive(p[0]);
+      bool ok = recv != NULL && recv->checksum_algo == cases[i].algo &&
+                recv->checksum_seed == cases[i].seed;
+      config_delete(recv);
+      close(p[0]);
+      _exit(ok ? 0 : 1);
+    } else {
+      close(p[0]);
+      io_set_fds(p[1], p[1]);
+      Config* send_cfg = config_create();
+      EXPECT_NOT_NULL(send_cfg);
+      send_cfg->send_directory = str_dup("/src");
+      send_cfg->receive_root_directory = str_dup("/dst");
+      send_cfg->checksum_algo = cases[i].algo;
+      send_cfg->checksum_seed = cases[i].seed;
+      bool sent = config_send(p[1], send_cfg);
+      int status;
+      waitpid(pid, &status, 0);
+      close(p[1]);
+      config_delete(send_cfg);
+      EXPECT_TRUE(sent);
+      EXPECT_TRUE(WIFEXITED(status) && WEXITSTATUS(status) == 0);
+    }
+  }
+}
+
+/* An out-of-range algorithm id on the wire must be rejected on receive, never
+   accepted as-is (prevents mixing unsupported digests on a path). */
+static void test_config_receive_rejects_invalid_checksum_algo() {
+  if (is_running_under_valgrind())
+    return;
+  Config* c = config_create();
+  EXPECT_NOT_NULL(c);
+  c->send_directory = str_dup("/src");
+  c->receive_root_directory = str_dup("/dst");
+  c->checksum_algo = 99;
+  EXPECT_FALSE(roundtrip_config_ok(c));
+  config_delete(c);
+}
+
 void test_config() {
   test_config_lifecycle();
   test_config_ssh_dest();
@@ -774,6 +836,8 @@ void test_config() {
     test_config_basis_roundtrip();
     test_config_basis_wire_rejects_escaping();
     test_config_basis_normalization();
+    test_config_checksum_options_wire_roundtrip();
+    test_config_receive_rejects_invalid_checksum_algo();
   }
   test_config_delete_timing_early_helper();
   test_config_is_remote_dest();
diff --git a/tests/test_delta.c b/tests/test_delta.c
index ac522f8..cfc8846 100644
--- a/tests/test_delta.c
+++ b/tests/test_delta.c
@@ -634,12 +634,65 @@ static void test_delta_hash_index_large_mostly_matching() {
   free(new_data);
 }
 
+/* --checksum-seed: the delta strong (block) hash is genuinely seed-aware.  A
+ * nonzero seed changes the per-block xxHash32, and a signature + delta computed
+ * with the same seed still reconstruct the file exactly (symmetric), while a
+ * mismatched seed produces a delta that does not match the signature blocks. */
+static void test_delta_xxhash32_seeded() {
+  const char* data = "seedme";
+  uint32_t a = delta_xxhash32(data, 6);
+  uint32_t b = delta_xxhash32_seeded(data, 6, 42);
+  uint32_t c = delta_xxhash32_seeded(data, 6, 42);
+  EXPECT_TRUE(a != b);
+  EXPECT_EQ_INT((int)b, (int)c);
+  /* Unseeded == seeded with 0 (default reproduces today's behavior). */
+  EXPECT_EQ_INT((int)delta_xxhash32(data, 6), (int)delta_xxhash32_seeded(data, 6, 0));
+}
+
+static void test_delta_seeded_signature_compute_matches() {
+  uint32_t block_size = 1024;
+  /* Identical old/new data with a non-zero seed: the receiver builds a seeded
+     signature and the sender computes a seeded delta over the same bytes, so
+     every block matches and applying the delta rebuilds the file exactly. */
+  char data[4096];
+  for (int i = 0; i < 4096; i++)
+    data[i] = (char)(i % 256);
+
+  DeltaSignature* sig = delta_signature_create_seeded(data, 4096, block_size, 99);
+  EXPECT_NOT_NULL(sig);
+  Delta* delta = delta_compute_seeded(data, 4096, sig, block_size, 99);
+  EXPECT_NOT_NULL(delta);
+  void* rebuilt = delta_apply(data, 4096, delta, block_size);
+  EXPECT_NOT_NULL(rebuilt);
+  EXPECT_TRUE(memcmp(rebuilt, data, 4096) == 0);
+  free(rebuilt);
+  delta_destroy(delta);
+  delta_signature_destroy(sig);
+
+  /* A MISMATCHED seed means the sender's window xxHash32 never equals the
+     receiver's signature-block xxHash32: no block can match, so the delta is
+     not worthwhile / has no block matches.  This proves the seed really gates
+     the block comparison rather than being an inert parameter. */
+  sig = delta_signature_create_seeded(data, 4096, block_size, 99);
+  EXPECT_NOT_NULL(sig);
+  delta = delta_compute_seeded(data, 4096, sig, block_size, 7);
+  EXPECT_NOT_NULL(delta);
+  bool any_match = false;
+  for (uint32_t i = 0; i < delta->instruction_count; i++)
+    if (delta->instructions[i].type == DELTA_INSTR_BLOCK_MATCH)
+      any_match = true;
+  EXPECT_FALSE(any_match);
+  delta_destroy(delta);
+  delta_signature_destroy(sig);
+}
+
 void test_delta() {
   test_adler32_basic();
   test_adler32_different_data();
   test_xxhash32_basic();
   test_xxhash32_different_data();
   test_xxhash64_different_data();
+  test_delta_xxhash32_seeded();
   test_signature_roundtrip();
   test_delta_identical_files();
   test_delta_small_edit();
@@ -653,4 +706,5 @@ void test_delta() {
   test_delta_apply_rejects_output_overflow();
   test_delta_hash_index_matches_linear_reference();
   test_delta_hash_index_large_mostly_matching();
+  test_delta_seeded_signature_compute_matches();
 }