diff --git a/src/cli/cli.c b/src/cli/cli.c
index 142d8a880..884849e0b 100644
--- a/src/cli/cli.c
+++ b/src/cli/cli.c
@@ -2968,15 +2968,16 @@ static bool prompt_yn(const char *question) {
 /* Minimum line length in checksums.txt: CBM_SZ_64 hex + 2 spaces + 1 char filename */
 #define CHECKSUM_LINE_MIN (SHA256_HEX_LEN + 2)
 
-/* Compute SHA-CBM_SZ_256 of a file using platform tools (sha256sum/shasum).
- * Writes CBM_SZ_64-char hex digest + NUL to out. Returns 0 on success. */
-static int sha256_file(const char *path, char *out, size_t out_size) {
+/* Compute the SHA-256 of a file using platform tools (sha256sum/shasum).
+ * Writes a 64-char hex digest + NUL to out. Returns 0 on success. Not static:
+ * exercised directly by the self-update checksum regression test. */
+int cbm_cli_sha256_file(const char *path, char *out, size_t out_size) {
     if (out_size < SHA256_BUF_SIZE) {
         return CLI_ERR;
     }
     char cmd[CLI_BUF_1K];
 #ifdef __APPLE__
-    snprintf(cmd, sizeof(cmd), "shasum -a CBM_SZ_256 '%s' 2>/dev/null", path);
+    snprintf(cmd, sizeof(cmd), "shasum -a 256 '%s' 2>/dev/null", path);
 #else
     snprintf(cmd, sizeof(cmd), "sha256sum '%s' 2>/dev/null", path);
 #endif
@@ -3107,8 +3108,8 @@ static int verify_download_checksum(const char *archive_path, const char *archiv
     }
 
     char actual[SHA256_BUF_SIZE] = {0};
-    if (sha256_file(archive_path, actual, sizeof(actual)) != 0) {
-        (void)fprintf(stderr, "warning: sha256sum/shasum not available — skipping verification\n");
+    if (cbm_cli_sha256_file(archive_path, actual, sizeof(actual)) != 0) {
+        (void)fprintf(stderr, "error: could not compute checksum (sha256 tool unavailable)\n");
         return CLI_ERR;
     }
 
@@ -4250,8 +4251,12 @@ static int download_verify_install(const char *url, const char *ext, const char
     const char *portable = (strcmp(os, "linux") == 0) ? "-portable" : "";
     snprintf(archive_name, sizeof(archive_name), "codebase-memory-mcp-%s%s-%s%s.%s",
              want_ui ? "ui-" : "", os, arch, portable, ext);
+    /* Fail closed: install only a positively-verified download. A mismatch,
+     * a missing checksum entry, or an unavailable hash tool (crc != 0) all
+     * abort rather than install an unverified binary. */
     int crc = verify_download_checksum(tmp_archive, archive_name);
-    if (crc == CLI_TRUE) {
+    if (crc != 0) {
+        (void)fprintf(stderr, "error: refusing to install an unverified download\n");
         cbm_unlink(tmp_archive);
         return CLI_TRUE;
     }
diff --git a/tests/test_cli.c b/tests/test_cli.c
index 1aa4ea716..6f2b19c4a 100644
--- a/tests/test_cli.c
+++ b/tests/test_cli.c
@@ -3010,11 +3010,38 @@ TEST(cli_print_tool_help_issue680) {
     PASS();
 }
 
+/* The self-update path verifies a downloaded archive against a published
+ * checksum. That check is only meaningful if the digest is actually computed —
+ * a broken hash command (it once invoked `shasum -a CBM_SZ_256`, an invalid
+ * algorithm, from a bad macro rename inside the shell string) makes every
+ * digest fail, and the caller then falls through and installs unverified.
+ * Guard the digest itself against a known vector. */
+extern int cbm_cli_sha256_file(const char *path, char *out, size_t out_size);
+
+TEST(cli_sha256_file_matches_known_vector) {
+    char path[512];
+    snprintf(path, sizeof(path), "%s/cbm_sha_XXXXXX", cbm_tmpdir());
+    int fd = cbm_mkstemp(path);
+    ASSERT_TRUE(fd >= 0);
+    FILE *fp = fdopen(fd, "wb");
+    ASSERT_NOT_NULL(fp);
+    fwrite("abc", 1, 3, fp); /* sha256("abc") is a NIST test vector */
+    fclose(fp);
+
+    char digest[128] = {0};
+    int rc = cbm_cli_sha256_file(path, digest, sizeof(digest));
+    remove(path);
+    ASSERT_EQ(rc, 0);
+    ASSERT_STR_EQ(digest, "ba7816bf8f01cfea414140de5dae2223b00361a396177a9cb410ff61f20015ad");
+    PASS();
+}
+
 /* ═══════════════════════════════════════════════════════════════════
  *  Suite definition
  * ═══════════════════════════════════════════════════════════════════ */
 
 SUITE(cli) {
+    RUN_TEST(cli_sha256_file_matches_known_vector);
     /* Version (2 tests — selfupdate_test.go) */
     RUN_TEST(cli_compare_versions);
     RUN_TEST(cli_version_get_set);