diff --git a/cmp11sht.c b/cmp11sht.c
index d619687..fd502bd 100644
--- a/cmp11sht.c
+++ b/cmp11sht.c
@@ -89,7 +89,8 @@ cmp11sht v20251221.0718 (C) 2025 by Ruben C. Benante (MIT Lic)\n\n"
     {
         if(opt==3) printf("cmp11sht: string\n");
         errno = 0;
-        res = sequal_full(argv[1], argv[2], delta, &ratio, s1, s2);
+        res = sequal_full(argv[1], argv[2], delta, &ratio,
+                          s1, LEVN_SBUFF, s2, LEVN_SBUFF);
         if(errno == EINVAL)
             res = 3;                /* error → CLI exit 3 */
         else if(res > 0)
diff --git a/lib11sht.c b/lib11sht.c
index 048864d..342d149 100644
--- a/lib11sht.c
+++ b/lib11sht.c
@@ -42,44 +42,48 @@ int sequal(char *a, char *b, float shold)
 {
     float ratio;
     char s1[LEVN_SBUFF], s2[LEVN_SBUFF];
-    return sequal_full(a, b, shold, &ratio, s1, s2);
+    return sequal_full(a, b, shold, &ratio, s1, LEVN_SBUFF, s2, LEVN_SBUFF);
 }
 
 /* ---------------------------------------------------------------------- */
-int sequal_full(char *a, char *b, float shold, float *ratio, char *s1, char *s2)
+int sequal_full(char *a, char *b, float shold, float *ratio,
+                char *s1, size_t s1_size,
+                char *s2, size_t s2_size)
 {
-    int i;
+    size_t i;
 
-    if(!a || !b || !s1 || !s2 || !ratio)
+    if(!a || !b || !s1 || !s2 || !ratio || s1_size == 0 || s2_size == 0)
     {
         errno = EINVAL;
         return 0;
     }
 
-    /* remove accents */
-    asciify(a, s1, LEVN_SBUFF);
-    asciify(b, s2, LEVN_SBUFF);
+    /* remove accents (asciify NUL-terminates within s1_size / s2_size) */
+    asciify(a, s1, s1_size);
+    asciify(b, s2, s2_size);
 
     /* trim spaces (leading, trailing, and internal collapse) */
     trim(s1);
     trim(s2);
 
-    /* lowercase */
-    for(i=0; i<LEVN_SBUFF && s1[i]; i++)
+    /* lowercase, bounded by each buffer's actual size */
+    for(i=0; i<s1_size && s1[i]; i++)
         s1[i] = (char)tolower((unsigned char)s1[i]);
-    for(i=0; i<LEVN_SBUFF && s2[i]; i++)
+    for(i=0; i<s2_size && s2[i]; i++)
         s2[i] = (char)tolower((unsigned char)s2[i]);
 
-    i=strcmp(s1, s2);
-    if(!i)
     {
-        *ratio=1.0;
-        return 0;
+        int cmp = strcmp(s1, s2);
+        if(cmp == 0)
+        {
+            *ratio = 1.0;
+            return 0;
+        }
+        *ratio = shit11(s1, s2);
+        if(*ratio > shold)
+            return 0;
+        return (cmp < 0) ? -1 : 1;
     }
-    *ratio = shit11(s1, s2);
-    if(*ratio > shold)
-        return 0;
-    return (i < 0)? -1 : 1;
 }
 
 /* ---------------------------------------------------------------------- */
diff --git a/lib11sht.h b/lib11sht.h
index 30fbcd1..9f91605 100644
--- a/lib11sht.h
+++ b/lib11sht.h
@@ -9,7 +9,9 @@
 #ifndef LIB11SHT_H
 #define LIB11SHT_H
 
-#define LEVN_SBUFF 256  /* min size for s1/s2 buffers and bounded input */
+#include <stddef.h>  /* size_t */
+
+#define LEVN_SBUFF 256  /* recommended size for s1/s2 buffers */
 
 /* Compare similarity between two strings (after asciify + trim + lowercase).
  * Symmetric in shape with fequal(a, b, delta).
@@ -31,11 +33,14 @@ int sequal(char *a, char *b, float shold);
  * (e.g. cmp11sht CLI's -o / -n flags).
  *
  * Extra parameters:
- *   ratio   out: Levenshtein similarity 0.0..1.0 (1.0 on exact-after-normalize)
- *   s1, s2  out: caller-provided buffers (>= LEVN_SBUFF) filled with the
- *           normalized inputs
+ *   ratio    out: Levenshtein similarity 0.0..1.0 (1.0 on exact-after-normalize)
+ *   s1, s2   out: caller-provided buffers filled with the normalized inputs
+ *   s1_size  size of s1 in bytes (writes capped at s1_size-1 + final NUL)
+ *   s2_size  size of s2 in bytes (writes capped at s2_size-1 + final NUL)
  */
-int sequal_full(char *a, char *b, float shold, float *ratio, char *s1, char *s2);
+int sequal_full(char *a, char *b, float shold, float *ratio,
+                char *s1, size_t s1_size,
+                char *s2, size_t s2_size);
 
 /* Compare two floats within ±delta.
  * Returns: