diff --git a/lib/chibi/ast.c b/lib/chibi/ast.c
index c071436c..e29db840 100644
--- a/lib/chibi/ast.c
+++ b/lib/chibi/ast.c
@@ -440,6 +440,37 @@ static sexp sexp_string_contains (sexp ctx, sexp self, sexp_sint_t n, sexp x, se
   return res ? sexp_make_fixnum(res-sexp_string_data(x)) : SEXP_FALSE;
 }
 
+static sexp sexp_string_cursor_copy (sexp ctx, sexp self, sexp_sint_t n, sexp dst, sexp sfrom, sexp src, sexp sstart, sexp send) {
+  unsigned char *pfrom, *pto, *pstart, *pend, *prev, *p;
+  sexp_sint_t from = sexp_unbox_fixnum(sfrom), to = sexp_string_size(dst),
+    start = sexp_unbox_fixnum(sstart), end = sexp_unbox_fixnum(send);
+  sexp_assert_type(ctx, sexp_stringp, SEXP_STRING, dst);
+  sexp_assert_type(ctx, sexp_stringp, SEXP_STRING, src);
+  sexp_assert_type(ctx, sexp_fixnump, SEXP_FIXNUM, sfrom);
+  sexp_assert_type(ctx, sexp_fixnump, SEXP_FIXNUM, sstart);
+  sexp_assert_type(ctx, sexp_fixnump, SEXP_FIXNUM, send);
+  if (from < 0 || from > to)
+    return sexp_user_exception(ctx, self, "string-cursor-copy!: from out of range", sfrom);
+  if (start < 0 || start > sexp_string_size(src))
+    return sexp_user_exception(ctx, self, "string-cursor-copy!: start out of range", sstart);
+  if (end < start || end > sexp_string_size(src))
+    return sexp_user_exception(ctx, self, "string-cursor-copy!: end out of range", send);
+  pfrom = (unsigned char*)sexp_string_data(dst) + from;
+  pto = (unsigned char*)sexp_string_data(dst) + to;
+  pstart = (unsigned char*)sexp_string_data(src) + start;
+  pend = (unsigned char*)sexp_string_data(src) + end;
+  for ( ; pfrom < pto && pstart < pend; ++pfrom, ++pstart)
+    *pfrom = *pstart;
+  /* adjust for incomplete trailing chars */
+  prev = (unsigned char*)sexp_string_utf8_prev(pfrom);
+  if (sexp_utf8_initial_byte_count(*prev) > pfrom - prev) {
+    for (p = prev; p < pfrom; ++p)
+      *p = '\0';
+    pstart -= pfrom - prev;
+  }
+  return sexp_make_fixnum(pstart - (unsigned char*)sexp_string_data(src));
+}
+
 static sexp sexp_errno (sexp ctx, sexp self, sexp_sint_t n) {
 #ifdef PLAN9
   return SEXP_FALSE;
@@ -615,6 +646,7 @@ sexp sexp_init_library (sexp ctx, sexp self, sexp_sint_t n, sexp env, const char
 #endif
   sexp_define_foreign(ctx, env, "thread-list", 0, sexp_thread_list);
   sexp_define_foreign(ctx, env, "string-contains", 2, sexp_string_contains);
+  sexp_define_foreign(ctx, env, "string-cursor-copy!", 5, sexp_string_cursor_copy);
   sexp_define_foreign(ctx, env, "errno", 0, sexp_errno);
   sexp_define_foreign_opt(ctx, env, "integer->error-string", 1, sexp_error_string, SEXP_FALSE);
   sexp_define_foreign(ctx, env, "update-free-vars!", 1, sexp_update_free_vars);
diff --git a/lib/chibi/ast.sld b/lib/chibi/ast.sld
index 2ce1a223..b0091957 100644
--- a/lib/chibi/ast.sld
+++ b/lib/chibi/ast.sld
@@ -35,7 +35,7 @@
    env-define! env-push! env-syntactic? env-syntactic?-set! core-code
    type-name type-cpl type-parent type-slots type-num-slots type-printer
    object-size integer->immediate gc atomically thread-list
-   string-contains errno integer->error-string
+   string-contains string-cursor-copy! errno integer->error-string
    flatten-dot update-free-vars! setenv unsetenv safe-setenv)
   (import (chibi))
   (include-shared "ast")
diff --git a/lib/chibi/io/io.scm b/lib/chibi/io/io.scm
index dab3340d..988dd670 100644
--- a/lib/chibi/io/io.scm
+++ b/lib/chibi/io/io.scm
@@ -12,14 +12,15 @@
 ;; Copy whole characters from the given cursor positions.
 ;; Return the src cursor position of the next unwritten char,
 ;; which may be before `to' if the char would overflow.
-(define (string-cursor-copy! dst start src from to)
-  (let lp ((i from)
-           (j (string-offset->index dst start)))
-    (let ((i2 (string-cursor-next src i)))
-      (cond ((> i2 to) i)
-            (else
-             (string-set! dst j (string-cursor-ref src i))
-             (lp i2 (+ j 1)))))))
+;; Now provided as a primitive from (chibi ast).
+;; (define (string-cursor-copy! dst start src from to)
+;;   (let lp ((i from)
+;;            (j (string-offset->index dst start)))
+;;     (let ((i2 (string-cursor-next src i)))
+;;       (cond ((> i2 to) i)
+;;             (else
+;;              (string-set! dst j (string-cursor-ref src i))
+;;              (lp i2 (+ j 1)))))))
 
 (define (utf8->string vec . o)
   (if (pair? o)