diff --git a/README.md b/README.md
index f6ceb55..3e5d8f0 100644
--- a/README.md
+++ b/README.md
@@ -53,7 +53,7 @@ are regenerated on each release — patches cannot be merged directly.
 Report problems (or send patches) to the author; accepted changes are
 applied to the development tree and appear in a following snapshot.
 
-This snapshot was assembled from development commit `685ce778cf34`.
+This snapshot was assembled from development commit `3321e7c9e36e`.
 
 ## License
 
diff --git a/sks/document/document_class.cpp b/sks/document/document_class.cpp
index f7302eb..52d77ba 100644
--- a/sks/document/document_class.cpp
+++ b/sks/document/document_class.cpp
@@ -1,4 +1,6 @@
 #include "util.h"
+#include <functional>
+#include <sstream>
 #include "machine.h"
 #include "error.h"
 #include "kutil.h"
@@ -133,6 +135,30 @@ Document_class::Document_class(Machine& machine) : Klammer_base(machine)
 }
 
 
+// The cache directory of ONE document: the target's cache directory
+// (_latex_files, _html_pages) with a subdirectory per document, named by
+// the document's stem and a hash of its absolute path.  The cache used to
+// be one directory per target with an entry per chapter BASENAME, shared by
+// every document on the machine, and freshness is only "the source is
+// older than the entry": document A's kt/alpha.kt was served document B's
+// cached alpha whenever B had rendered later -- silently when the chapter
+// assembled, with B's error message when it did not (TODO 79, 2026-10-19).
+// The hash, not the path itself: a path can be long and hold any
+// character; the stem is for the eye.  A document given as a string
+// (ktext -s) has no file and is keyed by the working directory, where its
+// _text.kt is rewritten on every run.
+std::string Document_class::document_cache_directory(const std::string& target_cache) const
+{
+    const fs::path document = m_kt_root_filename.empty()
+        ? fs::current_path() / "_string_input"
+        : fs::absolute(fs::path(m_kt_root_filename)).lexically_normal();
+    std::ostringstream key;
+    key << document.stem().string() << "-" << std::hex
+        << std::hash<std::string>{}(document.generic_string());
+    return cache_directory(target_cache + "/" + key.str());
+}
+
+
 void Document_class::save_string_input_as_file()
 {
     if (!m_text.empty()) {
diff --git a/sks/document/document_class.h b/sks/document/document_class.h
index 1748266..6c9db5b 100644
--- a/sks/document/document_class.h
+++ b/sks/document/document_class.h
@@ -193,7 +193,10 @@ public:
     std::vector<std::pair<std::string, std::string>> m_string_components {};
     std::vector<std::tuple<std::string,std::string,std::string,std::string>> m_toc_components {};
 
+    // The page cache of THIS document under a target's cache directory
+    // (document_class.cpp); set by each target's create_*_output_directories().
     std::string m_cache_dir {};
+    std::string document_cache_directory(const std::string& target_cache) const;
 };
 
 fs::path parse_input_filename(const std::string& s, const std::string& input_dir);
diff --git a/sks/document/document_html.cpp b/sks/document/document_html.cpp
index c0b78b6..ce19d1e 100644
--- a/sks/document/document_html.cpp
+++ b/sks/document/document_html.cpp
@@ -190,7 +190,7 @@ std::string Document_class::create_html_output_directories()
         install_resolved_font(m_resolved_sans, output_directory);
         install_resolved_font(m_resolved_mono, output_directory);
     }
-    m_cache_dir = cache_directory("_html_pages");
+    m_cache_dir = document_cache_directory("_html_pages");
     if (m_no_cache && fs::exists(m_cache_dir)) {
         for (auto& entry : std::filesystem::directory_iterator(m_cache_dir)) {
             // What ":cache false" actually removed: a derived consequence of
diff --git a/sks/document/document_latex.cpp b/sks/document/document_latex.cpp
index feb3968..8ec9fc7 100644
--- a/sks/document/document_latex.cpp
+++ b/sks/document/document_latex.cpp
@@ -12,7 +12,7 @@ std::string Document_class::create_tex_output_directories()
     (void)K::log(3);
     std::string output_directory =
         absolute_pathname(get("K_output_dir")); // + "/" + get("K_output_basename");
-    m_cache_dir = cache_directory("_latex_files");
+    m_cache_dir = document_cache_directory("_latex_files");
     if (!m_toc_only) { 
         if (!file_exists(output_directory, false, true)) {
             fs::create_directory(output_directory);
diff --git a/tst/editor/vim_feature_test.vim b/tst/editor/vim_feature_test.vim
index 5489370..7aeacbd 100644
--- a/tst/editor/vim_feature_test.vim
+++ b/tst/editor/vim_feature_test.vim
@@ -20,6 +20,11 @@
 "
 " Writes PASS/FAIL lines to $KT_OUT; exits nonzero if anything failed.
 
+" The buffer text below is UTF-8 (the umlauts of the multibyte case).  Vim
+" takes 'encoding' from the locale, and under a C locale that is latin1,
+" where each umlaut is two characters and the jump lands two columns short
+" -- a failure of this file, not of the plugin (TODO 78, 2026-10-19).
+set encoding=utf-8
 let s:results = []
 function! s:Check(name, ok, ...) abort
   let detail = (a:ok || !a:0) ? '' : ' -- ' . string(a:1)
diff --git a/tst/kdesc_test.sh b/tst/kdesc_test.sh
index c36ea88..22d6e66 100755
--- a/tst/kdesc_test.sh
+++ b/tst/kdesc_test.sh
@@ -49,6 +49,9 @@ else
 fi
 
 # shows NAME PATTERN CMD... — exits 0 and the output matches PATTERN.
+# The matching grep alone runs under a UTF-8 locale (TODO 78): under C its
+# "." counts bytes, so the width test 10a fails and 10b can falsely pass; the
+# command itself keeps the caller's locale.
 shows() {
     local name="$1" pattern="$2"; shift 2
     local out status
@@ -56,7 +59,7 @@ shows() {
     if [ $status -gt 128 ]; then
         echo "${red}FAIL${reset} $name — died by signal $((status-128))"; FAIL=$((FAIL+1)); return
     fi
-    if printf '%s' "$out" | grep -Eq -- "$pattern"; then
+    if printf '%s' "$out" | LC_ALL=C.UTF-8 grep -Eq -- "$pattern"; then
         echo "${green}PASS${reset} $name"; PASS=$((PASS+1))
     else
         echo "${red}FAIL${reset} $name — no line matching [$pattern]"
@@ -67,7 +70,7 @@ shows() {
 # absent NAME PATTERN CMD... — the output does NOT match PATTERN.
 absent() {
     local name="$1" pattern="$2"; shift 2
-    if limited "$@" 2>&1 | plain | grep -Eq -- "$pattern"; then
+    if limited "$@" 2>&1 | plain | LC_ALL=C.UTF-8 grep -Eq -- "$pattern"; then
         echo "${red}FAIL${reset} $name — unexpected match [$pattern]"; FAIL=$((FAIL+1))
     else
         echo "${green}PASS${reset} $name"; PASS=$((PASS+1))
diff --git a/tst/kdiag_test.sh b/tst/kdiag_test.sh
index 0e6d754..43b1b7b 100755
--- a/tst/kdiag_test.sh
+++ b/tst/kdiag_test.sh
@@ -80,6 +80,11 @@ pass() { echo "${green}PASS${reset} $1"; PASS=$((PASS + 1)); }
 fail() { echo "${red}FAIL${reset} $1"; [ -n "$2" ] && echo "       $2"; FAIL=$((FAIL + 1)); }
 
 # shows NAME PATTERN KDIAG_ARGS... — exits 0 and the output matches PATTERN.
+# The MATCHING grep runs under a UTF-8 locale whatever the caller's: under C
+# its "." matches a byte, and a pattern like "⟨@i.⟩" over a three-byte
+# subscript fails as if kdiag were wrong (cases 28 and 30; TODO 78).  Only
+# the grep: kdiag itself runs under the caller's locale, so a locale
+# dependency in the command would still show.
 shows() {
     local name="$1" pattern="$2"; shift 2
     run "$KDIAG" "$@"
@@ -89,7 +94,7 @@ shows() {
     if [ "$STATUS" -ne 0 ]; then
         fail "$name" "exit $STATUS: $(printf '%s' "$OUT" | head -2)"; return
     fi
-    if printf '%s' "$OUT" | grep -Eq -- "$pattern"; then
+    if printf '%s' "$OUT" | LC_ALL=C.UTF-8 grep -Eq -- "$pattern"; then
         pass "$name"
     else
         fail "$name" "no match for [$pattern]; got: $(printf '%s' "$OUT" | head -2)"
@@ -106,7 +111,7 @@ absent() {
     if [ "$STATUS" -ne 0 ]; then
         fail "$name" "exit $STATUS: $(printf '%s' "$OUT" | head -2)"; return
     fi
-    if printf '%s' "$OUT" | grep -Eq -- "$pattern"; then
+    if printf '%s' "$OUT" | LC_ALL=C.UTF-8 grep -Eq -- "$pattern"; then
         fail "$name" "unexpected match [$pattern]"
     else
         pass "$name"
@@ -123,7 +128,7 @@ reports() {
     if [ "$STATUS" -eq 0 ]; then
         fail "$name" "expected a nonzero exit, got 0"; return
     fi
-    if printf '%s' "$OUT" | grep -Eq -- "$pattern"; then
+    if printf '%s' "$OUT" | LC_ALL=C.UTF-8 grep -Eq -- "$pattern"; then
         pass "$name"
     else
         fail "$name" "no match for [$pattern]; got: $(printf '%s' "$OUT" | head -2)"
@@ -245,7 +250,7 @@ reports "24a. the error advice names a way kdiag has" '@read sks/sks\.k @' \
 absent_in_error() {
     local name="$1" pattern="$2"; shift 2
     run "$KDIAG" "$@"
-    if printf '%s' "$OUT" | grep -Eq -- "$pattern"; then
+    if printf '%s' "$OUT" | LC_ALL=C.UTF-8 grep -Eq -- "$pattern"; then
         fail "$name" "unexpected match [$pattern]"
     else
         pass "$name"