diff --git a/runtime/lua/vim/lsp/completion.lua b/runtime/lua/vim/lsp/completion.lua
index ee13c915d9..3b2724b170 100644
--- a/runtime/lua/vim/lsp/completion.lua
+++ b/runtime/lua/vim/lsp/completion.lua
@@ -377,7 +377,7 @@ local function generate_kind(item)
 
   hex = hex:lower()
   local group = ('@lsp.color.%s'):format(hex)
-  if #api.nvim_get_hl(0, { name = group }) == 0 then
+  if next(api.nvim_get_hl(0, { name = group })) == nil then
     api.nvim_set_hl(0, group, { fg = '#' .. hex })
   end
 
@@ -435,9 +435,12 @@ local function commit_chars_str(chars)
     -- commit characters "should have `length=1`" and superfluous
     -- characters are ignored.  Keep the first codepoint.
     local b = type(ch) == 'string' and ch:byte(1)
-    if b then
-      local len = b < 0x80 and 1 or b < 0xe0 and 2 or b < 0xf0 and 3 or 4
-      result[#result + 1] = ch:sub(1, len)
+    if b and b ~= 0 then
+      local n = vim.str_utf_end(ch, 1)
+      -- Only whole characters: multibyte fragments pair up across entries, NUL truncates.
+      if b < 0x80 or n > 0 then
+        result[#result + 1] = ch:sub(1, 1 + n)
+      end
     end
   end
   return table.concat(result)
diff --git a/test/functional/plugin/lsp/completion_spec.lua b/test/functional/plugin/lsp/completion_spec.lua
index 1a6bd1f4f1..ddd20af87b 100644
--- a/test/functional/plugin/lsp/completion_spec.lua
+++ b/test/functional/plugin/lsp/completion_spec.lua
@@ -1807,7 +1807,8 @@ describe('vim.lsp.completion: integration', function()
       isIncomplete = false,
       items = {
         {
-          commitCharacters = { '.', ',', ';', '(' },
+          -- Only whole characters are kept: '\0' and '\169x' are dropped, '=>' becomes '='.
+          commitCharacters = { '.', ',', ';', '\0', '(', '=>', '\169x' },
           data = {
             cacheId = 1,
           },
@@ -1842,6 +1843,18 @@ describe('vim.lsp.completion: integration', function()
     wait_for_pum()
     feed('(')
     eq('f.(', n.api.nvim_get_current_line())
+
+    -- Test that a dropped fragment did not leak: 'x' must not commit, '=' must.
+    n.command('set completeopt=menuone,menu,noinsert')
+    feed('<ESC>Sf.')
+    wait_for_pum()
+    feed('x')
+    eq('f.x', n.api.nvim_get_current_line())
+
+    feed('<ESC>Sf.')
+    wait_for_pum()
+    feed('=')
+    eq('f.bar=', n.api.nvim_get_current_line())
   end)
 end)