Fix error message for invalid single-dash multi-char option

src/click/parser.py

@@ -386,6 +386,20 @@ def _match_long_opt(
 
         option.process(value, state)
 
+    def _get_single_prefix_long_opt_prefix(self, arg: str) -> str | None:
+        norm_arg = _normalize_opt(arg, self.ctx)
+        matches = [
+            opt
+            for opt in self._long_opt
+            if len(opt) > 2
+            and len(norm_arg) > len(opt)
+            and len(_split_opt(opt)[0]) == 1
+            and norm_arg.startswith(opt)
+        ]
+        if not matches:
+            return None
+        return max(matches, key=len)
+
     def _match_short_opt(self, arg: str, state: _ParsingState) -> None:
         stop = False
         i = 1
@@ -401,6 +415,10 @@ def _match_short_opt(self, arg: str, state: _ParsingState) -> None:
                 if self.ignore_unknown_options:
                     unknown_options.append(ch)
                     continue
+                if i == 2:
+                    long_opt_prefix = self._get_single_prefix_long_opt_prefix(arg)
+                    if long_opt_prefix is not None:
+                        raise NoSuchOption(long_opt_prefix, ctx=self.ctx)
                 raise NoSuchOption(opt, ctx=self.ctx)
             if option.takes_value:
                 # Any characters left in arg?  Pretend they're the

tests/test_options.py

@@ -144,6 +144,17 @@ def cli():
     assert f"No such option: {unknown_flag}" in result.output
 
 
+def test_unknown_single_dash_multichar_option_shows_full_option(runner):
+    @click.command()
+    @click.option("-dbg", is_flag=True)
+    def cli(dbg):
+        click.echo(f"dbg={dbg}")
+
+    result = runner.invoke(cli, ["-dbgwrong"])
+    assert result.exception
+    assert "No such option: -dbg" in result.output
+
+
 @pytest.mark.parametrize(
     ("value", "expect"),
     [