diff --git a/src/solidlsp/ls_config.py b/src/solidlsp/ls_config.py
index e46106e2..c1e02d7e 100644
--- a/src/solidlsp/ls_config.py
+++ b/src/solidlsp/ls_config.py
@@ -40,7 +40,7 @@ class FilenameMatcher:
         a *complete* extension — i.e. the extension must either end the string or be followed
         by a non-extension-character (anything other than a letter, digit, or underscore).
         """
-        if self._case_sensitive:
+        if not self._case_sensitive:
             string = string.lower()
         for ext in self._file_extensions:
             if re.search(rf"{re.escape(ext)}(?:\W|$)", string):
diff --git a/test/solidlsp/test_ls_config.py b/test/solidlsp/test_ls_config.py
new file mode 100644
index 00000000..680bb399
--- /dev/null
+++ b/test/solidlsp/test_ls_config.py
@@ -0,0 +1,46 @@
+from solidlsp.ls_config import FilenameMatcher
+
+
+class TestFilenameMatcherCaseSensitivity:
+    """Regression tests for FilenameMatcher case-sensitivity handling.
+
+    ``is_relevant_filename`` and ``string_contains_relevant_filename`` must apply the same
+    case-folding rule: fold to lower case only when ``case_sensitive=False``. A previously inverted
+    check in ``string_contains_relevant_filename`` folded the input when ``case_sensitive=True``
+    (and left it unfolded when ``case_sensitive=False``), so both modes could mismatch.
+    """
+
+    def test_case_insensitive_matches_uppercase_input(self) -> None:
+        matcher = FilenameMatcher(".py", ".pyi", case_sensitive=False)
+        # Uppercase / mixed-case input must match a lower-case extension when case-insensitive.
+        assert matcher.is_relevant_filename("FOO.PY")
+        assert matcher.is_relevant_filename("Foo.PyI")
+        assert matcher.string_contains_relevant_filename("opened FOO.PY for editing")
+        assert matcher.string_contains_relevant_filename("see Foo.PyI:10")
+
+    def test_case_sensitive_respects_extension_case(self) -> None:
+        # R uses case-significant extensions: .R and .r are distinct.
+        matcher = FilenameMatcher(".R", case_sensitive=True)
+        assert matcher.is_relevant_filename("script.R")
+        assert not matcher.is_relevant_filename("script.r")
+        # The same must hold for the substring variant (previously inverted).
+        assert matcher.string_contains_relevant_filename("edited script.R just now")
+        assert not matcher.string_contains_relevant_filename("edited script.r just now")
+
+    def test_two_methods_agree_on_case_folding(self) -> None:
+        """The two matcher methods must never disagree on a filename that ends the string."""
+        for case_sensitive in (True, False):
+            for extensions in ((".py",), (".R", ".Rmd"), (".ts", ".tsx")):
+                matcher = FilenameMatcher(*extensions, case_sensitive=case_sensitive)
+                for candidate in ("main.py", "Main.PY", "app.R", "app.r", "x.ts", "X.TSX"):
+                    assert matcher.is_relevant_filename(candidate) == matcher.string_contains_relevant_filename(candidate), (
+                        f"disagreement for {candidate!r} with extensions={extensions} case_sensitive={case_sensitive}"
+                    )
+
+    def test_string_contains_requires_complete_extension(self) -> None:
+        """A registered extension must be a *complete* extension, not a prefix of a longer word."""
+        matcher = FilenameMatcher(".py", case_sensitive=False)
+        assert matcher.string_contains_relevant_filename("run main.py now")
+        assert matcher.string_contains_relevant_filename("main.py")
+        # ".py" embedded in ".python" is not a complete extension occurrence.
+        assert not matcher.string_contains_relevant_filename("file.python")