diff --git a/roboco/conventions/custom.py b/roboco/conventions/custom.py
index 45af16c3..9e438a4f 100644
--- a/roboco/conventions/custom.py
+++ b/roboco/conventions/custom.py
@@ -18,6 +18,26 @@ if TYPE_CHECKING:
         CustomRule,
     )
 
+# Dialect relations: the validator tags a ``.tsx`` file as language ``tsx``
+# (the JSX grammar needs that tag, distinct from plain ``typescript``), but a
+# custom rule scoped to ``typescript`` — the language the scan *reports* for a
+# React+TS repo — must still apply to ``.tsx`` files. The relation is
+# one-directional: ``tsx`` is a TypeScript dialect, so a ``typescript``-scoped
+# rule fires on ``.tsx``, but a ``tsx``-scoped (JSX-only) rule does not fire on
+# plain ``.ts``. Map each dialect tag to the language family it belongs to.
+_DIALECT_OF: dict[str, str] = {"tsx": "typescript"}
+
+
+def _rule_applies(rule_languages: list[str], file_language: str) -> bool:
+    """Whether a custom rule scoped to ``rule_languages`` applies to a file of
+    ``file_language``. An unscoped rule (empty list) applies to everything."""
+    if not rule_languages:
+        return True
+    if file_language in rule_languages:
+        return True
+    family = _DIALECT_OF.get(file_language)
+    return family is not None and family in rule_languages
+
 
 def check_custom(
     rel_path: str, source: bytes, language: str, standard: ConventionsStandard
@@ -26,7 +46,7 @@ def check_custom(
     text = source.decode(errors="replace")
     findings: list[Finding] = []
     for rule in standard.custom:
-        if rule.languages and language not in rule.languages:
+        if not _rule_applies(rule.languages, language):
             continue
         findings.extend(_matches(rel_path, text, rule))
     return findings
diff --git a/tests/unit/conventions/test_custom.py b/tests/unit/conventions/test_custom.py
index f15973cc..b29e5a7e 100644
--- a/tests/unit/conventions/test_custom.py
+++ b/tests/unit/conventions/test_custom.py
@@ -47,3 +47,60 @@ def test_bad_regex_abstains_without_crashing() -> None:
     rule = CustomRule(id="bad", pattern=r"(unclosed", message="m", level="block")
     std = ConventionsStandard(custom=[rule])
     assert check_custom("a.py", b"anything\n", "python", std) == []
+
+
+# ---------------------------------------------------------------------------
+# .tsx is a TypeScript dialect: a rule scoped to ``typescript`` must fire on a
+# ``.tsx`` file. The validator tags a .tsx file as language ``tsx`` (the JSX
+# grammar needs that tag), so without dialect-awareness in the scoping check a
+# ``languages: [typescript]`` rule — the language the scan *reports* for a
+# React+TS repo — silently skipped every .tsx file.
+# ---------------------------------------------------------------------------
+
+_NO_CONSOLE = CustomRule(
+    id="no-console",
+    pattern=r"console\.",
+    message="no console in app code",
+    level="warn",
+    languages=["typescript"],
+)
+
+_JSX_ONLY = CustomRule(
+    id="jsx-only",
+    pattern=r"jsx",
+    message="jsx-specific",
+    level="warn",
+    languages=["tsx"],
+)
+
+
+def test_typescript_scoped_rule_fires_on_tsx_file() -> None:
+    """A ``typescript``-scoped rule fires on a ``.tsx`` (language ``tsx``)
+    file — tsx is a typescript dialect, and the scan reports the project as
+    ``typescript`` so operators scope rules to that."""
+    std = ConventionsStandard(custom=[_NO_CONSOLE])
+    findings = check_custom("a.tsx", b"console.log(1)\n", "tsx", std)
+    assert len(findings) == 1
+    assert findings[0].rule == "no-console"
+
+
+def test_typescript_scoped_rule_still_fires_on_ts_file() -> None:
+    """Sanity: the typescript-scoped rule still fires on a plain ``.ts``
+    (language ``typescript``) file — the dialect fix must not regress the
+    direct-match case."""
+    std = ConventionsStandard(custom=[_NO_CONSOLE])
+    assert len(check_custom("a.ts", b"console.log(1)\n", "typescript", std)) == 1
+
+
+def test_tsx_scoped_rule_does_not_fire_on_plain_typescript() -> None:
+    """A ``tsx``-scoped rule (JSX-only) must NOT fire on a plain ``.ts``
+    file — the dialect relation is one-directional (tsx ⊂ typescript), so a
+    JSX-specific rule does not apply to non-JSX TypeScript."""
+    std = ConventionsStandard(custom=[_JSX_ONLY])
+    assert check_custom("a.ts", b"var jsx = 1\n", "typescript", std) == []
+
+
+def test_tsx_scoped_rule_fires_on_tsx_file() -> None:
+    """Sanity: a ``tsx``-scoped rule still fires on a ``.tsx`` file directly."""
+    std = ConventionsStandard(custom=[_JSX_ONLY])
+    assert len(check_custom("a.tsx", b"var jsx = 1\n", "tsx", std)) == 1