diff --git a/fastmcp_slim/fastmcp/utilities/json_schema_type.py b/fastmcp_slim/fastmcp/utilities/json_schema_type.py
index de1db2900..a2dc98580 100644
--- a/fastmcp_slim/fastmcp/utilities/json_schema_type.py
+++ b/fastmcp_slim/fastmcp/utilities/json_schema_type.py
@@ -269,12 +269,14 @@ def _create_string_type(schema: Mapping[str, Any]) -> type | Annotated[Any, ...]
     if "const" in schema:
         return Literal[schema["const"]]  # type: ignore
 
+    base: Any = str
     if fmt := schema.get("format"):
         if fmt == "uri":
             return AnyUrl
         elif fmt == "uri-reference":
-            return str
-        return FORMAT_TYPES.get(fmt, str)
+            base = str
+        else:
+            base = FORMAT_TYPES.get(fmt, str)
 
     constraints = {
         k: v
@@ -286,10 +288,15 @@ def _create_string_type(schema: Mapping[str, Any]) -> type | Annotated[Any, ...]
         if v is not None
     }
 
-    if not constraints:
-        return str
+    # StringConstraints (min/max length, pattern) only apply to str-based types.
+    # Non-str formats (e.g. date-time -> datetime, json -> Json) carry no such
+    # constraints. Previously *any* format made this function return early,
+    # silently dropping minLength/maxLength/pattern even for plain-string
+    # formats (e.g. a custom format that falls back to str).
+    if not constraints or not (isinstance(base, type) and issubclass(base, str)):
+        return base
 
-    annotated: Any = Annotated[str, StringConstraints(**constraints)]
+    annotated: Any = Annotated[base, StringConstraints(**constraints)]
 
     if "pattern" in constraints:
         try:
@@ -307,10 +314,10 @@ def _create_string_type(schema: Mapping[str, Any]) -> type | Annotated[Any, ...]
             pattern_field = Field(json_schema_extra={"x-unsupported-pattern": pattern})
             if constraints:
                 annotated = Annotated[
-                    str, StringConstraints(**constraints), pattern_field
+                    base, StringConstraints(**constraints), pattern_field
                 ]  # type: ignore[valid-type]
             else:
-                annotated = Annotated[str, pattern_field]  # type: ignore[valid-type]
+                annotated = Annotated[base, pattern_field]  # type: ignore[valid-type]
 
     return annotated
 
diff --git a/tests/utilities/json_schema_type/test_constraints.py b/tests/utilities/json_schema_type/test_constraints.py
index 53a1d9a1e..fbb196eb9 100644
--- a/tests/utilities/json_schema_type/test_constraints.py
+++ b/tests/utilities/json_schema_type/test_constraints.py
@@ -130,3 +130,39 @@ class TestNumberConstraints:
         validator = TypeAdapter(exclusive_max_number)
         with pytest.raises(ValidationError):
             validator.validate_python(100)
+
+
+class TestStringFormatConstraints:
+    """A ``format`` must not silently drop string length/pattern constraints."""
+
+    def test_custom_format_keeps_max_length(self):
+        # An unrecognized format falls back to ``str``; maxLength must still apply.
+        t = json_schema_to_type({"type": "string", "format": "phone", "maxLength": 3})
+        validator = TypeAdapter(t)
+        assert validator.validate_python("abc") == "abc"
+        with pytest.raises(ValidationError):
+            validator.validate_python("abcd")
+
+    def test_custom_format_keeps_min_length(self):
+        t = json_schema_to_type({"type": "string", "format": "phone", "minLength": 3})
+        validator = TypeAdapter(t)
+        assert validator.validate_python("abc") == "abc"
+        with pytest.raises(ValidationError):
+            validator.validate_python("ab")
+
+    def test_uri_reference_format_keeps_max_length(self):
+        t = json_schema_to_type(
+            {"type": "string", "format": "uri-reference", "maxLength": 5}
+        )
+        validator = TypeAdapter(t)
+        assert validator.validate_python("a/b") == "a/b"
+        with pytest.raises(ValidationError):
+            validator.validate_python("too/long/path")
+
+    def test_non_string_format_ignores_length(self):
+        # date-time resolves to ``datetime`` (not str), so length constraints do
+        # not apply and the type must stay datetime (regression guard).
+        import datetime
+
+        t = json_schema_to_type({"type": "string", "format": "date-time"})
+        assert t is datetime.datetime