diff --git a/vm/src/builtins/str.rs b/vm/src/builtins/str.rs
index 2e4678d55..57bc1428b 100644
--- a/vm/src/builtins/str.rs
+++ b/vm/src/builtins/str.rs
@@ -15,7 +15,7 @@ use crate::{
     format::{format, format_map},
     function::{ArgIterable, ArgSize, FuncArgs, OptionalArg, OptionalOption, PyComparisonValue},
     intern::PyInterned,
-    object::{Traverse, TraverseFn},
+    object::{MaybeTraverse, Traverse, TraverseFn},
     protocol::{PyIterReturn, PyMappingMethods, PyNumberMethods, PySequenceMethods},
     sequence::SequenceExt,
     sliceable::{SequenceIndex, SliceableSequenceOp},
@@ -64,6 +64,9 @@ impl<'a> TryFromBorrowedObject<'a> for &'a Wtf8 {
     }
 }
 
+pub type PyStrRef = PyRef<PyStr>;
+pub type PyUtf8StrRef = PyRef<PyUtf8Str>;
+
 #[pyclass(module = false, name = "str")]
 pub struct PyStr {
     data: StrData,
@@ -80,30 +83,6 @@ impl fmt::Debug for PyStr {
     }
 }
 
-#[repr(transparent)]
-#[derive(Debug)]
-pub struct PyUtf8Str(PyStr);
-
-// TODO: Remove this Deref which may hide missing optimized methods of PyUtf8Str
-impl std::ops::Deref for PyUtf8Str {
-    type Target = PyStr;
-    fn deref(&self) -> &Self::Target {
-        &self.0
-    }
-}
-
-impl PyUtf8Str {
-    /// Returns the underlying string slice.
-    pub fn as_str(&self) -> &str {
-        debug_assert!(
-            self.0.is_utf8(),
-            "PyUtf8Str invariant violated: inner string is not valid UTF-8"
-        );
-        // Safety: This is safe because the type invariant guarantees UTF-8 validity.
-        unsafe { self.0.to_str().unwrap_unchecked() }
-    }
-}
-
 impl AsRef<str> for PyStr {
     #[track_caller] // <- can remove this once it doesn't panic
     fn as_ref(&self) -> &str {
@@ -241,8 +220,6 @@ impl Default for PyStr {
     }
 }
 
-pub type PyStrRef = PyRef<PyStr>;
-
 impl fmt::Display for PyStr {
     #[inline]
     fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
@@ -374,7 +351,7 @@ impl Constructor for PyStr {
     type Args = StrArgs;
 
     fn py_new(cls: PyTypeRef, args: Self::Args, vm: &VirtualMachine) -> PyResult {
-        let string: PyStrRef = match args.object {
+        let string: PyRef<PyStr> = match args.object {
             OptionalArg::Present(input) => {
                 if let OptionalArg::Present(enc) = args.encoding {
                     vm.state.codec_registry.decode_text(
@@ -458,7 +435,7 @@ impl PyStr {
         self.data.as_str()
     }
 
-    pub fn ensure_valid_utf8(&self, vm: &VirtualMachine) -> PyResult<()> {
+    fn ensure_valid_utf8(&self, vm: &VirtualMachine) -> PyResult<()> {
         if self.is_utf8() {
             Ok(())
         } else {
@@ -531,6 +508,22 @@ impl PyStr {
             .mul(vm, value)
             .map(|x| Self::from(unsafe { Wtf8Buf::from_bytes_unchecked(x) }).into_ref(&vm.ctx))
     }
+
+    pub fn try_as_utf8<'a>(&'a self, vm: &VirtualMachine) -> PyResult<&'a PyUtf8Str> {
+        // Check if the string contains surrogates
+        self.ensure_valid_utf8(vm)?;
+        // If no surrogates, we can safely cast to PyStr
+        Ok(unsafe { &*(self as *const _ as *const PyUtf8Str) })
+    }
+}
+
+impl Py<PyStr> {
+    pub fn try_as_utf8<'a>(&'a self, vm: &VirtualMachine) -> PyResult<&'a Py<PyUtf8Str>> {
+        // Check if the string contains surrogates
+        self.ensure_valid_utf8(vm)?;
+        // If no surrogates, we can safely cast to PyStr
+        Ok(unsafe { &*(self as *const _ as *const Py<PyUtf8Str>) })
+    }
 }
 
 #[pyclass(
@@ -980,7 +973,11 @@ impl PyStr {
     }
 
     #[pymethod(name = "__format__")]
-    fn __format__(zelf: PyRef<Self>, spec: PyStrRef, vm: &VirtualMachine) -> PyResult<PyStrRef> {
+    fn __format__(
+        zelf: PyRef<PyStr>,
+        spec: PyStrRef,
+        vm: &VirtualMachine,
+    ) -> PyResult<PyRef<PyStr>> {
         let spec = spec.as_str();
         if spec.is_empty() {
             return if zelf.class().is(vm.ctx.types.str_type) {
@@ -989,7 +986,7 @@ impl PyStr {
                 zelf.as_object().str(vm)
             };
         }
-
+        let zelf = zelf.try_into_utf8(vm)?;
         let s = FormatSpec::parse(spec)
             .and_then(|format_spec| {
                 format_spec.format_string(&CharLenStr(zelf.as_str(), zelf.char_len()))
@@ -1351,8 +1348,12 @@ impl PyStr {
     }
 
     #[pymethod]
-    fn expandtabs(&self, args: anystr::ExpandTabsArgs) -> String {
-        rustpython_common::str::expandtabs(self.as_str(), args.tabsize())
+    fn expandtabs(&self, args: anystr::ExpandTabsArgs, vm: &VirtualMachine) -> PyResult<String> {
+        // TODO: support WTF-8
+        Ok(rustpython_common::str::expandtabs(
+            self.try_as_utf8(vm)?.as_str(),
+            args.tabsize(),
+        ))
     }
 
     #[pymethod]
@@ -1480,20 +1481,6 @@ impl PyStr {
     }
 }
 
-struct CharLenStr<'a>(&'a str, usize);
-impl std::ops::Deref for CharLenStr<'_> {
-    type Target = str;
-
-    fn deref(&self) -> &Self::Target {
-        self.0
-    }
-}
-impl crate::common::format::CharLen for CharLenStr<'_> {
-    fn char_len(&self) -> usize {
-        self.1
-    }
-}
-
 #[pyclass]
 impl PyRef<PyStr> {
     #[pymethod]
@@ -1504,7 +1491,7 @@ impl PyRef<PyStr> {
     }
 }
 
-impl PyStrRef {
+impl PyRef<PyStr> {
     pub fn is_empty(&self) -> bool {
         (**self).is_empty()
     }
@@ -1526,6 +1513,20 @@ impl PyStrRef {
     }
 }
 
+struct CharLenStr<'a>(&'a str, usize);
+impl std::ops::Deref for CharLenStr<'_> {
+    type Target = str;
+
+    fn deref(&self) -> &Self::Target {
+        self.0
+    }
+}
+impl crate::common::format::CharLen for CharLenStr<'_> {
+    fn char_len(&self) -> usize {
+        self.1
+    }
+}
+
 impl Representable for PyStr {
     #[inline]
     fn repr_str(zelf: &Py<Self>, vm: &VirtualMachine) -> PyResult<String> {
@@ -1941,6 +1942,170 @@ impl AnyStrWrapper<AsciiStr> for PyStrRef {
     }
 }
 
+#[repr(transparent)]
+#[derive(Debug)]
+pub struct PyUtf8Str(PyStr);
+
+impl fmt::Display for PyUtf8Str {
+    #[inline]
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        self.0.fmt(f)
+    }
+}
+
+impl MaybeTraverse for PyUtf8Str {
+    fn try_traverse(&self, traverse_fn: &mut TraverseFn<'_>) {
+        self.0.try_traverse(traverse_fn);
+    }
+}
+
+impl PyPayload for PyUtf8Str {
+    #[inline]
+    fn class(ctx: &Context) -> &'static Py<PyType> {
+        ctx.types.str_type
+    }
+
+    fn payload_type_id() -> std::any::TypeId {
+        std::any::TypeId::of::<PyStr>()
+    }
+
+    fn downcastable_from(obj: &PyObject) -> bool {
+        obj.typeid() == Self::payload_type_id() && {
+            // SAFETY: we know the object is a PyStr in this context
+            let wtf8 = unsafe { obj.downcast_unchecked_ref::<PyStr>() };
+            wtf8.is_utf8()
+        }
+    }
+
+    fn try_downcast_from(obj: &PyObject, vm: &VirtualMachine) -> PyResult<()> {
+        let str = obj.try_downcast_ref::<PyStr>(vm)?;
+        str.ensure_valid_utf8(vm)
+    }
+}
+
+impl<'a> From<&'a AsciiStr> for PyUtf8Str {
+    fn from(s: &'a AsciiStr) -> Self {
+        s.to_owned().into()
+    }
+}
+
+impl From<AsciiString> for PyUtf8Str {
+    fn from(s: AsciiString) -> Self {
+        s.into_boxed_ascii_str().into()
+    }
+}
+
+impl From<Box<AsciiStr>> for PyUtf8Str {
+    fn from(s: Box<AsciiStr>) -> Self {
+        let data = StrData::from(s);
+        unsafe { Self::from_str_data_unchecked(data) }
+    }
+}
+
+impl From<AsciiChar> for PyUtf8Str {
+    fn from(ch: AsciiChar) -> Self {
+        AsciiString::from(ch).into()
+    }
+}
+
+impl<'a> From<&'a str> for PyUtf8Str {
+    fn from(s: &'a str) -> Self {
+        s.to_owned().into()
+    }
+}
+
+impl From<String> for PyUtf8Str {
+    fn from(s: String) -> Self {
+        s.into_boxed_str().into()
+    }
+}
+
+impl From<char> for PyUtf8Str {
+    fn from(ch: char) -> Self {
+        let data = StrData::from(ch);
+        unsafe { Self::from_str_data_unchecked(data) }
+    }
+}
+
+impl<'a> From<std::borrow::Cow<'a, str>> for PyUtf8Str {
+    fn from(s: std::borrow::Cow<'a, str>) -> Self {
+        s.into_owned().into()
+    }
+}
+
+impl From<Box<str>> for PyUtf8Str {
+    #[inline]
+    fn from(value: Box<str>) -> Self {
+        let data = StrData::from(value);
+        unsafe { Self::from_str_data_unchecked(data) }
+    }
+}
+
+impl AsRef<Wtf8> for PyUtf8Str {
+    #[inline]
+    fn as_ref(&self) -> &Wtf8 {
+        self.0.as_wtf8()
+    }
+}
+
+impl AsRef<str> for PyUtf8Str {
+    #[inline]
+    fn as_ref(&self) -> &str {
+        self.0.as_str()
+    }
+}
+
+impl PyUtf8Str {
+    // Create a new `PyUtf8Str` from `StrData` without validation.
+    // This function must be only used in this module to create conversions.
+    // # Safety: must be called with a valid UTF-8 string data.
+    unsafe fn from_str_data_unchecked(data: StrData) -> Self {
+        Self(PyStr::from(data))
+    }
+
+    /// Returns the underlying string slice.
+    pub fn as_str(&self) -> &str {
+        debug_assert!(
+            self.0.is_utf8(),
+            "PyUtf8Str invariant violated: inner string is not valid UTF-8"
+        );
+        // Safety: This is safe because the type invariant guarantees UTF-8 validity.
+        unsafe { self.0.to_str().unwrap_unchecked() }
+    }
+
+    #[inline]
+    pub fn byte_len(&self) -> usize {
+        self.0.byte_len()
+    }
+
+    #[inline]
+    pub fn is_empty(&self) -> bool {
+        self.0.is_empty()
+    }
+
+    #[inline]
+    pub fn char_len(&self) -> usize {
+        self.0.char_len()
+    }
+}
+
+impl Py<PyUtf8Str> {
+    /// Upcast to PyStr.
+    pub fn as_pystr(&self) -> &Py<PyStr> {
+        unsafe {
+            // Safety: PyUtf8Str is a wrapper around PyStr, so this cast is safe.
+            &*(self as *const Self as *const Py<PyStr>)
+        }
+    }
+}
+
+impl PartialEq for PyUtf8Str {
+    fn eq(&self, other: &Self) -> bool {
+        self.as_str() == other.as_str()
+    }
+}
+impl Eq for PyUtf8Str {}
+
 impl AnyStrContainer<str> for String {
     fn new() -> Self {
         Self::new()
@@ -2302,7 +2467,8 @@ impl std::fmt::Display for PyStrInterned {
 impl AsRef<str> for PyStrInterned {
     #[inline(always)]
     fn as_ref(&self) -> &str {
-        self.as_str()
+        self.to_str()
+            .expect("Interned PyStr should always be valid UTF-8")
     }
 }
 
diff --git a/vm/src/convert/try_from.rs b/vm/src/convert/try_from.rs
index 3fda682d4..4f921e9c5 100644
--- a/vm/src/convert/try_from.rs
+++ b/vm/src/convert/try_from.rs
@@ -78,12 +78,12 @@ where
     #[inline]
     fn try_from_object(vm: &VirtualMachine, obj: PyObjectRef) -> PyResult<Self> {
         let class = T::class(&vm.ctx);
-        let result = if obj.fast_isinstance(class) {
-            obj.downcast()
+        if obj.fast_isinstance(class) {
+            T::try_downcast_from(&obj, vm)?;
+            Ok(unsafe { obj.downcast_unchecked() })
         } else {
-            Err(obj)
-        };
-        result.map_err(|obj| vm.new_downcast_type_error(class, &obj))
+            Err(vm.new_downcast_type_error(class, &obj))
+        }
     }
 }
 
diff --git a/vm/src/object/core.rs b/vm/src/object/core.rs
index 54a6657a9..57576ce70 100644
--- a/vm/src/object/core.rs
+++ b/vm/src/object/core.rs
@@ -448,7 +448,7 @@ impl<T: PyObjectPayload> PyInner<T> {
         let member_count = typ.slots.member_count;
         Box::new(Self {
             ref_count: RefCount::new(),
-            typeid: TypeId::of::<T>(),
+            typeid: T::payload_type_id(),
             vtable: PyObjVTable::of::<T>(),
             typ: PyAtomicRef::from(typ),
             dict: dict.map(InstanceDict::new),
@@ -541,6 +541,11 @@ impl PyObjectRef {
         }
     }
 
+    pub fn try_downcast<T: PyObjectPayload>(self, vm: &VirtualMachine) -> PyResult<PyRef<T>> {
+        T::try_downcast_from(&self, vm)?;
+        Ok(unsafe { self.downcast_unchecked() })
+    }
+
     /// Force to downcast this reference to a subclass.
     ///
     /// # Safety
@@ -720,10 +725,24 @@ impl PyObject {
         }
     }
 
+    #[inline]
+    pub(crate) fn typeid(&self) -> TypeId {
+        self.0.typeid
+    }
+
     /// Check if this object can be downcast to T.
     #[inline(always)]
     pub fn downcastable<T: PyObjectPayload>(&self) -> bool {
-        self.0.typeid == T::payload_type_id()
+        T::downcastable_from(self)
+    }
+
+    /// Attempt to downcast this reference to a subclass.
+    pub fn try_downcast_ref<'a, T: PyObjectPayload>(
+        &'a self,
+        vm: &VirtualMachine,
+    ) -> PyResult<&'a Py<T>> {
+        T::try_downcast_from(self, vm)?;
+        Ok(unsafe { self.downcast_unchecked_ref::<T>() })
     }
 
     /// Attempt to downcast this reference to a subclass.
diff --git a/vm/src/object/payload.rs b/vm/src/object/payload.rs
index f223af6e9..0b7bfe0dc 100644
--- a/vm/src/object/payload.rs
+++ b/vm/src/object/payload.rs
@@ -1,6 +1,6 @@
 use crate::object::{MaybeTraverse, Py, PyObjectRef, PyRef, PyResult};
 use crate::{
-    PyRefExact,
+    PyObject, PyRefExact,
     builtins::{PyBaseExceptionRef, PyType, PyTypeRef},
     types::PyTypeFlags,
     vm::{Context, VirtualMachine},
@@ -23,6 +23,31 @@ pub trait PyPayload:
     fn payload_type_id() -> std::any::TypeId {
         std::any::TypeId::of::<Self>()
     }
+
+    /// # Safety: this function should only be called if `payload_type_id` matches the type of `obj`.
+    #[inline]
+    fn downcastable_from(obj: &PyObject) -> bool {
+        obj.typeid() == Self::payload_type_id()
+    }
+
+    fn try_downcast_from(obj: &PyObject, vm: &VirtualMachine) -> PyResult<()> {
+        if Self::downcastable_from(obj) {
+            return Ok(());
+        }
+
+        #[cold]
+        fn raise_downcast_type_error(
+            vm: &VirtualMachine,
+            class: &Py<PyType>,
+            obj: &PyObject,
+        ) -> PyBaseExceptionRef {
+            vm.new_downcast_type_error(class, obj)
+        }
+
+        let class = Self::class(&vm.ctx);
+        Err(raise_downcast_type_error(vm, class, obj))
+    }
+
     fn class(ctx: &Context) -> &'static Py<PyType>;
 
     #[inline]