diff --git a/crates/capi/src/abstract_.rs b/crates/capi/src/abstract_.rs
index 08b4e5400..fd6e966bd 100644
--- a/crates/capi/src/abstract_.rs
+++ b/crates/capi/src/abstract_.rs
@@ -25,9 +25,11 @@ fn dict_to_kwargs(vm: &VirtualMachine, dict: &Py<PyDict>) -> PyResult<KwArgs> {
     dict.items_vec()
         .into_iter()
         .map(|(key, value)| {
+            // `to_string()` would replace lone surrogates with U+FFFD; keep the
+            // raw WTF-8 so surrogate keys round-trip (issue #8228).
             let key = key
                 .downcast_ref::<PyStr>()
-                .map(|s| s.to_string())
+                .map(|s| s.as_wtf8().to_owned())
                 .ok_or_else(|| vm.new_type_error("keywords must be strings"))?;
             Ok((key, value))
         })
diff --git a/crates/stdlib/src/_asyncio.rs b/crates/stdlib/src/_asyncio.rs
index 0c74310ea..3146e39b7 100644
--- a/crates/stdlib/src/_asyncio.rs
+++ b/crates/stdlib/src/_asyncio.rs
@@ -566,7 +566,7 @@ pub(crate) mod _asyncio {
                 let args = if let Some(ctx) = context {
                     FuncArgs::new(
                         vec![callback, future_arg],
-                        KwArgs::new(core::iter::once(("context".to_owned(), ctx)).collect()),
+                        KwArgs::new(core::iter::once((Wtf8Buf::from("context"), ctx)).collect()),
                     )
                 } else {
                     FuncArgs::new(vec![callback, future_arg], KwArgs::default())
@@ -1498,7 +1498,7 @@ pub(crate) mod _asyncio {
                 let args = if let Some(ctx) = context {
                     FuncArgs::new(
                         vec![callback, task_arg],
-                        KwArgs::new(core::iter::once(("context".to_owned(), ctx)).collect()),
+                        KwArgs::new(core::iter::once((Wtf8Buf::from("context"), ctx)).collect()),
                     )
                 } else {
                     FuncArgs::new(vec![callback, task_arg], KwArgs::default())
@@ -1527,7 +1527,7 @@ pub(crate) mod _asyncio {
                 let cancel_args = if let Some(ref m) = msg_value {
                     FuncArgs::new(
                         vec![],
-                        KwArgs::new(core::iter::once(("msg".to_owned(), m.clone())).collect()),
+                        KwArgs::new(core::iter::once((Wtf8Buf::from("msg"), m.clone())).collect()),
                     )
                 } else {
                     FuncArgs::new(vec![], KwArgs::default())
@@ -2213,7 +2213,7 @@ pub(crate) mod _asyncio {
                 let cancel_args = if let Some(ref m) = cancel_msg {
                     FuncArgs::new(
                         vec![],
-                        KwArgs::new(core::iter::once(("msg".to_owned(), m.clone())).collect()),
+                        KwArgs::new(core::iter::once((Wtf8Buf::from("msg"), m.clone())).collect()),
                     )
                 } else {
                     FuncArgs::new(vec![], KwArgs::default())
diff --git a/crates/vm/src/builtins/function.rs b/crates/vm/src/builtins/function.rs
index af359f111..43cc33a90 100644
--- a/crates/vm/src/builtins/function.rs
+++ b/crates/vm/src/builtins/function.rs
@@ -337,8 +337,13 @@ impl PyFunction {
         let mut posonly_passed_as_kwarg = Vec::new();
         // Handle keyword arguments
         for (name, value) in func_args.kwargs {
+            // Parameter names are plain identifiers, so a non-UTF-8 (surrogate) key
+            // can never match one and just falls through to **kwargs / the error path.
+            let name_str = name.as_str().ok();
             // Check if we have a parameter with this name:
-            if let Some(pos) = arg_pos(code.posonlyarg_count as usize..total_args, &name) {
+            if let Some(pos) =
+                name_str.and_then(|s| arg_pos(code.posonlyarg_count as usize..total_args, s))
+            {
                 let slot = &mut fastlocals[pos];
                 if slot.is_some() {
                     return Err(vm.new_type_error(format!(
@@ -350,7 +355,9 @@ impl PyFunction {
                 *slot = Some(value);
             } else if let Some(kwargs) = kwargs.as_ref() {
                 kwargs.set_item(&name, value, vm)?;
-            } else if arg_pos(0..code.posonlyarg_count as usize, &name).is_some() {
+            } else if name_str
+                .is_some_and(|s| arg_pos(0..code.posonlyarg_count as usize, s).is_some())
+            {
                 posonly_passed_as_kwarg.push(name);
             } else {
                 return Err(vm.new_type_error(format!(
diff --git a/crates/vm/src/builtins/function/jit.rs b/crates/vm/src/builtins/function/jit.rs
index 8432bb536..96c1465d4 100644
--- a/crates/vm/src/builtins/function/jit.rs
+++ b/crates/vm/src/builtins/function/jit.rs
@@ -184,6 +184,9 @@ pub(crate) fn get_jit_args<'a>(
     for (name, value) in &func_args.kwargs {
         let arg_pos =
             |args: &[&PyStrInterned], name: &str| args.iter().position(|arg| arg.as_str() == name);
+        // Parameter names are plain identifiers, so a non-UTF-8 (surrogate) key
+        // can never match one.
+        let name = name.as_str().map_err(|_| ArgsError::NotAKeywordArg)?;
         if let Some(arg_idx) = arg_pos(arg_names.args, name) {
             if jit_args.is_set(arg_idx) {
                 return Err(ArgsError::ArgPassedMultipleTimes);
diff --git a/crates/vm/src/frame.rs b/crates/vm/src/frame.rs
index 2743bf3d5..5df8b85e5 100644
--- a/crates/vm/src/frame.rs
+++ b/crates/vm/src/frame.rs
@@ -7112,11 +7112,13 @@ impl ExecutingFrame<'_> {
             let func_str = Self::object_function_str(callable, vm);
 
             Self::iterate_mapping_keys(vm, &kw_obj, &func_str, |key| {
+                // `PyStr`, not `PyUtf8Str`: CPython only checks that the key is a
+                // `str`, not that it is valid UTF-8, so surrogate keys are accepted.
                 let key_str = key
-                    .downcast_ref::<PyUtf8Str>()
+                    .downcast_ref::<PyStr>()
                     .ok_or_else(|| vm.new_type_error("keywords must be strings"))?;
                 let value = kw_obj.get_item(&*key, vm)?;
-                kwargs.insert(key_str.as_str().to_owned(), value);
+                kwargs.insert(key_str.as_wtf8().to_owned(), value);
                 Ok(())
             })?
         };
diff --git a/crates/vm/src/function/argument.rs b/crates/vm/src/function/argument.rs
index 88ef5581b..aabe484c2 100644
--- a/crates/vm/src/function/argument.rs
+++ b/crates/vm/src/function/argument.rs
@@ -1,6 +1,7 @@
 use crate::{
     AsObject, PyObjectRef, PyPayload, PyRef, PyResult, TryFromObject, VirtualMachine,
     builtins::{PyBaseExceptionRef, PyTupleRef, PyTypeRef},
+    common::wtf8::{Wtf8, Wtf8Buf},
     convert::ToPyObject,
     object::{Traverse, TraverseFn},
 };
@@ -155,10 +156,12 @@ impl FuncArgs {
                 .iter()
                 .zip(&args[nargs..nargs + names.len()])
                 .map(|(name, val)| {
+                    // `PyStr`, not `PyUtf8Str`: a surrogate key is a valid str and
+                    // must survive as WTF-8 rather than panic.
                     let key = name
-                        .downcast_ref::<crate::builtins::PyUtf8Str>()
+                        .downcast_ref::<crate::builtins::PyStr>()
                         .expect("kwnames must be strings")
-                        .as_str()
+                        .as_wtf8()
                         .to_owned();
                     (key, val.clone())
                 })
@@ -187,9 +190,9 @@ impl FuncArgs {
                 .zip(args.drain(nargs..nargs + kw_count))
                 .map(|(name, val)| {
                     let key = name
-                        .downcast_ref::<crate::builtins::PyUtf8Str>()
+                        .downcast_ref::<crate::builtins::PyStr>()
                         .expect("kwnames must be strings")
-                        .as_str()
+                        .as_wtf8()
                         .to_owned();
                     (key, val)
                 })
@@ -268,7 +271,7 @@ impl FuncArgs {
         self.kwargs.swap_remove(name)
     }
 
-    pub fn remaining_keywords(&mut self) -> impl Iterator<Item = (String, PyObjectRef)> + '_ {
+    pub fn remaining_keywords(&mut self) -> impl Iterator<Item = (Wtf8Buf, PyObjectRef)> + '_ {
         self.kwargs.drain(..)
     }
 
@@ -406,8 +409,12 @@ impl<T: TryFromObject> FromArgOptional for T {
 /// KwArgs is only for functions that accept arbitrary keyword arguments. For
 /// functions that accept only *specific* named arguments, a rust struct with
 /// an appropriate FromArgs implementation must be created.
+// Keys are stored as `Wtf8Buf`, not `String`, so that a lone-surrogate keyword
+// name coming through `f(**d)` is preserved instead of being rejected (see
+// issue #8228). `PyStr` is WTF-8 backed, and CPython only requires that a
+// keyword key be a `str`, not that it be valid UTF-8.
 #[derive(Clone, Debug)]
-pub struct KwArgs<T = PyObjectRef>(IndexMap<String, T>);
+pub struct KwArgs<T = PyObjectRef>(IndexMap<Wtf8Buf, T>);
 
 impl<T> Default for KwArgs<T> {
     fn default() -> Self {
@@ -416,7 +423,7 @@ impl<T> Default for KwArgs<T> {
 }
 
 impl<T> Deref for KwArgs<T> {
-    type Target = IndexMap<String, T>;
+    type Target = IndexMap<Wtf8Buf, T>;
 
     fn deref(&self) -> &Self::Target {
         &self.0
@@ -440,24 +447,47 @@ where
 
 impl<T> KwArgs<T> {
     #[must_use]
-    pub const fn new(map: IndexMap<String, T>) -> Self {
+    pub const fn new(map: IndexMap<Wtf8Buf, T>) -> Self {
         Self(map)
     }
 
+    // `String` keys accepted `&str` lookups for free via `Borrow<str>`; `Wtf8Buf`
+    // borrows only as `Wtf8`, so these inherent methods restore the `&str` interface
+    // via the zero-cost `Wtf8::new` cast, keeping every call site unchanged.
+    #[must_use]
+    pub fn get(&self, name: &str) -> Option<&T> {
+        self.0.get(Wtf8::new(name))
+    }
+
+    #[must_use]
+    pub fn contains_key(&self, name: &str) -> bool {
+        self.0.contains_key(Wtf8::new(name))
+    }
+
+    pub fn swap_remove(&mut self, name: &str) -> Option<T> {
+        self.0.swap_remove(Wtf8::new(name))
+    }
+
+    pub fn shift_remove(&mut self, name: &str) -> Option<T> {
+        self.0.shift_remove(Wtf8::new(name))
+    }
+
     pub fn pop_kwarg(&mut self, name: &str) -> Option<T> {
         self.swap_remove(name)
     }
 }
 
-impl<T> FromIterator<(String, T)> for KwArgs<T> {
-    fn from_iter<I: IntoIterator<Item = (String, T)>>(iter: I) -> Self {
-        Self(iter.into_iter().collect())
+// Accept any key that converts into `Wtf8Buf` (notably `String`), so existing
+// call sites that build kwargs from string literals keep compiling unchanged.
+impl<K: Into<Wtf8Buf>, T> FromIterator<(K, T)> for KwArgs<T> {
+    fn from_iter<I: IntoIterator<Item = (K, T)>>(iter: I) -> Self {
+        Self(iter.into_iter().map(|(k, v)| (k.into(), v)).collect())
     }
 }
 
 impl<'a, T> IntoIterator for &'a KwArgs<T> {
-    type Item = (&'a String, &'a T);
-    type IntoIter = indexmap::map::Iter<'a, String, T>;
+    type Item = (&'a Wtf8Buf, &'a T);
+    type IntoIter = indexmap::map::Iter<'a, Wtf8Buf, T>;
 
     fn into_iter(self) -> Self::IntoIter {
         self.0.iter()
@@ -465,8 +495,8 @@ impl<'a, T> IntoIterator for &'a KwArgs<T> {
 }
 
 impl<T> IntoIterator for KwArgs<T> {
-    type Item = (String, T);
-    type IntoIter = indexmap::map::IntoIter<String, T>;
+    type Item = (Wtf8Buf, T);
+    type IntoIter = indexmap::map::IntoIter<Wtf8Buf, T>;
 
     fn into_iter(self) -> Self::IntoIter {
         self.0.into_iter()
diff --git a/crates/vm/src/stdlib/_ast/python.rs b/crates/vm/src/stdlib/_ast/python.rs
index e7697e4cf..db92f20db 100644
--- a/crates/vm/src/stdlib/_ast/python.rs
+++ b/crates/vm/src/stdlib/_ast/python.rs
@@ -8,8 +8,9 @@ use super::{
 pub(crate) mod _ast {
     use crate::{
         AsObject, Context, Py, PyObject, PyObjectRef, PyPayload, PyRef, PyResult, VirtualMachine,
-        builtins::{PyDictRef, PySet, PyStr, PyTupleRef, PyType, PyTypeRef, PyUtf8Str},
+        builtins::{PyDictRef, PySet, PyStr, PyTupleRef, PyType, PyTypeRef},
         class::{PyClassImpl, StaticType},
+        common::wtf8::Wtf8Buf,
         function::{ArgIterable, FuncArgs, KwArgs, PyMethodDef, PyMethodFlags},
         stdlib::_ast::repr,
         types::{Constructor, Initializer},
@@ -229,7 +230,7 @@ pub(crate) mod _ast {
         ast_replace_set_update(&expecting, attributes.as_ref(), vm)?;
 
         for (key, _value) in &args.kwargs {
-            let key_obj: PyObjectRef = vm.ctx.new_str(key.as_str()).into();
+            let key_obj: PyObjectRef = vm.ctx.new_str(key.as_ref()).into();
             if !ast_replace_set_discard(&expecting, &key_obj, vm)? {
                 return Err(vm.new_type_error(format!(
                     "{}.__replace__ got an unexpected keyword argument '{}'.",
@@ -290,11 +291,11 @@ pub(crate) mod _ast {
             .into_iter()
             .map(|(key, value)| {
                 let key = key
-                    .downcast::<PyUtf8Str>()
+                    .downcast::<PyStr>()
                     .map_err(|_| vm.new_type_error("keywords must be strings"))?;
-                Ok((key.as_str().to_owned(), value))
+                Ok((key.as_wtf8().to_owned(), value))
             })
-            .collect::<PyResult<IndexMap<String, PyObjectRef>>>()?;
+            .collect::<PyResult<IndexMap<Wtf8Buf, PyObjectRef>>>()?;
         let result = type_obj.call(FuncArgs::new(vec![], KwArgs::new(kwargs)), vm)?;
         Ok(result)
     }
@@ -418,7 +419,7 @@ pub(crate) mod _ast {
                 ast_replace_set_discard(&remaining_fields, &name, vm)?;
             }
             for (key, value) in args.kwargs {
-                let key_obj: PyObjectRef = vm.ctx.new_str(key.as_str()).into();
+                let key_obj: PyObjectRef = vm.ctx.new_str(key.as_ref()).into();
                 let contains = fields_seq.contains(&key_obj, vm)?;
                 if contains {
                     if !ast_replace_set_discard(&remaining_fields, &key_obj, vm)? {
diff --git a/crates/vm/src/stdlib/_ctypes/structure.rs b/crates/vm/src/stdlib/_ctypes/structure.rs
index 39bb8a574..34f53f52d 100644
--- a/crates/vm/src/stdlib/_ctypes/structure.rs
+++ b/crates/vm/src/stdlib/_ctypes/structure.rs
@@ -1,5 +1,6 @@
 use super::base::{CDATA_BUFFER_METHODS, PyCData, PyCField, StgInfo, StgInfoFlags};
 use crate::builtins::{PyList, PyStr, PyTuple, PyType, PyTypeRef, PyUtf8Str};
+use crate::common::wtf8::Wtf8Buf;
 use crate::convert::ToPyObject;
 use crate::function::{FuncArgs, OptionalArg, PySetterValue};
 use crate::protocol::{BufferDescriptor, PyBuffer, PyNumberMethods};
@@ -712,7 +713,7 @@ impl PyCStructure {
         self_obj: &Py<Self>,
         type_obj: &Py<PyType>,
         args: &[PyObjectRef],
-        kwargs: &indexmap::IndexMap<String, PyObjectRef>,
+        kwargs: &indexmap::IndexMap<Wtf8Buf, PyObjectRef>,
         index: usize,
         vm: &VirtualMachine,
     ) -> PyResult<usize> {
@@ -746,7 +747,7 @@ impl PyCStructure {
                     && let Some(name) = tuple.first()
                     && let Some(name_str) = name.downcast_ref::<PyUtf8Str>()
                 {
-                    let field_name = name_str.as_str().to_owned();
+                    let field_name = name_str.as_wtf8().to_owned();
                     // Check for duplicate in kwargs
                     if kwargs.contains_key(&field_name) {
                         return Err(
@@ -784,9 +785,9 @@ impl Initializer for PyCStructure {
         }
 
         // 2. Process keyword arguments
-        for (key, value) in &args.kwargs {
+        for (key, value) in args.kwargs {
             zelf.as_object()
-                .set_attr(vm.ctx.intern_str(key.as_str()), value.clone(), vm)?;
+                .set_attr(vm.ctx.intern_str(key), value, vm)?;
         }
 
         Ok(())
diff --git a/crates/vm/src/stdlib/_ctypes/union.rs b/crates/vm/src/stdlib/_ctypes/union.rs
index 8d37178d5..727ad0118 100644
--- a/crates/vm/src/stdlib/_ctypes/union.rs
+++ b/crates/vm/src/stdlib/_ctypes/union.rs
@@ -1,6 +1,7 @@
 use super::base::{CDATA_BUFFER_METHODS, StgInfoFlags};
 use super::{PyCData, PyCField, StgInfo};
 use crate::builtins::{PyList, PyStr, PyTuple, PyType, PyTypeRef, PyUtf8Str};
+use crate::common::wtf8::Wtf8Buf;
 use crate::convert::ToPyObject;
 use crate::function::{ArgBytesLike, FuncArgs, OptionalArg, PySetterValue};
 use crate::protocol::{BufferDescriptor, PyBuffer};
@@ -581,7 +582,7 @@ impl PyCUnion {
         self_obj: &Py<Self>,
         type_obj: &Py<PyType>,
         args: &[PyObjectRef],
-        kwargs: &indexmap::IndexMap<String, PyObjectRef>,
+        kwargs: &indexmap::IndexMap<Wtf8Buf, PyObjectRef>,
         index: usize,
         vm: &VirtualMachine,
     ) -> PyResult<usize> {
@@ -617,7 +618,7 @@ impl PyCUnion {
                     && let Some(name) = tuple.first()
                     && let Some(name_str) = name.downcast_ref::<PyUtf8Str>()
                 {
-                    let field_name = name_str.as_str().to_owned();
+                    let field_name = name_str.as_wtf8().to_owned();
                     // Check for duplicate in kwargs
                     if kwargs.contains_key(&field_name) {
                         return Err(
@@ -655,9 +656,9 @@ impl Initializer for PyCUnion {
         }
 
         // 2. Process keyword arguments
-        for (key, value) in &args.kwargs {
+        for (key, value) in args.kwargs {
             zelf.as_object()
-                .set_attr(vm.ctx.intern_str(key.as_str()), value.clone(), vm)?;
+                .set_attr(vm.ctx.intern_str(key), value, vm)?;
         }
 
         Ok(())
diff --git a/crates/vm/src/stdlib/_functools.rs b/crates/vm/src/stdlib/_functools.rs
index 7c6914c2f..94b9565e7 100644
--- a/crates/vm/src/stdlib/_functools.rs
+++ b/crates/vm/src/stdlib/_functools.rs
@@ -373,7 +373,7 @@ mod _functools {
 
             // Add new keywords
             for (key, value) in args.kwargs {
-                final_keywords.set_item(vm.ctx.intern_str(key.as_str()), value, vm)?;
+                final_keywords.set_item(vm.ctx.intern_str(key), value, vm)?;
             }
 
             Ok(Self {
@@ -436,10 +436,11 @@ mod _functools {
 
             // Add keywords from self.keywords
             for (key, value) in &*keywords {
+                // `expect_str()` would panic on surrogate keys; keep them as WTF-8.
                 let key_str = key
                     .downcast_ref::<crate::builtins::PyStr>()
                     .ok_or_else(|| vm.new_type_error("keywords must be strings"))?;
-                final_kwargs.insert(key_str.expect_str().to_owned(), value);
+                final_kwargs.insert(key_str.as_wtf8().to_owned(), value);
             }
 
             // Add keywords from args.kwargs (these override self.keywords)
diff --git a/crates/vm/src/stdlib/_operator.rs b/crates/vm/src/stdlib/_operator.rs
index e4db04605..5e72ef03e 100644
--- a/crates/vm/src/stdlib/_operator.rs
+++ b/crates/vm/src/stdlib/_operator.rs
@@ -610,7 +610,7 @@ mod _operator {
                 }
                 for (key, value) in kwargs {
                     result.push_str(", ");
-                    result.push_str(key);
+                    result.push_wtf8(key);
                     result.push_char('=');
                     result.push_wtf8(value.repr(vm)?.as_wtf8());
                 }
diff --git a/crates/vm/src/stdlib/_typing.rs b/crates/vm/src/stdlib/_typing.rs
index 45740f7eb..04b9208b3 100644
--- a/crates/vm/src/stdlib/_typing.rs
+++ b/crates/vm/src/stdlib/_typing.rs
@@ -353,9 +353,9 @@ pub(crate) mod decl {
             // typealias(name, value, *, type_params=())
             // name and value are positional-or-keyword; type_params is keyword-only.
 
-            // Reject unexpected keyword arguments
+            // Reject unexpected keyword arguments.
             for key in args.kwargs.keys() {
-                if key != "name" && key != "value" && key != "type_params" {
+                if !matches!(key.as_str(), Ok("name" | "value" | "type_params")) {
                     return Err(vm.new_type_error(format!(
                         "typealias() got an unexpected keyword argument '{key}'"
                     )));
diff --git a/crates/vm/src/types/structseq.rs b/crates/vm/src/types/structseq.rs
index b7468d4a7..703cc79c1 100644
--- a/crates/vm/src/types/structseq.rs
+++ b/crates/vm/src/types/structseq.rs
@@ -1,4 +1,5 @@
 use crate::common::lock::LazyLock;
+use crate::common::wtf8::Wtf8;
 use crate::{
     AsObject, Py, PyObject, PyObjectRef, PyPayload, PyRef, PyResult, VirtualMachine, atomic_func,
     builtins::{PyBaseExceptionRef, PyStr, PyStrRef, PyTuple, PyTupleRef, PyType, PyTypeRef},
@@ -276,7 +277,7 @@ pub trait PyStructSequence: StaticType + PyClassImpl + Sized + 'static {
 
         // Check for unexpected keyword arguments
         if !kwargs.is_empty() {
-            let names: Vec<&str> = kwargs.keys().map(|k| k.as_str()).collect();
+            let names: Vec<&Wtf8> = kwargs.keys().map(|k| k.as_ref()).collect();
             return Err(vm.new_type_error(format!("Got unexpected field name(s): {names:?}")));
         }
 
diff --git a/crates/wasm/src/convert.rs b/crates/wasm/src/convert.rs
index 17ad5b629..3e07b27d4 100644
--- a/crates/wasm/src/convert.rs
+++ b/crates/wasm/src/convert.rs
@@ -119,9 +119,12 @@ pub fn py_to_js(vm: &VirtualMachine, py_obj: PyObjectRef) -> JsValue {
                 if let Some(ref kwargs) = kwargs {
                     for pair in object_entries(kwargs) {
                         let (key, val) = pair?;
-                        py_func_args
-                            .kwargs
-                            .insert(js_sys::JsString::from(key).into(), js_to_py(vm, val));
+                        py_func_args.kwargs.insert(
+                            // JS strings coming in are UTF-16; go through Rust `String`
+                            // (kwargs keys are now WTF-8, so convert String -> Wtf8Buf).
+                            String::from(js_sys::JsString::from(key)).into(),
+                            js_to_py(vm, val),
+                        );
                     }
                 }
                 let result = py_obj.call(py_func_args, vm);
@@ -229,7 +232,9 @@ pub fn js_to_py(vm: &VirtualMachine, js_val: JsValue) -> PyObjectRef {
             move |args: FuncArgs, vm: &VirtualMachine| -> PyResult {
                 let this = Object::new();
                 for (k, v) in args.kwargs {
-                    Reflect::set(&this, &k.into(), &py_to_js(vm, v))
+                    // WTF-8 -> JS string: lone surrogates in the key become U+FFFD
+                    // (wasm-bindgen only accepts Rust `String`); acceptable at this boundary.
+                    Reflect::set(&this, &k.to_string().into(), &py_to_js(vm, v))
                         .expect("property to be settable");
                 }
                 let js_args = args