diff --git a/fix/src/runtime.rs b/fix/src/runtime.rs
index 0cc0bb4..847c1ac 100644
--- a/fix/src/runtime.rs
+++ b/fix/src/runtime.rs
@@ -47,6 +47,7 @@ pub struct Runtime {
     fuel: usize,
     error_contexts: Stack<8192, ErrorFrame>,
     arena: Arena<Rootable![GcRoot<'_>]>,
+    force_mode: ForceMode,
 }
 
 impl Runtime {
@@ -81,6 +82,7 @@ impl Runtime {
             fuel: Self::DEFAULT_FUEL_AMOUNT,
             error_contexts: Stack::new(),
             arena,
+            force_mode: ForceMode::default(),
         })
     }
 
diff --git a/fix/src/runtime/vm.rs b/fix/src/runtime/vm.rs
index 9c7f57b..24d65e1 100644
--- a/fix/src/runtime/vm.rs
+++ b/fix/src/runtime/vm.rs
@@ -578,13 +578,181 @@ impl Runtime {
                 let n = self.read_u16() as usize;
                 let _span_id = self.read_u32();
                 let keys = self.read_attr_keys(n);
-                todo!("implement Select (force + lookup)");
+
+                // Move dynamic key values from stack to temp_stack
+                let dyn_count = keys
+                    .iter()
+                    .filter(|k| matches!(k, SelectKeyData::Dynamic))
+                    .count();
+                let temp_base = self.arena.mutate_root(|_, root| {
+                    let base = root.temp_stack.len();
+                    for _ in 0..dyn_count {
+                        let v = root.pop_stack();
+                        root.temp_stack.push(v);
+                    }
+                    base
+                });
+
+                // Force the expr (now TOS)
+                self.force_tos();
+
+                for (i, key) in keys.iter().enumerate() {
+                    let key_sid = match key {
+                        &SelectKeyData::Static(sid) => sid,
+                        SelectKeyData::Dynamic => {
+                            self.arena.mutate_root(|_, root| {
+                                let v =
+                                    root.temp_stack.pop().expect("missing dynamic key");
+                                root.push_stack(v);
+                            });
+                            self.force_tos();
+                            let key_data: std::result::Result<StringId, String> =
+                                self.arena.mutate_root(|_, root| {
+                                    let v = root.pop_stack();
+                                    if let Some(sid) = v.as_inline::<StringId>() {
+                                        Ok(sid)
+                                    } else if let Some(ns) = v.as_gc::<NixString>() {
+                                        Err(ns.as_str().to_owned())
+                                    } else {
+                                        panic!("dynamic select key must be a string")
+                                    }
+                                });
+                            match key_data {
+                                Ok(sid) => sid,
+                                Err(s) => StringId(self.strings.get_or_intern(&s)),
+                            }
+                        }
+                    };
+
+                    let result = self.arena.mutate_root(|_, root| {
+                        let val = root.pop_stack();
+                        let Some(attrset) = val.as_gc::<AttrSet<'_>>() else {
+                            return Err(vm_err(
+                                "value is not a set while a set was expected",
+                            ));
+                        };
+                        match attrset.lookup(key_sid) {
+                            Some(v) => {
+                                root.push_stack(v);
+                                Ok(true)
+                            }
+                            None => Ok(false),
+                        }
+                    });
+
+                    match result {
+                        Err(e) => {
+                            self.arena
+                                .mutate_root(|_, root| root.temp_stack.truncate(temp_base));
+                            return Runtime::handle_vm_error(self, e);
+                        }
+                        Ok(false) => {
+                            self.arena
+                                .mutate_root(|_, root| root.temp_stack.truncate(temp_base));
+                            let name =
+                                self.strings.resolve(key_sid.0).unwrap_or("«unknown»");
+                            return Runtime::handle_vm_error(
+                                self,
+                                vm_err(format!("attribute '{name}' missing")),
+                            );
+                        }
+                        Ok(true) => {
+                            if i < n - 1 {
+                                self.force_tos();
+                            }
+                        }
+                    }
+                }
+
+                self.arena
+                    .mutate_root(|_, root| root.temp_stack.truncate(temp_base));
             }
             SelectDefault => {
                 let n = self.read_u16() as usize;
                 let _span_id = self.read_u32();
                 let keys = self.read_attr_keys(n);
-                todo!("implement SelectDefault (force + lookup with default)");
+
+                let dyn_count = keys
+                    .iter()
+                    .filter(|k| matches!(k, SelectKeyData::Dynamic))
+                    .count();
+                let temp_base = self.arena.mutate_root(|_, root| {
+                    let base = root.temp_stack.len();
+                    // Default value is on top of the stack (pushed last by codegen)
+                    let default_val = root.pop_stack();
+                    root.temp_stack.push(default_val);
+                    // Then dynamic keys
+                    for _ in 0..dyn_count {
+                        let v = root.pop_stack();
+                        root.temp_stack.push(v);
+                    }
+                    base
+                });
+
+                // Force the expr (now TOS)
+                self.force_tos();
+
+                let mut use_default = false;
+                for (i, key) in keys.iter().enumerate() {
+                    let key_sid = match key {
+                        &SelectKeyData::Static(sid) => sid,
+                        SelectKeyData::Dynamic => {
+                            self.arena.mutate_root(|_, root| {
+                                let v =
+                                    root.temp_stack.pop().expect("missing dynamic key");
+                                root.push_stack(v);
+                            });
+                            self.force_tos();
+                            let key_data: std::result::Result<StringId, String> =
+                                self.arena.mutate_root(|_, root| {
+                                    let v = root.pop_stack();
+                                    if let Some(sid) = v.as_inline::<StringId>() {
+                                        Ok(sid)
+                                    } else if let Some(ns) = v.as_gc::<NixString>() {
+                                        Err(ns.as_str().to_owned())
+                                    } else {
+                                        panic!("dynamic select key must be a string")
+                                    }
+                                });
+                            match key_data {
+                                Ok(sid) => sid,
+                                Err(s) => StringId(self.strings.get_or_intern(&s)),
+                            }
+                        }
+                    };
+
+                    let found = self.arena.mutate_root(|_, root| {
+                        let val = root.pop_stack();
+                        if let Some(attrset) = val.as_gc::<AttrSet<'_>>() {
+                            if let Some(v) = attrset.lookup(key_sid) {
+                                root.push_stack(v);
+                                return true;
+                            }
+                        }
+                        // Not a set or key missing → use default
+                        false
+                    });
+
+                    if !found {
+                        use_default = true;
+                        break;
+                    }
+
+                    if i < n - 1 {
+                        self.force_tos();
+                    }
+                }
+
+                if use_default {
+                    self.arena.mutate_root(|_, root| {
+                        let default_val = root.temp_stack[temp_base];
+                        root.temp_stack.truncate(temp_base);
+                        root.push_stack(default_val);
+                    });
+                } else {
+                    self.arena
+                        .mutate_root(|_, root| root.temp_stack.truncate(temp_base));
+                }
             }
             HasAttr => {
                 let n = self.read_u16() as usize;