diff --git a/crates/api/src/command.rs b/crates/api/src/command.rs
index a38425ab..42900cf8 100644
--- a/crates/api/src/command.rs
+++ b/crates/api/src/command.rs
@@ -77,8 +77,17 @@ pub fn del_user_command(name: &str) -> Result<()> {
 /// Returns an iterator over the infos of the global ex commands. Only
 /// user-defined commands are returned, not builtin ones.
 ///
+/// # Safety
+///
+/// The underlying C API function creates a Lua registry slot for each
+/// command and completion callback, which it expects the caller to clean up.
+/// Currently `nvim-oxi` does not perform this cleanup, consequentially
+/// calling this function creates a serious leak of Lua registry slots which
+/// in turn also prevents the callbacks from being garbage collected if their
+/// commands are deleted.
+///
 /// [1]: https://neovim.io/doc/user/api.html#nvim_get_commands()
-pub fn get_commands(
+pub unsafe fn get_commands(
     opts: &GetCommandsOpts,
 ) -> Result<impl SuperIterator<CommandInfos> + use<>> {
     let mut err = nvim::Error::new();
diff --git a/crates/api/src/opts/create_command.rs b/crates/api/src/opts/create_command.rs
index 1cbd3e6c..cefafdee 100644
--- a/crates/api/src/opts/create_command.rs
+++ b/crates/api/src/opts/create_command.rs
@@ -4,7 +4,7 @@ use crate::Buffer;
 use crate::types::{
     CommandAddr,
     CommandArgs,
-    CommandComplete,
+    CommandCompleteOrFunction,
     CommandNArgs,
     CommandRange,
 };
@@ -27,8 +27,9 @@ pub struct CreateCommandOpts {
     bar: types::Boolean,
 
     #[builder(
-        argtype = "CommandComplete",
-        inline = "{0}.to_object().unwrap()"
+        generics = "C: CommandCompleteOrFunction",
+        argtype = "C",
+        inline = "{0}.to_object()"
     )]
     complete: types::Object,
 
diff --git a/crates/api/src/types/command_complete.rs b/crates/api/src/types/command_complete.rs
index cd096597..b6d66514 100644
--- a/crates/api/src/types/command_complete.rs
+++ b/crates/api/src/types/command_complete.rs
@@ -1,25 +1,33 @@
-use serde::Serialize;
+use serde::{Deserialize, Serialize, de::Error};
 use types::{
     Function,
     Object,
-    conversion::{self, ToObject},
-    serde::Serializer,
+    conversion::{self, FromObject, ToObject},
+    serde::{Deserializer, Serializer},
 };
 
+use crate::ToFunction;
+
+pub type CompleteCallbackArgs = (String, String, usize);
+pub type CompleteCallbackRet = Vec<String>;
+pub type CompleteCallbackFunc =
+    Function<CompleteCallbackArgs, CompleteCallbackRet>;
+
 /// See `:h command-complete` for details.
 #[non_exhaustive]
-#[derive(Clone, Debug, Eq, PartialEq, Hash, Serialize)]
+#[derive(Clone, Debug, Eq, PartialEq, Hash, Serialize, Deserialize)]
 #[serde(rename_all = "snake_case")]
 pub enum CommandComplete {
     Arglist,
     Augroup,
     Buffer,
-    Behave,
+    Breakpoint,
     Color,
     Command,
     Compiler,
-    Cscope,
+    DiffBuffer,
     Dir,
+    DirInPath,
     Environment,
     Event,
     Expression,
@@ -30,6 +38,7 @@ pub enum CommandComplete {
     Help,
     Highlight,
     History,
+    Keymap,
     Locale,
     Lua,
     Mapclear,
@@ -38,7 +47,12 @@ pub enum CommandComplete {
     Messages,
     Option,
     Packadd,
+    #[cfg(feature = "neovim-0-12")] // On 0.12 and Nightly
+    Retab,
+    Runtime,
+    Scriptnames,
     Shellcmd,
+    Shellcmdline,
     Sign,
     Syntax,
     Syntime,
@@ -48,7 +62,27 @@ pub enum CommandComplete {
     Var,
 
     /// See `:h command-completion-customlist` for details.
-    CustomList(Function<(String, String, usize), Vec<String>>),
+    /// *Note*: This variant contains a nvim_oxi::Function.
+    #[serde(untagged, deserialize_with = "deserialize_callback")]
+    Callback(CompleteCallbackFunc),
+
+    /// See `:h command-completion-customlist` for details.
+    /// *Note*: This variant contains the name of a Vim Script function.
+    #[serde(
+        untagged,
+        serialize_with = "serialize_customlist",
+        deserialize_with = "deserialize_customlist"
+    )]
+    Customlist(String),
+
+    /// See `:h command-completion-custom` for details.
+    /// *Note*: This variant contains the name of a Vim Script function.
+    #[serde(
+        untagged,
+        serialize_with = "serialize_custom",
+        deserialize_with = "deserialize_custom"
+    )]
+    Custom(String),
 }
 
 impl ToObject for CommandComplete {
@@ -56,3 +90,70 @@ impl ToObject for CommandComplete {
         self.serialize(Serializer::new()).map_err(Into::into)
     }
 }
+
+impl FromObject for CommandComplete {
+    fn from_object(obj: Object) -> Result<Self, conversion::Error> {
+        Self::deserialize(Deserializer::new(obj)).map_err(Into::into)
+    }
+}
+
+pub trait CommandCompleteOrFunction {
+    fn to_object(self) -> Object;
+}
+
+impl<T> CommandCompleteOrFunction for T
+where
+    T: ToFunction<CompleteCallbackArgs, CompleteCallbackRet>,
+{
+    fn to_object(self) -> Object {
+        Object::from_luaref(self.into_luaref())
+    }
+}
+
+impl CommandCompleteOrFunction for CommandComplete {
+    fn to_object(self) -> Object {
+        ToObject::to_object(self).unwrap()
+    }
+}
+
+fn deserialize_callback<'de, D>(
+    deserializer: D,
+) -> Result<CompleteCallbackFunc, D::Error>
+where
+    D: serde::de::Deserializer<'de>,
+{
+    CompleteCallbackFunc::deserialize(deserializer)
+}
+
+macro_rules! serde_complete_custom {
+    ($ser_fn_name:ident, $de_fn_name:ident, $variant:literal) => {
+        fn $ser_fn_name<S>(
+            vim_fn_name: &str,
+            serializer: S,
+        ) -> Result<S::Ok, S::Error>
+        where
+            S: serde::ser::Serializer,
+        {
+            serializer.serialize_str(&[$variant, vim_fn_name].join(","))
+        }
+
+        fn $de_fn_name<'de, D>(deserializer: D) -> Result<String, D::Error>
+        where
+            D: serde::de::Deserializer<'de>,
+        {
+            let value = String::deserialize(deserializer)?;
+            if let Some(remainder) = value.strip_prefix($variant) {
+                if remainder.is_empty() {
+                    return Ok(String::new());
+                }
+                if let Some(vim_fn_name) = remainder.strip_prefix(",") {
+                    return Ok(vim_fn_name.to_string());
+                }
+            }
+            Err(D::Error::custom("not custom or customlist"))
+        }
+    };
+}
+
+serde_complete_custom! {serialize_custom, deserialize_custom, "custom"}
+serde_complete_custom! {serialize_customlist, deserialize_customlist, "customlist"}
diff --git a/crates/api/src/types/command_infos.rs b/crates/api/src/types/command_infos.rs
index 2d2f9696..fd7a54c7 100644
--- a/crates/api/src/types/command_infos.rs
+++ b/crates/api/src/types/command_infos.rs
@@ -10,6 +10,8 @@ use types::{
 };
 
 use super::{CommandAddr, CommandArgs, CommandNArgs, CommandRange};
+#[cfg(feature = "neovim-0-12")] // on 0.12 and Nightly.
+use crate::types::CommandComplete;
 
 #[non_exhaustive]
 #[derive(Clone, Debug, Eq, PartialEq, Hash, Deserialize)]
@@ -26,7 +28,10 @@ pub struct CommandInfos {
     /// Callback triggered by the command.
     pub callback: Option<Function<CommandArgs, ()>>,
 
-    /// Command complletion strategy.
+    /// Command completion strategy.
+    #[cfg(feature = "neovim-0-12")] // on 0.12 and Nightly.
+    pub complete: Option<CommandComplete>,
+    #[cfg(not(feature = "neovim-0-12"))] // Only on 0.11
     pub complete: Option<String>,
 
     /// TODO: docs
diff --git a/tests/src/api/command.rs b/tests/src/api/command.rs
index 1358a719..53d62ff5 100644
--- a/tests/src/api/command.rs
+++ b/tests/src/api/command.rs
@@ -27,3 +27,49 @@ fn regression_1() {
         .build();
     api::create_user_command("Echo", "", &opts).unwrap();
 }
+
+#[cfg(feature = "neovim-0-12")]
+#[nvim_oxi::test]
+fn command_complete_customlist() {
+    api::command("comclear").unwrap();
+    let opts = CreateCommandOpts::builder()
+        .nargs(CommandNArgs::Any)
+        .complete(CommandComplete::Customlist("VimFunc".to_string()))
+        .build();
+    api::create_user_command("Foo", ":", &opts).unwrap();
+    let cmd_info = unsafe { api::get_commands(&Default::default()) }
+        .unwrap()
+        .find(|cmd| cmd.name == "Foo")
+        .unwrap();
+    let complete =
+        cmd_info.complete.expect("Missing `CommandInfos::complete` value");
+    assert_eq!(complete, CommandComplete::Customlist(String::new()));
+    let complete_arg = cmd_info
+        .complete_arg
+        .expect("Missing `CommandInfos::complete_arg` value");
+    assert_eq!(complete_arg, "VimFunc".to_string());
+}
+
+#[cfg(feature = "neovim-0-12")]
+#[nvim_oxi::test]
+fn command_complete_callback() {
+    api::command("comclear").unwrap();
+    let opts = CreateCommandOpts::builder()
+        .nargs(CommandNArgs::Any)
+        .complete(|_args: CompleteCallbackArgs| vec!["Bar".to_string()])
+        .build();
+    api::create_user_command("Foo", ":", &opts).unwrap();
+    let cmd_info = unsafe { api::get_commands(&Default::default()) }
+        .unwrap()
+        .find(|cmd| cmd.name == "Foo")
+        .unwrap();
+    let complete =
+        cmd_info.complete.expect("Missing `CommandInfos::complete` value");
+    match complete {
+        CommandComplete::Callback(fun) => {
+            let res = fun.call((String::new(), String::new(), 0)).unwrap();
+            assert_eq!(Some(&"Bar".to_string()), res.first())
+        },
+        _ => panic!("Wrong `complete::Callback` value"),
+    }
+}
diff --git a/tests/src/api/global.rs b/tests/src/api/global.rs
index 208a72fc..3bd7bb5d 100644
--- a/tests/src/api/global.rs
+++ b/tests/src/api/global.rs
@@ -17,8 +17,9 @@ fn create_del_user_command() {
     assert_eq!(Ok(()), res);
     api::command("Bar").unwrap();
 
-    let commands =
-        api::get_commands(&Default::default()).unwrap().collect::<Vec<_>>();
+    let commands = unsafe { api::get_commands(&Default::default()) }
+        .unwrap()
+        .collect::<Vec<_>>();
 
     assert!(commands.iter().any(|cmd| cmd.name == "Foo"));
     assert!(commands.iter().any(|cmd| cmd.name == "Bar"));
@@ -289,7 +290,7 @@ fn user_command_with_count() {
     let opts = CreateCommandOpts::builder().count(32).build();
     api::create_user_command("Foo", "echo 'foo'", &opts).unwrap();
 
-    let res = api::get_commands(&Default::default())
+    let res = unsafe { api::get_commands(&Default::default()) }
         .map(|cmds| cmds.collect::<Vec<_>>());
 
     assert!(res.is_ok(), "{res:?}");