diff --git a/rpc-toolkit-macro-internals/src/command/build.rs b/rpc-toolkit-macro-internals/src/command/build.rs
index 297d0d9..6c6778b 100644
--- a/rpc-toolkit-macro-internals/src/command/build.rs
+++ b/rpc-toolkit-macro-internals/src/command/build.rs
@@ -10,6 +10,172 @@ use syn::token::{Comma, Where};
 use super::parse::*;
 use super::*;
 
+fn metadata(full_options: &Options) -> TokenStream {
+    let options = match full_options {
+        Options::Leaf(a) => a,
+        Options::Parent(ParentOptions { common, .. }) => common,
+    };
+    let fallthrough = |ty: &str| {
+        let getter_name = Ident::new(&format!("get_{}", ty), Span::call_site());
+        match &*full_options {
+            Options::Parent(ParentOptions { subcommands, .. }) => {
+                let subcmd_handler = subcommands.iter().map(|subcmd| {
+                    let mut subcmd = subcmd.clone();
+                    subcmd.segments.last_mut().unwrap().arguments = PathArguments::None;
+                    quote_spanned!{ subcmd.span() =>
+                        [#subcmd::NAME, rest] => if let Some(val) = #subcmd::Metadata.#getter_name(rest, key) {
+                            return Some(val);
+                        },
+                    }
+                });
+                quote! {
+                    if !command.is_empty() {
+                        match command.splitn(2, ".").chain(std::iter::repeat("")).take(2).collect::<Vec<_>>().as_slice() {
+                            #(
+                                #subcmd_handler
+                            )*
+                            _ => ()
+                        }
+                    }
+                }
+            }
+            _ => quote! {},
+        }
+    };
+    fn impl_getter<I: Iterator<Item = TokenStream>>(
+        ty: &str,
+        metadata: I,
+        fallthrough: TokenStream,
+    ) -> TokenStream {
+        let getter_name = Ident::new(&format!("get_{}", ty), Span::call_site());
+        let ty: Type = syn::parse_str(ty).unwrap();
+        quote! {
+            fn #getter_name(&self, command: &str, key: &str) -> Option<#ty> {
+                #fallthrough
+                match key {
+                    #(#metadata)*
+                    _ => None,
+                }
+            }
+        }
+    }
+    let bool_metadata = options
+        .metadata
+        .iter()
+        .filter(|(_, lit)| matches!(lit, Lit::Bool(_)))
+        .map(|(name, value)| {
+            let name = LitStr::new(&name.to_string(), name.span());
+            quote! {
+                #name => Some(#value),
+            }
+        });
+    let number_metadata = |ty: &str| {
+        let ty: Type = syn::parse_str(ty).unwrap();
+        options
+            .metadata
+            .iter()
+            .filter(|(_, lit)| matches!(lit, Lit::Int(_) | Lit::Float(_) | Lit::Byte(_)))
+            .map(move |(name, value)| {
+                let name = LitStr::new(&name.to_string(), name.span());
+                quote! {
+                    #name => Some(#value as #ty),
+                }
+            })
+    };
+    let char_metadata = options
+        .metadata
+        .iter()
+        .filter(|(_, lit)| matches!(lit, Lit::Char(_)))
+        .map(|(name, value)| {
+            let name = LitStr::new(&name.to_string(), name.span());
+            quote! {
+                #name => Some(#value),
+            }
+        });
+    let str_metadata = options
+        .metadata
+        .iter()
+        .filter(|(_, lit)| matches!(lit, Lit::Str(_)))
+        .map(|(name, value)| {
+            let name = LitStr::new(&name.to_string(), name.span());
+            quote! {
+                #name => Some(#value),
+            }
+        });
+    let bstr_metadata = options
+        .metadata
+        .iter()
+        .filter(|(_, lit)| matches!(lit, Lit::ByteStr(_)))
+        .map(|(name, value)| {
+            let name = LitStr::new(&name.to_string(), name.span());
+            quote! {
+                #name => Some(#value),
+            }
+        });
+
+    let bool_getter = impl_getter("bool", bool_metadata, fallthrough("bool"));
+    let u8_getter = impl_getter("u8", number_metadata("u8"), fallthrough("u8"));
+    let u16_getter = impl_getter("u16", number_metadata("u16"), fallthrough("u16"));
+    let u32_getter = impl_getter("u32", number_metadata("u32"), fallthrough("u32"));
+    let u64_getter = impl_getter("u64", number_metadata("u64"), fallthrough("u64"));
+    let usize_getter = impl_getter("usize", number_metadata("usize"), fallthrough("usize"));
+    let i8_getter = impl_getter("i8", number_metadata("i8"), fallthrough("i8"));
+    let i16_getter = impl_getter("i16", number_metadata("i16"), fallthrough("i16"));
+    let i32_getter = impl_getter("i32", number_metadata("i32"), fallthrough("i32"));
+    let i64_getter = impl_getter("i64", number_metadata("i64"), fallthrough("i64"));
+    let isize_getter = impl_getter("isize", number_metadata("isize"), fallthrough("isize"));
+    let f32_getter = impl_getter("f32", number_metadata("f32"), fallthrough("f32"));
+    let f64_getter = impl_getter("f64", number_metadata("f64"), fallthrough("f64"));
+    let char_getter = impl_getter("char", char_metadata, fallthrough("char"));
+    let str_fallthrough = fallthrough("str");
+    let str_getter = quote! {
+        fn get_str(&self, command: &str, key: &str) -> Option<&'static str> {
+            #str_fallthrough
+            match key {
+                #(#str_metadata)*
+                _ => None,
+            }
+        }
+    };
+    let bstr_fallthrough = fallthrough("bstr");
+    let bstr_getter = quote! {
+        fn get_bstr(&self, command: &str, key: &str) -> Option<&'static [u8]> {
+            #bstr_fallthrough
+            match key {
+                #(#bstr_metadata)*
+                _ => None,
+            }
+        }
+    };
+
+    let res = quote! {
+        #[derive(Clone, Copy, Default)]
+        pub struct Metadata;
+
+        #[allow(overflowing_literals)]
+        impl ::rpc_toolkit::Metadata for Metadata {
+            #bool_getter
+            #u8_getter
+            #u16_getter
+            #u32_getter
+            #u64_getter
+            #usize_getter
+            #i8_getter
+            #i16_getter
+            #i32_getter
+            #i64_getter
+            #isize_getter
+            #f32_getter
+            #f64_getter
+            #char_getter
+            #str_getter
+            #bstr_getter
+        }
+    };
+    // panic!("{}", res);
+    res
+}
+
 fn build_app(name: LitStr, opt: &mut Options, params: &mut [ParamType]) -> TokenStream {
     let about = opt.common().about.clone().into_iter();
     let (subcommand, subcommand_required) = if let Options::Parent(opt) = opt {
@@ -835,6 +1001,7 @@ pub fn build(args: AttributeArgs, mut item: ItemFn) -> TokenStream {
             .map(|a| a.span())
             .unwrap_or_else(Span::call_site),
     );
+    let metadata = metadata(&mut opt);
     let build_app = build_app(command_name_str.clone(), &mut opt, &mut params);
     let rpc_handler = rpc_handler(fn_name, fn_generics, &opt, &params);
     let cli_handler = cli_handler(fn_name, fn_generics, &mut opt, &params);
@@ -847,6 +1014,8 @@ pub fn build(args: AttributeArgs, mut item: ItemFn) -> TokenStream {
             pub const NAME: &'static str = #command_name_str;
             pub const ASYNC: bool = #is_async;
 
+            #metadata
+
             #build_app
 
             #rpc_handler
diff --git a/rpc-toolkit-macro-internals/src/command/mod.rs b/rpc-toolkit-macro-internals/src/command/mod.rs
index 35b5a04..ba8d4f2 100644
--- a/rpc-toolkit-macro-internals/src/command/mod.rs
+++ b/rpc-toolkit-macro-internals/src/command/mod.rs
@@ -1,3 +1,5 @@
+use std::collections::HashMap;
+
 use syn::*;
 
 pub mod build;
@@ -24,6 +26,7 @@ pub struct LeafOptions {
     rename: Option<LitStr>,
     exec_ctx: ExecutionContext,
     display: Option<Path>,
+    metadata: HashMap<Ident, Lit>,
 }
 
 pub struct SelfImplInfo {
diff --git a/rpc-toolkit-macro-internals/src/command/parse.rs b/rpc-toolkit-macro-internals/src/command/parse.rs
index ffa76b1..422df68 100644
--- a/rpc-toolkit-macro-internals/src/command/parse.rs
+++ b/rpc-toolkit-macro-internals/src/command/parse.rs
@@ -305,6 +305,47 @@ pub fn parse_command_attr(args: AttributeArgs) -> Result<Options> {
             NestedMeta::Meta(Meta::Path(p)) if p.is_ident("rename") => {
                 return Err(Error::new(p.span(), "`rename` must be assigned to"));
             }
+            NestedMeta::Meta(Meta::List(list)) if list.path.is_ident("metadata") => {
+                for meta in list.nested {
+                    match meta {
+                        NestedMeta::Meta(Meta::NameValue(metadata_pair)) => {
+                            let ident = metadata_pair.path.get_ident().ok_or(Error::new(
+                                metadata_pair.path.span(),
+                                "must be an identifier",
+                            ))?;
+                            if opt
+                                .common()
+                                .metadata
+                                .insert(ident.clone(), metadata_pair.lit)
+                                .is_some()
+                            {
+                                return Err(Error::new(
+                                    ident.span(),
+                                    format!("duplicate metadata `{}`", ident),
+                                ));
+                            }
+                        }
+                        a => {
+                            return Err(Error::new(
+                                a.span(),
+                                "`metadata` takes a list of identifiers to be assigned to",
+                            ))
+                        }
+                    }
+                }
+            }
+            NestedMeta::Meta(Meta::Path(p)) if p.is_ident("metadata") => {
+                return Err(Error::new(
+                    p.span(),
+                    "`metadata` takes a list of identifiers to be assigned to",
+                ));
+            }
+            NestedMeta::Meta(Meta::NameValue(nv)) if nv.path.is_ident("metadata") => {
+                return Err(Error::new(
+                    nv.path.span(),
+                    "`metadata` cannot be assigned to",
+                ));
+            }
             _ => {
                 return Err(Error::new(arg.span(), "unknown argument"));
             }
diff --git a/rpc-toolkit-macro-internals/src/rpc_server/build.rs b/rpc-toolkit-macro-internals/src/rpc_server/build.rs
index c81c629..48c901c 100644
--- a/rpc-toolkit-macro-internals/src/rpc_server/build.rs
+++ b/rpc-toolkit-macro-internals/src/rpc_server/build.rs
@@ -10,6 +10,7 @@ pub fn build(args: RpcServerArgs) -> TokenStream {
         &mut command.segments.last_mut().unwrap().arguments,
         PathArguments::None,
     );
+    let command_module = command.clone();
     command.segments.push(PathSegment {
         ident: Ident::new("rpc_handler", command.span()),
         arguments,
@@ -22,14 +23,18 @@ pub fn build(args: RpcServerArgs) -> TokenStream {
         .map(|i| Ident::new(&format!("middleware_pre_{}", i), Span::call_site()))
         .take(args.middleware.len());
     let middleware_name_pre2 = middleware_name_pre.clone();
-    let middleware_name = (0..)
-        .map(|i| Ident::new(&format!("middleware_{}", i), Span::call_site()))
+    let middleware_name_post = (0..)
+        .map(|i| Ident::new(&format!("middleware_post_{}", i), Span::call_site()))
         .take(args.middleware.len());
-    let middleware_name_inv = middleware_name
+    let middleware_name_post_inv = middleware_name_post
         .clone()
         .collect::<Vec<_>>()
         .into_iter()
         .rev();
+    let middleware_name = (0..)
+        .map(|i| Ident::new(&format!("middleware_{}", i), Span::call_site()))
+        .take(args.middleware.len());
+    let middleware_name2 = middleware_name.clone();
     let middleware = args.middleware.iter();
     let res = quote! {
         {
@@ -41,9 +46,10 @@ pub fn build(args: RpcServerArgs) -> TokenStream {
                 async move {
                     Ok::<_, ::rpc_toolkit::hyper::Error>(::rpc_toolkit::hyper::service::service_fn(move |mut req| {
                         let ctx = ctx.clone();
+                        let metadata = #command_module::Metadata::default();
                         async move {
                             #(
-                                let #middleware_name_pre = match ::rpc_toolkit::rpc_server_helpers::constrain_middleware(#middleware)(&mut req).await? {
+                                let #middleware_name_pre = match ::rpc_toolkit::rpc_server_helpers::constrain_middleware(#middleware)(&mut req, metadata).await? {
                                     Ok(a) => a,
                                     Err(res) => return Ok(res),
                                 };
@@ -52,28 +58,35 @@ pub fn build(args: RpcServerArgs) -> TokenStream {
                             match rpc_req {
                                 Ok(mut rpc_req) => {
                                     #(
-                                        let #middleware_name = match #middleware_name_pre2(&mut rpc_req).await? {
+                                        let #middleware_name_post = match #middleware_name_pre2(&mut rpc_req).await? {
                                             Ok(a) => a,
                                             Err(res) => return Ok(res),
                                         };
                                     )*
-                                    let mut rpc_res = ::rpc_toolkit::rpc_server_helpers::to_response(
+                                    let mut rpc_res = #command(
+                                        ctx,
+                                        ::rpc_toolkit::yajrc::RpcMethod::as_str(&rpc_req.method),
+                                        rpc_req.params,
+                                    )
+                                    .await;
+                                    #(
+                                        let #middleware_name = match #middleware_name_post_inv(&mut rpc_res).await? {
+                                            Ok(a) => a,
+                                            Err(res) => return Ok(res),
+                                        };
+                                    )*
+                                    let mut res = ::rpc_toolkit::rpc_server_helpers::to_response(
                                         &req,
                                         Ok((
                                             rpc_req.id,
-                                            #command(
-                                                ctx,
-                                                ::rpc_toolkit::yajrc::RpcMethod::as_str(&rpc_req.method),
-                                                rpc_req.params,
-                                            )
-                                            .await,
+                                            rpc_res,
                                         )),
                                         status_fn,
                                     )?;
                                     #(
-                                        #middleware_name_inv(&mut rpc_res).await?;
+                                        #middleware_name2(&mut res).await?;
                                     )*
-                                    Ok::<_, ::rpc_toolkit::hyper::http::Error>(rpc_res)
+                                    Ok::<_, ::rpc_toolkit::hyper::http::Error>(res)
                                 }
                                 Err(e) => ::rpc_toolkit::rpc_server_helpers::to_response(
                                     &req,
diff --git a/rpc-toolkit/src/lib.rs b/rpc-toolkit/src/lib.rs
index 962a993..894cfad 100644
--- a/rpc-toolkit/src/lib.rs
+++ b/rpc-toolkit/src/lib.rs
@@ -42,7 +42,9 @@ pub use rpc_toolkit_macro::run_cli;
 pub use {clap, hyper, reqwest, serde, serde_json, tokio, url, yajrc};
 
 pub use crate::context::Context;
+pub use crate::metadata::Metadata;
 
 pub mod command_helpers;
 mod context;
+mod metadata;
 pub mod rpc_server_helpers;
diff --git a/rpc-toolkit/src/metadata.rs b/rpc-toolkit/src/metadata.rs
new file mode 100644
index 0000000..c300f76
--- /dev/null
+++ b/rpc-toolkit/src/metadata.rs
@@ -0,0 +1,68 @@
+macro_rules! getter_for {
+    ($($name:ident => $t:ty,)*) => {
+        $(
+            #[allow(unused_variables)]
+            fn $name(&self, command: &str, key: &str) -> Option<$t> {
+                None
+            }
+        )*
+    };
+}
+
+pub trait Metadata: Copy + Default + Send + Sync + 'static {
+    fn get<Ty: Primitive>(&self, command: &str, key: &str) -> Option<Ty> {
+        Ty::from_metadata(self, command, key)
+    }
+    getter_for!(
+        get_bool => bool,
+        get_u8 => u8,
+        get_u16 => u16,
+        get_u32 => u32,
+        get_u64 => u64,
+        get_usize => usize,
+        get_i8 => i8,
+        get_i16 => i16,
+        get_i32 => i32,
+        get_i64 => i64,
+        get_isize => isize,
+        get_f32 => f32,
+        get_f64 => f64,
+        get_char => char,
+        get_str => &'static str,
+        get_bstr => &'static [u8],
+    );
+}
+
+macro_rules! impl_primitive_for {
+    ($($name:ident => $t:ty,)*) => {
+        $(
+            impl Primitive for $t {
+                fn from_metadata<M: Metadata + ?Sized>(m: &M, command: &str, key: &str) -> Option<Self> {
+                    m.$name(command, key)
+                }
+            }
+        )*
+    };
+}
+
+pub trait Primitive: Copy {
+    fn from_metadata<M: Metadata + ?Sized>(m: &M, command: &str, key: &str) -> Option<Self>;
+}
+impl_primitive_for!(
+    get_bool => bool,
+    get_u8 => u8,
+    get_u16 => u16,
+    get_u32 => u32,
+    get_u64 => u64,
+    get_usize => usize,
+    get_i8 => i8,
+    get_i16 => i16,
+    get_i32 => i32,
+    get_i64 => i64,
+    get_isize => isize,
+    get_f32 => f32,
+    get_f64 => f64,
+    get_char => char,
+    get_str => &'static str,
+    get_bstr => &'static [u8],
+);
diff --git a/rpc-toolkit/src/rpc_server_helpers.rs b/rpc-toolkit/src/rpc_server_helpers.rs
index ed4d626..174b435 100644
--- a/rpc-toolkit/src/rpc_server_helpers.rs
+++ b/rpc-toolkit/src/rpc_server_helpers.rs
@@ -1,6 +1,7 @@
 use std::future::Future;
 
 use futures::future::BoxFuture;
+use futures::FutureExt;
 use hyper::body::Buf;
 use hyper::http::Error as HttpError;
 use hyper::server::conn::AddrIncoming;
@@ -12,7 +13,7 @@ use serde_json::Value;
 use url::Host;
 use yajrc::{AnyRpcMethod, GenericRpcMethod, Id, RpcError, RpcRequest, RpcResponse};
 
-use crate::Context;
+use crate::{Context, Metadata};
 
 lazy_static! {
     #[cfg(feature = "cbor")]
@@ -100,15 +101,20 @@ pub fn to_response<F: Fn(i32) -> StatusCode>(
 }
 
 // &mut Request<Body> -> Result<Result<Future<&mut RpcRequest<...> -> Future<Result<Result<&mut Response<Body> -> Future<Result<(), HttpError>>, Response<Body>>, HttpError>>>, Response<Body>>, HttpError>
-pub type DynMiddleware<Params> = Box<
+pub type DynMiddleware<Params, Metadata> = Box<
     dyn for<'a> FnOnce(
             &'a mut Request<Body>,
+            Metadata,
         ) -> BoxFuture<
             'a,
             Result<Result<DynMiddlewareStage2<Params>, Response<Body>>, HttpError>,
         > + Send
         + Sync,
 >;
+pub fn noop<Params: for<'de> Deserialize<'de> + 'static, M: Metadata>() -> DynMiddleware<Params, M>
+{
+    Box::new(|_, _| async { Ok(Ok(noop2())) }.boxed())
+}
 pub type DynMiddlewareStage2<Params> = Box<
     dyn for<'a> FnOnce(
             &'a mut RpcRequest<GenericRpcMethod<String, Params>>,
@@ -118,23 +124,45 @@ pub type DynMiddlewareStage2<Params> = Box<
         > + Send
         + Sync,
 >;
+pub fn noop2<Params: for<'de> Deserialize<'de> + 'static>() -> DynMiddlewareStage2<Params> {
+    Box::new(|_| async { Ok(Ok(noop3())) }.boxed())
+}
 pub type DynMiddlewareStage3 = Box<
+    dyn for<'a> FnOnce(
+            &'a mut Result<Value, RpcError>,
+        ) -> BoxFuture<
+            'a,
+            Result<Result<DynMiddlewareStage4, Response<Body>>, HttpError>,
+        > + Send
+        + Sync,
+>;
+pub fn noop3() -> DynMiddlewareStage3 {
+    Box::new(|_| async { Ok(Ok(noop4())) }.boxed())
+}
+pub type DynMiddlewareStage4 = Box<
     dyn for<'a> FnOnce(&'a mut Response<Body>) -> BoxFuture<'a, Result<(), HttpError>>
         + Send
         + Sync,
 >;
+pub fn noop4() -> DynMiddlewareStage4 {
+    Box::new(|_| async { Ok(()) }.boxed())
+}
 
 pub fn constrain_middleware<
     'a,
     'b,
     'c,
+    'd,
     Params: for<'de> Deserialize<'de> + 'static,
-    ReqFn: Fn(&'a mut Request<Body>) -> ReqFut,
+    M: Metadata,
+    ReqFn: Fn(&'a mut Request<Body>, M) -> ReqFut,
     ReqFut: Future<Output = Result<Result<RpcReqFn, Response<Body>>, HttpError>> + 'a,
     RpcReqFn: FnOnce(&'b mut RpcRequest<GenericRpcMethod<String, Params>>) -> RpcReqFut,
-    RpcReqFut: Future<Output = Result<Result<ResFn, Response<Body>>, HttpError>> + 'b,
-    ResFn: FnOnce(&'c mut Response<Body>) -> ResFut,
-    ResFut: Future<Output = Result<(), HttpError>> + 'c,
+    RpcReqFut: Future<Output = Result<Result<RpcResFn, Response<Body>>, HttpError>> + 'b,
+    RpcResFn: FnOnce(&'c mut Result<Value, RpcError>) -> RpcResFut,
+    RpcResFut: Future<Output = Result<Result<ResFn, Response<Body>>, HttpError>> + 'c,
+    ResFn: FnOnce(&'d mut Response<Body>) -> ResFut,
+    ResFut: Future<Output = Result<(), HttpError>> + 'd,
 >(
     f: ReqFn,
 ) -> ReqFn {
diff --git a/rpc-toolkit/tests/test.rs b/rpc-toolkit/tests/test.rs
index c260621..65018ec 100644
--- a/rpc-toolkit/tests/test.rs
+++ b/rpc-toolkit/tests/test.rs
@@ -7,11 +7,13 @@ use hyper::Request;
 use rpc_toolkit::clap::Arg;
 use rpc_toolkit::hyper::http::Error as HttpError;
 use rpc_toolkit::hyper::{Body, Response};
-use rpc_toolkit::rpc_server_helpers::{DynMiddlewareStage2, DynMiddlewareStage3};
+use rpc_toolkit::rpc_server_helpers::{
+    DynMiddlewareStage2, DynMiddlewareStage3, DynMiddlewareStage4,
+};
 use rpc_toolkit::serde::{Deserialize, Serialize};
 use rpc_toolkit::url::Host;
 use rpc_toolkit::yajrc::RpcError;
-use rpc_toolkit::{command, rpc_server, run_cli, Context};
+use rpc_toolkit::{command, rpc_server, run_cli, Context, Metadata};
 
 #[derive(Debug, Clone)]
 pub struct AppState<T, U> {
@@ -89,21 +91,28 @@ fn dothething2<U: Serialize + for<'a> Deserialize<'a> + FromStr<Err = E>, E: Dis
     ))
 }
 
-async fn cors<Params: for<'de> Deserialize<'de> + 'static>(
+async fn cors<Params: for<'de> Deserialize<'de> + 'static, M: Metadata + 'static>(
     req: &mut Request<Body>,
+    _: M,
 ) -> Result<Result<DynMiddlewareStage2<Params>, Response<Body>>, HttpError> {
     if req.method() == hyper::Method::OPTIONS {
         Ok(Err(Response::builder()
             .header("Access-Control-Allow-Origin", "*")
             .body(Body::empty())?))
     } else {
-        Ok(Ok(Box::new(|_req| {
+        Ok(Ok(Box::new(|_| {
             async move {
-                let res: DynMiddlewareStage3 = Box::new(|res| {
+                let res: DynMiddlewareStage3 = Box::new(|_| {
                     async move {
-                        res.headers_mut()
-                            .insert("Access-Control-Allow-Origin", "*".parse()?);
-                        Ok::<_, HttpError>(())
+                        let res: DynMiddlewareStage4 = Box::new(|res| {
+                            async move {
+                                res.headers_mut()
+                                    .insert("Access-Control-Allow-Origin", "*".parse()?);
+                                Ok::<_, HttpError>(())
+                            }
+                            .boxed()
+                        });
+                        Ok::<_, HttpError>(Ok(res))
                     }
                     .boxed()
                 });