diff --git a/crates/twirp/src/client.rs b/crates/twirp/src/client.rs
index 1aa6831..4ee5f5b 100644
--- a/crates/twirp/src/client.rs
+++ b/crates/twirp/src/client.rs
@@ -3,6 +3,10 @@ use std::sync::Arc;
 use std::vec;
 
 use async_trait::async_trait;
+use http::header::Entry;
+use http::header::IntoHeaderName;
+use http::HeaderMap;
+use http::HeaderValue;
 use reqwest::header::CONTENT_TYPE;
 use url::Host;
 use url::Url;
@@ -43,21 +47,21 @@ impl ClientBuilder {
         }
     }
 
+    /// Set the HTTP client. Without this a default HTTP client is used.
+    pub fn with_http_client(mut self, http_client: reqwest::Client) -> Self {
+        self.http_client = Some(http_client);
+        self
+    }
+
     /// Add middleware to the client that will be called on each request.
     /// Middlewares are invoked in the order they are added as part of the
     /// request cycle.
-    pub fn with_middleware<M>(self, middleware: M) -> Self
+    pub fn with_middleware<M>(mut self, middleware: M) -> Self
     where
         M: Middleware,
     {
-        let mut mw = self.middleware;
-        mw.push(Box::new(middleware));
-        Self {
-            base_url: self.base_url,
-            http_client: self.http_client,
-            handlers: self.handlers,
-            middleware: mw,
-        }
+        self.middleware.push(Box::new(middleware));
+        self
     }
 
     /// Add a handler for a service using the default host.
@@ -83,9 +87,16 @@ impl ClientBuilder {
         self
     }
 
-    /// Set the HTTP client. Without this a default HTTP client is used.
-    pub fn with_http_client(mut self, http_client: reqwest::Client) -> Self {
-        self.http_client = Some(http_client);
+    /// Set a default header for use in direct mode.
+    pub fn with_default_header<K>(mut self, key: K, value: HeaderValue) -> Self
+    where
+        K: IntoHeaderName,
+    {
+        if let Some(handlers) = &mut self.handlers {
+            handlers.default_headers.insert(key, value);
+        } else {
+            panic!("you must use `ClientBuilder::direct()` to register handler default headers");
+        }
         self
     }
 
@@ -315,9 +326,15 @@ impl<'a> Next<'a> {
 }
 
 async fn execute_handlers(
-    req: reqwest::Request,
+    mut req: reqwest::Request,
     request_handlers: &RequestHandlers,
 ) -> Result<reqwest::Response> {
+    let req_headers = req.headers_mut();
+    for (key, value) in &request_handlers.default_headers {
+        if let Entry::Vacant(entry) = req_headers.entry(key) {
+            entry.insert(value.clone());
+        }
+    }
     let url = req.url().clone();
     let Some(mut segments) = url.path_segments() else {
         return Err(crate::bad_route(format!(
@@ -344,6 +361,7 @@ async fn execute_handlers(
 
 #[derive(Clone, Default)]
 pub struct RequestHandlers {
+    default_headers: HeaderMap,
     /// A map of host/service names to handlers.
     handlers: HashMap<String, Arc<dyn DirectHandler>>,
 }
@@ -351,6 +369,7 @@ pub struct RequestHandlers {
 impl RequestHandlers {
     pub fn new() -> Self {
         Self {
+            default_headers: HeaderMap::new(),
             handlers: HashMap::new(),
         }
     }