From a539cfde14cf8c9b0025fcbd1425415d3a37573f Mon Sep 17 00:00:00 2001
From: Timothy Clem <timothy.clem@gmail.com>
Date: Wed, 11 Jun 2025 17:58:42 -0700
Subject: [PATCH 1/8] First pass at a design

---
 Cargo.lock                    |  1 +
 crates/twirp-build/Cargo.toml |  1 +
 crates/twirp-build/src/lib.rs | 32 ++++++++++++++++++++++++++++----
 crates/twirp/src/client.rs    | 22 +++++++++++++++-------
 crates/twirp/src/test.rs      |  3 ++-
 example/src/bin/client.rs     | 31 ++++++++++++++++++++++++++-----
 6 files changed, 73 insertions(+), 17 deletions(-)

diff --git a/Cargo.lock b/Cargo.lock
index b1894de..df694cf 100644
--- a/Cargo.lock
+++ b/Cargo.lock
@@ -1294,6 +1294,7 @@ dependencies = [
  "proc-macro2",
  "prost-build",
  "quote",
+ "reqwest",
  "syn",
 ]
 
diff --git a/crates/twirp-build/Cargo.toml b/crates/twirp-build/Cargo.toml
index 908c318..67ebe7a 100644
--- a/crates/twirp-build/Cargo.toml
+++ b/crates/twirp-build/Cargo.toml
@@ -16,6 +16,7 @@ license-file = "./LICENSE"
 [dependencies]
 prost-build = "0.13"
 prettyplease = { version = "0.2" }
+reqwest = { version = "0.12", default-features = false }
 quote = "1.0"
 syn = "2.0"
 proc-macro2 = "1.0"
diff --git a/crates/twirp-build/src/lib.rs b/crates/twirp-build/src/lib.rs
index 0540c67..e614770 100644
--- a/crates/twirp-build/src/lib.rs
+++ b/crates/twirp-build/src/lib.rs
@@ -164,21 +164,45 @@ impl prost_build::ServiceGenerator for ServiceGenerator {
         let client_name = service.client_name;
         let mut client_trait_methods = Vec::with_capacity(service.methods.len());
         let mut client_methods = Vec::with_capacity(service.methods.len());
+        client_trait_methods.push(quote! {
+            async fn request<O>(&self, req: twirp::reqwest::RequestBuilder) -> Result<O, twirp::ClientError> where O: prost::Message + Default;
+        });
+        client_methods.push(quote! {
+            async fn request<O>(&self, req: twirp::reqwest::RequestBuilder) -> Result<O, twirp::ClientError> where O: prost::Message + Default {
+                self.make_request(req).await
+            }
+        });
         for m in &service.methods {
             let name = &m.name;
+            // let name_ext = format_ident!("{}_ext", name);
+            let build_name = format_ident!("build_{}", name);
             let input_type = &m.input_type;
             let output_type = &m.output_type;
             let request_path = format!("{}/{}", service.fqn, m.proto_name);
 
             client_trait_methods.push(quote! {
-                async fn #name(&self, req: #input_type) -> Result<#output_type, twirp::ClientError>;
+                async fn #name(&self, req: #input_type) -> Result<#output_type, twirp::ClientError> {
+                    let builder = self.#build_name(req)?;
+                    self.request(builder).await
+                }
+            });
+            // client_trait_methods.push(quote! {
+            //     async fn #name_ext(&self, req: twirp::reqwest::RequestBuilder) -> Result<#output_type, twirp::ClientError>;
+            // });
+            client_trait_methods.push(quote! {
+                fn #build_name(&self, req: #input_type) -> Result<twirp::reqwest::RequestBuilder, twirp::ClientError>;
             });
 
+            // client_methods.push(quote! {
+            //     async fn #name_ext(&self, req: twirp::reqwest::RequestBuilder) -> Result<#output_type, twirp::ClientError> {
+            //         self.request(req).await
+            //     }
+            // });
             client_methods.push(quote! {
-                async fn #name(&self, req: #input_type) -> Result<#output_type, twirp::ClientError> {
-                    self.request(#request_path, req).await
+                fn #build_name(&self, req: #input_type) -> Result<twirp::reqwest::RequestBuilder, twirp::ClientError> {
+                    self.build_request(#request_path, req)
                 }
-            })
+            });
         }
         let client_trait = quote! {
             #[twirp::async_trait::async_trait]
diff --git a/crates/twirp/src/client.rs b/crates/twirp/src/client.rs
index 5f8ac5b..cfc84e8 100644
--- a/crates/twirp/src/client.rs
+++ b/crates/twirp/src/client.rs
@@ -3,7 +3,7 @@ use std::vec;
 
 use async_trait::async_trait;
 use reqwest::header::{InvalidHeaderValue, CONTENT_TYPE};
-use reqwest::StatusCode;
+use reqwest::{RequestBuilder, StatusCode};
 use thiserror::Error;
 use url::Url;
 
@@ -155,23 +155,31 @@ impl Client {
         }
     }
 
-    /// Make an HTTP twirp request.
-    pub async fn request<I, O>(&self, path: &str, body: I) -> Result<O>
+    pub fn build_request<I>(&self, path: &str, body: I) -> Result<RequestBuilder>
     where
         I: prost::Message,
-        O: prost::Message + Default,
     {
         let mut url = self.inner.base_url.join(path)?;
         if let Some(host) = &self.host {
             url.set_host(Some(host))?
         };
-        let path = url.path().to_string();
+
         let req = self
             .http_client
             .post(url)
             .header(CONTENT_TYPE, CONTENT_TYPE_PROTOBUF)
-            .body(serialize_proto_message(body))
-            .build()?;
+            .body(serialize_proto_message(body));
+        Ok(req)
+    }
+
+    /// Make an HTTP twirp request.
+    // pub async fn request<I, O>(&self, ctx: Context, path: &str, body: I) -> Result<O>
+    pub async fn make_request<O>(&self, builder: RequestBuilder) -> Result<O>
+    where
+        O: prost::Message + Default,
+    {
+        let req = builder.build()?;
+        let path = req.url().path().to_string();
 
         // Create and execute the middleware handlers
         let next = Next::new(&self.http_client, &self.inner.middlewares);
diff --git a/crates/twirp/src/test.rs b/crates/twirp/src/test.rs
index e80effd..6178e91 100644
--- a/crates/twirp/src/test.rs
+++ b/crates/twirp/src/test.rs
@@ -121,7 +121,8 @@ pub trait TestApiClient {
 #[async_trait]
 impl TestApiClient for Client {
     async fn ping(&self, req: PingRequest) -> Result<PingResponse> {
-        self.request("test.TestAPI/Ping", req).await
+        let req = self.build_request("test.TestAPI/Ping", req)?;
+        self.make_request(req).await
     }
 
     async fn boom(&self, _req: PingRequest) -> Result<PingResponse> {
diff --git a/example/src/bin/client.rs b/example/src/bin/client.rs
index 89c6e71..28ea4a3 100644
--- a/example/src/bin/client.rs
+++ b/example/src/bin/client.rs
@@ -1,6 +1,6 @@
 use twirp::async_trait::async_trait;
 use twirp::client::{Client, ClientBuilder, Middleware, Next};
-use twirp::reqwest::{Request, Response};
+use twirp::reqwest::{self, Request, Response};
 use twirp::url::Url;
 use twirp::GenericError;
 
@@ -38,6 +38,15 @@ pub async fn main() -> Result<(), GenericError> {
         .await;
     eprintln!("{:?}", resp);
 
+    // TODO: Figure out where `with_host` goes in all this...
+    let req = client
+        .with_host("localhost")
+        .build_make_hat(MakeHatRequest { inches: 1 })?
+        .header("x-custom-header", "a");
+    // Make a request with context
+    let resp: MakeHatResponse = client.request(req).await?;
+    eprintln!("{:?}", resp);
+
     Ok(())
 }
 
@@ -69,23 +78,35 @@ impl Middleware for PrintResponseHeaders {
     }
 }
 
+// NOTE: This is just to demonstrate manually implementing the client trait. You don't need to do this as this code will
+// be generated for you by twirp-build.
 #[allow(dead_code)]
 #[derive(Debug)]
 struct MockHaberdasherApiClient;
 
 #[async_trait]
 impl HaberdasherApiClient for MockHaberdasherApiClient {
-    async fn make_hat(
+    fn build_make_hat(
         &self,
         _req: MakeHatRequest,
-    ) -> Result<MakeHatResponse, twirp::client::ClientError> {
+    ) -> Result<reqwest::RequestBuilder, twirp::ClientError> {
         todo!()
     }
 
-    async fn get_status(
+    fn build_get_status(
         &self,
         _req: GetStatusRequest,
-    ) -> Result<GetStatusResponse, twirp::client::ClientError> {
+    ) -> Result<reqwest::RequestBuilder, twirp::ClientError> {
+        todo!()
+    }
+
+    async fn request<O>(
+        &self,
+        _req: reqwest::RequestBuilder,
+    ) -> Result<O, twirp::client::ClientError>
+    where
+        O: prost::Message + Default,
+    {
         todo!()
     }
 }

From ce4848747de4128416a3a573fbcfc6a19aa6706e Mon Sep 17 00:00:00 2001
From: Timothy Clem <timothy.clem@gmail.com>
Date: Thu, 12 Jun 2025 13:01:57 -0700
Subject: [PATCH 2/8] WIP: handle both types

---
 crates/twirp-build/src/lib.rs | 37 ++++++++++++++++++----------
 crates/twirp/src/client.rs    | 46 ++++++++++++++++++++++++++++++++---
 crates/twirp/src/lib.rs       |  2 +-
 example/src/bin/client.rs     | 22 ++++++++++++-----
 4 files changed, 83 insertions(+), 24 deletions(-)

diff --git a/crates/twirp-build/src/lib.rs b/crates/twirp-build/src/lib.rs
index e614770..a1ab865 100644
--- a/crates/twirp-build/src/lib.rs
+++ b/crates/twirp-build/src/lib.rs
@@ -165,16 +165,35 @@ impl prost_build::ServiceGenerator for ServiceGenerator {
         let mut client_trait_methods = Vec::with_capacity(service.methods.len());
         let mut client_methods = Vec::with_capacity(service.methods.len());
         client_trait_methods.push(quote! {
-            async fn request<O>(&self, req: twirp::reqwest::RequestBuilder) -> Result<O, twirp::ClientError> where O: prost::Message + Default;
+            async fn request<I, O>(&self, req: twirp::RequestBuilder<I, O>) -> Result<O, twirp::ClientError>
+            where
+                I: prost::Message,
+                O: prost::Message + Default;
+        });
+        client_trait_methods.push(quote! {
+            fn build<I, O>(&self, req: I) -> Result<twirp::RequestBuilder<I, O>, twirp::ClientError>
+            where
+                I: prost::Message,
+                O: prost::Message + Default;
         });
         client_methods.push(quote! {
-            async fn request<O>(&self, req: twirp::reqwest::RequestBuilder) -> Result<O, twirp::ClientError> where O: prost::Message + Default {
+            async fn request<I, O>(&self, req: twirp::RequestBuilder<I, O>) -> Result<O, twirp::ClientError>
+            where
+                I: prost::Message,
+                O: prost::Message + Default {
                 self.make_request(req).await
             }
         });
+        client_methods.push(quote! {
+            fn build<I, O>(&self, req: I) -> Result<twirp::RequestBuilder<I, O>, twirp::ClientError>
+            where
+                I: prost::Message,
+                O: prost::Message + Default {
+                    todo!()
+            }
+        });
         for m in &service.methods {
             let name = &m.name;
-            // let name_ext = format_ident!("{}_ext", name);
             let build_name = format_ident!("build_{}", name);
             let input_type = &m.input_type;
             let output_type = &m.output_type;
@@ -186,20 +205,12 @@ impl prost_build::ServiceGenerator for ServiceGenerator {
                     self.request(builder).await
                 }
             });
-            // client_trait_methods.push(quote! {
-            //     async fn #name_ext(&self, req: twirp::reqwest::RequestBuilder) -> Result<#output_type, twirp::ClientError>;
-            // });
             client_trait_methods.push(quote! {
-                fn #build_name(&self, req: #input_type) -> Result<twirp::reqwest::RequestBuilder, twirp::ClientError>;
+                fn #build_name(&self, req: #input_type) -> Result<twirp::RequestBuilder<#input_type, #output_type>, twirp::ClientError>;
             });
 
-            // client_methods.push(quote! {
-            //     async fn #name_ext(&self, req: twirp::reqwest::RequestBuilder) -> Result<#output_type, twirp::ClientError> {
-            //         self.request(req).await
-            //     }
-            // });
             client_methods.push(quote! {
-                fn #build_name(&self, req: #input_type) -> Result<twirp::reqwest::RequestBuilder, twirp::ClientError> {
+                fn #build_name(&self, req: #input_type) -> Result<twirp::RequestBuilder<#input_type, #output_type>, twirp::ClientError> {
                     self.build_request(#request_path, req)
                 }
             });
diff --git a/crates/twirp/src/client.rs b/crates/twirp/src/client.rs
index cfc84e8..9a839b7 100644
--- a/crates/twirp/src/client.rs
+++ b/crates/twirp/src/client.rs
@@ -2,8 +2,9 @@ use std::sync::Arc;
 use std::vec;
 
 use async_trait::async_trait;
+use http::{HeaderName, HeaderValue};
 use reqwest::header::{InvalidHeaderValue, CONTENT_TYPE};
-use reqwest::{RequestBuilder, StatusCode};
+use reqwest::StatusCode;
 use thiserror::Error;
 use url::Url;
 
@@ -145,6 +146,8 @@ impl Client {
         &self.inner.base_url
     }
 
+    // TODO: Move this to the `ClientBuilder`
+    //
     /// Creates a new `twirp::Client` with the same configuration as the current
     /// one, but with a different host in the base URL.
     pub fn with_host(&self, host: &str) -> Self {
@@ -155,9 +158,10 @@ impl Client {
         }
     }
 
-    pub fn build_request<I>(&self, path: &str, body: I) -> Result<RequestBuilder>
+    pub fn build_request<I, O>(&self, path: &str, body: I) -> Result<RequestBuilder<I, O>>
     where
         I: prost::Message,
+        O: prost::Message + Default,
     {
         let mut url = self.inner.base_url.join(path)?;
         if let Some(host) = &self.host {
@@ -169,13 +173,14 @@ impl Client {
             .post(url)
             .header(CONTENT_TYPE, CONTENT_TYPE_PROTOBUF)
             .body(serialize_proto_message(body));
-        Ok(req)
+        Ok(RequestBuilder::new(req))
     }
 
     /// Make an HTTP twirp request.
     // pub async fn request<I, O>(&self, ctx: Context, path: &str, body: I) -> Result<O>
-    pub async fn make_request<O>(&self, builder: RequestBuilder) -> Result<O>
+    pub async fn make_request<I, O>(&self, builder: RequestBuilder<I, O>) -> Result<O>
     where
+        I: prost::Message,
         O: prost::Message + Default,
     {
         let req = builder.build()?;
@@ -214,6 +219,39 @@ impl Client {
     }
 }
 
+pub struct RequestBuilder<I, O> {
+    inner: reqwest::RequestBuilder,
+    host: Option<String>,
+    _input: std::marker::PhantomData<I>,
+    _output: std::marker::PhantomData<O>,
+}
+
+impl<I, O> RequestBuilder<I, O> {
+    pub fn new(inner: reqwest::RequestBuilder) -> Self {
+        Self {
+            inner,
+            host: None,
+            _input: std::marker::PhantomData,
+            _output: std::marker::PhantomData,
+        }
+    }
+
+    pub fn header<K, V>(self, key: K, value: V) -> RequestBuilder<I, O>
+    where
+        HeaderName: TryFrom<K>,
+        <HeaderName as TryFrom<K>>::Error: Into<http::Error>,
+        HeaderValue: TryFrom<V>,
+        <HeaderValue as TryFrom<V>>::Error: Into<http::Error>,
+    {
+        RequestBuilder::new(self.inner.header(key, value))
+    }
+
+    /// Builds the request.
+    pub fn build(self) -> Result<reqwest::Request, reqwest::Error> {
+        self.inner.build()
+    }
+}
+
 // This concept of reqwest middleware is taken pretty much directly from:
 // https://github.com/TrueLayer/reqwest-middleware, but simplified for the
 // specific needs of this twirp client.
diff --git a/crates/twirp/src/lib.rs b/crates/twirp/src/lib.rs
index 5b66b2b..6cbbb52 100644
--- a/crates/twirp/src/lib.rs
+++ b/crates/twirp/src/lib.rs
@@ -10,7 +10,7 @@ pub mod test;
 #[doc(hidden)]
 pub mod details;
 
-pub use client::{Client, ClientBuilder, ClientError, Middleware, Next, Result};
+pub use client::{Client, ClientBuilder, ClientError, Middleware, Next, RequestBuilder, Result};
 pub use context::Context;
 pub use error::*; // many constructors like `invalid_argument()`
 pub use http::Extensions;
diff --git a/example/src/bin/client.rs b/example/src/bin/client.rs
index 28ea4a3..977e252 100644
--- a/example/src/bin/client.rs
+++ b/example/src/bin/client.rs
@@ -1,6 +1,6 @@
 use twirp::async_trait::async_trait;
 use twirp::client::{Client, ClientBuilder, Middleware, Next};
-use twirp::reqwest::{self, Request, Response};
+use twirp::reqwest::{Request, Response};
 use twirp::url::Url;
 use twirp::GenericError;
 
@@ -44,7 +44,7 @@ pub async fn main() -> Result<(), GenericError> {
         .build_make_hat(MakeHatRequest { inches: 1 })?
         .header("x-custom-header", "a");
     // Make a request with context
-    let resp: MakeHatResponse = client.request(req).await?;
+    let resp = client.request(req).await?;
     eprintln!("{:?}", resp);
 
     Ok(())
@@ -89,22 +89,32 @@ impl HaberdasherApiClient for MockHaberdasherApiClient {
     fn build_make_hat(
         &self,
         _req: MakeHatRequest,
-    ) -> Result<reqwest::RequestBuilder, twirp::ClientError> {
+    ) -> Result<twirp::RequestBuilder<MakeHatRequest, MakeHatResponse>, twirp::ClientError> {
         todo!()
     }
 
     fn build_get_status(
         &self,
         _req: GetStatusRequest,
-    ) -> Result<reqwest::RequestBuilder, twirp::ClientError> {
+    ) -> Result<twirp::RequestBuilder<GetStatusRequest, GetStatusResponse>, twirp::ClientError>
+    {
         todo!()
     }
 
-    async fn request<O>(
+    async fn request<I, O>(
         &self,
-        _req: reqwest::RequestBuilder,
+        _req: twirp::RequestBuilder<I, O>,
     ) -> Result<O, twirp::client::ClientError>
     where
+        I: prost::Message,
+        O: prost::Message + Default,
+    {
+        todo!()
+    }
+
+    fn build<I, O>(&self, _req: I) -> Result<twirp::RequestBuilder<I, O>, twirp::ClientError>
+    where
+        I: prost::Message,
         O: prost::Message + Default,
     {
         todo!()

From 7ea433d683ac3f64886893698d6e6d9c06a72759 Mon Sep 17 00:00:00 2001
From: Timothy Clem <timothy.clem@gmail.com>
Date: Thu, 12 Jun 2025 13:44:56 -0700
Subject: [PATCH 3/8] Clean up

---
 crates/twirp-build/src/lib.rs | 39 +++++++----------------------------
 crates/twirp/src/client.rs    |  5 +----
 crates/twirp/src/test.rs      |  2 +-
 example/src/bin/client.rs     | 24 +++++++--------------
 4 files changed, 17 insertions(+), 53 deletions(-)

diff --git a/crates/twirp-build/src/lib.rs b/crates/twirp-build/src/lib.rs
index a1ab865..0d69e86 100644
--- a/crates/twirp-build/src/lib.rs
+++ b/crates/twirp-build/src/lib.rs
@@ -164,34 +164,6 @@ impl prost_build::ServiceGenerator for ServiceGenerator {
         let client_name = service.client_name;
         let mut client_trait_methods = Vec::with_capacity(service.methods.len());
         let mut client_methods = Vec::with_capacity(service.methods.len());
-        client_trait_methods.push(quote! {
-            async fn request<I, O>(&self, req: twirp::RequestBuilder<I, O>) -> Result<O, twirp::ClientError>
-            where
-                I: prost::Message,
-                O: prost::Message + Default;
-        });
-        client_trait_methods.push(quote! {
-            fn build<I, O>(&self, req: I) -> Result<twirp::RequestBuilder<I, O>, twirp::ClientError>
-            where
-                I: prost::Message,
-                O: prost::Message + Default;
-        });
-        client_methods.push(quote! {
-            async fn request<I, O>(&self, req: twirp::RequestBuilder<I, O>) -> Result<O, twirp::ClientError>
-            where
-                I: prost::Message,
-                O: prost::Message + Default {
-                self.make_request(req).await
-            }
-        });
-        client_methods.push(quote! {
-            fn build<I, O>(&self, req: I) -> Result<twirp::RequestBuilder<I, O>, twirp::ClientError>
-            where
-                I: prost::Message,
-                O: prost::Message + Default {
-                    todo!()
-            }
-        });
         for m in &service.methods {
             let name = &m.name;
             let build_name = format_ident!("build_{}", name);
@@ -200,10 +172,7 @@ impl prost_build::ServiceGenerator for ServiceGenerator {
             let request_path = format!("{}/{}", service.fqn, m.proto_name);
 
             client_trait_methods.push(quote! {
-                async fn #name(&self, req: #input_type) -> Result<#output_type, twirp::ClientError> {
-                    let builder = self.#build_name(req)?;
-                    self.request(builder).await
-                }
+                async fn #name(&self, req: #input_type) -> Result<#output_type, twirp::ClientError>;
             });
             client_trait_methods.push(quote! {
                 fn #build_name(&self, req: #input_type) -> Result<twirp::RequestBuilder<#input_type, #output_type>, twirp::ClientError>;
@@ -214,6 +183,12 @@ impl prost_build::ServiceGenerator for ServiceGenerator {
                     self.build_request(#request_path, req)
                 }
             });
+            client_methods.push(quote! {
+                async fn #name(&self, req: #input_type) -> Result<#output_type, twirp::ClientError> {
+                    let builder = self.#build_name(req)?;
+                    self.request(builder).await
+                }
+            });
         }
         let client_trait = quote! {
             #[twirp::async_trait::async_trait]
diff --git a/crates/twirp/src/client.rs b/crates/twirp/src/client.rs
index 9a839b7..38c2e73 100644
--- a/crates/twirp/src/client.rs
+++ b/crates/twirp/src/client.rs
@@ -177,8 +177,7 @@ impl Client {
     }
 
     /// Make an HTTP twirp request.
-    // pub async fn request<I, O>(&self, ctx: Context, path: &str, body: I) -> Result<O>
-    pub async fn make_request<I, O>(&self, builder: RequestBuilder<I, O>) -> Result<O>
+    pub async fn request<I, O>(&self, builder: RequestBuilder<I, O>) -> Result<O>
     where
         I: prost::Message,
         O: prost::Message + Default,
@@ -221,7 +220,6 @@ impl Client {
 
 pub struct RequestBuilder<I, O> {
     inner: reqwest::RequestBuilder,
-    host: Option<String>,
     _input: std::marker::PhantomData<I>,
     _output: std::marker::PhantomData<O>,
 }
@@ -230,7 +228,6 @@ impl<I, O> RequestBuilder<I, O> {
     pub fn new(inner: reqwest::RequestBuilder) -> Self {
         Self {
             inner,
-            host: None,
             _input: std::marker::PhantomData,
             _output: std::marker::PhantomData,
         }
diff --git a/crates/twirp/src/test.rs b/crates/twirp/src/test.rs
index 6178e91..4a8ecd5 100644
--- a/crates/twirp/src/test.rs
+++ b/crates/twirp/src/test.rs
@@ -122,7 +122,7 @@ pub trait TestApiClient {
 impl TestApiClient for Client {
     async fn ping(&self, req: PingRequest) -> Result<PingResponse> {
         let req = self.build_request("test.TestAPI/Ping", req)?;
-        self.make_request(req).await
+        self.request(req).await
     }
 
     async fn boom(&self, _req: PingRequest) -> Result<PingResponse> {
diff --git a/example/src/bin/client.rs b/example/src/bin/client.rs
index 977e252..cff9dd4 100644
--- a/example/src/bin/client.rs
+++ b/example/src/bin/client.rs
@@ -80,6 +80,8 @@ impl Middleware for PrintResponseHeaders {
 
 // NOTE: This is just to demonstrate manually implementing the client trait. You don't need to do this as this code will
 // be generated for you by twirp-build.
+//
+// This is here so that we can visualize changes to the generated client code
 #[allow(dead_code)]
 #[derive(Debug)]
 struct MockHaberdasherApiClient;
@@ -92,6 +94,9 @@ impl HaberdasherApiClient for MockHaberdasherApiClient {
     ) -> Result<twirp::RequestBuilder<MakeHatRequest, MakeHatResponse>, twirp::ClientError> {
         todo!()
     }
+    async fn make_hat(&self, _req: MakeHatRequest) -> Result<MakeHatResponse, twirp::ClientError> {
+        todo!()
+    }
 
     fn build_get_status(
         &self,
@@ -100,23 +105,10 @@ impl HaberdasherApiClient for MockHaberdasherApiClient {
     {
         todo!()
     }
-
-    async fn request<I, O>(
+    async fn get_status(
         &self,
-        _req: twirp::RequestBuilder<I, O>,
-    ) -> Result<O, twirp::client::ClientError>
-    where
-        I: prost::Message,
-        O: prost::Message + Default,
-    {
-        todo!()
-    }
-
-    fn build<I, O>(&self, _req: I) -> Result<twirp::RequestBuilder<I, O>, twirp::ClientError>
-    where
-        I: prost::Message,
-        O: prost::Message + Default,
-    {
+        _req: GetStatusRequest,
+    ) -> Result<GetStatusResponse, twirp::ClientError> {
         todo!()
     }
 }

From 05e4f6ad11bf4d89254a16fd4ee6e7ec8a49b9d1 Mon Sep 17 00:00:00 2001
From: Timothy Clem <timothy.clem@gmail.com>
Date: Thu, 12 Jun 2025 13:47:28 -0700
Subject: [PATCH 4/8] Minor cleanup

---
 Cargo.lock                    | 2 --
 crates/twirp-build/Cargo.toml | 2 --
 2 files changed, 4 deletions(-)

diff --git a/Cargo.lock b/Cargo.lock
index df694cf..8f7a016 100644
--- a/Cargo.lock
+++ b/Cargo.lock
@@ -1291,10 +1291,8 @@ name = "twirp-build"
 version = "0.8.0"
 dependencies = [
  "prettyplease",
- "proc-macro2",
  "prost-build",
  "quote",
- "reqwest",
  "syn",
 ]
 
diff --git a/crates/twirp-build/Cargo.toml b/crates/twirp-build/Cargo.toml
index 67ebe7a..900843e 100644
--- a/crates/twirp-build/Cargo.toml
+++ b/crates/twirp-build/Cargo.toml
@@ -16,7 +16,5 @@ license-file = "./LICENSE"
 [dependencies]
 prost-build = "0.13"
 prettyplease = { version = "0.2" }
-reqwest = { version = "0.12", default-features = false }
 quote = "1.0"
 syn = "2.0"
-proc-macro2 = "1.0"

From 49a738d839ca93abef50f4477aa04a9548666c6c Mon Sep 17 00:00:00 2001
From: Timothy Clem <timothy.clem@gmail.com>
Date: Thu, 12 Jun 2025 20:02:26 -0700
Subject: [PATCH 5/8] second attempt

---
 crates/twirp-build/src/lib.rs | 11 +++---
 crates/twirp/src/client.rs    | 67 +++++++++++++++++++----------------
 crates/twirp/src/test.rs      |  3 +-
 example/src/bin/client.rs     | 15 ++++----
 4 files changed, 50 insertions(+), 46 deletions(-)

diff --git a/crates/twirp-build/src/lib.rs b/crates/twirp-build/src/lib.rs
index 0d69e86..4506ac9 100644
--- a/crates/twirp-build/src/lib.rs
+++ b/crates/twirp-build/src/lib.rs
@@ -166,7 +166,7 @@ impl prost_build::ServiceGenerator for ServiceGenerator {
         let mut client_methods = Vec::with_capacity(service.methods.len());
         for m in &service.methods {
             let name = &m.name;
-            let build_name = format_ident!("build_{}", name);
+            let name_request = format_ident!("{}_request", name);
             let input_type = &m.input_type;
             let output_type = &m.output_type;
             let request_path = format!("{}/{}", service.fqn, m.proto_name);
@@ -175,18 +175,17 @@ impl prost_build::ServiceGenerator for ServiceGenerator {
                 async fn #name(&self, req: #input_type) -> Result<#output_type, twirp::ClientError>;
             });
             client_trait_methods.push(quote! {
-                fn #build_name(&self, req: #input_type) -> Result<twirp::RequestBuilder<#input_type, #output_type>, twirp::ClientError>;
+                fn #name_request(&self, req: #input_type) -> Result<twirp::RequestBuilder<#input_type, #output_type>, twirp::ClientError>;
             });
 
             client_methods.push(quote! {
-                fn #build_name(&self, req: #input_type) -> Result<twirp::RequestBuilder<#input_type, #output_type>, twirp::ClientError> {
-                    self.build_request(#request_path, req)
+                fn #name_request(&self, req: #input_type) -> Result<twirp::RequestBuilder<#input_type, #output_type>, twirp::ClientError> {
+                    self.request(#request_path, req)
                 }
             });
             client_methods.push(quote! {
                 async fn #name(&self, req: #input_type) -> Result<#output_type, twirp::ClientError> {
-                    let builder = self.#build_name(req)?;
-                    self.request(builder).await
+                    self.#name_request(req)?.send().await
                 }
             });
         }
diff --git a/crates/twirp/src/client.rs b/crates/twirp/src/client.rs
index 38c2e73..e46bbce 100644
--- a/crates/twirp/src/client.rs
+++ b/crates/twirp/src/client.rs
@@ -158,31 +158,10 @@ impl Client {
         }
     }
 
-    pub fn build_request<I, O>(&self, path: &str, body: I) -> Result<RequestBuilder<I, O>>
+    pub(super) async fn execute<O>(&self, req: reqwest::Request) -> Result<O>
     where
-        I: prost::Message,
         O: prost::Message + Default,
     {
-        let mut url = self.inner.base_url.join(path)?;
-        if let Some(host) = &self.host {
-            url.set_host(Some(host))?
-        };
-
-        let req = self
-            .http_client
-            .post(url)
-            .header(CONTENT_TYPE, CONTENT_TYPE_PROTOBUF)
-            .body(serialize_proto_message(body));
-        Ok(RequestBuilder::new(req))
-    }
-
-    /// Make an HTTP twirp request.
-    pub async fn request<I, O>(&self, builder: RequestBuilder<I, O>) -> Result<O>
-    where
-        I: prost::Message,
-        O: prost::Message + Default,
-    {
-        let req = builder.build()?;
         let path = req.url().path().to_string();
 
         // Create and execute the middleware handlers
@@ -216,36 +195,64 @@ impl Client {
             }),
         }
     }
+
+    // Start building a request...
+    pub fn request<I, O>(&self, path: &str, body: I) -> Result<RequestBuilder<I, O>>
+    where
+        I: prost::Message,
+        O: prost::Message + Default,
+    {
+        let mut url = self.inner.base_url.join(path)?;
+        if let Some(host) = &self.host {
+            url.set_host(Some(host))?
+        };
+
+        let req = self
+            .http_client
+            .post(url)
+            .header(CONTENT_TYPE, CONTENT_TYPE_PROTOBUF)
+            .body(serialize_proto_message(body));
+        Ok(RequestBuilder::new(self.clone(), req))
+    }
 }
 
-pub struct RequestBuilder<I, O> {
+pub struct RequestBuilder<I, O>
+where
+    O: prost::Message + Default,
+{
+    client: Client,
     inner: reqwest::RequestBuilder,
     _input: std::marker::PhantomData<I>,
     _output: std::marker::PhantomData<O>,
 }
 
-impl<I, O> RequestBuilder<I, O> {
-    pub fn new(inner: reqwest::RequestBuilder) -> Self {
+impl<I, O> RequestBuilder<I, O>
+where
+    O: prost::Message + Default,
+{
+    pub fn new(client: Client, inner: reqwest::RequestBuilder) -> Self {
         Self {
+            client,
             inner,
             _input: std::marker::PhantomData,
             _output: std::marker::PhantomData,
         }
     }
 
-    pub fn header<K, V>(self, key: K, value: V) -> RequestBuilder<I, O>
+    pub fn header<K, V>(mut self, key: K, value: V) -> RequestBuilder<I, O>
     where
         HeaderName: TryFrom<K>,
         <HeaderName as TryFrom<K>>::Error: Into<http::Error>,
         HeaderValue: TryFrom<V>,
         <HeaderValue as TryFrom<V>>::Error: Into<http::Error>,
     {
-        RequestBuilder::new(self.inner.header(key, value))
+        self.inner = self.inner.header(key, value);
+        self
     }
 
-    /// Builds the request.
-    pub fn build(self) -> Result<reqwest::Request, reqwest::Error> {
-        self.inner.build()
+    pub async fn send(self) -> Result<O> {
+        let req = self.inner.build()?;
+        self.client.execute(req).await
     }
 }
 
diff --git a/crates/twirp/src/test.rs b/crates/twirp/src/test.rs
index 4a8ecd5..7489a76 100644
--- a/crates/twirp/src/test.rs
+++ b/crates/twirp/src/test.rs
@@ -121,8 +121,7 @@ pub trait TestApiClient {
 #[async_trait]
 impl TestApiClient for Client {
     async fn ping(&self, req: PingRequest) -> Result<PingResponse> {
-        let req = self.build_request("test.TestAPI/Ping", req)?;
-        self.request(req).await
+        self.request("test.TestAPI/Ping", req)?.send().await
     }
 
     async fn boom(&self, _req: PingRequest) -> Result<PingResponse> {
diff --git a/example/src/bin/client.rs b/example/src/bin/client.rs
index cff9dd4..a959fd8 100644
--- a/example/src/bin/client.rs
+++ b/example/src/bin/client.rs
@@ -38,13 +38,12 @@ pub async fn main() -> Result<(), GenericError> {
         .await;
     eprintln!("{:?}", resp);
 
-    // TODO: Figure out where `with_host` goes in all this...
-    let req = client
+    let resp = client
         .with_host("localhost")
-        .build_make_hat(MakeHatRequest { inches: 1 })?
-        .header("x-custom-header", "a");
-    // Make a request with context
-    let resp = client.request(req).await?;
+        .make_hat_request(MakeHatRequest { inches: 1 })?
+        .header("x-custom-header", "a")
+        .send()
+        .await?;
     eprintln!("{:?}", resp);
 
     Ok(())
@@ -88,7 +87,7 @@ struct MockHaberdasherApiClient;
 
 #[async_trait]
 impl HaberdasherApiClient for MockHaberdasherApiClient {
-    fn build_make_hat(
+    fn make_hat_request(
         &self,
         _req: MakeHatRequest,
     ) -> Result<twirp::RequestBuilder<MakeHatRequest, MakeHatResponse>, twirp::ClientError> {
@@ -98,7 +97,7 @@ impl HaberdasherApiClient for MockHaberdasherApiClient {
         todo!()
     }
 
-    fn build_get_status(
+    fn get_status_request(
         &self,
         _req: GetStatusRequest,
     ) -> Result<twirp::RequestBuilder<GetStatusRequest, GetStatusResponse>, twirp::ClientError>

From 3ecd726dfba718a8e2995bffaaa43fe2d228f848 Mon Sep 17 00:00:00 2001
From: Timothy Clem <timothy.clem@gmail.com>
Date: Fri, 13 Jun 2025 10:11:27 -0700
Subject: [PATCH 6/8] Default impl

---
 crates/twirp-build/src/lib.rs | 9 +++------
 example/src/bin/client.rs     | 2 ++
 2 files changed, 5 insertions(+), 6 deletions(-)

diff --git a/crates/twirp-build/src/lib.rs b/crates/twirp-build/src/lib.rs
index 4506ac9..78216aa 100644
--- a/crates/twirp-build/src/lib.rs
+++ b/crates/twirp-build/src/lib.rs
@@ -172,7 +172,9 @@ impl prost_build::ServiceGenerator for ServiceGenerator {
             let request_path = format!("{}/{}", service.fqn, m.proto_name);
 
             client_trait_methods.push(quote! {
-                async fn #name(&self, req: #input_type) -> Result<#output_type, twirp::ClientError>;
+                async fn #name(&self, req: #input_type) -> Result<#output_type, twirp::ClientError> {
+                    self.#name_request(req)?.send().await
+                }
             });
             client_trait_methods.push(quote! {
                 fn #name_request(&self, req: #input_type) -> Result<twirp::RequestBuilder<#input_type, #output_type>, twirp::ClientError>;
@@ -183,11 +185,6 @@ impl prost_build::ServiceGenerator for ServiceGenerator {
                     self.request(#request_path, req)
                 }
             });
-            client_methods.push(quote! {
-                async fn #name(&self, req: #input_type) -> Result<#output_type, twirp::ClientError> {
-                    self.#name_request(req)?.send().await
-                }
-            });
         }
         let client_trait = quote! {
             #[twirp::async_trait::async_trait]
diff --git a/example/src/bin/client.rs b/example/src/bin/client.rs
index a959fd8..51b132a 100644
--- a/example/src/bin/client.rs
+++ b/example/src/bin/client.rs
@@ -93,6 +93,7 @@ impl HaberdasherApiClient for MockHaberdasherApiClient {
     ) -> Result<twirp::RequestBuilder<MakeHatRequest, MakeHatResponse>, twirp::ClientError> {
         todo!()
     }
+    // implementing this one is optional
     async fn make_hat(&self, _req: MakeHatRequest) -> Result<MakeHatResponse, twirp::ClientError> {
         todo!()
     }
@@ -104,6 +105,7 @@ impl HaberdasherApiClient for MockHaberdasherApiClient {
     {
         todo!()
     }
+    // implementing this one is optional
     async fn get_status(
         &self,
         _req: GetStatusRequest,

From c71285a83bea198fbdb3c9beeaa6083d710810bf Mon Sep 17 00:00:00 2001
From: Timothy Clem <timothy.clem@gmail.com>
Date: Fri, 13 Jun 2025 11:01:27 -0700
Subject: [PATCH 7/8] Some docs

---
 crates/twirp/src/client.rs | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/crates/twirp/src/client.rs b/crates/twirp/src/client.rs
index e46bbce..c6875a7 100644
--- a/crates/twirp/src/client.rs
+++ b/crates/twirp/src/client.rs
@@ -146,8 +146,6 @@ impl Client {
         &self.inner.base_url
     }
 
-    // TODO: Move this to the `ClientBuilder`
-    //
     /// Creates a new `twirp::Client` with the same configuration as the current
     /// one, but with a different host in the base URL.
     pub fn with_host(&self, host: &str) -> Self {
@@ -158,6 +156,7 @@ impl Client {
         }
     }
 
+    /// Executes a `Request`.
     pub(super) async fn execute<O>(&self, req: reqwest::Request) -> Result<O>
     where
         O: prost::Message + Default,
@@ -239,6 +238,7 @@ where
         }
     }
 
+    /// Add a `Header` to this Request.
     pub fn header<K, V>(mut self, key: K, value: V) -> RequestBuilder<I, O>
     where
         HeaderName: TryFrom<K>,

From f3b0e716e13618650f2a1c64697dde2a881b46f3 Mon Sep 17 00:00:00 2001
From: Timothy Clem <timothy.clem@gmail.com>
Date: Fri, 13 Jun 2025 15:16:21 -0700
Subject: [PATCH 8/8] Docs

---
 crates/twirp/src/client.rs | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/crates/twirp/src/client.rs b/crates/twirp/src/client.rs
index c6875a7..c38176b 100644
--- a/crates/twirp/src/client.rs
+++ b/crates/twirp/src/client.rs
@@ -195,7 +195,9 @@ impl Client {
         }
     }
 
-    // Start building a request...
+    /// Start building a `Request` with a path and a request body.
+    ///
+    /// Returns a `RequestBuilder`, which will allow setting headers before sending.
     pub fn request<I, O>(&self, path: &str, body: I) -> Result<RequestBuilder<I, O>>
     where
         I: prost::Message,

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