fix(relay): avoid override with header passthrough (#4931)

The current implementation causes duplicate `Content-Type` headers when users
override headers in the UI or use OAuth2 authentication with the agent.
Web servers receive multiple `Content-Type` headers which causes
undefined behavior and 400 errors for backends that don't accept duplicate headers.

This also fixes inconsistent behavior when overriding the `Content-Type` header
with custom values (e.g., `application/json;v=2`).

While HTTP/1.1 headers are case-insensitive per RFC 7230, inconsistent handling
across server implementations can treat differently-cased variations (e.g.,
"Content-Type" vs "content-type") as distinct headers. HTTP/2 (RFC 7540) mandates
converting all header field names to lowercase, which would prevent this issue.

This patch removes the automatic content-type header insertion, allowing user-defined
headers to take precedence without duplication. The is a temporary
workaround until we implement a HTTP/2-compliant solution with proper normalization.

This was implemented initially to support moving lower level handling
towards the kernel, although since the larger refactor has been slightly
deferred in favor of stability, this change is suitable for current
state.

This will be revisited when we implement HTTP/2 compliant header handling in the
kernel layer as part of our upcoming kernel efforts.

Use the following request to test this out on Desktop app and Agent and
override `Content-Type` header to `application/json;=v2`:

```
curl --request POST \
  --url 'https://echo.qubit.codes/?qp=1' \
  --header 'Content-Type: application/json;v=2' \
  --data '{ "test-key": "test-value" }'
```

packages/hoppscotch-agent/src-tauri/Cargo.lock

@@ -4098,7 +4098,7 @@ checksum = "2b15c43186be67a4fd63bee50d0303afffcef381492ebe2c5d87f324e1b8815c"
 [[package]]
 name = "relay"
 version = "0.1.1"
-source = "git+https://github.com/CuriousCorrelation/relay.git#78f2360c022e974c34413d68355dcbc625ba1acd"
+source = "git+https://github.com/CuriousCorrelation/relay.git#893cec31865dc396a3d351781ec39b7625f59862"
 dependencies = [
  "bytes",
  "curl",
@@ -6209,7 +6209,7 @@ version = "0.1.9"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "cf221c93e13a30d793f7645a0e7762c55d169dbb0a49671918a2319d289b10bb"
 dependencies = [
- "windows-sys 0.59.0",
+ "windows-sys 0.48.0",
 ]
 
 [[package]]

packages/hoppscotch-desktop/plugin-workspace/relay/src/content.rs

@@ -109,10 +109,10 @@ impl<'a> ContentHandler<'a> {
     }
 
     fn set_text_content(&mut self, content: &str, media_type: &MediaType) -> Result<()> {
-        let mut headers = HashMap::new();
-        headers.insert("content-type".to_string(), media_type.to_string());
-
-        self.merge_headers(headers);
+        /* TODO: Look into reintroducing this when auth handling is done by kernel */
+        // let mut headers = HashMap::new();
+        // headers.insert("content-type".to_string(), media_type.to_string());
+        // self.merge_headers(headers);
 
         self.handle
             .post_fields_copy(content.as_bytes())
@@ -141,10 +141,10 @@ impl<'a> ContentHandler<'a> {
             }
         })?;
 
-        let mut headers = HashMap::new();
-        headers.insert("content-type".to_string(), media_type.to_string());
-
-        self.merge_headers(headers);
+        /* TODO: Look into reintroducing this when auth handling is done by kernel */
+        // let mut headers = HashMap::new();
+        // headers.insert("content-type".to_string(), media_type.to_string());
+        // self.merge_headers(headers);
 
         self.handle
             .post_fields_copy(json_str.as_bytes())
@@ -174,16 +174,18 @@ impl<'a> ContentHandler<'a> {
                 .and_then(|n| n.to_str())
                 .unwrap_or(name);
 
-            headers.insert("content-type".to_string(), media_type.to_string());
+            /* TODO: Look into reintroducing this when auth handling is done by kernel */
+            // headers.insert("content-type".to_string(), media_type.to_string());
             headers.insert(
                 "Content-Disposition".to_string(),
                 format!("attachment; filename=\"{}\"", safe_name),
             );
         } else {
-            headers.insert("content-type".to_string(), media_type.to_string());
+            /* TODO: Look into reintroducing this when auth handling is done by kernel */
+            // headers.insert("content-type".to_string(), media_type.to_string());
         }
 
-        self.merge_headers(headers);
+        // self.merge_headers(headers);
 
         self.handle.post_fields_copy(content).map_err(|e| {
             tracing::error!(error = %e, "Failed to set binary content");
@@ -202,10 +204,10 @@ impl<'a> ContentHandler<'a> {
         content: &IndexMap<String, Vec<FormValue>>,
         media_type: &MediaType,
     ) -> Result<()> {
-        let mut headers = HashMap::new();
-        headers.insert("content-type".to_string(), media_type.to_string());
-
-        self.merge_headers(headers);
+        /* TODO: Look into reintroducing this when auth handling is done by kernel */
+        // let mut headers = HashMap::new();
+        // headers.insert("content-type".to_string(), media_type.to_string());
+        // self.merge_headers(headers);
 
         let mut form = curl::easy::Form::new();
 
@@ -282,10 +284,10 @@ impl<'a> ContentHandler<'a> {
     }
 
     fn set_urlencoded_content(&mut self, content: &String, media_type: &MediaType) -> Result<()> {
-        let mut headers = HashMap::new();
-        headers.insert("content-type".to_string(), media_type.to_string());
-
-        self.merge_headers(headers);
+        /* TODO: Look into reintroducing this when auth handling is done by kernel */
+        // let mut headers = HashMap::new();
+        // headers.insert("content-type".to_string(), media_type.to_string());
+        // self.merge_headers(headers);
 
         tracing::debug!(content_length = content.len(), "URL-encoded form data");
 

packages/hoppscotch-desktop/plugin-workspace/relay/src/header.rs

@@ -66,12 +66,4 @@ impl<'a> HeadersBuilder<'a> {
             }
         })
     }
-
-    #[tracing::instrument(skip(self), level = "debug")]
-    pub(crate) fn add_content_type(&mut self, content_type: &str) -> Result<()> {
-        tracing::info!(content_type = %content_type, "Adding content-type header");
-        let mut headers = HashMap::new();
-        headers.insert("Content-Type".to_string(), content_type.to_string());
-        self.add_headers(Some(&headers))
-    }
 }

packages/hoppscotch-desktop/plugin-workspace/tauri-plugin-relay/Cargo.lock

@@ -2888,7 +2888,7 @@ checksum = "2b15c43186be67a4fd63bee50d0303afffcef381492ebe2c5d87f324e1b8815c"
 [[package]]
 name = "relay"
 version = "0.1.1"
-source = "git+https://github.com/CuriousCorrelation/relay.git#78f2360c022e974c34413d68355dcbc625ba1acd"
+source = "git+https://github.com/CuriousCorrelation/relay.git#893cec31865dc396a3d351781ec39b7625f59862"
 dependencies = [
  "bytes",
  "curl",

packages/hoppscotch-desktop/src-tauri/Cargo.lock

@@ -3956,7 +3956,7 @@ checksum = "2b15c43186be67a4fd63bee50d0303afffcef381492ebe2c5d87f324e1b8815c"
 [[package]]
 name = "relay"
 version = "0.1.1"
-source = "git+https://github.com/CuriousCorrelation/relay.git#78f2360c022e974c34413d68355dcbc625ba1acd"
+source = "git+https://github.com/CuriousCorrelation/relay.git#893cec31865dc396a3d351781ec39b7625f59862"
 dependencies = [
  "bytes",
  "curl",
@@ -5082,7 +5082,7 @@ dependencies = [
 [[package]]
 name = "tauri-plugin-relay"
 version = "0.1.0"
-source = "git+https://github.com/CuriousCorrelation/tauri-plugin-relay#86e1ccf327e60442c6ae9c2911d9359bf9e32c1b"
+source = "git+https://github.com/CuriousCorrelation/tauri-plugin-relay#fee58601fb4b0c129b5a9d87aaa0f9f87c26cd09"
 dependencies = [
  "relay",
  "serde",

pnpm-lock.yaml

@@ -1180,7 +1180,7 @@ importers:
     dependencies:
       '@hoppscotch/plugin-relay':
         specifier: github:CuriousCorrelation/tauri-plugin-relay
-        version: '@CuriousCorrelation/plugin-relay@https://codeload.github.com/CuriousCorrelation/tauri-plugin-relay/tar.gz/86e1ccf327e60442c6ae9c2911d9359bf9e32c1b'
+        version: '@CuriousCorrelation/plugin-relay@https://codeload.github.com/CuriousCorrelation/tauri-plugin-relay/tar.gz/fee58601fb4b0c129b5a9d87aaa0f9f87c26cd09'
       '@tauri-apps/api':
         specifier: 2.1.1
         version: 2.1.1
@@ -1809,8 +1809,8 @@ packages:
     resolution: {tarball: https://codeload.github.com/CuriousCorrelation/tauri-plugin-appload/tar.gz/1c2e8b19db7f1b6af6d00abb907f15cdc2017298}
     version: 0.1.0
 
-  '@CuriousCorrelation/plugin-relay@https://codeload.github.com/CuriousCorrelation/tauri-plugin-relay/tar.gz/86e1ccf327e60442c6ae9c2911d9359bf9e32c1b':
-    resolution: {tarball: https://codeload.github.com/CuriousCorrelation/tauri-plugin-relay/tar.gz/86e1ccf327e60442c6ae9c2911d9359bf9e32c1b}
+  '@CuriousCorrelation/plugin-relay@https://codeload.github.com/CuriousCorrelation/tauri-plugin-relay/tar.gz/fee58601fb4b0c129b5a9d87aaa0f9f87c26cd09':
+    resolution: {tarball: https://codeload.github.com/CuriousCorrelation/tauri-plugin-relay/tar.gz/fee58601fb4b0c129b5a9d87aaa0f9f87c26cd09}
     version: 0.1.0
 
   '@alloc/quick-lru@5.2.0':
@@ -13213,7 +13213,7 @@ snapshots:
     dependencies:
       '@tauri-apps/api': 2.1.1
 
-  '@CuriousCorrelation/plugin-relay@https://codeload.github.com/CuriousCorrelation/tauri-plugin-relay/tar.gz/86e1ccf327e60442c6ae9c2911d9359bf9e32c1b':
+  '@CuriousCorrelation/plugin-relay@https://codeload.github.com/CuriousCorrelation/tauri-plugin-relay/tar.gz/fee58601fb4b0c129b5a9d87aaa0f9f87c26cd09':
     dependencies:
       '@tauri-apps/api': 2.1.1