feat: omit Content-Type header for empty responses

Author: cablehead

CLAUDE.md

@@ -26,7 +26,7 @@ Prefer calm, matter-of-fact technical tone.
 ## Code Quality
 
 Always run `./scripts/check.sh` before committing. Use `cargo fmt` to fix
-formatting issues.
+formatting issues. Use ASCII characters only in code, comments, and documentation.
 
 ## Release Process
 

README.md

@@ -254,10 +254,14 @@ Content-type is determined in the following order of precedence:
 
 2. Pipeline metadata content-type (e.g., from `to yaml` or
    `metadata set --content-type`)
-3. For Record values with no content-type, defaults to `application/json`
-4. For lists or streams of records, defaults to `application/x-ndjson` (JSONL)
-5. For binary values or byte streams, defaults to `application/octet-stream`
-6. Otherwise defaults to `text/html; charset=utf-8`
+
+3. Inferred from value type:
+   - Record -> `application/json`
+   - List or stream of records -> `application/x-ndjson` (JSONL)
+   - Binary or byte stream -> `application/octet-stream`
+   - Empty (null) -> no Content-Type header
+
+4. Default: `text/html; charset=utf-8`
 
 Examples:
 
@@ -268,16 +272,13 @@ Examples:
 # 2. Pipeline metadata
 {|req| ls | to yaml }  # Returns as application/x-yaml
 
-# 3. Record auto-converts to JSON
-{|req| {foo: "bar"} }  # Returns as application/json
-
-# 4. List of records auto-converts to JSONL (newline-delimited JSON)
-{|req| [{a: 1}, {b: 2}, {c: 3}] }  # Returns as application/x-ndjson
-
-# 5. Binary data
-{|req| 0x[deadbeef] }  # Returns as application/octet-stream
+# 3. Inferred from value type
+{|req| {foo: "bar"} }                 # Record -> application/json
+{|req| [{a: 1}, {b: 2}, {c: 3}] }     # List of records -> application/x-ndjson
+{|req| 0x[deadbeef] }                 # Binary -> application/octet-stream
+{|req| null }                         # Empty -> no Content-Type header
 
-# 6. Default
+# 4. Default
 {|req| "Hello" }  # Returns as text/html; charset=utf-8
 ```
 

src/handler.rs

@@ -330,9 +330,10 @@ async fn build_normal_response(
     let mut header_map = hyper::header::HeaderMap::new();
 
     // Content-type precedence:
-    // 1. Explicit in http.response headers (highest priority)
-    // 2. Pipeline metadata content_type (from `to json`, etc.)
-    // 3. Auto-detection default
+    // 1. Explicit in http.response headers
+    // 2. Pipeline metadata (from `to json`, etc.)
+    // 3. Inferred: record->json, binary->octet-stream, list/stream of records->ndjson, empty->None
+    // 4. Default: text/html
     let content_type = http_meta
         .headers
         .get("content-type")
@@ -342,12 +343,20 @@ async fn build_normal_response(
             crate::response::HeaderValue::Multiple(v) => v.first().cloned(),
         })
         .or(inferred_content_type)
-        .unwrap_or("text/html; charset=utf-8".to_string());
+        .or_else(|| {
+            if matches!(body, ResponseTransport::Empty) {
+                None
+            } else {
+                Some("text/html; charset=utf-8".to_string())
+            }
+        });
 
-    header_map.insert(
-        hyper::header::CONTENT_TYPE,
-        hyper::header::HeaderValue::from_str(&content_type)?,
-    );
+    if let Some(ref ct) = content_type {
+        header_map.insert(
+            hyper::header::CONTENT_TYPE,
+            hyper::header::HeaderValue::from_str(ct)?,
+        );
+    }
 
     // Add compression headers if using brotli
     if use_brotli {
@@ -362,7 +371,7 @@ async fn build_normal_response(
     }
 
     // Add SSE-required headers for event streams
-    let is_sse = content_type == "text/event-stream";
+    let is_sse = content_type.as_deref() == Some("text/event-stream");
     if is_sse {
         header_map.insert(
             hyper::header::CACHE_CONTROL,

src/test_handler.rs

@@ -224,6 +224,20 @@ async fn test_content_type_precedence() {
         .await
         .unwrap();
     assert_eq!(resp4.headers()["content-type"], "text/html; charset=utf-8");
+
+    // 5. Empty body has no Content-Type header
+    let req5 = Request::builder()
+        .uri("/")
+        .body(Empty::<Bytes>::new())
+        .unwrap();
+    let engine = Arc::new(ArcSwap::from_pointee(test_engine(r#"{|req| null}"#)));
+    let resp5 = handle(engine.clone(), None, no_trusted_proxies(), req5)
+        .await
+        .unwrap();
+    assert!(
+        resp5.headers().get("content-type").is_none(),
+        "Empty body should not have Content-Type header"
+    );
 }
 
 #[tokio::test]