diff --git a/crates/proc-macro-api/src/msg.rs b/crates/proc-macro-api/src/msg.rs
index dd882d82fb64..f266050012fd 100644
--- a/crates/proc-macro-api/src/msg.rs
+++ b/crates/proc-macro-api/src/msg.rs
@@ -17,8 +17,9 @@ pub const NO_VERSION_CHECK_VERSION: u32 = 0;
 pub const VERSION_CHECK_VERSION: u32 = 1;
 pub const ENCODE_CLOSE_SPAN_VERSION: u32 = 2;
 pub const HAS_GLOBAL_SPANS: u32 = 3;
+pub const EXPAND_MACRO_RESPONSE_PREFIX: u32 = 4;
 
-pub const CURRENT_API_VERSION: u32 = HAS_GLOBAL_SPANS;
+pub const CURRENT_API_VERSION: u32 = EXPAND_MACRO_RESPONSE_PREFIX;
 
 #[derive(Debug, Serialize, Deserialize)]
 pub enum Request {
@@ -96,13 +97,21 @@ pub trait Message: Serialize + DeserializeOwned {
         })
     }
     fn write(self, out: &mut impl Write) -> io::Result<()> {
-        let text = serde_json::to_string(&self)?;
-        write_json(out, &text)
+        write_message_as_json_to(out, self)
     }
 }
 
 impl Message for Request {}
-impl Message for Response {}
+
+impl Message for Response {
+    fn write(self, out: &mut impl Write) -> io::Result<()> {
+        match (&self, std::env::var("RA_EXPAND_MACRO_RESPONSE_PREFIX")) {
+            (Response::ExpandMacro(..), Ok(prefix)) => out.write_all(prefix.as_bytes())?,
+            _ => {}
+        }
+        write_message_as_json_to(out, self)
+    }
+}
 
 fn read_json<'a>(inp: &mut impl BufRead, buf: &'a mut String) -> io::Result<Option<&'a String>> {
     loop {
@@ -126,6 +135,11 @@ fn read_json<'a>(inp: &mut impl BufRead, buf: &'a mut String) -> io::Result<Opti
     }
 }
 
+fn write_message_as_json_to(out: &mut impl Write, m: impl Message) -> io::Result<()> {
+    let text = serde_json::to_string(&m)?;
+    write_json(out, &text)
+}
+
 fn write_json(out: &mut impl Write, msg: &str) -> io::Result<()> {
     tracing::debug!("> {}", msg);
     out.write_all(msg.as_bytes())?;