diff --git a/adapter/rest/src/main.rs b/adapter/rest/src/main.rs
index 847a0eb..e19a985 100644
--- a/adapter/rest/src/main.rs
+++ b/adapter/rest/src/main.rs
@@ -14,6 +14,7 @@ use hyper::{
     server::conn::http1,
 };
 use hyper_util::rt::TokioIo;
+use serde_json::{Map, Number, Value as J};
 use std::collections::HashMap;
 use std::convert::Infallible;
 use std::net::SocketAddr;
@@ -58,6 +59,45 @@ fn json_error(status: StatusCode, msg: &str) -> Response<Full<Bytes>> {
         .unwrap()
 }
 
+fn pb_value_to_json(v: &Value) -> serde_json::Value {
+    match v.kind.as_ref() {
+        None => J::Null,
+
+        Some(Kind::NullValue(_)) => J::Null,
+
+        Some(Kind::BoolValue(b)) => J::Bool(*b),
+
+        Some(Kind::StringValue(s)) => J::String(s.clone()),
+
+        Some(Kind::NumberValue(n)) => {
+            if let Some(num) = Number::from_f64(*n) {
+                J::Number(num)
+            } else if n.is_nan() {
+                J::String("NaN".to_string())
+            } else if n.is_infinite() {
+                if n.is_sign_positive() {
+                    J::String("Infinity".to_string())
+                } else {
+                    J::String("-Infinity".to_string())
+                }
+            } else {
+                // Fallback for unexpected cases where from_f64 returns None.
+                J::Null
+            }
+        }
+
+        Some(Kind::ListValue(lv)) => J::Array(lv.values.iter().map(pb_value_to_json).collect()),
+
+        Some(Kind::StructValue(st)) => {
+            let mut obj = Map::new();
+            for (k, v) in &st.fields {
+                obj.insert(k.clone(), pb_value_to_json(v));
+            }
+            J::Object(obj)
+        }
+    }
+}
+
 fn build_response(
     status: StatusCode,
     headers: HashMap<String, String>,
@@ -210,8 +250,13 @@ async fn execute_flow_to_hyper_response(
             };
 
             // payload -> json bytes
-            let json = serde_json::to_vec_pretty(payload_val).unwrap_or_else(|err| {
-                format!(r#"{{"error":"Serialization failed: {}"}}"#, err).into_bytes()
+            let json_val = pb_value_to_json(payload_val);
+            let json = serde_json::to_vec_pretty(&json_val).unwrap_or_else(|err| {
+                let fallback = serde_json::json!({
+                    "error": format!("Serialization failed: {}", err),
+                });
+                serde_json::to_vec(&fallback)
+                    .unwrap_or_else(|_| br#"{"error":"Serialization failed"}"#.to_vec())
             });
 
             let status =