diff --git a/crates/common/src/interop/ssv/utils.rs b/crates/common/src/interop/ssv/utils.rs
index cfe1f812..add840c5 100644
--- a/crates/common/src/interop/ssv/utils.rs
+++ b/crates/common/src/interop/ssv/utils.rs
@@ -17,8 +17,13 @@ pub async fn request_ssv_pubkeys_from_ssv_node(
     http_timeout: Duration,
 ) -> eyre::Result<SSVNodeResponse> {
     let client = reqwest::ClientBuilder::new().timeout(http_timeout).build()?;
+    // The SSV node API expects operator IDs as numeric (uint64) values. Serializing
+    // the U256 directly emits a JSON string, which the node rejects with a 400,
+    // so narrow it to a u64 first.
+    let operator_id = u64::try_from(node_operator_id)
+        .map_err(|e| eyre::eyre!("SSV node operator ID does not fit in u64: {e}"))?;
     let body = json!({
-        "operators": [node_operator_id]
+        "operators": [operator_id]
     });
     let response = client.get(url).json(&body).send().await.map_err(|e| {
         if e.is_timeout() {
@@ -51,3 +56,25 @@ pub async fn request_ssv_pubkeys_from_public_api(
     serde_json::from_slice::<SSVPublicResponse>(&body_bytes)
         .wrap_err("failed to parse SSV response")
 }
+
+#[cfg(test)]
+mod tests {
+    use alloy::primitives::U256;
+    use serde_json::json;
+
+    #[test]
+    fn ssv_node_request_serializes_operator_as_number() {
+        let node_operator_id = U256::from(100u64);
+
+        // Regression guard: serializing the U256 directly emits a (hex) JSON string,
+        // which the SSV node rejects ("cannot unmarshal string into ...
+        // uint64").
+        let stringy = serde_json::to_string(&json!({ "operators": [node_operator_id] })).unwrap();
+        assert_eq!(stringy, r#"{"operators":["0x64"]}"#);
+
+        // The fix narrows to u64 so the operator ID is emitted as a numeric value.
+        let operator_id = u64::try_from(node_operator_id).unwrap();
+        let numeric = serde_json::to_string(&json!({ "operators": [operator_id] })).unwrap();
+        assert_eq!(numeric, r#"{"operators":[100]}"#);
+    }
+}