fix(a2a-pb): ignore unknown fields in ProtoJSON decode (spec §5.7)

a2a-pb/build.rs

@@ -120,6 +120,10 @@ fn main() -> Result<(), Box<dyn std::error::Error>> {
     let mut builder = pbjson_build::Builder::new();
     builder.out_dir(&out_dir);
     builder.register_descriptors(&descriptor_set)?;
+    // Spec §5.7: implementations SHOULD ignore unrecognized fields in
+    // messages, allowing for forward compatibility. Unknown enum string
+    // values are still rejected.
+    builder.ignore_unknown_fields();
     builder.build(PROTOJSON_PACKAGES)?;
     patch_generated_protojson_serde(&out_dir)?;
 

a2a-pb/tests/protojson.rs

@@ -227,6 +227,46 @@ fn agent_card_protojson_deserializes_null_skills_as_empty() {
     assert_eq!(parsed, expected);
 }
 
+/// Spec §5.7: implementations SHOULD ignore unrecognized fields in messages,
+/// allowing for forward compatibility. Unknown fields must be tolerated both
+/// at the top level and inside nested messages.
+#[test]
+fn task_protojson_decode_ignores_unknown_fields() {
+    let json = json!({
+        "id": "task-1",
+        "contextId": "ctx-1",
+        "status": {
+            "state": "TASK_STATE_COMPLETED",
+            "futureStatusField": "ignored"
+        },
+        "futureTopLevelField": {"nested": ["values", 1, true]}
+    });
+
+    let task = a2a_pb::protojson_conv::from_value::<a2a::Task>(json)
+        .expect("unknown fields must be ignored");
+    assert_eq!(task.id, "task-1");
+    assert_eq!(task.context_id, "ctx-1");
+    assert_eq!(task.status.state, a2a::TaskState::Completed);
+}
+
+/// Unknown *enum string values* are still rejected: only unrecognized fields
+/// are tolerated (`ignore_unknown_enum_variants` is intentionally not
+/// enabled).
+#[test]
+fn task_protojson_decode_rejects_unknown_enum_values() {
+    let json = json!({
+        "id": "task-1",
+        "contextId": "ctx-1",
+        "status": {"state": "TASK_STATE_FROM_THE_FUTURE"}
+    });
+
+    let result = a2a_pb::protojson_conv::from_value::<a2a::Task>(json);
+    assert!(
+        result.is_err(),
+        "unknown enum values must still be rejected"
+    );
+}
+
 #[test]
 fn stream_response_protojson_roundtrip_serializes_oneof_payloads() {
     let response = StreamResponse {

a2a-server/src/jsonrpc.rs

@@ -410,12 +410,37 @@ mod tests {
     #[tokio::test]
     async fn test_invalid_params() {
         let app = make_app();
-        let params = serde_json::json!({"bogus": true});
+        // Type mismatch: `message` must be an object. A merely-unknown field
+        // is not invalid params; it is ignored per spec §5.7.
+        let params = serde_json::json!({"message": "not-an-object"});
         let resp = post_jsonrpc(app, methods::SEND_MESSAGE, params).await;
         assert!(resp.error.is_some());
         assert_eq!(resp.error.unwrap().code, error_code::PARSE_ERROR);
     }
 
+    /// Spec §5.7: unrecognized fields in request params are ignored for
+    /// forward compatibility instead of being rejected.
+    #[tokio::test]
+    async fn test_unknown_params_fields_are_ignored() {
+        let app = make_app();
+        let params = serde_json::json!({
+            "message": {
+                "messageId": "m1",
+                "role": "ROLE_USER",
+                "parts": [{"text": "hi"}],
+                "futureField": true
+            },
+            "anotherFutureField": {"nested": 1}
+        });
+        let resp = post_jsonrpc(app, methods::SEND_MESSAGE, params).await;
+        assert!(
+            resp.error.is_none(),
+            "unknown fields must be ignored: {:?}",
+            resp.error
+        );
+        assert!(resp.result.is_some());
+    }
+
     #[tokio::test]
     async fn test_list_tasks() {
         let app = make_app();